Extrusion of a parison (hollow melt tube) which is forced to a mold caving by internal pressure and cooled down to form a hollow article. Used for the production of bottles, wide-mouth containers, petrol tanks etc.

 

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.

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 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.

 

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 (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.

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.

 

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.

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.

 

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.

 

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.

 

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 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.

 

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.

 

 

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.

 

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

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.

 

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°).

 

 

 

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).

 

Something relating to two ore more phases, i.e. not mixed so as to be homogeneous.

 

Something relating to only one phase, i.e. so well mixed that the mixture have the same composition throughout.

 

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.

 

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 (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 (HOMO) consist only of propylene units; their properties can be regulated via molar mass distribution and the degree of isotacticity (stereoregular structure).

 

 

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).

 

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 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.

Jam polypropylene company’s brand name for polypropylene homopolymer, random copolymer and heterophasic copolymer grades.

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.

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.

 

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.

 

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 forms a repeating unit in a polymer chain.

 

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.

 

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.

 

A catalyst which contains a metallocene compound of a transition metal. Often used as a synonym for single site.

 

Multimodal polymer includes more than one molecular weight fraction.

 

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.

Substances with the ability to act as substrates for crystal formation by epitaxy, thus increasing the nucleation density. Improve crystallisationbehaviour in processing, i.e., cycle time or line speed, as well as crystallinity and morphology in final product, improving optical and mechanical properties.

A relative measure of a level or a degree of stabilisation (amount of remaining active antioxidant).

 

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 (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.

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).

 

Material constant characterising gas penetration, like water vapour, oxygen or carbondioxide.

 

Polymerisation is a chemical reaction where polymer is synthesised from monomers.

 

Polypropylenes are thermoplastic polymers consisting mainly of propylene (C3H6), come frome a low pressure process based on catalysts and have generallya linear polymer chain.

 

A property of the catalyst which indicates how much the catalyst has produced polymer in the process (kg polymer/g catalyst).

 

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.

 

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.

 

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.

 

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.

 

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.

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.

 

the study of the change in form and the flow of matter, embracing elasticity, viscosity, and plasticity.

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)

 

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.

 

(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.

 

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.

 

Abehaviour where the viscosity decreases when the shear stress increases. Typically broadening of the molecular weight distribution increases shear thinning.

 

Formation of sheet-like agglomerates in the gas phase reactor.

 

• Produces polymer having very narrow MWD

• Extremely hydrogen and comonomer sensitive

• Very uniform comonomer distribution

• Usually cocatalyst required

• Produces stereospecific polymers

 

Decrease of dimension, created by the introduction of internal stress to the material during processing and cooling, or by ageing and post-crystallisation processes.

 

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.

 

 

A process for making polypropylene and propylene copolymers.Homopolymerization is conducted in the liquid phase in a loop tubular reactor; copolymerization is conducted in the gas phase in a fluidized-bed reactor. The catalyst is treated with a special silane.

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.

 

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.

Terpolymer

A terpolymer is a polymer composed of three different mer units.

 

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.

 

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 (TMA) is is a technique for monitoring changes of physical dimensions (expansion, schrinkage, penetration) while the temperature of the sample is changed.

 

To protect polyolefins for outdoor applications against light induced degradation. Hindered amine light stabilizers (HALS) are the most efficient.

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.

• Produces polymer having relatively narrow MWD

• Hydrogen is used to control MW

• Relatively comonomer sensitive

• Aluminum alkyl cocatalyst required