Light scattering in pulp and paper

Abstract: The pulp and paper industry of today is facing a highly competitive market where manufacturers are constantly seeking ways to reduce costs as well as to improve product specifications and quality. On-line quality control is one of many areas being actively developed, from the wood delivered to the plant to the fibers in the finished product. In general, on-line property measurements are best utilized by establishing an understanding of the underlying physics and using that knowledge in an optimal way to determine the parameter or property sought for. One specific area of development is to determine fiber orientation in a paper sheet. The fiber orientation is set by the production Process and is an important parameter since it defines e.g. strength and Optical properties of the paper. In the paper mill today, improvements can be made if the fiber orientation could be determined on-line and in real time. One way of doing this is to use non-contact, full-optical techniques to determine the light scattering pattern in a paper sheet. The objective of this work is to determine the properties of light scattering in paper and how this knowledge could be utilized in extracting information on the properties of paper. A numerical model utilizing light scattering in a sphere-cylinder medium is presented. The simulated medium can represent scattering in both wood pulp and paper. Wood fibers are represented as long, straight cylinders and smaller particles, like fines, are represented as small spherical particles. Scattering from fibers are determined by an analytical solution of Maxwell’s equations for scattering on infinitely long cylinders. The small spherical particles are described by Mie theory. Fibers can have random orientation as in the case of pulp, or aligned orientation as in paper. The layer-like anisotropic microstructure in paper is considered in the model. The model also employ the Stokes-Mueller formalism for the scattering particles, making the state of polarization possible to track. The effects of varyingvolume concentration and size of the scattering components on reflection, transmission and polarization of the incident light are investigated. The findings on the differences in depolarization and its spatial distribution opens for techniques that enables the relative proportions of fibers and fines in pulp to be determined. For aligned fiber structures it is shown both theoretically and experimentally that spatially resolved reflectance and transmittance exhibits directional dependence. This information could possibly be used in a robust, rapid and cheap device for on-line characterization in the paper production process.