The melting point of plastic depends on the polymer makeup of the plastic, and it can be low to high. The temperature at which a plastic melts is called the melt transition temperature because it is not defined.
Plastics are polymers, or large molecules that are made up of many subunits. A polymer is a chain of the units carbon, hydrogen, oxygen and sometimes silicon. These components are polymerized together or "joined" together. Natural polymers can be found as tar, shellac, tortoise horns, tree sap amber and even latex. These polymers were processed and made into materials such as vulcanized rubber, celluloid and gun cotton.
When the world went to war in World War II, the supply of natural polymers was cut off and many countries, including the United States, were forced to create their own polymers through synthetic polymers. This is when acrylic, neoprene, styrene-butadiene rubber (SBR) and polyethylene came into existence.
There are two different polymer or plastic groups: thermoplastics and thermosets. Most plastics are thermoplastic. This means that they can be heated over and over again once the polymers have formed and are set. If thermoset plastics are reheated, the material will begin to scorch. All polymers have their own unique characteristics based on the way that they were made and if they were combined with any other materials.