Effect of paper structure on the load elongation behaviour of fibre-to-fibre bonds

The deformation of paper and its fibre bonds was studied by straining thin paper structures inside the specimen chamber of a scanning electron microscope. The average bond strength values of different sheet structures were characterized by several methods. The handsheet structures were varied by refining and by addition of bond strength chemicals. The preliminary results obtained show that the structure of the sheet has a pronounced effect on the elongation properties of paper and of its fibres. The coarseness of the fibres had a distinct effect on the loading capacity of fibres and on the simultaneous straining behaviour. The thin wall springwood fibres often became inactive only after the final rupture of the structure. The summerwood fibres tended to become inactive earlier through breakage of the fibre bonds. A blinking light phenomenon was observed during the SEM straining of some paper specimens. The light blinks were interpreted as complete breakages of fibre-to-fibre bonds. The structural features of the studied handsheets had different effects on the bond strength values obtained by the various methods. These results seemed to indicate that refining produced a sheet structure which could be loaded in a more homogeneous manner. This was also reflected as higher values of bond strength. All bond strength methods used showed that starch increases the bond strength and that the debonding chemical decreases it.