Morphology of injection‐molded polypropylene

• Published in: Journal of polymer science. Polymer physics edition 1973-05, Vol.11 (5), p.951-971
• Main Authors: Fitchmun, D. R., Mencik, Z.
• Format: Article
• Language: English
• ISSN: 0098-1273; 1542-9385
• DOI: 10.1002/pol.1973.180110512

A surface zone composed of at least three distinct layers has been detected in injection‐molded polypropylene. The morphology of these layers as determined by optical and scanning electron microscopy forms a continuous picture from the “as‐molded” surface to the core. With the exception of the first layer which is always featureless, the observed morphology is mostly spherulitic. Starting in the second layer, the spherulites decrease steadily in size until they reach a minimum somewhere near the middle of the third layer. After passing this point, they increase in size—rather rapidly—as the boundary between layer 3 and the core is approached. In samples prepared at the shorter fill times, the spherulites continue to decrease in size until they are replaced by the mixed matted straw–flat plate texture. The overall morphological features of this surface zone are explained by using an extension of a model for the crystallization of a quiescent polymer melt. In this model, growth originates from heterogeneous nuclei whose density varies throughout the sample as a function of both a temperature gradient and the induction time. Interpretation of anomalies in the fine structure of the individual layers indicates that shear is influencing the crystalline morphology also.