Canadian Plastics

Polypropylene: What a difference an atom makes

By Jim Anderton, technical editor   



Last month I touched on the basics of the simplest of thermoplastics, polyethylene. I like the versatility of polyethylene, but I love the simplicity of its chemistry. PE is nothing but a long chain o...

Last month I touched on the basics of the simplest of thermoplastics, polyethylene. I like the versatility of polyethylene, but I love the simplicity of its chemistry. PE is nothing but a long chain of carbon atoms, like a string of pearls in a bowl.

To understand a little about polypropylene, consider the PE structure. Carbon atoms like to hook up to their two nearest neighbors to form the backbone chain of the polymer, but in fact carbon would prefer to attach to four neighboring atoms. Hydrogen is a favorite partner. Hydrogen is a small atom, so it essentially stays out of the way when the polymer chains fold.

So what does this have to do with polypropylene? Every processor knows PP is a very different material from PE. But if you could see it at the atomic level, there would be only one difference: Instead of a couple of hydrogens hanging off the two spare sites on each carbon atom in the chain, one hydrogen is replaced by another carbon with three hydrogens hanging on to it, rather like the charms on a charm bracelet.

A chemist might call it a “pendant methyl group”, but that dangling carbon is more than just decoration. It alters the way the tangled mass of polymer chains interact, altering the properties of the resin. It turns out that the presence of the extra carbon “methyl groups” is important, but just as important is where they’re attached. They can line up all on one side (isotactic), alternately on either side (syndiotactic) or just randomly (atactic). Completely isotactic PP is very crystalline, while atactic resin is amorphous. Common commercial grades are 50 to 60% crystalline. Crystallinity controls many properties of thermoplastics, but in the case of polypropylene, one consequence of a high crystallinity level is very important: high-temperature stability. Completely isotactic PP melts at 175C, which means that polypropylene dinnerware can survive a trip through the dishwasher, whereas a similar polyethylene part would warp.

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Syndiotactic PP from Dow is just coming on-stream, promising high-level crystallinity, clarity, room temperature impact strength, and a controllable melting point.

There’s current research into using metallocenes to create PP with “blocks” of isotactic and atactic regions on the same atomic backbone. The atactic blocks would give soft, rubbery properties, while the isotactic regions would tie the atactic regions together giving the mass cohesion and strength. The result? Polypropylene thermoplastic elastomers, combining the low cost of PP with the market potential of TPEs.

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