Light-scattering coefficient as a measure of specific surface area in mechanical pulp laboratory sheets

Relative bonded area (RBA) is an important paper structure defining parameter that controls strength and optical properties of the paper. The traditional determination of RBA relies on the measurement of specific surface area at various bonding states (Ingmansson-Thode method), and is significantly easier if the scattering coefficient can be used as a measure of specific surface area. However, in mechanical pulps, where the absorption coefficient might affect the scattering coefficient, and the pore size structure is variable, the use of scattering coefficient as a measure of specific surface area cannot be taken for granted. In this paper, the fundamental relationship between scattering coefficient and specific surface area in mechanical pulps was studied using mercury porosimetry and BET nitrogen absorption to measure specific surface area. It is shown that the scattering coefficient is a measure of specific surface area when the wavelength of light used is above that which the scattering coefficient is not limited by significant absorption. The limit depends on the composition of the sheet and the pressing level and type used to induce bonding. However, there was no significant reduction in scattering coefficient in any of the samples when the wavelength of light used to measure scattering was above 600nm. The scattering efficiency, defined as the ratio of scattering coefficient and specific surface area, was fully explained by the wavelength used to measure scattering coefficient. A plausible explanation for the wavelength dependency of the scattering efficiency was given, which relates the increase in scattering efficiency to the change in refractive index of the material when the wavelength of light to measure scattering coefficient is altered. The implications of this mechanism were discussed.