Degradation of carboxylated styrene butadiene rubber based water born paints: Part 2 - Models to predict UV stability and water absorption through central composite design

Central composite experimental design methods have been used to examine the simultaneous effects of talc (Viaton Viatalc® 30), titanium dioxide (modified Rutile, Tioxide® TR92) and additional hindered phenolic stabiliser (Aquanox® L, a 50% w/w aqueous dispersion of Winstay® L) on the water uptake and UV stability of composite films based on a carboxylated styrene butadiene rubber (c-SBR) latex. The talc and TR92 were in situ treated as pre-dispersions with Solsperse® S27000 and Solplus® D540 dispersants, respectively. For water uptake related responses, quadratic models were found to provide the most accurate prediction of effects associated with interactions between talc and TR92. It was found that the addition of TR92 to formulations with high talc loading reduced the water uptake, this was attributed a packing effect that arose due to the vast difference in pigment and filler particle size. For responses related to photo-oxidation (Microscal unit/mercury lamp, carboxylic acid carbonyl growth was monitored by IR), linear models gave the best data fit, thus indicating negligible interaction between the three variables. Within the experimental space explored, the level of talc had by far the strongest influence; increasing talc level led to a proportional increase in rate of carbonyl growth. This corroborates previous single variable studies, where the iron impurities present in the talc were suspected to be associated with the pro-degradant effect observed. Interestingly, the addition of dispersants amplified the latter effect and strongly muted the UV stabilising effect of TR92. An optimised formulation based on c-SBR was determined from the response equations and subsequently evaluated; in general the actual response trends matched those predicted. The suitability of experimental design as a tool to discover effects, interactions and responses of the ingredients of a paint system, and to optimise its formulation was thus confirmed.