Thursday, June 30, 2011
Thursday, September 30, 2010
Polymer Dictionary
Acid scavengers
Neutralise acidic residues to protect processing equipment from corrosion.
Activity
is a measure which indicates how much polymer a certain amount of catalyst produces in a specified time. For a full definition the polymerisation conditions (mode, temperature, pressure etc.) have to be defined.
Additive
Additives are added to the polymer to protect it from degradation and to give the material desired properties. Normally the additives are mixed with the powder before the extruder.
Adiabatic
An adiabatic process is a process that occurs without heat entering or leaving a system. In general, an adiabatic change involves a fall or rise in temperature of the system. An adiabatic chemical process does not have any external heaters or coolers.
Active site
The site on the catalyst at which the polymerisation takes place.
Anti-blocking and slip agents
Surface-modifying additives to reduce friction and tackiness of polyolefin films.
Antioxidants
A group of substances being able to inhibit radical reactions in the polymer and thus prevent degradation processes. Different types are available: sterically hindered phenols and phosphites as base for polyolefins, sulphur based heat stabilisers and C-radical scavengers for special applications.
Antistatic agents
Surface-modifying additives to reduce surface resistance of polyolefins, eliminating charge build-up and dust collection; an example of a widely used antistatic agent is glycerine-monostearate (GMS).Different types of antistatic agents are used in polymerisation reactors to prevent the formed polymer powder from adhering to the reactor wall.
BorAPC
Borealis’ tailor-made Advanced Process Control (APC) for polyolefin polymerisation processes. APC is process control utilising detailed knowledge, compared with traditional process control where the method is standard and not specifically adapted to the process. The basis for APC is knowledge of a process expressed as mathematical models. Model predictive control (MPC) ensures that control actions also compensate for the future behaviour of the process, leading to optimal grade changes. BorAPC includes non-linear MPC, making it well-suited for polymerisation processes and ahead of what "normal" control vendors offer.
Blow moulding
Extrusion of a parison (hollow melt tube) which is forced to a mould caving by internal pressure and cooled down to form a hollow article. Used for the production of bottles, wide-mouth containers, petrol tanks etc.
Bimodal
A polyethylene with a relatively high molecular weight fraction and a relatively low molecular weight fraction is called bimodal polyethylene. The comonomer contents of the two fractions are often different.
Comonomer sensitivity
A property of the catalyst which indicates how eagerly the catalyst reacts with a comonomer, defining the relation between the relation of partial concentrations (pressures) of different monomers in the polymerisation process and the relative content in the produced polymer.. Comonomer sensitivity is determined in a test polymerisation in standard conditions.
Capillary rheometry
Actual capillary rheometry gives information about flow properties of polymer melts. Results are given as a viscosity function, that is viscosity as a function of shear rate or shear stress. Compared to MFR measurement the shear rate range is much wider and the rates in processing applications are reached. With many products, however, flow instabilities (melt fracture) at high rates limit the useful range. A capillary rheometer consists of a barrel, piston and die. The sample is put into the barrel, heated and pushed by the piston through the die. The piston is driven by hydraulic pressure which allows more flexible choice of rates compared to MFR measurements. In addition, longer dies can be used to avoid the errors coming from the entrance and exit pressure losses.
Chromium Catalysts
• Produces polymer having relatively broad molecular distribution (MWD)
• Control of Molecular Weight (MW) by heat and elimination
• Comonomer sensitive
• Does not give stereospecific polymers (e.g. isotactic PP)
Catalyst
A substance added into the reactor to initiate or accelerate a polymerisation reaction. It’s used in small quantities and is not chemically consumed during the polymerisation. Because of the catalyst’s high activity, residues in the polymer are so low that they do not need to be removed.
There are several different types of catalysts used in olefin polymerisation. The three main groups are Ziegler-Natta, chromium and single-site (metallocene).
To make a catalyst active in polymerisation, a cocatalyst is often used. Cocatalysts are highly reactive compounds, usually group I-III metal alkyls.They are used in excess compared to the catalyst, and prevent the catalyst from being poisoned by the impurities in the polymerisation reactor.
Comonomer
Comonomer is a monomer which is used to modify the polymer properties, e.g. to reduce the density of the polymer. Different types of copolymerisation processes are possible, leading to different structures and properties of the produced copolymer. In polyethylene, higher a-olefins like butene, hexene or octene are used as comonomers, in polypropylene, mostly ethylene and sometimes butene.
Compounding
In compounding a final homogenised and stabilised product in pellet form is made. Also, this term is used for the production of compounds incorporating reinforcing fibres or fillers as well as blends with other polymers (in the case of polyolefins, frequently elastomers).
Copolymer
Two monomers polymerised together to form a polymer.
Catalyst support
A support material on which the active compounds of the catalyst are deposited. Silica is commonly used as support material. Also called a carrier.
Creep tests
(a) Solid-state creep tests are used to measure long term behaviour of mechanically loaded material. Creep is time-dependent strain resulting from constant loading. Tensile loading is most common here but flexural loading can be used as well, testing time is typically 1000 hrs. Several loading levels are used to get an overview of creep behaviour of the material.
(b) Melt creep tests are a type of rheological measurement for the determination of the creep compliance of a polymer melt; they are carried out in the region of linear viscoelasticity.
Carbon Black
Black pigment used in concentrations of about 2.5% in order to ensure excellent UV-resistance. In higher concentrations, 30-40%, Carbon Black is used to obtain semiconductive properties used in Cable applications. The type of CB and the particle size is very important for the end-product properties.
Cocatalyst
Most often an organometallic compound that is used to activate the catalyst.
Crystallinity
also designated “degree of crystallinity”, it gives the relative fraction of a solid polymer having crystalline regularity. The crystallinity can be determined with various measuring techniques like DSC, density measurement or WAXS, which has the additional advantage that the relative amounts of the different crystal modifications can be determined as well. All crystallinity values are relative and the applied reference values should be quoted.
Density
An important parameter for polyethylene, affecting most end product physical properties like stiffness, impact strength and optical properties. The density is normally given in g/cm3, although the correct SI unit would be kg/m3. Two basic measuring methods are used: (a) Density gradient column is the basic method for density measurement. In it a density gradient is prepared by mixing two liquids of different densities so that density increases uniformly from top to bottom. Small test pieces are immersed into this density gradient, and the density of the polymer is determined based on the equilibrium position of the pieces and calibration floats. (b) Pyknometer measurements rely on calibrated volumes and the density difference to a reference fluid; compression-moulded samples are normally used here.
Dynamic rheometry
Dynamic rheometry gives information both about flow and elastic properties of polymer melts. In a dynamic measurement the sample is put between two round plates or between a cone and a plate in the oven. The system is heated to a desired temperature and a sinusoidal deformation at different frequencies is applied.
As a result we get storage and loss modulus as a function of frequency. Storage modulus is connected with elastic energy, while loss modulus corresponds to viscose energy. From the above parameters we can calculate complex viscosity as a function of frequency.
With most of polyolefin products this is the same as the conventional viscosity function, which is viscosity as a function of shear rate. The dynamic measurements are limited to relatively low shear rates. On the other hand, a benefit compared to capillary rheometry is, that we get reliable information also about elasticity.
In addition, the obtained properties are much more sensitive for small differences in polymer structure which play an important role in processing and end-use properties.
Differential Scanning Calorimetry
In Differential Scanning Calorimetry (DSC) the sample and a reference material are heated and cooled in a defined manner. DSC provides a rapid method for the determination of the temperature profile of a polymeric material. This includes e.g. measurement of heat capacity, specific heat, melting, crystallisation and glass transition temperatures, heat (enthalpy) of fusion, crystallisation, study of thermal stability or reaction kinetics and in certain cases identification of the polymer or polymer mixture.
Dynamic Mechanical Thermal Analysis
Dynamic Mechanical Thermal Analysis (DMTA): Many polymeric materials exhibit time-dependent, reversible viscoelastic properties in deformation. Dynamic mechanical tests are characterized by an application of a small stress in sinusoidal fashion and a continuous scan of resonant frequency of vibration and damping vs. temperature is produced. This is transformed into dynamic storage modulus, dynamic loss modulus and damping factor by the instrument software.
Extruder
Equipment for melting, pressurising and homogenising plastics by means of a rotating screw. Different configurations are possible; the most simple one being a „_ single screw extruder mainly used for conversion processes like cast film extrusion, fibre spinning, film blowing or pipe and profile extrusion. For more demanding applications like compounding of polymer powder before „_ pelletisation or the mixing of polymers, twin screw extruders are used where the two normally intermeshing screws can be moved in a corotating (same direction) or counterrotating (opposite direction) fashion. These machines normally contain different mixing elements, most important being kneading blocks for the local creation of extensional flow.
Environmental stress cracking
Under certain conditions of stress and environment like oils, detergents or soaps, ethylene plastics may fail mechanically by cracking. This phenomenon is called environmental stress cracking resistance (ESCR). ESCR properties can be tested in accelerated tests by presence of starter notches, surface active agent, elevated temperature and mechanical loading. Test result is the time when 50 % of the samples have failed.
Flexural test
Like tensile test, flexural test is also used to characterize strength and stiffness of the material. Flexural properties are obtained by placing a specimen horizontally on two supports. A load is applied at a specified rate in the center, normal to specimen longitudinal axis. Measured force - deformation data is used to calculate flexural modulus and flexural strength. Flexural tests can be used if material is too brittle to withstand compressive forces of fixing in tensile grips.
Friction
When handling and storing plastic packages it is important that friction between packages and pallets, shelves etc. or between packages itself is suitable. Coefficient of starting and sliding friction (static and dynamic friction) of film is measured by using a moving sled, force transducer and a static horizontal testing table. Friction can be measured either film against film or film versus metal or other material. It is determined by the material structure and the additivation, especially the addition of slip agents and antiblocking agents.
FTIR Spectroscopy
Infrared radiation induces vibrations of molecules and molecular segments. Absorption of radiation occurs only if the vibration results in the change of the molecular dipole moment. The strength of the vibrating bonds and their dipole moment affect the absorption band intensities and positions which makes FTIR spectroscopy a very useful method in qualitative polymer analysis. Also several fast quantitative FTIR methods have been developed for the structural characterisation of polyolefins.
Polymer crystallinity and orientation implies a restriction in the movement of some molecules and consequent improvement in polymer mechanical properties. This restriction in the movements reflects also to the FTIR spectrum and thus the amorphous and crystalline polymer phases can be identified.
Fines
Polymer particles having a small diameter. Those particles whose diameter is < 0.105 mm are usually classified as fines. A high amount of fines can severely reduce the operability of a plant.
Film impact
Film impact testing is the very basic, simple and fast method to find out overall mechanical strength of the film product. Impact resistance of film is determined by using free-falling dart method. In this method the impact is generated by dropping a dart from constant height on clamped film sample. Mass of the dart is varied and a mass causing failure of 50 % of the specimen tested is the result.
Fee radical
An atom or group of atoms with an unpaired valence electron (Valence electron is an electron in the outer shell of the atom that takes part in forming chemical bonds). Free radicals are formed in High-pressure polyolefin polymerisation by decomposing organic peroxides initiating the polymerisation process and then the propagation of the ethylene monomer onto the radicals forming long chains.
Fractionation
Analysis of the whole polymer gives the average structure and composition of the material. Polymer fractionation is useful when detailed information about polymer structure is desired. Fractionation serves as a general term for the methods of separating macromolecules according to their molar mass, chemical composition or microstructural parameters. The choice of the fractionation method depends on the polymeric material and the problem to be solved.
Film Application tests
Mechanical properties of film are very much dependent on film blowing parameters like extrusion screw, die construction, blow-up ratio and cooling system. Customary film tests are carried out by using film samples.
Film blowing
Extrusion of the melt through a ring from die, blowing up, and cooling to form a thin film. Film blowing is used to produce very thin films of high mechanial strength, mostly from polyethylene.
Gas phase reactor
Type of reactor to polymerise in the gas phase (i.e. all monomers are in the gaseous form); different designs are possible. In processes for the production of polyolefins, often fluidised bed reactors are used. Other constructions involve stirred or slow-flowing beds in different reactor geometries.
Gloss
Brightness or luster of a plastic resulting from a smooth surface. Gloss is determined by measuring the amount of light reflected from a surface, where the position and strength of the illumination has to be defined as well as the reflection angle covered by the measurement (normally 20°).
Gel
A heterogeneity in the polymer that appear as local thickness increase in films or thin-walled articles. Gels are caused by inclusion of high molecular weight particles with a higher viscosity than the surrounding, in some instances also including cross-linked molecules; normally the melting point is also higher than the surrounding. In some instances one also refer to contamination as gels, as examples can be mentioned fibre gels or dirt particles. The cause of this can be badly dispersed additives (agglomeration, dispersion) or other substances. This is not actually a gel, but sometimes called so for simplicity.
Heterophasic copolymers
Heterophasic copolymers (HECO) also contain ethylene (and possibly higher a-olefins) as comonomer. The production is however carried out in a two-stage process, resulting in a multiphase structure with a homopolymer matrix and inclusions consisting of amorphous EP-copolymer (“rubber”) and crystalline PE. Variations of molar mass and composition of the elastomeric phase in relation to the matrix allow a wide variation of properties (stiffness, toughness and transparency).
Heterogeneous
Something relating to two ore more phases, i.e. not mixed so as to be homogeneous.
Homogeneous
Something relating to only one phase, i.e. so well mixed that the mixture have the same composition throughout.
Haze
Cloudiness in polymer films or thin-walled injection-moulded parts, resulting from light scattering in the material (reducing the transparency). Measured in transmission with a well-defined light source, results are given on a percentage scale.
Hydrogen sensitivity
A property of the catalyst which indicates how sensitive the molecular weight of the polymer is to hydrogen concentration, (also: hydrogen response). The property also depends on the polymerisation conditions (mode, temperature, pressure etc.).
Heat deflection temperature
Heat deflection temperature (HDT) indicates how loaded material deforms at higher temperatures. Test bars are placed in a heating bath, resting horizontally on two supports. A constant load is applied in the center of the specimen and the bath temperature is raised at a constant rate. The temperature of the bath at which the flexural deflection of the loading point has reached a predefined level is the heat deflection temperature of the material.
Homopolymers
Homopolymers (HOMO) consist only of propylene units; their properties can be regulated via molar mass distribution and the degree of isotacticity (stereoregular structure).
High pressure polyethylene (HPPE)
The collective name given to polyethylene produced by high pressure processes. The process is using free radical polymerisation and the polymerisation is performed at pressures of 120-350 MPa (1200-3500 bar) and at temperatures of 150-3500C. Homopolymer and copolymers of ethylene is made by this process. (See more descriptions in process description).
HDPE
High density polyethylene
Coming from a low pressure process based on special catalysts (Ti- or Cr-based), HDPE has a linear polymeric chain, combined with a higher density and melting point. To fit special applications, the molar mass distribution can be adjusted in a wide range by means of catalysts or polymerisation technology. New applications are opened for HDPE as well as other low pressure PEs by applying metallocene (single-site) catalysts, which allow the production of very narrow molar mass distributions and the better distribution of comonomers.
Hardness
Resistance of a (polymer) surface to deformation. The different hardness measures applied for characterising polymers are:
(a) Shore hardness (two scales, A for softer and D for harder materials)
(b) Ball indentation hardness (also useable on profiled surfaces because of bigger measuring device).
(c) Vickers hardness (normally used for steel).
Injection moulding
A repetitive process in which plastic is melted and injected into a mould cavity where the article is cooled down. After cooling, the mould opens and the article is ejected. ). The melt is prepared in a screw setup similar to an extruder; the screw acts in melting and homogenising the polymer while slowly retracted to build up the melt reservoir necessary for the injection step. Then, the screw is used as a plunger in a forward movement to inject the melt through the runner , a manifold in case of a multiple mould and the gate into the mould. In the injection moulding process, filling of the mould and solidification takes place partially at the ssame time, resulting in a very complex morphology in case of semicrystalline polymers.
Impact Strength
Impact Strength characterizes material behaviour under high speed loading. Pendulum and falling weight type testers are applied here. Specimen can be either plaques, notched or unnotched bars or parts of finished products. There are several impact methods like Charpy impact test, instrumented puncture test and tensile impact test. Generally impact test shows the energy which is needed to break or puncture specimen under specified conditions. In some tests it is possible to accurately record force - deformation transient curves and calculate specific parameters describing material behaviour under impact loading.
LLDPE
Linear low density polyethylene
Based on the same technology as HDPE, but the density is reduced through the incorporation of longer a-olefins as comonomers. With this, another way of adapting the products to the final applications turns up. Single-site catalysts allow an expansion of the application range, mainly by allowing higher comonomer contents and a more even distribution of these.
LDPE
(Low density polyethylene)
A product from a high-pressure polymerisation process, LDPE is characterised by its highly branched chain structure. As a result of this, density and crystallinity are reduced while the processability in a variety of conversion technologies is greatly facilitated. The addition of special comonomers can further expand the property range of these grades.
Low pressure polyethylene (LPPE)
The collective name given to polyethylene produced in low pressure processes. The polymerisation is utilising a catalyst to get the neccessary rate of polymerisation. Homopolymer and copolymers of ethylene are made by these processes.
Long chain branch
A brach with many (>20) carbon atoms. Typical for low density PE.
Loop reactor
A bulk (slurry) polymerisation reactor which is made of a closed long tube through which the slurry is circulated. Loop reactors are characterised by a very narrow residence time distribution, leading to uniform product characteristics.
Metal deactivators
Some metal ions may contribute to accelerated degradation. For polyolefins in contact with metals, such as cable insulation of copper wires, metal deactivators are added to passivate the degradation reactions.
Melt strength
A property of the polymer melt which indicates its ability to withstand drawing without breaking. Melt strength is improved by the presence of high molecular weight tail or long chain branches.
MDPE
Medium low density polyethylene
Based on the same technology as HDPE, but the density is reduced through the incorporation of longer a-olefins as comonomers. With this, another way of adapting the products to the final applications turns up. Single-site catalysts allow an expansion of the application range, mainly by allowing higher comonomer contents and a more even distribution of these.
MFR
Melt mass flow rate or melt flow rate (MFR) gives an indication of melt processing properties of a polymer. Shear rate range is, however, narrow and much lower than in the most of processing applications. It gives thus very limited information about melt flow behaviour. The method is a kind of capillary measurement.
The equipment consists of a barrel, a piston with a place for loading masses and a die. The sample is charged into the barrel and heated up to a specified temperature. After that the piston is loaded extruding the polymer through the die.
Melt flow rate (MFR) is defined as a mass flown through the die in 10 min. The weights mostly used with polyolefins are 2.16 kg, 5 kg, 10 kg and 21.6 kg. In addition to the evaluation of flow properties MFR serves as a rough estimation of molecular weight.
Usually also Flow Rate Ratio (FRR) is determined. This ia defined as a ratio of two MFR values obtained with two different weights. FRR can be used as a rough estimation of molecular weight distribution.
Monomer
Monomer forms a repeating unit in a polymer chain.
Morphology
Morphology is the internal or external structure of a solid substance. Important sub-definitions include
(a)Crystal morphology of a polymer film or part (b)Phase morphology of heterophasic polymer systems (blend, copolymer) (c) Powder morphology of a polymer powder from the polymerisation reactor.
Molecular Weight Distribution
The relation between the number of molecules in a polymer and their individual chain length. It is often also called “molar mass distribution” (MMD). Technical polymers are always polydisperse as a result of the nature of the catalyst and the polymerisation conditions. The MWD is often characterised by size exclusion chromatography (SEC), but it can also be calculated approximately from rheological data for purely linear polymers.
Metallocene
A catalyst which contains a metallocene compound of a transition metal. Often used as a synonym for single site.
Multimodal polymer
Multimodal polymer includes more than one molecular weight fraction.
Melt tension test
Extensional properties play an important role in some processes such as melt spinning, blow moulding and film blowing. Melt tension test is a simple method for characterisation of extensional properties. The measuring principal copies industrial spinning process. A fluid is pressed or extruded through a round hole die, the resulting thread is hauled off with increasing speed and the applied extensional force is measured. The force is recorded as a function of haul-off speed or draw-down ratio, which is the ratio of the haul-off speed to the initial speed. Melt strength is defined as a force at filament break, while drawability can be estimated qualitatively from the maximum draw-down ratio at the failure.
Nucleating agents
Substances with the ability to act as substrates for crystal formation by epitaxy, thus increasing the nucleation density. Improve crystallisation behaviour in processing, i.e., cycle time or line speed, as well as crystallinity and morphology in final product, improving optical and mechanical properties.
Oxygen induction time (OIT)
A relative measure of a level or a degree of stabilisation (amount of remaining active antioxidant).
Optical properties
Optical properties are important in packaging applications where decorative printing is required or good visibility of the packed product is essential. Gloss and haze are used to characterize optical appearance of film products.
Optical Microscopy
Optical Microscopy (OM) is usually the most familiar tool to study material morphology and it offers many techniques for polymer characterisation including transmission, reflected light, dark field, polarized light, phase contrast and interference contrast microscopy. These techniques give information based upon the variations in light scattering, differences in refractive index and differential phase shifts of reflected or transmitted light. Light microscopy indicates the physical aspects of polymer composition, such as multiple phases present, the sizes and shapes of the particles and overall characteristics of the individual particles.
Pelletising
A process where the polymer powder is homogenised, addiviated and made to pellets. The polymer melt coming from the extruder is pressed through dies in a die plate and cut to pellets by knives under water (underwater pelletising) or cooled to solid strands and then cut to pellets (strand pelletising).
Permeability
Material constant characterising gas penetration, like water vapour, oxygen or carbondioxide.
Polymerisation
Polymerisation is a chemical reaction where polymer is synthesised from monomers.
Polypropylenes
Polypropylenes are thermoplastic polymers consisting mainly of propylene (C3H6), come frome a low pressure process based on catalysts and have generallya linear polymer chain.
Productivity
A property of the catalyst which indicates how much the catalyst has produced polymer in the process (kg polymer/g catalyst).
Prepolymerisation
A process where polymerisation is started to increase the activity of the catalyst getting over the induction time of the catalyst and not to overheat the catalyst particle by to sudden onset of polymerisation. Prepolymerisation will also help in ensuring more even polymerisation on the catalyst particles reducing the probability of creating a high fraction of fines in the later reaction steps.
Peroxide
Peroxides are a group of compounds that contain the O22- ion. It is used to initiate the polymerisation for high-pressure free radical polymerisation processes.
Polydispersity
Broadness of the molecular weight distribution. A commonly used measure, polydispersity index, is defined as the ratio of weight average molecular weight to the number average molecular weight.
Processability
A concept to describe the behaviour of the polymer in a converting process, such as blow moulding machine and film line. A good processability means a stable behaviour in the process and a high processing speed.
Puncture
In certain packaging applications film is stretched and wrapped around multiform items which can have sharp corners or other protrusions causing high point loading for the film. Puncture test is used to simulate this kind of film loading. Puncture resistance is measured by pressing a specially shaped probe through a clamped film sample. The maximum force, force at break, penetration distance and energy to break are determined.
Polyethylenes
Polyethylenes (or polythenes) are thermoplastic polymers consisting mainly of ethylene (C2H4), which are divided according to their density, melting point and molecular structure.
Random Copolymers
Random copolymers (RACO) are produced by statistical insertion of units of ethylene or higher a-olefins (mostly butene, also in combination with ethyle to give terpolymers). Through this, the glass transition temperature and melting point are lower than in homopolymers, allowing better sealability and application at temperatures below 0°C.
Shore Hardness
Resistance of a (polymer) surface to deformation. The different hardness measures applied for characterising polymers are:
(a) Shore hardness (two scales, A for softer and D for harder materials)
(b) Ball indentation hardness (also useable on profiled surfaces because of bigger measuring device).
(c) Vickers hardness (normally used for steel)
Slip agent
type of additive, which provides surface lubrication to the polymer during and after conversion; especially important for films. Usual slip agents are amides of fatty acids.
Size Exclusion Chromatography
(SEC, also called Gel Permeation Chromatography, GPC) is used to separate dissolved polymer molecules according to their coil diameter in solution. The chromatograms can be converted to molar mass distributions, which is an important parameter because it has a significant effect on the physical, mechanical and rheological properties of a polymer. It is also possible to calculate the average molar masses, Mn, Mv, Mw and Mz, and when on-line viscometric detection is used, to study long chain branching (LCB) and its distribution.
Solution Viscometry
In Solution Viscometry the rate at which a dilute polymer solution flows through a capillary is measured. Temperature and concentration of the polymer are carefully controlled. The parameter most often determined by dilute solution viscometry is intrinsic viscosity. Intrinsic viscosity is related to molar mass through a semi-empirical relationship called Mark-Houwink’s equation.
Shear thinning
A behaviour where the viscosity decreases when the shear stress increases. Typically broadening of the molecular weight distribution increases shear thinning.
Supercritical state
The critical state of a fluid is when the liquid and gas phase both have the same density. The fluid is then at its critical temperature, critical pressure, and critical volume. When the pressure or temperature exceeds this, the fluid is in its supercritical state. The loop reactor in the BORSTAR process is operated in supercritical state for some products.
Sheeting
Formation of sheet-like agglomerates in the gas phase reactor.
Single Site Catalysts
• Produces polymer having very narrow MWD
• Extremely hydrogen and comonomer sensitive
• Very uniform comonomer distribution
• Usually cocatalyst required
• Produces stereospecific polymers
Shrinkage
Decrease of dimension, created by the introduction of internal stress to the material during processing and cooling, or by ageing and post-crystallisation processes.
Scanning Electron Microscopy
In Scanning Electron Microscopy (SEM) a focused beam of electrons scans across the surface of the sample. It is detected synchronously to the position of the scanning electron beam and recorded in an image storage device. Differencies in the topography of the sample give rise to the differencies in the intensities of the signals formed by the secondary electrons in the detector resulting in a three dimensional image. Scanning electron microscopy is suited to most types of surface morphology studies. Rough topographic features, void content and particle agglomerations are easily revealed as well as the compositional differencies within material. Energy Dispersive X-ray Spectroscopy (EDS) or Wavelength Dispersive X-Ray Spectroscopy (WDS) in conjunction with SEM allows elemental analyses to be performed directly over the surface of the sample.
Tensile
Tensile properties are the most important indication of the strength and stiffnes of the material. The force necessary to pull the specimen apart is determined as well the deformation before breaking. Tensile modulus is used as a measure of stiffness and it is an important design parameter for end use applications. It is calculated based on measured force - deformation data. Tensile tests can be carried out at different temperatures according to application needs. Test automatisation and modern data logging systems are used in order to ensure high quality, reliable results.
Tear strength
Film must have certain resistance to tear loading. Tear strength indicates the average force required to propagate tearing through a specified length of the film sample. This test is suitable for investigation of orientation balance of the film. Normally tear strength is measured in two directions, parallel and transverse to the extrusion direction of the film.
Thermogravimetry
Thermogravimetry (TG, TGA) can be used to measure any reaction involving mass change. TG analysis is often complementary test to DSC, the weight loss curve giving a picture of the make-up of the material by showing the temperatures at which individual components volatilize.
Thermal Stability
Thermoanalytical methods such as Differential Scanning Calorimetry (DSC) and thermogravimetry (TG) provide information on the thermal stability of the polymers, their shelf-life under particular conditions and the effect of additives on thermal stability. The thermal stability of a polymer is usually measured as the oxidative induction time (OIT) at set temperature.
Thermomechanical Analysis
Thermomechanical Analysis (TMA) is is a technique for monitoring changes of physical dimensions (expansion, schrinkage, penetration) while the temperature of the sample is changed.
ULDPE
Ultra low density polyethylene
Based on the same technology as HDPE, but the density is reduced through the incorporation of longer a-olefins as comonomers. With this, another way of adapting the products to the final applications turns up. Single-site catalysts allow an expansion of the application range, mainly by allowing higher comonomer contents and a more even distribution of these.
UV light stabilizers
To protect polyolefins for outdoor applications against light induced degradation. Hindered amine light stabilizers (HALS) are the most efficient.
Vicat softening temperature
Vicat softening temperature shows heat softening characteristics of thermoplastics. Flat specimen are placed in a temperature regulated heating bath, a needle type, loaded penetrator is set on the specimen surface and bath temperature is raised at a constant rate. The temperature of the bath at which the penetration of the needle has reached a predefined level is the Vicat softening temperature of the material.
VLDPE
Very low density polyethylene
Based on the same technology as HDPE, but the density is reduced through the incorporation of longer a-olefins as comonomers. With this, another way of adapting the products to the final applications turns up. Single-site catalysts allow an expansion of the application range, mainly by allowing higher comonomer contents and a more even distribution of these.
XLPE
Crosslinked (X-linked) Polyethylene, is a polyethylene where the thermoplastic structure with separate polymer chains have been modified to a form a network structure that is no longer thermoplastic and will not melt when heated. This modification is used to increase service temperature for products as Hot Water Pipes and Power Cables. Crosslinking is manly achieved by using silane copolymers or by free-radical-crosslinking using a peroxide.
Ziegler-Natta Catalyst
• Produces polymer having relatively narrow MWD
• Hydrogen is used to control MW
• Relatively co-monomer sensitive
• Aluminum alkyl co-catalyst required
Neutralise acidic residues to protect processing equipment from corrosion.
Activity
is a measure which indicates how much polymer a certain amount of catalyst produces in a specified time. For a full definition the polymerisation conditions (mode, temperature, pressure etc.) have to be defined.
Additive
Additives are added to the polymer to protect it from degradation and to give the material desired properties. Normally the additives are mixed with the powder before the extruder.
Adiabatic
An adiabatic process is a process that occurs without heat entering or leaving a system. In general, an adiabatic change involves a fall or rise in temperature of the system. An adiabatic chemical process does not have any external heaters or coolers.
Active site
The site on the catalyst at which the polymerisation takes place.
Anti-blocking and slip agents
Surface-modifying additives to reduce friction and tackiness of polyolefin films.
Antioxidants
A group of substances being able to inhibit radical reactions in the polymer and thus prevent degradation processes. Different types are available: sterically hindered phenols and phosphites as base for polyolefins, sulphur based heat stabilisers and C-radical scavengers for special applications.
Antistatic agents
Surface-modifying additives to reduce surface resistance of polyolefins, eliminating charge build-up and dust collection; an example of a widely used antistatic agent is glycerine-monostearate (GMS).Different types of antistatic agents are used in polymerisation reactors to prevent the formed polymer powder from adhering to the reactor wall.
BorAPC
Borealis’ tailor-made Advanced Process Control (APC) for polyolefin polymerisation processes. APC is process control utilising detailed knowledge, compared with traditional process control where the method is standard and not specifically adapted to the process. The basis for APC is knowledge of a process expressed as mathematical models. Model predictive control (MPC) ensures that control actions also compensate for the future behaviour of the process, leading to optimal grade changes. BorAPC includes non-linear MPC, making it well-suited for polymerisation processes and ahead of what "normal" control vendors offer.
Blow moulding
Extrusion of a parison (hollow melt tube) which is forced to a mould caving by internal pressure and cooled down to form a hollow article. Used for the production of bottles, wide-mouth containers, petrol tanks etc.
Bimodal
A polyethylene with a relatively high molecular weight fraction and a relatively low molecular weight fraction is called bimodal polyethylene. The comonomer contents of the two fractions are often different.
Comonomer sensitivity
A property of the catalyst which indicates how eagerly the catalyst reacts with a comonomer, defining the relation between the relation of partial concentrations (pressures) of different monomers in the polymerisation process and the relative content in the produced polymer.. Comonomer sensitivity is determined in a test polymerisation in standard conditions.
Capillary rheometry
Actual capillary rheometry gives information about flow properties of polymer melts. Results are given as a viscosity function, that is viscosity as a function of shear rate or shear stress. Compared to MFR measurement the shear rate range is much wider and the rates in processing applications are reached. With many products, however, flow instabilities (melt fracture) at high rates limit the useful range. A capillary rheometer consists of a barrel, piston and die. The sample is put into the barrel, heated and pushed by the piston through the die. The piston is driven by hydraulic pressure which allows more flexible choice of rates compared to MFR measurements. In addition, longer dies can be used to avoid the errors coming from the entrance and exit pressure losses.
Chromium Catalysts
• Produces polymer having relatively broad molecular distribution (MWD)
• Control of Molecular Weight (MW) by heat and elimination
• Comonomer sensitive
• Does not give stereospecific polymers (e.g. isotactic PP)
Catalyst
A substance added into the reactor to initiate or accelerate a polymerisation reaction. It’s used in small quantities and is not chemically consumed during the polymerisation. Because of the catalyst’s high activity, residues in the polymer are so low that they do not need to be removed.
There are several different types of catalysts used in olefin polymerisation. The three main groups are Ziegler-Natta, chromium and single-site (metallocene).
To make a catalyst active in polymerisation, a cocatalyst is often used. Cocatalysts are highly reactive compounds, usually group I-III metal alkyls.They are used in excess compared to the catalyst, and prevent the catalyst from being poisoned by the impurities in the polymerisation reactor.
Comonomer
Comonomer is a monomer which is used to modify the polymer properties, e.g. to reduce the density of the polymer. Different types of copolymerisation processes are possible, leading to different structures and properties of the produced copolymer. In polyethylene, higher a-olefins like butene, hexene or octene are used as comonomers, in polypropylene, mostly ethylene and sometimes butene.
Compounding
In compounding a final homogenised and stabilised product in pellet form is made. Also, this term is used for the production of compounds incorporating reinforcing fibres or fillers as well as blends with other polymers (in the case of polyolefins, frequently elastomers).
Copolymer
Two monomers polymerised together to form a polymer.
Catalyst support
A support material on which the active compounds of the catalyst are deposited. Silica is commonly used as support material. Also called a carrier.
Creep tests
(a) Solid-state creep tests are used to measure long term behaviour of mechanically loaded material. Creep is time-dependent strain resulting from constant loading. Tensile loading is most common here but flexural loading can be used as well, testing time is typically 1000 hrs. Several loading levels are used to get an overview of creep behaviour of the material.
(b) Melt creep tests are a type of rheological measurement for the determination of the creep compliance of a polymer melt; they are carried out in the region of linear viscoelasticity.
Carbon Black
Black pigment used in concentrations of about 2.5% in order to ensure excellent UV-resistance. In higher concentrations, 30-40%, Carbon Black is used to obtain semiconductive properties used in Cable applications. The type of CB and the particle size is very important for the end-product properties.
Cocatalyst
Most often an organometallic compound that is used to activate the catalyst.
Crystallinity
also designated “degree of crystallinity”, it gives the relative fraction of a solid polymer having crystalline regularity. The crystallinity can be determined with various measuring techniques like DSC, density measurement or WAXS, which has the additional advantage that the relative amounts of the different crystal modifications can be determined as well. All crystallinity values are relative and the applied reference values should be quoted.
Density
An important parameter for polyethylene, affecting most end product physical properties like stiffness, impact strength and optical properties. The density is normally given in g/cm3, although the correct SI unit would be kg/m3. Two basic measuring methods are used: (a) Density gradient column is the basic method for density measurement. In it a density gradient is prepared by mixing two liquids of different densities so that density increases uniformly from top to bottom. Small test pieces are immersed into this density gradient, and the density of the polymer is determined based on the equilibrium position of the pieces and calibration floats. (b) Pyknometer measurements rely on calibrated volumes and the density difference to a reference fluid; compression-moulded samples are normally used here.
Dynamic rheometry
Dynamic rheometry gives information both about flow and elastic properties of polymer melts. In a dynamic measurement the sample is put between two round plates or between a cone and a plate in the oven. The system is heated to a desired temperature and a sinusoidal deformation at different frequencies is applied.
As a result we get storage and loss modulus as a function of frequency. Storage modulus is connected with elastic energy, while loss modulus corresponds to viscose energy. From the above parameters we can calculate complex viscosity as a function of frequency.
With most of polyolefin products this is the same as the conventional viscosity function, which is viscosity as a function of shear rate. The dynamic measurements are limited to relatively low shear rates. On the other hand, a benefit compared to capillary rheometry is, that we get reliable information also about elasticity.
In addition, the obtained properties are much more sensitive for small differences in polymer structure which play an important role in processing and end-use properties.
Differential Scanning Calorimetry
In Differential Scanning Calorimetry (DSC) the sample and a reference material are heated and cooled in a defined manner. DSC provides a rapid method for the determination of the temperature profile of a polymeric material. This includes e.g. measurement of heat capacity, specific heat, melting, crystallisation and glass transition temperatures, heat (enthalpy) of fusion, crystallisation, study of thermal stability or reaction kinetics and in certain cases identification of the polymer or polymer mixture.
Dynamic Mechanical Thermal Analysis
Dynamic Mechanical Thermal Analysis (DMTA): Many polymeric materials exhibit time-dependent, reversible viscoelastic properties in deformation. Dynamic mechanical tests are characterized by an application of a small stress in sinusoidal fashion and a continuous scan of resonant frequency of vibration and damping vs. temperature is produced. This is transformed into dynamic storage modulus, dynamic loss modulus and damping factor by the instrument software.
Extruder
Equipment for melting, pressurising and homogenising plastics by means of a rotating screw. Different configurations are possible; the most simple one being a „_ single screw extruder mainly used for conversion processes like cast film extrusion, fibre spinning, film blowing or pipe and profile extrusion. For more demanding applications like compounding of polymer powder before „_ pelletisation or the mixing of polymers, twin screw extruders are used where the two normally intermeshing screws can be moved in a corotating (same direction) or counterrotating (opposite direction) fashion. These machines normally contain different mixing elements, most important being kneading blocks for the local creation of extensional flow.
Environmental stress cracking
Under certain conditions of stress and environment like oils, detergents or soaps, ethylene plastics may fail mechanically by cracking. This phenomenon is called environmental stress cracking resistance (ESCR). ESCR properties can be tested in accelerated tests by presence of starter notches, surface active agent, elevated temperature and mechanical loading. Test result is the time when 50 % of the samples have failed.
Flexural test
Like tensile test, flexural test is also used to characterize strength and stiffness of the material. Flexural properties are obtained by placing a specimen horizontally on two supports. A load is applied at a specified rate in the center, normal to specimen longitudinal axis. Measured force - deformation data is used to calculate flexural modulus and flexural strength. Flexural tests can be used if material is too brittle to withstand compressive forces of fixing in tensile grips.
Friction
When handling and storing plastic packages it is important that friction between packages and pallets, shelves etc. or between packages itself is suitable. Coefficient of starting and sliding friction (static and dynamic friction) of film is measured by using a moving sled, force transducer and a static horizontal testing table. Friction can be measured either film against film or film versus metal or other material. It is determined by the material structure and the additivation, especially the addition of slip agents and antiblocking agents.
FTIR Spectroscopy
Infrared radiation induces vibrations of molecules and molecular segments. Absorption of radiation occurs only if the vibration results in the change of the molecular dipole moment. The strength of the vibrating bonds and their dipole moment affect the absorption band intensities and positions which makes FTIR spectroscopy a very useful method in qualitative polymer analysis. Also several fast quantitative FTIR methods have been developed for the structural characterisation of polyolefins.
Polymer crystallinity and orientation implies a restriction in the movement of some molecules and consequent improvement in polymer mechanical properties. This restriction in the movements reflects also to the FTIR spectrum and thus the amorphous and crystalline polymer phases can be identified.
Fines
Polymer particles having a small diameter. Those particles whose diameter is < 0.105 mm are usually classified as fines. A high amount of fines can severely reduce the operability of a plant.
Film impact
Film impact testing is the very basic, simple and fast method to find out overall mechanical strength of the film product. Impact resistance of film is determined by using free-falling dart method. In this method the impact is generated by dropping a dart from constant height on clamped film sample. Mass of the dart is varied and a mass causing failure of 50 % of the specimen tested is the result.
Fee radical
An atom or group of atoms with an unpaired valence electron (Valence electron is an electron in the outer shell of the atom that takes part in forming chemical bonds). Free radicals are formed in High-pressure polyolefin polymerisation by decomposing organic peroxides initiating the polymerisation process and then the propagation of the ethylene monomer onto the radicals forming long chains.
Fractionation
Analysis of the whole polymer gives the average structure and composition of the material. Polymer fractionation is useful when detailed information about polymer structure is desired. Fractionation serves as a general term for the methods of separating macromolecules according to their molar mass, chemical composition or microstructural parameters. The choice of the fractionation method depends on the polymeric material and the problem to be solved.
Film Application tests
Mechanical properties of film are very much dependent on film blowing parameters like extrusion screw, die construction, blow-up ratio and cooling system. Customary film tests are carried out by using film samples.
Film blowing
Extrusion of the melt through a ring from die, blowing up, and cooling to form a thin film. Film blowing is used to produce very thin films of high mechanial strength, mostly from polyethylene.
Gas phase reactor
Type of reactor to polymerise in the gas phase (i.e. all monomers are in the gaseous form); different designs are possible. In processes for the production of polyolefins, often fluidised bed reactors are used. Other constructions involve stirred or slow-flowing beds in different reactor geometries.
Gloss
Brightness or luster of a plastic resulting from a smooth surface. Gloss is determined by measuring the amount of light reflected from a surface, where the position and strength of the illumination has to be defined as well as the reflection angle covered by the measurement (normally 20°).
Gel
A heterogeneity in the polymer that appear as local thickness increase in films or thin-walled articles. Gels are caused by inclusion of high molecular weight particles with a higher viscosity than the surrounding, in some instances also including cross-linked molecules; normally the melting point is also higher than the surrounding. In some instances one also refer to contamination as gels, as examples can be mentioned fibre gels or dirt particles. The cause of this can be badly dispersed additives (agglomeration, dispersion) or other substances. This is not actually a gel, but sometimes called so for simplicity.
Heterophasic copolymers
Heterophasic copolymers (HECO) also contain ethylene (and possibly higher a-olefins) as comonomer. The production is however carried out in a two-stage process, resulting in a multiphase structure with a homopolymer matrix and inclusions consisting of amorphous EP-copolymer (“rubber”) and crystalline PE. Variations of molar mass and composition of the elastomeric phase in relation to the matrix allow a wide variation of properties (stiffness, toughness and transparency).
Heterogeneous
Something relating to two ore more phases, i.e. not mixed so as to be homogeneous.
Homogeneous
Something relating to only one phase, i.e. so well mixed that the mixture have the same composition throughout.
Haze
Cloudiness in polymer films or thin-walled injection-moulded parts, resulting from light scattering in the material (reducing the transparency). Measured in transmission with a well-defined light source, results are given on a percentage scale.
Hydrogen sensitivity
A property of the catalyst which indicates how sensitive the molecular weight of the polymer is to hydrogen concentration, (also: hydrogen response). The property also depends on the polymerisation conditions (mode, temperature, pressure etc.).
Heat deflection temperature
Heat deflection temperature (HDT) indicates how loaded material deforms at higher temperatures. Test bars are placed in a heating bath, resting horizontally on two supports. A constant load is applied in the center of the specimen and the bath temperature is raised at a constant rate. The temperature of the bath at which the flexural deflection of the loading point has reached a predefined level is the heat deflection temperature of the material.
Homopolymers
Homopolymers (HOMO) consist only of propylene units; their properties can be regulated via molar mass distribution and the degree of isotacticity (stereoregular structure).
High pressure polyethylene (HPPE)
The collective name given to polyethylene produced by high pressure processes. The process is using free radical polymerisation and the polymerisation is performed at pressures of 120-350 MPa (1200-3500 bar) and at temperatures of 150-3500C. Homopolymer and copolymers of ethylene is made by this process. (See more descriptions in process description).
HDPE
High density polyethylene
Coming from a low pressure process based on special catalysts (Ti- or Cr-based), HDPE has a linear polymeric chain, combined with a higher density and melting point. To fit special applications, the molar mass distribution can be adjusted in a wide range by means of catalysts or polymerisation technology. New applications are opened for HDPE as well as other low pressure PEs by applying metallocene (single-site) catalysts, which allow the production of very narrow molar mass distributions and the better distribution of comonomers.
Hardness
Resistance of a (polymer) surface to deformation. The different hardness measures applied for characterising polymers are:
(a) Shore hardness (two scales, A for softer and D for harder materials)
(b) Ball indentation hardness (also useable on profiled surfaces because of bigger measuring device).
(c) Vickers hardness (normally used for steel).
Injection moulding
A repetitive process in which plastic is melted and injected into a mould cavity where the article is cooled down. After cooling, the mould opens and the article is ejected. ). The melt is prepared in a screw setup similar to an extruder; the screw acts in melting and homogenising the polymer while slowly retracted to build up the melt reservoir necessary for the injection step. Then, the screw is used as a plunger in a forward movement to inject the melt through the runner , a manifold in case of a multiple mould and the gate into the mould. In the injection moulding process, filling of the mould and solidification takes place partially at the ssame time, resulting in a very complex morphology in case of semicrystalline polymers.
Impact Strength
Impact Strength characterizes material behaviour under high speed loading. Pendulum and falling weight type testers are applied here. Specimen can be either plaques, notched or unnotched bars or parts of finished products. There are several impact methods like Charpy impact test, instrumented puncture test and tensile impact test. Generally impact test shows the energy which is needed to break or puncture specimen under specified conditions. In some tests it is possible to accurately record force - deformation transient curves and calculate specific parameters describing material behaviour under impact loading.
LLDPE
Linear low density polyethylene
Based on the same technology as HDPE, but the density is reduced through the incorporation of longer a-olefins as comonomers. With this, another way of adapting the products to the final applications turns up. Single-site catalysts allow an expansion of the application range, mainly by allowing higher comonomer contents and a more even distribution of these.
LDPE
(Low density polyethylene)
A product from a high-pressure polymerisation process, LDPE is characterised by its highly branched chain structure. As a result of this, density and crystallinity are reduced while the processability in a variety of conversion technologies is greatly facilitated. The addition of special comonomers can further expand the property range of these grades.
Low pressure polyethylene (LPPE)
The collective name given to polyethylene produced in low pressure processes. The polymerisation is utilising a catalyst to get the neccessary rate of polymerisation. Homopolymer and copolymers of ethylene are made by these processes.
Long chain branch
A brach with many (>20) carbon atoms. Typical for low density PE.
Loop reactor
A bulk (slurry) polymerisation reactor which is made of a closed long tube through which the slurry is circulated. Loop reactors are characterised by a very narrow residence time distribution, leading to uniform product characteristics.
Metal deactivators
Some metal ions may contribute to accelerated degradation. For polyolefins in contact with metals, such as cable insulation of copper wires, metal deactivators are added to passivate the degradation reactions.
Melt strength
A property of the polymer melt which indicates its ability to withstand drawing without breaking. Melt strength is improved by the presence of high molecular weight tail or long chain branches.
MDPE
Medium low density polyethylene
Based on the same technology as HDPE, but the density is reduced through the incorporation of longer a-olefins as comonomers. With this, another way of adapting the products to the final applications turns up. Single-site catalysts allow an expansion of the application range, mainly by allowing higher comonomer contents and a more even distribution of these.
MFR
Melt mass flow rate or melt flow rate (MFR) gives an indication of melt processing properties of a polymer. Shear rate range is, however, narrow and much lower than in the most of processing applications. It gives thus very limited information about melt flow behaviour. The method is a kind of capillary measurement.
The equipment consists of a barrel, a piston with a place for loading masses and a die. The sample is charged into the barrel and heated up to a specified temperature. After that the piston is loaded extruding the polymer through the die.
Melt flow rate (MFR) is defined as a mass flown through the die in 10 min. The weights mostly used with polyolefins are 2.16 kg, 5 kg, 10 kg and 21.6 kg. In addition to the evaluation of flow properties MFR serves as a rough estimation of molecular weight.
Usually also Flow Rate Ratio (FRR) is determined. This ia defined as a ratio of two MFR values obtained with two different weights. FRR can be used as a rough estimation of molecular weight distribution.
Monomer
Monomer forms a repeating unit in a polymer chain.
Morphology
Morphology is the internal or external structure of a solid substance. Important sub-definitions include
(a)Crystal morphology of a polymer film or part (b)Phase morphology of heterophasic polymer systems (blend, copolymer) (c) Powder morphology of a polymer powder from the polymerisation reactor.
Molecular Weight Distribution
The relation between the number of molecules in a polymer and their individual chain length. It is often also called “molar mass distribution” (MMD). Technical polymers are always polydisperse as a result of the nature of the catalyst and the polymerisation conditions. The MWD is often characterised by size exclusion chromatography (SEC), but it can also be calculated approximately from rheological data for purely linear polymers.
Metallocene
A catalyst which contains a metallocene compound of a transition metal. Often used as a synonym for single site.
Multimodal polymer
Multimodal polymer includes more than one molecular weight fraction.
Melt tension test
Extensional properties play an important role in some processes such as melt spinning, blow moulding and film blowing. Melt tension test is a simple method for characterisation of extensional properties. The measuring principal copies industrial spinning process. A fluid is pressed or extruded through a round hole die, the resulting thread is hauled off with increasing speed and the applied extensional force is measured. The force is recorded as a function of haul-off speed or draw-down ratio, which is the ratio of the haul-off speed to the initial speed. Melt strength is defined as a force at filament break, while drawability can be estimated qualitatively from the maximum draw-down ratio at the failure.
Nucleating agents
Substances with the ability to act as substrates for crystal formation by epitaxy, thus increasing the nucleation density. Improve crystallisation behaviour in processing, i.e., cycle time or line speed, as well as crystallinity and morphology in final product, improving optical and mechanical properties.
Oxygen induction time (OIT)
A relative measure of a level or a degree of stabilisation (amount of remaining active antioxidant).
Optical properties
Optical properties are important in packaging applications where decorative printing is required or good visibility of the packed product is essential. Gloss and haze are used to characterize optical appearance of film products.
Optical Microscopy
Optical Microscopy (OM) is usually the most familiar tool to study material morphology and it offers many techniques for polymer characterisation including transmission, reflected light, dark field, polarized light, phase contrast and interference contrast microscopy. These techniques give information based upon the variations in light scattering, differences in refractive index and differential phase shifts of reflected or transmitted light. Light microscopy indicates the physical aspects of polymer composition, such as multiple phases present, the sizes and shapes of the particles and overall characteristics of the individual particles.
Pelletising
A process where the polymer powder is homogenised, addiviated and made to pellets. The polymer melt coming from the extruder is pressed through dies in a die plate and cut to pellets by knives under water (underwater pelletising) or cooled to solid strands and then cut to pellets (strand pelletising).
Permeability
Material constant characterising gas penetration, like water vapour, oxygen or carbondioxide.
Polymerisation
Polymerisation is a chemical reaction where polymer is synthesised from monomers.
Polypropylenes
Polypropylenes are thermoplastic polymers consisting mainly of propylene (C3H6), come frome a low pressure process based on catalysts and have generallya linear polymer chain.
Productivity
A property of the catalyst which indicates how much the catalyst has produced polymer in the process (kg polymer/g catalyst).
Prepolymerisation
A process where polymerisation is started to increase the activity of the catalyst getting over the induction time of the catalyst and not to overheat the catalyst particle by to sudden onset of polymerisation. Prepolymerisation will also help in ensuring more even polymerisation on the catalyst particles reducing the probability of creating a high fraction of fines in the later reaction steps.
Peroxide
Peroxides are a group of compounds that contain the O22- ion. It is used to initiate the polymerisation for high-pressure free radical polymerisation processes.
Polydispersity
Broadness of the molecular weight distribution. A commonly used measure, polydispersity index, is defined as the ratio of weight average molecular weight to the number average molecular weight.
Processability
A concept to describe the behaviour of the polymer in a converting process, such as blow moulding machine and film line. A good processability means a stable behaviour in the process and a high processing speed.
Puncture
In certain packaging applications film is stretched and wrapped around multiform items which can have sharp corners or other protrusions causing high point loading for the film. Puncture test is used to simulate this kind of film loading. Puncture resistance is measured by pressing a specially shaped probe through a clamped film sample. The maximum force, force at break, penetration distance and energy to break are determined.
Polyethylenes
Polyethylenes (or polythenes) are thermoplastic polymers consisting mainly of ethylene (C2H4), which are divided according to their density, melting point and molecular structure.
Random Copolymers
Random copolymers (RACO) are produced by statistical insertion of units of ethylene or higher a-olefins (mostly butene, also in combination with ethyle to give terpolymers). Through this, the glass transition temperature and melting point are lower than in homopolymers, allowing better sealability and application at temperatures below 0°C.
Shore Hardness
Resistance of a (polymer) surface to deformation. The different hardness measures applied for characterising polymers are:
(a) Shore hardness (two scales, A for softer and D for harder materials)
(b) Ball indentation hardness (also useable on profiled surfaces because of bigger measuring device).
(c) Vickers hardness (normally used for steel)
Slip agent
type of additive, which provides surface lubrication to the polymer during and after conversion; especially important for films. Usual slip agents are amides of fatty acids.
Size Exclusion Chromatography
(SEC, also called Gel Permeation Chromatography, GPC) is used to separate dissolved polymer molecules according to their coil diameter in solution. The chromatograms can be converted to molar mass distributions, which is an important parameter because it has a significant effect on the physical, mechanical and rheological properties of a polymer. It is also possible to calculate the average molar masses, Mn, Mv, Mw and Mz, and when on-line viscometric detection is used, to study long chain branching (LCB) and its distribution.
Solution Viscometry
In Solution Viscometry the rate at which a dilute polymer solution flows through a capillary is measured. Temperature and concentration of the polymer are carefully controlled. The parameter most often determined by dilute solution viscometry is intrinsic viscosity. Intrinsic viscosity is related to molar mass through a semi-empirical relationship called Mark-Houwink’s equation.
Shear thinning
A behaviour where the viscosity decreases when the shear stress increases. Typically broadening of the molecular weight distribution increases shear thinning.
Supercritical state
The critical state of a fluid is when the liquid and gas phase both have the same density. The fluid is then at its critical temperature, critical pressure, and critical volume. When the pressure or temperature exceeds this, the fluid is in its supercritical state. The loop reactor in the BORSTAR process is operated in supercritical state for some products.
Sheeting
Formation of sheet-like agglomerates in the gas phase reactor.
Single Site Catalysts
• Produces polymer having very narrow MWD
• Extremely hydrogen and comonomer sensitive
• Very uniform comonomer distribution
• Usually cocatalyst required
• Produces stereospecific polymers
Shrinkage
Decrease of dimension, created by the introduction of internal stress to the material during processing and cooling, or by ageing and post-crystallisation processes.
Scanning Electron Microscopy
In Scanning Electron Microscopy (SEM) a focused beam of electrons scans across the surface of the sample. It is detected synchronously to the position of the scanning electron beam and recorded in an image storage device. Differencies in the topography of the sample give rise to the differencies in the intensities of the signals formed by the secondary electrons in the detector resulting in a three dimensional image. Scanning electron microscopy is suited to most types of surface morphology studies. Rough topographic features, void content and particle agglomerations are easily revealed as well as the compositional differencies within material. Energy Dispersive X-ray Spectroscopy (EDS) or Wavelength Dispersive X-Ray Spectroscopy (WDS) in conjunction with SEM allows elemental analyses to be performed directly over the surface of the sample.
Tensile
Tensile properties are the most important indication of the strength and stiffnes of the material. The force necessary to pull the specimen apart is determined as well the deformation before breaking. Tensile modulus is used as a measure of stiffness and it is an important design parameter for end use applications. It is calculated based on measured force - deformation data. Tensile tests can be carried out at different temperatures according to application needs. Test automatisation and modern data logging systems are used in order to ensure high quality, reliable results.
Tear strength
Film must have certain resistance to tear loading. Tear strength indicates the average force required to propagate tearing through a specified length of the film sample. This test is suitable for investigation of orientation balance of the film. Normally tear strength is measured in two directions, parallel and transverse to the extrusion direction of the film.
Thermogravimetry
Thermogravimetry (TG, TGA) can be used to measure any reaction involving mass change. TG analysis is often complementary test to DSC, the weight loss curve giving a picture of the make-up of the material by showing the temperatures at which individual components volatilize.
Thermal Stability
Thermoanalytical methods such as Differential Scanning Calorimetry (DSC) and thermogravimetry (TG) provide information on the thermal stability of the polymers, their shelf-life under particular conditions and the effect of additives on thermal stability. The thermal stability of a polymer is usually measured as the oxidative induction time (OIT) at set temperature.
Thermomechanical Analysis
Thermomechanical Analysis (TMA) is is a technique for monitoring changes of physical dimensions (expansion, schrinkage, penetration) while the temperature of the sample is changed.
ULDPE
Ultra low density polyethylene
Based on the same technology as HDPE, but the density is reduced through the incorporation of longer a-olefins as comonomers. With this, another way of adapting the products to the final applications turns up. Single-site catalysts allow an expansion of the application range, mainly by allowing higher comonomer contents and a more even distribution of these.
UV light stabilizers
To protect polyolefins for outdoor applications against light induced degradation. Hindered amine light stabilizers (HALS) are the most efficient.
Vicat softening temperature
Vicat softening temperature shows heat softening characteristics of thermoplastics. Flat specimen are placed in a temperature regulated heating bath, a needle type, loaded penetrator is set on the specimen surface and bath temperature is raised at a constant rate. The temperature of the bath at which the penetration of the needle has reached a predefined level is the Vicat softening temperature of the material.
VLDPE
Very low density polyethylene
Based on the same technology as HDPE, but the density is reduced through the incorporation of longer a-olefins as comonomers. With this, another way of adapting the products to the final applications turns up. Single-site catalysts allow an expansion of the application range, mainly by allowing higher comonomer contents and a more even distribution of these.
XLPE
Crosslinked (X-linked) Polyethylene, is a polyethylene where the thermoplastic structure with separate polymer chains have been modified to a form a network structure that is no longer thermoplastic and will not melt when heated. This modification is used to increase service temperature for products as Hot Water Pipes and Power Cables. Crosslinking is manly achieved by using silane copolymers or by free-radical-crosslinking using a peroxide.
Ziegler-Natta Catalyst
• Produces polymer having relatively narrow MWD
• Hydrogen is used to control MW
• Relatively co-monomer sensitive
• Aluminum alkyl co-catalyst required
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Activity
is a measure which indicates how much polymer a certain amount of catalyst produces in a specified time. For a full definition the polymerisation conditions (mode, temperature, pressure etc.) have to be defined.
Additive
Additives are added to the polymer to protect it from degradation and to give the material desired properties. Normally the additives are mixed with the powder before the extruder.
Adiabatic
An adiabatic process is a process that occurs without heat entering or leaving a system. In general, an adiabatic change involves a fall or rise in temperature of the system. An adiabatic chemical process does not have any external heaters or coolers.
Active site
The site on the catalyst at which the polymerisation takes place.
Anti-blocking and slip agents
Surface-modifying additives to reduce friction and tackiness of polyolefin films.
Antioxidants
A group of substances being able to inhibit radical reactions in the polymer and thus prevent degradation processes. Different types are available: sterically hindered phenols and phosphites as base for polyolefins, sulphur based heat stabilisers and C-radical scavengers for special applications.
Antistatic agents
Surface-modifying additives to reduce surface resistance of polyolefins, eliminating charge build-up and dust collection; an example of a widely used antistatic agent is glycerine-monostearate (GMS).Different types of antistatic agents are used in polymerisation reactors to prevent the formed polymer powder from adhering to the reactor wall.
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BorAPC
Borealis’ tailor-made Advanced Process Control (APC) for polyolefin polymerisation processes. APC is process control utilising detailed knowledge, compared with traditional process control where the method is standard and not specifically adapted to the process. The basis for APC is knowledge of a process expressed as mathematical models. Model predictive control (MPC) ensures that control actions also compensate for the future behaviour of the process, leading to optimal grade changes. BorAPC includes non-linear MPC, making it well-suited for polymerisation processes and ahead of what "normal" control vendors offer.
Blow moulding
Extrusion of a parison (hollow melt tube) which is forced to a mould caving by internal pressure and cooled down to form a hollow article. Used for the production of bottles, wide-mouth containers, petrol tanks etc.
Bimodal
A polyethylene with a relatively high molecular weight fraction and a relatively low molecular weight fraction is called bimodal polyethylene. The comonomer contents of the two fractions are often different.
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Comonomer sensitivity
A property of the catalyst which indicates how eagerly the catalyst reacts with a comonomer, defining the relation between the relation of partial concentrations (pressures) of different monomers in the polymerisation process and the relative content in the produced polymer.. Comonomer sensitivity is determined in a test polymerisation in standard conditions.
Capillary rheometry
Actual capillary rheometry gives information about flow properties of polymer melts. Results are given as a viscosity function, that is viscosity as a function of shear rate or shear stress. Compared to MFR measurement the shear rate range is much wider and the rates in processing applications are reached. With many products, however, flow instabilities (melt fracture) at high rates limit the useful range. A capillary rheometer consists of a barrel, piston and die. The sample is put into the barrel, heated and pushed by the piston through the die. The piston is driven by hydraulic pressure which allows more flexible choice of rates compared to MFR measurements. In addition, longer dies can be used to avoid the errors coming from the entrance and exit pressure losses.
Chromium Catalysts
Produces polymer having relatively broad molecular distribution (MWD)
Control of Molecular Weight (MW) by heat and elimination
Comonomer sensitive
Does not give stereospecific polymers (e.g. isotactic PP)
Catalyst
A substance added into the reactor to initiate or accelerate a polymerisation reaction. It’s used in small quantities and is not chemically consumed during the polymerisation. Because of the catalyst’s high activity, residues in the polymer are so low that they do not need to be removed.
There are several different types of catalysts used in olefin polymerisation. The three main groups are Ziegler-Natta, chromium and single-site (metallocene).
To make a catalyst active in polymerisation, a cocatalyst is often used. Cocatalysts are highly reactive compounds, usually group I-III metal alkyls.They are used in excess compared to the catalyst, and prevent the catalyst from being poisoned by the impurities in the polymerisation reactor.
Comonomer
Comonomer is a monomer which is used to modify the polymer properties, e.g. to reduce the density of the polymer. Different types of copolymerisation processes are possible, leading to different structures and properties of the produced copolymer. In polyethylene, higher a-olefins like butene, hexene or octene are used as comonomers, in polypropylene, mostly ethylene and sometimes butene.
Compounding
In compounding a final homogenised and stabilised product in pellet form is made. Also, this term is used for the production of compounds incorporating reinforcing fibres or fillers as well as blends with other polymers (in the case of polyolefins, frequently elastomers).
Copolymer
Two monomers polymerised together to form a polymer.
Catalyst support
A support material on which the active compounds of the catalyst are deposited. Silica is commonly used as support material. Also called a carrier.
Creep tests
(a) Solid-state creep tests are used to measure long term behaviour of mechanically loaded material. Creep is time-dependent strain resulting from constant loading. Tensile loading is most common here but flexural loading can be used as well, testing time is typically 1000 hrs. Several loading levels are used to get an overview of creep behaviour of the material.
(b) Melt creep tests are a type of rheological measurement for the determination of the creep compliance of a polymer melt; they are carried out in the region of linear viscoelasticity.
Carbon Black
Black pigment used in concentrations of about 2.5% in order to ensure excellent UV-resistance. In higher concentrations, 30-40%, Carbon Black is used to obtain semiconductive properties used in Cable applications. The type of CB and the particle size is very important for the end-product properties.
Cocatalyst
Most often an organometallic compound that is used to activate the catalyst.
Crystallinity
also designated “degree of crystallinity”, it gives the relative fraction of a solid polymer having crystalline regularity. The crystallinity can be determined with various measuring techniques like DSC, density measurement or WAXS, which has the additional advantage that the relative amounts of the different crystal modifications can be determined as well. All crystallinity values are relative and the applied reference values should be quoted.
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Density
An important parameter for polyethylene, affecting most end product physical properties like stiffness, impact strength and optical properties. The density is normally given in g/cm3, although the correct SI unit would be kg/m3. Two basic measuring methods are used: (a) Density gradient column is the basic method for density measurement. In it a density gradient is prepared by mixing two liquids of different densities so that density increases uniformly from top to bottom. Small test pieces are immersed into this density gradient, and the density of the polymer is determined based on the equilibrium position of the pieces and calibration floats. (b) Pyknometer measurements rely on calibrated volumes and the density difference to a reference fluid; compression-moulded samples are normally used here.
Dynamic rheometry
Dynamic rheometry gives information both about flow and elastic properties of polymer melts. In a dynamic measurement the sample is put between two round plates or between a cone and a plate in the oven. The system is heated to a desired temperature and a sinusoidal deformation at different frequencies is applied.
As a result we get storage and loss modulus as a function of frequency. Storage modulus is connected with elastic energy, while loss modulus corresponds to viscose energy. From the above parameters we can calculate complex viscosity as a function of frequency.
With most of polyolefin products this is the same as the conventional viscosity function, which is viscosity as a function of shear rate. The dynamic measurements are limited to relatively low shear rates. On the other hand, a benefit compared to capillary rheometry is, that we get reliable information also about elasticity.
In addition, the obtained properties are much more sensitive for small differences in polymer structure which play an important role in processing and end-use properties.
Differential Scanning Calorimetry
In Differential Scanning Calorimetry (DSC) the sample and a reference material are heated and cooled in a defined manner. DSC provides a rapid method for the determination of the temperature profile of a polymeric material. This includes e.g. measurement of heat capacity, specific heat, melting, crystallisation and glass transition temperatures, heat (enthalpy) of fusion, crystallisation, study of thermal stability or reaction kinetics and in certain cases identification of the polymer or polymer mixture.
Dynamic Mechanical Thermal Analysis
Dynamic Mechanical Thermal Analysis (DMTA): Many polymeric materials exhibit time-dependent, reversible viscoelastic properties in deformation. Dynamic mechanical tests are characterized by an application of a small stress in sinusoidal fashion and a continuous scan of resonant frequency of vibration and damping vs. temperature is produced. This is transformed into dynamic storage modulus, dynamic loss modulus and damping factor by the instrument software.
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Extruder
Equipment for melting, pressurising and homogenising plastics by means of a rotating screw. Different configurations are possible; the most simple one being a „_ single screw extruder mainly used for conversion processes like cast film extrusion, fibre spinning, film blowing or pipe and profile extrusion. For more demanding applications like compounding of polymer powder before „_ pelletisation or the mixing of polymers, twin screw extruders are used where the two normally intermeshing screws can be moved in a corotating (same direction) or counterrotating (opposite direction) fashion. These machines normally contain different mixing elements, most important being kneading blocks for the local creation of extensional flow.
Environmental stress cracking
Under certain conditions of stress and environment like oils, detergents or soaps, ethylene plastics may fail mechanically by cracking. This phenomenon is called environmental stress cracking resistance (ESCR). ESCR properties can be tested in accelerated tests by presence of starter notches, surface active agent, elevated temperature and mechanical loading. Test result is the time when 50 % of the samples have failed.
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Flexural test
Like tensile test, flexural test is also used to characterize strength and stiffness of the material. Flexural properties are obtained by placing a specimen horizontally on two supports. A load is applied at a specified rate in the center, normal to specimen longitudinal axis. Measured force - deformation data is used to calculate flexural modulus and flexural strength. Flexural tests can be used if material is too brittle to withstand compressive forces of fixing in tensile grips.
Friction
When handling and storing plastic packages it is important that friction between packages and pallets, shelves etc. or between packages itself is suitable. Coefficient of starting and sliding friction (static and dynamic friction) of film is measured by using a moving sled, force transducer and a static horizontal testing table. Friction can be measured either film against film or film versus metal or other material. It is determined by the material structure and the additivation, especially the addition of slip agents and antiblocking agents.
FTIR Spectroscopy
Infrared radiation induces vibrations of molecules and molecular segments. Absorption of radiation occurs only if the vibration results in the change of the molecular dipole moment. The strength of the vibrating bonds and their dipole moment affect the absorption band intensities and positions which makes FTIR spectroscopy a very useful method in qualitative polymer analysis. Also several fast quantitative FTIR methods have been developed for the structural characterisation of polyolefins.
Polymer crystallinity and orientation implies a restriction in the movement of some molecules and consequent improvement in polymer mechanical properties. This restriction in the movements reflects also to the FTIR spectrum and thus the amorphous and crystalline polymer phases can be identified.
Fines
Polymer particles having a small diameter. Those particles whose diameter is < 0.105 mm are usually classified as fines. A high amount of fines can severely reduce the operability of a plant.
Film impact
Film impact testing is the very basic, simple and fast method to find out overall mechanical strength of the film product. Impact resistance of film is determined by using free-falling dart method. In this method the impact is generated by dropping a dart from constant height on clamped film sample. Mass of the dart is varied and a mass causing failure of 50 % of the specimen tested is the result.
Fee radical
An atom or group of atoms with an unpaired valence electron (Valence electron is an electron in the outer shell of the atom that takes part in forming chemical bonds). Free radicals are formed in High-pressure polyolefin polymerisation by decomposing organic peroxides initiating the polymerisation process and then the propagation of the ethylene monomer onto the radicals forming long chains.
Fractionation
Analysis of the whole polymer gives the average structure and composition of the material. Polymer fractionation is useful when detailed information about polymer structure is desired. Fractionation serves as a general term for the methods of separating macromolecules according to their molar mass, chemical composition or microstructural parameters. The choice of the fractionation method depends on the polymeric material and the problem to be solved.
Film Application tests
Mechanical properties of film are very much dependent on film blowing parameters like extrusion screw, die construction, blow-up ratio and cooling system. Customary film tests are carried out by using film samples.
Film blowing
Extrusion of the melt through a ring from die, blowing up, and cooling to form a thin film. Film blowing is used to produce very thin films of high mechanial strength, mostly from polyethylene.
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Gas phase reactor
Type of reactor to polymerise in the gas phase (i.e. all monomers are in the gaseous form); different designs are possible. In processes for the production of polyolefins, often fluidised bed reactors are used. Other constructions involve stirred or slow-flowing beds in different reactor geometries.
Gloss
Brightness or luster of a plastic resulting from a smooth surface. Gloss is determined by measuring the amount of light reflected from a surface, where the position and strength of the illumination has to be defined as well as the reflection angle covered by the measurement (normally 20°).
Gel
A heterogeneity in the polymer that appear as local thickness increase in films or thin-walled articles. Gels are caused by inclusion of high molecular weight particles with a higher viscosity than the surrounding, in some instances also including cross-linked molecules; normally the melting point is also higher than the surrounding. In some instances one also refer to contamination as gels, as examples can be mentioned fibre gels or dirt particles. The cause of this can be badly dispersed additives (agglomeration, dispersion) or other substances. This is not actually a gel, but sometimes called so for simplicity.
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Heterophasic copolymers
Heterophasic copolymers (HECO) also contain ethylene (and possibly higher a-olefins) as comonomer. The production is however carried out in a two-stage process, resulting in a multiphase structure with a homopolymer matrix and inclusions consisting of amorphous EP-copolymer (“rubber”) and crystalline PE. Variations of molar mass and composition of the elastomeric phase in relation to the matrix allow a wide variation of properties (stiffness, toughness and transparency).
Heterogeneous
Something relating to two ore more phases, i.e. not mixed so as to be homogeneous.
Homogeneous
Something relating to only one phase, i.e. so well mixed that the mixture have the same composition throughout.
Haze
Cloudiness in polymer films or thin-walled injection-moulded parts, resulting from light scattering in the material (reducing the transparency). Measured in transmission with a well-defined light source, results are given on a percentage scale.
Hydrogen sensitivity
A property of the catalyst which indicates how sensitive the molecular weight of the polymer is to hydrogen concentration, (also: hydrogen response). The property also depends on the polymerisation conditions (mode, temperature, pressure etc.).
Heat deflection temperature
Heat deflection temperature (HDT) indicates how loaded material deforms at higher temperatures. Test bars are placed in a heating bath, resting horizontally on two supports. A constant load is applied in the center of the specimen and the bath temperature is raised at a constant rate. The temperature of the bath at which the flexural deflection of the loading point has reached a predefined level is the heat deflection temperature of the material.
Homopolymers
Homopolymers (HOMO) consist only of propylene units; their properties can be regulated via molar mass distribution and the degree of isotacticity (stereoregular structure).
High pressure polyethylene (HPPE)
The collective name given to polyethylene produced by high pressure processes. The process is using free radical polymerisation and the polymerisation is performed at pressures of 120-350 MPa (1200-3500 bar) and at temperatures of 150-3500C. Homopolymer and copolymers of ethylene is made by this process. (See more descriptions in process description).
HDPE
High density polyethylene
Coming from a low pressure process based on special catalysts (Ti- or Cr-based), HDPE has a linear polymeric chain, combined with a higher density and melting point. To fit special applications, the molar mass distribution can be adjusted in a wide range by means of catalysts or polymerisation technology. New applications are opened for HDPE as well as other low pressure PEs by applying metallocene (single-site) catalysts, which allow the production of very narrow molar mass distributions and the better distribution of comonomers.
Hardness
Resistance of a (polymer) surface to deformation. The different hardness measures applied for characterising polymers are:
(a) Shore hardness (two scales, A for softer and D for harder materials)
(b) Ball indentation hardness (also useable on profiled surfaces because of bigger measuring device).
(c) Vickers hardness (normally used for steel).
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Injection moulding
A repetitive process in which plastic is melted and injected into a mould cavity where the article is cooled down. After cooling, the mould opens and the article is ejected. ). The melt is prepared in a screw setup similar to an extruder; the screw acts in melting and homogenising the polymer while slowly retracted to build up the melt reservoir necessary for the injection step. Then, the screw is used as a plunger in a forward movement to inject the melt through the runner , a manifold in case of a multiple mould and the gate into the mould. In the injection moulding process, filling of the mould and solidification takes place partially at the ssame time, resulting in a very complex morphology in case of semicrystalline polymers.
Impact Strength
Impact Strength characterizes material behaviour under high speed loading. Pendulum and falling weight type testers are applied here. Specimen can be either plaques, notched or unnotched bars or parts of finished products. There are several impact methods like Charpy impact test, instrumented puncture test and tensile impact test. Generally impact test shows the energy which is needed to break or puncture specimen under specified conditions. In some tests it is possible to accurately record force - deformation transient curves and calculate specific parameters describing material behaviour under impact loading.
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LLDPE
Linear low density polyethylene
Based on the same technology as HDPE, but the density is reduced through the incorporation of longer a-olefins as comonomers. With this, another way of adapting the products to the final applications turns up. Single-site catalysts allow an expansion of the application range, mainly by allowing higher comonomer contents and a more even distribution of these.
LDPE
(Low density polyethylene)
A product from a high-pressure polymerisation process, LDPE is characterised by its highly branched chain structure. As a result of this, density and crystallinity are reduced while the processability in a variety of conversion technologies is greatly facilitated. The addition of special comonomers can further expand the property range of these grades.
Low pressure polyethylene (LPPE)
The collective name given to polyethylene produced in low pressure processes. The polymerisation is utilising a catalyst to get the neccessary rate of polymerisation. Homopolymer and copolymers of ethylene are made by these processes.
Long chain branch
A brach with many (>20) carbon atoms. Typical for low density PE.
Loop reactor
A bulk (slurry) polymerisation reactor which is made of a closed long tube through which the slurry is circulated. Loop reactors are characterised by a very narrow residence time distribution, leading to uniform product characteristics.
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Metal deactivators
Some metal ions may contribute to accelerated degradation. For polyolefins in contact with metals, such as cable insulation of copper wires, metal deactivators are added to passivate the degradation reactions.
Melt strength
A property of the polymer melt which indicates its ability to withstand drawing without breaking. Melt strength is improved by the presence of high molecular weight tail or long chain branches.
MDPE
Medium low density polyethylene
Based on the same technology as HDPE, but the density is reduced through the incorporation of longer a-olefins as comonomers. With this, another way of adapting the products to the final applications turns up. Single-site catalysts allow an expansion of the application range, mainly by allowing higher comonomer contents and a more even distribution of these.
MFR
Melt mass flow rate or melt flow rate (MFR) gives an indication of melt processing properties of a polymer. Shear rate range is, however, narrow and much lower than in the most of processing applications. It gives thus very limited information about melt flow behaviour. The method is a kind of capillary measurement.
The equipment consists of a barrel, a piston with a place for loading masses and a die. The sample is charged into the barrel and heated up to a specified temperature. After that the piston is loaded extruding the polymer through the die.
Melt flow rate (MFR) is defined as a mass flown through the die in 10 min. The weights mostly used with polyolefins are 2.16 kg, 5 kg, 10 kg and 21.6 kg. In addition to the evaluation of flow properties MFR serves as a rough estimation of molecular weight.
Usually also Flow Rate Ratio (FRR) is determined. This ia defined as a ratio of two MFR values obtained with two different weights. FRR can be used as a rough estimation of molecular weight distribution.
Monomer
Monomer forms a repeating unit in a polymer chain.
Morphology
Morphology is the internal or external structure of a solid substance. Important sub-definitions include
(a)Crystal morphology of a polymer film or part (b)Phase morphology of heterophasic polymer systems (blend, copolymer) (c) Powder morphology of a polymer powder from the polymerisation reactor.
Molecular Weight Distribution
The relation between the number of molecules in a polymer and their individual chain length. It is often also called “molar mass distribution” (MMD). Technical polymers are always polydisperse as a result of the nature of the catalyst and the polymerisation conditions. The MWD is often characterised by size exclusion chromatography (SEC), but it can also be calculated approximately from rheological data for purely linear polymers.
Metallocene
A catalyst which contains a metallocene compound of a transition metal. Often used as a synonym for single site.
Multimodal polymer
Multimodal polymer includes more than one molecular weight fraction.
Melt tension test
Extensional properties play an important role in some processes such as melt spinning, blow moulding and film blowing. Melt tension test is a simple method for characterisation of extensional properties. The measuring principal copies industrial spinning process. A fluid is pressed or extruded through a round hole die, the resulting thread is hauled off with increasing speed and the applied extensional force is measured. The force is recorded as a function of haul-off speed or draw-down ratio, which is the ratio of the haul-off speed to the initial speed. Melt strength is defined as a force at filament break, while drawability can be estimated qualitatively from the maximum draw-down ratio at the failure.
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Nucleating agents
Substances with the ability to act as substrates for crystal formation by epitaxy, thus increasing the nucleation density. Improve crystallisation behaviour in processing, i.e., cycle time or line speed, as well as crystallinity and morphology in final product, improving optical and mechanical properties.
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Oxygen induction time (OIT)
A relative measure of a level or a degree of stabilisation (amount of remaining active antioxidant).
Optical properties
Optical properties are important in packaging applications where decorative printing is required or good visibility of the packed product is essential. Gloss and haze are used to characterize optical appearance of film products.
Optical Microscopy
Optical Microscopy (OM) is usually the most familiar tool to study material morphology and it offers many techniques for polymer characterisation including transmission, reflected light, dark field, polarized light, phase contrast and interference contrast microscopy. These techniques give information based upon the variations in light scattering, differences in refractive index and differential phase shifts of reflected or transmitted light. Light microscopy indicates the physical aspects of polymer composition, such as multiple phases present, the sizes and shapes of the particles and overall characteristics of the individual particles.
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Pelletising
A process where the polymer powder is homogenised, addiviated and made to pellets. The polymer melt coming from the extruder is pressed through dies in a die plate and cut to pellets by knives under water (underwater pelletising) or cooled to solid strands and then cut to pellets (strand pelletising).
Permeability
Material constant characterising gas penetration, like water vapour, oxygen or carbondioxide.
Polymerisation
Polymerisation is a chemical reaction where polymer is synthesised from monomers.
Polypropylenes
Polypropylenes are thermoplastic polymers consisting mainly of propylene (C3H6), come frome a low pressure process based on catalysts and have generallya linear polymer chain.
Productivity
A property of the catalyst which indicates how much the catalyst has produced polymer in the process (kg polymer/g catalyst).
Prepolymerisation
A process where polymerisation is started to increase the activity of the catalyst getting over the induction time of the catalyst and not to overheat the catalyst particle by to sudden onset of polymerisation. Prepolymerisation will also help in ensuring more even polymerisation on the catalyst particles reducing the probability of creating a high fraction of fines in the later reaction steps.
Peroxide
Peroxides are a group of compounds that contain the O22- ion. It is used to initiate the polymerisation for high-pressure free radical polymerisation processes.
Polydispersity
Broadness of the molecular weight distribution. A commonly used measure, polydispersity index, is defined as the ratio of weight average molecular weight to the number average molecular weight.
Processability
A concept to describe the behaviour of the polymer in a converting process, such as blow moulding machine and film line. A good processability means a stable behaviour in the process and a high processing speed.
Puncture
In certain packaging applications film is stretched and wrapped around multiform items which can have sharp corners or other protrusions causing high point loading for the film. Puncture test is used to simulate this kind of film loading. Puncture resistance is measured by pressing a specially shaped probe through a clamped film sample. The maximum force, force at break, penetration distance and energy to break are determined.
Polyethylenes
Polyethylenes (or polythenes) are thermoplastic polymers consisting mainly of ethylene (C2H4), which are divided according to their density, melting point and molecular structure.
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Random Copolymers
Random copolymers (RACO) are produced by statistical insertion of units of ethylene or higher a-olefins (mostly butene, also in combination with ethyle to give terpolymers). Through this, the glass transition temperature and melting point are lower than in homopolymers, allowing better sealability and application at temperatures below 0°C.
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Shore Hardness
Resistance of a (polymer) surface to deformation. The different hardness measures applied for characterising polymers are:
(a) Shore hardness (two scales, A for softer and D for harder materials)
(b) Ball indentation hardness (also useable on profiled surfaces because of bigger measuring device).
(c) Vickers hardness (normally used for steel)
Slip agent
type of additive, which provides surface lubrication to the polymer during and after conversion; especially important for films. Usual slip agents are amides of fatty acids.
Size Exclusion Chromatography - (SEC, also called Gel Permeation Chromatography, GPC) is used to separate dissolved polymer molecules according to their coil diameter in solution. The chromatograms can be converted to molar mass distributions, which is an important parameter because it has a significant effect on the physical, mechanical and rheological properties of a polymer. It is also possible to calculate the average molar masses, Mn, Mv, Mw and Mz, and when on-line viscometric detection is used, to study long chain branching (LCB) and its distribution.
Solution Viscometry - In Solution Viscometry the rate at which a dilute polymer solution flows through a capillary is measured. Temperature and concentration of the polymer are carefully controlled. The parameter most often determined by dilute solution viscometry is intrinsic viscosity. Intrinsic viscosity is related to molar mass through a semi-empirical relationship called Mark-Houwink’s equation.
Shear thinning - A behaviour where the viscosity decreases when the shear stress increases. Typically broadening of the molecular weight distribution increases shear thinning.
Supercritical state - The critical state of a fluid is when the liquid and gas phase both have the same density. The fluid is then at its critical temperature, critical pressure, and critical volume. When the pressure or temperature exceeds this, the fluid is in its supercritical state. The loop reactor in the BORSTAR process is operated in supercritical state for some products.
Sheeting - Formation of sheet-like agglomerates in the gas phase reactor.
Single Site Catalysts - Produces polymer having very narrow MWD
Extremely hydrogen and comonomer sensitive
Very uniform comonomer distribution
Usually cocatalyst required
Produces stereospecific polymers
Shrinkage - Decrease of dimension, created by the introduction of internal stress to the material during processing and cooling, or by ageing and post-crystallisation processes.
Scanning Electron Microscopy - In Scanning Electron Microscopy (SEM) a focused beam of electrons scans across the surface of the sample. It is detected synchronously to the position of the scanning electron beam and recorded in an image storage device. Differencies in the topography of the sample give rise to the differencies in the intensities of the signals formed by the secondary electrons in the detector resulting in a three dimensional image. Scanning electron microscopy is suited to most types of surface morphology studies. Rough topographic features, void content and particle agglomerations are easily revealed as well as the compositional differencies within material. Energy Dispersive X-ray Spectroscopy (EDS) or Wavelength Dispersive X-Ray Spectroscopy (WDS) in conjunction with SEM allows elemental analyses to be performed directly over the surface of the sample.
Tensile - Tensile properties are the most important indication of the strength and stiffnes of the material. The force necessary to pull the specimen apart is determined as well the deformation before breaking. Tensile modulus is used as a measure of stiffness and it is an important design parameter for end use applications. It is calculated based on measured force - deformation data. Tensile tests can be carried out at different temperatures according to application needs. Test automatisation and modern data logging systems are used in order to ensure high quality, reliable results.
Tear strength - Film must have certain resistance to tear loading. Tear strength indicates the average force required to propagate tearing through a specified length of the film sample. This test is suitable for investigation of orientation balance of the film. Normally tear strength is measured in two directions, parallel and transverse to the extrusion direction of the film.
Thermogravimetry - Thermogravimetry (TG, TGA) can be used to measure any reaction involving mass change. TG analysis is often complementary test to DSC, the weight loss curve giving a picture of the make-up of the material by showing the temperatures at which individual components volatilize.
Thermal Stability - Thermoanalytical methods such as Differential Scanning Calorimetry (DSC) and thermogravimetry (TG) provide information on the thermal stability of the polymers, their shelf-life under particular conditions and the effect of additives on thermal stability. The thermal stability of a polymer is usually measured as the oxidative induction time (OIT) at set temperature.
Thermomechanical Analysis - Thermomechanical Analysis (TMA) is is a technique for monitoring changes of physical dimensions (expansion, schrinkage, penetration) while the temperature of the sample is changed.
ULDPE - Ultra low density polyethylene - Based on the same technology as HDPE, but the density is reduced through the incorporation of longer a-olefins as comonomers. With this, another way of adapting the products to the final applications turns up. Single-site catalysts allow an expansion of the application range, mainly by allowing higher comonomer contents and a more even distribution of these.
UV light stabilizers - To protect polyolefins for outdoor applications against light induced degradation. Hindered amine light stabilizers (HALS) are the most efficient.
Vicat softening temperature - Vicat softening temperature shows heat softening characteristics of thermoplastics. Flat specimen are placed in a temperature regulated heating bath, a needle type, loaded penetrator is set on the specimen surface and bath temperature is raised at a constant rate. The temperature of the bath at which the penetration of the needle has reached a predefined level is the Vicat softening temperature of the material.
VLDPE - Very low density polyethylene - Based on the same technology as HDPE, but the density is reduced through the incorporation of longer a-olefins as comonomers. With this, another way of adapting the products to the final applications turns up. Single-site catalysts allow an expansion of the application range, mainly by allowing higher comonomer contents and a more even distribution of these.
XLPE Crosslinked (X-linked) Polyethylene, is a polyethylene where the thermoplastic structure with separate polymer chains have been modified to a form a network structure that is no longer thermoplastic and will not melt when heated. This modification is used to increase service temperature for products as Hot Water Pipes and Power Cables. Crosslinking is manly achieved by using silane copolymers or by free-radical-crosslinking using a peroxide.
Ziegler-Natta Catalyst: Produces polymer having relatively narrow MWD Hydrogen is used to control MW Relatively comonomer sensitive Aluminum alkyl cocatalyst required
is a measure which indicates how much polymer a certain amount of catalyst produces in a specified time. For a full definition the polymerisation conditions (mode, temperature, pressure etc.) have to be defined.
Additive
Additives are added to the polymer to protect it from degradation and to give the material desired properties. Normally the additives are mixed with the powder before the extruder.
Adiabatic
An adiabatic process is a process that occurs without heat entering or leaving a system. In general, an adiabatic change involves a fall or rise in temperature of the system. An adiabatic chemical process does not have any external heaters or coolers.
Active site
The site on the catalyst at which the polymerisation takes place.
Anti-blocking and slip agents
Surface-modifying additives to reduce friction and tackiness of polyolefin films.
Antioxidants
A group of substances being able to inhibit radical reactions in the polymer and thus prevent degradation processes. Different types are available: sterically hindered phenols and phosphites as base for polyolefins, sulphur based heat stabilisers and C-radical scavengers for special applications.
Antistatic agents
Surface-modifying additives to reduce surface resistance of polyolefins, eliminating charge build-up and dust collection; an example of a widely used antistatic agent is glycerine-monostearate (GMS).Different types of antistatic agents are used in polymerisation reactors to prevent the formed polymer powder from adhering to the reactor wall.
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BorAPC
Borealis’ tailor-made Advanced Process Control (APC) for polyolefin polymerisation processes. APC is process control utilising detailed knowledge, compared with traditional process control where the method is standard and not specifically adapted to the process. The basis for APC is knowledge of a process expressed as mathematical models. Model predictive control (MPC) ensures that control actions also compensate for the future behaviour of the process, leading to optimal grade changes. BorAPC includes non-linear MPC, making it well-suited for polymerisation processes and ahead of what "normal" control vendors offer.
Blow moulding
Extrusion of a parison (hollow melt tube) which is forced to a mould caving by internal pressure and cooled down to form a hollow article. Used for the production of bottles, wide-mouth containers, petrol tanks etc.
Bimodal
A polyethylene with a relatively high molecular weight fraction and a relatively low molecular weight fraction is called bimodal polyethylene. The comonomer contents of the two fractions are often different.
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Comonomer sensitivity
A property of the catalyst which indicates how eagerly the catalyst reacts with a comonomer, defining the relation between the relation of partial concentrations (pressures) of different monomers in the polymerisation process and the relative content in the produced polymer.. Comonomer sensitivity is determined in a test polymerisation in standard conditions.
Capillary rheometry
Actual capillary rheometry gives information about flow properties of polymer melts. Results are given as a viscosity function, that is viscosity as a function of shear rate or shear stress. Compared to MFR measurement the shear rate range is much wider and the rates in processing applications are reached. With many products, however, flow instabilities (melt fracture) at high rates limit the useful range. A capillary rheometer consists of a barrel, piston and die. The sample is put into the barrel, heated and pushed by the piston through the die. The piston is driven by hydraulic pressure which allows more flexible choice of rates compared to MFR measurements. In addition, longer dies can be used to avoid the errors coming from the entrance and exit pressure losses.
Chromium Catalysts
Produces polymer having relatively broad molecular distribution (MWD)
Control of Molecular Weight (MW) by heat and elimination
Comonomer sensitive
Does not give stereospecific polymers (e.g. isotactic PP)
Catalyst
A substance added into the reactor to initiate or accelerate a polymerisation reaction. It’s used in small quantities and is not chemically consumed during the polymerisation. Because of the catalyst’s high activity, residues in the polymer are so low that they do not need to be removed.
There are several different types of catalysts used in olefin polymerisation. The three main groups are Ziegler-Natta, chromium and single-site (metallocene).
To make a catalyst active in polymerisation, a cocatalyst is often used. Cocatalysts are highly reactive compounds, usually group I-III metal alkyls.They are used in excess compared to the catalyst, and prevent the catalyst from being poisoned by the impurities in the polymerisation reactor.
Comonomer
Comonomer is a monomer which is used to modify the polymer properties, e.g. to reduce the density of the polymer. Different types of copolymerisation processes are possible, leading to different structures and properties of the produced copolymer. In polyethylene, higher a-olefins like butene, hexene or octene are used as comonomers, in polypropylene, mostly ethylene and sometimes butene.
Compounding
In compounding a final homogenised and stabilised product in pellet form is made. Also, this term is used for the production of compounds incorporating reinforcing fibres or fillers as well as blends with other polymers (in the case of polyolefins, frequently elastomers).
Copolymer
Two monomers polymerised together to form a polymer.
Catalyst support
A support material on which the active compounds of the catalyst are deposited. Silica is commonly used as support material. Also called a carrier.
Creep tests
(a) Solid-state creep tests are used to measure long term behaviour of mechanically loaded material. Creep is time-dependent strain resulting from constant loading. Tensile loading is most common here but flexural loading can be used as well, testing time is typically 1000 hrs. Several loading levels are used to get an overview of creep behaviour of the material.
(b) Melt creep tests are a type of rheological measurement for the determination of the creep compliance of a polymer melt; they are carried out in the region of linear viscoelasticity.
Carbon Black
Black pigment used in concentrations of about 2.5% in order to ensure excellent UV-resistance. In higher concentrations, 30-40%, Carbon Black is used to obtain semiconductive properties used in Cable applications. The type of CB and the particle size is very important for the end-product properties.
Cocatalyst
Most often an organometallic compound that is used to activate the catalyst.
Crystallinity
also designated “degree of crystallinity”, it gives the relative fraction of a solid polymer having crystalline regularity. The crystallinity can be determined with various measuring techniques like DSC, density measurement or WAXS, which has the additional advantage that the relative amounts of the different crystal modifications can be determined as well. All crystallinity values are relative and the applied reference values should be quoted.
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Density
An important parameter for polyethylene, affecting most end product physical properties like stiffness, impact strength and optical properties. The density is normally given in g/cm3, although the correct SI unit would be kg/m3. Two basic measuring methods are used: (a) Density gradient column is the basic method for density measurement. In it a density gradient is prepared by mixing two liquids of different densities so that density increases uniformly from top to bottom. Small test pieces are immersed into this density gradient, and the density of the polymer is determined based on the equilibrium position of the pieces and calibration floats. (b) Pyknometer measurements rely on calibrated volumes and the density difference to a reference fluid; compression-moulded samples are normally used here.
Dynamic rheometry
Dynamic rheometry gives information both about flow and elastic properties of polymer melts. In a dynamic measurement the sample is put between two round plates or between a cone and a plate in the oven. The system is heated to a desired temperature and a sinusoidal deformation at different frequencies is applied.
As a result we get storage and loss modulus as a function of frequency. Storage modulus is connected with elastic energy, while loss modulus corresponds to viscose energy. From the above parameters we can calculate complex viscosity as a function of frequency.
With most of polyolefin products this is the same as the conventional viscosity function, which is viscosity as a function of shear rate. The dynamic measurements are limited to relatively low shear rates. On the other hand, a benefit compared to capillary rheometry is, that we get reliable information also about elasticity.
In addition, the obtained properties are much more sensitive for small differences in polymer structure which play an important role in processing and end-use properties.
Differential Scanning Calorimetry
In Differential Scanning Calorimetry (DSC) the sample and a reference material are heated and cooled in a defined manner. DSC provides a rapid method for the determination of the temperature profile of a polymeric material. This includes e.g. measurement of heat capacity, specific heat, melting, crystallisation and glass transition temperatures, heat (enthalpy) of fusion, crystallisation, study of thermal stability or reaction kinetics and in certain cases identification of the polymer or polymer mixture.
Dynamic Mechanical Thermal Analysis
Dynamic Mechanical Thermal Analysis (DMTA): Many polymeric materials exhibit time-dependent, reversible viscoelastic properties in deformation. Dynamic mechanical tests are characterized by an application of a small stress in sinusoidal fashion and a continuous scan of resonant frequency of vibration and damping vs. temperature is produced. This is transformed into dynamic storage modulus, dynamic loss modulus and damping factor by the instrument software.
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Extruder
Equipment for melting, pressurising and homogenising plastics by means of a rotating screw. Different configurations are possible; the most simple one being a „_ single screw extruder mainly used for conversion processes like cast film extrusion, fibre spinning, film blowing or pipe and profile extrusion. For more demanding applications like compounding of polymer powder before „_ pelletisation or the mixing of polymers, twin screw extruders are used where the two normally intermeshing screws can be moved in a corotating (same direction) or counterrotating (opposite direction) fashion. These machines normally contain different mixing elements, most important being kneading blocks for the local creation of extensional flow.
Environmental stress cracking
Under certain conditions of stress and environment like oils, detergents or soaps, ethylene plastics may fail mechanically by cracking. This phenomenon is called environmental stress cracking resistance (ESCR). ESCR properties can be tested in accelerated tests by presence of starter notches, surface active agent, elevated temperature and mechanical loading. Test result is the time when 50 % of the samples have failed.
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Flexural test
Like tensile test, flexural test is also used to characterize strength and stiffness of the material. Flexural properties are obtained by placing a specimen horizontally on two supports. A load is applied at a specified rate in the center, normal to specimen longitudinal axis. Measured force - deformation data is used to calculate flexural modulus and flexural strength. Flexural tests can be used if material is too brittle to withstand compressive forces of fixing in tensile grips.
Friction
When handling and storing plastic packages it is important that friction between packages and pallets, shelves etc. or between packages itself is suitable. Coefficient of starting and sliding friction (static and dynamic friction) of film is measured by using a moving sled, force transducer and a static horizontal testing table. Friction can be measured either film against film or film versus metal or other material. It is determined by the material structure and the additivation, especially the addition of slip agents and antiblocking agents.
FTIR Spectroscopy
Infrared radiation induces vibrations of molecules and molecular segments. Absorption of radiation occurs only if the vibration results in the change of the molecular dipole moment. The strength of the vibrating bonds and their dipole moment affect the absorption band intensities and positions which makes FTIR spectroscopy a very useful method in qualitative polymer analysis. Also several fast quantitative FTIR methods have been developed for the structural characterisation of polyolefins.
Polymer crystallinity and orientation implies a restriction in the movement of some molecules and consequent improvement in polymer mechanical properties. This restriction in the movements reflects also to the FTIR spectrum and thus the amorphous and crystalline polymer phases can be identified.
Fines
Polymer particles having a small diameter. Those particles whose diameter is < 0.105 mm are usually classified as fines. A high amount of fines can severely reduce the operability of a plant.
Film impact
Film impact testing is the very basic, simple and fast method to find out overall mechanical strength of the film product. Impact resistance of film is determined by using free-falling dart method. In this method the impact is generated by dropping a dart from constant height on clamped film sample. Mass of the dart is varied and a mass causing failure of 50 % of the specimen tested is the result.
Fee radical
An atom or group of atoms with an unpaired valence electron (Valence electron is an electron in the outer shell of the atom that takes part in forming chemical bonds). Free radicals are formed in High-pressure polyolefin polymerisation by decomposing organic peroxides initiating the polymerisation process and then the propagation of the ethylene monomer onto the radicals forming long chains.
Fractionation
Analysis of the whole polymer gives the average structure and composition of the material. Polymer fractionation is useful when detailed information about polymer structure is desired. Fractionation serves as a general term for the methods of separating macromolecules according to their molar mass, chemical composition or microstructural parameters. The choice of the fractionation method depends on the polymeric material and the problem to be solved.
Film Application tests
Mechanical properties of film are very much dependent on film blowing parameters like extrusion screw, die construction, blow-up ratio and cooling system. Customary film tests are carried out by using film samples.
Film blowing
Extrusion of the melt through a ring from die, blowing up, and cooling to form a thin film. Film blowing is used to produce very thin films of high mechanial strength, mostly from polyethylene.
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Gas phase reactor
Type of reactor to polymerise in the gas phase (i.e. all monomers are in the gaseous form); different designs are possible. In processes for the production of polyolefins, often fluidised bed reactors are used. Other constructions involve stirred or slow-flowing beds in different reactor geometries.
Gloss
Brightness or luster of a plastic resulting from a smooth surface. Gloss is determined by measuring the amount of light reflected from a surface, where the position and strength of the illumination has to be defined as well as the reflection angle covered by the measurement (normally 20°).
Gel
A heterogeneity in the polymer that appear as local thickness increase in films or thin-walled articles. Gels are caused by inclusion of high molecular weight particles with a higher viscosity than the surrounding, in some instances also including cross-linked molecules; normally the melting point is also higher than the surrounding. In some instances one also refer to contamination as gels, as examples can be mentioned fibre gels or dirt particles. The cause of this can be badly dispersed additives (agglomeration, dispersion) or other substances. This is not actually a gel, but sometimes called so for simplicity.
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Heterophasic copolymers
Heterophasic copolymers (HECO) also contain ethylene (and possibly higher a-olefins) as comonomer. The production is however carried out in a two-stage process, resulting in a multiphase structure with a homopolymer matrix and inclusions consisting of amorphous EP-copolymer (“rubber”) and crystalline PE. Variations of molar mass and composition of the elastomeric phase in relation to the matrix allow a wide variation of properties (stiffness, toughness and transparency).
Heterogeneous
Something relating to two ore more phases, i.e. not mixed so as to be homogeneous.
Homogeneous
Something relating to only one phase, i.e. so well mixed that the mixture have the same composition throughout.
Haze
Cloudiness in polymer films or thin-walled injection-moulded parts, resulting from light scattering in the material (reducing the transparency). Measured in transmission with a well-defined light source, results are given on a percentage scale.
Hydrogen sensitivity
A property of the catalyst which indicates how sensitive the molecular weight of the polymer is to hydrogen concentration, (also: hydrogen response). The property also depends on the polymerisation conditions (mode, temperature, pressure etc.).
Heat deflection temperature
Heat deflection temperature (HDT) indicates how loaded material deforms at higher temperatures. Test bars are placed in a heating bath, resting horizontally on two supports. A constant load is applied in the center of the specimen and the bath temperature is raised at a constant rate. The temperature of the bath at which the flexural deflection of the loading point has reached a predefined level is the heat deflection temperature of the material.
Homopolymers
Homopolymers (HOMO) consist only of propylene units; their properties can be regulated via molar mass distribution and the degree of isotacticity (stereoregular structure).
High pressure polyethylene (HPPE)
The collective name given to polyethylene produced by high pressure processes. The process is using free radical polymerisation and the polymerisation is performed at pressures of 120-350 MPa (1200-3500 bar) and at temperatures of 150-3500C. Homopolymer and copolymers of ethylene is made by this process. (See more descriptions in process description).
HDPE
High density polyethylene
Coming from a low pressure process based on special catalysts (Ti- or Cr-based), HDPE has a linear polymeric chain, combined with a higher density and melting point. To fit special applications, the molar mass distribution can be adjusted in a wide range by means of catalysts or polymerisation technology. New applications are opened for HDPE as well as other low pressure PEs by applying metallocene (single-site) catalysts, which allow the production of very narrow molar mass distributions and the better distribution of comonomers.
Hardness
Resistance of a (polymer) surface to deformation. The different hardness measures applied for characterising polymers are:
(a) Shore hardness (two scales, A for softer and D for harder materials)
(b) Ball indentation hardness (also useable on profiled surfaces because of bigger measuring device).
(c) Vickers hardness (normally used for steel).
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Injection moulding
A repetitive process in which plastic is melted and injected into a mould cavity where the article is cooled down. After cooling, the mould opens and the article is ejected. ). The melt is prepared in a screw setup similar to an extruder; the screw acts in melting and homogenising the polymer while slowly retracted to build up the melt reservoir necessary for the injection step. Then, the screw is used as a plunger in a forward movement to inject the melt through the runner , a manifold in case of a multiple mould and the gate into the mould. In the injection moulding process, filling of the mould and solidification takes place partially at the ssame time, resulting in a very complex morphology in case of semicrystalline polymers.
Impact Strength
Impact Strength characterizes material behaviour under high speed loading. Pendulum and falling weight type testers are applied here. Specimen can be either plaques, notched or unnotched bars or parts of finished products. There are several impact methods like Charpy impact test, instrumented puncture test and tensile impact test. Generally impact test shows the energy which is needed to break or puncture specimen under specified conditions. In some tests it is possible to accurately record force - deformation transient curves and calculate specific parameters describing material behaviour under impact loading.
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LLDPE
Linear low density polyethylene
Based on the same technology as HDPE, but the density is reduced through the incorporation of longer a-olefins as comonomers. With this, another way of adapting the products to the final applications turns up. Single-site catalysts allow an expansion of the application range, mainly by allowing higher comonomer contents and a more even distribution of these.
LDPE
(Low density polyethylene)
A product from a high-pressure polymerisation process, LDPE is characterised by its highly branched chain structure. As a result of this, density and crystallinity are reduced while the processability in a variety of conversion technologies is greatly facilitated. The addition of special comonomers can further expand the property range of these grades.
Low pressure polyethylene (LPPE)
The collective name given to polyethylene produced in low pressure processes. The polymerisation is utilising a catalyst to get the neccessary rate of polymerisation. Homopolymer and copolymers of ethylene are made by these processes.
Long chain branch
A brach with many (>20) carbon atoms. Typical for low density PE.
Loop reactor
A bulk (slurry) polymerisation reactor which is made of a closed long tube through which the slurry is circulated. Loop reactors are characterised by a very narrow residence time distribution, leading to uniform product characteristics.
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Metal deactivators
Some metal ions may contribute to accelerated degradation. For polyolefins in contact with metals, such as cable insulation of copper wires, metal deactivators are added to passivate the degradation reactions.
Melt strength
A property of the polymer melt which indicates its ability to withstand drawing without breaking. Melt strength is improved by the presence of high molecular weight tail or long chain branches.
MDPE
Medium low density polyethylene
Based on the same technology as HDPE, but the density is reduced through the incorporation of longer a-olefins as comonomers. With this, another way of adapting the products to the final applications turns up. Single-site catalysts allow an expansion of the application range, mainly by allowing higher comonomer contents and a more even distribution of these.
MFR
Melt mass flow rate or melt flow rate (MFR) gives an indication of melt processing properties of a polymer. Shear rate range is, however, narrow and much lower than in the most of processing applications. It gives thus very limited information about melt flow behaviour. The method is a kind of capillary measurement.
The equipment consists of a barrel, a piston with a place for loading masses and a die. The sample is charged into the barrel and heated up to a specified temperature. After that the piston is loaded extruding the polymer through the die.
Melt flow rate (MFR) is defined as a mass flown through the die in 10 min. The weights mostly used with polyolefins are 2.16 kg, 5 kg, 10 kg and 21.6 kg. In addition to the evaluation of flow properties MFR serves as a rough estimation of molecular weight.
Usually also Flow Rate Ratio (FRR) is determined. This ia defined as a ratio of two MFR values obtained with two different weights. FRR can be used as a rough estimation of molecular weight distribution.
Monomer
Monomer forms a repeating unit in a polymer chain.
Morphology
Morphology is the internal or external structure of a solid substance. Important sub-definitions include
(a)Crystal morphology of a polymer film or part (b)Phase morphology of heterophasic polymer systems (blend, copolymer) (c) Powder morphology of a polymer powder from the polymerisation reactor.
Molecular Weight Distribution
The relation between the number of molecules in a polymer and their individual chain length. It is often also called “molar mass distribution” (MMD). Technical polymers are always polydisperse as a result of the nature of the catalyst and the polymerisation conditions. The MWD is often characterised by size exclusion chromatography (SEC), but it can also be calculated approximately from rheological data for purely linear polymers.
Metallocene
A catalyst which contains a metallocene compound of a transition metal. Often used as a synonym for single site.
Multimodal polymer
Multimodal polymer includes more than one molecular weight fraction.
Melt tension test
Extensional properties play an important role in some processes such as melt spinning, blow moulding and film blowing. Melt tension test is a simple method for characterisation of extensional properties. The measuring principal copies industrial spinning process. A fluid is pressed or extruded through a round hole die, the resulting thread is hauled off with increasing speed and the applied extensional force is measured. The force is recorded as a function of haul-off speed or draw-down ratio, which is the ratio of the haul-off speed to the initial speed. Melt strength is defined as a force at filament break, while drawability can be estimated qualitatively from the maximum draw-down ratio at the failure.
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Nucleating agents
Substances with the ability to act as substrates for crystal formation by epitaxy, thus increasing the nucleation density. Improve crystallisation behaviour in processing, i.e., cycle time or line speed, as well as crystallinity and morphology in final product, improving optical and mechanical properties.
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Oxygen induction time (OIT)
A relative measure of a level or a degree of stabilisation (amount of remaining active antioxidant).
Optical properties
Optical properties are important in packaging applications where decorative printing is required or good visibility of the packed product is essential. Gloss and haze are used to characterize optical appearance of film products.
Optical Microscopy
Optical Microscopy (OM) is usually the most familiar tool to study material morphology and it offers many techniques for polymer characterisation including transmission, reflected light, dark field, polarized light, phase contrast and interference contrast microscopy. These techniques give information based upon the variations in light scattering, differences in refractive index and differential phase shifts of reflected or transmitted light. Light microscopy indicates the physical aspects of polymer composition, such as multiple phases present, the sizes and shapes of the particles and overall characteristics of the individual particles.
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Pelletising
A process where the polymer powder is homogenised, addiviated and made to pellets. The polymer melt coming from the extruder is pressed through dies in a die plate and cut to pellets by knives under water (underwater pelletising) or cooled to solid strands and then cut to pellets (strand pelletising).
Permeability
Material constant characterising gas penetration, like water vapour, oxygen or carbondioxide.
Polymerisation
Polymerisation is a chemical reaction where polymer is synthesised from monomers.
Polypropylenes
Polypropylenes are thermoplastic polymers consisting mainly of propylene (C3H6), come frome a low pressure process based on catalysts and have generallya linear polymer chain.
Productivity
A property of the catalyst which indicates how much the catalyst has produced polymer in the process (kg polymer/g catalyst).
Prepolymerisation
A process where polymerisation is started to increase the activity of the catalyst getting over the induction time of the catalyst and not to overheat the catalyst particle by to sudden onset of polymerisation. Prepolymerisation will also help in ensuring more even polymerisation on the catalyst particles reducing the probability of creating a high fraction of fines in the later reaction steps.
Peroxide
Peroxides are a group of compounds that contain the O22- ion. It is used to initiate the polymerisation for high-pressure free radical polymerisation processes.
Polydispersity
Broadness of the molecular weight distribution. A commonly used measure, polydispersity index, is defined as the ratio of weight average molecular weight to the number average molecular weight.
Processability
A concept to describe the behaviour of the polymer in a converting process, such as blow moulding machine and film line. A good processability means a stable behaviour in the process and a high processing speed.
Puncture
In certain packaging applications film is stretched and wrapped around multiform items which can have sharp corners or other protrusions causing high point loading for the film. Puncture test is used to simulate this kind of film loading. Puncture resistance is measured by pressing a specially shaped probe through a clamped film sample. The maximum force, force at break, penetration distance and energy to break are determined.
Polyethylenes
Polyethylenes (or polythenes) are thermoplastic polymers consisting mainly of ethylene (C2H4), which are divided according to their density, melting point and molecular structure.
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Random Copolymers
Random copolymers (RACO) are produced by statistical insertion of units of ethylene or higher a-olefins (mostly butene, also in combination with ethyle to give terpolymers). Through this, the glass transition temperature and melting point are lower than in homopolymers, allowing better sealability and application at temperatures below 0°C.
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Shore Hardness
Resistance of a (polymer) surface to deformation. The different hardness measures applied for characterising polymers are:
(a) Shore hardness (two scales, A for softer and D for harder materials)
(b) Ball indentation hardness (also useable on profiled surfaces because of bigger measuring device).
(c) Vickers hardness (normally used for steel)
Slip agent
type of additive, which provides surface lubrication to the polymer during and after conversion; especially important for films. Usual slip agents are amides of fatty acids.
Size Exclusion Chromatography - (SEC, also called Gel Permeation Chromatography, GPC) is used to separate dissolved polymer molecules according to their coil diameter in solution. The chromatograms can be converted to molar mass distributions, which is an important parameter because it has a significant effect on the physical, mechanical and rheological properties of a polymer. It is also possible to calculate the average molar masses, Mn, Mv, Mw and Mz, and when on-line viscometric detection is used, to study long chain branching (LCB) and its distribution.
Solution Viscometry - In Solution Viscometry the rate at which a dilute polymer solution flows through a capillary is measured. Temperature and concentration of the polymer are carefully controlled. The parameter most often determined by dilute solution viscometry is intrinsic viscosity. Intrinsic viscosity is related to molar mass through a semi-empirical relationship called Mark-Houwink’s equation.
Shear thinning - A behaviour where the viscosity decreases when the shear stress increases. Typically broadening of the molecular weight distribution increases shear thinning.
Supercritical state - The critical state of a fluid is when the liquid and gas phase both have the same density. The fluid is then at its critical temperature, critical pressure, and critical volume. When the pressure or temperature exceeds this, the fluid is in its supercritical state. The loop reactor in the BORSTAR process is operated in supercritical state for some products.
Sheeting - Formation of sheet-like agglomerates in the gas phase reactor.
Single Site Catalysts - Produces polymer having very narrow MWD
Extremely hydrogen and comonomer sensitive
Very uniform comonomer distribution
Usually cocatalyst required
Produces stereospecific polymers
Shrinkage - Decrease of dimension, created by the introduction of internal stress to the material during processing and cooling, or by ageing and post-crystallisation processes.
Scanning Electron Microscopy - In Scanning Electron Microscopy (SEM) a focused beam of electrons scans across the surface of the sample. It is detected synchronously to the position of the scanning electron beam and recorded in an image storage device. Differencies in the topography of the sample give rise to the differencies in the intensities of the signals formed by the secondary electrons in the detector resulting in a three dimensional image. Scanning electron microscopy is suited to most types of surface morphology studies. Rough topographic features, void content and particle agglomerations are easily revealed as well as the compositional differencies within material. Energy Dispersive X-ray Spectroscopy (EDS) or Wavelength Dispersive X-Ray Spectroscopy (WDS) in conjunction with SEM allows elemental analyses to be performed directly over the surface of the sample.
Tensile - Tensile properties are the most important indication of the strength and stiffnes of the material. The force necessary to pull the specimen apart is determined as well the deformation before breaking. Tensile modulus is used as a measure of stiffness and it is an important design parameter for end use applications. It is calculated based on measured force - deformation data. Tensile tests can be carried out at different temperatures according to application needs. Test automatisation and modern data logging systems are used in order to ensure high quality, reliable results.
Tear strength - Film must have certain resistance to tear loading. Tear strength indicates the average force required to propagate tearing through a specified length of the film sample. This test is suitable for investigation of orientation balance of the film. Normally tear strength is measured in two directions, parallel and transverse to the extrusion direction of the film.
Thermogravimetry - Thermogravimetry (TG, TGA) can be used to measure any reaction involving mass change. TG analysis is often complementary test to DSC, the weight loss curve giving a picture of the make-up of the material by showing the temperatures at which individual components volatilize.
Thermal Stability - Thermoanalytical methods such as Differential Scanning Calorimetry (DSC) and thermogravimetry (TG) provide information on the thermal stability of the polymers, their shelf-life under particular conditions and the effect of additives on thermal stability. The thermal stability of a polymer is usually measured as the oxidative induction time (OIT) at set temperature.
Thermomechanical Analysis - Thermomechanical Analysis (TMA) is is a technique for monitoring changes of physical dimensions (expansion, schrinkage, penetration) while the temperature of the sample is changed.
ULDPE - Ultra low density polyethylene - Based on the same technology as HDPE, but the density is reduced through the incorporation of longer a-olefins as comonomers. With this, another way of adapting the products to the final applications turns up. Single-site catalysts allow an expansion of the application range, mainly by allowing higher comonomer contents and a more even distribution of these.
UV light stabilizers - To protect polyolefins for outdoor applications against light induced degradation. Hindered amine light stabilizers (HALS) are the most efficient.
Vicat softening temperature - Vicat softening temperature shows heat softening characteristics of thermoplastics. Flat specimen are placed in a temperature regulated heating bath, a needle type, loaded penetrator is set on the specimen surface and bath temperature is raised at a constant rate. The temperature of the bath at which the penetration of the needle has reached a predefined level is the Vicat softening temperature of the material.
VLDPE - Very low density polyethylene - Based on the same technology as HDPE, but the density is reduced through the incorporation of longer a-olefins as comonomers. With this, another way of adapting the products to the final applications turns up. Single-site catalysts allow an expansion of the application range, mainly by allowing higher comonomer contents and a more even distribution of these.
XLPE Crosslinked (X-linked) Polyethylene, is a polyethylene where the thermoplastic structure with separate polymer chains have been modified to a form a network structure that is no longer thermoplastic and will not melt when heated. This modification is used to increase service temperature for products as Hot Water Pipes and Power Cables. Crosslinking is manly achieved by using silane copolymers or by free-radical-crosslinking using a peroxide.
Ziegler-Natta Catalyst: Produces polymer having relatively narrow MWD Hydrogen is used to control MW Relatively comonomer sensitive Aluminum alkyl cocatalyst required
Wednesday, September 8, 2010
Correct method and material to clean extruders without damage
Plastic processors often clean screws wrongly, causing serious and expensive damage. A common tool used as part of the cleaning procedure is an acetylene torch that unfortunately, affects the metallurgical properties of the base metal of the screw and closely machined screw tolerances. During manufacturing of tool-steel screws, the base metal is heat treated to increase its hardness. If heat from an acetylene torch is used to remove plastic from the root of the screw, it will undo the annealing of the base metal, reduce the yield strength of the steel at that point and increase the likelihood that the screw will break because of torque.
If the area where the torch was applied turns a permanent blue, it means heat has caused metallurgical changes to the base metal and reduced the wear resistance in that area of the screw. Occasionally, torching has caused delamination of the metal wear coating from the base metal of the screw. Heat from an acetylene torch also causes the metal to expand on the side of the screw where it is applied, which in turn causes the screw to bend. Another big source of damage to screws during cleaning is using steel screw drivers, scrapers or pries bars to remove plastic.
Tools and cleaning materials for "pristine" cleaning of a screw and barrel are few and simple: high-heat gloves, safety glasses, a brass putty knife, brass wire brush, copper gauze, stearic acid flakes, an electric drill, cotton rags, a round brass wire brush about the ID of the barrel that is mounted on a rod as long as the barrel.
A more efficient alternative to blow torching is to purge and clean the screw immediately after use, while it is still evenly heated from production. In high-pressure applications like wire and cable and blown film, commercial purging compounds can be used. In low-pressure applications, a fractional-melt (0.35-MI) HDPE may be sufficient as a purging material instead.
For larger extruders, it may be more practical and economical to use a commercial purging compound for product changeovers, without pulling the screw. The first step in purging the screw is to close off the flow of the polymer being processed, by closing the slide gate at the bottom of the hopper. Screw speed needs to be reduced to 15-25 rpm and run at this speed until melt stops flowing from the end of the die. All barrel zones should be set to about 200°C. Once the barrel zones have reached that temperature, purging can begin. Depending on the type of extrusion process, it may be necessary to remove the die or head tooling to reduce risk of over-pressurizing the end of the extruder. The screw should be turning at 15 to 20 rpm. In a low-pressure die application, the die can remain on the extruder during purging until there is a complete change from the processing material to the purging material.
Once the die has been completely purged, stop the screw so that the die can be removed and the end of the screw exposed. Once the die is removed, the screw can be restarted and run at about 10 rpm to allow the remaining purge material to be pumped out.
Once the purging material has stopped extruding from the screw, the screw needs to be removed from the machine. For an extruder with a screw cooling system, this apparatus of hoses, rotary union, and piping needs to be removed before the screw extractor mechanism can be attached to the extruder gearbox. Using the screw extractor, the screw should now be pushed forward until 4 or 5 turns of the screw are exposed for cleaning. Clean the purge material from the channels of the screw using the brass putty knife and brass wire brush. When the purge material has been removed from the exposed portion of the screw, push another 4 to 5 turns of the screw forward using the screw extractor, and continue the cleaning process.
The screw can eventually be pushed down most of the length of the barrel. When the large amounts of purge or HDPE have been removed with the brass putty knife and wire brush, stearic acid flakes need to be sprinkled onto the root of the hot screw. The copper gauze is to be used to remove any remaining residue. After the entire screw has been polished with copper gauze, a final clean-up to "pristine" condition needs to be done using a soft cotton rag to ensure that there is no contamination in the next production run. Once the screw is completely cleaned, it can be set aside until the barrel has been cleaned, or placed on a screw rack. If the screw is stored, it should be sprayed and wiped down with light oil, such as WD-40 or PB Blaster to prevent rusting.
Chrome-plated screws are less likely to rust during storage and don't need oiling.
Cleaning the barrel is much easier than cleaning the screw, but just as important. With barrel temperatures still set at 200°C, the barrel is ready to clean. A wire brush is to be wrapped with copper gauze. Before inserting the brush and gauze assembly into the bore of the barrel, throw in a handful of stearic acid. Stearic acid can be sprinkled over the copper gauze before pushing the brush/gauze assembly into the barrel. Once the brush/gauze assembly is inside the barrel, the electric drill is to be used to rotate it while moving it in and out until it moves easily. Additional stearic acid may be needed to be added before the bore is thoroughly cleaned.
When the brush/gauze assembly is removed from the barrel, push a bundle of cotton rags back and forth inside the length of the barrel to remove any purge or stearic acid residue. When the rags have been passed back and forth several times and return totally clean, barrel cleaning is complete. The entire screw and barrel assembly are pristine and ready for the next production run.
Many purge materials are commercially available. It is the processor's job to identify the appropriate purge to use with the resin being processed. Purge materials work by one of four mechanisms:
• Mechanical purge
• Abrasive mineral filled material
• Chemical purge to break down resins and contaminants
• Hard resin filled with surfactants
Mechanical purges are stiff materials, normally polyethylene (PE) based, containing cleaning and release agents. Fractional melt flow high density polyethylene (HDPE) functions very well as a purge material over a wide temperature range. Low viscosity abrasive mineral or glass filled materials will force most resins out of an extruder while scouring the screw, barrel, and die. Cast acrylic is an abrasive purge, as it does not completely melt in the extruder. Due to the high melt viscosity of cast acrylic, it is better to remove the die prior to purging the screw. Typical amounts required are 5 to 10 lbs/inch of screw diameter. Once cast acrylic is in the barrel, it has to be purged out or the screw pulled and mechanically cleaned. The third purging material uses a surfactant that penetrates and loosens residue on the screw, barrel, and die, dispersing it in the melt. The surfactant is mixed with a melt flow resin that is 0.1 to 0.3 times that of the original melt to provide maximum purging effectiveness. The fourth material is a chemical purge that attacks the material left in the barrel.
This may be through plastication of the resin in the barrel, lowering the viscosity and making it easier to force out of the barrel, or the additive may actually cause polymer depolymerization into lower-molecular-weight components. Mechanical purging commonly uses a fractional melt flow HDPE. The stiff material pushes the resin being purged out in front of the HDPE. Fractional melt flow HDPE is cheap, works well, and has a wide processing temperature range. After the resin being purged is out of the extruder, the barrel temperatures can be lowered and more HDPE added to remove the earlier purge material.
As the screw is being pulled from the extruder, any residual HDPE can be brushed off the screw with a brass wire brush. Alternatively an electric or air-powered brass wire brush can be used in cleaning. Several commercially available materials are produced as purge compounds based on mechanical purging
If the area where the torch was applied turns a permanent blue, it means heat has caused metallurgical changes to the base metal and reduced the wear resistance in that area of the screw. Occasionally, torching has caused delamination of the metal wear coating from the base metal of the screw. Heat from an acetylene torch also causes the metal to expand on the side of the screw where it is applied, which in turn causes the screw to bend. Another big source of damage to screws during cleaning is using steel screw drivers, scrapers or pries bars to remove plastic.
Tools and cleaning materials for "pristine" cleaning of a screw and barrel are few and simple: high-heat gloves, safety glasses, a brass putty knife, brass wire brush, copper gauze, stearic acid flakes, an electric drill, cotton rags, a round brass wire brush about the ID of the barrel that is mounted on a rod as long as the barrel.
A more efficient alternative to blow torching is to purge and clean the screw immediately after use, while it is still evenly heated from production. In high-pressure applications like wire and cable and blown film, commercial purging compounds can be used. In low-pressure applications, a fractional-melt (0.35-MI) HDPE may be sufficient as a purging material instead.
For larger extruders, it may be more practical and economical to use a commercial purging compound for product changeovers, without pulling the screw. The first step in purging the screw is to close off the flow of the polymer being processed, by closing the slide gate at the bottom of the hopper. Screw speed needs to be reduced to 15-25 rpm and run at this speed until melt stops flowing from the end of the die. All barrel zones should be set to about 200°C. Once the barrel zones have reached that temperature, purging can begin. Depending on the type of extrusion process, it may be necessary to remove the die or head tooling to reduce risk of over-pressurizing the end of the extruder. The screw should be turning at 15 to 20 rpm. In a low-pressure die application, the die can remain on the extruder during purging until there is a complete change from the processing material to the purging material.
Once the die has been completely purged, stop the screw so that the die can be removed and the end of the screw exposed. Once the die is removed, the screw can be restarted and run at about 10 rpm to allow the remaining purge material to be pumped out.
Once the purging material has stopped extruding from the screw, the screw needs to be removed from the machine. For an extruder with a screw cooling system, this apparatus of hoses, rotary union, and piping needs to be removed before the screw extractor mechanism can be attached to the extruder gearbox. Using the screw extractor, the screw should now be pushed forward until 4 or 5 turns of the screw are exposed for cleaning. Clean the purge material from the channels of the screw using the brass putty knife and brass wire brush. When the purge material has been removed from the exposed portion of the screw, push another 4 to 5 turns of the screw forward using the screw extractor, and continue the cleaning process.
The screw can eventually be pushed down most of the length of the barrel. When the large amounts of purge or HDPE have been removed with the brass putty knife and wire brush, stearic acid flakes need to be sprinkled onto the root of the hot screw. The copper gauze is to be used to remove any remaining residue. After the entire screw has been polished with copper gauze, a final clean-up to "pristine" condition needs to be done using a soft cotton rag to ensure that there is no contamination in the next production run. Once the screw is completely cleaned, it can be set aside until the barrel has been cleaned, or placed on a screw rack. If the screw is stored, it should be sprayed and wiped down with light oil, such as WD-40 or PB Blaster to prevent rusting.
Chrome-plated screws are less likely to rust during storage and don't need oiling.
Cleaning the barrel is much easier than cleaning the screw, but just as important. With barrel temperatures still set at 200°C, the barrel is ready to clean. A wire brush is to be wrapped with copper gauze. Before inserting the brush and gauze assembly into the bore of the barrel, throw in a handful of stearic acid. Stearic acid can be sprinkled over the copper gauze before pushing the brush/gauze assembly into the barrel. Once the brush/gauze assembly is inside the barrel, the electric drill is to be used to rotate it while moving it in and out until it moves easily. Additional stearic acid may be needed to be added before the bore is thoroughly cleaned.
When the brush/gauze assembly is removed from the barrel, push a bundle of cotton rags back and forth inside the length of the barrel to remove any purge or stearic acid residue. When the rags have been passed back and forth several times and return totally clean, barrel cleaning is complete. The entire screw and barrel assembly are pristine and ready for the next production run.
Many purge materials are commercially available. It is the processor's job to identify the appropriate purge to use with the resin being processed. Purge materials work by one of four mechanisms:
• Mechanical purge
• Abrasive mineral filled material
• Chemical purge to break down resins and contaminants
• Hard resin filled with surfactants
Mechanical purges are stiff materials, normally polyethylene (PE) based, containing cleaning and release agents. Fractional melt flow high density polyethylene (HDPE) functions very well as a purge material over a wide temperature range. Low viscosity abrasive mineral or glass filled materials will force most resins out of an extruder while scouring the screw, barrel, and die. Cast acrylic is an abrasive purge, as it does not completely melt in the extruder. Due to the high melt viscosity of cast acrylic, it is better to remove the die prior to purging the screw. Typical amounts required are 5 to 10 lbs/inch of screw diameter. Once cast acrylic is in the barrel, it has to be purged out or the screw pulled and mechanically cleaned. The third purging material uses a surfactant that penetrates and loosens residue on the screw, barrel, and die, dispersing it in the melt. The surfactant is mixed with a melt flow resin that is 0.1 to 0.3 times that of the original melt to provide maximum purging effectiveness. The fourth material is a chemical purge that attacks the material left in the barrel.
This may be through plastication of the resin in the barrel, lowering the viscosity and making it easier to force out of the barrel, or the additive may actually cause polymer depolymerization into lower-molecular-weight components. Mechanical purging commonly uses a fractional melt flow HDPE. The stiff material pushes the resin being purged out in front of the HDPE. Fractional melt flow HDPE is cheap, works well, and has a wide processing temperature range. After the resin being purged is out of the extruder, the barrel temperatures can be lowered and more HDPE added to remove the earlier purge material.
As the screw is being pulled from the extruder, any residual HDPE can be brushed off the screw with a brass wire brush. Alternatively an electric or air-powered brass wire brush can be used in cleaning. Several commercially available materials are produced as purge compounds based on mechanical purging
Thursday, July 29, 2010
Upcomming Plastic Exhibitions and Events
ICE South America 2010
Venue: Sao Paolo - Brazil
Aug 04 2010 - Aug 06 2010
China Plastics & Rubber Industry Exhibition
Venue: Qingdao - China
Aug 11 2010 - Aug 13 2010
The 10th China Plastics Exhibition and Conference (China PEC 2010)
Venue: Taizhou - China
Sep 03 2010 - Sep 05 2010
Asiamold 2010
Venue: Guang Zhou - China
Sep 15 2010 - Sep 17 2010
China Injection Moulding Expo
Venue: Tian Jin City - China
Sep 15 2010 - Sep 17 2010
Vietnam Plas
Venue: Ho Chi Minh - Vietnam
Sep 22 2010 - Sep 25 2010
Plastic Industry Show-Moscow
Venue: Moscow - Russia
Sep 27 2010 - Sep 30 2010
Plastec Midwest
Venue: Illinois - USA
Sep 28 2010 - Sep 30 2010
IndiaPack
Venue: Mumbai - India
Sep 30 2010 - Oct 03 2010
CHILEPLAST 2010
Venue: SANTIAGO - Chile
Oct 01 2010 - Oct 03 2010
Plast Imagen Mexico
Venue: Mexico - Mexico
Oct 04 2010 - Oct 07 2010
Plastics Philippines
Venue: Makati City - Philippines
Oct 06 2010 - Oct 09 2010
Interplast Expo
Venue: bucharest - Romania
Oct 06 2010 - Oct 09 2010
Cheminnovations 2010
Venue: HOUSTON - USA
Oct 19 2010 - Oct 21 2010
3P Indonesia Packaging, Printing & Plastic Machinery Expo
Venue: Medan - Indonesia
Oct 21 2010 - Oct 23 2010
Plastex Ukraine
Venue: Kiev - Ukraine
Oct 26 2010 - Oct 29 2010
Allplas Indonesia
Venue: jakarta - Indonesia
Oct 27 2010 - Oct 30 2010
K-2010
Venue: Dusseldorf - Germany
Oct 27 2010 - Nov 03 2010
Pack Expo International 2010
Venue: Chicago - USA
Oct 31 2010 - Nov 03 2010
PLASTEX UZBEKISTAN
Venue: Tashkent - Uzbekistan
Nov 16 2010 - Nov 18 2010
International Packtech India 2010
Venue: Mumbai - India
Nov 18 2010 - Nov 20 2010
Autoplas-2010
Venue: Pune - India
Nov 26 2010 - Nov 28 2010
PLASTEX SIBERIA 2010
Venue: Novosibirsk - Russia
Nov 30 2010 - Dec 03 2010
Plastic & Rubber Indonesia
Venue: jakarta - Indonesia
Dec 01 2010 - Dec 04 2010
Plast Eurasia Istanbul
Venue: Istanbul - Turkey
Dec 02 2010 - Dec 05 2010
PackPlus 2010
Venue: New Delhi - India
Dec 03 2010 - Dec 06 2010
Plast Show
Venue: Gujarat - India
Dec 03 2010 - Dec 06 2010
NILE-PLAST 2010
Venue: Khartoum - Sudan
Dec 06 2010 - Dec 09 2010
ArabPlast 2011
Venue: Dubai - UAE
Jan 08 2011 - Jan 11 2011
Printing, Plastic, Packaging & Food Processing Machinery-Sri Lanka
Venue: Colombo - Sri Lanka
Jan 12 2011 - Jan 14 2011
Printpack India 2011
Venue: New Delhi - India
Jan 16 2011 - Jan 21 2011
Plastivision
Venue: Mumbai - India
Jan 20 2011 - Jan 24 2011
Interplastica
Venue: Moscow - Russia
Jan 25 2011 - Jan 28 2011
International Fair of Plastics & Rubber Processing
Venue: poznan - Poland
Feb 01 2011 - Feb 04 2011
COMPOSITES 2011.
Venue: Florida - USA
Feb 02 2011 - Feb 04 2011
Global Plastics & Rubber 2011
Venue: Bangkok - Thailand
Mar 03 2011 - Mar 04 2011
Plastex Central Asia
Venue: Almaty - Kazakisatan
Mar 02 2011 - Mar 05 2011
Molding 2011
Venue: SAN DIEGO - USA
Mar 07 2011 - Mar 09 2011
PU TECH
Venue: Delhi - India
Mar 09 2011 - Mar 11 2011
Wenzhou International Plastics Industry & Materials Exhibition
Venue: wenzhou - China
Mar 11 2011 - Mar 13 2011
Plastic, Packaging, Paper & Print Asia
Venue: Karachi - Pakistan
Mar 12 2011 - Mar 14 2011
Mass Plastics 2011
Venue: MA - USA
Mar 15 2011 - Mar 16 2011
Suedtec
Venue: Stuttgart - Germany
Mar 22 2011 - Mar 24 2011
Plastixexpo
Venue: Italy - Italy
Mar 24 2011 - Mar 26 2011
International Plastic & Packaging Industry Exhibition
Venue: Karachi - Pakistan
Apr 05 2011 - Apr 07 2011
PLASTIC EXPO
Venue: Tunis - Tunisia
Apr 12 2011 - Apr 16 2011
BANGLAPLAST 2011
Venue: Dhaka - Bangladesh
Apr 20 2011 - Apr 23 2011
ANTEC 2011
Venue: Massachusetts - USA
May 01 2011 - May 05 2011
Plastteknik Oresund
Venue: Malmo - Sweden
May 04 2011 - May 05 2011
Brasilplast
Venue: Sao Paolo - Brazil
May 09 2011 - May 13 2011
Venue: Sao Paolo - Brazil
Aug 04 2010 - Aug 06 2010
China Plastics & Rubber Industry Exhibition
Venue: Qingdao - China
Aug 11 2010 - Aug 13 2010
The 10th China Plastics Exhibition and Conference (China PEC 2010)
Venue: Taizhou - China
Sep 03 2010 - Sep 05 2010
Asiamold 2010
Venue: Guang Zhou - China
Sep 15 2010 - Sep 17 2010
China Injection Moulding Expo
Venue: Tian Jin City - China
Sep 15 2010 - Sep 17 2010
Vietnam Plas
Venue: Ho Chi Minh - Vietnam
Sep 22 2010 - Sep 25 2010
Plastic Industry Show-Moscow
Venue: Moscow - Russia
Sep 27 2010 - Sep 30 2010
Plastec Midwest
Venue: Illinois - USA
Sep 28 2010 - Sep 30 2010
IndiaPack
Venue: Mumbai - India
Sep 30 2010 - Oct 03 2010
CHILEPLAST 2010
Venue: SANTIAGO - Chile
Oct 01 2010 - Oct 03 2010
Plast Imagen Mexico
Venue: Mexico - Mexico
Oct 04 2010 - Oct 07 2010
Plastics Philippines
Venue: Makati City - Philippines
Oct 06 2010 - Oct 09 2010
Interplast Expo
Venue: bucharest - Romania
Oct 06 2010 - Oct 09 2010
Cheminnovations 2010
Venue: HOUSTON - USA
Oct 19 2010 - Oct 21 2010
3P Indonesia Packaging, Printing & Plastic Machinery Expo
Venue: Medan - Indonesia
Oct 21 2010 - Oct 23 2010
Plastex Ukraine
Venue: Kiev - Ukraine
Oct 26 2010 - Oct 29 2010
Allplas Indonesia
Venue: jakarta - Indonesia
Oct 27 2010 - Oct 30 2010
K-2010
Venue: Dusseldorf - Germany
Oct 27 2010 - Nov 03 2010
Pack Expo International 2010
Venue: Chicago - USA
Oct 31 2010 - Nov 03 2010
PLASTEX UZBEKISTAN
Venue: Tashkent - Uzbekistan
Nov 16 2010 - Nov 18 2010
International Packtech India 2010
Venue: Mumbai - India
Nov 18 2010 - Nov 20 2010
Autoplas-2010
Venue: Pune - India
Nov 26 2010 - Nov 28 2010
PLASTEX SIBERIA 2010
Venue: Novosibirsk - Russia
Nov 30 2010 - Dec 03 2010
Plastic & Rubber Indonesia
Venue: jakarta - Indonesia
Dec 01 2010 - Dec 04 2010
Plast Eurasia Istanbul
Venue: Istanbul - Turkey
Dec 02 2010 - Dec 05 2010
PackPlus 2010
Venue: New Delhi - India
Dec 03 2010 - Dec 06 2010
Plast Show
Venue: Gujarat - India
Dec 03 2010 - Dec 06 2010
NILE-PLAST 2010
Venue: Khartoum - Sudan
Dec 06 2010 - Dec 09 2010
ArabPlast 2011
Venue: Dubai - UAE
Jan 08 2011 - Jan 11 2011
Printing, Plastic, Packaging & Food Processing Machinery-Sri Lanka
Venue: Colombo - Sri Lanka
Jan 12 2011 - Jan 14 2011
Printpack India 2011
Venue: New Delhi - India
Jan 16 2011 - Jan 21 2011
Plastivision
Venue: Mumbai - India
Jan 20 2011 - Jan 24 2011
Interplastica
Venue: Moscow - Russia
Jan 25 2011 - Jan 28 2011
International Fair of Plastics & Rubber Processing
Venue: poznan - Poland
Feb 01 2011 - Feb 04 2011
COMPOSITES 2011.
Venue: Florida - USA
Feb 02 2011 - Feb 04 2011
Global Plastics & Rubber 2011
Venue: Bangkok - Thailand
Mar 03 2011 - Mar 04 2011
Plastex Central Asia
Venue: Almaty - Kazakisatan
Mar 02 2011 - Mar 05 2011
Molding 2011
Venue: SAN DIEGO - USA
Mar 07 2011 - Mar 09 2011
PU TECH
Venue: Delhi - India
Mar 09 2011 - Mar 11 2011
Wenzhou International Plastics Industry & Materials Exhibition
Venue: wenzhou - China
Mar 11 2011 - Mar 13 2011
Plastic, Packaging, Paper & Print Asia
Venue: Karachi - Pakistan
Mar 12 2011 - Mar 14 2011
Mass Plastics 2011
Venue: MA - USA
Mar 15 2011 - Mar 16 2011
Suedtec
Venue: Stuttgart - Germany
Mar 22 2011 - Mar 24 2011
Plastixexpo
Venue: Italy - Italy
Mar 24 2011 - Mar 26 2011
International Plastic & Packaging Industry Exhibition
Venue: Karachi - Pakistan
Apr 05 2011 - Apr 07 2011
PLASTIC EXPO
Venue: Tunis - Tunisia
Apr 12 2011 - Apr 16 2011
BANGLAPLAST 2011
Venue: Dhaka - Bangladesh
Apr 20 2011 - Apr 23 2011
ANTEC 2011
Venue: Massachusetts - USA
May 01 2011 - May 05 2011
Plastteknik Oresund
Venue: Malmo - Sweden
May 04 2011 - May 05 2011
Brasilplast
Venue: Sao Paolo - Brazil
May 09 2011 - May 13 2011
Friday, May 21, 2010
PE bags less damaging to environment vs bags based on biodegradable polymers?
Bags made from Polyethylene are found to be less damaging in the environment terms when compared to bags based on biodegradable polymers currently in the market in the European region. This is an interesting finding by Germany’s Institute for Energy & Environment Research (IFEU) during a detailed life cycle analysis of plastics waste bags. The study shows that, when all environmental factors are considered, the least environmentally damaging option for plastics waste bags is to use recycled PE, followed by virgin PE. The current biodegradable options, including both petrochemical and renewably-sourced plastics, performed the worst in this analysis. Oxo-degradable bags/products have not been considered. The study looked at 20 and 120 liter plastics waste bags on the market in Germany and a 30-litre size popular in France, comparing both virgin and recycled PE resins with commercially-available biodegradable alternatives including starch and PLA-based formulations. According to IFEU, the environmental profile of each waste bag is dominated by the raw material production process, with conversion to bags and transport to point-of-sale relatively minor contributors in most cases. The environmental impact of each bag type considered in the study will be lower for thinner bags. In fact, virgin PE and recycled PE score well in environmental terms because they enable significant down-gauging. Bio degradable film of 20 liter is in the range of 15-25 microns compared to virgin PE bags of only 12.5 micron thickness. The IFEU analysis also takes into account future anticipated improvements in the biodegradable plastics sector in terms of polymer manufacturing efficiency and material performance. However, the study authors point out that none of its forecast future scenarios result in the materials outperforming PE in general waste bag applications. In a separate study, the European Plastics Recyclers Association warned that oxo-degradable polymers have the potential to do more harm to the environment than good. Oxo-degradable plastics are made to degrade in the presence of oxygen and sunlight. If these bags get buried in a landfill, they probably won't degrade at all because there is no light or oxygen. Studies of one brand in the US, commissioned by the Biodegradable Products Institute, found that breakdown is very dependent on temperature and humidity. It goes slow in cold weather, while high humidity virtually stops the process. A recent Swedish study found that polyethylene containing manganese additive stops breaking down when put in compost, probably due to the influence of ammonia or other gases generated by microorganisms in the compost. The National Association for PET Container Resources- PET being a widely used plastic packaging material. NAPCOR recently called on food companies to eschew use of these degrading chemicals in packaging until their efficacy is proven. One of NAPCOR's concerns is that unless degradable and nondegradable plastics are handled separately, they could wind up being recycled together - creating plastic products that fall apart in sunlight and air. Another potential hazard is that the substance in the plastic causing its degradation could become airborne and enter the food chain, with unknown effects. Oxo-biodegradable plastics are traditional plastics that incorporate additives which affect their chemical stability. Thus, they are identified and classified according to their chemical structure and finish together with the other plastic waste in the recycling streams. In this way, they bring their degration additives to the recyclate feedstock. As a consequence the recyclates may be destabilised, which will hinder acceptance and lead to reduced value. Studies have shown that these degraded plastics can accumulate toxic chemicals such as PCB, DDE and others from the environment and act as transport medium in marine environments. Such persistent organic pollutants in the marine environment were found to have negative effects on marine resources. The United Nations Environment Programme (UNEP) stresses that littering is a behavioural problem and must be resolved by raising environmental awareness and by the establishment of appropriate waste management systems. "Oxo-biodegradable" plastics are not specified as a solution by UNEP. Long standing efforts for the prevention of littering could actually be damaged by giving users of plastic items the impression that those items might vanish harmlessly if discarded in the environment.
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