Plastic is an important organic synthetic polymer material with a wide range of applications. However, "white pollution" caused by waste plastics is becoming more and more serious. If we can understand composition and classification of plastics in detail, it will not only help us use plastic products scientifically, but also facilitate classification and recycling of plastics, and effectively control and reduce "White pollution".
According to different use characteristics of various plastics, plastics are usually divided into three types: general plastics, engineering plastics and special plastics.

 

Generally refers to plastics with large output, wide use, good formability, and low price. There are five types of general plastic pellets, namely polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS) and acrylonitrile-butadiene-styrene copolymer (ABS) .
These five types of plastics account for vast majority of plastic raw materials, and the rest can basically be classified into special plastic varieties, such as: PPS, PPO, PA, PC, POM, etc., they are rarely used in daily life products, are mainly used in high-end fields such as engineering industry and national defense technology, such as automobiles, aerospace, construction, and communications.
According to its plasticity classification, plastics can be divided into thermoplastics and thermosetting plastics. Under normal circumstances, thermoplastic products can be recycled, while thermosetting plastics cannot. According to optical properties of plastics, they can be divided into transparent, translucent and opaque raw materials。 For example, PS, PMMA, AS, PC, etc. are transparent plastics, while most other plastics are opaque plastics.

1. Polyethylene: Commonly used polyethylene can be divided into low-density polyethylene (LDPE), high-density polyethylene (HDPE) and linear low-density polyethylene (LLDPE). Among three, HDPE has better thermal, electrical and mechanical properties, while LDPE and LLDPE have better flexibility, impact properties, film-forming properties, etc. LDPE and LLDPE are mainly used in packaging films, agricultural films, plastic modification, etc., while HDPE has a wide range of uses, such as films, pipes, and injection daily necessities.
2. Polypropylene: Relatively speaking, polypropylene has more varieties, more complex uses, and various fields. Varieties mainly include homopolymer polypropylene (homopp), block copolymer polypropylene (copp) and random copolymer polypropylene (rapp). According to different uses, homopolymerization is mainly used in the fields of wire drawing, fiber, injection, BOPP film, etc. Copolymer polypropylene is mainly used in household appliances injection parts, modified raw materials, daily injection products, pipes, etc. Atactic polypropylene is mainly used for transparent products, high-performance products, high-performance pipes, etc.
3. Polyvinyl chloride: Because of its low cost and self-flame retardant properties, it has a wide range of uses in construction field, especially for sewer pipes, plastic steel doors and windows, plates, and artificial leather.
4. Polystyrene: As a kind of transparent raw material, it has a wide range of uses when there is a need for transparency, such as automobile lampshades, daily transparent parts, transparent cups, cans, etc.
5. ABS: It is an engineering plastic with a wide range of uses, with outstanding physical mechanical and thermal properties. It is widely used in household appliances, panels, masks, assemblies, accessories, etc., especially household appliances, such as washing machines, air conditioners, refrigerators, electric fans, etc., with a huge amount of use. In addition, it has a wide range of uses in plastic modification.

Generally refers to plastics that can withstand a certain external force, have good mechanical properties, high and low temperature resistance, have good dimensional stability, can be used as engineering structures, such as polyamide and polysulfone. In engineering plastics, it is divided into two categories: general engineering plastics and special engineering plastics.
Engineering plastics can meet higher requirements in terms of mechanical properties, durability, corrosion resistance, heat resistance, etc., are more convenient to process and can replace metal materials. Engineering plastics are widely used in electrical and electronic, automotive, construction, office equipment, machinery, aerospace and other industries. Substituting plastic for steel and plastic for wood has become an international trend.
General engineering plastics include: polyamide, polyoxymethylene, polycarbonate, modified polyphenylene ether, thermoplastic polyester, ultra-high molecular weight polyethylene, methylpentene polymer, vinyl alcohol copolymer, etc.
Special engineering plastics are divided into cross-linked and non-cross-linked types. Cross-linked types are: polyamino bismaleamide, polytriazine, cross-linked polyimide, heat-resistant epoxy resin and so on. Non-crosslinked types are: polysulfone, polyethersulfone, polyphenylene sulfide, polyimide, polyether ether ketone (PEEK) and so on.

 

Generally refers to plastics that have special functions and can be used in special applications such as aviation and aerospace. For example, fluoroplastics and silicones have outstanding high temperature resistance, self-lubricating and other special functions, reinforced plastics and foamed plastics have special properties such as high strength and high cushioning. These plastics belong to category of special plastics.

Reinforced plastic raw materials can be divided into granular (such as calcium plastic reinforced plastic), fiber (such as glass fiber or glass cloth reinforced plastic), and flake (such as mica reinforced plastic) in appearance. According to material, it can be divided into cloth-based reinforced plastics (such as rag reinforced or asbestos reinforced plastics), inorganic mineral filled plastics (such as quartz or mica filled plastics), fiber reinforced plastics (such as carbon fiber reinforced plastics).

Foam plastics can be divided into three types: rigid, semi-rigid and flexible foams. Rigid foam has no flexibility, and its compression hardness is very large. It will deform only when it reaches a certain stress value and cannot return to its original state after stress is relieved. Flexible foam is flexible, with low compression hardness, and is easy to deform. It can return to its original shape after stress is relieved, and residual deformation is small. Flexibility and other properties of semi-rigid foam are between rigid and soft foams.
In addition, according to different physical and chemical properties of various plastics, plastics can be divided into two types: thermosetting plastics and thermoplastics.
1. Thermoplastics (Thermo plastics): refers to plastics that will melt after heating, can flow to mold after cooling, then be formed, and will melt after heating; heating and cooling can be used to produce reversible changes (liquid ←→solid) , which is so-called physical change.
2. Thermosetting plastics refer to plastics that can be cured under heat or other conditions or have insoluble (melting) characteristics, such as phenolic plastics, epoxy plastics, etc. Thermosetting plastics are divided into formaldehyde cross-linked type and other cross-linked types. After thermal processing and molding, an infusible and insoluble solidified product is formed, resin molecules are cross-linked from a linear structure into a network structure, then heat will be decomposed and destroyed.