Pressure polymerisation processes

The different available high pressure polymerisation processes of polyethylene (PE) yield LDPE (low density PE), LLDPE (linear low density PE) and copolymer features of the same. The various process variations have been developed during recent decades and introduced a number of well developed steps and devices to achieve safe and economical operating conditions at the very high reaction pressures of 1500 to 3000 bar. 

Low density polyethylene material has branched chains and limited crystallinity, which lead to an open structure and the low density. It is particularly soft and flexible, transparent to translucent, has good impact resistance and relatively low melting points, which give good heat sealability. Most LDPEs are made by a high pressure polymerisation process starting from ethylene gas. The proportion of crystallinity to amorphous is around 3 2 (i.e. 60-65% crystalline). Recently new linear polyethylene copolymers of 0.89-0.91 (ultra or very low densities) have been developed. Special antioxidant free grades are available for pharmaceutical applications. 

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... 

High molecular weight polymers are produced by an adiabatic bulk polymerisation process ° using di-tert-butyl peroxide (0.02%) and 2,2 -azo-bisdi-isobutyronitrile (0.01%) as initiators and pressurised with N2. Heating to 80-90°C causes an onset of polymerisation and a rapid increase in temperature. After the maximum temperature has been reached the mass is allowed to cool under pressure. A typical current commercial material (Luvican M.170) has a A -value of about 70 (as assessed in a 1% tetrahydrofuran solution). 

Dartnell, R. C. et al., Loss Prev., 1971, 5, 53-56 MCA Case History No. 1649 A batch of 8 t of material accumulated in storage at 154°C during 72 h decomposed explosively. Stability tests showed that thermal instability developed when 3-methyl-4-nitrophenol is stored molten at temperatures above 140°C. Decomposition set in after 14 h at 185° or 45 h at 165°, with peak temperatures of 593 and 521°C, respectively. In a closed vessel, a peak pressure of 750 bar was attained, with a maximum rate of increase of 40 kbar/s. Thermal degradation involves an initially slow exothermic free radical polymerisation process, followed by a rapid and violently exothermic decomposition at take-off. 

In a first full scale attempt at a new polymerisation process, the thermally unstable initiator was charged and heated to reaction temperature, but there was then an unforeseen delay of an horn before monomer addition was started. The rate of polymerisation effected by the depleted initiator was lower than the addition rate of the monomer, and the concentration of the latter reached a level at which an uncontrollable polymerisation set in which eventually led to pressure-failure of the vessel seals. Precautions to prevent such occurrences are detailed. In another incident, operator error led to catalyst, condensing styrene and acrylonitrile being ducted into an unstirred weighing tank instead of a reactor. When the error was recognised, the reacting mixture was dropped into drums containing inhibitor. One of the sealed drums had insufficient inhibitor to stop the reaction, and it slowly heated and eventually burst [1], The features and use of... 

Fast high-pressure fractionation of tails in the molar-mass-distribution and/or the chemical-composition-distribution of a (co)polymer [47] during the polymerisation process will indicate eventual drifts in (co)polymer composition. Several important polymer properties depend strongly on such tails in the (co)polymer distribution. On--line fractionation data are strongly needed. 

The polymerisation of branched polyethylene was discovered by ICI in 1933, when chemical reactions under very high pressures were studied. Development of commercial free-radical-initiated, high pressure PE process technology by the end of 1930s was the foundation of the polyolefins industry. 

In all the low pressure PE processes the polymer is formed through coordination polymerisation. Three basic catalyst types are used chromium oxide, Ziegler-Natta and single-site catalysts. The catalyst type together with the process defines the basic structure and properties of the polyethylene produced. Apart from the MWD and comonomer distribution that a certain catalyst produces in polymerisation in one reactor, two or more cascaded reactors with different polymerisation conditions increase the freedom to tailor... 

Figure 9 shows a typical pressure curve and indicates that very low pressure rises occur during the polymerisation process. 

Polymerisation is one area where the introduction of supercritical fluids is especially appealing. Not only have many polymers traditionally been synthesised using solvents which are now considered environmentally unacceptable for long term use, but also the increased demand for so-called specialty polymers has led to the need for ever greater control of the polymerisation process itself. Supercritical fluids offer both environmental acceptability and the potential of extra control of the process. This potential arises from the gas-like nature of the fluids which allows pressure (and hence, fluid density) to be used as an additional parameter in the tuning of... 

The polyethylene fibre is produced by either high pressure polymerisation of ethylene with a peroxide-catalysed process or low pressure polymerisation of ethylene using new catalysts systems. The molecular structure of polyethylene is a linear polymer of ethylene units with repeat unit of... 

A closely related method to the in situ polymerisation processing of composites is based on epoxy resins thermosets. In this approach, CNTs can be dispersed in a liquid epoxy precursor and then the mixtures can be cured by the addition of hardener, such as triethylene tetramine (TETA), and the application of temperature or pressure. In most cases, the epoxy monomer exists in liquid state, facilitating nanotube dispersion. Curing is then carried out to... 

The process of destraction dates back to a discovery by Kurt Zosel (1913-1989) at the Max-Planck-Institut fur Kohlenforschung in Miilheim, who was engaged in normal-pressure polymerisation of ethylene. He had incidentally noticed, that residues from the distillation of oU, but also waste oil, vegetable fats and oils from natural products, could be extracted very well with supercritical gases. [514] This methodology offered correspondingly a number of other applications, as to extract hop aroma from hops, unsaturated fatty acids from fish oUs, vitamin E from vegetable oils, and paraffins or phenols from bituminous tar. 

The polymerisation process consists of a series of sequential stages with the main ones being initiation, propagation, and material chain termination. In order to produce a quality product it is necessary to provide optimum conditions for corresponding stages. These conditions include operation temperatures, pressure, concentrations of ingredients, and a sufficient degree of field uniformity for all characteristics. At the... 

The effects of the pressure on the plasma polymerisation process include ... 

The system involved in the bulk polymerisation process is the simplest from the point of view of composition and is used for large-scale radical polymerisation. In this process, the initiator is mixed with the monomer, usually under pressure of an inert gas, and the mixture is heated to induce generation of free radicals by thermal decomposition of the initiator. Depending on whether the monomer is a gas or a liquid, the system can be homogenous or heterogeneous... 

A density-based method has been introduced for polyethylene classification relying on ASTM-standards (Table 2.2). The density-based classification coincides to some extent with that based on the polymerisation processes. Consequently, polyethylene with low density (LDPE) is usually a polyethylene material produced by high-pressure polymerisation and polyethylene with high density (HDPE) is a polyethylene material manufactured by low-pressure polymerisation. Since the end of the 1970s co-... 

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