Plastic with high crystallinity are crystalline plastic. The attraction between molecules is easy to interact with each other, making them strong and tough plastic.

Basic definition

Crystallization refers to the regular arrangement of molecules, which becomes a crystalline structure after cooling. The crystalline structure of general plastic is a collection of many linear and slender polymer compounds. The degree of regular arrangement of molecules is called the degree of crystallization (crystallinity). It also means that each molecule is only arranged neatly, so crystalline resins are actually only partially crystalline. The proportion of the crystalline part is the crystallinity. The degree of crystallization can be measured by X-ray reflection. The structure of organic compounds is complex, and the structure of plastic is more complex. The structure of molecular chains (linear, hair ball, folded, spiral, etc.) varies a lot.

, so its structure also varies greatly due to different molding conditions. Plastic with high crystallinity are crystalline plastic. The attraction between molecules is easy to interact with each other, making them strong and tough plastic. In order to achieve crystallization and regular correct arrangement, the volume becomes smaller, and the molding shrinkage rate and thermal expansion rate become larger. Therefore, if the crystallinity is higher, the transparency is worse, but the strength is greater. Crystalline plastic have a clear melting point (Tm), the molecules are arranged regularly when solid, the strength is strong, and the tensile force is also strong. The specific volume changes greatly when melting, it is easier to shrink after solidification, the internal stress is not easy to release, the finished product is opaque, the heat dissipation is slow during forming, the shrinkage is large after cold mold production, and the shrinkage is small after hot mold production. Compared with crystalline plastic, there is another type of non-crystalline plastic, which has no obvious melting point, the molecules are arranged irregularly when solid, the specific volume does not increase much when melting, it is not easy to shrink after solidification, the finished product has good transparency, the higher the material temperature, the yellower the color, and the heat dissipation is fast during forming. The following is a comparison of the physical properties of the two.

Main characteristics

1. The molecules are close together in the crystal structure, so the structure is more solid. The density, strength, stiffness, and hardness increase, but the transparency decreases.

2. The crystalline resin has a sharp drop in specific volume at the melting point temperature, and the non-crystalline resin specific volume does not change sharply at the melting point temperature. Specific volume refers to the volume per unit mass, and the unit is cm2/g.

The crystallinity varies depending on the type of resin and the cooling rate. The crystallinity of rigid polyethylene is as high as 90%, while the crystallinity of refractory polyethylene is only about 20-30%. The slower the cooling rate, the higher the crystallinity.

A. Comparison of physical properties of crystalline and non-crystalline plastic

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