Enhancing the wet strength of lignocellulosic fibrous networks using photo-crosslinkable polymers

Wet strengthening of paper products is typically performed by thermo-crosslinking reactive polymers or polymer precursors. In this article we introduce a novel concept for generating wet-strength paper grades by using photoreactive polymers and light as the energy source. Statistical, hydrophilic copolymers of dimethylacrylamide and benzophenone-methacrylate monomer, respectively, were synthesised and surface-applied to model paper handsheets. Benzophenone, as a photoreactive group, was selected because of its high crosslinking efficiency when illuminated with UV light (365 or 254 nm). After photo-crosslinking of the polymer-modified handsheets, first results of mechanical testing showed a significant increase of the wet tensile strength. The wet strength is a function of the molar content of the photoreactive group (benzophenone) within the polymer, the mass of the polymer relative to the dry fibre, the wavelength of irradiation and the intensity of UV exposure. Hence, there are a number of options to control and modulate the wet strength of paper sheets between less than 2 % (relative to the dry strength of the paper sheet) up to more than 15 %. In comparison to commercially available wet strength additives, crosslinking can be achieved even in the presence of humidity (up to 10 % water in the handsheet by weight) and at room temperature.