Crystallization is considered a common unit operation for the isolation and purification of products. However, the scientific principles of crystallization are generally not as well understood as other unit operations such as distillation or absorption. The crystallization step directly affects product attributes such as crystal size, shape, yield and purity, and can improve final product quality and process efficiency.
Next, Xiaobian will introduce a concept-crystallinity. The so-called crystallinity is used to indicate the proportion of crystalline regions in the polymer. The range of polymer crystallinity varies widely, generally from 30% to 85%. For the same material, the higher the crystallinity, the higher the melting point. Crystallization is an orderly arrangement of molecular chains, while melting point destroys all the molecular assembly structures to form molecular chains.
Generally, the higher the degree of crystallinity, the more regular the arrangement of molecular chains, the higher the temperature is required to destroy, so the higher the melting point. Many factors can affect crystallization, including:
1. Temperature: The polymer moves from the disordered coils to the surface of the growing crystal. When the mold temperature is high, the mobility of the polymer is improved and the crystallization is accelerated.
2. Polymer chain structure: Macromolecules with good symmetry, no branched chain or few branched chains, small side groups, and large intermolecular forces tend to be close to each other and crystallize easily.
3. Nucleating agent: Because low temperature is conducive to rapid nucleation, but it slows down the growth of grains, so in order to eliminate this contradiction, a nucleating agent is added to the molding material, so that the plastic can be rapidly molded at high mold temperature. crystallization.
4. Pressure: If there is an external force during the cooling process, it can also promote the crystallization of the polymer, so the injection pressure and holding pressure can be increased during production to control the crystallinity of the crystalline plastic.
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