Polysaccharides Dissolved from Norway Spruce in Thermomechanical Pulping and Peroxide Bleaching

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Abstract
Methanolysis, followed by GC analysis, was used to determine the amount and composition of sugar units comprising the carbohydrates present in water samples from unbleached and peroxide bleached thermomechanical pulp (TMP). Circulation experiments were carried out in the laboratory to simulate white water system closure in mechanical pulp mills. Water samples from simulated “closed” water systems were fractionated to determine the composition of the neutral and anionic polysaccharides released from the TMP samples. Decreasing fresh water consumption from 100 to 10 m3/t, for both unbleached and peroxide bleached TMP, had little influence on the overall sugar composition of the carbohydrates in the water samples. Alkaline peroxide bleaching resulted in substantial changes in the sugar composition of the carbohydrates comprising the dissolved and colloidal substances (DCS) in the water samples. The polysaccharides released from unbleached TMP were comprised of neutral (88%) and anionic (12%) polysaccharides. The neutral polysaccharides were mostly O-acetyl-galactoglucomannans and the anionic polysaccharides were mostly arabinogalactans. The galactoglucomannans, partially acetylated when dissolved in the unbleached suspension, were deacetylated by the alkaline conditions of peroxide bleaching, thereby causing their substantial adsorption onto the TMP fibers. The anionic arabinogalactans probably originated from heartwood present in the TMP furnish. About half of the dissolved polysaccharides present after peroxide bleaching were neutral, probably comprised mostly of galactoglucomannans. Anionic galacturonans and arabino-(4-O-methylglucurono)xylans were released from the pulp in peroxide bleaching. Anionic arabinogalactans were also present after alkaline peroxide bleaching.