The types and main characteristics of commonly used engineering plastics​

                           The types and main characteristics of commonly used engineering plastics

1.Classification of plastics

Classified by the nature of the resin
   Thermoplastic: A plastic that can be repeatedly heated to soften and cool to harden within a specific temperature range. Such as polyethylene plastic, polyvinyl chloride plastic.
   Thermosetting plastic: Plastic that can be cured into insoluble and insoluble materials due to heat or other conditions. Such as phenolic plastic, epoxy plastic, etc.
2. Classified by plastic use
    General-purpose plastics: plastics with large output, wide use, good formability, and low price. Such as polyethylene, polypropylene, and polyvinyl chloride.
    Engineering plastics: refers to plastics that can withstand certain external forces, have good mechanical properties and dimensional stability, and can maintain their excellent performance at high and low temperatures. Such as ABS, NYLON, polyalum, 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, silicones, etc.

Basic properties of plastics

    1. Light weight and high specific strength. Plastic is light. The density of general plastics is between 0.9 ~ 2.3 g / cm3, only 1/8 ~ 1/4 of steel and 1/2 of aluminum. The density of various foam plastics is lower, about 0.01 ~ 0.5 g / cm3. The strength calculated per unit mass is called specific strength, and the specific strength of some reinforced plastics is close to or even exceeds that of steel. For example, the tensile strength per unit mass of alloy steel is 160 MPa, while 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 minimal dielectric loss and excellent arc resistance. These properties are comparable to ceramics.

    3. Excellent chemical stability. Generally, plastics have good corrosion resistance to chemicals such as acids and alkalis, especially polytetrafluoroethylene has better chemical resistance than gold. Plastic King ".

    4. Good friction and wear resistance. Most plastics have excellent friction reduction, wear resistance and self-lubricating properties. Many engineering plastics' friction-resistant parts use these characteristics of plastics. When certain solid lubricants and fillers are added to abrasion-resistant plastics, their friction coefficient can be reduced or their abrasion resistance can be further improved.

    5. Light transmission and protection performance. Most plastics can be used as transparent or translucent products, of which polystyrene and acrylic plastics are as transparent as glass. The chemical name of plexiglass is polymethyl methacrylate, 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. Plastic has a variety of protective properties, so it is often used as protective equipment, such as plastic films, boxes, buckets, 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, internal plasticity is generated internally, which converts mechanical energy into thermal energy. Therefore, it is used as a vibration damping material in engineering. For example, bearings and teeth made of Engineering Plastic can reduce noise, and various foam plastics are widely used as excellent shock-absorbing and sound-absorbing materials.

     The excellent properties of the above plastic make it widely used in industrial and agricultural production and people's daily life; it has become a substitute for materials such as metals, glass, ceramics, wood and fibers, and has become a modern life and cutting-edge Indispensable materials for industry.

             However, plastics also have shortcomings. For example, heat resistance is worse than materials such as metals. Generally plastic can only be used at temperatures below 100 ° C, and a few are used at about 200 ° C. The thermal expansion coefficient of plastics is 3 to 10 times larger than that of metals, which is easily affected by temperature changes and affects dimensional stability. Under the load, plastic will slowly produce viscous flow or deformation, that is, creep phenomenon; In addition, plastic will aging under the atmosphere, sunlight, long-term pressure or certain mass effects, which will deteriorate performance. These shortcomings of plastic more or less affect or limit its application. However, with the development of the plastics industry and the deepening of research work on plastic materials, these shortcomings are gradually being overcome, and novel plastics and various plastic composite materials with excellent properties are constantly emerging.