758 sept 26 1978 the heavy chemical treatment will result in a dissolution of wood components giving a considerably lower yield furthermore to achieve such a degree of lignin sulfonation as stated by ford et al a pressurized device for chip treatment is needed including the use of concentrated sulfite solutions high temperature during prolonged treatment time 34 cooking 34 and a system for recovery of chemicals and waste liquor treatment thus a properly performed sulfonation in good balance with thermal softening is an efficient tool for improving thermomechanical pulping from the discussion and the results presented it also follows that the use of a pressurized first stage refiner with a higher degree of thermal softening must be compensated for by a lower degree of chemical softening i e to retain optimum conditions a lower sulfite impregnating level is needed cf fig1 if a two stage operation is performed it is not essential whether the second refining stage is carried out in an open refiner or not although a non pressurized refiner is mostly used in this stage thus it must be borne in mind that only the conditions in the first stage refiner pressurized or nonpressurized are essential with respect to the critical balance between chemical and thermal softening as can be seen from the illustrative examples the partial substitution of pine for spruce in the raw material called for an increase.