POLYESTER WEATHERABILITY - COUPLING FRONTIER MOLECULAR-ORBITAL CALCULATIONS OF OXIDATIVE STABILITY WITH ACCELERATED TESTING

A series of polyesters were formulated into melamine baking enamels an d evaluated by QUV-B, QUV-A, Xenon arc, Emmaqua(R) NTW, and Florida ex posures in order to determine the effect of polyester composition outd oor durability. Specifically, 2-methyl-1,3-porpanediol was compared to propylene glycol and neopentyl glycol in several compositions that ut ilized adipic acid, isophthalic acid, and phthalic anhydride as the di acid constituents. Unexpected positive results for polyesters based up on 2-methyl-1,3-propanediol led to a detailed analysis of polyester de composition mechanisms and raised questions concerning the true signif icance of beta-hydrogen degradation pathways. Semiempirical frontier m olecular orbital calculations demonstrated that the beta-hydrogen of d iols and diol esters are less susceptible to oxidative attack than the alpha hydrogens. The molecular orbital calculations fit well with exp erimental evidence and provided a deeper understanding of the effect t hat the components of polyester have upon the exterior durability of t he coating.