Effect of tannin addition on the rheological properties of starch-based adhesives

Starch-based adhesives play a relevant role in paperboard production and are becoming more and more interesting, for different uses, because they are based on renewable biopolymers. Starch modifications or additive addition are becoming frequent to obtain the macroscopic properties desired for specific uses. In this paper the effects of the addition of four different tannins on a typical adhesive, adopted for corrugated paperboard production, were investigated by using fundamental rheological techniques, both in dynamic and steady conditions. It was found that tannins increase the onset of starch gelatinisation, estimated as the knee point of the storage modulus in a dynamic temperature ramp test, and decrease the steady shear viscosity. This is due to the interactions between tannin and starch that affect the gelatinisation and retrogradation reactions weakening the starch network. Even though a partial reinforcement effect was also observed, owing to the polymeric nature of tannin components, a lower consistency, with respect to the neat adhesive, was found for all modified samples. Tannin has shown itself able to modify technological properties such as gelatinization temperature and viscosity, since the specific results are determined by the nature and amount of tannin; therefore it could be used to adapt adhesive characteristics to specific applications, potentially improving starch-based adhesive competitiveness with respect to different adhesives.