Plastics Materials 7 Episode 5 ppsx

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

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 high- temperature 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%

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

(3) The Phillips process

(4) The Standard Oil (Indiana) process

(5) 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

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

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 gas- liquid 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

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 copoly- mers 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 cyclopentadiene- based 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

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

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)

Table 10.1 Crystallinity data for polyethylene

Molecular disposition

Unit cell dimensions

Cell density (unbranched polymer) (25°C)

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

(2) Variation in the degree of long chain branching

(3) Variation in the average molecular weight

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

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

(6) The presence of impurities or polymerisation residues, some of which may the long chain branching)

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.)

The presence of these branch points is bound to interfere with the ease of crystallisation and this is clearly shown in differences between the polymers The branched high-pressure polymers have the lowest density (since close-packing due

In addition to the short chain branches there is some evidence in high-pressure polyethylenes for the presence per chain of a few long branches which are probably several tens of carbon atoms long These probably arise from the transfer mechanism during polymerisation shown in Figure 10.4 Such side chains may be as long as the original main chain and like the original main chain will produce a wide distribution of lengths It is therefore possible to obtain fairly short chains grafted on to short main chains, long side chains on to long main chains and a wide variety of intermediate situations

In addition, subsequent chain transfer reactions may occur on side chains and the larger the resulting polymer, the more likely will it be to be attacked These features tend to cause a wide molecular weight distribution for these materials and it is sometimes difficult to check whether an effect is due inherently to a wide molecular weight distribution or simply due to long chain branching

One further effect of long chain branches is on flow properties Unbranched polymers have higher melt viscosities than long-branched polymers of similar weight average molecular weight This would be expected since the long- branched molecules would be more compact and be expected to entangle less with other molecules

The more recently developed so-called linear low-density polyethylenes are virtually free of long chain branches but do contain short side chains as a result

of copolymerising ethylene with a smaller amount of a higher alkene such as oct- I-ene Such branching interferes with the ability of the polymer to crystallise as with the older low-density polymers and like them have low densities The word linear in this case is used to imply the absence of long chain branches For reference purposes the polymer produced from diazomethane is particularly useful in that it is free from both long and short branches and apart from the end groups consists only of methylene groups This material is generally known as polymethylene, which is also the name now being recommended by IUPAC to describe polyethylenes in general The diazomethane polymer has the highest density of this family of materials, it being about 0.98 g/cm3 Copolymerisation with diazoethane and higher homologues provides an alternative method for producing a polymer with short chains but with no long ones

Differences in molecular weight will also give rise to differences in properties The higher the molecular weight, the greater the number of points of attraction and

entanglement between molecules Whereas differences in short chain branching and hence degree of crystallinity largely affect properties characterised by small solid displacement, molecular weight differences will affect properties that involve large deformations such as ultimate tensile strength, elongation at break, melt viscosity and low-temperature brittle point There is also an improvement in resistance to environmental stress cracking with increase in molecular weight Before the advent of Ziegler and Phillips polymers it was common practice to

characterise the molecular weight for technological purposes by the melt flow

index (MFI), the weight in grams extruded under a standard load in a standard plastometer at 190°C in 10 minutes This test had also proved useful for quality control and as a very rough guide to processability From measurements of MFI various workers have calculated the apparent viscosity of the polymer and correlated these figures with both number average and weight average molecular weight (It should be noted that estimation of apparent viscosities from melt flow index data is rather hazardous since large corrections have to be made for end effects, pressure losses in the main cylinder and friction of the plunger It would

be better to use a high shear viscometer designed to minimise the sources of error and to compare results at equal shear rate.) Suffice it to say that the higher the melt flow index, the lower the molecular weight

With the availability of the higher density polymers the value of the melt flow index as a measure of molecular weight diminishes For example, it has been found* that with two polymers of the same weight average molecular weight (4.2

X lo')), the branched polymer (density = 0.92 g/cm3) had only 1/50 the viscosity

of the more or less unbranched polymer (density = 0.96g/cm3) This is due to long chain branches as explained above

Commercial polyethylenes also vary in their molecular weight distribution

(MWD) Whilst for some purposes a full description of the distribution is required, the ratio-of _weight average molecular weight to number average molecular weight (M,/M,) provides a useful parameter Its main deficiency is that it provides no information about any unusual high or low molecular weight tail which might have profound significance For polymethylenes the ratio is about 2 whilst with low-density polymers values varying from 1.9 to 100 have been reported with values of 20-50 being said to be typical High-density polymers have values of 4-15

The very high figures for low-density materials are in part a result of long chain branching and, as has already been stated, it is sometimes not clear if an effect is due to branching or to molecular weight distribution It is generally considered, however, that with other structural factors constant a decrease in

M,/M, leads to an increase in impact strength, tensile strength, toughness, softening point and resistance to environmental stress cracking There is also a pronounced influence on melt flow properties, the narrower distribution materials being less sensitive to shear rate but more liable to sharkskin effects The general principles outlined in the previous paragraph (which has been unchanged since the first edition of this book) have been found to be particularly relevant for the metallocene polyethylenes being introduced in the late 1990s

These have M w / M n ratios in the range 2-3 and while they do exhibit enhanced toughness they show higher melt viscosities at high shear rates than correspond- ing traditional polymers and suffer from problems with melt defects

Much of recent development in polymerisation technology has been devoted to establishing control of the MWD of LLDPE polymers With such polymers, narrowing the MWD confers higher toughness, greater clarity, lower heat seal

initiation temperatures and, where this is important, higher cross-link efficiency

As with LDPE there is lower melt shear sensitivity and poorer melt strength Catalyst systems have been used which result in polymers with a bimodal (double-peaked) molecular weight distribution in an attempt to improve flow properties, whilst another approach combines the use of polymers with narrow molecular weight distribution but with a broad side-chain length distribution

A number of comonomers have been used in conjunction with ethylene Such comonomers are either hydrocarbons such as propylene or but-1 -ene non- hydrocarbons such as vinyl acetate Small amounts of a second alkene are sometimes used to produce a controlled degree of short chain branching and some retardation in the growth of large crystal structures As will be described in the next chapter, copolymers of this type produced by the Phillips process have better creep, environmental stress cracking and thermal cracking resistance than the corresponding homopolymer The use of hydrocarbon comonomers such as oct-1-ene became very common with the development of LLDPE and this approach is also being used with metallocene polyethylenes Properties of metallocene polyethylenes such as low density (cf standard homopolymers), lower melt temperatures, clarity and heat sealability would be expected to be more related to the presence of copolymers than the narrow molecular weight distribution (which has a more significant effect on toughness and melt flow properties) Small amounts of vinyl acetate also impede crystallisation and, as with the alkene copolymers, substantial amounts of the second comonomer lead

to rubbery materials

The final variable to be mentioned here is the presence of impurities These may be metallic fragments residual from Ziegler-type processes or they can be trace materials incorporated into the polymer chain Such impurities as catalyst fragments and carbonyl groups incorporated into the chain can have a serious adverse influence on the power factor of the polymer, whilst in other instances impurities can have an effect on aging behaviour

Polyethylene is a wax-like thermoplastic softening at about 80-130°C with a density less than that of water It is tough but has moderate tensile strength, is an excellent electrical insulator and has very good chemical resistance In the mass

it is translucent or opaque but thin films may be transparent

10.5.1 Mechanical Properties

The mechanical properties are very dependent on the molecular weight and on

the degree of branching of the polymer As with other polymers these properties

are also dependent on the rate of testing, the temperature of test, the method of specimen preparation, the size and shape of the specimen and, to only a small degree with polyethylene, the conditioning of samples before testing The data in

Table 10.2, although not all obtained from the same source, has been obtained using only one test method for each property The figures given show clearly the general effects of branching (density) and molecular weight on some polymer properties but it should be remembered that under different test conditions different results may be obtained It should also be remembered that polymers of different density but with the same melt flow index do not have the same molecular weight The general effects of changing rate of testing, temperature

Figure 10.5 Effect of polymer density, testing rate and temperature on the shape of the stress-strain

curve for polyethylene’

18.48 11.03

18.96 10.90 20.00 10.34

and density on the tensile stress-strain curves are shown schematically in Figure

10.5 It is seen in particular that as the test temperature is lowered or the testing rate increased, a pronounced ‘hump’ in the curve becomes apparent, the apex of the hump A being the yield point Up to the yield point deformations are

recoverable and the polymer is almost Hookean in its behaviour The working of the sample, however, causes ‘strain softening’ by, for example, spherulite breakdown or in some cases by crystal melting so that the polymer extends at constant stress This cold drawing, however, causes molecular orientation and induces crystallisation so that there is a stiffening of the sample and an upward sweep of the stress-strain curve The effect of temperature on a sample of low- density polyethylene with an MFI of 2 is shown in Figure 10.6 The varying influence of rate of strain on tests results can be clearly shown from figures obtained with two commercial polyethylene samples (Table 10.3) It is seen that

in one case an increase in rate of strain is accompanied by increase in tensile strength and in the other case, reduction

Figure 10.6 Effect of temperature on the tensile stress-strain curve for polyethylene (Low-density

polymer -0.92g/cm3 MFI = 2.) Rate of extension 190% per minute”

The elongation at break of polyethylene is strongly dependent on density

(Figure 10.7), the more highly crystalline high-density materials being less

ductile This lack of ductility results in high-density polymers tending to be

brittle, particularly with low molecular weight materials The tough-brittle

dependence on melt flow index and density is shown in Figure 10.8

Under load polyethylene will deform continuously with time (‘creep’) A

knowledge of creep behaviour is important when considering load-bearing

applications, water piping being a case in point with polyethylene In general

there will be an increase in creep with increased load, increased temperature and

decreased density A large amount of creep data has been made available in

specialised monographs and in trade literature

Figure 10.7 Effect of density and melt flow index on elongation at break (Separation rate 45 cm/min

on specimen of l i n gauge length.) A, constant density (0.92 glcm’) B , constant MFI (0.7) C,

constant density (0.94 g/cm3).’ (Reproduced by permission of ICI)

10.5.2 Thermal Properties

As mentioned in Section 10.4 there are conflicting data on the position of the T,

of polyethylene It is the author’s belief that a transition at about -20°C is probably the true T , but another transition at about -120°C is also to be observed Tough at room temperature, the polymers become brittle on cooling but some specimens do not appear to become brittle until temperatures as low as -70°C

have been reached In general the higher the molecular weight and the more the branching the lower the brittle point Measured brittle points also depend on the method of sample preparation, thus indicating that the polymer is notch sensitive, i.e sensitive to surface imperfections

The specific heat of polyethylene is higher than for most thermoplastics and is strongly dependent on temperature Low-density materials have a value of about

2.3 J/g at room temperature and a value of 2.9 J/g at 120-140°C A somewhat schematic representation is given in Figure 10.9 The peaks in these curves may

u -u

0 9 0 0 9 1 092 093 0 9 4 0 9 5 0 9 6

D E N S I T Y AT 23 ‘C I N g/cmJ

Figure 10.8 Effects of melt flow index and density on the room temperature tough-brittle transition

of polyethylene.’ (Reproduced by permission of ICI)

0

Figure 10.9 Specific heat-temperature relationships for low-density polyethylene, high-density

polyethylene and polystyrene." (The Distillers Company Ltd.)

be considered to be due to a form of latent heat of fusion of the crystalline zones Melting point (T,) data are given in Table 10.2 and the T , is seen to vary with density

Flow properties of polyethylene have been widely studied Because of the wide range of average molecular weights amongst commercial polymers the viscosities vary widely The most commonly used materials, however, have viscosities lower than for unplasticised PVC and poly(methy1 methacrylate) and higher than for the nylons

Typical of thermoplastics (see Chapter 8) the melts are pseudoplastic and also

in common with most thermoplastics the zero shear rate apparent viscosity of

linear polyethylene is related to the weight average molecular weight by the relationship

log ( Y ~ , ~ ) = K + 3.4 log G,

for polymers with a molecular weight in excess of about 5000

Polymers with long branches do not fit these equations and different relations

exist with polymers of different degrees of long branching In many cases the equation

log (qa,o) = A + BG,,1'2

gives a good fit to the data

It is interesting to note that so-called linear low-density polyethylenes are said

to be less pseudoplastic than conventional low-density polyethylenes Thus on

comparing the two materials at the same melt flow index the ‘linear’ polymer will be found to be more viscous at the higher shear rates usually encountered during processing

As usual, an increase in temperature reduces melt viscosity and equations of the type discussed in Chapter 8 fit data very well Melt processing is usually carried out in the range 150-210°C but temperatures as high as 300°C may be used in some paper-coating applications In an inert atmosphere the polymer is stable at temperatures up to 300°C so that the high processing temperatures do not lead to severe problems due to degradation, providing contact of the melt with oxygen is reduced to a minimum

The elastic melt effects mentioned briefly in Chapter 8 are commonly encountered with polyethylene Some typical experimental results on die swell are shown in Figure 8.8 The phenomenon of elastic turbulence (waviness, bambooing, melt fracture) is also observed in low-temperature processes (e.g bottle blowing) and when extruding at very high rates (wire covering) This situation is generally aggravated by high molecular weights and low temperature but reduced by long chain branching and increasing the molecular weight distribution

In addition to elastic turbulence (characterised by helical deformation) another phenomenon known as ‘sharkskin’ may be observed This consists of a number

of ridges transverse to the extrusion direction which are often just barely discernible to the naked eye These often appear at lower shear rates than the critical shear rate for elastic turbulence and seem more related to the linear extrudate output rate, suggesting that the phenomenon may be due to some form

of slip-stick at the die exit It appears to be temperature dependent (in a complex manner) and is worse with polymers of narrow molecular weight distribution Melt elasticity is of considerable importance in understanding much of the behaviour of polyethylene when processing by film extrusion techniques and when blow moulding The complex relationships observed experimentally here have been summarised by the author elsewhere ’*

10.5.3 Chemical Properties

The chemical resistance of polyethylene is, to a large measure, that expected of

an alkane It is not chemically attacked by non-oxidising acids, alkalis and many aqueous solutions Nitric acid oxidises the polymer, leading to a rise in power

factor and to a deterioration in mechanical properties As with the simple alkanes,

halogens combine with the hydrocarbon by means of substitution mechanisms

Since polyethylene is a crystalline hydrocarbon polymer incapable of specific interaction and with a melting point of about 1OO"C, there are no solvents at room temperature Low-density polymers will dissolve in benzene at about 60°C but the more crystalline high-density polymers only dissolve at temperatures some 20-30°C higher Materials of similar solubility parameter and low molecular weight will, however, cause swelling, the more so in low-density polymers (Table

10.5)

Low-density polyethylene has a gas permeability in the range normally

expected with rubbery materials (Table 5.11) This is because in the amorphous

zones the free volume and segmental movements facilitate the passage of small molecules Polymers of the Phillips type (density 0.96 g/cm3) have a permeabil- ity of about one-fifth that of the low-density materials

Exposure of polyethylene to ultraviolet light causes eventual embrittlement of the polymer This is believed to be due to the absorption of energy by carbonyl groups introduced into the chain during polymerisation andlor processing The carbonyl groups absorb energy from wavelengths in the range 220-320 nm Fortunately very little energy from wavelengths below 300 nm strikes the earth's surface and so the atmosphere offers some protection However, in different climates and in different seasons there is some variation in the screening effect

of the atmosphere and this can give rise to considerable variation in the outdoor weathering behaviour of the polymer

Table 10.5 Absorption of liquids by polyethylenes of density 0.92 and

0.96 g/cm3 at 20°C after 30 days immersion

6 MPa'I2

17.5 18.7 19.4 15.1 20.3 18.6 20.4 26.0 48.0

0.92 g/cm3

42.4 14.6 13.8 12.8 8.5 4.9 3.9 2.9 1.81 1.24 1.01 0.7

<0.01

0.96 gfcm3

13.5 5.0 4.6 5.8 2.6 0.95 2.4 1.6 1.53 0.79 0.85

0.4

<0.01

When polyethylene is subjected to high-energy irradiation, gases such as hydrogen and some lower hydrocarbons are evolved, there is an increase in unsaturation and, most important, cross-linking occurs by the formation of C-C

bonds between molecules The formation of cross-link points interferes with crystallisation and progressive radiation will eventually yield an amorphous but cross-linked polymer Extensive exposure may lead to colour formation and in the presence of air surface oxidation will occur Oxygen will cause polymer degradation during irradiation and this offsets the effects of cross-linking Long exposure to low radiation doses on thin film in the presence of oxygen may lead

to serious degradation but with short exposure, high radiation doses and thicker specimens the degradation effects become less significant Since cross-linking is accompanied by a loss of crystallisation, irradiation does not necessarily mean an increased tensile strength at room temperature However, at temperatures about 130°C irradiated polymer still has some strength (it is quite rubbery), whereas the untreated material will have negligible tenacity It is found that incorporation of carbon black into polyethylene which is subsequently irradiated can give substantial reinforcement whereas corresponding quantities in the untreated product lead to brittleness

If polyethylene is exposed to a mechanical stress in certain environments, fracture of the sample occurs at stresses much lower than in the absence of the

environment As a corollary if a fixed stress, or alternatively fixed strain, is

imposed on a sample the time for fracture is much less in the 'active

environment' than in its absence This phenomenon is referred to as environmen-

tal stress cracking An example of this effect can be given by considering one of

the tests used (the Bell Telephone Laboratory Test) to measure the resistance of

a specific polymer to this effect A small moulded rectangle is nicked to a fixed length and depth with a sharp blade and the nicked sample is then bent through

180 degrees so that the nick is on the outside of the bend and at right angles to the line of the bend The bent sample is held in a jig and immersed in a specific

detergent, usually an alkyl aryl polyethylene glycol ether (e.g Igepal CA) and placed in an oven at 50°C Low-density polymers with an MFI of 20 and above will often be observed to crack in an hour or two Amongst materials which appear to be active environments are alcohols, liquid hydrocarbons, organic esters, metallic soaps, sulphated and sulphonated alcohols, polyglycol ethers and silicone fluids This is rather a formidable list and at one time it was thought that this would lead to some limitation in the use of polyethylene for bottles and other containers However, for a number of reasons this has not proved a problem except with high-density homopolymers and the main reason for concern about the cracking phenomenon is in fact associated with cables when the polyethylene insulator is in contact with greases and oils

The reason for the activity of the above named classes of liquids is not fully understood but it has been noted that the most active liquids are those which reduce the molecular cohesion to the greatest extent It is also noticed that the effect is far more serious where biaxial stresses are involved (a condition which invariably causes a greater tendency to brittleness) Such stresses may be frozen

in as a result of molecular orientation during processing or may be due to

distortion during use

Different polyethylenes vary considerably in the environmental stress cracking resistance It has been found that with low-density polymers the Bell Test generally shows that the higher the molecular weight the greater the resistance, low-density polymers with a melt flow index of 0.4 being immune to the common detergents Narrow molecular weight distributions considerably improve resistance of a polymer of given density and average molecular weight Large crystalline structures and molecular orientations appear to aggravate the problem The effect of polymer density is somewhat complicated The Bell Test

is performed at constant strain and hence much higher stresses will be involved

in the high-density polymers It is thus not surprising that these materials often appear to be inferior by this test but in constant stress tests different results may

be expected Paradoxically, Phillips-type homopolymers have often been less satisfactory in service than indicated by the Bell Test

It may seem surprising that low-density, comparatively low molecular weight (MFI 20) materials have been successfully used for detergent bottles in view of the stress cracking phenomenon (Nevertheless higher molecular weight materials are usually used here, i.e with an MFI <0.7.) The reason for this lies

in the fact that good processing conditions and good design result in low stresses being imparted to the products Under these conditions stress cracking times are

invariably longer than the required service life of the product

10.5.4 Electrical Properties

The insulating properties of polyethylene compare favourably with those of any

other dielectric material As it is a non-polar material, properties such as power

factor and dielectric constant are almost independent of temperature and frequency Dielectric constant is linearly dependent on density and a reduction of density on heating leads to a small reduction in dielectric constant Some typical

data are given in Table 10.6

Oxidation of polyethylene with the formation of carbonyl groups can lead to

a serious increase in power factor Antioxidants are incorporated into compounds for electrical applications in order to reduce the effect

Table 10.6 Electrical properties of polyethylene

Volume resistivity Dielectric strength Dielectric constant density = 0.92 g/cm’

density = 0.96 g/cm3

Power factor

>1Oz2nm

700 k V / m 2.28 2.35 -1-2 x 10-4

10.5.5 Properties of LLDPE and VLDPE

As with LDPE and HDPE materials, there is a wide range of linear low-density polyethylenes (LLDPEs) Primarily competitive with LDPE, the ‘linear low’ materials have found rapid acceptance because of their high toughness (at low, normal and high temperatures), tensile strength, elongation at break and puncture resistance compared to LDPE materials of similar melt flow index and density More specifically the improved resistance to environmental stress cracking has been emphasised by suppliers as also has the ability to use dishwashers to clean LLDPE kitchen utensils, a consequence of the higher heat deformation resistance

The very low density materials (VLDPEs) introduced in the mid-1980s are generally considered as alternatives to plasticised PVC (Chapter 12) and ethylene-vinyl acetate (EVA) plastics (see Chapter 11) They have no volatile or extractable plasticisers as in plasticised PVC nor do they have the odour or moulding problems associated with EVA Whilst VLDPE materials can match the flexibility of EVA they also have better environmental stress cracking resistance, improved toughness and a higher softening point

Some comparative data for a VLDPE copolymer based on ethylene and oct- 1-ene and an EVA material (91% ethylene, 9% vinyl acetate) are given in Table

475

5 1 -130

32

240

10.5.6 Properties of Metallocene-catalysed Polyethylenes

Metallocene-catalysed polyethylenes exhibit the general characteristics of

polyethylene as noted in the introductory paragraph of Section 10.5 Furthermore

they are more like low density polyethylenes (LDPE and LLDPE) than HDPE

As with LLDPE they are usually copolymers containing small quantities of a low molecular weight a-olefin such as but- 1 -ene, hex- I-ene and oct- 1 -ene The property differences largely arise from the narrow molecular weight distribution,

the more uniform incorporation of the a-olefin and the low level of

polymerisation residues (about one-tenth that of Ziegler-Natta catalysed LLDPE)

It is generally claimed that metallocene polyethylenes (often abbreviated to m-PE) exhibit superior mechanical and optical properties as well as better organoleptic properties (resulting from the lower residue levels) As an example m-LLDPE is particularly favoured as a stretch film for wrapping because of the good prestretchability, high puncture resistance and tear strength, all of which are claimed to be better than with conventional LLDPE

As previously mentioned, narrow molecular weight distribution polymers such

as m-PE are less pseudoplastic in their melt flow behaviour than conventional polyethylenes so that given an m-LLDPE and a conventional LLDPE of similar melt index (measured at low shear rates), the m-LLDPE will have a much higher melt viscosity at the high shear rates involved in film processing The polymers are also more susceptible to melt fracture and sharkskin This difference requires that such steps be taken as to use more highly powered extruders, to use special processing aids such as fluoroelastomers or to make compromises in the polymer structure which may, however, reduce the advantages of m-PE materials One obvious approach would be to produce bi-, tri- or other polymodal blends (see the Appendix to Chapter 2 for explanations) to overcome the inherent disadvantages

of narrow molecular weight distribution polymers It is of interest that ‘bimodal’ polymers produced by a two-reactor system have become available which have enhanced resistance to cracking and are rapidly finding use in pipe applications

Metallocene-catalysed very low density polyethylene (m-VLDPE) has become available with densities of as low as 0.903 This is of use for sealing layers of multi-layer films since sealing can commence at lower temperatures than with conventional materials such as LLDPE and EVA (see Section 11.6) with the polymer seal exhibiting both cold strength and hot tack strength

Fillers, important constituents of many plastics materials, are rarely used with polyethylene since they interfere with the crystallinity of the polymer and often give rather brittle products of low ductility Carbon black has some reinforcing effect and is of use in cross-linked polymers It is also of some use in introducing

a measure of conductivity to the polymer Somewhat better results with non- black fillers may be achieved with the use of silane and titanate coupling agents and compounds with increased rigidity and tensile strength compared with unfilled polymer may be obtained However, unlike polypropylene, mineral- filled polyethylene has remained unimportant A number of pigments are available for use in polyethylene The principal requirements of a pigment are that it should have a high covering power/cost ratio and that it should withstand processing and service conditions In the case of polyethylene special care should

be taken to ensure that the pigment does not catalyse oxidation, an effect observed with a number of pigments based on cobalt, cadmium and manganese Other adverse effects have also been reported with hydrated chromic oxide, iron blues, ultramarine and anatase titanium dioxide For electrical insulation applications pigments such as cobalt blues, which cause a rapid rise of power factor on aging, should be avoided

Polyethylene bums readily and a number of materials have been used asflame retarders These include antimony trioxide and a number of halogenated materials

Layers of low-density polyethylene film often show high cohesion, or

‘blocking’, a feature which is often a nuisance on both processing and use One way of overcoming this defect is to incorporate anti-blocking agents such as fine silicas In addition slip agents may be added to reduce the friction between layers

of film Fatty acid amides such as oleamide and, more importantly, erucamide, are widely used for this purpose Polymers with densities of above 0.935 g/cm3 show good slip properties and slip agents are not normally required for these products

Products with very low dielectric constant (about 1.45) can be obtained by the use of cellular polymers Blowing agents such as 4,4’-oxybisbenzenesulphono-

hydrazide and azocarbonamide are incorporated into the polymer On extrusion the blowing agent decomposes with the evolution of gas and gives rise to a cellular extrudate Cellular polyethylene is a useful dielectric in communication cables Although many rubbery materials show varying compatibility with poly- ethylene the only elastomeric materials used in commercial compounds are polyisobutylene (PIB) and butyl rubber Polyisobutylene was originally used as

a ‘plasticiser’ for polyethylene but was later found also to improve the environmental stress cracking resistance Ten per cent PIB in polyethylene gives

a compound resistant to stress cracking as assessed by the severe BTL test It has been shown13 that within broad limits the higher the molecular weight of the PIB the greater the beneficial effect Very high molecular weight polyisobutylenes are, however, less effective, possibly due to the difficulty in obtaining satisfactory blends PIB may or may not increase the ‘ease of flow’ of polyethylene, this depending on the molecular weights of the two polymers Because of its lower cost butyl rubber is preferred to polyisobutylene at the present time, use of the latter in polyethylene being largely restricted to cable applications

Polyethylene is sometimes blended with ethylene-propylene rubber (see

Chapter 11) In this application it is most commonly used as an additive to the

rubber, which in turn is added to polypropylene to produce rubber-modified

polypropylenes In addition up to 20% of ethylene-propylene rubber may be used in blown film applications

Vulcanised (cross-linked) polyethylene is being used for cable application where service temperatures up to 90°C are encountered Typical cross-linking agents for this purpose are peroxides such as dicumyl peroxide The use of such agents is significantly cheaper than irradiation processes for the cross-linking of the polymer

An alternative process involves the use of vinyl silanes (see Section 10.9)

When polyethylene is to be used in long-term applications where a low power factor is to be maintained and/or where it is desired to provide thermal protection during processing, antioxidants are incorporated into the polymers These were discussed extensively in Chapter 7 but a few particular points with regard to their use in polyethylene should be made Although amines have been used widely in the past phenols are now used almost exclusively

For protection against degradation during processing 4-methyl-2,6-t-butyl- phenol is widely used It causes only a low level of staining and is also used in

non-toxic formulations Its volatility restricts its use for long-term and/or high- temperature work For service use 1,1,3-tris-(4-hydroxy-2-rnethyl-5-t-butyl-

pheny1)butane (Topanol CA) and bis-[2-hydroxy-5-methyl-3-( l-methylcyclo- hexy1)phenyllmethane (Nonox WSP) are widely used Only small amounts (of

the order of 0.1 %) are required of these chain-breaking antioxidants, which may

be used in conjunction with a peroxide-decomposing antioxidant such as dilauryl-P$'-thiodipropionate (DLTP) The phenols show little tendency to bloom, bleed, discolour or stain but Topanol CA/DLTP blends cause some discolouration which can be minimised by incorporation of certain phosphorus compounds (e.g 0.1-0.2% of Phosclere T268)

In the presence of carbon black the phenols and phenol/DLTP combinations are much less effective whilst some phenolic sulphides (e.g Santonox) show positive synergism with carbon black However, in general terms the phenol systems tend

to be reduced to about the same levels as to those to which the phenol sulphide systems are raised Some typical figures are given in Table 10.8

ruison d'&tre for incorporating antioxidants is often to prevent an increase in power factor on aging, power factor measurements are widely used In addition the property is very sensitive to oxidation Figure 10.11 shows the change in power factor of polyethylenes containing various antioxidants after air aging

Figure 10.12 shows the effect of varying the antioxidant concentration The sharp increase in power factor after an induction time during which little change occurs

is to be noted in particular In practice about 0.1% of antioxidant is employed in electrical grade compounds

The weathering properties of polyethylene are improved by the incorporation

of carbon blacks Maximum protection is obtained using blacks with a particle

size of 25 p m and below In practice finely divided channel or furnace blacks are used at 2-3% concentration and to be effective they must be very well dispersed into the polymer The use of more than 3% black leads to little improvement in weathering resistance and may adversely affect other properties

TIME IN h

Figure 10.12 Oxidation o f polyethylene in air at 105°C Effect of antioxidant concentration (N,"

diphenyl-p-phenylenediamine) A, blank B, 50ppm C, 100ppm D, 500ppm E, lOWppm

Antistatic additives are widely used to reduce dust attraction and also in films

to improve handling behaviour on certain types of bag making and packaging equipment Whilst at one time it was more common to apply an antistatic agent

to the surface by wiping, dipping or spraying, increasing use is now being made

of antistatic agents which are incorporated into the polymer mass during normal compounding and which migrate to the surface with the passage of time This approach has the advantage that the extra coating process is avoided and also that any layer of material removed by normal handling will be replaced by material which will migrate out of the mass The selection of such ‘antistats’ is critical and will depend particularly on the polymers used and on the thermal stability required For a given polymer the agent should have a limited compatibility and

a high diffusion rate in order to produce an antistatic layer as soon as possible after manufacture Whereas quaternary ammonium compounds are widely used for polystyrene, polyethylene glycol alkyl esters would appear to be preferred in polyethylene compositions The actual chemical composition of the rather small number of antistatic agents so far found suitable is rarely disclosed by the suppliers

Polyethylene can be compounded on any of the standard types of mixing equipment used for visco-elastic materials For laboratory purposes a two-roll mill is suitable; operating temperatures varying from about 90°C to about 140°C according to the type of polymer On the industrial scale, compounding is undertaken either in internal mixers, or more particularly, extrusion compounders

(3) Although processing temperatures are low compared with many plastics the specific heat, which varies with temperature, is high As a consequence, as

was shown in Chapter 8, more heat needs to be put in and taken out of

polyethylene during processing than for other major thermoplastics This means higher energy costs to raise the temperature and longer cooling times after shaping

(4) The melt viscosity is highly nowNewtonian in that the apparent viscosity drops considerably with increasing shear rate Melt viscosities are about the average encountered with plastics materials but there is a considerable variation between grades

(5) The high degree of crystallisation, which leads, among other things, to a high shrinkage on cooling

(6) The short polymer relaxation times

(7) A rather sharp melting point

Polyethylene is processed by a wide variety of techniques, most of which were outlined briefly in Chapter 8 There is insufficient space here to deal adequately with the principles and practice of these processes or even with the particular characteristics of polyethylene being fabricated by these processes For this reason a list of books giving further details is given at the end of Chapter 8

Compression moulding is used only occasionally with polyethylene In this process the polymer is heated in a mould at about 150”C, compressed to shape and cooled The process is slow since heating and cooling of the mould must be carried out in each cycle and it is employed only for the manufacture of large blocks and sheets, for relatively strain-free objects such as test-pieces and where alternative processes cannot be used because of lack of equipment

A very large number of products are produced by injection moulding In this process the polymer is melted and injected into a mould which is at a temperature below the freezing point of the polymer so that the latter can harden For mouldings with a minimum frozen-in strain, operating conditions should be so selected that the mould cavity pressure drops to zero as the material sets Because

of the tendency of the material to crystallise, high shrinkage values are observed, ranging from 0.01 5-0.050 cm/cm with low-density materials to 0.025-0.060 cm/

cm with high-density polymers High mould temperatures, desirable to reduce strains through freezing of oriented molecules, lead to increased shrinkage since there is more time available for crystallisation High cavity pressures reduce shrinkage since during much of the cooling part of the cycle there is only pressure reduction in the polymer and not physical contraction High cavity pressures also reduce packing near to the point of entry into the mould which can occur as the material in the cavity shrinks Since it is partially solidified material which is packed into the mould, and which often freezes while the molecules are oriented, a weakness of the part around the gate can occur and so ‘packing’ should be reduced as far as possible Melt temperatures are of the order of

160-1 90°C for low-density polymers and up to 50°C higher with high-density materials In order to achieve these melt temperatures cylinder temperatures may

be anything from 30 to 100°C higher

Many articles, bottles and containers in particular, are made by blow moulding

techniques of which there are many variations In one typical process a hollow tube is extruded vertically downwards on to a spigot Two mould halves close on

to the extrudate (known in this context as the ‘parison’) and air is blown through the spigot to inflate the parison so that it takes up the shape of the mould As in injection moulding, polymers of low, intermediate and high density each find use according to the flexibility required of the finished product

Another moulding process based on the extruder is ‘extrusion moulding’

Molten polymer is extruded into a mould where it sets Since satisfactory mouldings can be produced using low moulding pressures, cheap cast moulds can be used The process has been used to produce very large objects from polyethylene The techniques of screw-preplasticising with injection moulding can be considered as a development of this process

Approximately three-quarters of the polyethylene produced is formed into products by means of extrusion processes These processes will differ according

to the product being made, Le according to whether the end-product is film, coated paper, sheet, tube, rod or wire covering In principle the extrusion process consists of metering polymer (usually in granular form) into a heated barrel in which a screw is rotating The rotation of the screw causes the granules to move

up the barrel, where they are compacted and plasticised The resultant melt is

then forced under pressure through an orifice to give a product of constant cross- section Although the polymer may be processed on a variety of different machines, screws usually have a length-diameter ratio in excess of 16:l and a compression ratio of between 2.5:l and 4:l (The compression ratio can be considered as the volume of one turn of the screw channel at the feed end to the volume of one turn at the delivery end of the barrel.) Since well over half the polyethylene extruded is converted into film, film extrusion processes will be considered in somewhat greater detail There are basically two processes, the tubular process and the flat film process, which are shown schematically in

Figure 10.13

In the tubular process a thin tube is extruded (usually in a vertically upward direction) and by blowing air through the die head the tube is inflated into a thin bubble This is cooled, flattened out and wound up The ratio of bubble diameter

to die diameter is known as the blow-up ratio, the ratio of the haul-off rate to the natural extrusion rate is referred to as the draw-down ratio and the distance between the die and the frost line (when the extrudate becomes solidified and which can often be seen by the appearance of haziness), the freeze-line distance

The properties of the film are strongly dependent on the polymer used and on

processing conditions The higher the density the lower the flexibility, the greater the brittleness and to some extent, up to densities of about 0.94 g/cm3, the greater the clarity (in the absence of sharkskin effects) and the lower the tensile strength

at rupture High-density materials are also less susceptible to ‘blocking’ The higher the molecular weight the greater the melt viscosity, tensile strength and resistance to film brittleness at low temperatures but the lower the transparency and the less the ability of the melt to draw down Wide molecular weight distributions have been claimed to improve the resistance to film brittleness but this view is not universally accepted

For general purpose work, polymers with a density of about 0.923g/cm3, a melt flow index of about 2, a reasonably wide molecular weight distribution, and which are free from high molecular weight oxidised ‘blobs’, are most commonly used The requirements of a film, however, differ according to the application Whereas in some instances toughness may be of greater importance, in other cases high optical clarity may be the paramount requirement The choice of both polymer and processing conditions can greatly influence the properties of the product

There has been extensive investigationi4 into the effect of processing conditions on clarity, haze and gloss of polyethylene film It can easily be demonstrated that the presence of haze and lack of clarity of low- and intermediate-density polymers is due to surface irregularities which can arise either as a melt roughness (which tends to disappear after extrusion if the polymer remains molten) or due to crystallisation (which though not developing structures large enough to impede the passage of light tends to cause surface distortions)

The effects of melt roughness and surface crystallisation are shown more clearly in Figure 10.14, in which haze is plotted against freeze-line distance, all other operating variables being constant It will be observed that initially as the freeze-line distance increases there is a reduction in haze since the extra period that the polymer is molten allows a reduction in melt roughness Eventually, however, there is an increase in haze because the longer cooling times allow larger crystal structures to build up and distort the surface An increase in

INCREASING OUTPUT RATE INCREASING BLOW

RATIO INCREASING HAUL-OFF SPEED

INCREASING EXTRUSION TEMPERATURE

1

FREEZE-LINE DISTANCE +

Figure 10.14 Effect of freeze-line distance and other operating variables on the haze of low-density

polyethylene film.' (Reproduced by permission of ICI)

extrusion temperature reduces haze because there is a reduction in melt imperfections and because the time available for crystallisation is reduced (although the total cooling time is the same, the polymer will be above its melting point for a longer fraction of this time) An increased output rate with constant freeze-line distance would increase melt imperfections but reduce surface crystallinity effects and thus shift the curve to the right The effects of most other operating variables can also be explained in terms of melt roughness and surface crystallinity

The effect of extrusion conditions on the impact strength of tubular film has also been studied and found to be related to molecular orientation Polyethylene molecules in the melt have a very short relaxation time (a measure of the time taken for molecules to coil after release of an orienting stress) Thus in the tubular film process only molecules that have been oriented just before the melt freezes will remain in the oriented state Because of this the order in which drawing down and transverse stretching of the film occur will affect the impact strength These factors can be adjusted by varying freeze-line distance, blow ratio and output rate, the shape of the bubble giving a guide to the sequence of events In

recent years the technology of film manufacture has been extended by the appearance of ethylene-propylene rubber modified LDPE and also linear low- density polyethylene Both materials can show high levels of toughness Considerable amounts of polyethylene film are produced using coaxial extrusion processes in which two or more melt streams are combined in the die

to produce extruded film of two or more layers of plastics materials Layers in such a composite may be included, for example, to improve barrier properties, to enhance sealability or even simply to act as an adhesive between dissimilar layers

[NB Some technical reviews refer to polymer composites being used in film manufacture when it is not always obvious whether the reference is to the use of

a physical blend of the component polymers or whether the polymers are separated in layers (as in coaxial extrusion) or in some combination Clearly the effects can be quite different.]

Although a large proportion of polyethylene film is made by the tubular process some film is produced by extruding flat film from a slit die either into a water bath or on to a chilled casting roll Although extrusion directly into water results in the most rapid quenching and tends to give products of highest clarity, the presence of antistatic and slip additives tends to cause water to carry over