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The introduction of hard particles into the polymer matrix creates stress concentration, which induces local micromechanical deformation processes. Occasionally, these might be advantageous for increasing plastic deformation and impact resistance, but they usually deteriorate the properties of the composite. The encapsulation of the filler particles by an elastomer layer changes the stress distribution around the particles and modifies local deformation processes. Encapsulation can take place spontaneously, it can be promoted by the use of functionalized elastomers or the filler can be treated in advance. Such a surface modification is rarely done directly by covering the filler with a soft layer, but forms spontaneously during the preparation of multicomponent polymer/filler/elastomer composites (Pukanszky 1995; Voros and Pukanszky 2001).
The introduction of fillers or reinforcements into a polymer matrix results in a heterogeneous system. Under the effect of external load, heterogeneities induce stress concentration, the magnitude of which depends on the geometry of the inclusions, the elastic properties of the components, and interfacial adhesion (Goodier 1933). Heterogeneous stress distribution and local stress maxima initiate local micromechanical deformations, which determine the deformation and failure behavior, as well as the overall performance of the composites. Stress concentration and local stress distribution can be estimated by the use of theoretical models or by finite element analysis (Bucknall 1977; Pukanszky and Voros 1993). The interacting stress fields of neighboring particles are very complicated and change with composition. The most often used approach is the analysis of stresses around a single particle embedded in an infinite matrix, which was first proposed by (Goodier 1933). According to his model, radial stress has a maximum at the pole, where it exceeds almost twice the external stress. Micromechanical deformation processes initiated by local stress maxima around the particles are influenced also by thermal stresses induced by the different thermal expansion coefficients of the components, crystallization, or shrinkage during the curing of thermoset matrices (Kerch andIrgen 1985; Stoklasa et al. 1985). Although the importance of inhomogeneous stress distribution developing in particulate-filled composites is pointed out in numerous publications, the exact role of stress concentration is not completely clear, and contradictory information is published claiming either beneficial (Nakagawa and Sano 1985), neutral (Trantina 1984), or detrimental effect on properties (Riley et al. 1990; Maiti and Mahapatro 1991).
In particulate-filled polymers, the dominating micromechanical deformation process is debonding. The stress necessary to initiate debonding, the number of debonded particles, and the size of the voids formed all influence the macroscopic properties of composites. Several models exist for the prediction of debonding stress including the one below (Pukanszky and Voros 1993):
where aD and aT are debonding and thermal stresses, respectively, WAB is the reversible work of adhesion, and R denotes the radius of the particle. C1 and C2 are constants which depend on the geometry of the debonding process. The validity of the model was checked in various particulate-filled composites. Initiation stress
Fig. 15 Dependence of debonding stress derived from volume strain measurements on the stiffness of the matrix (see Eq. 9)
determined in PP/CaCO3 composites from volume strain measurements is plotted against the stiffness of the matrix in Fig. 15 in the representation predicted by the model (Sudar et al. 2007). The correlation is close and corresponds to the prediction. Similarly, good correlations can be obtained if we plot debonding stress against the reversible work of adhesion (Pukanszky and Voros 1996) or the particle size of the filler (Pukanszky et al. 1994b) (see Eq. 9).
Micromechanical deformations are competitive processes, and the dominating one depends on material properties and loading conditions. Several fiber-related processes, like fiber breakage, pull out, buckling, etc., may take place in short- and long-fiber-reinforced composites. Quite a few of these can be observed also in wood fiber-reinforced polymers or layered silicate nanocomposites as well. The complexity of deformation and failure in such materials is demonstrated well by the number of processes detected in wood flour-reinforced PP composites (Danyadi et al. 2007b). The stress versus strain correlation of a PP composite containing 20 wt% unmodified wood flour is presented in Fig. 16 together with the acoustic signals detected during deformation. Since the adhesion between wood and PP is poor and the particles are large, the majority of the signals is emitted by the debonding of the wood particles. The cumulative number of acoustic events versus elongation plot clearly indicates that at least two processes occur in this composite shown by the two steps in the correlation. The application of a coupling agent, which improves interfacial adhesion between the components, changes the mechanism of deformation completely; the fracture of the fibers dominates under those conditions. The analysis of a large number of results showed that at least four processes take place during the deformation of PP/wood composites. The PP matrix deforms mainly by shear yielding, debonding, and fiber pull out dominate when the adhesion is poor, while mainly fiber fracture takes place in the presence of MAPP coupling agent, which creates strong bond between the matrix and the wood particles
Fig. 16 Acoustic emission testing of a PP composite containing 20 wt% wood without MAPP (poor adhesion). Small circles indicate individual acoustic events. Stress vs. strain and cumulative number of signal vs. strain traces
(Danyadi et al. 2007b; Renner et al. 2009, 2010). The fracture and the fibrillation of a particle are shown in Fig. 17 in order to support the analysis. The importance of local deformations is strongly supported by Fig. 18 in which composite strength is plotted against the initiation stress of the dominating process of a large number of PP and PLA composites reinforced with wood. It is obvious that micromechanical processes initiated by local stress maxima determine the final properties of particulate-filled and reinforced composites, and only the analysis of the resulting processes can help the development of stronger and better materials.
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