I vividly remember a science fiction book I read as a kid about the destruction of civilization by a new strain of bacteria. It didn’t kill people: it ate plastic. Electronic equipment disintegrated, clothes dissolved, airplanes fell apart, buildings burned–modern society ceased to function, so dependent had it become on plastic. If our ancestors lived in the Stone, Bronze and Iron Ages, we live in the Plastic Age.
Alexander Parkes invented the first synthetic plastic, celluloid, in 1856. This mixture of cellulose nitrate and camphor substituted for ivory in billiard balls, combs and piano keys. Early motion picture film was also made of celluloid, but its extreme flammability meant that more than one early movie house went up in smoke.
The second synthetic plastic, Bakelite, came along in 1909. Invented by Leo Baekeland, who simply heated a mixture of phenol and formaldehyde, it was heat-resistant, and is still used in pot handles, bottle caps and electrical hardware.
But the real boom in plastic began just before the Second World War, when nylon, vinyl, polystyrene, polyethylene and polyester all appeared.
All plastic is based on the chemistry of carbon, whose atoms can form long chains. These chains give most plastics (“most” because there are lots of exceptions) these characteristics:
1. Low strength–most plastics are about one-sixth as strong as steel.
2. Low stiffness–about one-tenth that of metals.
3. A tendency to stretch under stress.
4. Low hardness.
5. Low density–about the same as water.
6. Brittleness at low temperatures.
7. A loss of strength and hardness at moderately elevated temperatures–plastics expand about 10 times as much as metals do when heated.
8. Flammability.
9. High resistance to electricity.
10. A tendency to degrade when exposed to ultraviolet radiation.
The exceptions arise because carbon also interacts easily with other atoms. The most basic plastics are synthesized from hydrocarbons: oil, natural gas, etc. By replacing some of the hydrogen atoms in the hydrocarbon with other elements, you can create various kinds of plastics: polyvinyl chloride (PVC), for example, in which chlorine has replaced some of the hydrogen atoms.
You can get other effects by changing other aspects of the plastic. Most plastics are “thermoplastic,” that is, they soften when heated. By adding cross-links between the long molecular chains (usually oxygen atoms), however, you can create a “thermoset,” which can only be molded once, then hardens and cannot be reshaped. Teflon, which also contains flouring atoms in place of all hydrogen atoms, is a familiar cross-linked thermoset. (Teflon’s full name is polytetrafluoroethylene. I can’t imagine why they shortened it…)
Most thermoplastics are particularly vulnerable to ultraviolet radiation, which breaks apart the long carbon chains, causing brittleness. (Carbon black is usually added to polyethelyne pipe to keep ultraviolet radiation from penetrating it.)
The general softness and flexibility of plastics was overcome just before the Second World War with the invention of the first of the “engineering plastics,” nylon. Incorporating nitrogen into the long molecular chains along with carbon resulted in a strong, crystalline, abrasion-resistant material.
Polyester appeared at about the same time. We think of polyester as something ugly clothes were made out of in the ’70s, but reinforced with glass, polyesters have also been used to make large objects from boats to furniture, because their synthesis requires neither heat nor pressure. Reinforced polyester can be as strong as metal. (No surprise to anyone who ever had a pair of polyester pants that simply refused to wear out!)
Most plastics are actually composites, whether reinforced with glass or mixed with wood flour, clay filler, aluminum or various gases.
Mixing plastic with gases creates plastic foams such as Styrofoam, used not only to make coffee cups, but also as insulation or a cushioning material. Other higher-density foams can even be used as structural materials. Some foams are completely water resistant because all the gas bubbles are self-contained, while in some the bubbles run into each other, allowing the foam to absorb water like as sponge.
What does the future hold? Maybe “oriented plastics,” in which the long molecular chains are parallel to each other, instead of twisted and tangled. Ever since the 1930s, scientists have known that oriented plastics could be made stiffer and stronger than steel or ceramics, but it’s proven very hard to accomplish. The most famous oriented plastic to date is Kevlar, used to make lightweight bullet-proof vests. But although it’s stronger and stiffer than aluminum and glass-fibre composites, it’s still less stiff than steel.
But research is continuing–and considering what has already been accomplished with plastic, expect many more uses for this remarkable material to come.
The Age of Plastic is just beginning.
