Weakening of paper strength by wood resin

The effects of retained wood resin and polysaccharides on paper strength, optical properties and friction were studied. Wood resin, glucomannans and pectic acids were extracted from TMP. The wood resin was added in colloidal form to the fibre suspensions and aggregated by the addition of cationic polymers or electrolytes. In part of the tests, polysaccharides isolated from TMP were also added. Hand sheets were made and tested. The amounts of resin and polysaccharides in the sheets were determined by extraction and gas chromatography. The strength properties of the sheets weakened when the amount of wood resin was increased up to 2 mg/g fibres. Above 2 mg/g, no significant further decrease in strength properties was obtained. The static friction was lowered when the amount of wood resin was increased in the sheets. However, the optical properties were not significantly affected, even at high resin contents. When polysaccharides were added to fibre suspensions, together with wood resin, the strength properties were higher compared, at the same resin content, to sheets without added polysaccharides. Glucomannans and pectic acids especially diminished the negative effect on paper strength caused by high resin content. Some of the polysaccharides were probably adsorbed onto the surface of the TMP fibres or wood resin droplets, which increased the hydrophilicity of the surfaces, thereby promoting the formation of the hydrogen, or other, bonds between fibres. Aggregates of polysaccharides, wood resin and cationic polymer could also be retained in the fibre network.