A new analysis of filler effects on paper strength

Information about the disruption of fiber/fiber bonds by fillers was derived from measurements of the peel force required to delaminate two-ply handsheets with a sparse layer of fillers between the plies. Confirming evidence in the literature, the smallest filler particles have the greatest negative effect on delamination strength. A surprising finding was that polystyrene spheres had little effect on strength. Electron microscopic evidence suggests that the particles rolled away from fiber/fiber bonding regions during sheet lamination. From a modeling perspective, the decline in peel force with filler content follows a simple exponential decay with an exponent -βΓ where Γ (g/m2) is the filler coverage, and β is the specific debonding index, a measure of the fillers' tendency to weaken paper. A model based on the Poisson deposition of disks on a surface indicates that each filler particle disrupted a fiber/fiber bonding area between 4 and 10 times its own projected area. The behaviors of different fillers could not be normalized by plotting as a function of the total surface area or total number of particles in paper.