Rigid fillers

In addition to the boundary conditions (5.82a-5.82d), it is required that the displacement components should vanish on the surface of a rigid filler. 

The mechanical properties of pure polymeric materials are often inadequate for particular applications, and to overcome this problem these materials may be reinforced in some way. The most common method is to include a substantial amount of a rigid filler or fillers, generally as finely divided powder, or as rods or fibres. For certain materials, elastomeric particles may be used, and these have the effect of reducing brittleness. 

It is important to note here that the presence of rigid filler clusters, with bonds in the virgin, unbroken state of the sample, gives rise to hydrodynamic reinforcement of the mbber matrix. This must be specified by the strain amplification factor X, which relates the external strain of the... 

Also, mechanical data on the influence of low volume fractions (0.03-0.05) of rigid filler particles provide evidence of a localized plastic deformation which would not seem understandable by reference to a uniformly crosslinked network. A non-uniformly crosslinked matrix might also be invoked to account for insensitivity of the rate of diffusion of water on the apparent degree of crosslinking. However, an observed increase in the uptake of water with apparent degree of crosslinking remains unexplained. 

Fig. 4 Influence of volume fraction of rigid filler particles on Vickers hardness number of a BIS-GMA conolymer.

Stars with 32% PSt, (PSt/15-h-PIB/34)g-C8, showed -26 MPa tensile stress. There was no appreciable difference in the tensile properties of unextracted and MEK extracted star blocks [see (PSt/15-h-PIB/34)g-C8 and (PSt/16-h-PIB/34)g-C8]. The modulus and Shore A hardness were slightly higher for the unextracted star, (PSt/16-h-PIB/34)g-C8, which maybe due to the presence of PSt contamination which acts as a rigid filler. A dramatic difference in the tensile behavior was observed when PSt content was increased. Stars with low PIB block molecular weight and high PSt content (46%), e.g., (PSt/21-h-PIB/25)g-C8, showed plasticlike behavior, i.e., it showed a high modulus, a yield point, and a short draw. Ex-... 

Non-Newtonian Polymer Suspensions Containing Rigid Fillers... 

The two systems discussed above demonstrate two mechanisms whereby the tensile strength of elastomers can be reinforced by the presence of rigid fillers. For the polymeric fillers dispersed within a vulcani-zate, the filler operates by raising the viscosity of the matrix, analogous to a decrease in temperature, but without affecting the dynamic, high frequency response (there is ample experimental evidence of the independence of Ty on presence of filler). There is also some indication that the rigidity of the filler affects the extent of reinforcement. 

The above interpretations of the Mullins effect of stress softening ignore the important results of Haarwood et al. [73, 74], who showed that a plot of stress in second extension vs ratio between strain and pre-strain of natural rubber filled with a variety of carbon blacks yields a single master curve [60, 73]. This demonstrates that stress softening is related to hydrodynamic strain amplification due to the presence of the filler. Based on this observation a micro-mechanical model of stress softening has been developed by referring to hydrodynamic reinforcement of the rubber matrix by rigid filler... 

Hydrodynamic reinforcement of the rubber matrix by the fraction of hard, rigid filler clusters with strong filler-filler bonds that have not been broken during previous deformations. 

Dewey, J. M., "The Elastic Constants of Materials Loaded with Non-Rigid Fillers," J. Appl. Phys.. 1946, ], pp. 578-581. 

The effective area modd predicts that the stress concentration factor 7 should be independent of composition in conqmsites containing well-bonded rigid filler paitides. This prediction is supported by the compressive yield data for silica-loaded epoxy resins presented in Fig. 8 yield stress is linear with log (strain rate) for eadi material, and the dopes are identical in each case. The increase of yield stress with silica content must therefore be interpreted as a decrease in the pre-exponential factor rather than in 7. Young and Beaumont observed a similar large increase in the yidd stress of silica-loaded epoxy resins, and suggested an analc with precipi-taticm-hardening in metals ... 

Unlike PPO, epoxy resins are toughened by the addition of rigid filler particles. DiBenedetto found that Gjc increased monotonically on adding gpiass beads ... 

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