Meltrheology of filled thermoplastics

The rheological properties of filled thermoplastic melts are reviewed. The response of filled melts in steady, dynamic and transient shear flows as well as uniaxial extensional flows are studied in terms of three important material variables: the volume concentration of the fillers, filier size and shape, and polymer-filler'interfacial characteristics. The fillers, the shapes of which strongly influence the overall rheology especially the elastic behavior, can be classified into those having directionality such as fibers and those without it which we denote as particles. Within each category, increasing filier loading increases the viscous dissipation and elastic energy storage capacity of the system eventually giving rise to a yield stress at high concentrations. Decreasing filier size increases the importance of interfacial interactions and generally results in larger values of viscous dissipation and elasticity. The ratio of the normal stress difference to shear stress is smaller for a particle filled system than for the pure melt, but is larger for a fiber filled melt compared to the pure melt. Extrudate swell decreases with the presence of fillers regardless of shape. Explanations for these contradictory observations, both of which relate to the elasticity of the system, is provided. The effect of the filler-polymer interface (interphase) on rheology is even more complex and depends on the specific interactions between the polymer, filier, and surface chemical additives.