Influence of thermal history and molecular weight on mechanical properties of polypropylene (PP)

Several variables affect the solidification process of plastics, being the thermal history given to the polymer and its molecular weight the more important. Therefore, this paper is aimed at: 1) analyzing the effect of the crystallinity degree reached at different cooling rates and at different molecular weights on the mechanical properties of a crystalline polymer, polypropylene (PP); and 2) developing a model of a relation between thermal history, mechanical properties, and molecular weight of PP, of the mechanical property =f(molecular weight, crystallinity) type. The results obtained lead us to infer that the crystallinity degree tends to increase with a decrease in the cooling rate, and, in turn, decreases when PP molecular weight increases. The models obtained were found to be reasonably adjusted to the results of the mechanical properties and crystallinity. Therefore, they represent an important tool to estimate the mechanical behavior of these polymers with regard to the degree of crystallinity and the molecular weight, particularly for an industry that is always demanding products which meet its specific requirements.