Definition of Plastic
Definition of plastic: It is mainly composed of carbon, oxygen, hydrogen, nitrogen and other organic or inorganic elements. The finished product is solid. It is a molten liquid during the manufacturing process. Therefore, it can be melted by heating, flowed by pressure, and solidified by cooling to form various shapes. This large and varied family of materials is called plastic.
Plastic: It refers to a plastic (flexible) material with high molecular weight synthetic resin as the main component, added with appropriate additives such as plasticizers, stabilizers, flame retardants, lubricants, colorants, etc., and processed into a plastic (flexible) material, or a rigid material formed by curing and cross-linking.Plastic parts are widely used in every field of life today, such as household appliances, instruments and meters, wires and cables, construction equipment, communications electronics, automobile industry, aerospace, daily hardware, etc.
1. Characteristics of plastic
(1) It is a high molecular weight organic compound.
(2) It can exist in various forms, such as liquid, solid, gas, colloid, solution, etc.
(3) It can be formed.
(4) There are many types of plastics. Different monomer compositions can produce different plastics.
(5) Widely used, and the products are diversified.
(6) Have different properties.
(7) The processing methods can be diversified.
2. Advantages of plastics
(1) Most plastics have strong corrosion resistance and do not react with acids and alkalis.
(2) Plastics are cheap to manufacture.
(3) Durable, waterproof, and lightweight.
(4) Easy to be molded into different shapes.
(5) Good insulators.
(6) Some plastics can be used to make fuel oil and fuel gas, which can reduce crude oil consumption.
3. Disadvantages of plastics
(1) When recycling waste plastics, classification is very difficult and economically uneconomical.
(2) Plastics are easy to burn, and some plastics are easy to produce toxic gases when burned. For example, polystyrene produces toluene when burned. A small amount of toluene can cause blindness and vomiting after inhalation. PVC produces toxic hydrogen chloride gas when burned. In addition to combustion, high temperature environments can also cause plastics to decompose into toxic components, such as benzene rings.
(3) Plastics are products made from petroleum, and petroleum resources are limited.
(4) Plastics cannot be naturally decomposed.
4. Classification of plastics
(1) Plastics are classified into the following categories based on their performance after being heated.
① Thermoplastic plastics. Thermoplastic plastics refer to plastics that melt after heating, flow into molds, are molded after cooling, and melt again after heating. That is, heating and cooling can be used to make them plastic (liquid ←→ solid), that is, physical changes. The continuous use temperature of general thermoplastic plastics is below 100°. Polyethylene, polyvinyl chloride, polypropylene, and polystyrene are called the four general plastics.
② Thermosetting plastics. Thermosetting plastics refer to plastics that are insoluble in any solvent after being cured under heat or other conditions and cannot be converted again by heating. Thermosetting plastics will decompose if the heating temperature is too high. Such as phenolic plastics (commonly known as bakelite), epoxy plastics, etc.
(2) According to the different usage characteristics of plastics, plastics are usually divided into the following categories
① General plastics. General plastics generally refer to plastics with large output, wide application, good formability and low price. General plastics usually include polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS), etc.
② Engineering plastics. Engineering plastics generally refer to plastics that can withstand certain external forces, have good mechanical properties and high and low temperature resistance, good dimensional stability, can be used as plastics for engineering structures, and can be used in special application fields such as aviation and aerospace. For example, fluoroplastics and silicones have outstanding special functions such as high temperature resistance and self-lubrication. Reinforced plastics and foam plastics have special functions such as high strength and high cushioning. These plastics are all special plastics.