There are two different types of Plastics: Thermoplastics and thermosetting plastics. Hot melt plastics become soft and flow when heated, and harden when cooled. This process is reversible and can be repeated. Polyethylene, polypropylene, polyvinyl chloride, polystyrene, polyformaldehyde, polycarbonate, polyamide, acrylic plastics, other polyolefins and their copolymers, polysulfone, polyphenylene ether, chlorinated polyether, etc. are thermoplastics. In thermoplastics, the resin molecular chains are linear or branched, and there is no chemical bond between the molecular chains. The process of softening and flowing when heated and hardening when cooled is a physical process of morphological transformation. Thermosetting plastics can soften and flow when heated to a certain temperature, produce chemical reaction, cross chain curing and harden. This change is irreversible. After that, when heated again, they can no longer soften and flow. This material is called thermosetting plastics.

对于热固性聚合物的这种交联反应，粘度反映了它的固化程度。借助这种特性可以对热固性塑料进行成型加工，一次加热时的塑化流动，在压力下充满型腔，进而固化成为确定形状和尺寸的制品。热固性聚合物在成型中不仅有物理状态的变化，还有化学变化，并且是不可逆的。

For this crosslinking reaction of thermosetting polymer, the viscosity reflects its curing degree. With this feature, thermosetting plastics can be shaped and processed. The plasticizing flow during one-time heating will fill the mold cavity under pressure, and then solidify into products with certain shape and size. Thermosetting polymers have not only physical state changes, but also chemical changes, and are irreversible.

热固性聚合物在未交联前与热塑性聚合物相似，都是线型聚合物。但热固性聚合物在分子链中带有反应基团或反应活点，成型时分子链通过自带的反应基团的作用或反应活点与交联剂（硬化剂）的作用而发生交联，使线型变成体型结构，不仅不能再熔触，在溶剂中也不能溶解。

Thermosetting polymers are similar to thermoplastic polymers before crosslinking. They are linear polymers. However, thermosetting polymers have reaction groups or reaction active points in the molecular chain. During molding, the molecular chain is crosslinked through the action of its own reaction groups or reaction active points and crosslinking agents (hardeners), so that the linear structure becomes a body structure, which can not be fused and dissolved in the solvent.

酚醛、服醛、三聚氰胺甲醛、环氧、不饱和聚酯、有机硅等塑料都是热固性塑料。热固性塑料可以用作隔热、耐磨、绝缘、耐高压电材料，如炒锅锅把手和高低压电器。

Phenolic, formaldehyde, melamine formaldehyde, epoxy, unsaturated polyester, silicone and other plastics are thermosetting plastics. Thermosetting plastics can be used as heat insulation, wear resistance, insulation and high-voltage electrical materials, such as frying pan handles and high-voltage and low-voltage electrical appliances.

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