ROTATIONAL MOLDING OF POLYOLEFINS: PROCESSING, MORPHOLOGY, AND PROPERTIES

Abstract

In the rotational molding of semicrystalline polymers, the slow heating and cooling rates and an almost absence of shear stresses lead to coarse spherulitic morphologies free from molecular orientation. This molding process, which is suitable for short runs of large parts, is not favorable for the dispersion of additives like pigments. This article reviews the type of morphologies that develop during the rotational molding of polyethylene (PE) and polypropylene (PP) when mixed with nucleating pigments or with different amounts of antioxidants. Over a range of typical processing conditions, it was observed that the morphology is affected by the processing temperature. Especially at the inner surface that is in contact with air, the sensitivity to temperature is higher, and degradation is more likely to occur. In the case of PE, the degradation is revealed in the morphology by the suppression of crystallization, whereas for PP, a change in the appearance and the level of birefringence of the spherulites occurs. Degradation may be delayed by increasing the antioxidant content or using an inert atmosphere, but without significant mechanical strength improvement. The morphology is affected by the way the nucleating pigments are incorporated during the mixing process. For example, the poor mixing capability of turboblending leads to the pigment concentrating around the polymer particles and to development of transcrystalline textures. Conversely, the more effective extrusion compounding allows the pigment to disperse and distribute better and leads to the reduction of the spherulite size. In the case of PP, which is a candidate for rotational molding, an improvement of the impact strength is achieved only if the finer morphology is associated with a significantly lower degree of crystallinity.