Novel Processing Additives for Rotational Molding of Polyethylene

Abstract

Over 85% of the articles manufactured via rotational molding employ Polyethylene (PE) resins. PE resins used for rotomolding have usually particle sizes less than about 0.8 mm and used as powders or micro-pellets. During heating in a rotating mold, these particles get fused and merge into one piece. A disadvantage of the rotomolding process is long cycle times that affect not only the production rate but also increase thermal degradation of the polymer due to its long exposure to heat. One of the problems in rotomolding is bubbles of gasses trapped during sintering of the PE powders which reduce mechanical strength of the article produced. We propose to use reacting mixtures of Polyethylene Glycol with citric acid as an additive to rotomolding grades of PE. The additive accelerates sintering of the PE particulates and greatly reduces number of bubbles in the melt but at high concentrations (>0.4 wt.%) it impedes flow of the particles. Fine powders with particles of irregular shape and wide distribution of sizes are characterized by low flowability while micro-pellets of oval shape with smooth surface have too high flowability for rotomolding. To improve cost-efficiency and quality of the rotomolding process we propose to use micro-pellets instead of powders and to fabricate micro-pellets by extrusion at reduced temperatures and cutting the produced strands in conditions of air-cooling. Amazingly, the same processing additives improve extrusion of PE resins and rotomolding of the PE micro-pellets. The additives also adjust flowability of micro-pellets in the direction to optimum. Silica fume, vinyl-silanes and stearates can be ingredients of the additive package. Mechanisms of the observed improvements are discussed.