MILD KRAFT TREATMENT OF recycled fibers. Implementation of such a multiple-use scheme LINERBOARD, CORRUGATED MEDIUM, will require information on the behavior of OCC components AND BOX PLANT CLIPPINGS during delignification under kraft pulping conditions.

Thomas Reichert William T. McKean The complex composition of OCC will influence pulping Graduate Student Professor results. The linerboard portion of OCC typically contains about University of Washington University of Washington one-half to two-thirds kraft pulp at kappa number 70 to 100. Seattle, WA 98105 Seattle, WA 98105 The kraft pulp may be up to 30% hardwood. The remaining material is recycled OCC.The OCC portion includesfibers origi­ Said M. Abubakr nating from linerboard and corrugated medium derived from Project Leader earlier-generation box material. As much as 100% recycled USDA Forest Service fiber may be present in test linerboard or bogus board. The Forest Products Laboratory1 corrugated medium usually consists of semichemicalhigh-yield Madison, WI 53705-2398 pulp at kappa number 95 to 120.

Typical North American high-quality OCC produced by kraft ABSTRACT and semichemical processes consists of 60%to 80% softwood and 20% to 40% hardwood. Lower grade OCC may be con­ taminated with mechanical pulps, ink, and small amounts of Recycling of old corrugated containers (OCC) is complicated hot-melt tape and wax. If the OCC is derived from a source by the nature of OCC components, which respond differently outside the United States, it may also contain a variety of to chemical recycling treatments. We studied the pulping re­ nonwood fibers. sponses of recycled box plant clippings (BPC), linerboard, and corrugated medium. Each material was kraft pulped to kappa The effects of recycling on chemical pulp fibers like OCC are numbers of 40 to 70. The BPC produced pulp with higher kappa well-documented (2). Recycling by solely physical processes numbers and lower yields compared to pulps made from results in strength loss in later-generation boards. The losses linerboard and corrugated medium. Compression, burst, and are primarily attributedto the presence of previously dried, stiff, tensile strength of paper made from BPC increased at lower and hornified fibers (2). Mild chemical treatments such as alka­ kappa numbers, compared to the properties of paper made from line soaking can reverse these effects to a great extent, can mini­ linerboard and corrugated medium, which remained constant mize fines generation during recycling, and can help to retain or decreased at lower kappa numbers. The results indicate that most of the strength potential (3). These treatments generally changes in composition and source of corrugated boxes have a cause little or no change in lignin content, but are restricted to strong impact on physical properties. 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 INTRODUCTION products. Oxygen delignification lowers kappa number and produces more flexible fibers capable of greater bonding and Recovery and recycling rates for United States wastepaper have strength (4). Kraft pulping of whole OCC to bleachable grades grown sharply over the last decade. Recycling of old corrugated may offer process, product, and economic advantages relative containers (OCC) into container and board products has led this to virgin fiber (1,5). trend. An additional use of this high quality resource could involve pulping and conversion to lower kappa number, Despite changes in the quality of virgin fibers and OCC, manu­ unbleached products. Ultimately, this multiple recycled mate­ rial could be converted to bleached products (1), which may facturers must maintain established paper and board performance tolerate fines and strength characteristics associated with standards. Understanding the response of various OCC compo­ nents 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. 1The Forest Products Laboratory is maintained in cooperation In this report, we describe the separate pulping responses of with the University of Wisconsin. This article was written and box plant clippings (BPC), linerboard, and corrugated medium. prepared by U.S. Government employees on official time, and Conditions were selected to reach kappa numbers of processed it is therefore in the public domain and not subject to pulp in the range 40 to 70. Physical properties of the resulting copyright. pulps are discussed.

1995 Recycling Symposium / 125 METHODSAND MATERIALS Pulping The BPC, linerboard, and corrugated medium were each Box plants typically acquire corrugated medium and linerboard laboratory-pulped over a range of digester conditions. Each from several sources and match properties of available inven­ material was hydropulped, centrifuge dewatered to 30% con­ tory to achieve target combined board specifications. Conse­ sistency, fluffed, and stored at 3°C. Kappa numbers of BPC, quently, the corrugated medium and linerboard samples for this linerboard, and corrugated medium were 120, 85 and 95, study were not the same material contained in the BPC sample. respectively. Thus, the BPC kappa number was not a weighted average of the linerboard and corrugated medium kappa values. Neverthe­ Vapor-phase kraft pulping was conducted in 10-L batch digest­ less, the results showed expected reductions in lignin content ers. Fluffed pulp (150 g, ovendry [OD]) was thoroughly mixed with higher active alkali concentration (Table I, Fig. 1). 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) low­ In kraft pulping of wood, 50% to 60% of the alkali consumed ered kappa numbers by 10 to 50 units, depending on the fur­ neutralizes easily-formed carbohydrate- and lignin- related acidic nish. The pulp was washed and passed through 0.15-mm screens, groups early in the pulping process (6). This is a “one-time’’ resulting in virtually no rejects. Pulp black liquor contained trace alkali demand. Consequently, alkali demand of OCC pulps is amounts of residual alkali. lower than that of wood pulps. In our study, pulps required about 40%to 50%of the active alkali compared to pulping wood Pulps were refined to 550 ml Canadian standard freeness (CSF) chips to the same kappa value (Fig. 1). and converted to 60-gkm2 handsheets using TAPPI standard procedures. Tensile index, burst index, and compression strength Figures 2 and 3 show relationships between yield, kappa num­ (STFI) were determined. ber, 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 RESULTS AND DISCUSSION contained substantial amounts of virgin kraft fiber, yield losses Results of pulping and strength tests are shown in Tables I and observed in our study were primarily the result of lignin II and Figures 1 to 6. 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/m2) (mil) (g/m3) number

Linerboard 0 — — — — — 3 90.69 63.35 22.42 556.22 74.6 4 83.39 63.68 21.89 572.66 63.2 4.5 87.80 58.82 23.60 490.58 61.0 5 84.39 60.75 24.03 497.70 58.1 5.25 85.06 59.62 23.27 504.35 56.6 5.5 85.53 58.78 22.93 504.64 58.8 6 79.86 60.91 21.57 555.87 46.6 Corrugated medium 0 — 66.58 24.44 536.26 — 3 86.23 65.09 25.86 495.48 75.5 4 83.80 70.37 27.31 507.23 69.4 5 80.58 68.23 25.98 516.98 61.8 6 78.52 67.25 25.32 522.84 59.7 BPC 0 — 64.28 24.19 523.09 — 3 83.84 63.14 22.58 550.45 101 4 80.39 64.35 21.90 578.42 91 5 77.78 63.19 20.07 593.18 80 6 74.78 67.23 21.71 609.59 54 8 71.40 60.26 19.41 611.14 52

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

1995 Recycling Symposium / 127 Table II. Results of Strength Tests Burst strength. Com­ Both fiber strength and bonding contribute substantially to pression strength of paper subjected to tensile stresses. Handsheets Alkali Tensile Burst strength formed from pulped linerboard and BPC contained substantial Sample (%) index index index amounts of kraft fibers, which retained strength and became Linerboard 0 — — — more flexible at lower kappa numbers. These pulps were ca­ 3 45.41 3.78 24.92 pable of high bonding potential; consequently, burst strength 4 41.07 3.87 23.74 increased in the higher density sheets (Table II). Furthermore, BPC burst index increased at lower kappa numbers, presum­ 4.5 43.32 3.54 24.90 ably because of improved bonding (Fig. 5). Linerboard burst 5 39.74 3.26 24.20 index decreased slightly with kappa number. The burst strength 5.25 44.95 3.58 24.00 of paper made from delignified corrugated medium was virtu­ 5.5 43.41 3.71 24.00 ally independent of sheet density and kappa number. This trend 6 34.37 3.60 22.90 is probably related to the very poor bonding potential of this Corrugated medium 0 29.50 3.40 17.50 material relative to other furnishes, even at lower kappa num­ bers. 3 28.12 3.39 18.30 4 26.01 3.09 17.10 Tensile strength. 5 24.91 3.20 — 6 25.87 3.18 19.20 Tensile strength is dependent on the presence of long, strong, and well-bonded fibers. Approximately 60% of the material in BPC 0 43.21 4.51 23.50 recycled, pulped linerboard should fall in this category. The re­ 3 36.24 4.51 20.10 sults showed that tensile behavior varied with fiber type 4 42.67 4.52 23.50 (Table II, Fig. 6). The tensile index of recycled linerboard 5 44.49 4.89 24.60 declined at lower kappa numbers; linerboard values were 6 45.71 5.07 25.10 significantly below values obtained with virgin softwood kraft 8 47.74 5.49 25.30 fibers (10) but similar to those of unbleached hardwood prod­ ucts. The difference is likely caused by the lower strength frac­ tions 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 contribution of fiber properties relative to bond strength means index. In our study, the tensile index of sheets made from BPC that compression strength of linerboardand BPC should be only was very similar to that of linerboard. The corrugated medium slightly dependent on changes in sheet density (9). These trends showed the least potential for increasing tensile strength. are confirmed in Table II. Semichemical pulps seemed to retain poor bonding character­ istics even after pulping to low lignin levels. Conversely,compression strength of pulped corrugated medium rises with increase in sheet density. Semichemical pulps in cor­ CONCLUDING REMARKS rugated medium normally bond poorly. Cooking these pulps to lower kappa numbers probably increases fiber flexibility and The high kappa number box plant clippings (BPC) used in this swelling. The lower lignin content fibers bond and conform study produced pulp with higher kappa numbers and lower yields better during pressing and thus reach higher density levels. Im­ compared to pulps made from linerboard and corrugated me­ proved compression strength of corrugated medium could have dium. The latter clearly originated from different sources and resulted from moderate increases in density and fiber bonding contained materials with lower lignin content materials com­ (Fig. 4). These results show that kraft delignification of BPC pared to BPC materials. The differences in composition and caused an increase in compression strength, presumably for the pulping response were also reflected in paper properties. Com­ same reasons discussed for corrugated medium. Conversely, pression, burst, and tensile strength of paper made from BPC linerboard compression strength tended to decrease slightly at increased at lower kappa numbers. Conversely, the strength lower kappa numbers. Kraft fibers, which dominate in properties of paper made from linerboard and corrugated me­ linerboard, probably became more flexible during dium remained constant or decreased at lower kappa numbers. delignification. Bonding improved, but stiffness and compres­ The results indicate that the composition of old corrugated con­ sion strength decreased. tainers has a strong influence on kraft pulping.

128 / TAPPIProceedings 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 Cor­ rugated Containers (OCC) for Strength Improvement during Recycling,” Master of Science Thesis, University of Washington (1993).

4. de Ruvo, A., Farnstrand, D.A., Hagen, N., and Haglund, N., Fig. 5. Burst index as a function of kappa number. TAPPI Journal, “Upgrading Pulp From Corrugated Containers by Oxygen Delignification,” 69(6): 100 (1986).

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. 588.

7. 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, Fig. 6. Tensile index as a function of kappa number. P., (eds.), Marcel Dekker, New York, 1986, p. 281. 1995 Recycling Symposium Proceedings

TAPPI PRESS Technology Park/Atlanta P. O. Box 105113 1995 Recycling Symposium / 129 on recycled paper Atlanta. GA 30348-5113, USA