MILD KRAFT TREATMENT OF LINERBOARD, CORRUGATED MEDIUM, AND BOX PLANT CLIPPINGS ABSTRACT INTRODUCTION

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1 MILD KRAFT TREATMENT OF LINERBOARD, CORRUGATED MEDIUM, AND BOX PLANT CLIPPINGS recycled fibers. Implementation of such a multiple-use scheme will require information on the behavior of OCC components during delignification under kraft pulping conditions. Thomas Reichert Graduate Student University of Washington Seattle, WA Said M. Abubakr Project Leader USDA Forest Service Forest Products Laboratory 1 Madison, WI William T. McKean Professor University of Washington Seattle, WA The complex composition of OCC will influence pulping results. The linerboard portion of OCC typically contains about one-half to two-thirds kraft pulp at kappa number 70 to 100. The kraft pulp may be up to 30% hardwood. The remaining material is recycled OCC.The OCC portion includesfibers originating from linerboard and corrugated medium derived from earlier-generation box material. As much as 100% recycled fiber may be present in test linerboard or bogus board. The corrugated medium usually consists of semichemicalhigh-yield pulp at kappa number 95 to 120. ABSTRACT Recycling of old corrugated containers (OCC) is complicated by the nature of OCC components, which respond differently to chemical recycling treatments. We studied the pulping responses of recycled box plant clippings (BPC), linerboard, and corrugated medium. Each material was kraft pulped to kappa numbers of 40 to 70. The BPC produced pulp with higher kappa numbers and lower yields compared to pulps made from linerboard and corrugated medium. Compression, burst, and tensile strength of paper made from BPC increased at lower kappa numbers, compared to the properties of paper made from linerboard and corrugated medium, which remained constant or decreased at lower kappa numbers. The results indicate that changes in composition and source of corrugated boxes have a strong impact on physical properties. INTRODUCTION Recovery and recycling rates for United States wastepaper have grown sharply over the last decade. Recycling of old corrugated containers (OCC) into container and board products has led this trend. An additional use of this high quality resource could involve pulping and conversion to lower kappa number, unbleached products. Ultimately, this multiple recycled material could be converted to bleached products (1), which may tolerate fines and strength characteristics associated with 1The Forest Products Laboratory is maintained in cooperation with the University of Wisconsin. This article was written and prepared by U.S. Government employees on official time, and it is therefore in the public domain and not subject to copyright. Typical North American high-quality OCC produced by kraft and semichemical processes consists of 60%to 80% softwood and 20% to 40% hardwood. Lower grade OCC may be contaminated with mechanical pulps, ink, and small amounts of hot-melt tape and wax. If the OCC is derived from a source outside the United States, it may also contain a variety of nonwood fibers. The effects of recycling on chemical pulp fibers like OCC are well-documented (2). Recycling by solely physical processes results in strength loss in later-generation boards. The losses are primarily attributedto the presence of previously dried, stiff, and hornified fibers (2). Mild chemical treatments such as alkaline soaking can reverse these effects to a great extent, can minimize fines generation during recycling, and can help to retain most of the strength potential (3). These treatments generally cause little or no change in lignin content, but are restricted to production of container and board products. More severe chemical treatments can lower lignin content, resulting in pulps with potential value for other types of paper products. Oxygen delignification lowers kappa number and produces more flexible fibers capable of greater bonding and strength (4). Kraft pulping of whole OCC to bleachable grades may offer process, product, and economic advantages relative to virgin fiber (1,5). Despite changes in the quality of virgin fibers and OCC, manufacturers must maintain established paper and board performance standards. Understanding the response of various OCC components to chemical recycling treatments will help in selecting processing steps and conditions for enhancing the strength potential of paper and board products containing recycled OCC. In this report, we describe the separate pulping responses of box plant clippings (BPC), linerboard, and corrugated medium. Conditions were selected to reach kappa numbers of processed pulp in the range 40 to 70. Physical properties of the resulting pulps are discussed Recycling Symposium / 125

2 METHODSAND MATERIALS The BPC, linerboard, and corrugated medium were each laboratory-pulped over a range of digester conditions. Each material was hydropulped, centrifuge dewatered to 30% consistency, fluffed, and stored at 3 C. Kappa numbers of BPC, linerboard, and corrugated medium were 120, 85 and 95, respectively. Vapor-phase kraft pulping was conducted in 10-L batch digesters. Fluffed pulp (150 g, ovendry [OD]) was thoroughly mixed with kraft liquor at a 3.5:1 liquor:solid ratio. Kraft liquor charges of 25% sulfidity and 3%-8%active alkali (on OD pulp) lowered kappa numbers by 10 to 50 units, depending on the furnish. The pulp was washed and passed through 0.15-mm screens, resulting in virtually no rejects. Pulp black liquor contained trace amounts of residual alkali. Pulps were refined to 550 ml Canadian standard freeness (CSF) and converted to 60-gkm 2 handsheets using TAPPI standard procedures. Tensile index, burst index, and compression strength (STFI) were determined. RESULTS AND DISCUSSION Results of pulping and strength tests are shown in Tables I and II and Figures 1 to 6. Pulping Box plants typically acquire corrugated medium and linerboard from several sources and match properties of available inventory to achieve target combined board specifications. Consequently, the corrugated medium and linerboard samples for this study were not the same material contained in the BPC sample. Thus, the BPC kappa number was not a weighted average of the linerboard and corrugated medium kappa values. Nevertheless, the results showed expected reductions in lignin content with higher active alkali concentration (Table I, Fig. 1). In kraft pulping of wood, 50% to 60% of the alkali consumed neutralizes easily-formed carbohydrate- and lignin-related acidic groups early in the pulping process (6). This is a one-time alkali demand. Consequently, alkali demand of OCC pulps is lower than that of wood pulps. In our study, pulps required about 40%to 50%of the active alkali compared to pulping wood chips to the same kappa value (Fig. 1). Figures 2 and 3 show relationships between yield, kappa number, and active alkali. Linerboard produced the highest yield at a given kappa number. Kraft pulping of virgin fibers eliminates easily-removed carbohydrates. Assuming that the linerboard contained substantial amounts of virgin kraft fiber, yield losses observed in our study were primarily the result of lignin removal. Table I. Result of Pulping Linerboard, Corrugated Medium, and Box Plant Clippings (BPC) at Various Alkali Contents Alkali Percent Grammage Caliper Density Kappa Sample (%) yield (g/m 2 ) (mil) (g/m 3 ) number Linerboard Corrugated medium BPC / TAPPI Proceedings

3 Fig. 1. Kappa number as a function of percent active alkali Fig. 3. Percent yield as a function of kappa number. Fig. 2. Percent yield as a function of percent active alkali. By contrast, semichemical pulps used in corrugated medium retain larger amounts of the original wood carbohydrates. In our study, removal of these materials during kraft pulping of OCC apparently caused lower yields than those observed with linerboard. Furthermore, the steeper slope in Figure 3 supports the contention that more carbohydrates were removed per unit of lignin removed. Generally, OCC yield should be a weighted average of the linerboard and corrugated medium that constituted the original box material. Pulp yields ranged from about 65% to 85% when OCC was kraft pulped to kappa number 20 to 70, respectively. These results agree with previous work (1, 5). In the study reported here, BPC kappa numbers fell at lower yields, confirming that the linerboard and corrugated medium of the BPC were of different origin than that of the linerboard and corrugated medium actually pulped. Nevertheless, at the lowerkapparange, pulp yields were similar to values expected for OCC pulping. Fig. 4. Compression strength index as a function of kappa number. Paper Properties Most linerboards contain some OCC fiber, with the z-direction distribution dependent on the number of plys in the sheet and the machine construction. Searcy (8) showed that compression strength increases with OCC fiber and fines content up to peak levels of about 40% and 30%, respectively. In our study, tensile and burst strength values were generally less influenced by OCC content. Edgewise compression strength. Compression strength of unbleached paper and board products depends on a balance of fiber strength and stiffness compared to bond strength. Modest amounts of high-lignin, stiff OCC fibers can increase linerboard compression strength relative to that of paper containing only virgin kraft fiber (8). The heavy 1995 Recycling Symposium / 127

4 Table II. Results of Strength Tests Compression Alkali Tensile Burst strength Sample (%) index index index Linerboard Corrugated medium BPC contribution of fiber properties relative to bond strength means that compression strength of linerboardand BPC should be only slightly dependent on changes in sheet density (9). These trends are confirmed in Table II. Conversely,compression strength of pulped corrugated medium rises with increase in sheet density. Semichemical pulps in corrugated medium normally bond poorly. Cooking these pulps to lower kappa numbers probably increases fiber flexibility and swelling. The lower lignin content fibers bond and conform better during pressing and thus reach higher density levels. Improved compression strength of corrugated medium could have resulted from moderate increases in density and fiber bonding (Fig. 4). These results show that kraft delignification of BPC caused an increase in compression strength, presumably for the same reasons discussed for corrugated medium. Conversely, linerboard compression strength tended to decrease slightly at lower kappa numbers. Kraft fibers, which dominate in linerboard, probably became more flexible during delignification. Bonding improved, but stiffness and compression strength decreased. Burst strength. Both fiber strength and bonding contribute substantially to strength of paper subjected to tensile stresses. Handsheets formed from pulped linerboard and BPC contained substantial amounts of kraft fibers, which retained strength and became more flexible at lower kappa numbers. These pulps were capable of high bonding potential; consequently, burst strength increased in the higher density sheets (Table II). Furthermore, BPC burst index increased at lower kappa numbers, presumably because of improved bonding (Fig. 5). Linerboard burst index decreased slightly with kappa number. The burst strength of paper made from delignified corrugated medium was virtually independent of sheet density and kappa number. This trend is probably related to the very poor bonding potential of this material relative to other furnishes, even at lower kappa numbers. Tensile strength. Tensile strength is dependent on the presence of long, strong, and well-bonded fibers. Approximately 60% of the material in recycled, pulped linerboard should fall in this category. The results showed that tensile behavior varied with fiber type (Table II, Fig. 6). The tensile index of recycled linerboard declined at lower kappa numbers; linerboard values were significantly below values obtained with virgin softwood kraft fibers (10) but similar to those of unbleached hardwood products. The difference is likely caused by the lower strength fractions of hardwood and semichemical pulps contained in the linerboard.the mixture of virgin kraft softwood and hardwood, and recycled and semichemical pulps in BPC influences tensile index. In our study, the tensile index of sheets made from BPC was very similar to that of linerboard. The corrugated medium showed the least potential for increasing tensile strength. Semichemical pulps seemed to retain poor bonding characteristics even after pulping to low lignin levels. CONCLUDING REMARKS The high kappa number box plant clippings (BPC) used in this study produced pulp with higher kappa numbers and lower yields compared to pulps made from linerboard and corrugated medium. The latter clearly originated from different sources and contained materials with lower lignin content materials compared to BPC materials. The differences in composition and pulping response were also reflected in paper properties. Compression, burst, and tensile strength of paper made from BPC increased at lower kappa numbers. Conversely, the strength properties of paper made from linerboard and corrugated medium remained constant or decreased at lower kappa numbers. The results indicate that the composition of old corrugated containers has a strong influence on kraft pulping. 128 / TAPPIProceedings

5 LITERATURE CITED 1. Bisner, H.M., Campbell, R., and McKean, W.T., Progress in Paper Recycling, Bleached Kraft Pulp From OCC, 3(1): 27 (1993). 2. Ferguson, L.D., R: The Effects on Fibres from Multiple Recycles, In Proceedings of TAPPI Recycling Symposium, 215 (1993). 3. Freeland, S.A., Sodium Hydroxide Treatment of Old Corrugated Containers (OCC) for Strength Improvement during Recycling, Master of Science Thesis, University of Washington (1993). Fig. 5. Burst index as a function of kappa number. 4. de Ruvo, A., Farnstrand, D.A., Hagen, N., and Haglund, N., TAPPI Journal, Upgrading Pulp From Corrugated Containers by Oxygen Delignification, 69(6): 100 (1986). Fig. 6. Tensile index as a function of kappa number. 5. Nguyen, X.T., Shariff, A., Earl, P.F., and Eamer, R.J., Progress in Paper Recycling, Bleached Pulps for Printing and Writing Papers From Old Corrugated Containers, 2(3): 25 (1993). 6. Rydholm, S.A., Pulping Processes, Interscience Publishers, New York, 1965, p Agarwal, N., Gustafson, R., and Arasakesari, S., Paperi ja Puu, Modelling the Effect of Chip Size in Kraft Pulping, 76(6-7): 410 (1994). 8. Searcy, S.M., The Impact of OCC on the Mechanical and Visual Properties of Multiply Linerboard, Master of Science Thesis, University of Washington (1993). 9. Paper: Structure and Properties, Bristow, J.A. and Kolseth, P., (eds.), Marcel Dekker, New York, 1986, p Recycling Symposium Proceedings on recycled paper TAPPI PRESS Technology Park/Atlanta P. O. Box Atlanta. GA , USA 1995 Recycling Symposium / 129