Effect of Fiber Hornification in Recycling on Bonding Potential at Interfiber Crossings. Confocal Laser-scanning Microscopy.

Abstract

Hardwood bleached kraft pulp (HBKP) handsheets were recycled with heat treatment as a model of paper recycling. The apparent density and the tensile index decreased from 0.67 to 0.51 g/cm3 and 46.3 to 12.9 Nm/g, respectively, after they were recycled for four times. Decrease in fines content and hornification of fibers with recycling, which were related to the reduction in total bonding strength of recycled handsheets, were investigated as possibly influential factors on this behavior. A possibility of strengthening role of fines by covering fiber surfaces including filling peripheral regions of interfiber crossings was presented by confocal laser-scanning microscopy (CLSM). The CLSM micrographs of cross-sections of HBKP fibers evidently showed hornification effect on re-swelling in the wet state, which strongly affected their conformability during wet web forming, in each cycle of handsheets recycling. Recycled fibers showed inferior re-swelling capability and conformability, i.e. they could not provide sufficient interfiber contacts in recycled handsheets, as in CLSM micrographs clearly demonstrated. Because no decrease in fines content was noticeable by handsheets recycling, the increase in un-bonded areas of recycled handsheets is suggested to rely on the reduction in re-swelling capability or conformability of rewetted recycled fibers. The decrease in strength of recycled handsheets was also certainly attributed to the increase in un-bonded areas of interfiber crossings by CLSM.