The mass production of plastics began in 1950 and has since increased almost exponentially. In 1950, global plastic production was 1.7 million tons per year, and by 2014 it had reached 311 million tons annually. Plastic is lightweight, durable, inexpensive, and easy to mold, which is why its usage grew rapidly and continues to expand. Plastics are widely used in everyday life — almost everywhere you look, you will find something made of plastic. As new uses for plastic materials have developed and products have become more accessible, the amount of plastic entering the environment has also increased.

Plastic consists of polymers, which are large organic molecules made of repeating carbon-based units or chains. Polymers are formed when molecules called monomers join together in a process known as polymerization. Monomers act as the building blocks of polymers.

A polymer made from identical repeating monomers is called a homopolymer, while a polymer made from different types of monomers is called a copolymer. The type of monomers used determines the polymer’s fundamental properties, structure, and size.

Understanding the specific gravities of plastics is important when considering how plastic waste distributes in the ocean. The density of plastic particles relative to seawater largely determines their vertical position within the marine environment.