Effects of crosslinking on adhesion behavior of waterborne polyurethane ink binder

A series of waterborne polyurethanes (WPUs) with different crosslinking densities were synthesized by varying the amount of trifunctional crosslinker trimethylolpropane (TMP). The effects of the TMP content on the adhesion behavior of WPUs were investigated in terms of the cohesive strength of WPUs, and the interface adhesion strength between WPUs and the substrate–polyethylene glycol terephthalate (PET) film. It was found that with the increase of TMP content, the interface adhesion strength decreased due to the decreased mobility of PU molecular chains, the cohesive strength of WPUs films increased because of the increase in the crosslinking density (υe), and the T-peel strength (T) initially increased and then decreased with the change of debonding failure from a cohesive failure to an interface failure. An empirical equation, revealing the relation between peel strength and TMP content (x), was found to be T = 1.94 + 0.81x − 0.22x2, and another empirical equation, T=1.59+750.44υe−128313.62υe2, showing the relation between peel strength and crosslinking density was obtained. Moreover, crosslinking density was mainly determined by TMP content, and it was found that the relationship between υe and x could be showed by the equation υe = 0.00045 + 0.0013x or υe = 0.00050 + 0.0013x. On the whole, crosslinking played an important role in the adhesion behavior of WPUs, and an optimal crosslinking density was a key for preparing the WPUs with outstanding adhesion property. In this study, the optimal υe was 0.00292 mol/cm3.