Rubber particle cavitation

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).

Figure 13.9 Sequence of events in a croid formation, (a) Initial state at the crack tip. (b) Cavitation ofthe rubber particles dueto loading head of the crack tip. (c) Cavitation of rubber particles near the already cavitated particles due to stress-concentration effect. The croid is forming, (d) Croids are propagating ahead ofthe crack and inside the craze-like damaged zone many shear bands develop between cavitated rubber particles. (Sue, 1992 with kind permission from Kluwer Academic Publisher.)...

Figure 19.12 TEM images of crazes and cavitated rubber particles after deformation of sPS rubber modified with 35 % core-shell BA-S rubber particles (BA S = 60 40)...

This study of stress-whitening in rubber-modified epoxies showed that the size of the whitened zone at the root of a notch decreases with increasing rubber content. Stress-whitening has been shown to be caused by hydrostatic stress. Two different species of stress whitening were found. One is reversible by heating and is deduced to be due to matrix cavitation the other is irreversible by heating and is due to highly cavitated rubber particles and shear bands. 

Case b stress-induced formation of homogeneous crazes. The stress concentration at the particles causes homogeneous crazes to start at the particle-matrix interfaces. Propagation of these crazes into the matrix is accomplished by an increase of volume, which arises from cavitation inside the particles (the possible mechanism of cavitation inside the originally homogeneous crazes is unlikely). Therefore, these crazes are closely connected to the cavitated rubber particles—they cannot propagate for distances as long as those of the fibrillated crazes—and appear mainly between particles. 

A slightly different mechanism of deformation has been observed with some specific combinations of epoxy resins and core-shell particles [125,126] crazelike features were observed in the damage zone at the crack tip and shown to consist of line arrays of cavitated rubber particles. The matrix material around the cavitated particles had undergone plastic deformation, whereas the material outside the line array was left undeformed. These line arrays of cavitated particles were termed croids , derived from the words crack and void [125]. 

Fig. 11.19 Energy curves for a cavitating rubber particle, calculated using Eq. 11.14 in blends with D = 0.3 pm, r, = 35 and Sv = 0.255 %, showing the effect of varying rubber particle concentration at a critical particle size.

Since a blend containing high concentration of cavitated rubber particles becomes cellular solid (porous) rather than continuous material, Eq. 11.15 does not apply to it any longer and any analysis of the plastic zone size must be based on yield criteria appropriate for the porous solid. Free from the constraints of continuum mechanics, the cavitated plastic zones formed in polymer blends are able to increase substantially in radius even under plane-strain conditions (Bucknall and Paul 2009). 

Fig. 11.23 Critical tensile stress for craze initiation as a function of (cavitated) rubber particle diameter, calculated using Eq. 11.26 with three different values of Gcraze the specific energy of craze initiation (From Bucknall and Paul (2009) reproduced with permission of Elsevier)...

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