Debonding rubber particles

The effect of simply having a void, or a hole, in the epoxy cell was also ascertained in order to determine the effect of a cavitated, or interfacially debonded, rubber particle. 

Pofycarbonate is exceptional in this respect. Most pofymers are embrittled by large voids or debonded rubber particles, which act as nuclei for crazes and cracks, whereas the same pofymers are toughened by small well-bonded rubber particles, which generate similar stress concentrations, and are also capable of forming voids, by cavitating under stress. The key hictor appears to be the size of the voids, which determines whether they nucleate crazes and cracks (see Sections 5.3 and 5.4), or promote shear yielding. 

Fig. 8.1. Toughening mechanisms in rubber-modified polymers (1) shear band formation near rubber particles (2) fracture of rubber particles after cavitation (3) stretching, (4) debonding and (5) tearing of rubber particles (6) transparticle fracture (7) debonding of hard particles (8) crack deflection by hard particles (9) voided/cavitated rubber particles (10) crazing (II) plastic zone at craze tip (12) diffuse shear yielding (13) shear band/craze interaction. After Garg and Mai (1988a).

If the adhesion is low, debonding at the rubber particle matrix interface can occur. In both cases voids are formed and this reduces the degree of stress triaxiality in the surrounding matrix and favors the further growth of shear bands. 

A threshold level of interfacial adhesion is also necessary to produce a triaxial tensile state around rubber particles as the result of the cure process. When the two-phase material is cooled from the cure temperature to room temperature, internal stresses around particles are generated due to the difference of thermal expansion coefficients of both phases. If particles cannot debond from the matrix, this stress field magnifies the effect produced upon mechanical loading. 

Fig. 13.38 (a) The cavitated layer just underneath the flank of the fracture plane in an Izod impact sample of a rubber-particle-modified HDPE blend (Bartczak et al. (1999a) courtesy of Elsevier), (b) The cavitated layer, again just underneath the fracture plane, in an Izod sample of a CaCOs-particle-modified HDPE blend, showing the extensive debonding of particles prior to fracture (from Bartczak et al. (1999b) courtesy of Elsevier). 

Fig. 13.41 The temperature dependence of the critical fracture energy Gic in DGEBA epoxy-resin thermosets, modified either by rubber particles or by debonding glass spheres, either in tests of conventional extension rates or in Izod impact tests, compared with the generally flat behavior of unmodified epoxy resin (Kinloch (1985) courtesy of Springer).

Toughening mechanisms due to the elastomer spheres include shear-band formation, fracture of rubber particles, stretching, debonding and tearing of rubber particles, rubber cavitation, transparticle fi acture, crazing, formation of a plastic zone at the craze tip, diffuse shear-yielding, as well as shear band/craze interaction. 

It is sometimes suggested that rubber particles lose completely their ability to sustain a stress once they have cavitated. This is actually not true except only for a very few cases. First exception is when voids are formed along particle-matrix interfaces due to debonding (poor adhesion). Transfer of stress between the matrix... 

Generation of microvoids due to cavitation or debonding of rubber particles that... 

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