Rubber-modified polymers

Table 15.3 Comparison of properties of some rubber-modified polymers with polyimethyl methacrylate)...

Transition from liquid behavior to solid behavior has been reported with fine particle suspensions with increased filler content in both Newtonian and non-Newtonian liquids. Industrially important classes are rubber-modified polymer melts (small rubber particles embedded in a polymer melt), e.g. ABS (acrylo-nitrile-butadiene-styrene) or HIPS (high-impact polystyrene) and fiber-reinforced polymers. Another interesting suspension is present in plasticized polyvinylchloride (PVC) at low temperatures, when suspended PVC particles are formed in the melt [96], The transition becomes evident in the following... 

Liquid-solid transitions in suspensions are especially complicated to study since they are accompanied by additional phenomena such as order-disorder transition of particulates [98,106,107], anisotropy [108], particle-particle interactions [109], Brownian motion, and sedimentation-particle convection [109], Furthermore, the size, size distribution, and shape of the filler particles strongly influence the rheological properties [108,110]. More comprehensive reviews on the rheology of suspensions and rubber modified polymer melts were presented by Metzner [111] and Masuda et al. [112], respectively. 

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).

Processing Stability. As with elastomers or other rubber modified polymers, the presence of double bonds in the elastomeric phase increases sensitivity to thermal oxidation either during processing or end use. Antioxidants are generally added at the compounding step to ensure retention of physical properties. Physical effects can also have marked effects on mechanical properties due to orientation, molded-in stress, and the agglomeration of dispersed rubber particles under very severe conditions. Proper drying conditions are essential to prevent... 

Aggression of a liquid upon a rubber-modified polymer under stress is well studied and depends upon capillary transport of the liquid to the crack tip. Crack propagation is resisted by the molecular weight of the continuous phase and aided by the plasticization effect of the aggressive liquid or plasticizer in the polymer matrix. High molecular weight, unplasticized products can be expected to have enhanced ESCR to aggressive liquids. 

Some studies of fatigue behavior in rubber modified polymers, such as ABS, HIPS and rubber reinforced PVC, have been reported However, there appear... 

At the same stress amplitude, rubber modified polymers fail sooner in fatigue than do the unmodified polymers even though they have superior resistance to fatigue crack propagation. This is a result of much earlier initiation of crazing, localized plastic deformation, and subsequent crack development due to the stress concentrating effect of the dispersed second phase particles. 

The second half of this volume is reserved to a discussion of specific craze problems encountered in practical application of polymer materials. J. A. Sauer and C. C. Chen analyze the fatigue behavior (mostly of rubber modified polymers). They show quantitatively the important effects of test variables and sample morphology on fatigue response. K. Friedrich gives an overview on the shear and craze phenomena in semicrystalline polymers. 

Fig. 1. da/dn vs. AK curves for the homogeneous glassy polymers, PS and PC, for two rubber modified polymers, HIPS and ABS, and for a crosslinked polystyrene XLPS. Data from Ref... 

ABS is another polymer in which the associated thermal effects can become quite large even at modest stress levels and frequencies. In this polymer, in tests made at 27.6 MPa, it has been noted that AT increases linearly with frequency, in accord with Eq. (5), from a value of about 2.5 °C at 2 Hz to a value above 25 °C at 21 Hz. The influence of stress magnitude on the temperatiare rise, for a constant frequency of 21 Hz, is shown for two different polymers, PSAN and ABS, in Fig. 5. In PSAN over the whole stress range investigated, and in ABS in the range where thermal equilibrium is achieved, AT varies approximately as the square of the stress, as predicted by the preceding equations. It may also be noted from the figure that the associated thermal effects are much more severe for the rubber-modified polymer than for the unmodified PSAN. 

The principal mode of deformation in fatigue cycling of glassy polymers is crazing. In some polymers, like PC and PSF, and in some rubber-modified polymers, like ABS, crazing may be accompanied by localized shear yielding. 

Unexpected advantages of bound antioxidants have been observed due to the selective protection of the most oxidatively sensitive regions of the polymer in rubber-modified polymer blends. 

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