Opportunities in Wet-End Chemistry: Feature Essay, from Jan. 2001
"Do Recycled Fibers Require Higher Levels of Additives"
Martin A. Hubbe
Dept. Wood & Paper Sci., N.C. State Univ., Box 8005, Raleigh, NC 27695-8005
Citation (public domain): http://www4.ncsu.edu/~hubbe/new
Recycled papermaking fibers are just like virgin fibers, only different. Despite the differences, papermakers often produce similar grades with and without recycled fibers. Sometimes this is made possible by adjustments in the levels of chemical additives. So today's column will address a related question, "Do recycled fibers require higher levels of chemical additives?"
First of all, let's consider ways in which recycled fibers differ from those that are produced for the first time from wood chips. Let's assume that the fibers are kraft fibers, though much of the discussion that follows will be true of various fiber types.
The differences between recycled and virgin fibers often lie in two general areas:
1. Recycled fibers tend to be contaminated. The potential contaminants include stickies, waxes, ink, dirt, fillers, and process additives. See other columns in this issue for discussion of some of these contaminants.
2. Recycled kraft fibers lack some of their original ability to bond with each other. The decreased bonding ability is related to such factors as closure of submicroscopic, slit-like pores in the cell wall, crystallization of cellulose chains, and reduced capacity of the fibers to hold water. These factors can be reversed, at least to some extent, by additional refining, but there is always a concern that refining of waste furnish will hurt freeness and create excessive levels of fiber fines. Speaking of fines, recent work has shown that recycled fiber fines act a lot like fillers; they have a poorer than usual contribution to inter-fiber bonding .
So, what about chemical requirements?
Let's first tackle an issue that ought to be easy - strength additives. I just told you about changes that occur to kraft fibers when they are dried, making them less suited for inter-fiber bonding. In addition, the presence of fillers, previously dried fiber fines, and surface-active agents in the furnish all tend to hurt strength. That is, if those items have not been removed during a de-inking treatment.
Cationic starch is widely used to enhance the bonding strength of either virgin or recycled papers, though it is likely that papermakers using waste furnish use higher levels of it. But let's keep things in perspective: Regardless of the furnish type, papermakers know that they get diminishing returns if starch is added above some critical level, often in the range of 20 to 30 lb/ton. To get the next increment of additional strength, the solution may be to replace some of the starch with something more expensive, e.g. and acrylamide-based dry-strength additive.
Sizing agent usage is often found to be higher when one is using waste furnish in place of virgin stock. This is exactly contrary to expectations based on the work of Sjöström and Wågberg . These authors showed that alkylketene dimer size in waste paper retains some of its effect when recycled, potentially decreasing the demand for sizing agents in the next cycle. The difference may be due to the anti-sizing effects of surfactants used in the de-inking of waste furnish.
Drainage and retention aid use tends to become more critical with waste furnish, especially if most of the fines are intended to become product, not sludge from de-inking or wastewater treatment.
A possible exception to the rule of "recycled fibers require more" is the case of cationic materials such as alum. The original cationic demand of a fiber furnish tends to be reduced by two factors during the production of paper. First, the cationic demand is partly neutralized by cationic additives used in making the virgin paper. Second, the paper machine itself acts as a washing operation; not all of the dissolved and colloidal anionic materials are retained. So in certain recycled grades such as linerboard, where the waste paper does not contain highly anionic coating ingredients, one should not be surprised by a lower requirement for such things as alum, PAC, or polyamines.
1. Rundlöf, M., Htun, M., Höglund, H., and Wågberg, L., "Mechanical Pulp Fines of Poor Quality - Characteristics and Influence of White Water," J. Pulp Paper Sci. 26 (9): 308 (2000).
2. Sjöström, L., and Ödberg,
L., "Influence of Wet-End Chemicals on the Recyclability of Paper,"
Papier 51 (6A): V69 (1997).