Striped‐pattern deterioration and morphological analysis of injection molding comprising polypropylene/ethylene–α‐olefin rubber blends. II. Influence of heating

Abstract

Unique deterioration with a striped pattern caused by thermal heating is reported for injection molding comprising polypropylene/ethylene–α‐olefin rubber blends. This stripe is due to the formation of glossy and cloudy parts in alternation approximately perpendicular to the flow direction of the molten resin. On the surface of the cloudy part, a number of micro‐objects appear. They have a dome shape about 10 μm in diameter and 2 μm height. Some experimental evidence indicates that the objects are composed of the rubber contained in the blends. On the other hand, only a few objects appear in the glossy region after heating. This is the reason that the stripe with the micro‐objects (migrating rubber to the surface) is examined in terms of a higher order structure of injection molding with laser scanning microscopy, wide‐angle X‐ray scattering, and microhardness (Martens hardness). Consequently, we have concluded that this thermally deteriorated stripe depends on periodic changes in the crystallinity along the flow direction. The evidence indicates that a snakelike flow occurs during the injection process of the molding. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2008

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

“…A serious and known problem encountered with flow marks is that their visibility can be enhanced during aging, i.e., as a result of exposure to ultraviolet light, temperature changes and outdoor conditions. 15,16 In addition to the processing conditions, the composition of the polymer melt and its rheological behavior will influence the formation and the appearance of the flow marks. This will be discussed in more detail in the following.…”

Numerical simulation of the fountain flow instability in injection molding

“…The reason for this type of defect to occur is the time periodic asymmetric motion of the fountain flow [12][13][14], which has been shown to be a viscoelastic instability [15][16][17]. Bogaerds et al [17] studied the linear stability of a viscoelastic fluid at relatively low Weissenberg numbers and hence only predicted the critical conditions for which the flow becomes unstable.…”

Ageing of polymeric surfaces exhibiting flow marks—Effects on surface characteristics

“…Therefore, a thorough understanding of the weather resistance and the degradation of the used polymeric materials is of great importance, not least in connection with the appearance of flow marks. In some cases, flow marks are initially almost invisible (right after molding) but they become more and more apparent after exposure to outdoor conditions 10, 11. More general, it is well known that polymeric materials can undergo degradation owing to sunlight and heat.…”

“…Several investigations have been carried out to determine the degradation behavior of (PP)‐based materials, also with regard to the visibility of flow marks and similar defects 10, 11, 13. Surfaces exposed to outdoor conditions generally show a rougher and less bright appearance compared to the original parts, but light‐stabilizers and antioxidant agents can improve their weather resistance 10, 11.…”

“…Several investigations have been carried out to determine the degradation behavior of (PP)‐based materials, also with regard to the visibility of flow marks and similar defects 10, 11, 13. Surfaces exposed to outdoor conditions generally show a rougher and less bright appearance compared to the original parts, but light‐stabilizers and antioxidant agents can improve their weather resistance 10, 11. In some studies on elastomer‐modified PP blends, it was clearly seen that ageing owing to UV light and heat exposure increased the visibility of flow marks changing the surface aspect of dull and glossy bands,10, 11 the first ones being affected more during the heat ageing as a result of their initially lower degree of crystallinity 11.…”