Plastic is a polymer organic material with resin as its main component, which can be molded into a certain shape under a certain temperature and pressure, and can maintain a certain shape at room temperature.
Resin refers to an organic polymer that usually has a transformation or melting range when heated. It is fluid under action of external force during transformation and is solid, semi-solid or liquid at room temperature. It is the most basic and important component of plastics. Broadly speaking, any polymer that is basic material of plastics in plastics industry can be called a resin.

 

There is currently no exact classification of plastics, but general classification is as follows:
1. Thermoplastics are classified according to their physical and chemical properties: plastics that can be softened by repeated heating and hardened by cooling within a specific temperature range, such as polyethylene plastics and polyvinyl chloride plastics. Thermosetting plastics: Plastics that can solidify into infusible and insoluble materials due to heat or other conditions, such as phenolic plastics, epoxy plastics, etc.
2. General plastics are classified according to plastic use: generally refers to plastics with large output, wide range of uses, good formability and low price, such as polyethylene, polypropylene, polyvinyl chloride, etc. Engineering plastics: generally refers to plastics that can withstand certain external forces, have good mechanical properties and dimensional stability, can still maintain their excellent properties at high and low temperatures, and can be used as engineering structural parts, such as ABS, nylon, polyvinyl, etc.
Special plastics: generally refers to plastics with special functions (such as heat resistance, self-lubricating, etc.) and used in special requirements, such as fluoroplastics, silicone, etc.
3. According to plastic molding method, molded plastics are divided into: resin mixture for molding, such as general thermosetting plastics. Laminated plastic: refers to fiber fabrics impregnated with resin, which can be combined by lamination and hot pressing to become a whole material. Injection, extrusion and blow molding plastics: generally refers to resin mixes that can melt and flow at barrel temperature and harden quickly in mold, such as general thermoplastics.
Casting plastic: A liquid resin mixture that can be poured into a mold without pressure or with a little pressure and hardened into a product of a certain shape, such as MC nylon. Reaction injection molding compound: generally refers to liquid raw materials that are injected into mold cavity under pressure to react and solidify to produce finished products, such as polyurethane.
4. Molding powder is divided into plastic semi-finished products and finished products: also known as plastic powder, it is mainly obtained by fully mixing, pressing and crushing thermosetting resins (such as phenolic resin) and fillers, such as phenolic plastic powder. Reinforced plastic: A type of plastic that has reinforced materials and some mechanical properties are greatly improved compared to original resin. Foam plastic: A plastic with countless micropores inside the whole body. Film: generally refers to a flat and soft plastic product with a thickness of less than 0.25 mm.

1. Light weight and high specific strength. Plastic is light in weight. Density of general plastics is between 0.9 ~ 2.3 g/cm3, which is only 1/8 ~ 1/4 of steel and about 1/2 of aluminum. Density of various foam plastics is even lower, about Between 0.01 ~ 0.5 g/cm3.
Strength calculated per unit mass is called specific strength. Specific strength of some reinforced plastics is close to or even exceeds that of steel. For example, tensile strength of alloy steel per unit mass is 160 MPa, while plastic reinforced with glass fiber can reach 170 ~ 400 MPa.
2. Excellent electrical insulation properties. Almost all plastics have excellent electrical insulation properties, such as minimal dielectric loss and excellent arc resistance, which are comparable to ceramics.
3. Excellent chemical stability. Generally, plastics have good corrosion resistance to chemicals such as acids and alkalis. In particular, polytetrafluoroethylene has better chemical corrosion resistance than gold, and can even withstand corrosion of highly corrosive electrolytes such as aqua regia, making it known as "King of Plastics."
4. Good friction reduction and wear resistance. Most plastics have excellent friction-reducing, wear-resistant and self-lubricating properties. Many friction-resistant parts made of engineering plastics take advantage of these characteristics of plastics. When certain solid lubricants and fillers are added to wear-resistant plastics, their friction coefficients can be reduced or their wear-resistant properties can be further improved.
5. Light transmission and protective properties. Most plastics are available as transparent or translucent products, with polystyrene and acrylic plastics being as transparent as glass. Chemical name of organic glass is polymethyl methacrylate, which can be used as aviation glass material.
Plastic films such as polyvinyl chloride, polyethylene, polypropylene have good light transmission and warmth retention properties and are widely used as agricultural films. Plastic has a variety of protective properties, so it is often used as protective packaging products, such as plastic films, boxes, barrels, bottles, etc.

 

6. Excellent shock absorption and noise reduction performance. Some plastics are flexible and elastic. When they are subjected to frequent mechanical impact and vibration from outside world, viscous internal friction occurs inside, converting mechanical energy into thermal energy. Therefore, they are used as shock-absorbing and sound-absorbing materials in engineering. For example, bearings and teeth made of engineering plastics can reduce noise, various foam plastics are widely used as excellent shock-absorbing and sound-absorbing materials.

Excellent properties of above-mentioned plastics make it widely used in industrial and agricultural production and people's daily life; it has been used as a substitute for materials such as metal, glass, ceramics, wood and fiber in the past, has become an indispensable material in modern life and cutting-edge industry.
However, plastic also has its drawbacks. For example, heat resistance is worse than that of metals and other materials. Generally, plastics can only be used at temperatures below 100℃, and a few can be used at temperatures around 200℃;
Thermal expansion coefficient of plastic is 3 to 10 times larger than that of metal, and its dimensional stability is easily affected by temperature changes. Under load, plastic will slowly produce viscous flow or deformation, that is, creep phenomenon; in addition, plastics will age under influence of atmosphere, sunlight, long-term pressure or certain substances, causing performance deterioration.
These shortcomings of plastic more or less affect or limit its application. However, with development of plastics industry and in-depth research on plastic materials, these shortcomings are gradually being overcome, novel plastics and various plastic composite materials with excellent properties are constantly emerging.

Plastics have been widely used in various fields such as agriculture, industry, construction, packaging, defense cutting-edge industries, and people's daily lives. Agriculture: A large amount of plastics are used to make mulch films, seedling-raising films, greenhouse films, drainage and irrigation pipes, fish nets, breeding floats, etc. Industrial aspect: Plastics are widely used in electrical and electronics industry to make insulation materials and packaging materials;
In machinery industry, plastics are used to make transmission gears, bearings, bearings and many parts to replace metal products; in chemical industry, plastics are used to make pipes, various containers and other anti-corrosion materials; in construction industry, plastics are used to make doors and windows, stair handrails, floor tiles, ceilings, thermal and sound insulation panels, wallpaper, downspout fittings and pit pipes, decorative panels and sanitary ware, etc.
In defense industry and cutting-edge technology, whether it is conventional weapons, aircraft, ships, rockets, missiles, satellites, spacecraft and atomic energy industries, plastics are indispensable materials. In people's daily life, plastics are widely used, such as plastic sandals, slippers, raincoats, handbags, children's toys, toothbrushes, soap dishes, thermos shells, etc. sold on the market.
At present, it has also been widely used in various household appliances, such as televisions, cassette players, electric fans, washing machines, refrigerators, etc. As a new type of packaging material, plastic has been widely used in the field of packaging, such as various hollow containers, injection molded containers (turnover boxes, containers, barrels, etc.), packaging films, woven bags, corrugated boxes, foam plastics, strapping ropes and strapping tapes.

As early as the 19th century, people had been using natural resins such as asphalt, rosin, amber, and shellac. In 1868, natural cellulose was nitrated and camphor was used as a plasticizer to create the world's first plastic variety, called celluloid. This began history of human use of plastics.
In 1909, the first synthetic plastic - phenolic plastic - appeared. In 1920, another synthetic plastic-aminoplastic (aniline formaldehyde plastic) was born. These two plastics played an active role in promoting development of electrical industry and instrument manufacturing industry at that time.
By the 1920s and 1930s, plastics such as alkyd resin, polyvinyl chloride, acrylics, polystyrene, and polyamine appeared one after another. From the 1940s to present, with development of science, technology and industry, extensive development and utilization of petroleum resources, plastics industry has developed rapidly. In terms of varieties, polyethylene, polypropylene, unsaturated polyester, fluoroplastics, epoxy resin, polyformaldehyde, polycarbonate, polyimide, etc. have appeared.