Page 340 - Failure Analysis Case Studies II

325

Fig. 1 1. ‘Press’ surface of release button showing discoloration and crazing.

Fig. 12. Light microscope closeup of the button in Fig. 11 showing crazing.

As noted earlier, the presence of the discontinuous rubber phase in ABS is necessary in order to
improve the impact resistance of the otherwise brittle SAN matrix. However, the polybutadiene
phase in ABS also reduces its environmental resistance for two reasons. First, the backbone of the
butadiene chain contains unsaturated C=C bonds, which have the effect of destabilizing adjacent
bonds. This makes those bonds more susceptible to attack by, for example, dissolved oxygen.
Second, the polybutadiene phase has a very low glass transition temperature (the T, for poly-
butadiene is -9OOC). This is why this phase improves the toughness of the glassy SAN phase,