High-Pressure Polyethylene Manufacturing Process

If the monomer and polymer are not mutually soluble, the bulk reaction mixture will be heterogeneous. The high pressure free radical process for the manufacture of low density polyethylene is an example of such reactions. This polyethylene is branched because of self-branching processes illustrated in reaction (6-89). Branches longer than methyls cannot fit into the polyethylene crystal lattice, and the solid polymer is therefore less crystalline and rigid than higher density (0.935-0.96 g cm ) species that are made by coordination polymerization (Section 9.5). 

Barriers to entry into the pseudocommodity business include all the barriers for commodities plus the all-important customer know-how. Lack of technical expertise and patent protection can be formidable barriers to potential producers of a pseudocommodity. Du Pont was the sole producer of nylon from 1939 to 1951. Barriers to entry were removed under threat of government antitrust action in 1951, with the licensing of Chemstrand, which later became part of Monsanto. Since then a number of companies have entered the nylon business. Technical know-how and patent protection played a major role in both high- and low-pressure polyethylene manufacture in the years shortly after World War II. ICI developed polyethylene, but Union Carbide had a superior high-pressure process. Ziegler, Du Pont, and Phillips Petroleum all developed low-pressure processes, which they subsequently licensed to other manufacturers. Many pseudocommodities eventually become commodities by the diffusion of technology, standardization of the product, and the entry of many firms into the business. 

These high reactor efficiencies reported for the high-pressure process have significantly improved the cost analysis for this process compared to the gas-phase fluidized-bed process, which is often considered the low-cost polyethylene manufacturing process. 

The main use of propylene is for polymerization to polypropylene, a process similar to the manufacture of high-density polyethylene (i.e., a low-pressure, catalytic process). Textile hhers made from polypropylene are relatively low-cost and have particularly good properties, such as high resistance to abrasion and soiling for use in furniture upholstery and indoor/outdoor carpeting. 

In 1953, Ziegler in Germany, Phillips Petroleum Co. (USA) and Standard Oil Co. (USA) were able to discover the process of manufacturing polyethylene by avoiding the use of high pressure. The polyethylene so obtained is termed as the high density polyethylene. 

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. 

In recent years metallocene catalysts have been introduced into low-pressure gas-phase-, solution-, and slurry-processes to manufacture polyethylene and polypropylene. The new technology extends not only the range of conventional materials but generates new speciality polymers. Some companies have also retro-fitted high-pressure reactors to make use of the advantages of metallocene catalysts. 

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. 

A variety of technological processes arc used for polyethylene manufacture. They include polymerization in supercritical ethylene at a high ethylene pressure and temperature above the PE melting point (110-140°C), polymerization in solution at 120-150°C or in slurry, and polymerization in the gas phase... 

Chapter 1 is used to review the history of polyethylene, to survey quintessential features and nomenclatures for this versatile polymer and to introduce transition metal catalysts (the most important catalysts for industrial polyethylene). Free radical polymerization of ethylene and organic peroxide initiators are discussed in Chapter 2. Also in Chapter 2, hazards of organic peroxides and high pressure processes are briefly addressed. Transition metal catalysts are essential to production of nearly three quarters of all polyethylene manufactured and are described in Chapters 3, 5 and 6. Metal alkyl cocatalysts used with transition metal catalysts and their potentially hazardous reactivity with air and water are reviewed in Chapter 4. Chapter 7 gives an overview of processes used in manufacture of polyethylene and contrasts the wide range of operating conditions characteristic of each process. Chapter 8 surveys downstream aspects of polyethylene (additives, rheology, environmental issues, etc.). However, topics in Chapter 8 are complex and extensive subjects unto themselves and detailed discussions are beyond the scope of an introductory text. 

Discovery of Linear (High-Density) Polyethylene Use of Comonomers Linear Low-Density Polyethylene Stereospecific Polymerization Discovery of Polypropylene Manufacturing Processes High-Pressure LDPE Low-Pressure, Linear HOPE LLDPE... 

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