Page 342 - Failure Analysis Case Studies II

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can sever the covalent bond between a polybutadiene molecule (rubber phase) and the styrene-
acrylonitrile copolymer matrix. The various chemical reactions involve the creation of free radicals
and the incorporation of oxygen into the polymer molecules. The reactions also tend to be
autocatalytic-once they start, they feed on themselves and tend to accelerate. In addition, high
temperatures tend to increase the rates of the various degradation reactions.
When the loss of the chemical bond between the rubber phase and the matrix phase becomes
widespread, the rubber phase loses its ability to absorb impact energy for the material as a whole.
That is, once the rubber phase becomes disbonded from the matrix, the impact properties of ABS
are, for all practical purposes, reduced to those of the brittle SAN matrix.
Most polymers are treated to prevent or slow the effects of photo-oxidation and other forms of
degradation-both in service and during fabrication. Chemicals known as anti-oxidants and
stabilizers are blended into polymers in order to slow the creation of free radicals and the
incorporation of oxygen into polymer molecules and/or to absorb damaging radiation. Some of
the rather complex mechanisms by which these additives work are described by Grassie and Scott
PI.
Like most polymers, ABS can and usually does contain antioxidants and stabilizers designed to
retard the effects of photo-oxidation. However, the presence of the rubbery phase (for the reasons
noted earlier), still tends to make ABS more susceptible to environmental degradation than most
non-rubber-modified polymers. It is possible that the subject series of failures were exacerbated by
improper stabilization of random batches of ABS release buttons installed on the subject cars.
This possibility was not investigated as a part of this report.

4. Short- and long-term industry solutions

4.1. Short-term solution

As noted earlier, no formal automobile recalls resulted in the U.S. from this series of failures.
Instead, automakers (such as Honda in the letter quoted earlier), instituted a voluntary recall in
which car owners could take their vehicles to authorized dealers for inspection and repair (free of
charge). One of the authors owned an affected vehicle (a 1989 Honda Accord). When this vehicle
was returned to the dealer as part of the voluntary recall, a Honda mechanic made a very brief
visual inspection of the front seat belt receptacles. Noting no breakage of the release buttons, the
mechanic quickly installed a small plastic impact guard (Fig. 14) on each seat belt receptacle. The
mechanic noted that, had he seen evidence of severe degradation of the release buttons, he would
have replaced the seat belt receptacles from the floorboards up with new assemblies. Having the
discretion to simply add the impact guard instead of replacing the entire assembly saves Honda
roughly $100 per vehicle in parts and labor. As can be seen in Fig. 14, the impact guard is unlikely
to be completely effective in eliminating impact damage to the seat belt release buttons.

4.2. Long-term solution

The basic design of many Takata seat belt receptacles remains very similar to those that are the
subject of this report. However, it appears that ABS has been phased out and replaced by more
environmentally resistant polymers such as various acetal copolymers [6].