Rheological properties of elastomer‐modified polypropylene and their influence on the formation of flow marks

Abstract A premium appearance is essential for many polymeric products and for this reason surface defects associated with the manufacturing process, e.g. injection molding, are detrimental. In this study, the interest is focussed on defects arising during injection molding of such elastomer‐modified polymers that are often used in the automotive sector to produce interior and exterior components. In particular, defects denoted as “flow marks” or “tiger stripes” were investigated. These defects appear on the surface of the injection‐molded components, especially if long flow lengths are involved and consist of alternating glossy and dull bands. Here an attempt was made to elucidate to what extent some important rheological properties of the polymer melts influence the generation of such flow marks. Hence, the flow properties, mainly in shear; of three different grades of elastomer‐modified polypropylene containing mineral fillers were correlated to their propensity for defect generation. In summary, it was noted that a higher melt elasticity, as reflected in pressure losses during flow through a capillary, the degree of die swell and to some extent the dynamic‐mechanical behavior, leads to less severe flow marks or retards the formation of such defects. Elongational draw‐down experiments also indicated a more stable flow in the case of the melt that exhibited the highest elasticity. Furthermore, subjecting the measured flow behavior to a Mooney analysis, the results obtained pointed to that wall‐slip in itself is not a primary cause for the appearance of the type of flow marks studied here. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010