"A little knowledge about plastics" is dedicated to beginners

"A little knowledge about plastics" is dedicated to beginners

First, definition of plastic

Plastic is a high molecular weight organic material with a resin as its main component, which is shaped under a certain temperature and pressure, and can maintain a given shape at room temperature.

Resin refers to an organic polymer that generally has a range of transformation or melting when heated, and becomes liquid when reformed by an external force. It is solid, semi-solid, or liquid at room temperature. It is most basic and most important component of plastics. Generally speaking, any polymer used as a base material for plastics in plastics industry can be called a resin.

Secondly, classification of plastic

There is no exact classification of plastics yet, general classification is as follows:

1. Thermoplastics are classified according to physical and chemical properties of plastics: plastics that can soften when heated and harden when cooled within a certain temperature range. Such as polyethylene plastic, PVC plastic. Thermosetting Plastics: Plastics that can solidify into infusible and insoluble materials when exposed to heat or other conditions. For example, phenolic plastics, epoxy plastics, etc.

2. According to choice of plastics, general-purpose plastics: - Usually refers to plastics with large yield, wide application, good moldability and low price. Such as polyethylene, polypropylene, polyvinyl chloride, etc. Engineering plastics: - usually refers to plastics that can withstand certain external forces, have good mechanical properties and dimensional stability, can maintain their excellent properties at high and low temperatures, and can be used as engineering structural parts. For example, ABS, nylon, polyaluminum, etc.

Specialty Plastics: - Generally refers to plastics with special features (such as heat resistance, self-lubrication, etc.) and applied to special requirements. Such as fluoroplastics, silicone, etc.

3. According to plastic injection molding method, molded plastic: resin mixture for injection. For example, common thermosetting plastics. Laminated plastic: Refers to resin-impregnated fibrous fabric that can be combined into a solid material by lamination and hot pressing. Plastics for Injection Molding, Extrusion and Blow Molding: - Generally refers to resin compounds that can melt and flow at barrel temperature and quickly solidify in mold. Just like regular thermoplastics.

Injection molded plastic: A liquid mixture of resins that can be poured into a mold and solidified into a shaped product with little or no pressure. For example, nylon MC. Injection Molding Reaction Mixture: Generally refers to liquid raw material whiche is injected into cavity under pressure to cause it to react and solidify into finished product. For example, polyurethane.

4. According to plastic semi-finished products and finished products, molding powder, also known as plastic powder, is mainly obtained by completely mixing, pressing and grinding thermosetting resins (such as phenolics) and fillers. For example, phenolic plastic powder. Reinforced Plastics: A type of plastic that has reinforced materials and some mechanical properties are greatly improved compared to original resin. Styrofoam: A plastic with countless micropores in general. Film: Generally refers to a flat and soft plastic product less than 0.25mm thick.

Third, basic properties of plastic

1. Light weight, high specific strength. The plastic is lightweight, and density of ordinary plastic is 0.9 to 2.3g/cm3, which is only 1/8 to 1/4 of steel and about 1/2 of aluminum, while density of various foams is lower, about 1 /2 0.01 ~ 0.5 g/cm3.

The strength per unit mass is called specific strength, and specific strength of some reinforced plastics is close to or even exceeds specific strength of steel. For example, alloy steel has a tensile strength of 160 MPa per unit mass, while fiberglass reinforced plastic can reach 170-400 MPa.

2. Excellent electrical insulating properties. Almost all plastics have excellent electrical insulating properties, such as extremely low dielectric losses and excellent arc resistance characteristics comparable to ceramics.

3. Excellent chemical stability. In general, plastics have good corrosion resistance to acids, alkalis and other chemicals, especially chemical corrosion resistance of PTFE is better than that of gold, and it can even withstand corrosion of strong corrosive electrolytes such as aqua regia. King of Plastics.

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

5. Light transmission and protection. Most plastics can be used as transparent or translucent products, among which polystyrene and acrylic plastics are transparent as glass. The chemical name for plexiglass is polymethyl methacrylate, which can be used as a material for aircraft glass.

Plastic films such as polyvinyl chloride, polyethylene and polypropylene have good light transmission and heat preservation properties, and are widely used asagricultural films. Plastic has a variety of protective properties, so it is often used as protective packaging products such as plastic film, boxes, barrels, bottles, etc.

6. Excellent cushioning and noise reduction performance. Some plastics are flexible and elastic. When exposed to frequent mechanical shocks and vibrations from outside, viscous internal friction will occur and convert mechanical energy into heat. 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, and various foams are excellent shock-absorbing and sound-absorbing materials that are widely used.

The excellent properties of above plastics make them widely used in industrial and agricultural production and people's lives, indispensable materials for everyday life and advanced industries.

However, plastic has its drawbacks. For example, heat resistance is worse than that of metal and other materials. As a general rule, plastics can only be used at temperatures below 100 °C, and some can be used at around 200 °C.

The thermal expansion coefficient of plastics is 3-10 times that of metals, and dimensional stability is easily affected by temperature changes; under load, plastics slowly cause a viscous flow or deformation, i.e. a creep phenomenon; in addition, aging of plastic will occur under influence of atmosphere, sunlight, long-term pressure or certain quality impacts, leading to deterioration in performance, and so on.

These shortcomings of plastics affect or limit their use to a greater or lesser extent. However, with development of plastics industry and deepening of plastics research, these shortcomings are gradually overcome, and new plastics with excellent performance and various plastic composite materials are emerging.

Fourthly, use of plastic

Plastic is widely used in various fields such as agriculture, industry, construction, packaging, advanced defense industries and people's daily life. Agriculture: A large amount of plastic is used to make mulch film, seedling film, greenhouse film, as well as drainage and irrigation pipes, fishing nets, breeding floats, etc. Industry. The electrical and electrical industries make extensive use of plastics for manufacture of insulating and packaging materials.

In mechanical engineering, plastics are used to make transmission gears, bearings, bearing bushings and many parts to replace metal products; in chemical industry, plastics are used as pipes, various containers and other anti-corrosion materials; in construction industry,nor are used as doors, windows, stair railings, floor tiles, ceilings, heat and sound insulation panels, wallpaper, drain fittings and wells, decorative panels and sanitary ware, etc.

In defense industry and advanced technologies, whether conventional weapons, aircraft, ships or missiles, missiles, artificial satellites, spacecraft and nuclear power, plastics are indispensable materials. Plastics are widely used in people's daily life, such as plastic sandals, slippers, raincoats, bags, children's toys, toothbrushes, soap dishes, thermos shells and so on.

Currently, it is also widely used in various household appliances such as televisions, radios, electric fans, washing machines and refrigerators. As a new type of packaging material, plastic has found wide application in field of packaging, such as various hollow containers, injection molded containers (return boxes, containers, barrels, etc.), packaging films, woven bags, corrugated boxes, foam plastics, strapping ropes and packing tapes, etc.

5. Development history and current state of plastics industry

Natural resins such as resin, rosin, amber and shellac have been used since before 19th century. In 1868, natural cellulose was nitrided and camphor was used as a plasticizer to make world's first type of plastic, called celluloid, and history of human use of plastic began.

In 1909, first synthetic plastic appeared - phenolic plastic. In 1920, another synthetic plastic was born - aminoplast (aniline-formaldehyde plastic). These two plastics have played an active role in promoting development of electrical and instrument industries.

By 1920s and 1930s, plastics such as alkyds, polyvinyl chloride, acrylates, polystyrene, and polyamines appeared one after other. From 1940s to present, with development of science and technology and industry, extensive development and use of oil resources, plastics industry has developed rapidly. Polyethylene, polypropylene, unsaturated polyester, fluoroplastics, epoxy resin, polyoxymethylene, polycarbonate, polyimide, etc. appeared in varieties.

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