Crazing in HIPS

Fig. 2.10. Stress distribution along a craze in HIPS grown from a rubber particle calculated by two different methods...

Figure 5. Rubber particles with crazes in HIPS. The rubber particles are strongly elongated by cavitation and fibrillation in the rubber network around PS inclusions (HVEM image). The deformation direction is vertical.

Figure 17.6 Micrographs of HIPS samples imaged by using different microscopic techniques. (a) Crazes in HIPS-imaged using stained ultrathin sections byTEM [64] ...

The formation of voids in the rubbery phase in HIPS influences its mechanical properties. The formation of voids is believed to facilitate the energy dissipating deformation processes, i.e., crazing and shearing. Crazing and shearing are facilitated under conditions in that the rubber particles can easily cavitate. 

The proposed normalized dispersion index (NDI) has been shown to be a sensitive measure of the "goodness" of particle dispersion in space. An NDI value of. 142 was shown to be a critical level of dispersion that demarcates two totally different types of crazing behavior in HIPS. 

For comparing we microscoped in the described manner the specimen of homopolymer of styrene where there is no rubber. In the not annealled specimens of the material the crazes were formed which were the same as those in HIPS (according to their sight).In those annealled at 80 C for 24 hours crazes don t exist. It shows that the cause for forming residual microtension in HIPS is presence of rubber particles in it. 

When chemicals are weak solvents for polystyrene, stress cracks are formed in the products. Chemical resistances to practical chemicals are compared in Table 18.5. HIPS-SPS blend exhibits better resistance to chemicals which are used in the kitchen and bathroom. In HIPS/SPS blend, SPS may work to prevent the formation of crazes and their propagation to cracks initiated from contact with chemicals. 

On the tension side, the loop width begins to rise at about 7 cycles. This is a result of strain softening due to craze initiation. With continued cycling, the plastic deformation increases continuously and rapidly with increase of N. On the compression half cycle, no changes are observed for about 30 cycles and only very small changes are noted thereafter. This comparative behavior indicates clearly that crazing, as noted also by Bucknall and Stevens is by far the dominant mode of deformation in HIPS samples subject to alternating stresses. 

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. 

Owing to the difficulty of moulding thick sheets, most fixture measurements on pdymers have been made on specimens less than 7 mm thick. This thickness is not sufficient to produce plane strain conditions in most tou or ductile polymers. The difference between plane stress and plane strain fracture has hitherto been widely regarded as unimportant in HIPS and other rubber-modified jdastics, since crazing is itself a plane-strain process, and little attempt has been made to work with thick specimens. However, recent work by Parvin and Williams has shown that there is a very marked transition between plane stress and plane strain fracture in and it is clear that this aspect of rubber-toughening will requite closer attention in the future. 

Stress State at Particles. If the modifier particles consist of rubber-like material, they act as stress concentrators as in HIPS and ABS. Whereas in HIPS and related polymers the maximum stress component, aee, at the equatorial regions around the particles is responsible for initiating crazes, in polymers with a tendency to shear deformation the maximum shear stress at the particles, yielding the formation of shear bands, must be considered (see Figure 17a). But in contrast to crazes, which have a stress-concentrating ability, shear bands to not increase the stress between particles as effectively. Therefore, the formation of microvoids inside the particles is necessary as an additional mechanism to increase the stress at and between particles. To make the polymeric material between particles yield, not only is the stress concentration at particles necessary (as in craze formation), but so is the stress field between particles. 

The three mechanisms described above briefly contribute to the overall dilatation of the material under tension, and it is not easy to assign to each process its relative importance in the recorded damage rate A. However, some authors like Keskkula and Schwarz (37) for HIPS, showed from detailed morphological observation that the crazing in the PS matrix is not the only active source of damage, but that the decohesion at the PS/PB interface and cavitation in the PB nodules play a significant role as well. 

The structure of ABS is similar to that of HIPS but with a SAN matrix instead of the PSt matrix in HIPS. PB grafted with SAN acts as a compatibilizer between the rubber particles and the SAN matrix. The rubber particle morphology in ABS can be similar to that in HIPS, with salami-type particles, but ABS particles can also be of the core-shell type, with a core of solid PB and a shell of graft copolymer, especially if the ABS is produced by the emulsion process [34]. In addition to craze formation, an important fracture mechanism in ABS polymers is shear yielding, which leads to tougher materials [46]. 

---