A thermoplastic (sometimes written as thermo plastic) is a type of plastic made from polymer resins that becomes a homogenized liquid when heated and hard when cooled. When frozen, however, a thermoplastic becomes glass-like and subject to fracture. These characteristics, which lend the material its name, are reversible. That is, it can be reheated, reshaped, and frozen repeatedly. This quality also makes thermoplastics recyclable.
There are dozens of kinds of thermoplastics, with each type varying in crystalline organization and density. Some types that are commonly produced today are polyurethane, polypropylene, polycarbonate, and acrylic. Celluloid, which is considered the first thermoplastic, made its appearance in the mid-1800s and reigned in the industry for approximately 100 years. During its peak production, it was used as a substitute for ivory. Today, it is used to make guitar picks.
Sometimes, thermoplastics are confused with thermosetting plastics. Although they may sound the same, they actually possess very different properties. While thermoplastics can be melted to a liquid and cooled to a solid, thermosetting plastics chemically deteriorate when subjected to heat. Ironically, however, thermosetting plastics tend to be more durable when allowed to cool than many thermoplastics.
Thermoplastics also differ from elastomers, even though some are considered both. While many thermoplastics can be stretched to a point, they generally tend to both resist, and stay in the shape they are stretched to. Elastomers, as the name suggests, bounce back. However, the addition of plasticizers to the melt can render a more pliable thermoplastic. In fact, this is usually the case when a thermoplastic is being used for plastic injection molding or extrusion.
The specific action of a plasticizer is to lower the material’s glass transition temperature (Tg), which is the point it becomes brittle when cooled and soft when heated. Tg varies with each type of thermoplastic and is dictated by its crystallization structure. However, Tg can also be adjusted by introducing a thermoplastic into a copolymer, such as polystyrene. Until the use of plasticizers, some molded thermoplastic parts were prone to crack in cold weather.
Thermoplastics have been around for a long time, but are a huge component of everyday life today. For example, acrylonitrile butadiene styrene (ABS) is a type of thermoplastic used to manufacture sports equipment, toys (i.e., LEGO® blocks), and various automobile parts. Polycarbonate is used to make compact discs (CDs), drinking bottles, food storage containers, and eyeglass lenses, among other things. Polyethylene is likely the most commonly encountered thermoplastic and is used to make shampoo bottles, plastic grocery bags, and even bullet proof vests.