Polymers

Polymers are the chemist’s contribution to the materials world. The fact that most are derived from oil (a nonrenewable resource) and the difficulty of disposing of them at the end of their life (they don’t easily degrade) has led to a view that polymers are environmental villains. There is some truth in this, but the present problems are soluble. Using oil to make polymers is a better primary use than just burning it for heat; the heat can still be recov­ered from the polymer at the end of its life. There are alternatives to oil; polymer feed stocks can be synthesized from agricultural products (notably starch and sugar, via methanol and ethanol). And thermoplastics—provided they are not contaminated—can be (and, to some extent, are) recycled.

Thermoplastics soften when heated and harden again to their origi­nal state when cooled. This allows them to be molded to complex shapes. Some are crystalline, some amorphous, some a mixture of both. Most accept coloring agents and fillers, and many can be blended to give a wide range of physical, visual, and tactile effects. Their sensitivity to sunlight is decreased by adding UV filters, and their flammability is decreased by add­ing flame retardants. The properties of thermoplastics can be controlled by chain length (measured by molecular weight), by degree of crystallinity, and by blending and plasticizing. As the molecular weight increases, the resin becomes stiffer, tougher, and more resistant to chemicals, but it is more difficult to mold. Crystalline polymers tend to have better chemical resis­tance, greater stability at high temperature, and better creep resistance than those that are amorphous. For transparency, the polymer must be amor­phous; partial crystallinity gives translucency.

Thermosets. If you are a do-it-yourself type, you have Araldite in your toolbox—two tubes, one a sticky resin, the other an even stickier hardener. Mix and warm them and they react to give a stiff, strong, durable polymer, stuck to whatever it is put on. Araldite is an epoxy resin. It typifies thermo­sets: resins that polymerize when catalyzed and heated; when reheated they do not melt, they degrade. Polyurethane thermosets are produced in the highest volume; polyesters come second; phenolics (Bakelite), epoxies, and silicones follow, and, not surprisingly, the cost rises in the same order. Once shaped, thermosets cannot be reshaped. They cannot easily be recycled.

Elastomers were originally called "rubbers" because they could rub out pencil marks, but that is the least of their many remarkable and useful properties. Unlike any other class of solid, elastomers remember their shape when they are stretched and return to it when released. This allows con- formability—hence their use for seals and gaskets. High-damping elastomers recover slowly; those with low damping snap back, returning the energy it took to stretch them—hence their use for springs, catapults, and bouncy things. Conformability gives elastomers high friction on rough surfaces, part of the reason (along with comfort) that they are used for pneumatic tires and footwear, their two largest markets.

Elastomers are thermosets; once cured, you can’t remold them or recy­cle them, a major problem with car tires. Tricks can be used to make them behave in some ways like thermoplastics (TPOs—thermoplastic elastomers, of which EVA is an example). Blending or copolymerizing elastomer mol­ecules with a thermoplastic like polypropylene (PP), if done properly, gives separated clumps of elastomer stuck together by a film of PP (Santoprene). The material behaves like an elastomer, but if heated so that the PP melts, it can be remolded and even recycled.

Profiles for 17 polymers follow, in the order in which they appear in Table 12.1.