Plastics are everywhere. While you’re reading this article, there are probably numerous plastic items within your reach (your computer, your pen, your phone). A plastic is any material that can be shaped or molded into any form — some are naturally occurring, but most are man-made.
Plastics are made from oil. Oil is a carbon-rich raw material, and plastics are large carbon-containing compounds. They’re large molecules called polymers, which are composed of repeating units of shorter carbon-containing compounds called monomers. Chemists combine various types of monomers in many different arrangements to make an almost infinite variety of plastics with different chemical properties. Most plastic is chemically inert and will not react chemically with other substances — you can store alcohol, soap, water, acid or gasoline in a plastic container without dissolving the container itself. Plastic can be molded into an almost infinite variety of shapes, so you can find it in toys, cups, bottles, utensils, wiring, cars, even in bubble gum. Plastics have revolutionized the world.
Because plastic doesn’t react chemically with most other substances, it doesn’t decay. Therefore, plastic disposal poses a difficult and significant environmental problem. Plastic hangs around in the environment for centuries, so recycling is the best method of disposal. However, new technologies are being developed to make plastic from biological substances like corn oil. These types of plastics would be biodegradable and better for the environment.
In this article, we’ll examine the chemistry of plastic, how it’s made, how it’s used, and how it’s disposed of and recycled. We’ll also look at some new biologically based plastics and their role in the future of plastic.
Before the invention of plastic, the only substances that could be molded were clay (pottery) and glass. Hardened clay and glass were used for storage, but they were heavy and brittle. Some natural substances, like tree gums and rubber, were sticky and mold-able. Rubber wasn’t very useful for storage because it eventually lost its ability to bounce back into shape and became sticky when heated.
In 1839, Charles Goodyear accidentally discovered a process in which sulfur reacted with crude rubber when heated and then cooled. The rubber became resilient upon cooling — it could stretch, but it snapped back to its original shape. It also retained its resilience when heated. We now know that the sulfur forms chemical bonds between adjacent rubber polymer strands. The bonds cross-link the polymer strands, allowing them to “snap back” when stretched. Charles Goodyear had discovered the process now known as vulcanization, which made rubber more durable.
In 1846, Charles Schonbein, a Swiss chemist, accidentally discovered another polymer when he spilled a nitric acid-sulfuric acid mixture on some cotton. A chemical reaction occurred in which the hydroxyl groups of the cellulose fibers in the cotton were converted to nitrate groups catalyzed by the sulfur. The resultant polymer, nitrocellulose, could burst into a smokeless flame and was used by the military in place of gunpowder. In 1870, chemist John Hyatt reacted nitrocellulose with camphor to make celluloid, a plastic polymer that was used in photographic film, billiard balls, dental plates and Ping-Pong balls.
In 1909, a chemist named Leo Baekeland synthesized Bakelite, the first truly synthetic polymer, from a mixture of phenol and formaldehyde. The condensation reaction between these monomers allows the formaldehyde to bind the phenol rings into rigid three-dimensional polymers. So, Bakelite can be molded when hot and solidified into a hard plastic that can be used for handles, phones, auto parts, furniture and even jewelry. Bakelite is hard, resistant to heat and electricity, and can’t be easily melted or scorched once cooled. The invention of Bakelite led to a whole class of plastics with similar properties, known as phenolic resins.
In the 1930s, a Dupont chemist named Wallace Carruthers invented a plastic polymer made from the condensation of adipic acid and a certain type of diaminohexane monomers that could be drawn out into strong fibers, like silk. This plastic became known as nylon. Nylon is lightweight, strong and durable and became the basis of many types of clothing, coverings (tents), luggage, bags and ropes.
The use of these early polymers became widespread following World War II and continues today. They lead to the creation of many other plastics, like Dacron, Styrofoam, polystyrene, polyethylene and vinyl.
In the next two sections, we’ll learn about the chemistry of plastic.