Although the structure of particulate-filled polymers is usually thought to be very simple, often structure-related phenomena determine their properties. Structure is strongly influenced by the particle characteristics of the filler, the composition, and the processing technology used. The most important structure-related phenomena are homogeneity, the attrition of the filler or reinforcement, aggregation, and the orientation of anisotropic particles. Occasionally, fillers might modify the structure of crystalline polymers as well. All structure-related effects must be controlled in order to prepare products of high quality.
Crystalline Matrices, Nucleation
The properties of crystalline polymers are determined by the relative amount of the amorphous and crystalline phases, crystal modification, the size and perfection of crystallites, the dimensions of spherulites, and the number of tie molecules (Samuels 1974). The most important effect of particulate fillers is their ability to act as nucleating agents. The very strong nucleating effect of talc in PP was proved many times (Menczel and Varga 1983; Fujiyama and Wakino 1991). Similarly to talc, layered silicates, and especially montmorillonite (MMT), were shown to
Fig. 4 Correlation between the heat of crystallization and yield stress of PP/CaCO3 composites (Maiti and Mahapatro 1990)
nucleate polypropylene quite strongly (Maiti et al. 2002; Pozsgay et al. 2002). The influence of other fillers and reinforcements is not so clear. Many fillers have shown weak nucleation effect in PP (Rybnikar 1991; Avella et al. 2006), while some others have shown no effect (Bajaj et al. 1987). A closer scrutiny of literature information and experimental data indicates that mostly physical and especially topological factors determine the nucleation effect of fillers. Fillers may influence also the crystal modification of the matrix. Introduction of talc into the p-modification of PP resulted in a complete change of crystalline structure; the higher crystallization temperature of the а-modification prevented the formation of the p-form (Varga and Toth 1991). Similarly, in the presence of montmorillonite, polyamide was shown to crystallize mainly in the у-form (Lincoln et al. 2001; Miltner et al. 2006) irrespective of the presence and/or type of organophilization (Choi et al. 2006).
Occasionally, strong correlation is claimed between the crystalline structure of the matrix and composite properties. (Hutley and Darlington 1984) found a more or less linear correlation between the crystallization temperature and the falling weight impact strength of particulate-filled PP, while (Maiti and Mahapatro 1990) observed an even better linear correlation between the crystallinity and tensile characteristics of PP filled with CaCO3 (Fig. 4). However, the similar effect of the filler on two or more composite characteristics might lead to the linear correlation between tensile yield stress and crystallinity as observed by (Maiti and Mahapatro 1990) and often to erroneous conclusions. The detailed analysis of experimental results obtained on PP composites containing different fillers indicated that the effect of changes in crystalline structure may be neglected especially at large filler contents (Pukanszky et al. 1994a). A very efficient nucleating agent may change the modulus of PP from 1.4 to 1.9 GPa (Pukanszky et al. 1997), while the introduction of 30 vol.% talc results in a composite with a stiffness of almost 8 GPa (Pukanszky et al. 1994a).