The term “plastics” includes materials composed of various elements such as carbon, hydrogen, oxygen, nitrogen, chlorine, and sulfur. Plastics typically have high molecular weight, meaning each molecule can have thousands of atoms bound together. Naturally occurring materials, such as wood, horn and rosin, are also composed of molecules of high molecular weight. The manufactured or synthetic plastics are often designed to mimic the properties of natural materials. Plastics, also called polymers, are produced by the conversion of natural products or by the synthesis from primary chemicals generally coming from oil, natural gas, or coal.
Most plastics are based on the carbon atom. Silicones, which are based on the silicon atom, are an exception. The carbon atom can link to other atoms with up to four chemical bonds. When all of the bonds are to other carbon atoms, diamonds or graphite or carbon black soot may result. For plastics the carbon atoms are also connected to the aforementioned hydrogen, oxygen, nitrogen, chlorine, or sulfur. When the connections of atoms result in long chains, like pearls on a string of pearls, the polymer is called a thermoplastic. Thermoplastics are characterized by being meltable. The thermoplastics all have repeat units, the smallest section of the chain that is identical. We call these repeat units unit cells. The vast majority of plastics, about 92%, are thermoplastics1.
The groups of atoms that are used to make unit cells are called monomers. For some plastics, such as polyethylene, the repeat unit can be just one carbon atom and two hydrogen atoms. For other plastics, such as nylons, the repeat unit can involve 38 or more atoms. When we combine monomers, we generate polymers or plastics. Raw materials form monomers that can be or are used to form unit cells. Monomers are used form polymers or plastics
When the connection of the carbon atoms forms two and three-dimensional networks instead of one-dimension chains, the polymer will be a thermoset plastic. Thermoset plastics are characterized by not being meltable. Thermoset plastics, such as epoxy adhesives or unsaturated polyester boat hulls and bathtubs or the phenolic adhesives used to make plywood, are created by the user mixing two chemicals and immediately using the mixture before the plastic “sets up” or cures.
The formation of the repeat units for thermoplastics usually begins with the formation of small carbon-based molecules that can be combined to form monomers. The monomers, in turn, are joined together by chemical polymerization mechanisms to form polymers. The raw material formation may begin by separating the hydrocarbon chemicals from natural gas, petroleum, or coal into pure streams of chemicals. Some are then processed in a “cracking process.” Here, in the presence of a catalyst, raw materials molecules are converted into monomers such as ethylene (ethene) C2H4, propylene (propene) C3H6, and butene C4H8 and others. All of these monomers contain double bonds between carbon atoms such that the carbon atoms can subsequently react to form polymers.
Other raw material chemicals are isolated from petroleum, such as benzene and xylenes. These chemicals are reacted with others to form the monomers for polystyrene, nylons, and polyesters. The raw materials have been changed into monomers and no longer contain the petroleum fractions. Still other raw materials can be obtained from renewable resources, such as cellulose from wood to make cellulose butyrate. For the polymerization step to work efficiently, the monomers must be very pure. All manufacturers purify raw materials and monomers, capturing unused raw materials for reuse and byproducts for proper disposition.