Stress-whitening of polypropylene block copolymer in impact tests and its measurement

Abstract This paper describes two methods—one based on a simple measurement and another requiring a special instrument—for measuring the degree of stress-whitening in impacted propylene-ethylene block copolymer (PEBC) specimens. These methods have been applied to determine the response of PEBC specimens, containing various nucleating agents, to measured stress-whitening. Difficulties in measuring stress-whitening have been resolved. Accordingly, low dart weights (60 and 80 g) are preferable for producing whitening. The area method, requiring measurement of the whitened spot, gives better reproducibility when measurements are done on the reverse side. In the differential opacity method, any side may be used. This difference between the two methods arises because area is a surface property whereas opacity is a property averaged across the specimen thickness. However, it is shown that even the area method can be used to evaluate and compare stress-whitening (studied as a function of dart weight or nucleating agent type).