Rheological and Morphological Evaluation of Natural Rubber Latex-Based PSAs Containing Waterborne Aliphatic Hydrocarbon Tackifier Dispersions

Abstract

The effect of three waterborne aliphatic hydrocarbon tackifier dispersions each with different softening points (70°C, 85°C, and 95°C) were evaluated with natural rubber latex at two addition levels (25% and 50%) for pressure-sensitive adhesive (PSA) applications. No other additives were incorporated into the PSA formulations so that rheological effects of waterborne aliphatic hydrocarbon tackifier resin dispersions in natural rubber-based PSAs could be clearly understood. Application of these waterborne PSAs was evaluated, in terms of rheology, since flow parameters have a very important influence on the convertibility (coating ability) of such adhesives. Morphological correlations with wet rheology for these waterborne PSA formulations and starting materials revealed that the interaction between the latex particle and tackifier dispersion particle has a major influence in determining the viscosity characteristics at low to medium shear rate, where stirring, pumping, and filtration processes occur. A shear-thinning effect was also predominant in formulations with lower tackifier dispersion levels. The extent of shear-thinning can be correlated well to morphology. Interestingly, all the PSA formulations tend to follow Newtonian behavior above a shear rate of 1000 s−1 and no shear-thinning or shear-thickening at higher shear rates was observed. The minimal change in viscosity at higher shear rates is a key parameter for high-speed coating techniques such as curtain coating and reverse gravure, since any change in viscosity can affect coating uniformity and the wetting of the substrate surface during coating.