3. Classification of plastics
Classify according to application characteristics, physical and chemical characteristics, and processing methods

3.1 Classification by application characteristics
According to the different application characteristics of different types of plastics, plastics are usually divided into three types: general plastics, engineering plastics, and special plastics.

① Universal plastics
Usually refers to plastics with high production capacity, wide applicability, good formability, and low price, such as polyethylene, polypropylene, phenolic aldehyde, etc. PP, HDPE, LDPE, PVC, and PS are known as the five universal plastics.

② Engineering plastics
Usually, fats can withstand certain external forces, have good mechanical properties and resistance to high and low temperatures, have good scale stability, and can be used as engineering plastics, such as ABS, polyamide, polysulfone, etc. In engineering plastics, they are further divided into two categories: general engineering plastics and special engineering plastics.

General engineering plastics include: polyamide, polyoxymethylene, polycarbonate, modified polyphenylene ether, thermoplastic polyester, ultra-high molecular weight polyethylene, methylpentene polymer, ethylene alcohol copolymer, etc. Nylon, polyester, polyoxymethylene, polycarbonate, and modified polyphenylene ether are known as the five major engineering plastics

Special engineering plastics can be divided into cross-linked and non cross-linked types. Crosslinked types include: polyamino bismaleimide, polytriazine, cross-linked polyimide, heat-resistant epoxy resin, etc. Non crosslinked types include: polysulfone, polyethersulfone, polyphenylene sulfide, polyimide, polyether ether ketone (PEEK), etc

③ Special plastics
Usually refers to plastics with special functions that can be used in special application fields such as aviation and aerospace. For example, fluoroplastics and organosilicon have outstanding high temperature resistance, self-lubricating and other special functions, and reinforced plastics and foam plastics have high strength, high cushioning and other special functions. These plastics belong to the field of special plastics.

a. Reinforced plastics: Reinforced plastic materials can be divided into three types in appearance: granular (such as calcium plastic reinforced plastics), fibrous (such as glass fiber or glass cloth reinforced plastics), and sheet-like (such as mica reinforced plastics). According to the raw materials, it can be divided into three types: cloth based reinforced plastics (such as rag reinforced or asbestos reinforced plastics), inorganic mineral filled plastics (such as quartz or mica filled plastics), and fiber reinforced plastics (such as carbon fiber reinforced plastics).

b. Foam plastic: Foam plastic can be divided into hard, semi hard and soft foam plastic. Rigid foamed plastic has no flexibility and great shrinkage hardness. It will not deform until it reaches a certain stress value. It cannot be restored after stress relief; Soft foamed plastic has flexibility, low shrinkage hardness and simple deformation. It can be restored after stress relief, and the residual deformation is small; The flexibility and other functions of semi-rigid foams are between those of rigid and soft foams.

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