MICROGEL BASED ADHESIVES FOR WET PAPER STRENGTH

Abstract

<p>The interactions of microgel based adhesives with cellulose were studied by peel test of cellulose laminates and tensile test of handsheets. The objective of this project was to create design rules for microgel based adhesives so as to improve the wet paper strength. Colloidal microgel based adhesives were formed by coating carboxylated poly(Nisopropylacrylamide) (PNIPAM) microgels with polyvinylamine (PVAm). The characterization of the microgel base adhesives were performed by electrophoretic mobilities, dynamic light scattering, and potentiometric titration. The microgel based adhesives were pH sensitive and their swelling behaviour was related to the composition of PVAm in the microgels. The maximum amount of PVAm binding to microgels depends on the location of charges in the microgels and the molecular weight of PVAm. The binding process of PVAm to microgels was monitored by quartz crystal microbalance measurements. It is proposed that the binding of PVAm to microgels is controlled by the rate of initial attachment of PVAm and the rate of reconfiguration of PVAm on the microgels. The microgel based adhesives were laminated between oxidized cellulose films and the wet adhesion of microgel based adhesives with cellulose was studied by a 90° peel test. The wet delamination force was measured as a function of PVAm content, PVAm molecular weight, coverage of adhesives on cellulose films, size of adhesives, stiffness of adhesives and the roughness of cellulose films. The wet adhesion of microgel based adhesives with cellulose increased with PVAm content in the microgels, and decreased with microgel stiffness. The molecular weight of PVAm did not influence the performance of adhesives. The effect of microgel size on wet adhesion with cellulose was related to the roughness of cellulose films. Larger microgels did fill the voids between rough cellulose films to create more contact area with these films resulting in higher wet adhesion. By contrast, for smooth cellulose films, the size of microgels didn’t affect the wet adhesion. Finally, this basic research was extended to a practical situation. The microgel based adhesives were added to unbeaten, bleached softwood pulp to prepare handsheets and their ability to enhance wet paper strength was evaluated by tensile test. The wet paper strength increased with PVAm content of the microgels. For linear PVAm, high molecular weight PVAm was more effective as a wet strength adhesive while for PVAm coated microgels, the molecular weight was not significant for wet paper strength. With the aid of PVAm coating, solid carboxylated polystyrene particles improved the wet paper strength. However the wet strength of paper treated with PVAm coated microgels was larger than that treated with PVAm coated polystyrene by a factor of 2.</p>