What are the classifications of plastics and their respective characteristics?

Classification and Characteristics of Plastics

Based on the molecular structure and thermal properties of synthetic resins, plastics are divided into two categories: thermoplastics and thermosetting plastics.

Characteristics of Thermoplastics

1. The resin molecules adopt a linear or branched structure.

2. They soften when heated and melt into a fluid viscous state at a certain temperature. They can be shaped under pressure and retain the formed shape after cooling. The materials can be softened and melted again upon reheating for repeated molding.

3. Only physical changes occur during heating, cooling and molding, with no chemical reactions involved.

Characteristics of Thermosetting Plastics

1. The resin molecules form a three-dimensional network structure.

2. At the initial stage of heating, the resin molecules remain linear and possess plasticity like thermoplastics. With continuous heating, chemical cross-links form between the main chains of linear molecules to build a network structure. When the temperature rises to a certain level, the molecules develop into a three-dimensional structure. The material becomes infusible and insoluble, and its shape is permanently fixed. It will no longer soften or regain plasticity when reheated repeatedly.

3. Both physical and chemical changes take place during the molding process.

Common thermoplastics include polyethylene, polyvinyl chloride, ABS resin, polypropylene, polystyrene, nylon and polyoxymethylene.

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