DETERMINATION OF LIGHT-SCATTERING COEFFICIENT OF DARK AND HEAVY SHEETS

Strength and stiffness in paper products depend on the state of bonding in the sheet. There exists no universally accepted method for measuring the state of bonding, but the specific light scattering coefficient is useful for this purpose. The standard procedure uses reflectance measurements. This method cannot be used with heavy boards, nor with unbleached grades beyond very low basis weights, simply because the opacity of such grades is too high. A practical method for measuring the specific light scattering coefficient of dark and heavy products is based on a combination of light transmission and reflectance measurements and on the use of the Kubelka-Munk equations in the traditional manner for evaluating the light scattering coefficient. A new instrument has been developed. The theory and the design of the instrument are reviewed as well as the results of demonstration measurements on unbleached board and other grades.