Process Economics Program Report 19
Linear Polyethylene and Polyethylene
Published: November 1966
Chemical Industries Ltd., holder of the basic patent (3511), together with members of the original research group, Fawcett, Gibson, Perrin, Paton, and Williams, had a continuously running pilot plant in operation by 1937. In 1938 and 1939, polyethylene was evaluated for insulation of submarine cable. Full scale production was under way in Britain in early 1942 and in the United States in 1943. During the war, most of the polyethylene was used as insulation for high-frequency cables in radar applications. The reaction for producing this polyethylene involves high pressures (1,000-3,000 atm), high temperatures (150�-230�C), and a free radical mechanism of polymerization. The product itself is a partially crystalline, thermoplastic material usually melting above 110�C. The polymer produced by the high-pressure process is not perfectly linear in that it contains about two chain branches per hundred carbon atoms. The degree of crystallinity is determined, in fact, by the number of chain branches, Crystallinities are usually in the range of 6oW-7m, and densities are usually below 0.94 g/cc for polyethylenes produced at high pressures and temperatures. Although all polyethylenes with densities below 0.94 are usually referred to as low-density resins, polyethylenes with densities in the 0.926-0.940 range are sometimes referred to as medium-density resins.
For this study, the chromium oxide catalyst was selected for process evaluation of the supported metal oxide catalyst system. The chromium oxide catalyst normally gives a product in the density range of 0.960-0.970 (3303), somewhat higher than the Ziegler product, which under some conditions is as low as 0.940. Ziegler processes can be modified to produce polymer as high in density as 0.965 (B-3, p. 365; B-4, p. 595). Chromium oxide processes can also be modified by copolymerization with small amounts of butene-l.