Sometimes it's a little more complicated. Sometimes some of the carbons,
instead of having hydrogens attached to them, will
have long chains or branches of polyethylene attached to them. This is called
branched, or low-density polyethylene, or LDPE. When there is
no branching, it is called linear polyethylene, or HDPE. Linear
polyethylene is much stronger than branched polyethylene, but
branched polyethylene is cheaper and easier to make. It is also more flexible and works great for sandwich wrap.
Linear polyethylene is normally produced with molecular weights in the range of 200,000 to 500,000, but it can be made even higher. Polyethylene with molecular weights of three to six million is referred to as ultra-high molecular weight polyethylene, or UHMWPE. UHMWPE can be used to make fibers which are so strong they replaced Kevlar for use in bullet proof vests. Large sheets of it can be used instead of ice for skating rinks.
Polyethylene is vinyl polymer, made from the monomer ethylene. Here's a model of the ethylene monomer. It looks like some sort of four-legged headless animal if you ask me.
Branched polyethylene is often made by free radical vinyl polymerization. Linear polyethylene is made by a more complicated procedure called Ziegler-Natta polymerization. UHMWPE is made using metallocene catalysis polymerization.
But Ziegler-Natta polymerization can be used to make LDPE, too. By copolymerizing ethylene monomer with a alkyl-branched comonomer one gets a copolymer which has short hydrocarbon branches. Copolymers like this are called linear low-density polyethylene, or LLDPE. BP produces LLDPE using a comonomer with the catchy name 4-methyl-1-pentene, and sells it under the trade name Innovex¨. LLDPE is often used to make things like plastic films.
|Other polymers used as plastics include:||Other polymers used as fibers include:|
|Polycarbonate||Kevlar and Nomex|
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