The influence of interfacial adhesion, particle size and size distribution on the predicted mechanical properties of particulate thermoplastic composites

Various factors influence the mechanical properties of thermoplastic composites. Of these, the aspect ratios of the reinforcement, interfacial adhesion, porosity and binder content are most important. Other than these, the particle size and particle size distribution of the reinforcement also influence the mechanical properties. A previously developed model used micro-mechanical analysis to predict the Young's modulus and tensile strength of particulate composites. This model predicts that aspect ratio, rather than particle size, influences the Young's modulus and stress at break, assuming perfect adhesion between the reinforcement and binder. The model accounts for particle size distribution only in terms of the maximum packing fraction of the dry reinforcement. In this paper, compression-moulded phlogopite composites were investigated, using linear low density polyethylene (LLDPE) as a binder. The aspect ratio of the particles, binder content and processing conditions were kept constant. It was found that particle sizes between 60 and 300 mum had little effect on the Young's modulus. Particles smaller than 240 mum had lower tensile strengths than the particles above 240 mum. The results were, however, very close to the values predicted by the model. Acrylic acid was used to modify the adhesion between the reinforcement and the binder by blending it with the polymer prior to compression moulding. It was found that by using 100 mmol acrylic acid/100 g binder the tensile strength approached the value predicted by the micro-mechanical model and the Young's modulus was greater than the predicted value by a factor 5. No further improvement in the mechanical properties could be obtained by using more than 100 mmol acrylic acid/100 g binder. (C) 2002 Elsevier Science B.V. All rights reserved.