INFLUENCE OF IONIC GROUPS AND THEIR COUNTERIONS ON THE SOFTENING PROPERTIES OF WOOD MATERIALS

In order to reveal the influence of ionic groups acid their counterion s on lignin softening, the mechanical properties of wood materials und er moist conditions have been investigated at 90 degrees C. The soften ing of the wood fibre is one of the important factors in mechanical pu lping, and the effects on lignin softening of the content of ionic gro ups, i.e, carboxylic acid groups and sulphonic acid groups, and the co unterions to these, i.e. Ca2+, Ma(+) and H+, have therefore been inves tigated. The softening has been studied by impregnating glass braids w ith homogenized wood material from an aqueous suspension and measuring the torsional rigidity of this body at 90 degrees C as a function of the relative humidity. It was shown that with increasing content of io nic groups, the softening is moved to lower relative humidity to the s ame extent both for carboxylic acid groups and sulphonic acid groups. With sodium as the counterion, the ionic materials always have a softe ning at lower relative humidity than with calcium as the counterion. W hen the metal ions have been removed by acid washing, the sulphonated wood materials have a softening lower than that with sodium or calcium as counterion due to the dissociation of the strong acid. In the case of carboxylic acids on the other hand, the softening is higher when t he metal ions have been removed than with sodium or calcium as counter ion due to the lower tendency of this weak acid to dissociate.