I. Definition of plastics
Plastic is a polymer organic material with resin as its main component, which can be molded into a certain shape at a certain temperature and pressure, and can keep a certain shape at normal temperature.
Resin refers to an organic polymer which usually has a range of transformation or melting when heated, has fluidity when transformed by external force, and is solid or semi-solid or liquid at normal temperature. It is the most basic and important component of plastics. Broadly speaking, any polymer which is the basic material of plastics in the plastics industry can be called resin.
Second, the classification of plastics
At present, there is no exact classification of plastics, and the general classification is as follows:
1. According to the physical and chemical properties of plastics, thermoplastic plastics: plastics that can be heated, softened and cooled and hardened repeatedly within a specific temperature range. Such as polyethylene plastic and polyvinyl chloride plastic. Thermosetting plastics: plastics that can be solidified into insoluble materials due to heat or other conditions. Such as phenolic plastics, epoxy plastics, etc.
2. Classify general plastics according to their use:-general refers to plastics with large output, wide use, good formability and low price. Such as polyethylene, polypropylene and polyvinyl chloride. Engineering plastics:-generally refers to plastics that can bear 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, poly alum, etc.
Special plastics:-Generally refers to plastics with special functions (such as heat resistance, self-lubrication, etc.) and applied to special requirements. Such as fluoroplastics and organic silicon.
3. Molding plastic according to plastic molding method: resin mixture for molding. Such as general thermosetting plastics. Laminated plastics: refers to fiber fabric impregnated with resin, which can be laminated and bonded by hot pressing to form a whole material. Injection, extrusion and blow molding plastics:-generally refers to the resin mixing department which can melt and flow at the barrel temperature and harden rapidly in the mold. Such as general thermoplastics.
Casting plastic: a liquid resin mixture that can be poured into a mold and hardened into a product with a certain shape without pressure or a little pressure. E.g. MC nylon. Reaction injection molding compound: generally refers to the liquid raw material, which is injected into the mold cavity under pressure, and then reacted and solidified to obtain the finished product. Such as polyurethane.
4. Molding powder according to plastic semi-products and products: also called plastic powder, which is mainly obtained by fully mixing, pressing and crushing thermosetting resin (such as phenolic resin) and filler. Such as phenolic plastic powder. Reinforced plastics: a kind of plastics with reinforced materials and some mechanical properties greatly improved compared with the original resin. Foam plastic: plastic with numerous micropores in the whole. Thin film: generally refers to flat and soft plastic products with thickness below 0.25mm ..
Third, the basic properties of plastics
1. Light weight and high specific strength. Plastics are light, and the density of general plastics is between 0.9 ~ 2.3 g/cm3, which is only about 1/8 ~ 1/4 of steel and 1/2 of aluminum, while the density of various foam plastics is even lower, which is between 0.01 ~ O.5 g/cm3.
The strength calculated by unit mass is called specific strength, and the specific strength of some reinforced plastics approaches or even exceeds that of steel. For example, the tensile strength per unit mass of alloy steel is 160 MPa, while the plastic reinforced with glass fiber can reach 170 ~ 400 MPa.
2. Excellent electrical insulation performance. Almost all plastics have excellent electrical insulation properties, such as very small 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 acid and alkali, especially polytetrafluoroethylene, which has better chemical corrosion resistance than gold, and even can resist the corrosion of strong corrosive electrolytes such as aqua regia, so it is called "plastic king".
4. Good antifriction and wear resistance. Most plastics have excellent antifriction, wear resistance and self-lubricating properties. Many anti-friction parts made of engineering plastics make use of these characteristics of plastics. When some solid lubricants and fillers are added to the anti-friction plastics, their friction coefficient can be reduced or their wear resistance can be further improved.
5. Light transmission and protective performance. Most plastics can be used as transparent or translucent products, among which polystyrene and acrylate plastics are as transparent as glass. The chemical name of plexiglass is polymethylmethacrylate, which can be used as aviation glass material.
Polyvinyl chloride, polyethylene, polypropylene and other plastic films have good light transmission and thermal insulation properties, and are widely used as agricultural films. Plastics have various protective properties, so they are often used as protective and protective articles, 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 external mechanical shocks and vibrations, viscous internal friction occurs inside them, and mechanical energy is converted into heat energy. Therefore, they are used as damping and silencing materials in engineering. For example, bearings and teeth made of engineering plastics can reduce noise, and various foam plastics are widely used as excellent damping and silencing materials.
The excellent properties of the above plastics make it widely used in industrial and agricultural production and people's daily life; It has been used as a substitute for metal, glass, ceramics, wood and fiber in the past, and has become an indispensable material for modern life and cutting-edge industry.
However, plastics also have shortcomings. For example, the 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 around 200℃;
The thermal expansion coefficient of plastic is 3 ~ 10 times larger than that of metal, and it is easy to be affected by temperature change and its dimensional stability is affected. Under load, plastic will slowly produce viscous flow or deformation, that is, creep phenomenon; In addition, plastics will age under the action of the atmosphere, sunlight, long-term pressure or some quality, which will deteriorate the performance and so on.
These shortcomings of plastics more or less affect or limit its application. However, with the development of plastic industry and the deepening of research work on plastic materials, these shortcomings are being gradually overcome, and novel plastics with excellent performance and various plastic composite materials are constantly emerging.
Fourth, the use of plastics
Plastics have been widely used in agriculture, industry, construction, packaging, national defense cutting-edge industries and people's daily life and other fields. Agriculture: A large number of plastics are used to manufacture mulching film, seedling raising film, greenhouse film, irrigation and drainage pipeline, fishing net, breeding float, etc. Industrial aspects: electrical and electrical industries widely use plastics to make insulation materials and packaging materials;
In the machinery industry, the transmission gears, bearings, bearing shells and many parts made of plastic are used to replace metal products; In the chemical industry, plastic is used as pipes, various containers and other anti-corrosion materials; In the construction industry, it is used as doors and windows, stair handrails, floor tiles, ceilings, heat insulation and sound insulation boards, wallpaper, water pipe fittings and pit pipes, decorative boards and sanitary ware, etc.
In the national defense industry and advanced technology, plastics are indispensable materials, whether it is conventional weapons, aircraft, ships, rockets, missiles, artificial satellites, spaceships and atomic energy industries. In people's daily life, plastics are widely used, such as plastic sandals, slippers, raincoats, handbags, children's toys, toothbrushes, soap boxes, thermos shells and so on.
At present, it has been widely used in various household appliances, such as televisions, tape recorders, electric fans, washing machines, refrigerators and so on. As a new packaging material, plastic has been widely used in the packaging field, such as various hollow containers, injection molded containers (turnover boxes, containers, barrels, etc.), packaging films, woven bags, corrugated boxes, foam plastics, tying ropes and packing belts, etc.
V. Development history and present situation of plastics industry
As early as the 19th century, people had already used natural resins such as asphalt, rosin, amber and shellac. In 1868, natural cellulose was nitrated, and camphor was used as plasticizer to make the world's first plastic variety, called celluloid. From then on, the history of human using plastics began.
In 1909, the first synthetic plastic-phenolic plastic appeared. In 1920, another synthetic plastic-aminoplast (aniline formaldehyde plastic) was born. These two kinds of plastics played a positive role in promoting the development of electrical industry and instrument manufacturing industry at that time.
By the 1920s and 1930s, alkyd resin, polyvinyl chloride, acrylate, polystyrene and polyamine appeared one after another. Since the 1940s, with the development of science, technology and industry, and the extensive development and utilization of petroleum resources, the plastic industry has developed rapidly. Polyethylene, polypropylene, unsaturated polyester, fluoroplastics, epoxy resin, polyoxymethylene, polycarbonate, polyimide and so on appeared in the varieties.
