The disintegration of single fibres to fibre fragments and fines in mechanical pulping provides a high specific surface area. With increasing treatment (energy input) in the processes the
amount of fibre fragments and fines is increasing, at the same time the fibres become more slender
and more flexible. Increasing amount of fibre fragments and fines increases both the bounded
and the unbounded surface area in a sheet with mechanical pulp. The high fraction of
unbounded surface area in the relatively loose sheet structure, that can be built with the relatively
long stiff fibres, gives the sheet its high light scattering ability, Figure 4.6.