Measurement of Elastic Constant of Pulp Fiber Wall by Scanning Acoustic Microscope

Abstract

An acoustic micro-metrology technique of a scanning acoustic microscope, as acoustic material signature, was applied to measure the elastic constant of softwood bleached kraft pulp fiber wall. The Rayleigh wave speed measurement of cross sectional surface of heat-treated and untreated single softwood bleached kraft pulp fibers, which were 3, 520±170m/s and 3, 240±180m/s, respectively, was achieved by this technique. This evidently means that the wall of heat-treated fibers was stiffer than that of the untreated ones because the higher the Rayleigh wave speed, the higher the elastic constant (C44). Furthermore. if the ideal component of fiber wall is supposedly composed from a majority of a transversely isotropic S2 layer, the calculation of C44of the fiber wall could possibly be demonstrated by applying a Rayleigh wave speed to shear wave speed ratio of 0.93 and a density of 1.5g/cm3for a close-packed cellulosic material of the S2layer according to the theoretical equation. The calculated C44 values for the heat-treated and untreated fiber wall were 22±2 GPa and 18+2 GPa, respectively. However, handsheets made from unbeaten heat-treated fibers had a lower in-plane shear modulus than those from unbeaten unheated fibers. These results indicate the effect of heat treatment on elasticity of a fiber wall which is generally known as hornification.