8. MAJOR COMMERCIAL POLYMERS
8.1 POLYETHYLENE
Development
The conversion of ethylene to high molecular weight polymer was first accomplished in 1933 by Fawcett and Gibson of Imperial Chemical Industries Ltd. (U.K.) during an investigation of the reaction between ethylene and benzaldehyde at high pressure.
The first commercial high pressure polyethylene plant began production in 1939 and until 1955 all commercial polyethylene was produced by high pressure processes. In 1953 Ziegler in Germany,
Phillips Petroleum Co. (U.S.A.) and Standard Oil Co. (Indiana) (U.S.A.) almost simultaneously discovered methods whereby high pressures could be avoided.
The first plant using the Ziegler process came on stream in 1955 (Farbwerke Hoechst A.G.) whilst the first Phillips and Standard Oil plants began production in 1957 and 1961 respectively. At the present time, however, the bulk of commercial polyethylene is still produced by high pressure methods.
Polyethylene is now one of the major commercial polymers and is used in such diverse applications as chemical plant, domestic goods, electrical insulation, packaging film and toys.
Preparation
High pressure processes
Commercial high pressure polyethylene processes are generally operated at pressures of 1000-3000 atmospheres and temperatures of 80-3000C.
Polymerization is initiated by free radical producing agents such as azobisobutyronitrile, benzoyl peroxide or oxygen (which probably leads to in situ peroxide formation), and the choice of initiator determines the appropriate reaction conditions.
For the oxygen-initiated reaction, for example, the optimum conditions are about 1500 atmospheres and 2000C with 0.03-0.1% oxygen.
Generally, a continuous process is operated and polymerization is carried out either in narrow-bore tubular reactors or in autoclaves fitted with stirrers.
The reaction is highly exothermic and efficient heat dissipation is essential. Insufficient control of reaction variables is liable to lead to explosive formation of carbon, hydrogen and methane. In some processes a diluent such as benzene or water is added, mainly to serve as a heat-exchange medium but also to assist in the removal of polymer from the reactor. In a typical process, 10-30% of the monomer is converted to polymer per cycle. After pressure letdown, the polymer is separated from unreacted ethylene (which is recycled) and diluent, if any, and then the polymer is extruded as ribbon and granulated.
The most note worthy feature of the above type of process is, of course, the very high pressure involved. It is necessary to use high pressure in order to obtain high molecular weight polymer. It seems that growing polyethylene radicals have a very limited life available for reaction with monomer and unless they have reacted within a certain interval termination occurs. Thus high molecular weights are favored by the increased monomer concentration which obtains at high pressures. It may be noted that whilst commercial processes involve what is strictly the gas phase (since temperatures are above the critical temperature of 9.70C), at the high pressures used the gas resembles a liquid and contains polymer in solution. Thus these processes resemble conventional bulk polymerization.
In the previous chapter the polymerization of ethylene is shown as leading to a linear polymer as follows:
n CH2 = CH2 [~[CH2-CH2~]n
Whilst this is an essentially correct representation of the high pressure processes, the polymer obtained by these methods is, in fact, somewhat branched. Examination of the infrared spectrum of the polymer shows the presence of about 30 methyl groups per 1000 carbon atoms in the chain. Methyl groups must be terminal groups, but this number is larger than can be accounted for by methyl chain ends in a linear polymer; hence branches must be present.
In 1940 infrared spectroscopy revealed that low-density polyethylene contains branched chains. These branches are of two distinct type. Branching due to intermolecular chain transfer, arising from reactions of the type.
The second branching mechanism in polyethylene is postulated to produce short-chain branching by intramolecular chain transfer(backbite).
Ziegler processes
Commercial Ziegler polyethylene processes are generally operated at pressures only slightly above atmospheric, namely 2-4 atmospheres and at temperatures of 50-750C.
For the polymerization of ethylene the catalyst is usually based on titanium tetrachloride/aluminium alkyl (e.g., diethylaluminium chloride). The catalyst may be prepared in situ by adding the components separately to the reactor as solutions in diluents such as diesel oil, heptane or toluene or the components may be pre-reacted and the catalyst added as a slurry in a liquid diluent.
These operations must be conducted in an inert atmosphere (usually of nitrogen) since oxygen and water reduce the effectiveness of the catalyst and may even cause explosive decomposition. In a typical process, ethylene and the catalyst and diluent are fed continuously into the reactor. At the reaction temperatures generally used, the polymer is only sparingly soluble in the hydrocarbon diluent and therefore forms as a slurry, which is continuously removed. The reaction is quenched by the addition of alcohols such as methanol, ethanol or isopropanol and the resulting metallic residues are extracted with alcohol hydrochloric acid. This purification is extremely important when the polymer is intended for use in high frequency electrical insulation. Finally, the polymer is centrifuged, dried, extruded and granulated.
The mechanism of vinyl polymerization effected by Zieger-Natta catalysts is discussed in the previous chapter. The polyethylene obtained by Ziegler processes differs significantly from the polymer obtained by high pressure processes in that much less branching is present. Spectroscopic evidence indicates the presence of 57 ethyl groups per 1000 carbon atoms; butyl side groups appear to be absent.
Phillips processes
Phillips processes are usually operated under conditions which are intermediate between those used in high pressure processes and Ziegler processes.
Pressures are generally 30-40 atmospheres and temperatures are 90-1600C. Polymerization is effected by means of a chromium oxide catalyst. Typically, the catalyst is prepared by impregnating silica or silica-alumina (which is the support) with an aqueous solution of a chromium salt and heating the product in air at 400-8000C. The final product contains about 5% of chromium oxides, mainly chromium trioxide. In a typical process, ethylene is passed into a suspension of the catalyst in a hydrocarbon such as cyclohexane.
Depending mainly on the temperature at which polymerization is carried out, the polymer is obtained either in solution or as a slurry. Solution processes are normally run at 120-l600C, at which temperatures the polymer is soluble in the diluent. Hot polymer solution is continuously drawn from the reactor; unreacted ethylene is flashed off and suspended catalyst is removed by filtration or centrifuging. The solution is then cooled to precipitate the polymer which is separated by centrifuging. Slurry processes, on the other hand, are usually run at 901000C, at which temperatures the polymer has low solubility in the diluent. The slurry, which consists of polymer granules each formed around separate catalyst particles, is drawn off continuously. Unreacted ethylene is flashed off and then the polymer is separated by centrifuging. The polymer so obtained is contaminated with a small amount of catalyst which is not usually removed.
At the present time the nature of polymerization with Phillips catalysts is incompletely understood. Whatever their mode of reaction may be, these catalysts lead to almost completely linear polyethylene. No ethyl or butyl branches have been definitely detected although the number of methyl groups (up to about 3 per 1000 carbon atoms) may be somewhat greater than the number of main chain ends.
Properties
As has been noted in the preceding section, the polyethylenes obtained in the various commercial processes differ in the amount of branching which is present.
The presence of branches reduces the ability of polymer chains to pack together closely and regularly, i.e., to crystallize. As a result the various types of polyethylene have somewhat different properties.
Thus the more highly branched polymers have the lower density, crystalline melting point, stiffness, surface hardness, and softening temperature and greater permeability to gases and vapors.
To a first approximation, these properties are dependent only on the degree of branching. Other physical properties are more difficult to correlate since they are also affected by variations in average molecular weight and molecular weight distribution. Comparative values for some properties of typical commercial grades of polyethylenes are given in Table 2.2.
The electrical insulating properties of polyethylene are outstanding. Since it is a non-polar material, properties such as dielectric constant and power factor are almost independent of the frequency and temperature. Dielectric constant increases slightly with increasing density.
Chemically, polyethylene can be regarded as a high molecular weight paraffin and as may be expected it is a rather inert material. Since there is no specific interaction, such as hydrogen bonding, with any solvent and since the polymer is crystalline, there is no solvent for polyethylene at room temperature. At elevated temperatures, however, there is appreciable solubility in hydrocarbons and halogenated hydrocarbons, e.g., toluene, xylene and dichloroethylene. The temperature necessary to dissolve polyethylene in a given solvent increases as the crystallinity of the polymer increases and ranges from about 600C to 800C for commercial materials. It may be noted here that some liquids cause environ-mental stress cracking of polyethylene. If the polymer is stressed in the presence of liquids such as alcohols, esters and ketones, fracture occurs at stresses much lower than those required to cause failure in the absence of the liquid.
Polyethylene is unaffected by most acids, alkalis and aqueous solutions. Strong oxidizing agents such as concentrated nitric acid and concentrated solutions of hydrogen peroxide and potassium permanganate oxidize the polymer, resulting in an increase in power factor and a deterioration of mechanical properties. Resistance to these reagents increases with increase in density because of diminished permeability. Oxidation of polyethylene also occurs in air on exposure to ultraviolet light and/or elevated temperature.
8.2Low-Density(Branched) Polyethylene
The first commercial ethylene polymer was branched poly ethylene, commonly designated as low density or high pressure material to distinguish it from the essentially linear material described below.
After a period a relatively slow growth in the 1940’s, the production of branched polyethylene expanded rapidly; this was the first plastic with annual production exceeding 1 billion lb(in 1959)
Polymerization
Ethylene(b.p. –104) is made from the thermal(stream) and catalytic cracking of a variety of hydrocarbons, ranging from ethane derived from natural gas to fuel oil. About 25 billion lb was produced in 1982, almost half of it used to produce of ethylene polymers and copolymers.
High polymers of ethylene are made commercially at pressures between 1000 and 3000 atm or possibly higher, and temperatures as high as 250 ºC.
Traces of oxygen initiate the polymerization of ethylene readily. Rapid exothermic reaction can occur, and violent explosions have taken place. Many other possible impurities in the monomer, such as hydrogen and acetylene, act as chain transfer agents and must be carefully removed if high-molecular-weight products are to be obtained. Besides oxygen peroxides(benzoyl, diethyl), hydroperoxides, and azo compounds have been used as initiators.
Ethylene polymerization can be carried out with benzene or chlorobenzene as solvent.
Batch polymerization of ethylene can not be carried out rapidly with reproducibility and good control. Long reaction times, consistent with good control, are not economical. In addition, chain branching becomes excessive at high conversion and results in poor physical properties of the product. As a result, balanced, continuos polymerization systems are preferred. Emulsion polymerization has had little success.
Structure
Low density PE is a partially(50-60%) crystal-line solid melting at about 115ºC, with density in the range 0.91-0.94. It is soluble in many solvents at temperatures above 100ºC, but only a few solvent mixtures provide borderline solubility at or near room temperature.
Properties
The physical properties of low-density polyethylene are function of three independent structural variables :
Molecular weight
Molecular-weight distribution or long chain branching and short chain branching.
Short chain branching ha a predominant effect on the degree of crystallinity and therefore on the density of polyethylene.
The effect of molecular weight is largely evidence in properties of the melt and properties involving large deformations of the solid. As molecular weight increases, so do tensile strength, tear strength, low- temperature toughness, softening temperature, impact strength and resistance to environmental stress cracking increases.
Applications
Almost two-thirds of the low-and medium density branched polyethylene produced has gone into film and sheeting uses for many years.
Polyethylene has filled a long-standing need for a material that would effectively insulate electrical cables without introducing electrical losses at high frequencies.
In addition to the bight-frequency uses, polyethylene is being more generally utilised for mechanical protection of wire and cables, where its chemical inertness and light weight are advantageous. About 5% of the branched polyethylene produced is used for wire and cable insulation.
8.3 POLYPROPYLENE
Development
It has been previously noted that co-ordination catalysts were originally employed by Ziegler in 1953 to effect the polymerization of ethylene. Natta extended the use of the catalysts to higher olefins and obtained isotactic polypropylene in 1954. The first commercial production of the polymer was by Montecatini in 1957. Polypropylene is now an important commercial polymer, being used principally in the injection moulding of diverse articles and for fibers and films.
Preparation
High pressure, free radical processes of the type used to prepare polyethylene are not satisfactory when applied to propylene and other n-olefins bearing a hydrogen atom on the carbon atom adjacent to the double bond. This is attributed to extensive transfer of this hydrogen to propagating centers
R. + H-CH2-CH=CH2 RH + CH2-CH=CH2
CH2=CH-CH2
The resulting allyl radical is resonance stabilized and has a reduced tendency to react with another monomer molecule.
Although the Phillips and Standard Oil processes can be used to prepare polypropylene. the polymer yields tend to be low and it appears that these processes are not currently used for commercial production of polypropylene. At the present time, commercial production of polypropylene is exclusively by Ziegler-type processes. Commonly a slurry process is used and is carried out in much the same manner as described previously for the preparation of polyethylene. In the case of polypropylene. some atactic polymer is fo~ed in addition to the required isotactic polymer; but much of this atactic material is soluble in the diluent so that the product isolated is largely isotactic polymer. A common method of characterizing polypropylenes is by 'isotactic index' which is the percentage by weight of polymer remaining undissolved after extraction by boiling n-heptane. Most commercial polypropylenes have isotactic indices of 95~98%. This index is not an accurate measure of the true isotactic content since partially-ordered chains which are in part incorporated into crystalline regions will not be extracted by n-heptane.
Properties
As a first approximation, the physical properties of polypropylene resemble those of high density polyethylene but there art some significant differences.
For comparison, typical values for some properties of a standard commercial grade of polypropylene are given in Table The most noticeable attribute of polypropylene is the increased softening point and consequent higher maximum service temperature. Properties such as tensile strength and stiffness, whilst comparable to high density polyethylene at room temperature, are maintained to a greater extent at elevated temperatures. At 1400C polypropylene is still sufficiently stiff for a strain-free article to retain its shape, whereas polyethylene is molten at this temperature. At room temperature the impact strength of polypropylene is comparable to that of high density polyethylene; however, the impact strength of polypropylene falls markedly as the temperature is reduced whereas polyethylene shows little change.
The solubility characteristics of polypropylene are similar to those of polyethylene. Thus polypropylene is insoluble at room temperature but is soluble in hydrocarbons and chlorinated hydrocarbons at temperatures above about 800C. Polypropylene does not suffer environ mental stress cracking of the kind shown by polyethylene.
In polypropylene, each alternate carbon atom in the polymer chain bears a tertiary hydrogen atom which is relatively labile. Thus, compared to polyethylene, polypropylene is more prone to attack by oxidizing agents. Of particular significance is the susceptibility of the polymer to oxidation by air at elevated temperatures, and antioxidants are generally added. As has been mentioned earlier, when polyethylene is exposed to high energy radiation or is heated with a peroxide cross-linking occurs to give a useful material. When polypropylene is similarly treated, the cross-linking reactions are accompanied by an approximately equal amount of chain scission and useful properties are not developed.
Polypropylene can be chlorinated and chlorosulphonated in much the same way as polyethylene, but the reactions are accompanied by severe degradation and some cross-linking. These processes have not, therefore, found commercial utilization.
8.4 POLYSTYRENE
Development
Commercial interest in polystyrene began in the 1930's when the material was found to have good electrical insulation characteristics, and limited production was started by I. G. Farben industrie (Germany) and Dow Chemical Company (U.S.A.) shortly before the Second World War.
During the war enormous quantities of styrene were produced in the U.S.A. for use in the synthetic rubber programme which was undertaken when supplies of natural rubber were no longer available. Thus when the war ended and natural rubber supplies were resumed, a large styrene capacity was available for civilian use and new outlets were sought. Applications for polystyrene were vigorously explored and the material was quickly adopted in many fields. Polystyrene has now become one of the major commercial plastics, being very extensively used in such diverse applications as domestic appliances, food containers, packaging, toys and, in expanded form, thermal insulation.