In order to facilitate the research and use of plastics, it is necessary to classify plastics from different perspectives. There are two common classification methods: firstly, based on the performance characteristics of plastics after heating, plastics can be divided into two categories: thermoplastic and thermosetting plastics.

(1) Thermoplastics The molecular structure of polymers in thermoplastics is linear or branched chain structure, often called linear polymers. It can be molded into a certain shape by heating, and after cooling, it maintains its fixed shape. If heated again, it can soften and melt, and can be remade into plastic parts of a certain shape, which can be repeated multiple times and has reversibility. During the above molding process, there are generally no chemical changes, only physical changes.

Due to the fact that thermoplastics are materials that can be repeatedly heated, softened, and cooled to harden, they can be repeatedly solidified and formed by heating and melting. Therefore, the waste of thermoplastics can usually be recycled and reused, known as "secondary materials".

(2) Thermosetting plastics also have a chain or dendritic structure at the beginning of heating, which also has plasticity and fusibility, and can be molded into plastic parts of a certain shape. When heating continues, chemical bond are formed between the main chains of these chain like or dendritic molecules, which gradually become a network structure (called cross-linking reaction). When the temperature rises to a certain value, the cross-linking reaction proceeds further and eventually becomes a body structure, becoming a substance that neither melts nor melts (known as curing). When reheated, the shape of the plastic part is fixed and does not change due to chemical reactions between the chains of the molecules. Plastics no longer have plasticity until they are charred and carbonized at high temperatures, making them irreversible. During the molding process, there are both physical and chemical changes. Due to the aforementioned characteristics of thermosetting plastics, the leftover materials and waste during processing cannot be recycled and reused.

Obviously, thermosetting plastics have better heat resistance than thermoplastic plastics. Commonly used phenolic, melamine formaldehyde, unsaturated polyester, etc. are all thermosetting plastics. Due to the fact that the solidified thermosetting plastic cannot change its state even if it continues to be heated, it cannot become molten again. Therefore, thermosetting plastics cannot be repeatedly formed by reheating, so the waste of thermosetting plastics is usually not recyclable. Secondly, according to the specific usage occasions and characteristics of plastics, they can generally be divided into three categories: general plastics, engineering plastics, and special plastics.

(1) General plastics General plastics generally refer to a type of plastic with large production capacity, wide use, relatively low performance, and low price, such as polyethylene, polypropylene, polyvinyl chloride, polystyrene, phenolic plastics, amino plastics, etc., which account for about 60% of the total plastic production.

(2) Engineering plastics Engineering plastics refer to the plastics that can be used as structural materials. There is no obvious boundary between engineering plastics and general plastics. Engineering plastics have good strength, impact resistance, heat resistance, hardness and aging resistance, and can replace some metal materials as engineering materials, such as nylon, polycarbonate, polyformaldehyde, ABS, etc.

(3) Special plastics refer to plastics that have special functions and are suitable for a particular occasion, mainly including medical plastics, photosensitive plastics, magnetic conductive plastics, super conductive plastics, radiation resistant plastics, high-temperature resistant plastics, etc. Its main component is resin, some of which are specially synthesized resins, while others obtain special properties through special treatment or modification using the above-mentioned resins for general plastics and engineering plastics. This type of plastic has a small production capacity, excellent performance, and high price.