GENERAL
`PROPERTIES
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`ECHNICAL GUIDE
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`EPL LIMITED EX1013
`U.S. Patent No. 10,889,093
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`Front Cover
`Tensile testing reveals much about the mechanical properties of
`polyethylene products. Qenos manufactures and tests product
`samples in accordance with standards such as ASTM D638 and
`ASTM D882. Data collected includes tensile stress and tensile
`modulus, elongation at break as well as yield stress and strain.
`The resulting stress/strain curves are used by Qenos for product
`integrity, specifi cation development and product and process
`improvement purposes.
`
`Qenos, the Qenos brandmark, Alkathene, Alkatuff, Alkamax,
`Alkadyne and Alkatane are trade marks of Qenos Pty Ltd.
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`GENERAL
`GENERAL
`PROPERTIES
`PROPERTIES
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`TABLE OF CONTENTS
`WHAT IS POLYETHYLENE?
`Types of Polyethylene
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`MANUFACTURE OF THE VARIOUS TYPES OF POLYETHYLENE
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`STRUCTURAL DIFFERENCES BETWEEN THE POLYETHYLENES
`
`HISTORY OF POLYETHYLENE
`ICI Discovers Polyethylene
`Low Pressure High Density Polyethylene
`Linear Low Density Polyethylene
`Metallocene Linear Low Density Polyethylene
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`FORMS OF LDPE
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`FORMS OF mLLDPE AND LLDPE
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`FORMS OF MDPE AND HDPE
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`THE NATURE OF POLYETHYLENE
`Molecular Structure of Polyethylene
`Branching
`Molecular Weight and Molecular Weight Distribution
`Crystallinity
`Density
`Behaviour on Heating
`Behaviour on Cooling from the Molten State
`
`MELT FLOW (PROCESSING) PROPERTIES
`Melt Processing and Rheology
`Flow in Shear
`Melt Flow Index
`Melt Flow Index Ratio
`Extensional Flow
`Temperature Effects
`Pressure Effects
`Melt Elasticity and Memory
`Flow Defects
`Properties of Polyethylene
`
`LIMITATIONS OF TEST DATA
`Effect of Temperature
`Effect of Rate of Testing
`Effect of Strain
`Effect of Specimen Fabrication
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`MECHANICAL PROPERTIES
`Introduction
`Tensile Behaviour
`Creep in Tension
`Behaviour in Compression
`Flexural Behaviour
`Impact Properties
`Fracture Mechanics Analysis
`Environmental Stress Cracking Resistance (ESCR)
`Surface Hardness
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`THERMAL PROPERTIES
`Vicat Softening Point
`Thermal Conductivity
`Thermal Diffusivity
`Coefficient of Thermal Expansion
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`ELECTRICAL PROPERTIES
`Permittivity and Dielectric Loss
`Volume and Surface Resistivity
`Dielectric Strength
`
`CHEMICAL RESISTANCE
`Introduction
`Water Absorption
`Resistance to Oils
`Resistance to Solvents and Organic Chemicals
`Resistance to Inorganic Chemicals
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`OXIDATIVE STABILITY
`Oxidation of Polyethylene
`Stabilisation of Polyethylene
`Evaluation of Oxidative Stability
`Resistance to Weathering (Ultra-Violet Degradation)
`Use of Carbon Black
`Use of Ultra-Violet Stabilisers
`Typical Useful Life
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`FLAMMABILITY
`Combustion of Polyethylenes
`Flame Retardants
`Flammability Tests
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`GENERAL PROPERTIES 1
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`PERMEABILITY
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`APPROVAL FOR USE IN CONTACT WITH FOODSTUFFS
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`APPENDIX 1: MANUFACTURE OF POLYETHYLENE
`High Pressure Processes
`Low Pressure Processes
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`BIBLIOGRAPHY/FURTHER READING
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`GENERAL PROPERTIES 1
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`INTRODUCTION
`Polyethylene is a versatile thermoplastic polymer
`consisting of long hydrocarbon chains. It is chemically
`synthesised from ethylene, a compound that is usually
`derived from petroleum or natural gas. Polyethylene
`polymers can be broadly categorised into branched and
`linear polyethylenes, however, categorisation is more
`commonly based on polymer density. The physical
`properties of the polymer depend significantly on
`variables such as the extent and type of branching,
`the crystal structure and the molecular weight. LDPE,
`LLDPE, mLLDPE and HDPE grades are the most common
`types of polyethylene.
`
`Disclaimer
`
`All information contained in this publication and any further information, advice, recommendation or assistance given by Qenos either orally or
`in writing in relation to the contents of this publication is given in good faith and is believed by Qenos to be as accurate and up-to-date as possible.
`
`The information is offered solely for your information and is not all-inclusive. The user should conduct its own investigations and satisfy itself as to
`whether the information is relevant to the user’s requirements. The user should not rely upon the information in any way. The information shall not
`be construed as representations of any outcome. Qenos expressly disclaims liability for any loss, damage, or injury (including any loss arising out of
`negligence) directly or indirectly suffered or incurred as a result of or related to anyone using or relying on any of the information, except to the extent
`Qenos is unable to exclude such liability under any relevant legislation.
`
`Freedom from patent rights must not be assumed.
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`• Medium Density Polyethylene (MDPE) refers to polymer
`with a density between 0.930 and 0.940 g/cm3.
`• High Density Polyethylene (HDPE) refers to polymer with
`a density between 0.940 and 0.970 g/cm3.
`
`MANUFACTURE OF THE VARIOUS TYPES
`OF POLYETHYLENE
`The various types of polyethylene are made by different
`processes. These processes are described in Appendix 1,
`and are summarised below.
`
`LDPE is produced by high pressure free-radical
`polymerisation of ethylene, with pressures up to 200 MPa
`and temperatures up to 300°C. The polymer is highly
`branched, with both short and long chain branches. This
`process is used by Qenos to produce the Alkathene range of
`LDPE. Such LDPE is sometimes called branched
`polyethylene or high pressure polyethylene.
`
`HDPE is produced at low pressures by slurry, or
`gas-phase processes using Ziegler-Natta transition
`metal catalysts. These processes are used by Qenos
`to produce the Alkatane and Alkadyne range of HDPE.
`The polymer is basically linear, with little or no branching,
`depending on whether comonomer was used during
`the polymerisation process.
`
`mLLDPE is also produced by the low pressure
`polymerisation technology using metallocene catalyst
`to copolymerise ethylene and another monomer such
`as butene-1, or hexene-1. This process is used by Qenos
`to produce the Alkamax range of mLLDPE. The metallocene
`catalyst produces resins with very consistent and
`specific properties such as superior toughness and
`stiffness balance.
`
`MDPE and LLDPE products are produced by the low
`pressure polymerisation technology using transition-metal
`catalysts, however, comonomers are introduced into the
`reaction to create small short chain branches on the linear
`molecule, with the effect of reducing the density. This
`process is used by Qenos to produce the Alkatuff range of
`LLDPE. Qenos produces these products in Australia using
`the gas-phase polymerisation process, with hexene-1 as
`the comonomer.
`
`WHAT IS POLYETHYLENE?
`Polyethylene or polythene, as it is also known, is a polymer
`produced by the polymerisation of ethylene gas, a derivative
`of the petroleum industry. The polymer consists essentially
`of long-chain molecules of very high molecular weight,
`made up of many thousands of the -CH2- repeating unit
`(see Figure 1).
`
`Figure 1: Polymerisation of Ethylene to Polyethylene
`
`Ethylene can be polymerised to produce polymers of any
`desired molecular weight, from oils, greases and soft
`waxes at low molecular weights to tough flexible polymers,
`the polyethylenes, at high molecular weights.
`
`Types of Polyethylene
`Depending on the polymerisation process used to produce
`the polyethylene (see later), the polymer can be a linear
`molecule or it can be highly branched. The degree of
`branching affects how the molecules pack together, i.e. the
`density of the polymer. Polyethylenes can range in density
`from about 0.900 g/cm3 to 0.970 g/cm3.
`
`There are several basic types of polyethylene, classified by
`means of the density of the polymer:
`
`• Low Density Polyethylene (LDPE) refers to polymer with
`a density between 0.915 and 0.930 g/cm3.
`• Linear Low Density Polyethylene (LLDPE) refers to
`low density type polymer with a density between about
`0.915 and 0.940 g/cm3, made via an HDPE type
`manufacturing process.
`• Metallocene Linear Low Density Polyethylene (mLLDPE)
`refers to tougher LLDPE type polymer with a density
`between about 0.915 and 0.940 g/cm3, made using
`metallocene catalysts.
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`STRUCTURAL DIFFERENCES BETWEEN
`THE POLYETHYLENES
`The variations in chemistry between the processes
`produce important differences in the structure, and
`consequently the final properties, of the various
`polyethylenes. The different branching structures for
`LDPE, LLDPE and HDPE are illustrated in Figure 2.
`
`Figure 2: Schematic Representation of Different
`Branching Structures in Polyethylene
`
`LDPE made by the high pressure route is characterised
`by a significant level of long chain branching (typical branch
`length of several hundred carbon atoms, long chain
`branching frequency up to 5 per 1,000 carbon atoms), as
`well as short chain branching (2 to 6 carbon atoms long).
`
`HDPE is essentially a linear molecule with a very low level
`of short chain branching.
`
`LLDPE is again a linear molecule with a higher level of
`short chain branching than HDPE (specifically introduced
`by polymerisation with comonomer), but without the long
`chain branches which characterise LDPE.
`
`MDPE and mLLDPE resemble LLDPE in structure.
`
`The specific effects of these differences in branching on
`the crystallinity, rheology and mechanical properties of the
`polymers will be highlighted in the following text.
`
`HISTORY OF POLYETHYLENE
`ICI Discovers Polyethylene
`Polyethylene was first discovered in 1933 at ICI’s research
`laboratory at Winnington in England, as a result of
`experiments on ethylene gas at very high pressures. By
`chance, the scientists, Fawcett and Gibson, found a white
`waxy powder in the reactor and this proved to be a polymer
`of ethylene, today known as low density polythene or LDPE.
`
`By the late 1930s, developments had progressed and in
`1938 a large potential market was recognized to replace
`gutta-percha for submarine cable insulation, because of
`the polymer’s remarkable electrical properties. As a result
`of initial successful trials, a full-scale polyethylene plant
`with a capacity of 100 tons came on stream in 1939.
`
`Almost the entire production of polyethylene during
`World War II was used for the insulation of high frequency
`radar cables. Union Carbide Corporation and Du Pont
`commenced full-scale production in the United States in
`1943. By 1955 there were 13 producers worldwide and total
`output was about 200,000 tons; development accelerated
`as processing by film extrusion and injection moulding was
`established and new applications were found.
`
`Low Pressure High Density Polyethylene
`In 1953, it was discovered that ethylene could be
`polymerised at low pressures to form a linear polymer with
`high crystallinity and density. This new polyethylene was
`high density polyethylene (HDPE) and it resulted from the
`work of Ziegler on the low pressure polymerisation of
`ethylene using organo-metallic catalysts. This process
`was commercialised by Hoechst A G in Germany in 1955.
`Other processes using different catalysts were developed
`by Phillips Petroleum Co. and Standard Oil of Indiana.
`
`Since the early work of Ziegler, many improvements in
`catalyst types and efficiencies have occurred. In 1972,
`Union Carbide Corporation introduced their fluidised-bed
`gas-phase process for producing HDPE, whereas the earlier
`processes were carried out in solution or slurry.
`
`Linear Low Density Polyethylene
`The challenge for the scientists was to make LDPE by the
`low pressure organo-metallic catalyst route. In the early
`1960s, Du Pont of Canada successfully developed their
`“Sclair” range of polyethylenes made by a low pressure
`solution process, but they only marketed the resins in
`specialty market areas.
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`In 1977, Union Carbide Corporation announced that they
`could produce a low density polyethylene by their low
`pressure gas-phase process, with considerable cost savings
`over the original high pressure process. This polymer was a
`copolymer of ethylene with a small amount of propylene and
`had a linear structure with greater toughness than traditional
`low density polyethylene; it has since been called linear low
`density polyethylene (LLDPE). Later Dow Chemical introduced
`their “Dowlex” LLDPE, which was produced by a different
`(solution) process using octene-1 as the comonomer.
`
`Subsequently, other comonomers such as butene-1,
`hexene-1, octene-1, and 4-methyl-pentene-1 have been
`used in place of propylene by various manufacturers using
`several different process technologies to produce a wide
`range of different LLDPEs. Ultra low density polyethylene
`and very low density polyethylene with densities at the
`bottom end of the spectrum have been introduced in
`recent years.
`
`Each process and each comonomer leads to slightly
`different polymer structures. For example, Figure 3
`shows how the toughness of LLDPE improves as the
`comonomer type is changed (and hence the short chain
`branch length increased).
`
`Metallocene Linear Low Density Polyethylene
`In the early 1990s, Exxon Chemical Company, now
`ExxonMobil Chemical Company, developed revolutionary
`single-site metallocene catalysts for use in these
`UNIPOL™ gas-phase PE Process. Metallocene catalysts
`have enabled better control over the structure of the
`polyethylene molecule to achieve tailored performance
`and improved properties.
`
`Figure 3: Effect of Comonomer Type and Short Chain
`Branch Length on Toughness Properties of LLDPE
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`FORMS OF LDPE
`Alkathene LDPE is supplied in a number of different
`forms. All grades are available as pellets, either as virgin
`polymer (without any additives) or as formulated polymer
`(containing an antioxidant and, for some film grades, slip,
`antiblock or possibly antistatic additives).
`
`The range of grades available for Alkathene can cover
`densities from 0.915 to 0.928 g/cm3 and melt flow indices
`from 0.2 to 70 g/10 min.
`
`• Note for convenience, the term Melt Flow Index or MFI
`defined the melt flow of polymer extrudate in g/10 min
`when subjected to a load of 2.16 kg – otherwise referred
`to as MI2.
`
`FORMS OF mLLDPE AND LLDPE
`mLLDPE and LLDPE are supplied in pellet form, containing
`a basic stabiliser package for protection during processing
`and end-use. Film grades may be formulated with slip,
`antiblock, and process aid additives, as well as a basic
`stabiliser package.
`
`The range of grades available for Alkamax mLLDPE
`and Alkatuff LLDPE can cover densities from 0.915 up to
`0.938 g/cm3 and melt flow indices from 1.0 to 20 g/10 min.
`
`FORMS OF MDPE AND HDPE
`MDPE and HDPE are supplied in pellet form, containing a
`basic stabiliser package for protection during processing
`and end-use. Some grades are formulated with additional
`additives that perform unique functions such as UV
`protection. Alkadyne HDF193B is supplied as a black
`compound formulated with a stabiliser package and carbon
`black for UV protection.
`
`The range of grades available for MDPE and HDPE can cover
`densities from 0.940 to 0.965 g/cm3 and melt flow indices
`from 0.06 to 10 g/10 min.
`
`THE NATURE OF POLYETHYLENE
`Molecular Structure of Polyethylene
`The properties of a given polyethylene depend primarily
`on four factors:
`a. Its molecular weight (or average length of molecular
`chains),
`b. Its molecular weight distribution (MWD) (or the
`distribution of different chain lengths),
`c. Its degree of long chain branching, and
`d. Its degree of short chain branching (i.e. the number,
`length and distribution of the short branches)
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`All of these factors can be controlled during the
`polymerisation process.
`
`The polymer molecular weight determines the mechanical
`properties and the melt flow behaviour of the polyethylene.
`The molecular weight distribution and degree of long chain
`branching also affect the melt flow properties such as
`shear thinning behaviour and melt elasticity. The degree
`of short chain branching determines properties such as
`crystallinity, density and stiffness. Some properties
`depend on all four factors.
`
`As it is difficult to measure these basic parameters
`directly, it is convenient to use Melt Flow Index (MFI) to
`represent the number average molecular weight, Density
`to represent the degree of short chain branching and Melt
`Flow Index Ratio (MFR) to represent the molecular weight
`distribution (see below for explanation of MFI & MFR). The
`product is made to a specified MFI and density and these
`are correlated with the other properties. The MFI and
`density can be varied independently by the appropriate
`choice of process conditions.
`
`Branching
`LDPE consists essentially of long chain molecules, with a
`-CH2- repeating unit, a number average molecular weight
`between 10,000 and 50,000, and branch points at intervals
`of every 25 to 100 carbon atoms in the chain (10 to 40
`methyl groups per 1,000 carbon atoms). Most of the
`branches are short, being mainly ethyl (C2), butyl (C4) or
`pentyl (C5). The frequency of each group depends on the
`polymerisation conditions. Roughly 2 to 10% of the
`branches (1 to 5 per 1,000 carbon atoms) are long chains
`containing several hundred carbon atoms and each of
`these long chains has both short and long chain branches
`at the same frequency as the main chain.
`
`The branches of a polyethylene molecule are terminated by
`either methyl groups or olefinic unsaturation. Since the
`latter is present in a very small proportion, the methyl group
`content will largely determine the total number of branches.
`
`The methyl content and the amount and type of
`unsaturation can be measured by infra-red spectroscopy
`and a typical infra-red scan of LDPE is given in Figure 4. The
`unsaturated olefinic groups consist of pendent vinylidene,
`pendent vinyl and vinylene, with the first predominating; the
`total unsaturation ranges between 0.4 and 2 double bonds
`per molecule.
`
`Figure 4: Typical Infra-red Spectrum for Low Density
`Polyethylene
`
`In linear polyethylenes (mLLDPE, LLDPE, MDPE and some
`HDPE grades), the branching in the molecule is achieved
`through copolymerisation with comonomers such as
`butene-1, hexene-1 or octene-1. These comonomers
`respectively give ethyl (C2), butyl (C4) and hexyl (C6)
`branches. The degree of branching increases as the
`proportion of comonomer in the polymer is increased.
`The branching structure will hence depend on the type
`and proportion of comonomer, and also the distribution of
`comonomer along the molecule (i.e. whether the branches
`are evenly distributed or clumped together) and between
`molecules. This means that the linear polyethylene grades
`supplied by different manufacturers can have noticeably
`different properties. Figure 3 demonstrates how the
`toughness of different LLDPEs is highly dependent on
`the short chain branch length, i.e. the comonomer used.
`
`In Alkatuff LLDPE, which is a hexene-1 copolymer, the
`branch points occur every 50 to 100 carbon atoms (10 to
`20 methyl groups per 1,000 carbon atoms). As indicated
`earlier, there is no long chain branching.
`
`Molecular Weight and Molecular
`Weight Distribution
`Polyethylene consists of a mixture of molecules with a
`distribution of different molecular weights. The distribution
`of these molecular weights can be measured by means
`of Gel Permeation Chromatography (GPC) of dilute
`polyethylene solutions at elevated temperatures, and is
`characterised by the number and weight average molecular
`weights (Mn and Mw, respectively); the ratio Mw/Mn is a
`measure of the width of the molecular weight distribution
`(see Figure 5).
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`Crystallinity
`The similarity in structure of the individual polyethylene
`molecules allows close packing of parts of the chain, giving
`a regular, ordered, three-dimensional network. However,
`because of the effects of chain entanglement and
`branching, restrictions are placed on the degree of ordering
`and this interferes with crystallisation. Polyethylenes are
`thus ‘semi-crystalline’ polymers, having a proportion of
`ordered, crystalline regions (often called crystallites) and
`also non-ordered ‘amorphous’ regions between these
`crystallites. LDPE will have smaller crystallites than LLDPE
`because of the different short chain branching structures.
`HDPE products can have even larger crystallites depending
`on the amount of comonomer that was used in the
`polymerisation process.
`
`Polyethylene crystallises by a folded-chain mechanism
`to form a lamella-type structure similar to that depicted
`in Figure 6. Each lamella layer is about 100 carbon atoms
`thick. The branched parts of the molecule are excluded
`from the lamellae, either as folded loops or as ‘tie-
`molecules’ participating in adjacent lamellae. The longer
`side chains in hexene-based LLDPE compared with
`butene-based LLDPEs of the same density are believed to
`increase the number of inter-lamellar tie molecules, giving
`a reinforcing effect and resulting in a tougher product.
`This beneficial effect of increasing the short-chain branch
`length is shown in Figure 3.
`
`Figure 6: Schematic Representation of Polyethylene
`Lamella Structure showing Folded Chains and
`Tie Molecules
`
`Figure 5: Typical Molecular Weight Distributions for
`Polyethylene, Showing Narrow (MFI 2) and Broad (MFI 6)
`Distributions
`
`Mn = Number average molecular weight = Σ(Mi Ni)/ΣNi
`Mw = Weight average molecular weight = Σ(Mi Wi)/ΣWi
`
`where Ni is the number of polymer molecules of molecular
`weight Mi,and Wi is the weight of these molecules = Mi Ni.
`
`The average molecular weight and the molecular weight
`distribution of polyethylene are governed by the
`polymerisation conditions. For LDPE, the reactor design
`also has an important effect and polymers made in a
`stirred vessel will have a different molecular structure
`to those made in a tubular reactor.
`
`By varying the polymerisation conditions, it is possible
`to make products covering a wide range of molecular
`weights, molecular weight distributions and branching
`contents. Figure 5 illustrates the extremes of wide and
`narrow molecular weight distributions. For LDPE, polymers
`with a wide molecular weight distribution in general also
`have a high degree of long chain branching.
`
`With linear polyethylenes, molecular weight distribution
`is basically controlled by the choice of the catalyst type.
`
`LDPE grades generally have broad molecular weight
`distributions and the linear polyethylenes typically have
`narrower molecular weight distribution.
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`The lamellar crystallites grow in an approximately radial
`manner to form relatively large three-dimensionally ordered
`aggregates, known as spherulites. These spherulites
`can markedly affect the optical properties of a sheet of
`polyethylene as they act as light-scattering centres. They
`may be seen as a pattern of dark Maltese crosses when
`a thin sheet of polyethylene is viewed in a polarising
`microscope. Their size (which can be up to 100 microns in
`diameter) and distribution may also have an effect on the
`physical properties of the sample.
`
`Typically, the higher the density of the polymer the higher
`the degree of crystallinity and the stiffer the solid polymer
`is. The overall size of the spherulites in the polymer
`crystals basically depends on the rate of cooling and
`branch length. When crystallisation occurs under stress,
`the lamellar and spherulitic structure is modified.
`
`The proportion of crystalline material developed in any
`polyethylene sample is dependent firstly upon the
`structural features of the molecules (molecular weight,
`degree of branching, etc.), and also upon the conditions
`under which it is crystallised and any subsequent thermal
`treatment. Hence linear polyethylene will normally have
`a higher crystalline content than branched polyethylene
`crystallised under the same conditions, while fast cooling
`from the melt will normally yield lower crystallinities than
`slower cooling.
`
`LDPE has crystallinity values in the range 40% to 65%,
`depending on the degree of branching and thermal history
`LLDPE may have similar crystallinities to LDPE. On the other
`hand, HDPE has much higher crystallinity, viz. 60 to 85%.
`
`Density
`There is a direct relationship between the density of the
`polyethylene and its crystallinity, as has been intimated
`in the previous discussion. As density is easier and more
`convenient to measure, it is usual to quote the density of
`a polyethylene rather than its crystallinity. The density of
`polyethylene can be measured by means of the density
`gradient method (ASTM D1505) or by the titration technique.
`
`Density figures for the LDPE polymers range from
`0.915 g/cm3 to 0.928 g/cm3 at 23°C. Density figures
`for the LLDPE polymers range from 0.915 g/cm3 to
`0.938 g/cm3 at 23°C. Density figures for the HDPE
`polymers typically range from 0.940 g/cm3 to
`0.970 g/cm3 at 23°C.
`
`Behaviour on Heating
`There is normally a wide variation in the size and perfection
`of the crystallites which form in a sample of polyethylene.
`During heating this results in a wide melting distribution
`which can commence at quite a low temperature. Examples
`of such distributions are shown by the Differential
`Scanning Calorimeter (DSC) heating scans in Figure 7.
`These scans show that while the melting point quoted for a
`polymer is generally the peak temperature, a considerable
`proportion of crystalline polymer has already been melted
`at much lower temperatures.
`
`Figure 7: DSC Heating Curves for Alkathene LDPE and
`Alkatuff LLDPE of Density 0.922 g/cm3 (Heating Rate
`20°C/min)
`
`Typical values of the peak melting temperature and
`enthalpy for different types of polyethylene are shown in
`Table 1 below:
`
`Table 1: Polyethylene – Peak Melting, Crystallisation
`Temperatures and Enthalpy of Melting
`
`Resin
`Type
`LDPE
`LLDPE
`HDPE
`
`Peak melting
`Temperature
`°C
`112
`126
`134
`
`Enthalpy of
`Melting
`(J/g)
`103
`160
`203
`
`Major
`Crystallisation
`Temperature
`°C
`95
`107
`118
`
`The perfection of the crystallites and the overall crystallinity
`are mainly influenced by the degree and distribution of
`branching in the molecule. The temperature at which the
`crystallites melt depends primarily on this branching. Thus a
`highly crystalline polymer, with a low degree of branching,
`will melt within a higher temperature range.
`
`11
`
`GENERAL PROPERTIES 1
`
`Qenos Technical Guides
`
`0013
`
`

`

`Alkathene LDPE polymers have peak melting temperatures
`between 100°C and 118°C. Alkatane High density
`polyethylenes, for comparison, have peak melting
`temperatures typically in the range of 130°C to 138°C.
`Alkatuff LLDPE grades have major melting peaks between
`125°C to 130°C. In some LLDPEs there may be a shoulder
`or a secondary endotherm at a lower temperature possibly
`down to about 105°C; these peaks represent the linear
`and branched components in the molecule. The
`relationship between the peak melting temperature as
`measured by DSC and the polymer density is shown in
`Figure 8 for lower density polymers.
`
`Figure 8: Melting Temperature vs. Density for lower
`density polyethylenes (Peak Melting Temperature
`Measured by Differential Scanning Calorimetry)
`
`Above the melting temperature, the polymer is an
`amorphous mass which behaves as both a viscous and
`an elastic fluid.
`
`Behaviour on Cooling from the Molten State
`As the amorphous molten polyethylene is cooled, it begins
`to crystallise about nuclei in the polymer. Typical cooling
`curves are shown in Figure 9. As for the melting stage,
`crystallisation occurs over a range of temperatures. LLDPE
`will crystallise and solidify at higher temperatures than
`LDPE, and higher density linear polyethylene will typically
`also crystallise at higher temperatures than lower density
`linear polyethylene grades. The level to which the polymer
`is able to crystallise depends primarily on the degree of
`branching and the rate of cooling, but is also influenced by
`the molecular weight.
`
`12
`
`Figure 9: DSC Cooling Curves for Polyethylene of Density
`0.922 g/cm3 (Cooling Rate 20°C/min)
`
`Often it is advantageous, for example in injection moulding
`when cycle times can be reduced, to include nucleating
`agents in the polyethylene so that the crystallisation point
`is raised; such agents can be particularly effective in LLDPE.
`
`The crystal structure achieved during the cooling is
`significantly affected by the rate of cooling – fast cooling
`(quenching) does not allow the spherulites to grow as much
`and a less crystalline lower density structure results. Slow
`cooling allows the crystallites to organise themselves into
`large spherulites, resulting in higher crystallinity and
`density. Some typical figures for LDPE are given in Table 2.
`These differences in crystalline structure have important
`effects on both optical and mechanical properties;
`increased crystallinity will increase haze and tensile
`strength, coupled with a reduction in impact properties.
`
`Table 2: Variation of the Density of Polyethylene with
`Cooling (Production) Conditions
`
`Cooling Condition
`Pellets before Processing
`Cooled at 50°C/hr
`Blown Film (moderate cooling rate)
`Cast Film (rapid cooling)
`
`Density (g/cm3)
`0.920
`0.925
`0.919
`0.916
`
`1 GENERAL PROPERTIES
`
`Qenos Technical Guides
`
`0014
`
`

`

`Table 3: Variation of Density with Time at 25°C for MFI 2
`Polyethylene Quenched from 190°C in Cold Water
`
`Time after Quenching
`Hours
`Days
`1.5
`6.5
`1
`25
`6
`145
`17
`410
`133
`3190
`Slow cooled from 165°C
`
`Density
`(g/cm3)
`0.9173
`0.9177
`0.9183
`0.9189
`0.9202
`0.9198
`0.9220
`
`Polyethylene which has been cooled rapidly from the melt
`will undergo a gradual increase in crystallinity at room
`temperature and hence an increase in density. The data
`in Table 3 illustrates this effect, the density increasing by
`0.03 g/cm3 during the first two weeks and then remaining
`effectively constant.
`
`MELT FLOW (PROCESSING) PROPERTIES
`Melt Processing and Rheology
`One of the most important characteristics of a
`thermoplastic is its ability to be processed as a melt
`into the final shaped product, generally via extrusion or
`moulding. Therefore it is essential that the polymer has
`easy melt processing properties. An understanding of the
`melt flow behaviour of the polymer is advantageous in
`achieving this characteristic. The general term relating to
`melt flow is rheology.
`
`The three major variables in polyethylene melt processing
`are time (i.e. output rate, line speed, etc.), temperature and
`pressure; none of these variables is really independent of
`the others.
`
`Polymer melts will flow when subjected to forces (or
`stresses) which can act in both shear and extension. This
`flow behaviour is markedly non-Newtonian, since the flow
`or viscosity depends on the flow rate or extension to which
`the melt is subjected. Polymer melts are also elastically
`deformed by these stresses and some of this deformation
`is recovered when the stress is removed. Polymers are thus
`visco-elastic materials – they have both viscous flow and
`elastic properties in the melt. These characteristics arise
`because of the very long molecular chains and the very high
`intermolecular forces.
`
`The rheological properties of a polymer melt are basically
`expressed by the melt viscosity (in both shear and
`extension) and the melt elasticity. These properties are
`usually measured as functions of shear rate (i.e. the flow
`rate or throughput) and the temperature of flow. They
`can also be influenced by the pressure, the degree of
`shear working, and degradation or oxidation experienced
`by the polymer.
`
`Because of the basic differences in molecular structure
`between highly branched LDPE and linear polyethylenes,
`in particular in molecular weight distribution and long chain
`branching, there are significant variations in the rheological
`performance between the two. The following discussion
`outlines these rheological differences.
`
`Flow in Shear
`In the melt, the polymer molecules can be considered
`as entangled random coils. The entanglements are
`an impediment to the flow of the polymer. These
`entanglements become greater as the molecular weight
`and the degree of long chain branching increase. The
`molecular weight is the