The main mineral additives used in the modification of PP plastics are calcium carbonate, talc powder, mica powder, etc., which have obvious effect on the molding shrinkage of PP plastics.
It is mainly reflected in three aspects: the first is that the mineral filler itself does not shrink, and its addition reduces the shrinkage of polypropylene modified material from the overall proportion; the second is that the addition of mineral filler will inevitably affect the crystallinity of polypropylene, thus affecting the shrinkage; Third, the addition of fine mineral agent acts as a nucleating agent, which changes the structural state of polypropylene. The formation of enlarged spherulites also affects the molding shrinkage of polypropylene.
Glass fiber reinforcement is an important means of PP plastic modification, and glass fiber has the greatest influence on the molding shrinkage of PP modified material. When the content of glass fiber is more than 30%, the molding shrinkage of PP decreases from 1.8 to 0.5, and the influence of surface treated glass fiber on the molding shrinkage is greater than that of untreated glass fiber. The addition of glass fiber destroys the crystallinity of PP and affects the shrinkage. At the same time, glass fiber limits the crystallization shrinkage of PP.
Polyethylene is also a kind of plastic with high crystallinity, and its molding shrinkage is also very large. However, the crystallinity of polyethylene is destroyed in different degrees after it is added into polypropylene, which makes the overall molding shrinkage decrease.
The molding shrinkage of polypropylene is affected by its crystallinity, which in turn is affected by its molecular weight. When Mi increases, the molecular weight decreases, the crystallization rate increases and the molding shrinkage increases. The change of MI also has a certain effect on the molding shrinkage of PP.