The initial softening of the wood material has a large impact on the fracture mechanics in the
wood and on the fibre structure in the subsequent processing. Processes therefore give pulps
with different quality profiles that are related to the different heating temperatures before the
mechanical processing. The quality profile can in addition be influenced by impregnating the
wood with lignin softening chemicals, as in the CTMP process, or with small amounts of chemicals
added in the refiner. With pre-treatment and processing at increasingly higher temperatures
the fractures in the wood structure is successively more concentrated to the lignin rich
areas, i.e. to the middle lamella area, (see Figure 4.13) which have been most effectively softened.
This is especially obvious with preheating and processing at temperatures well above the
softening temperature of the lignin, which for spruce chips is at 125–145 °C at the processing
conditions in question. The final pulp quality is controlled by the energy input to the mechanical
processing of the fibre material. The intensity and the speed of the processing have a large influence
on the final properties of the fibres as well