Review of Preparation Principles of Low-Shrinkage Polypropylene
Polypropylene(PP)is a semi-crystalline general polymer material,and its products usually have a high shrinkage rate.With the developments of precision and large-sized injection molding,the demands for low-shrinkage PP increase rapidly.From a physical perspective,the shrinkage rate of PP is influenced by crystallization and molecular chain orientation.The crystallization shrinkage is directly proportional to the crystallinity of PP,and the shrinkage caused by molecular chain orientation is directly proportional to the internal stress and the degree of system orientation.The preparation methods of low-shrinkage PP could be summarized as the performance control of PP matrix resin,filling modification with inorganic particles,and olefin blending modification.The principle of regulating the performances of PP matrix is to obtain a low-crystallinity and low-orientation system by regulating the crystal form,molecular weight,molecular weight distribution,and copolymer monomers of PP,resulting to reduce the material shrinkage rate.The principle of filling modification with inorganic particles could be summarized as the heterogeneous nucleation effect of inorganic particles,the induction/hindrance effects of the shape and distribution of inorganic particles on the orientation of PP molecular chains,and the filling effect of particles.The principle of olefin blending modification could be summarized as the influences of dispersed phase on matrix crystallization and the inducing/hindering effects of the shape and distribution of dispersed phase in the matrix on the orientation of PP molecular chains.In summary,although the preparation methods of low shrinkage PP vary,the principle is to regulate the crystallization and molecular chain orientation of PP and its composite materials,achieving the goal of hindering matrix crystallization,refining spherical crystals,and directing molecular chains to crystallize in specific directions.
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