3. Particulate Fillers in Thermoplastics

Although particulate-filled polymer composites are mature materials with a long history of application, their structure–property correlations are more complicated than usually assumed. The characteristics of all heterogeneous polymer systems including composites containing micro- or nanofillers are determined by four factors: component properties, composition, structure, and interfacial interactions. Several filler characteristics influence composite properties, but the most important ones are particle size, size distribution, specific surface area, and particle shape. The main matrix property is stiffness. Composite properties usually depend nonlinearly on composition, thus they must be always determined as a function of filler content. The structure of particulate-filled polymers is often more complicated than expected. Although segregation rarely occurs in practice, aggregation and the orientation of anisotropic particles take place quite frequently. The former usually deteriorates properties, while the latter determines reinforcement. Interfacial interactions invariably develop in composites; they lead to the formation of a stiff interphase considerably influencing properties. Interactions can be modified by surface treatment, which must be always system specific and selected according to the goal of the modification. Nonreactive coating helps dispersion and beneficial for impact strength, while coupling is needed for reinforcement. Particulate-filled polymers are heterogeneous materials in which inhomogeneous stress distribution and stress concentration develop under the effect of external load. These initiate local micromechanical deformation processes, which determine the macroscopic properties of the composites. Usually debonding is the dominating deformation mechanism in particulate-filled polymers. Although the number of reliable models to predict properties is relatively small, they offer valuable information about structure and interactions in particulate-filled composites. High compounding prices require the thorough consideration and utilization of all benefits of particulate fillers including large stiffness, strength, dimensional stability, increased productivity, etc.