Effect of recycling on the properties of paper surfaces

AFM surface adhesion measurements made on virgin and recycled bleached kraft pulp show that recycling increases the apparent hydrophilicity of the fiber surface. Yet, the water retention values and strength decreases as expected, which is consistent with internal cross-linking of the bonding sites and a reduction in hydrophilicity. Recycling does not affect the amount of monolayer water bound to the fiber surface indicating that the pore water is reduced but not the water bound to fiber surfaces. It is proposed that the contact area between the AFM tip and the fiber is greater for recycled material than for virgin. This could be caused by fiber shrinkage, changes in the angle of contact, lamination of fibrils, or other processes. Hence, in this instance, the surface adhesion values are more a measure of the topography of the surface than of its chemistry. An application to newsprint is illustrated.