Investigation of fibre-fibre bonding and effect on sheet mechanical properties

Paper is made from fibres and other components where its properties are controlled by the properties of these components, as well as their structural arrangement in the fibres network and the papermaking conditions. The lack of understanding in the role of fibre-fibre bond strength in paper mechanical properties has led to the need for developing an accurate way to quantify fibre-fibre bond strength. The first part of the research work presented in this thesis focuses on the validation of the assumption that fibre-fibre bonds are not affected by hydrochloric acid vapour when using acid gas exposure method to measure fibre-fibre bond strength. Peel test and acid gas exposure on regenerated cellulose films were carried out; however the results obtained from both approaches taken suggested that future work is required to improve the reliability of the acid gas method. The second part of this thesis concentrates on the studies of the effect of formation on strength and elastic modulus, as well as the effect of spatial non-uniformity on mechanical properties of paper sheets by loading cellulose sheet samples to fracture and then select the intact part of the same sample for retesting/s. The initial study was focused on investigating the bond degradation prior to fracture by comparing the experimental results of both elastic modulus and sheet strength of three types of pulp samples that have been previously been loaded to close to failure; with the analytical discrete transfer shear-lag model. The subsequent study used two of the same pulps from the first study with longer samples. Within experimental uncertainties, the retested strength was always higher than the preceding strength, which agrees well with the first study. Also, for both high and low aspect ratio fibres used showed that increasing the solids content of forming cellulose sheet sample, decreased the entire strength distribution, which implying that the increased solids content of forming decreased the strength uniformly through the sample, not just at the weakest point through the sample.