Surface Anisotropy Measurement on Paper and Cardboard: Looking at Two-Sideness, Mean Fibril Angle and Various Dimensional Stability Problems, 1999 International Paper Physics Conference Proceedings

Pierre Bernard, Alain Charlebois

Uniformity of physical properties of paper continues to be one of the most serious quality issues in today’s paper mills. The large-scale “average” profile can often be controlled effectively with today’s technologies. On the other hand, to detect and control sizes from several microns to a few centimeters remains the industry’s biggest challenge. The fiber orientation distribution is one such property. However, it is customary to encompass fiber orientation distribution and other derived or related properties into a more generic category called the paper anisotropy. The ultrasonic technique is probably the most wide spread approach for the measurement of paper anisotropy. It actually measures the ratio of ultrasonic velocity in the machine and transverse directions. Over the last several years, INO has developed and perfected an entirely new instrument (SAM) to measure paper anisotropy. It is a non-destructive, non-contact laser technique, which basically measures the difference in light propagation in various directions in the plane of the sheet. The SAM instrument then provides the user with two basic parameters: the Surface Anisotropy Index (SAI), a parameter closely related to the TSI index measured by the ultrasonic technique, and the Angle of maximum SAI, or what we have come to call the Surface Anisotropy Angle (SAA). These parameters are fundamentally related to the stiffness of the microfibril chains of the fiber’s S2 layer. As a consequence, the instrument also provides direct information on a third parameter, the mean fibril angle, a parameter important in itself because it directly affects the fiber mechanical strength. The major differences between this technique and more conventional ultrasonic measurements is that it can be used to study two-sideness problems since top and bottom sides of the paper are measured independently. The spatial resolution is also excellent: measurements with spatial resolution well below the millimeter are possible, although a 5 mm diameter laser beam is used in the standard configuration. This allows the study of various small-scale dimensional stability problems not easily tackled by other means. In this presentation, we will describe how the instrument works, what it measures and will show several applications, from small-scale mappings to cross-machine profiles and various two-sideness problems.