What Is Plastic? Definition in Chemistry

Understand plastic chemical composition and properties

Many of the containers you use are made from plastic, a synthetic organic polymer.
Many of the containers you use are made from plastic, a synthetic organic polymer. Image Source / Getty Images

Have you ever wondered about the chemical composition of plastic or how it is made? Here's a look a what plastic is and how it is formed.

Plastic Definition and Composition

Plastic is any synthetic or semisynthetic organic polymer. In other words, while other elements may be present, plastics always include carbon and hydrogen. While plastics may be made from just about any organic polymer, most industrial plastic is made from petrochemicals. Thermoplastics and thermosetting polymers are the two types of plastic. The name "plastic" refers to the property of plasticity, which is the ability to deform without breaking.

The polymer used to make a plastic is almost always mixed with additives, including colorants, plasticizers, stabilizers, fillers, and reinforcements. These additives affect the chemical composition, chemical properties, and mechanical properties of a plastic and also affect its cost.

Thermosets and Thermoplastics

Thermosetting polymers, also known as thermosets, solidify into a permanent shape. They are amorphous and considered to have infinite molecular weight. Thermoplastics, on the other hand, can be heated and remolded over and over again. Some thermoplastics are amorphous, while some have a partially crystalline structure. Thermoplastics typically have a molecular weight between 20,000 to 500,000 amu.

Examples of Plastics

Plastics are often referred to by the acronyms for their chemical formulae:

Polyethylene terephthalate—PET or PETE
High-density polyethylene—HDPE
Polyvinyl chloride—PVC
Low-density polyethylene—LDPE

Properties of Plastics

The properties of plastics depend on the chemical composition of the subunits, the arrangement of these subunits, and the processing method.

All plastics are polymers, but not all polymers are plastic. Plastic polymers consist of chains of linked subunits, called monomers. If identical monomers are joined, it forms a homopolymer. Difference monomers link to form copolymers. Homopolymers and copolymers may be either straight chains or branched chains.

  • Plastics are usually solids. They may be amorphous solids, crystalline solids, or semicrystalline solids (crystallites).
  • Plastics are usually poor conductors of heat and electricity. Most are insulators with a high dielectric strength.
  • Glassy polymers tend to be stiff (e.g., polystyrene). However, thin sheets of these polymers may be used as films (e.g., polyethylene).
  • Nearly all plastics display elongation when they are stressed that is not recovered after the stress is removed. This is called "creep." 
  • Plastics tend to be durable, with a slow rate of degradation.

Interesting Plastic Facts

  • The first completely synthetic plastic was bakelite, which was made in 1907 by Leo Baekeland. Baekeland also coined the word "plastics."
  • The word plastic comes from the Greek word plastikos, which means able to be shaped or molded.
  • Approximately a third of plastic that is produced is used to make packaging. Another third is used for siding and piping.
  • Pure plastics are generally insoluble in water and nontoxic. However, many of the additives in plastics are toxic and may leach into the environmental. Examples of toxic additives include phthalates. Nontoxic polymers may also degrade into chemicals when they are heated.