Lignin Interdiffusion– A Mechanism Behind Improved Wet Strength

We have studied ways of improving strength properties of paper made from high yield pulps and lignin-rich chemical pulps by utilizing the thermoplastic properties of the lignin present in the fibre walls. Both dry and wet strength can be improved by hot pressing of sheets made from lignin-rich pulps. In this paper, we focus on aspects of the wet-strength development as a function of lignin content and temperature. Here we apply an activation energy evaluation approach to study lignin intermixing or interdiffusion. By means of hot pressing, it is possible to reach wet strength levels up to 50% of the dry strength level, provided that we use pulps with high enough lignin content. Our study included hot pressing of high yield pulps such as thermomechanical pulp (TMP), chemithermomechanical pulp (CTMP), high-temperature chemithermomechanical pulps (HTCTMP), unbleached northern softwood kraft (NSK) and northern bleached softwood kraft (NBSK). The sheet pressing trials were performed for varied temperatures from room temperature up to 270°C. As the activation energy for the high yield pulps and the lignin-rich NSK were all in the range of 20-32 kJ/mol, we suggest that the wet strength development as function of temperature has a similar mechanism as long as the pulp fibres contain enough lignin. We also suggest that the phenomenon involves intermixing and/or interdiffusion of wood polymers between adjacent fibres when they are in a close contact. Most probably both the amorphous wood polymers, i.e. the linear hemicelluloses and the cross-linked lignin, mix with each other across the fibre-fibre or even more probable over the fibril-fibril contact surface. While the hemicellulose can intermix already at room temperature under moist conditions, the lignin intermixes more easily at the higher temperature we use. We do not know how far the hemicellulose or lignin could move within the fibre walls, but it seems that the amount of lignin present on the fibre surfaces plays an important role.