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 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%. 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) High-pressure processes. (2) Ziegler processes. (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 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 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 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 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 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 Tg, 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, 48"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 Tg. 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) 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.) 214 Polyethylene CH, f *CH,-CH CH2 / CHz \ CH, - CH * I I I /CH2 CH, *CH2 FH2 1 +C,H, - CH, - CH - (CH2)?- CH, I CH2 I CH,* Figure 10.1 / CH2 - / cH, - CH3 - CH H I -CH /CHz-cH* (a) /CH2 - CH CH, - CH, \ CH, - CH,* \ / Bu / Bu \ / cHz- CH /cH2-cH \ CH, - CH, CH, + -CH - CH I (b) Figure 10.2. (a) Transfer to the butyl group. (b) Transfer to the main chain / \H *CH, /Et /Et \ /CHz-cH \ * FH2 -C-Et CH, - -C-Et / \H *CH, / yy *C-CH2- *CH I I Et Et / CH2- - CHI* - CH,=C Et I Figure 10.3 Short chain branching is negligible with Ziegler and Phillips homopolymers although it is possible to introduce deliberately up to about seven ethyl side chains per 1000 carbon atoms in the Ziegler polymers. 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 Structure and Properties of Polyethylene 215 mCH,-CH,* + H-3 -&CH,-CH, + 3-* Growing Radical ’Bad’ Polymer ’Bad’ Radical Polymer CH, = CH, 3 - * - CH, = CH, - -CH, - CH,* - etc. etc Figure 10.4 to crystallisation is reduced), the least opacity (since the growth of large crystalline structures is impeded) and a lower melting point, yield point, surface hardness and Young’s modulus in tension (these properties being dependent on the degree of crystallinity). In addition the more the branching and the lower the crystallinity, the greater will be the permeability to gases and vapours. For general technological purposes the density of the polyethylene (as prepared from the melt under standard conditions) is taken as a measure of short chain branching. 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 2 16 Polyethylene 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 Mw/Mn 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 [...]... material (91% ethylene, 9% vinyl acetate) are given in Table 10 .7 Table 10 .7 Comparison of VLDPE and EVA (9%VA) VLDPE Density (g/cm3) MFI Tear strength (N/mm2) Elongation at break (%) Vicat temperature (“C) Low-temperature brittle point (“C) Hardness (Shore D) Stress crack time (h) 0.910 7 11.4 71 0 78 -1 35 42 600 EVA 0.926 9 6.1 4 75 51 -130 32 240 10 .5. 6 Properties of Metallocene-catalysed Polyethylenes Metallocene-catalysed... Cyclohexanone Ethyl acetate Oleic acid Acetone Acetic acid Ethanol Water 17. 5 18 .7 19.4 - 15. 1 - 20.3 18.6 - 20.4 - 26.0 48.0 0.96 gfcm3 42.4 14.6 13.8 12.8 8 .5 4.9 3.9 2.9 1.81 1.24 1.01 0 .7 . MPa'I2 17. 5 18 .7 19.4 15. 1 20.3 18.6 20.4 26.0 48.0 - - - - 470 increase in weight in polymers 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. 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,. oct-I-ene. LLDPE materials are now available in a range of densities from around 0.900 g/cm3 for VLDPE materials to 0.9 35 g/cm3 for ethylene-octene copoly- mers. The bulk of materials are of