Some plastics are pure resins, such as polyethylene, polystyrene, etc., called single-component plastics. Some plastics contain, in addition to synthetic resins, other auxiliary materials such as plasticizers, stabilizers, colorants, various fillers, etc., called multi-component plastics.
(1) Synthetic resin
Synthetic resin refers to a high molecular compound (monomer) which has a certain synthesis condition by using coal, calcium carbide, petroleum, natural gas and some agricultural and sideline products as the main raw materials, and then polymerized by a chemical or physical method. The properties of these compounds are similar to natural resins (such as rosin, amber, shellac, etc.), but their properties are superior to natural resins. For example: synthetic resin dentures.
The content of the synthetic resin accounts for 40% to 100% of the total composition of the plastic, and acts as a bond, which determines the main properties of the plastic, such as mechanical strength, hardness, aging resistance, elasticity, chemical stability, photoelectricity. Sex and so on.
(2) Plastic additives
The main purpose of adding additives to plastics in plastics is to improve processability, improve performance and reduce costs. Additives account for a small proportion of plastic materials, but have a great impact on the quality of plastic products. Different types of plastics, depending on the molding process and the conditions of use, the types and amounts of additives required are also different. The main additives are as follows:
Plasticizer
Plasticizers can increase the softness, extensibility, plasticity of plastics, reduce the flow temperature and hardness of plastics, and are beneficial to the molding of plastic products. Commonly used phthalates, sebacates, chlorinated paraffins and camphor. The most common thing we have is camphor.
2. Stabilizer
We all know that during the processing, storage and use of plastic products, under the action of light, heat and oxygen, aging, embrittlement and cracking will occur. In order to delay and prevent the occurrence of aging, stabilizers must be added. Mainly used to prevent heat aging, called heat stabilizers; mainly used to prevent oxidative aging, called antioxidants; mainly used to prevent photoaging, called light stabilizers, they are collectively referred to as stabilizers. Today's best-performing plastic stabilizers are methyltin heat stabilizers (181), which are very effective for calendering, extrusion, injection molding and blow molding of rigid polyethylene (PVC). Because of its high safety, it is especially used for food packaging and high-definition rigid polyethylene products. At the same time, it is also widely used in plastic doors and windows, water pipes, decorative materials to replace other highly toxic plastics. Heat stabilizers. It has been widely used in the United States, Europe, and Japan. In recent years, 181 methyl tin heat stabilizer has been widely used in China.
3. Flame retardant
Additives that improve the flame resistance of plastics are called flame retardants. Most of the plastics containing flame retardants are self-extinguishing, or the burning rate is slowed down. Commonly used flame retardants are cerium oxide and aluminum, boron compounds, halides and phosphates, tetrachlorophthalic anhydride, tetra phthalic anhydride and the like.
4. Antistatic agent
Antistatic agents act to eliminate or reduce the generation of static electricity on the surface of plastic articles. Most of the antistatic agents are electrolytes, and their compatibility with synthetic resins is limited, so that they can migrate to the surface of the plastic to absorb moisture and eliminate static electricity.
5. Blowing agent
A plastic foaming agent is a low molecular organic substance that can be vaporized at a certain temperature, such as dichlorodifluoromethane; or an organic compound that decomposes a gas when heated. These gases remain in the plastic matrix to form a foam of many fine foam structures. Commonly used are azo compounds, nitroso compounds, and the like.
6. Colorants
Colorants are used for coloring plastics. Mainly to beautify and modify. About 80% of the plastic products are colored to make the final product.
7. Lubricant
The lubricant is added to improve the mold release property of the plastic during thermoforming and to improve the surface finish of the product. Commonly used lubricants are: stearic acid and its salts, paraffin wax, synthetic wax and the like.
8. Reinforcing materials and fillers
In many plastics, reinforcing materials and fillers account for a considerable proportion, especially for reinforced plastics and calcium-plastic materials. The main purpose is to add various fiber materials or inorganic substances in order to improve the strength and rigidity of plastic products. The most commonly used reinforcing materials are: glass fiber, asbestos, quartz, carbon black, silicate, calcium carbonate, metal oxide and the like.
Plastic classification
At present, there are hundreds of plastics produced in the industry, and there are more than 60 commonly used ones. The common classification methods are as follows:
According to performance after heating
Thermosetting plastic
This type of plastic is characterized by curing at a certain temperature after a certain period of heating or adding a curing agent.
The cured plastic has a hard texture and is insoluble in any solvent and cannot be softened again by heating. When the heating temperature is too high, it will decompose. Common thermosetting plastics are: bakelite, jade, decorative panels and unsaturated polyester plastics. Figure
2. Thermoplastics
This type of plastic is characterized by heat softening, cooling and hardening, and generally only physical changes during processing to maintain its chemical nature. The plastics of this type of plastic are mostly made of polyaddition. Common thermoplastics are: nitrocellulose plastics, acetate plastics, polyethylene plastics, polypropylene plastics, polystyrene plastics, polyvinyl chloride plastics, etc. For example, our common mineral water bottles, plastic bags, etc.
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