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The concept and classification of plastics

Material is the foundation of human life and production, and an important symbol of a country's science and technology, economic development and people's living standards. It is listed as the three pillars of modern science alongside energy and information. Materials are usually divided into three categories: metallic materials, inorganic non-metallic materials and polymer materials. At present, in terms of the development speed and the wide range of applications, polymer materials have greatly exceeded the traditional cement, glass, ceramics and steel materials. As one of the main varieties of polymer materials, plastics have developed rapidly since the realization of industrial production in the early 20th century. They have become important materials in the fields of industry, agriculture, national defense, science and technology, and are used in various fields of the national economy and daily life. Play a huge role.

 

Plastics are "materials that use high polymers as the main component and are flowable and moldable at a certain stage of processing into finished products". It can also be considered as "a material that uses resin as the main component, contains additives, and can flow and shape during processing." Generally does not contain fibers, coatings and adhesives. Plastic materials are usually composed of two basic materials: one is the matrix material-resin; the other is auxiliary material-additives. The composition of the material and the ratio of each component have a certain influence on the performance of the product, and the polymer as the main component plays a dominant role in the performance of the product. The structure and composition of plastic materials determine its properties and performance. Under the action of temperature and pressure, plastic can be melted and flowed. It can be molded into a certain shape, and after cooling or solidification, it can maintain its shape to become a product.

Plastic, rubber, and synthetic fibers are collectively referred to as the three major synthetic materials, and plastics are the most widely used. The glass transition temperature of plastics is higher than room temperature (except for crystalline plastics). At room temperature, they are generally rigid solids (a few are flexible), with a wide range of mechanical properties and are greatly affected by temperature; the performance difference between rubber and plastics is that their glass transition temperature is lower than At room temperature, it is usually in a highly elastic state at room temperature and exhibits elasticity; synthetic fibers have high mechanical strength and heat resistance, and the macroscopic long diameter is relatively large. In fact, with the development of polymer materials and their processing technology, there is no obvious difference between the three. Many commonly used plastics are also materials for making synthetic fibers, and some plastics are also elastic at room temperature.

 

There are many types of plastics with different properties. The most commonly used classification methods are as follows.

1. Classified by plastic thermal behavior

According to the different morphological performance of plastic materials after being heated, they can be divided into thermoplastic plastics and thermosetting plastics. Thermoplastics can be repeatedly heated and softened, cooled and solidified within a specific temperature range. Various molding processes can be carried out in a softened and molten state. They can maintain a certain shape below the melting point and softening point to become products. There is almost no chemical in the molding process. reaction.

 

 

Therefore, this type of plastic is easy to mold and process. After the product loses its use performance, the plastic solid can be softened and melted.

Thermoplastics account for more than 70% of the total output of plastics, such as commonly used polyethylene, poly internal ene, polyoxyethylene, polyamide, etc.,

Thermoplastic plastic is to heat the monomer or prepolymer at a specific temperature to make it flow. When the temperature reaches a certain temperature, the intermolecular cross-linking turns into a network into a three-dimensional structure. This process is called curing. Once cured, it is formed Cross-linking can no longer be restored to a plastic state: plastic ingredients, black>melt flow, solidification.

Therefore, for thermo-round plastics, the polymerization process and the molding process are carried out at the same time, and the resulting products are insoluble and difficult to recycle. Commonly used thermosetting plastics include phenolic plastics, unsaturated polyester plastics, amino plastics, etc. .

 

2. Classified by plastic matrix resin

According to the different matrix resins of the plastics, it can be divided into polycarbonate plastics, styrene plastics, polyamide plastics, fluoroplastics, etc. The composition and structure of the matrix resins in each type of plastic are similar, and the properties are similar. Single structure such as propylene marriage and butene. The thermoplastic resin obtained by olefin polymerization is referred to as polyolefin for short, and the plastic based on polyolefin resin is called polycarbonate plastic. The main varieties are polyethylene plastic and polypropylene plastic. Polyolefin plastic has low relative density and dielectric constant. And the dielectric loss value is small, the insulation performance is excellent, and it is easy to form and process.

3. Classification according to the use of plastics

According to the scope of use and purpose of plastics, it can be divided into general plastics, engineering plastics, and functional plastics. General plastics have large output, low prices, and general performance. They are mainly used as non-structural materials, such as polyethylene, polypropylene, and polyvinyl chloride. , Polystyrene, etc., engineering plastics can be used as component materials, can withstand a wide range of temperature changes and harsher environmental conditions, have excellent mechanical properties, heat resistance, wear resistance and good dimensional stability. Engineering plastics have lower output and higher prices than general-purpose plastics. Its main varieties include polyamide, polyoxymethylene, polycarbonate, and polysulfone. Due to the excellent comprehensive performance of engineering plastics, its use value exceeds that of general-purpose plastics. After the 1960s, with the rapid development of high-tech fields such as aerospace and aviation, engineering plastics with high heat resistance and high strength have appeared one after another. They are called special engineering plastics, such as fluoroplastics, polyimide, polysulfone, polyphenylene sulfide, etc. But in fact, with the rapid development of science and technology, the boundary between engineering plastics and general-purpose plastics has become more and more blurred. Some general-purpose plastics (such as polypropylene, etc.) can also be used as structural materials after modification. Ultra-high molecular weight polyethylene with a relative molecular mass of 1 million to 3 million has the properties of special engineering plastics.

Functional plastics are plastics that have specific functions and can meet certain special performance requirements, such as liquid crystal polymers, silicone plastics, conductive plastics, medical polymer materials, and environmentally degradable plastics. Functional plastics are an important part of new polymer materials and are used as high-performance materials in many aspects of national defense, medical care, electronics, agriculture, and packaging.


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