What Is Plastic? All plastics are polymers that are long chain-like molecules. These molecules are made up of many units called monomers repeating about thousands of times. However, different types of plastics have different types of properties. Some melt on heating, whereas some don’t. Some are transparent, whereas some are opaque. Some are rigid, whereas some are flexible. Plastics Are Classified As: 1. Thermoplastics Heating makes them soft and cooling makes them hard. This can be done many times without letting the plastic change its chemical properties. They can even be liquefied on heating. Thus, they can be molded into any shape again and again. Examples Of Thermoplastics Liquid crystalline polymers, polypropylene, polyesters, polysulfones, polyurethanes, polyethylene, polyvinyl chloride, ketones, polystyrene, and polyphenylene sulfide. 2. Thermosetting Plastics They are initially soft or in a liquid state but become hard when heated. The plastic gets cu...