Plastic is a widely used synthetic polymer material, plastic products can be found everywhere in our daily life. From the toiletries we use after getting up, the tableware we use at breakfast, to the stationery we use at work and study, the cushions and mattresses we use at rest, as well as the shell of TV, washing machine and computer, as well as the lamps and lanterns that bring us light at night.
Plastic has gradually replaced many materials and utensils that have been used for decades or centuries with its excellent properties and become an indispensable assistant in people's life. Plastic sets the hardness of metal, the lightness of wood, the transparency of glass, the corrosion resistance of ceramics, the elasticity and toughness of rubber in a suit, so in addition to daily supplies, plastic is more widely used in aerospace, medical equipment, petrochemical, machinery manufacturing, national defense, construction and other industries.
I. classification of plastics
There are many kinds of plastics. So far, there are about 300 kinds of plastics put into production in the world. There are many classification methods for plastics, and there are two commonly used:
1, according to the different properties of plastic after heat can be divided into thermoplastics and thermosetting plastics
Thermoplastic molecular structure is linear structure, in the heat of softening or melting, can be molded into a certain shape, cooling and hardening. When heated to a certain extent, it softens again and then hardens again after cooling. This process can be repeated many times. If polyvinyl chloride, polyethylene, polystyrene and so on. Thermoplastic molding process is relatively simple, continuous production, and has a relatively high mechanical strength, so the rapid development.
The molecular structure of thermosetting plastic is bodily form structure, also produce bate when be heated, can model makes certain shape, but be heated to certain degree or join a few curing agent hind, harden finalize the design, reheat also won't change soft with change shape. Thermosetting plastic processing molding, no longer softened by heat, so can not be recycled reuse, such as phenolic plastic, amino plastic, epoxy resin and so on belong to this kind of plastic. Thermosetting plastic molding process is more complex, so continuous production has certain difficulties, but its heat resistance is good, not easy to deformation, and the price is relatively low.
2, according to the use of plastic divided into general plastic and engineering plastic
General plastic is to point to the plastic with large output, low price, wide application range, basically include polyolefin, polyvinyl chloride, polystyrene, phenolic plastic and amino plastic 5 kinds. Many products used in People's Daily life are made of these universal plastics.
Engineering plastics are plastics that can be used as engineering structural materials and replace parts of metal manufacturing machines. For example, polyamide, polycarbonate, polyformaldehyde, ABS resin, polytetrafluoroethylene, polyester, Polysulfone, polyimide and so on. Engineering plastics have the characteristics of small density, high chemical stability, good mechanical performance, excellent electrical insulation, easy processing and forming, etc., widely used in automotive, electrical, chemical, mechanical, instrument, instrument and other industries, but also used in aerospace navigation, rockets, missiles and other aspects.
II. The composition of plastic
The plastic we usually use is not a pure substance, it is made of many materials. Among them high polymer polymer (or call synthetic resin) it is plastic main composition, in addition, in order to improve the performance of plastics, but also in the polymer to add a variety of auxiliary materials, such as filler, plasticizer, lubricant, stabilizer, colorant, etc., in order to become a good performance of plastics.
A. synthetic resin
Synthetic resin is the most important component of plastics, its content in plastic is generally 40% ~ 100%.Because of its large content and the properties of resins that often determine the properties of plastics, resins are often considered synonymous with plastics. For example, polyvinyl chloride resin and polyvinyl chloride plastic, phenolic resin and phenolic plastic confused. Actually resin and plastic are two different concepts. Resins are raw polymers that are used not only to make plastics, but also to make paints, adhesives and synthetic fibers. And plastic contains 100% outside very few resins, great majority of plastics, besides main component resins, still need to add other material.
Fillers, also known as fillers, increase the strength and heat resistance of plastics and reduce their cost. For example, wood powder added into phenolic resin can greatly reduce the cost, making phenolic plastic become one of the cheapest plastics, but also can significantly improve the mechanical strength. Fillers can be divided into organic fillers and inorganic fillers, the former such as wood powder, rags, paper and a variety of fabric fibers, etc., the latter such as glass fiber, diatomaceous earth, asbestos, carbon black and so on.
Plasticizer can increase the plasticity and softness of plastics, reduce brittleness and make plastics easy to be processed and formed. Plasticizers are generally miscible with resin, non-toxic, odorless, light, heat stable high boiling point organic compounds, the most commonly used phthalates. When producing polyvinyl chloride plastic for example, if join more plasticizer to be able to get soft qualitative polyvinyl chloride plastic, if do not add or add plasticizer less (dosage < 10%), get hard polyvinyl chloride plastic.
In order to prevent synthetic resin in the process of processing and use by the role of light and heat decomposition and damage, extend the service life, to add stabilizer in the plastic. Commonly used are stearate, epoxy resin and so on.
Colorants give plastics a variety of bright, beautiful colors. Organic dyes and inorganic pigments are commonly used as colorants.
The function of lubricant is to prevent the plastic from sticking to the metal mold when forming, and to make the plastic surface smooth and beautiful. Commonly used lubricants are stearic acid and calcium magnesium salt.
In addition to the above additives, flame retardant, foaming agent, antistatic agent can be added to plastics to meet the different requirements of use.