Of plastics, polymers, and popcorn

It’s surprising how many plastics professionals I meet who don’t understand what these things we call “plastics” really are. That so many individuals can effectively manage, invent and develop plastics processes and products without knowledge of the intricacies of polymer chemistry would initially suggest that such knowledge is only useful to the academics and engineering types, but I disagree. A little high school chemistry can go a long way when sourcing, molding, or just talking plastics. See if you agree:

Do you need to know resin composition at the molecular level? Probably not, but a sense of the chemistry helps when faced with confusing acronyms, product data sheets, and stressed QA personnel. Most polymers are basically carbon, and when burned, produce smoke and a black residue like another well-known carbon-based material: wood. Atoms of carbon combine with more carbon and other elements, like hydrogen, to make molecules. A simple molecule of two carbon atoms connected together with two strong bonds and surrounded by four hydrogen atoms is ethylene. That ethylene monomer will lie around as a liquid or a gas unless it’s run through a process called polymerization. During polymerization, heat, pressure, and a catalyst (to speed things up) are applied to the ethylene monomer, and an interesting thing happens: one of the two bonds holding the two carbon atoms together breaks, and instead grabs hold of another neighboring ethylene to make a new polymer molecule with four carbons. And that molecule itself grabs another ethylene, then another, until the polymer molecule has thousands of ethylene monomers, making polyethylene. Substitute styrene, propylene, amide, or other monomers for ethylene, and you can see how many different resins can be created.

Why is this important? Put simply, many of the properties a product designer or processor needs to control are determined by how many monomers are linked to form that enormous polymer molecule. One way to understand the importance of this molecular weight is to think of a bowl of popcorn. If each kernel represents a single monomer molecule, say propylene, then spilling the bowl will give you popcorn everywhere, much like the gas or liquid state of the propylene monomer. Thread a string connecting the kernels, and when spilled, the popcorn will stay in a fairly solid mass. That’s polymerization, and the number of kernels threaded on each string represents the molecular weight of each polymer molecule. Link them all, and you have a solid, tough, difficult to process material. On the other hand, if you have too many short strings, i.e. very low molecular weight polymer molecules, the result is a viscous waxy mass which is equally useless. Low molecular weight polyethylene, for example, is easy to process at moderate temperatures and pressures, but it has relatively poor toughness and chemical resistance when compared to high molecular weight polyethylenes. Getting the big molecules of HMWPE to flow, however, generally requires more energy (i.e higher pressures and temperatures) which increases the risk of degradation of the polymer during processing.

Molecular weight is a good starting point, but knowing the molecular weight distribution of a polymer gives a compounder or processor a better tool for understanding how the material will perform, both in the finished product, and in the hellish environment of the screw and barrel. A narrow distribution (molecules all roughly the same size) gives a polymer excellent mechanical properties, but is more difficult to process.

The image of polymers as long chains of monomer molecules all tangled together can also illuminate other important concepts such as crystallinity, viscosity, thermal and light degradation, flammability, toxicity, and other properties, topics which I’ll tackle in future columns. Chances are, this is the all the chemistry you’ll ever need to know; just remember, it’s popcorn on a string. CPL