Zhang et al., Proc. TAPPI 2001 Papermakers Conf., digital doc. 1 EFFECT OF CHEMICAL PRETREATMENTS OF NEVER-DRIED PULP ON THE STRENGTH OF RECYCLED LINERBOARD Min Zhang Martin A. Hubbe Dept. of Wood & Paper Science Dept. of Wood & Paper Science North Carolina State University North Carolina State University Box 8005; Raleigh, NC 27695 Box 8005; Raleigh, NC 27695 mzhang@unity.ncsu.edu http://www4.ncsu.edu/~hubbe Richard A. Venditti John A. Heitmann Dept. of Wood & Paper Science Dept. of Wood & Paper Science North Carolina State University North Carolina State University Box 8005; Raleigh, NC 27695 Box 8005; Raleigh, NC 27695 richard_venditti@ ncsu.edu heitmann@ncsu.edu ABSTRACT According to the American Forest and Paper Association (AF&PA) the recovery rate of corrugated boxes used in the US now exceeds 75%. In principle the recycling of boxes saves fiber resources and requires less total energy. However, further progress in old corrugated container (OCC) recycling faces a potential barrier. It is known that recycled kraft fibers have a reduced bonding ability. The approach taken in this study was to pre-treat the never-dried fibers before the first cycle of papermaking. New data have been obtained with never-dried, refined, unbleached kraft pulp. Simple drying, low-shear disintegration, and forming without further refining yielded a loss in compression strength in the range 19 to 26%, depending on the pulp batch. Pretreatment with various chemical agents was able to compensate for some of the strength loss. Two general classes of treatment agent were identified that were able to favorably affect the strength of recycled sheets. Certain low-molecular weight materials such as sucrose appeared to interfere with the mechanism of pore closure during the initial drying. In contrast, certain high-mass polyelectrolytes such as guar gum products, cationic starch, and polyelectrolyte complexes appeared to affect the adhesiveness of the fiber exteriors of the repulped fibers. INTRODUCTION With increases in the recycling of waste paper and paperboard there has been increased interest in the changes of fiber properties during recycling. Previous work has shown a deterioration of paper strength properties due to recycling [1-7]. In many cases it has been shown that the main factor contributing to this change in sheet properties is a reduced bonding ability of the fibers. The process leading to reduced bonding has been described as irreversible hornification, which implies a stiffening or hardening of the fiber [8-10]. Using the water retention value (WRV) as measuring the internal fiber swelling capacity [11], recycled pulp has a less swollen nature in comparison with the never-dried pulp [1,3,8,9,12-14]. This closure of the pore structure has also been demonstrated by the fact that the fiber surfaces become less susceptible to enzymatic attack [15]. Ellis and Sedlachek found that the fiber-fiber bond strength of recycled fiber per unit of optically bonded area is equivalent to virgin fiber [8]. Both virgin and recycled pulp yielded the same unbonded scattering coefficient if the recycled fiber received no additional refining. This implies that the weakness of the recycled paper sheet is due largely to the inability of the fibers to develop bonded area. Lundberg and de Ruvo found that higher drying temperatures during drying of papers resulted in less swelling when the fibers were placed back into water [14]. Pulp prepared from the sheet dried at higher temperature could not achieve as high a level of WRV as the virgin pulp, even after prolonged beating. In the mechanism proposed by Stone and Scallan, it was suggested that slit-like pores in the cell wall closed up preferentially during drying [16]. The relative significance of this effect increases with increased refining [6, 17]. It appears that drying may lead to some sort of plastic flow at the crack interface that results in strain hardening of the fiber [8,18].