6 Distortion, Voids and Frozen-in Stress

Distortion, Voids and Frozen-in Stress



203



A



EDGE-GATED MOULDING



CENTRE-GATED MOULDING



Figure 9.12. Moulding shrinkage along flow lines is greater than on arc perpendicular to flow line,

and this will cause warping or bowing of moulding, The fault will be minimised by the use of a single

gate along one edge, by the use of several gates along one edge or by designing a slight dome on the

moulding if this is acceptable to the customer. (From IC1 Technical Service Note (3117, reproduced

by permission of IC1 Plastics Division)



points (gates) to the mould cavity in order that there be an appropriate symmetry

and balance of flow times and molecular orientation to avoid distortion (Figure

9.12).

Resistance to distortion may also be increased by enhancing the stiffness of the

product either by introduction of doubly curved surfaces or by the use of ribs

(Figure 9.13fa)). In turn this can lead to other problems since the cooling of the

melt in the rib section is accompanied by a contraction that pulls in material from

the original flat surface, causing sink marks at A. This may be obviated by the use

of narrower deeper ribs, or by building a step into the design of the moulding at

that point (Figure 9.13(h)).

A



A

r



(a1



(b)



Figure 9.13



Distortion in mouldings can be worse in crystalline polymers than in

amorphous plastics. This is because additional stresses may be set up as a result

of varying crystallinity from point to point in the moulding so that the shrinkage

on cooling from the melt also varies from point to point. This uneven shrinkage

sets up stresses which may lead to distortion.

Another common problem on moulding is voids. These occur when a

moulding or extrudate cools and hardens rapidly on the surface. On further

cooling the moulding or extrudate cannot shrink inward because the outside

dimensions are fixed and therefore the molecules are pulled towards the surface

of the product, generating internal voids. At the same time molecules in the inner



204



Principles of Product Design



layers tend to be oriented at right angles to the surface in contradistinction to

molecules adjacent to the surface which as a result of the original shaping process

have tended to be oriented parallel to the surface.



9.7 CONCLUSIONS

This chapter has attempted to describe briefly some properties of polymers

relevant to product design in terms of molecular behaviour. For in depth

consideration the reader should consult more detailed reviews (e.g. refs 4, 5 and

6). There also exist specialist monographs concerned with practical aspects of

product design (e.g. refs 7 and 11). Mention should also be made of excellent

booklets by materials suppliers (e.g. refs 2 and 8) concerned with design aspects.

Some material manufactures now supply comprehensive data books backed by

computer data bases for multi-point engineering data (e.g. ref. 12).



References

1.



Impact Tests and Service Performance of Plastics, Plastics Institute, London

(1971)

2. I.C.I. Plastics Division, Technical Science Note G 117 (1970)

3. BRYDSON, I. A., Flow Properties of Polymer Melts (2nd Edn), Geo. Godwin, London (1981)

4. NIELSEN, L. E., Mechanical Properties of Polymers, Reinhold, New York (1962)

5. JENKINS, A. D. (Ed.), Polymer Science Vol. 1 (particularly Chapters 10 and 1 I), North-Holland,

Amsterdam (1972)

6. TURNER, s., Mechanical Testing of Plastics, Iliffe, London (1973)

7. BECK, R. D., Plastic Product Design, Van Nostrand-Reinhold, New York (1970)

8. General Electric Company (U.S.A.),Lexan Polycarbonate-Design

9. WILLIAMS, J. G . , Stress Analysis of Polymers, Longman, London (1973)

10. WHELAN, A. and CRAFT, J. L., British Plasrics and Rubber, p. 29 (Nov. 1982)

11. MORTON-JONES, D. H. and ELLIS, J. w., Polymer Products-Design, Materials and Processing,

Chapman and Hall, London (1986)

12. GENERAL ELECTRIC COMPANY (USA), Materials Profile, in conjunction with the computer-based

Engineering Design Database (1992)

VINCENT, P I . ,

.



Bibliography

Plastic Product Design, Van Nostrand-Reinhold, New York (1970)

A Practical Guide to the Selection of High Temperature Engineering Thermoplastics,

Elsevier Advanced Technology, London (1990)

LUCKETT, F. I., Engineering Design Basis for Plastics Products, HMSO, London (1981)

MACDERMOTT, P., Selecting Thermoplastics for Engineering Applications, Marcel Dekker, New

c.

York and Basel (1984)

MARGOLIS, I. M. (Ed.), Engineering Thermoplastics, Marcel Dekker, New York and Basel (1985)

MORTON-JONES, H. and ELLIS,J. w., Polymer Products-Design, Materials and Processing,

D.

Chapman and Hall, London (1986)

POWELL, P c., Engineering with Polymers, Chapman and Hall, London (1983)

.

BECK, R. D.,



COLLYER, A. A,,



10

Polyethylene



10.1 INTRODUCTION

There are at least three reasons why it is appropriate that the first chapter

reviewing individual plastics materials should be concerned with polyethylene. It

has the simplest basic structure of any polymer, it is the largest tonnage plastics

material, and it is a polymer about which more has probably been written than

any other. The main attractive features of polyethylene, in addition to its low

price, are excellent electrical insulation properties over a wide range of

frequencies, very good chemical resistance, good processability, toughness,

flexibility and, in thin films of certain grades, transparency.

It may come as a surprise to the reader that this material which has basically

the simplest molecular structure of all polymers should not have an agreed

name. In most (English language) scientific publications it is known as

polyethylene-an indication that it is a polymer of ethylene. Although it is a

specially allowed description, the word ethylene does not accord with the

terminology for alkenes (olefins) adopted by the International Union of Pure

and Applied Chemistry and which would indicate the word ethene. For this

reason the word polyethene is sometimes used, rather more in chemistry text

books than in industry. More recently the IUPAC Commission on Macromolecular Nomenclature have recommended that the nomenclature of singlestrand organic polymers should be based on the constitutional repeating unit in

the structure rather than on the monomer used, and in an Appendix to their

rules they recommend the term poly(methy1ene). As will be seen this has

already by common usage come to mean a rather specific polymer obtained

from diazomethane. Finally, it is probably still true that many people in the

United Kingdom still refer to the material as polythene, the name promoted by

the first manufacturers, ICI. For consistency the most widely used term,

polyethylene, will be used in this text.

Although polyethylene is virtually defined by its very name as a polymer of

ethylene produced by addition polymerisation, linear polymers with the formula

(CH,), have also been prepared by condensation reactions. For example in 1898

von Pechmann' produced a white substance from an ethereal solution of



205



206 Polyethylene

diazomethane on standing. In 1900 Bamberger and Tschirne? analysed a similar

product, found it to have the formula (CH,), and termed it ‘polymethylene’. The

reaction can be considered to be fundamentally



Since 1900 other methods have been devised for producing ‘polymethylene’,

including the use of boron trifluoride-diethyl ether catalysts at 0°C. Some of

these methods give unbranched linear polymers, often of very high molecular

weight, which are useful for comparing commercial polyethylenes which have

molecules that are branched to varying extents.

Another condensation method was investigated by Carothers and co-workers3

and reported in 1930. They reacted decamethylene dibromide with sodium in a

Wurtz-type reaction but found it difficult to obtain polymers with molecular

weights above 1300.

n Br(CH2),,BR



+ 2n Na -+ ~ ( C H 2 ) l o+, 2n NaBr

~



Other routes have also been devised which are sometimes useful for research

purposes and include:



( I ) Modified Fischer-Tropsch reduction of carbon monoxide with h y d r ~ g e n . ~

(2) Reduction of poly(viny1 chloride) with lithium aluminium hydride.

(3) Hydrogenation of polybutadiene.

Commercially, polyethylene is produced from ethylene, the polymer being

produced by this route in March 1933 and reported verbally by Fawcett in 1935.5

The basic patent relating to the polymerisation of ethylene was applied for by

IC16 on 4th February 1936 and accepted on 6th September 1937.

Until the mid-1950s all commercial polyethylene was produced by highpressure processes developed from those described in the basic patent. These

materials were somewhat branched materials and of moderate number average

molecular weight, generally less than 50 000. However, about 1954 two other

routes were developed, one using metal oxide catalysts (e.g. the Phillips process)

and the other aluminium alkyl or similar materials (the Ziegler process).’ By

these processes polymers could be prepared at lower temperature and pressures

and with a modified structure. Because of these modifications these polymers

had a higher density, were harder and had high softening points. These materials

are known as high-density polyethylenes (HDPE), while the earlier materials are

known as low-density polyethylenes (LDPE).

At the end of the 1970s considerable interest developed in what became known

as linear low density polyethylenes (LLDPE) which are intermediate in properties

and structure to the high pressure and low pressure materials. While strictly

speaking these are copolymers it is most convenient to consider them alongside the

homopolymers. The LLDPE materials were rapidly accepted by industry

particularly in the manufacture of film. The very low density polyethylenes

(VLDPE) introduced by Union Carbide in 1985 were closely related.

During the 1990s there was enormous activity in the development of a further

type of polyethylene based on metallocene catalysis methods. One patent search



Preparation of Monomer 207

revealed that over 950 patent applications had been filed on the subject by the

summer of 1996 and has since shown no signs of abating. Commercial

production commenced in the late 1990s and it is estimated that in 2000

metallocene-catalysed polyethylenes will comprise about 2% of the total

polyethylene market. This is somewhat less spectacular than achieved by LLDPE

and reflects the fact that although these materials may have many superior

properties in the finished product they are more expensive than the traditional

materials and in some respects more difficult to process. Whereas the

metallocene polymers can be of LDPE, LLDPE and HDPE types it is anticipated

that LLDPE types (referred to as mLLDPE) will take over 50% of the market;

mainly for film application.

By the mid- 1990s capacity for polyethylene production was about

50 000 000 t.p.a, much greater than for any other type of plastics material. Of this

capacity about 40% was for HDPE, 36% for LDPE and about 24% for LLDPE.

Since then considerable extra capacity has been or is in the course of being built

but at the time of writing financial and economic problems around the world

make an accurate assessment of effective capacity both difficult and academic. It

is, however, apparent that the capacity data above is not reflected in consumption

of the three main types of material where usage of LLDPE is now of the same

order as the other two materials. Some 75% of the HDPE and LLDPE produced

is used for film applications and about 60% of HDPE for injection and blow

moulding.

Polymers of low molecular weight and of very high molecular weight are also

available but since they are somewhat atypical in their behaviour they will be

considered separately.



10.2 PREPARATION OF MONOMER

At one time ethylene for polymerisation was obtained largely from molasses,

a by-product of the sugar industry. From molasses may be obtained ethyl

alcohol and this may be dehydrated to yield ethylene. Today the bulk of

ethylene is obtained from petroleum sources. When supplies of natural or

petroleum gas are available the monomer is produced in high yield by hightemperature cracking of ethane and propane. Good yields of ethylene may also

be obtained if the gasoline (‘petrol’) fraction from primary distillation of oil

is ‘cracked’. The gaseous products of the reaction include a number of lower

alkanes and olefins and the mixture may be separated by low-temperature

fractional distillation and by selective absorption. Olefins, in lower yield, are

also obtained by cracking gas oil. At normal pressures (760mmHg) ethylene

is a gas boiling at -103.71”C and it has a very high heat of polymerisation

(3350-41 85 J/g). In polymerisation reactions the heat of polymerisation must

be carefully controlled, particularly since decomposition reactions that take

place at elevated temperatures are also exothermic and explosion can occur if

the reaction gets out of control.

Since impurities can affect both the polymerisation reaction and the properties

of the finished product (particularly electrical insulation properties and resistance

to heat aging) they must be rigorously removed. In particular, carbon monoxide,

acetylene, oxygen and moisture must be at a very low level. A number of patents

require that the carbon monoxide content be less than 0.02%.



208 Polyethylene

It was estimated in 1997 that by the turn of the century 185 million tonnes of

ethylene would be consumed annually on a global basis but that at the same time

production of polyethylene would be about 46000000t.p.a., i.e. about 25% of

the total. This emphasises the fact that although polyethylene manufacture is a

large outlet for ethylene the latter is widely used for other purposes.

10.3 POLYMERISATION

There are five quite distinct routes to the preparation of high polymers of

ethylene:



(1)

(2)

(3)

(4)

(5)



High-pressure processes.

Ziegler processes.

The Phillips process.

The Standard Oil (Indiana) process.

Metallocene processes.



10.3.1 High-pressure Polymerisation

Although there are a number of publications dealing with the basic chemistry of

ethylene polymerisation under high pressure, little information has been made

publicly available concerning details of current commercial processes. It may

however be said that commercial high polymers are generally produced under

conditions of high pressure (1000-3000 atm) and at temperatures of 80-300°C.

A free-radical initiator such as benzoyl peroxide, azodi-isobutyronitrile or

oxygen is commonly used. The process may be operated continuously by passing

the reactants through narrow-bore tubes or through stirred reactors or by a batch

process in an autoclave. Because of the high heat of polymerisation care must be

taken to prevent runaway reaction. This can be done by having a high cooling

surface-volume ratio in the appropriate part of a continuous reactor and in

addition by running water or a somewhat inert liquid such as benzene (which also

helps to prevent tube blockage) through the tubes to dilute the exotherm. Local

runaway reactions may be prevented by operating at a high flow velocity. In a

typical process 10-30% of the monomer is converted to polymer. After a

polymer-gas separation the polymer is extruded into a ribbon and then

granulated. Film grades are subjected to a homogenisation process in an internal

mixer or a continuous compounder machine to break up high molecular weight

species present.

Although in principle the high-pressure polymerisation of ethylene follows the

free-radical-type mechanism discussed in Chapter 2 the reaction has two

particular characteristics, the high exothermic reaction and a critical dependence

on the monomer concentration.

The highly exothermic reaction has already been mentioned. It is particularly

important to realise that at the elevated temperatures employed other reactions

can occur leading to the formation of hydrogen, methane and graphite. These

reactions are also exothermic and it is not at all difficult for the reaction to get

out of hand. It is necessary to select conditions favourable to polymer formation

and which allow a controlled reaction.

Most vinyl monomers will polymerise by free-radical initiation over a wide

range of monomer concentration. Methyl methacrylate can even be polymerised



Polymerisation



209



by photosensitised catalysts in the vapour phase at less than atmospheric pressure.

In the case of ethylene only low molecular weight polymers are formed at low

pressures but high molecular weights are possible at high pressures. It would

appear that growing ethylene polymer radicals have a very limited life available for

reaction with monomer. Unless they have reacted within a given interval they

undergo changes which terminate their growth. Since the rate of reaction of radical

with monomer is much greater with higher monomer concentration (higher

pressure) it will be appreciated that the probability of obtaining high molecular

weights is greater at high pressures than at low pressures.

At high reaction temperatures (e.g. 200°C) much higher pressures are required

to obtain a given concentration or density of monomer than at temperatures of

say 25°C and it might appear that better results would be obtained at lower

reaction temperatures. This is in fact the case where a sufficiently active initiator

is employed. This approach has an additional virtue in that side reactions leading

to branching can be suppressed. For a given system the higher the temperature

the faster the reaction and the lower the molecular weight.

By varying temperature, pressure, initiator type and composition, by

incorporating chain transfer agents and by injecting the initiator into the reaction

mixture at various points in the reactor it is possible to vary independently of

each other polymer characteristics such as branching, molecular weight and

molecular weight distribution over a wide range without needing unduly long

reaction times. In spite of the flexibility, however, most high-pressure polymers

are of the lower density range for polyethylenes (0.915-0.94g/cm3) and usually

also of the lower range of molecular weights.



10.3.2 Ziegler Processes

As indicated by the title, these processes are largely due to the work of Ziegler

and coworkers. The type of polymerisation involved is sometimes referred to as

co-ordination polymerisation since the mechanism involves a catalyst-monomer

co-ordination complex or some other directing force that controls the way in

which the monomer approaches the growing chain. The co-ordination catalysts

are generally formed by the interaction of the alkyls of Groups 1-111 metals with

halides and other derivatives of transition metals in Groups IV-VI11 of the

Periodic Table. In a typical process the catalyst is prepared from titanium

tetrachloride and aluminium triethyl or some related material.

In a typical process ethylene is fed under low pressure into the reactor which

contains liquid hydrocarbon to act as diluent. The catalyst complex may be

first prepared and fed into the vessel or may be prepared in situ by feeding

the components directly into the main reactor. Reaction is carried out at some

temperatures below 100°C (typically 70°C) in the absence of oxygen and

water, both of which reduce the effectiveness of the catalyst. The catalyst

remains suspended and the polymer, as it is formed, becomes precipitated from

the solution and a slurry is formed which progressively thickens as the reaction

proceeds. Before the slurry viscosity becomes high enough to interfere

seriously with removing the heat of reaction, the reactants are discharged into

a catalyst decomposition vessel. Here the catalyst is destroyed by the action

of ethanol, water or caustic alkali. In order to reduce the amount of metallic

catalyst fragments to the lowest possible values, the processes of catalyst

decomposition, and subsequent purification are all important, particularly

where the polymer is intended for use in high-frequency electrical insulation.



2 10 Polyethylene

A number of variations in this stage of the process have been described in the

literature.

The Ziegler polymers are intermediate in density (about 0.945 g/cm3) between

the high-pressure polyethylenes and those produced by the Phillips and Standard

Oil (Indiana) processes. A range of molecular weights may be obtained by

varying the AI-Ti ratio in the catalyst, by introducing hydrogen as a chain

transfer agent and by varying the reaction temperature.

Over the years, considerable improvements and extensions of the Ziegler

process have taken place. One such was the advent of metallocene single-site

catalyst technology in the late 1980s. In these systems the olefin only reacts at a

single site on the catalyst molecules and gives greater control over the process.

One effect is the tendency to narrower molecular weight distributions. In a

further extension of this process Dow in 1993 announced what they refer to as

constrained geometry homogeneous catalysts. The catalyst is based on Group IV

transition metals such as titanium, covalently bonded to a monocyclopentadiene

group bridged with a heteroatom such as nitrogen. The catalyst is activated by

strong Lewis acid systems. These systems are being promoted particularly for use

with linear low-density polyethylene (see Section 10.3.5).



10.3.3 The Phillips Process

In this process ethylene, dissolved in a liquid hydrocarbon such as cyclohexane,

is polymerised by a supported metal oxide catalyst at about 130-160°C and at

about 200-500 Ibf/in2 (1.4-3.5 MPa) pressure. The solvent serves to dissolve

polymer as it is formed and as a heat transfer medium but is otherwise inert.

The preferred catalyst is one which contains 5% of chromium oxides, mainly

Cr03, on a finely divided silica-alumina catalyst (75-90% silica) which has been

activated by heating to about 250°C. After reaction the mixture is passed to a gasliquid separator where the ethylene is flashed off, catalyst is then removed from the

liquid product of the separator and the polymer separated from the solvent by either

flashing off the solvent or precipitating the polymer by cooling.

Polymers ranging in melt flow index (an inverse measure of molecular weight)

from less than 0.1 to greater than 600 can be obtained by this process but

commercial products have a melt flow index of only 0.2-5 and have the highest

density of any commercial polyethylenes (- 0.96 g/cm3).

The polymerisation mechanism is largely unknown but no doubt occurs at or

near the catalyst surface where monomer molecules are both concentrated and

specifically oriented so that highly stereospecific polymers are obtained. It is

found that the molecular weight of the product is critically dependent on

temperature and in a typical process there is 40-fold increase in melt flow index,

and a corresponding decrease in molecular weight, in raising the polymerisation

temperature from 140°C to just over 170°C. Above 4001bf/in2 (2.8MPa) the

reaction pressure has little effect on either molecular weight or polymer yield but

at lower pressures there is a marked decrease in yield and a measurable decrease

in molecular weight. The catalyst activation temperature also has an effect on

both yield and molecular weight. The higher the activation temperature the

higher the yield and the lower the molecular weight. A number of materials

including oxygen, acetylene, nitrogen and chlorine are catalyst poisons and very

pure reactants must be employed.

In a variation of the process polymerisation is carried out at about 9O-10O0C,

which is below the crytalline melting point and at which the polymer has a low



Polymerisution



21 1



solubility in the solvent. The polymer is therefore formed and removed as a slurry

of granules each formed around individual catalyst particles. High conversion

rates are necessary to reduce the level of contamination of the product with

catalyst and in addition there are problems of polymer accumulation on reactor

surfaces. Because of the lower polymerisation temperatures, polymers of higher

molecular mass may be prepared.



10.3.4 Standard Oil Company (Indiana) Process

This process has many similarities to the Phillips process and is based on the use

of a supported transition metal oxide in combination with a promoter. Reaction

temperatures are of the order of 230-270°C and pressures are 40-80 atm.

Molybdenum oxide is a catalyst that figures in the literature and promoters

include sodium and calcium as either metals or as hydrides. The reaction is

carried out in a hydrocarbon solvent.

The products of the process have a density of about 0.96 g/cm3, similar to the

Phillips polymers. Another similarity between the processes is the marked effect

of temperature on average molecular weight. The process is worked by the

Furukawa Company of Japan and the product marketed as Staflen.



10.3.5 Processes for Making Linear Low-density Polyethylene and

Metallocene Polyethylene

Over the years many methods have been developed in order to produce

polyethylenes with short chain branches but no long chain branches. Amongst the

earliest of these were a process operated by Du Pont Canada and another

developed by Phillips, both in the late 1950s. More recently Union Carbide have

developed a gas phase process. Gaseous monomers and a catalyst are fed to a

fluid bed reactor at pressures of 100-300 Ibf/in2 (0.7-2.1 MPa) at temperatures

of 100°C and below. The short branches are produced by including small

amounts of propene, but-1-ene, hex-1-ene or oct-l -ene into the monomer feed.

Somewhat similar products are produced by Dow using a liquid phase process,

thought to be based on a Ziegler-type catalyst system and again using higher

alkenes to introduce branching.

As mentioned in Section 10.3.2, there has been recent interest in the use of

the Dow constrained geometry catalyst system to produce linear low-density

polyethylenes with enhanced properties based, particularly, on ethylene and

oct-I-ene.

LLDPE materials are now available in a range of densities from around

0.900 g/cm3 for VLDPE materials to 0.935 g/cm3 for ethylene-octene copolymers. The bulk of materials are of density approx. 0.920g/cm3 using butene in

particular as the comonomer.

In recent years the market for LLDPE has increased substantially and is now

more than half the total for LDPE and for HDPE.

Mention has already been made in this chapter of metallocene-catalysed

polyethylene (see also Chapter 2). Such metallocene catalysts are transition metal

compounds, usually zirconium or titanium, incorporated into a cyclopentadienebased structure. During the late 1990s several systems were developed where the

new catalysts could be employed in existing polymerisation processes for

producing LLDPE-type polymers. These include high pressure autoclave and



2 12 Polyethylene

solution processes as well as gas phase processes. At the present time it remains

to be seen what methods will become predominant.

Mention may also be made of catalyst systems based on iron and cobalt

announced in 1998 by BP Chemicals working in collaboration with Imperial

College London and, separately, by DuPont working in collaboration with the

University of North Carolina. The DuPontNNC catalysts are said to be based on

tridentate pyridine bis-imine ligands coordinated to iron and cobalt. These are

capable of polymerising ethylene at low pressures (200-600 psi) yielding

polymers with very low branching (0.4 branches per 1000 carbon atoms) and

melting points as high as 139°C. The BP/ICL team claim that their system

provides many of the advantages of metallocenes but at lower cost.



10.4 STRUCTURE AND PROPERTIES OF POLYETHYLENE

The relationship between structure and properties of polyethylene is largely in

accord with the principles enunciated in Chapters 4, 5 and 6. The polymer is

essentially a long chain aliphatic hydrocarbon of the type



and would thus be thermoplastic. The flexibility of the C-C bonds would be

expected to lead to low values for the glass transition temperature. The T g ,

however, is associated with the motion of comparatively long segments in

amorphous matter and since in a crystalline polymer there is only a small number

of such segments the Tg has little physical significance. In fact there is

considerable argument as to the position of the Tg and amongst the values quoted

in the literature are -130"C, -120"C, -105"C, -93"C, -81"C, -77"C, -63"C,

4 8 " C , -3O"C, -20°C and +60"C! In one publication Kambour and Robertson

and the author* independently concluded that -20°C was the most likely value

for the T g . Such a value, however, has little technological significance. This

comment also applies to another transition at about -120°C which is currently

believed to arise from the Schatzki crankshaft effect. Far more important is the

crystalline melting point T,, which is usually in the range 108-132°C for

commercial polymers, the exact value depending on the detailed molecular

structure. Such low values are to be expected of a structure with a flexible

backbone and no strong intermolecular forces. Some data on the crystalline

structure of polyethylene are summarised in Table 10.1. There are no strong

intermolecular forces and most of the strength of the polymer is due to the fact

that crystallisation allows close molecular packing. The high crystallinity also

leads to opaque structures except in the case of rapidly chilled film where the

development of large crystalline structures is prevented.

Polyethylene, in essence a high molecular weight alkane (paraffin), would be

expected to have a good resistance to chemical attack and this is found to be the

case.

The polymer has a low cohesive energy density (the solubility parameter 6 is

about 16.1 MPa'/*) and would be expected to be resistant to solvents of solubility

parameter greater than 18.5 MPa'I2. Because it is a crystalline material and does



* JENKINS, A. D. (Ed.), Polymer Science, North-Holland, Amsterdam (1972).



Structure and Properties of Polyethylene 2 13

Table 10.1 Crystallinity data for polyethylene

Molecular disposition

Unit cell dimensions



Cell density (unbranched polymer) (25°C)

Amorphous density (20°C)



planar zigzag

a = 1.368,

b = 4.928,

c = 2.548,

1.014

0.84



not enter into specific interaction with any liquids, there is no solvent at room

temperature. At elevated temperatures the thermodynamics are more favourable

to solution and the polymer dissolves in a number of hydrocarbons of similar

solubility parameter.

The polymer, in the absence of impurities, would also be expected to be an

excellent high-frequency insulator because of its non-polar nature. Once again,

fact is in accord with prediction.

At the present time there are available many hundreds of grades of

polyethylene, most of which differ in their properties in one way or another. Such

differences arise from the following variables:



(1)

(2)

(3)

(4)



Variation in the degree of short chain branching in the polymer.

Variation in the degree of long chain branching.

Variation in the average molecular weight.

Variation in the molecular weight distribution (which may in part depend on

the long chain branching).

(5) The presence of a small amount of comonomer residues.

(6) The presence of impurities or polymerisation residues, some of which may

be combined with the polymer.



Further variations can also be obtained by compounding and cross-linking the

polymer but these aspects will not be considered at this stage.

Possibilities of brunching in high-pressure polyethylenes were first expressed

when investigation using infrared spectroscopy indicated that there were about

20-30 methyl groups per 1000 carbon atoms. Therefore in a polymer molecule

of molecular weight 26 000 there would be about 40-60 methyl groups, which is

of course far in excess of the one or two methyl groups to be expected from

normal chain ends. More refined studies have indicated that the methyl groups

are probably part of ethyl and butyl groups. The most common explanation is that

these groups arise owing to a ‘back-biting’ mechanism during polymerisation

(Figure 10.1).

Polymerisation could proceed from the radical in the normal way or

alternatively chain transfer may occur by a second back-biting stage either to the

butyl group (Figure 10.2(a)) or to the main chain (Figure 10.2(b)).

According to this scheme a third back-bite is also possible (Figure 10.3).In the

first stage a tertiary radical is formed which could then depolymerise by

p-scission. This will generate vinylidene groups, which have been observed and

found to provide about 50% of the unsaturation in high-pressure polymers, the

rest being about evenly divided by vinyl and in-chain trans double bonds. (There

may be up to about three double bonds per 1000 carbon atoms.)