Star oligomers with different reactivity in low VOC polyurethane coatings: Part II

Conclusions  In this paper the effects of the reactivity of hydroxyl functional oligomers with a polyisocyanate crosslinking agent have been demonstrated by comparing the potlife/hardness of a 3.5 VOC two-component polyurethane clear coat based on a low TG, high reactive against a high TG, low reactive oligomer. The reaction rate of two polymers forming a crosslinked network becomes diffusion controlled when, during the reaction, the increasing TG comes close to the reaction temperature.⁴ If this reaction takes place in the presence of a solvent which evaporates during film formation and crosslinking, the speed of evaporation will affect both the reaction rate and the film TG. The results of this study have shown that the reactivity determines the potlife whilst the TG controls the hardness. The reactivity of blends of such oligomers against pure oligomers is different as can be calculated using the theory of branching processes and as shown from practical measurements. Blends of oligomers or a pure oligomer with up to 75% of the properties of the high TG, low reactive oligomer do show a hardness close to the properties of the low TG, high reactive oligomer which further proves that the chemical reactivity controls the overall drying performance. A fast chemical reactivity shortens the potlife with a risk of solvent entrapment near to and at the gel point which results in low hardness.