Manufacture of Polyethylenes

Ethylene for the manufacture of polyethylene is derived from cracking various components of petroleum oil such as the gasoline fraction, gas oil, or from hydrocarbons such as ethane. While petroleum remains the predominant source of the monomer at the present time, it can also be produced using biomass. In fact ethylene has been commercially derived from molasses, a by-product of sugar cane industry, via the dehydration of ethanol.

The polymerization of ethylene might be carried out in solution or in a slurry process. But these processes are complicated by the need for a separation step to isolate the resin product from solution. The newer installations favor the gasphase process that can produce both the low- and high-density resins. Older plants lack this versatility and are able to produce only either the high-density or the low-density type of polyethylene.

In the older process, LDPE resin was produced under high pressure (15,000-22,500 psi at 100°C-300°C) in stirred autoclave or tubular-type reactors, where the liquefied ethylene gas is polymerized via a free radical reaction initiated by peroxide or by oxygen.

The reaction is highly exothermic (22 kcal/mol) and therefore requires careful control of the temperature, especially in autoclave reactors. The product generally has a high level of long chain branching from chain transfer to polymer. Short-chain branches are methyl or alkyl groups formed by the active growing chain end abstracting a hydrogen atom from another part of the chain via "back-biting" reactions. As illustrated below, the back-biting process involves hydrogen abstraction by the growing macroradical chain to yield a short (four-carbon) branch on the main chain. Such transfer of the macroradical can also occur with a separate polymer chain in the mix, yielding to longer branches typically found in high-pressure polyethylenes. The tubular reactors yielded a product with a relatively higher amount of chain branching compared to the resin made in autoclave reactors. The molecular weight of the product is determined by the reaction conditions employed:

~ch2

" CH2 CH2

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