High-pressure polyethylene process LDPE

The high-pressure process was developed from the high-pressure polyethylene process (LDPE). The polymerization is carried out in bulk. By means of this process mainly copolymers with a vinyl acetate content of up to 45% are produced. The important range lies between 5% and 30% vinyl acetate content, giving copolymers with thermoplastic properties. The maximum molecular weight achieved by the high-pressure process is comparatively low due to the high chain-transfer activity of the vinyl acetate in bulk polymerization. Therefore, the vinyl acetate content is limited. 

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. 

A simplified schematic of a high-pressure polyethylene synthesis plant is illustrated in Figure 1. This scheme produces LDPE of density between 0.915 to 0.940 g/cc. The dashed lines on the diagram indicate heating control zones in the process. (Source Handbook of Polymer Science and Technology Volume 2 -... 

In 1979 UNIPOL process for gas-phase production of the linear low-density polyethylene, LLDPE, was introduced by the Union Carbide Corp. Since the new resins were difficult to process on the processing lines designed for the low-density (high pressure) polyethylenes, LDPE, soon several patents were... 

Low density polyethylene is the oldest type of polyethylene. It is produced in a high pressure process. It is a soft, tough and flexible kind of polyethylene due to its highly branched molecular structure. The typical density of LDPE lies between 915 and 935 kg/m. When it is deformed, it can recover its original shape due to its natural elasticity. The high pressure polyethylene shows a higher melt flow index (MFI) and therefore, processes easier than most other types of polyethylene. 

Theoretically, an almost infinite number of different grades can be produced in a high-pressure polyethylene plant, due to the high flexibility in polymerization conditions and the relative short transition times. In contrast to the lo v-pressure technology, where the polymerization is carried out by Ziegler/Natta, chromium, or metallocene catalysts, the high-pressure process can also use polar comonomers such as vinylacetate, acrylic acid, or N-butylacrylate to modify product properties. With all these opportunities, the resin producer can adjust the product portfolio to cover the market needs at the current economic environment. LDPE homopolymer resins with densities 0.917-0.934 g/cm at MFRs 0.15-100 g/10 min are available in the market. 

The concerned industries are chemical/petrochemical, power, nuclear, and armament industries, as well as high-pressure food processing. Autofrettage is common practice for equipment used in polyethylene (LDPE) plants, gun barrels, waterjet cutting pumps, and high-pressure pasteurization equipment. 

Low-density polyethylene (LDPE) has traditionally been made by a high-pressure process (25 000-50 000 psi) and high-density polyethylene (HDPE) by a low-pressure process ( 100 psi). Thus, LDPE is sometimes referred to as high-pressure polyethylene and HDPE as low-pressure polyethylene. 

Coordination copolymerization of ethylene with small amounts of an a-olefin such as 1-butene, 1-hexene, or 1-octene results in the equivalent of the branched, low-density polyethylene produced by radical polymerization. The polyethylene, referred to as linear low-density polyethylene (LLDPE), has controlled amounts of ethyl, n-butyl, and n-hexyl branches, respectively. Copolymerization with propene, 4-methyl-1-pentene, and cycloalk-enes is also practiced. There was little effort to commercialize linear low-density polyethylene (LLDPE) until 1978, when gas-phase technology made the economics of the process very competitive with the high-pressure radical polymerization process [James, 1986]. The expansion of this technology was rapid. The utility of the LLDPE process Emits the need to build new high-pressure plants. New capacity for LDPE has usually involved new plants for the low-pressure gas-phase process, which allows the production of HDPE and LLDPE as well as polypropene. The production of LLDPE in the United States in 2001 was about 8 billion pounds, the same as the production of LDPE. Overall, HDPE and LLDPE, produced by coordination polymerization, comprise two-thirds of all polyethylenes. 

The first section of this chapter describes the most important high pressure process run under homogeneous conditions to manufacture Low Density PolyEthylene (LDPE). The radical polymerization of ethylene to LDPE is carried out in tubular reactors or in stirred autoclaves. Tubular reactors exhibit higher capacities than stirred autoclaves. The latter are preferred to produce ethylene copolymers having a higher comonomer content. 

As in any other process, so also in the high-pressure polymerization of ethylene, do capital costs, utilities, maintenance, manpower, and costs of raw materials contribute to the production costs of low-density polyethylene (LDPE). The cost structure is typical for the production of bulk chemicals but is strongly influenced by the requirements of a high-pressure process. 

In many cases, the linear low-density polyethylene (LLDPE) produced in low-pressure processes competes for the same market as LDPE. For this reason, in Figure 8.2-7 capital- and operation costs of the high-pressure polymerization are compared with those of a low-pressure solution process having the same capacity. Also, the production costs of the low-pressure process are dominated by the costs of the monomer, but some differences can be noted which are typical for the economics of low- and high-pressure processes. 

It is expected that around 500,000 tons mPE will be manufactured in 2000 in Western Europe. Resins from high-pressure- as well as from low-pressure processes suffer more difficult processability. Their narrow molecular-weight distribution results in an unfavourable viscosity behaviour [8], As with linear low-density polyethylene (LLDPE), the viscosity of mPE is independent of the shear-rate over a wide range and reduces only at high shear-rates (Fig. 9.5-11). Therefore, energy consumption is high when mPE is processed with extruders which are designed for LDPE. 

Today, several high pressure and low pressure process technologies have been developed for the production of the different polyethylene grades [4, 8, 9, 10, 11]. In 2001 about 20 million tonnes of the PE capacity used high pressure and about 45 million tonnes used low pressure technology. In 2000, 32% of the worldwide PE capacity was estimated to be for the production of LDPE, 31% for HDPE and 37% for LLDPE [6, 12,13]. 

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