Stress cracking agents, analysis

PHOSPHATE ESTERS AS STRESS CRACK AGENTS -CASE STUDIES IN FAILURE ANALYSIS... 

With a recent push toward non-brominated flame retardants, phosphorus-based alternatives, such as phosphate esters, are used more frequently for various applications. Their use as plasticizers is also well known. However, their function as environmental stress crack agents of various thermoplastics is less well recognized. Two case studies, one - in which a triaryl phosphate was a component of the formulation, the other - in which it was migrating from an adjacent component illustrate some of the problems with their use. Fractographic analysis and various analytical techniques were used to determine a root cause of each of the two failures. 

A follow-up quantitative GC-MS analysis did not disclose excessive levels of TCPP in the unstained polyurethane foam, as compared with samples taken from other, problem-free enclosures. However, significant variability in the flame retardant amounts at different foam locations was detected. Therefore, based upon a follow-up review of the manufacturing process, it was hypothesized that the flame retardant had separated from the polyol prepolymer (since specific gravity of TCPP is close to 1.3 g/cc). It was not clear whether this separation was due to inadequate stirring, pauses in production of malfunction in the polyol / isocyanate mixhead. However, in the final analysis it was clear that the flame retardant on the ABS surface acted as a stress crack agent, and it was occasionally being delivered with the foam to the ABS surface in excess amounts. Polyurethane delivery equipment troubleshooting was subsequently undertaken by the enclosiu e manufacturer. 

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