Soy-protein is used, for the first time, as bulk binder in papermaking from wood pulps (hardwood and softwood pulps). Wood pulps represent the major pulps used for paper production. A green denaturing method, involving only biodegradable compounds, was found sufficient to expose the functional groups of proteins. Addition of denatured soy-protein caused a considerable increase in all strength properties of paper, at all beating degrees. The effect was highest in non-wood (for comparison) followed by hardwood and softwood pulps respectively. This could be correlated to ratio of fiber length/width, fines, and hemicellulose content of pulps. The less the ratio, and the higher the fines and hemicellulose content, the more was the increase in strength properties; due to more exposed surfaces. The outstanding effect of soy-protein was magnifying the desired opacifying effect and retention of inorganic fillers e.g. kaolin, while eliminating the loss in strength, which occurs normally due to fillers.
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