People are in contact with plastics almost every day, so what is plastic?
Plastics are polymers whose main components are synthetic resins. In addition, there are some special-purpose additives added as needed, such as plasticizers that can improve plasticity, anti-aging agents that prevent plastic aging, etc.
Although the relative molecular weight of the polymer is very large, the composition is not complicated, and the structure also has certain rules. They are made by the polymerization of small molecules, such as polyethylene plastic, which is made by polymerizing ethylene molecules.
When polyethylene plastic is heated to a certain temperature range, it will begin to soften until it melts into a flowing liquid. Molten polyethylene plastic turns solid again when cooled, and melts back to liquid when heated, a phenomenon known as thermoplasticity. Polyethylene, polyvinyl chloride, and polypropylene are all thermoplastics. And some plastics can only become soft when heated during the manufacturing process, and can be molded into a certain shape. After processing and molding, they will not be melted by heat and are thermosetting, such as phenolic resin.
Why do the above phenomena occur? This is because thermoplastics have a long chain-like linear structure. For example, the long chains of polyethylene and polyvinyl chloride are connected by C-C bonds, and these long chains are combined by intermolecular forces. When heated, these long chains will vibrate faster, weaken the intermolecular force between the chains, and the corresponding sliding will occur between the long chains, so the plastic will melt into a liquid. When cooled, the energy contained in the long chains decreases, the distance gets closer, and the mutual attraction increases, so they will harden again.
Thermosetting plastics are also in the form of long chains in the early stage of formation, and will soften when heated, and can be molded into a certain shape. However, when the thermosetting plastic is further heated, covalent bonds will be formed between the chains, resulting in some cross-linking, forming a body-shaped network structure, and hardening and shaping. When it is reheated, the sliding of the chain molecules is restricted and does not melt.
There are many types of plastics and their uses vary. For example, polyethylene has good electrical insulation, chemical corrosion resistance, cold resistance, and non-toxicity. It can be made into film, used as packaging materials for food and medicine, as well as daily necessities, insulating materials, pipes, etc.; while polyvinyl chloride is electrically insulating Good performance, chemical corrosion resistance, organic solvent resistance, wear resistance, but poor thermal stability, poor air permeability, and hardening when cold, it can be made into films, hoses, daily necessities, pipes, insulating materials, etc. Films cannot be used for packaging Food; Polypropylene has good mechanical strength, good electrical insulation, chemical corrosion resistance, light weight, and non-toxicity, but poor oil resistance, low temperature brittleness, and easy aging. It can be made into films, daily necessities, pipes, packaging materials, etc.; Polypropylene Styrene has good electrical insulation, good light transmission, chemical corrosion resistance, non-toxic, hard and brittle at room temperature, becomes soft when the temperature is high, and has poor oil resistance. It can be made into high-frequency insulating materials, TV, radar components, medical Sanitary utensils can also be made into foam plastics for shockproof, moisture-proof, sound insulation, packaging cushions, etc.; urea-formaldehyde plastics have good insulation, heat resistance, waterproof, and easy coloring, and can be made into electrical equipment, auto parts, daily necessities, etc.; Tetrafluoroethylene is resistant to low temperature, high temperature, chemical corrosion resistance, solvent resistance, and electrical insulation, but it is difficult to process. products.