The role of surface polymer compability in the formation of fiber/fiber bonds in paper

The tensile strength of unbeaten kraft pulp handsheets was measured as a function of the compatibility of polymers adsorbed on the fibers. Two types of fibers were prepared, one was treated with slightly cationic dextran and the second was treated with slightly hydrophobic cationic dextran. The dextrans were chosen because both are paper strength enhancing polymers and yet they are incompatible with each other. Either dextran alone significantly enhanced paper strength whereas sheets made with mixtures were weaker. The results are interpreted in terms of the polymer compatibility. When a fiber/fiber bond forms between two surfaces bearing incompatible adsorbed polymers, the polymers will not interdiffuse during sheet consolidation thus giving a relatively weak interfiber bond.