Star oligomers and hyperbranched polymers in low VOC polyurethane coatings: Part III

Conclusion

In this paper the effects of the functionality of hydroxyl functional star oligomers and dendritic polys have been demonstrated in a low VOC clear 2-component clear coat crosslinked with a polysocyanate. At a fixed initial composition of the overall system and with the same reactivity of hydroxyl and isocyanate functional groups, the molecular weight increase as a function of time and the conversion can be calculated using the theory of branching processes. The study has shown that the practical results agree quite well with theory and that the potlife is shortened in case of higher functionality as well as in the case of a broad functionality distribution. The hardness in such a system is not only the effect of functionality, but also of solvent evaporation during crosslinking. If the molecular weight increase upon drying is too fast due to a high functionality (and/or distribution), early gel formation can result in a low film glass transition temperature and hardness due to solvent entrapment.