Synthesis of polyvinylamine microgels and their effect on paper strength

Various polymers are routinely used in papermaking to improve paper strength. The effect of chemical composition of the polymers, e.g. functional groups, molecular weight etc., on the strength properties of paper has been intensively studied. However, the effect of the physical structure of polymers (such as linear polymer versus microgel) based on the same polymer chemistry is rarely understood. In this thesis, a novel approach, using polymeric microgels to improve paper strength, is proposed and studied. This is based on the idea that relatively large gel particles can fill in the voids between rough fibre surfaces in the fibre-fibre bonding areas.; Polyvinylamine (PVAm) was chosen as the polymer for this study. Four methods were explored for the preparation of PVAm microgels, including inverse emulsion polymerization, post-polymerization crosslinking, physical milling and precipitation polymerization. Finally, six completely hydrolyzed PVAm microgels with various mean size and crosslinking degree were synthesized by precipitation polymerization combining with physical milling. The transmission electron microscopy (TEM) and size measurements revealed that these gels had wide size distribution and irregular shapes.; The effect of the six gels on paper strength was studied by handsheet making experiments and the results were compared with those of a linear PVAm (MW=950kDa). It was found that at the polymer dosage above 0.2 wt%, the strength of handsheets with added some microgels increased with the polymer dosage; whereas the linear PVAm was not sensitive to the polymer dosage. The best microgel sample, LS (L=low crosslinking, S=small size), could double the dry tensile strength and increase the internal bond strength 3 times and the wet tensile strength 6 times at 0.65 wt% dosage. The size and the crosslinking degree of microgels had significant effect on their performance. The small size and low crosslinking degree sample showed the best results.; It was also found that the treatment to fibres with linear PVAm first can prevent the adsorption of PVAm microgels and the gels must be introduced into paper before the paper sheet formation so that they can enter the fibre-fibre joints. Two mechanisms were proposed to explain the superiority of microgels over the linear polymer in paper tests. Firstly, microgels can achieve higher adsorption on fibre surfaces because of their bulk volume; secondly, microgels can fill in the voids between rough fibre surfaces in the bonding areas and conform to create more contact areas in molecular level to increase paper strength.; Adhesion studies carried out on wet cellulose membranes revealed that PVAm microgels are not intrinsically stronger adhesives than the linear PVAm. The confocal laser scanning microscopy (CLSM) and delamination results further proved the adsorption and space-filling mechanisms.