Effect of polyurethane addition on the strength, extensibility and 3D formability of paper and board

The extensibility and strength of fibre networks can be improved by modifying fibre material mechanically and by adding of polymers. Polyurethane (PU) is known to have interesting properties, like high extensibility. It was found, that the polyurethane dispersions can be effectively added to wet and dry paper by spraying on, as well as by wet-end addition to pulp suspension. In the wet end addition the retention of PU in paper is quite poor; however, it can be improved by fixing of PU by using a low molecular weight highly charged polymer. The improvements in extensibility increase with the amount of polyurethane retained in paper. However, considering the high extensibility (450%) of polyurethane the stiffness of the fibres and fibre network dominate the result. The adhesion of PU with paper, and the softening behavior (by DMTA tests), and surface energy measurements suggest that the compatibility of PU and kraft pulp could be better. Improvements are assumed to be due to the reasonable adhesion to fibres and thus fibres are connected by a material that has higher compliance than cellulosic material. The respective increase in extensibility was accompanied by relevant improvements in 2D- and 3D-formability.