Physical Properties of Plastics（1）
Physical Properties of Plastics（1）
The molecular aggregation state of thermoplastic resin solid is sparse and dense. The dense part can be called crystalline part, while the over sparse part can be called amorphous part. Most polymers will have crystalline part to some extent;
Therefore, we call the content of crystalline part as crystallinity. But generally speaking, polymers with functional groups such as nylon and thermoplastic polyester, or polymers with regular molecular arrangement such as polypropylene and polyethylene have higher crystallinity, while copolymers or mixed polymers have lower crystallinity
In general, the actual crystallinity of polymer is lower than its inherent crystallinity. Therefore, its crystallinity can be improved by heat treatment or increasing mold temperature
Polymers with high crystallinity have increased strength, decreased elongation and reduced volume. The higher the crystallinity of plastics, the higher the density, the higher the melting temperature (melting point), and the higher the strength, low transparency and low elongation
It can be seen that there is a close relationship between crystallinity and physical properties. The change of tensile properties of various resins is related to the difference of crystallization of the resin in the molding process. The greater the difference of crystallization, the greater the change of tensile properties of polymers
Polymers with good crystallinity will produce volume shrinkage due to their crystallization, which will affect the dimensional stability of their products. Therefore, we must try to improve their crystallinity to the inherent crystallinity as much as possible during processing, so as to prevent the dimensional stability of products caused by post shrinkage. In fact, in order to improve the dimensional stability of products, Some accelerators (nucleating agents) that can play a role in crystallization are often added to the resin.
Glass transition temperature
There is a phenomenon of glass transition temperature (glass transition point) in thermoplastic resin, that is, in the process of increasing with temperature, the polymer will be in the rubber state of neither solid nor viscous liquid within a certain temperature range before melting. We call the initial temperature of rubber attitude as its glass transition temperature (TG). In this temperature range, the thermal expansion of the polymer will suddenly increase, And the deformation is different from that of rubber, which is irreversible
The glass transition temperature characteristic is very important for the use of polymer products; For example, placing the product at a temperature above the glass transition temperature will lead to unexpected deformation. On the contrary, if you want to change the shape of the product, it can be implemented above the glass transition temperature. In addition, if you want to improve the crystallinity of the product, it can also be processed in this temperature range.
