Chemical changes in thermomechanical pulp at alkaline conditions

Chemical changes in thermomechanical pulp (TMP) from Norway spruce at alkaline conditions were examined at pH levels 9, 10, and 11, and temperatures of 40, 50, and 60degreesC. Alkali treatments were also made in the presence of hydrogen peroxide. The anionic charge of the fibres increased rapidly, following the kinetics of ester bond cleavage. Already at pH 9 and 60degreesC, acetic acid, methanol, pectic acids, and some lignin material were dissolved. Dissolution of xylans required higher pH. Lignin and xylans slowly leached into the water without affecting fibre charge. Leaching caused substantial material loss during extended alkali treatment. At pH 11, hydrogen peroxide almost doubled the overall loss of wood material compared with alkali only. This was partly due to acceleration of the dissolution of xylans and lignin material, but also due to considerable formation of low-molar-mass oxidation products. The effect of hydrogen peroxide on fibre charge was considerable also during extended treatment.