Fracture rubber particles

Mooney used a parallel plate rheometer with a piece of ruhher, with one surface which was dyed. As the rotation continued, the colour of the dye was observed to spread in the direction of thickness of the specimen. The fractured rubber particles, as they rotated under the shear field, carried the dye like an ink-roll. This process imprinted the dye from the surface down to the successive layers of the rubber particles. On the basis of this model Mooney calculated the size of the rubber particles, i.e., the flow unit to be of the order of 10 pm [9]. Mooney s model on the generation of super-molecular flow units is one of models used as the basis for proposing the comminution model for mixing of rubber with fillers [10]. 

Heterogeneous compatible blends of preformed elastomers and brittle plastics are also an important route for the development of blends of enhanced performance with respect to crack or impact resistance. Polycarbonate blends with preformed rubber particles of different sizes have been used to provide an insight into the impact properties and the fracture modes of these toughened materials. Izod impact strength of the blends having 5-7.5 wt% of rubber particles exhibits best overall product performance over a wide range temperature (RT to -40°C) [151-154]. 

Composite Particles, Inc. reported the use of surface-modified rubber particles in formulations of thermoset systems, such as polyurethanes, polysulfides, and epoxies [95], The surface of the mbber was oxidized by a proprietary gas atmosphere, which leads to the formation of polar functional groups like —COOH and —OH, which in turn enhanced the dispersibility and bonding characteristics of mbber particles to other polar polymers. A composite containing 15% treated mbber particles per 85% polyurethane has physical properties similar to those of the pure polyurethane. Inclusion of surface-modified waste mbber in polyurethane matrix increases the coefficient of friction. This finds application in polyurethane tires and shoe soles. The treated mbber particles enhance the flexibility and impact resistance of polyester-based constmction materials [95]. Inclusion of treated waste mbber along with carboxyl terminated nitrile mbber (CTBN) in epoxy formulations increases the fracture toughness of the epoxy resins [96]. 

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).

The second noteworthy morphological feature is presented in Fig. 12b. This micrograph depicts the pre-crack front of 15-1500-70F, which had a value significantly above that of the control, as shown in Fig. 11 a. The holes may be examples of the dilatation effect observed in CTBN-modified epoxies l9,22> in which rubber particles dilate in mutually perpendicular directions under the application of a triaxial stress and then collapse into spherical cavities following fracture. Dilatation requires a mismatch in coefficients of thermal expansion of resin and rubber 11. This effect will therefore be most striking when the elastomeric phase is homogeneous, as is apparently the case here. 

The introduction of rubber particles increases the fracture energy of the networks at room temperature, but also decreases the temperature of the ductile-brittle transition (Van der Sanden and Meijer, 1993). This ductile-brittle transition is strongly dependent on the nature (and Tg) of the rubber-rich phase and the amount of rubber dissolved in the matrix. The lowest ductile-brittle transition is obtained with butadiene-based copolymers (Tg — 80°C), compared with butylacrylate copolymers (Tg —40°C). 

As in the case of rubber particles it was demonstrated that the fracture energy is roughly proportional to the volume fraction of TP rich phase, both in the case of epoxy/PEI networks (Bucknall and Gilbert, 1989) and bismaleimide networks toughened with various TPs (Stenzenberger et al., 1988). This improvement was evidenced in the range of volume fractions below [Pg.415]

Fig. 24 TEM micrograph showing crazing and rubber particle break-down close to the fracture surface in an iPP/EPR CT specimen deformed at about 7ms [19, 26]...

Figure 3. Scanning electron micrograph of a typical fracture surface of a highly crosslinked polyurethane resin containing 8% w/w of dispersed polymyrcene-based rubber particles.

Analysis of the fracture surfaces in Fig. 18c reveals the different size of the rubber particles in the two PS systems. Tested in air, in both cases the crack... 

Fig. 18 a Effect of small styrene-butadiene rubber particles incorporated in PS (PS 486M) on FCP with and without oil. b Effect of large styrene-butadiene rubber particles incorporated in PS (PS 2710) on FCP with and without oil. c SEM micrographs of PS fracture surfaces with smaller rubber particles (above) and larger rubber particles (right). Left without oil, right with oil... 

ABS and HIPS. The yield stress vs. W/t curves of ABS and HIPS are very similar. They are somewhat surprising because the yield stresses reach their respective maximum values near the W/t (or W/b) where plane strain predominates. This behavior is not predicted by either the von Mises-type or the Tresca-type yield criteria. This also appears to be typical of grafted-rubber reinforced polymer systems. A plausible explanation is that the rubber particles have created stress concentrations and constraints in such a way that even in very narrow specimens plane strain (or some stress state approaching it) already exists around these particles. Consequently, when plane strain is imposed on the specimen as a whole, these local stress state are not significantly affected. This may account for the similarity in the appearance of fracture surface electron micrographs (Figures 13a, 13b, 14a, and 14b), but the yield stress variation is still unexplained. 

Figure 25.15 Fracture mechanics plot of the deformation energy in impact as a function of BW for S/DPE(15) modified with rubber particles prepared in emulsion...

In ABS, where particle size is much smaller than in HIPS, the particles are less effective as craze initiators and the fatigue fracture surface shows evidence of considerable localized plastic deformation of the matrix polymer as well as of cavitation and/or loss of adhesion of the rubber particles. 

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