The addition of inorganic fillers has been deemed as an effective strategy for reducing the mold shrinkage of polypropylene (PP). However

the current understanding on the underlying mechanisms is still unclear. In this work

taking talc filled isotactic PP (i-PP) as an example

the effect and mechanisms of inorganic particles on the linear shrinkage of injection-molded i-PP products were investigated theoretically and experimentally. The results indicate that

at low contents (<6 vol%) of talc

the linear shrinkage of the injection-molded products in both in-plane (i.e.

along the machine direction MD and transverse direction TD) and out-of-plane directions (i.e.

along the thickness direction ND) is found to decrease linearly with increasing talc content. This is consistent with the theoretical prediction. However

at high contents (>6 vol%) of talc

the change trend of the linear shrinkage deviates significantly from the theoretical curves. The in-plane shrinkage decreases remarkably

while the out-of-plane shrinkage remains almost unchanged. Further analysis indicates that the talc plates in the injection-molded products show a strong tendency to align along the MD-TD plane and induce the growth of i-PP lamellae perpendicular to their surfaces. Accordingly

the dense talc network promotes the formation of highly oriented i-PP lamellae perpendicular to the MD-TD plane

ultimately leading to a much lower in-plane linear shrinkage but a higher out-of-plane shrinkage as compared to the theoretical values. Achieving low in-plane linear shrinkage is particularly beneficial for the development of large-sized thin-walled products. Our work not only breaks through the current knowledge (the shrinkage of composites is decreased based on the increased volume fraction of fillers) and gives a new insight on the role of the filler-induced orientation crystallization in tailoring the shrinkage of PP products

but also provides a solution for the development of low-shrinkage PP materials.