Polyethylene was first synthesized in 1922 by the British company ICI. In 1933, the British company Bnemen Chemical Industries discovered that ethylene can polymerize under high pressure to form polyethylene. This method was industrialized in 1939 and is commonly known as the high-pressure method. In 1953, K. Ziegler of the Federal Republic of Germany discovered that with TiCl4-Al (C2H5)3 as a catalyst, ethylene can also be polymerized at lower pressures. This method was put into industrial production by the Federal German company Hoechst in 1955, commonly known as low-pressure polyethylene. In the early 50s, Philip Oil Company of the United States discovered that using chromium oxide-silica aluminum gel as a catalyst, ethylene can polymerize under medium pressure to form high-density polyethylene, and achieved industrial production in 1957. In the 60s, DuPont of Canada began to make low-density polyethylene from ethylene and α-olefins in solution. In 1977, the United States Union Carbide Company and Dow Chemical Company successively used the low-pressure method to make low-density polyethylene, called linear low-density polyethylene, of which the gas phase method of Union Carbide Company was the most important. Linear low-density polyethylene has similar properties to low-density polyethylene, but combines some of the characteristics of high-density polyethylene, coupled with low energy consumption in production, so it has developed extremely rapidly and has become one of the most interesting new synthetic resins.
The core technology of the low-pressure method lies in the catalyst. The TiCl4-Al (C2H5)3 system invented by Ziegler in Germany is the first generation of polyolefin catalysts, with low catalytic efficiency, and about thousands of grams of polyethylene per gram of titanium. In 1963, Belgian Solvay Company pioneered the second generation catalyst with magnesium compound as the carrier, with a catalytic efficiency of tens of thousands to hundreds of thousands of grams of polyethylene per gram of titanium. The use of second-generation catalysts also eliminates the need for post-treatment to remove catalyst residues. Later, high-efficiency catalysts by gas phase were developed. In 1975, the Italian Monte Edison Group Company developed a catalyst that can directly produce spherical polyethylene without granulation, known as the third generation catalyst, which is another change in the production of high-density polyethylene.
