PLASTICS have developed an amazing presence in our lives. From the most commonplace tasks to our most unusual needs, plastics increasingly have provided the performance in products that consumers want. In fact, if you woke up tomorrow and there were no plastics, you would be in for quite a shock. Life would be much more expensive and much less comfortable. And many of the conveniences you had come to take for granted would be gone. Mostly, though, you would be surprised at the many products that had vanished—things you had never thought of as being plastic. That’s because, in just a few decades, consumers have come to consider the extraordinary properties of plastics as nothing out of the ordinary. Plastics’ popularity and wide usage can be attributed to one basic fact: Because of their range of properties and design technologies, plastics offer consumer benefits unsurpassed by other materials. Let’s take a look at the different types of plastics, usually referred to as “resins,” and see how they are made and used:
Plastics generally are organic high polymers (i.e., they consist of large chainlike molecules containing carbon) that are formed in a plastic state either during or after their transition from a small-molecule chemical to a solid material. Stated very simply, the large chainlike molecules are formed by hooking together short-chain molecules of chemicals (monomers: mono = one, mer = unit) in a reaction known as polymerization (poly = many). When units of a single monomer are hooked together, the resulting plastic is a homopolymer, such as polyethylene, which is made from the ethylene monomer. When more than one monomer is included in the process, for example, ethylene and propylene, the resulting plastic is a copolymer.
The two basic groups of plastic materials are the thermoplastics and the thermosets. Thermoplastic resins consist of long molecules, each of which may have side chains or groups that are not attached to other molecules (i.e., are not crosslinked). Thus, they can be repeatedly melted and solidified by heating and cooling so that any scrap generated in processing can be reused. No chemical change generally takes place during forming. Usually, thermoplastic polymers are supplied in the form of pellets, which often contain additives to enhance processing or to provide necessary characteristics in the finished product (e.g., color, conductivity, etc.). The termperature service range of thermoplastics is limited by their loss of physical strength and eventual melting at elevated temperatures.
Thermoset plastics, on the other hand, react during processing to form crosslinked structures that cannot be remelted and reprocessed. Thermoset scrap must be either discarded or used as a low-cost filler in other products. In some cases, it may be pyrolyzed to recover inorganic fillers such as glass reinforcements, which can be reused. Thermosets may be supplied in liquid form or as a partially polymerized solid molding powder. In their uncured condition, they can be formed to the finished product shape with or without pressure and polymerized by using chemicals or heat.
The distinction between thermoplastics and thermosets is not always clearly drawn. For example, thermoplastic polyethylene can be extruded as a coating for wire and subsequently crosslinked (either chemically or by irradiation) to form a thermoset material that no longer will melt when heated. Some plastic materials even have members in both families; there are, for instance, both thermoset and thermoplastic polyester resins.
Sources: Chemical Economics Handbook, SRI International, Modern Plastics Encyclopedia, Whittington’s Dictionary of Plastics, The Condensed Chemical Dictionary, The Story of the Plastics Industry (SPI).