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All four factors mentioned in the previous section are equally important in the determination of composite properties, and they must be adjusted to achieve optimum performance and economics.
The characteristics of the matrix strongly influence the effect of the filler on composite properties; reinforcing effect increases with decreasing matrix stiffness. True reinforcement takes place in elastomers, both stiffness and strength increases (Krysztafkiewicz 1988). This effect is demonstrated well by Fig. 1, in which the tensile yield stress of CaCO3 composites is plotted against composition for two different matrices. LDPE is reinforced by the filler, while the yield stress of PVC containing the same filler decreases continuously with increasing filler content (Voros et al. 1997). For the sake of easier comparison, the data were plotted on a relative scale, related to the yield stress of the matrix. The direction of the change in yield stress or strength is determined by the relative load-bearing capacity of the components (Pukanszky et al. 1988; Pukanszky 1990). In weak matrices, the filler carries a significant part of the load; it reinforces the polymer.
Fig. 1 Effect of matrix properties on the tensile yield stress of particulate-filled composites. Particle size of CaCO3, R = 1.8 pm
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