BZT 1 R: Partially (50%) primary hydroxyl functionalized Benzotriazole UV absorber, expected to exhibit 50% retention in topcoat on TPO.
Its lower molecular weight versus TZ 1 R gives it a high specific extinction versus both TZ 1 R and BZT 2 UR.
As expected, none of the primary hydroxyl-functional BZT 3 R and HALS 1 R were extractable, indicating 100% reaction with the isocyanate crosslinker.
ANALYTICAL QUANTIFICATION OF UVA IN WEATHERED AND UNWEATHERED COATINGS: Based on one's knowledge of coating chemistry, as well as on previously reported results, one would expect to observe the least migration for the fully reactable BZT 3 R, followed by the secondary hydroxyl functional TZ 1 R, then the partially reactable BZT 1 R, and finally the nonreactable UVAs TZ 2 UR and BZT 2 UR.
UV MICROSCOPY: Comparison of Figures 6 and 7 shows that little migration of the nonreactable UV absorber BZT 2 UR occurred from the clearcoat after initial bake on either the steel or TPO substrate.
After 3000 hr of exposure, approximately 65% of the BZT 2 UR in the clearcoat on TPO was gone, leaving only 35% of the original amount, or approximately 0.
After 3000 hours of exposure, the best retention was observed for the primary hydroxy functional BZT 3 R, followed by the secondary hydroxy functional TZ 1 R, and finally the 36% reactable BZT 1 R.
As indicated in Table 5, for BZT 2 UR and TZ 2 UR on TPO, the data obtained from both UV microscopy and microtomy/extraction are in good agreement for most of the samples analyzed.
The structure of the BZT 1 (R) includes a primary hydroxyl end group; however, only approximately 39% of the chromophore consists of this monoester structure.
Higher losses over TPO occurred for both HALS 2 (UR) and BZT 2 (UR), indicating further migration into the TPO during cure.
Analogous results were obtained for the reactable benzotriazole UV absorber BZT 1 (R) versus the nonreactable benzotriazole BZT 2 (UR).
The results agree reasonably well with the hydrolysis and extraction procedure, with the exception of BZT 1 (R), where the transmission method found higher levels than the hydrolysis procedure.