Investigating Paper Structure Using Light Scattering, 2006 Coating & Graphic Arts Conference

The structure of paper is critical in determining many important properties, such as strength, liquid absorption and optics. Although the size of the various components, such as the fibres and pigments, in a sheet are known, the overall structure and porosity can change depending on how they are assembled. Traditionally, the pore structure in paper has been estimated by using mercury porosimetry. However, this technique has many complications and does not always give reliable results. It is proposed that light scattering can be used to study changes in paper structure. Modern spectrophotometers allow the easy and quick measurement of light scattering at many different wavelengths. This may be exploited by using the knowledge that the negative gradient of a log/log plot of light scattering against wavelength depends on scattering size. The higher the gradient, normally referred to as the wavelength exponent, the smaller the scatter size. The illumination used in the spectrophotometer can affect the results, especially if UV-active species are present. Examples of changing scatter size are given for latices of different sizes in suspension, highly filled SC papers and coating systems produced from pigments of various particle sizes.