The Efficiency of Cellulosic Fillers in Common Thermoplastics. Part 1. Filling without Processing Aids or Coupling Agents

Abstract

The paper reports on the processing and properties of composites consisting of thermoplastics and cellulose-based fillers. The plastics used were HDPE, PP, PS, SB, PA 6, and PA 12; the fillers were wood flour (white spruce), cellulose flour (bleached sulphate, pine) and cellulose fibre (bleached sulphate, birch). The mixtures were homogenized (kneader, alternatively single-screw extruder) before being injection moulded into tensile test bars. The modulus increased with the filler content, while the yield and breaking stress remained relatively unaffected. For HDPE, SB, and PA 12 a moderate increase in the strength level was noted. The extension at rupture and the impact strength fell sharply when the filler content was increased. The compounding method had no influence on the modulus. Kneader compounding, producing a higher homogeneity, gave in general higher strength and impact values than compounding in a single-screw extruder. The temperature dependence of the modulus was highly reduced, as was also the mould shrinkage. At processing temperatures in excess of 200°C, severe discolouration was noted. Cellulosic fillers may be regarded as low-reinforcing fillers comparable to untreated mineral products such as calcium carbonate. On the other hand, the materials used here have the advantage of a substantially lower density.