Effect of progressive recycling on cellulose fiber surface properties

Abstract

Hornification is the term used to describe the irreversible changes that occur in cellulose fibers due to recycling. While the effects of hornification have been documented, there are several conflicting hypotheses that attempt to explain the causes. In this research, 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 tensile strength decrease 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. Image analysis of the fiber surface supports this conclusion, revealing a decrease in the surface roughness. The irregularity of fiber surfaces is apparent in AFM images at all but the smallest scales, with macroscopic fiber features dominating the roughness analyses of images larger than 2500 square nanometers. 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, revealing the limitations of AFM analysis of samples with a high degree of variability.