The impact of sulphonation and hot-pressing on low-energy high-temperature chemi-thermomechanical pulp

Abstract Hot-pressing high-yield pulp-based paper, at a temperature well above the softening temperature of lignin, increases paper density and strength. Here, it is investigated whether improved paper strength can be achieved and whether lower pressing temperatures can be used in combination with the increased sulphonation of high-temperature chemi-thermomechanical pulp (HTCTMP). Moist paper sheets from low-energy Norway spruce HTCTMP were hot pressed at temperatures up to 270 °C. Sulphite charges of 25–120 kg/bdt were used during impregnation, preheating, and refining at 180 °C with an electric energy demand of 370–500 kW h/bdt to a shive content of 1%. The pulps were mixed with 20% bleached unrefined kraft pulp to ensure that the sheet formation would not be hampered by the pulp coarseness. A tensile index of 70 kN m/kg was reached with the highest sulphite dosage at a pressing temperature of only 150 °C, versus 60 kN m/kg for the corresponding market CTMP. To obtain high wet strength, the highest temperature was required, while the sulphite charge was found to be of minor importance. This study has shown that it is possible to obtain strong and wet-stable paper products from HTCTMP, with a yield of 94–96% and a low energy demand at reduced pressing temperature.