Investigation of the effect of drying and refining on the fiber–fiber shear bond strength measured using tensile fracture line analysis of sheets weakened by acid gas exposure

No reliable method exists for measuring the cellulose fiber–fiber shear bond strength in paper. This paper reports a simple method for measuring the fiber–fiber shear bond strength by weakening the fibers independently of the bonds in a sheet of paper, using acid vapor, until all the fibers break across the fracture line. The bond strength is then calculated from the fiber strength, as measured by the zero span test, at the point where the fibers first are weakened such that they all break. The method was used to calculate the average bond strength of handsheets made out of two different pulps. The first pulp was a never dried, 60% yield, unbleached radiata pine. The bond strength was 25.0 ± 3.3 MPa. Drying the fibers before reslushing and making sheets reduced the bond strength by up to 33%, with the reduction depending on the severity of the drying treatment. The second pulp was a bleached dried softwood kraft and was used to investigate the effect of low consistency refining on bond strength. The bond strength increased from 13.7 ± 1.0 MPa for the sheets made from the unrefined pulp to 37.0 ± 1.0 MPa, for the sheets made from the most heavily refined pulp. The bond strength measurements are considerably higher than previous literature estimates for the shear bond strength. The causes for the discrepancy include stress concentrations in tests of single fiber–fiber bonds.