What is plastic?
Plastic is a polymer material that is an organic compound that has the characteristics of long chains, but cannot be seen as chains with the naked eye. These chains are made up of small units called monomers. Polymers can be synthesized by the polymerization process of monomers, using petrochemical raw materials as the main source. There are many types of plastics and they can replace many natural materials, such as polyvinyl chloride (PVC) used to produce PVC pipes, polyethylene terephthalate (PET) used to produce drinking water bottles, and polystyrene (PS) used to produce various containers such as plastic spoons, etc.
There are two types of plastic:
1. (Thermoplastic)
This type of polymer has a linear or short branched molecular structure. It can be dissolved well in some solvents. When heated, it will soften and melt into a viscous liquid because the polymer molecules that are tied together can move through each other more easily when heated. When cooled, it will harden. This melting and cooling can occur back and forth without changing the chemical and physical properties or the structure of the polymer much.
This type of plastic can be molded by injection while the plastic is softened and flows with heat and pressure into a mold with a gap in the desired shape. After the plastic has flowed into the mold, it is cooled and removed from the mold. The product has the desired shape and can be used. When finished using, it can be recycled by crushing and melting with heat to form new products again. However, this type of plastic has disadvantages and limitations in use. It cannot be used at high temperatures because it may be distorted or deformed. For example, drinking water bottles are not suitable for containing very hot or boiling water.
2. (Thermosetting)
This type of polymer has a network structure, which can only be melted in the first molding step. In this step, a chemical reaction occurs to create bonds between molecules, giving the polymer a permanent shape that cannot be melted again when heated. If heated too high, the bonds between atoms in the molecules will break, resulting in a substance that does not have the properties of a polymer. The production of thermoset plastics is different from thermoplastic plastics in that in the first step, only a partial polymerization reaction is required, with some molecular linkages occurring. It can also melt when heated, so it can be molded under high pressure and temperature. When the product has the desired shape, the temperature should be maintained at around 200-300 degrees Celsius to obtain a stable and strong network structure. The product can be removed from the mold without waiting for it to cool because the product will harden inside the mold. Therefore, heating in the thermoset plastic production process hardens the material, unlike the thermoplastic plastic production process where heating softens the plastic and melts it. Thermoset plastics cannot be melted and re-made into new products or recycled after use. If heated too much, the plastic will decompose or burn without melting. Examples of plastics in this group include Bakelite and Melamine.
What happens when plastic is heated?
When plastic is heated, it exhibits two main properties:
1) Plastics will permanently harden no matter how much heat is applied to them, making them impossible to re-melt and re-form as desired. This group of plastics is called “thermosetting plastics.
2) Plastics soften when heated and harden again when cooled, allowing them to be remelted and reshaped. Plastics that exhibit these properties are called “thermoplastics.” The plastics that can be recycled are called “thermoplastics.”
