Foaming Ability of Rotomolding Polyolefin Resins

The main focus of the research presented in this paper is the investigation of the ability of various polyolefin resins to be converted into integral-skin cellular composites by using the rotational foam molding process. Integral-skin foamed rotational moldings are formally denoted as cellular composites ideally having a clearly distinct surface layer of solid skin of uniform thickness that is encapsulating a seamlessly coupled fine-celled foamed core or layer of uniform cell density and distribution. A systematic comparative material characterization study that attempts to derive practical guidelines about determining the roto-foamability of polyolefins that would be useful for rotomolding processors is presented. The study included two experimental methods of characterization, a melt rheology-based and a rotational foam molding processing-based. The experimental results from both implemented characterization methods revealed good agreement. A comprehensive insight into the key polyolefin material characteristics that would ensure satisfactory results if processed using the rotational foam molding technology have been provided. The experimental results revealed that high quality polyethylene (PE) based cellular morphologies can be obtained from both dry blended and melt compounded foamable compositions for both 6-fold and 3-fold expanded foams. Unlike PE resins, it was observed that successful foaming of polypropylene (PP) resins in rotational foam molding can only be successfully accomplished over a very narrow range of melt temperatures that are close to the melting point of the polymer and by using PP grades with a quite limited range of Melt Flow Rates (MFR).