IMPACT OF COOKING CONDITIONS ON PHYSICAL STRENGTH OF EUCALYPTUS PULP

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1 IMPACT OF COOKING CONDITIONS ON PHYSICAL STRENGTH OF EUCALYPTUS PULP Nam Hee Shin and Bertil Stromberg Andritz Inc 13 Pruyn s Island Drive Glens Falls, NY 121 ABSTRACT Laboratory studies using multiple white liquor additions and multiple black liquor extractions were carried out to investigate the impact of cooking conditions on the physical strength of Eucalyptus pulp. Results of these studies indicate that the bleached pulp tensile index cannot be predicted by unbleached pulp tensile index. The variation of cooking conditions does not affect unbleached pulp tensile index, but they do have an effect on bleached pulp tensile index. Both unbleached pulp cellulose yield and pulp viscosity show a linear relationship with tensile index. However, unbleached pulp cellulose yield shows a better correlation than pulp viscosity. Xylan content of pulp does not show any effect on tensile index of either unbleached or bleached pulps. In contrast to tensile index, tear index decreases with increasing both cellulose yield or pulp viscosity. In addition, the bleached pulp tear index can be predicted by the unbleached pulp tear index. Key words: White liquor profile, Unbleached, Bleached, Viscosity, Xylan, Cellulose yield INTRODUCTION Since the introduction of Lo-Solids cooking in 1993, there are over digesters around world using this innovative cooking process. These mills reported many benefits such as better digester runability, higher pulp yield and better bleachability. In addition, both hardwood and softwood mills reported a pulp strength improvement as results of using the Lo-Solids cooking process. Especially hardwood mills reported a significant improvement of tensile The mechanical properties of paper depend primarily on the morphological character of the individual fiber and on the interaction between them. The general relationship between fiber morphology and paper strength properties has been studied for various wood species (1, 2, 3). Fiber length has been shown to have a significant impact on tear At a given apparent density, the tear strength increases with increasing fiber length. Meanwhile, the fiber length shows a minor influence on tensile In addition to the fiber length, fiber coarseness also has a morphological influence on the paper properties. At similar fiber length and strength, coarser fibers form bulky sheets of high tear strength but low tensile Another factor affecting the paper strength properties is fiber The fiber strength is measured as the wet zero-span tensile A lower wet zero-span tensile strength measurement is often obtained by either a lower cellulose content or a higher fiber damage. The correlation between fiber strength and cellulose content was found up to ~% cellulose content. However, above that level, factors other than cellulose content were found to be more important. In laboratory cooking, the fiber damage usually indicates chemical damage which can be measured by the DP or viscosity of fiber. There have been many attempts to compare softwood pulp strength properties with the varying carbohydrate composition of pulp (especially hemicellulose content). One of the widely accepted concepts is that increasing pulp yield with increasing hemicellulose yield decreases the tear index at a given tensile index. A lower tear index is explained by a lower number of fibers per unit weight of paper or higher rigidity in the fiber bonding due to higher hemicellulose content. However, this higher hemicellulose yield was obtained mainly by the addition of additives instead of by changing the cooking conditions.

2 As previously published paper (International Colloquium on Eucalyptus Kraft Pulp, 2003), the white liquor profilings significantly affect not only pulp yield and viscosity but also delignification rate. The pulp yield depends mainly on cellulose yield, and cellulose yield is significantly affected by alkali concentration in the beginning of the cook. A higher cellulose yield is obtained by decreasing the EA concentration. However, this lower alkali concentration negatively affects the delignification rate. This study also indicated clearly that the xylan yield is varied to a minor extent and the variation in xylan content does not reflect the variation in the pulp yield. In this study, the effect of cooking conditions on Eucalyptus pulp strength is investigated. RESULTS AND DISCUSSIONS As described in the previous paper, Eucalyptus Urograndis chips from Brazil were cooked to different kappa levels using the laboratory counter-current Lo-Solids cooking process. The total white liquor addition was divided into three charges with varying total EA charges (18 to 24% EA charges) at different cooking temperatures (145, 150, and 155 o C) and a 6 H-factor. One more set of cooks was done at 155 o C and 0 H-factor. Pulps of kappa range from 13.9 to 17.6 were oxygen delignified to a 10 kappa number and then bleached to %ISO using a D-E-D bleaching sequence. Tensile Index In order to study the impact of cooking conditions on pulp strength, the tensile index at 3000 PFI revolutions was plotted as a function of unbleached pulp kappa number (Figure 1). For unbleached pulps, all pulps show a similar tensile index. This means the variation of cooking conditions does not affect unbleached pulp tensile index. On the other hand, bleached pulps show a very different trend. The bleached pulp tensile index is significantly affected by the conditions in the cook. A decrease in kappa number from 17.6 to 13.9 during the cook decreases the bleached tensile index by 14% from 101 Nm/g to 87 Nm/g. Evidently, low kappa number pulps lose more tensile strength during bleaching than Bleached kappa number Xylan content, % in the pulp Figure 1. Impact of degree of cooking on tensile Figure 2. Impact of xylan content of pulp on tensile high kappa pulps. This finding was unexpected because both 13.9 and 17.6 kappa unbleached pulps have a similar tensile index and the 13.9 kappa unbleached pulp is oxygen delignified to the same target kappa number under milder conditions than the 17.6 kappa pulp. In addition, both oxygen

3 delignified pulps were bleached to % ISO under identical bleaching conditions. This clearly indicates that the variation of cooking conditions has a great influence on the bleached pulp strength properties. The variation of kappa number with varying cooking conditions affects the carbohydrate compositions of pulp (4) which is commonly believed to influence the tensile and tear The improvement of the pulp tensile strength is often explained by improving bonding strength and bonding area. Higher bonding strength is usually obtained by maintaining a higher hemicellulose content in the fiber. For this case, hemicellulose apparently acts to increase bonding However, as shown in Figure 2, the variation of xylan content of both unbleached pulp and bleached pulps does not affect the tensile index. For the unbleached pulp, despite a variation of xylan content of the pulp from 16.1 to 14.9%, the same tensile index is obtained. A similar trend can also be observed with the fully bleached pulps. The variation of xylan content from 15.4 to 14.5% does not affect tensile index. All bleached pulps show a similar tensile This finding can be possibly explained by noting that the xylan content of pulp may exceed a critical value that affects bonding strength, and as a result, xylan no longer impacts the tensile index. As compared to unbleached pulp, the bleached pulp shows a wider variation of tensile index at a given xylan content. The range of variation in the tensile index is from 87 to Nm/g. This clearly indicates that factors other than xylan content significantly affect tensile strength of pulp. Since the variation of pulp yield depends mainly on the cellulose yield under different cooking conditions, its impact on tensile strength of the bleached pulp is plotted in Figure 3. The bleached pulp tensile strength shows a good linear correlation with the unbleached pulp cellulose yield. A higher tensile index of bleached pulp is obtained from a higher unbleached pulp cellulose yield. When unbleached pulp cellulose yield increases from 41 to 43%, the tensile index of bleached pulp increases from 86 to 101 Nm/g. The effect of pulp viscosity as a fiber strength indicator (5, 6) on the tensile index was evaluated. Figure 4 clearly demonstrates that pulp viscosity significantly affect the tensile index, especially the bleached pulp tensile index. For unbleached pulps of different yields, tensile strength decreases slightly as pulp cellulose yield, % on wood Pulp viscosity, dm3/kg Figure 3. Impact of unbleached pulp cellulose yield on tensile strength of the bleached pulp. Figure 4. Effect of pulp viscosity on tensile viscosity decreases from 1640 to 1340 dm 3 /kg. However, bleached pulps show a quite different trend. Tensile strength of pulp is directly proportional to pulp viscosity. Changing bleached pulp viscosity from 1310 to 1 dm 3 /kg decreases the tensile strength by 15 units from 101 to 86 Nm/g. This clearly indicates that decreasing the viscosity value below a certain level significantly affects the tensile strength due to a loss of fiber

4 The positive impact of cooking to a higher kappa number on the bleached pulp tensile index seen in Figure 1 can be explained by the higher bleached pulp viscosity. Due to the fact that the viscosity loss during the bleaching is affected in a similar way for both high and low kappa pulps, a higher bleached viscosity is obtained from a high kappa pulp (Figure 5). These results clearly indicate that the selection of cooking conditions which produce higher pulp viscosity and higher cellulose yield is the key factor to produce a higher tensile index bleached pulp. Bleached 145 C 150 C 155 C Pulp viscosity, mpa.s kappa number Cellulose yield, % on wood EA concentration at the end of lower cook, g/l Figure 5. Effect of bleaching on pulp viscosity. Figure 6. Effect of black liquor concentration at the end of lower cook on cellulose yield. As described in the previous paper (4), the black liquor EA concentration at the end of the lower cook zone (at the end of bulk delignification) shows a good correlation with cellulose yield. Cellulose yield increases by decreasing the black liquor EA concentration (Figure 6) regardless of cooking temperature. When the tensile strength of bleached pulps is plotted as a function of PFI revolutions at a given white liquor charge and H-factor (Figure 7), it demonstrates clearly there is no effect of cooking temperature on the bleached pulp tensile All three cooking temperatures of 145, 150, and 155 o C show the Tensile index, Nm/g 145 C-6H 150 C-6H 155 C-6H PFI, revolutions Tensile index, Nm/g 18% 20% 22% 24% PFI, revolutions Figure 7. Impact of cooking temperature on tensile Figure 8. Impact of alkali charge on tensile same tensile strength development. On the other hand, a variation in cooking chemical charge shows a significant effect on the bleached pulp tensile strength (Figure 8). At 145 o C cooking temperature and 6 H-factor, an increase in the white liquor charges decreases the bleached pulp tensile This

5 result clearly indicates that a higher bleached pulp tensile strength can be obtained by reducing the white liquor charge. 145-D1-BL D1-BL D1-BL D1-BL-0 viscosity, dm3/kg Kappa number BL pulp tensile index, Nm/g viscosity, dm3/kg Figure 9. Effect of H-factor on bleached pulp viscosity. Figure10. Effect of bleached pulp viscosity on bleached pulp tensile In order to cook to the same target kappa number, decreasing white liquor charge requires an increase in H-factor. Since cooking temperature did not show any negative effect on the bleached pulp tensile strength, in this study, H-factor was increased only by increasing cooking temperature. As shown in Figure 9, a higher H-factor cooking at a higher temperature does not show any negative impact on the bleached pulp viscosity. Pulps produced from 0 H-factor at 155 o C show the same bleached pulp viscosity at a given kappa number as pulps produced from 6 H-factor at 145 o C. Bleaching the same kappa number pulps should show no difference in the bleached pulp tensile strength due to the same bleached pulp viscosity. However, results in Figure 10 show an unexpected trend. s produced from high H-factor cooking show a higher tensile index at a given 145-D1-BL D1-BL D1-BL D1-BL-0 44 Cellulose yield, % on wood BL pulp tensile index, Nm/g Kappa number UBL pulp cellulose yield, % on wood Figure 11. Effect of H-factor on unbleached pulp cellulose yield. Figure12. Effect of unbleached pulp cellulose yield on bleached pulp tensile bleached pulp viscosity than those produced from low H-factor cooking. The positive impact of a high H- factor on the bleached pulp tensile strength can be explained by a higher unbleached pulp cellulose yield (Figure 11). The results shown in Figure 12 clearly demonstrate that the bleached pulp tensile strength show a better correlation with the unbleached pulp cellulose yield than the bleached pulp

6 viscosity. This finding leads us to believe that the cellulose yield may have a greater influence on tensile index than pulp viscosity. However, this cellulose yield can be used only as a guiding factor to improve tensile index. It is still not clear which fiber characteristics are modified or changed with the variation of cellulose yield and how these characteristics affect the tensile Tear Index In contrast to tensile index, tear index shows quite a different trend when the tear index at 3000 PFI revolutions is plotted against unbleached pulp kappa number. For unbleached pulp, a linear correlation is found between tear index and unbleached pulp kappa number. tear index increases from 10 to11 mnm 2 /g with decreasing kappa number from 17.6 to A similar trend is also obtained from the bleached pulp (Figure 13). Another interesting finding is the bleached pulp shows a higher tear index than unbleached pulp and the bleached pulp increases one unit of tear index regardless of unbleached pulp kappa number. This means that the bleached pulp tear index can be predicted from the unbleached pulp tear index. Pulp viscosity and cellulose yield are an indirect measure of fiber As previously discussed, tensile index decreases with decreasing pulp viscosity below a critical value. Above this critical value, pulp viscosity does not affect tensile index. In addition, the cellulose yield also shows a similar trend as pulp viscosity. This means fiber strength shows an important role on tensile index. Bleached Tear 3000 rev., mnm2/g Tear 3000 rev., mnm2/g kappa number Pulp viscosity, dm3/kg Figure 13. Effect of degree of cooking on tear Figure 14. Effect of pulp viscosity on tear The effect of pulp viscosity on tear index is shown in Figure 14. Both brown and bleached pulps show that tear index increases with decreasing pulp viscosity. This finding is also true when tear index is plotted as a function of cellulose yield (Figure 15). Tear index decreases with increasing cellulose yield. Both results suggest that fiber strength does not have any significant role in the improvement of tear index. The trend shown in Figure 15 is consistent with the fact that a lower yield pulp contains more fibers per unit weight of pulp. A lower pulp yield is obtained with a lower cellulose yield. The effect of xylan content in pulp on tear index is shown in Figure 16. The general trend is that tear index increases with decreasing xylan content. s show the range of tear index from 10 to 11.5 mnm 2 /g with xylan content ranging from 16.2% to 14.8%. After bleaching, the pulps show a higher tear index range from 11 to 12.2 mnm 2 /g at a lower xylan content from 15.3% to 14.4%. However, the variation of tear index at a given xylan content is too large to conclude that xylan content significantly affects tear index.

7 Tear 3000 rev., mnm2/g Tear 3000 rev., mnm2/g cellulose yield, % on wood Xylan content, % in the pulp Figure 15. Effect of unbleached pulp cellulose yield on tear Figure 16. Effect of xylan content on tear Mill Results Lo-Solids Cooking is a unique cooking method maintaining both lower dissolved wood solids and lower alkali concentration throughout the cook. As shown in a four screen single vessel digester (Figure 17), post impregnation liquor which is relatively high in dissolved wood solids is removed through the first set of extraction screens. The first white liquor charge which is added through the chip feed system is controlled mainly by maintaining the black liquor EA concentration between 3 and 8 g/l as NaOH. White Liquor Chip In Impregnation Extraction Liquor Out Upper Cook White Liquor Lower Cook Cold Blow Extraction Liquor Out Wash zone White Liquor Cold Blow Pulp Out Figure 17. Four screen single vessel Lo-Solids digester configuration.

8 The second white liquor and cold blow are added into the lower cooking circulation: the second set of extraction screens. Makeup liquor is uniformly distributed throughout the column by means of the counter-current flow in the upper cook zone and bulk delignification is continued in the lower cooking zone between the second and main extraction (the third set) screens. The final white liquor charge is added at the bottom of digester with the counter-current flow in the wash zone or into the main extraction circulation with the con-current flow in the wash zone. As discussed previously, the laboratory study clearly demonstrated that the black liquor EA concentration at the end of lower cook has a significant impact on cellulose yield and bleached pulp tensile An increase in black liquor EA concentration results in decreasing both cellulose yield and bleached pulp tensile An interesting finding is that mill bleached pulp produced from the same Eucalyptus using other cooking processes also shows the same linear relationship between the bleached pulp tensile strength and black liquor EA concentration at the end of Lab bleached pulp Mill bleached pulp Lab bleached pulp Mill bleached pulp Lo-Solids Cooking Other processes EA concentration at the end of lower cook, g/l Pulp viscosity, dm3/kg Figure 18. Effect of black liquor EA concentration at the end of lower cook on mill pulp tensile Figure 19. Comparison of mill and lab pulps tensile strength against pulp viscosity. the lower cook stage as the lab produced pulp. The mill using this other cooking process maintains 14 to 16 g/l EA concentration at the end of the lower cook stage compared to 5 to 10 g/l in a Lo-Solids cook. As shown in Figure 18, an increase in black liquor concentration to 14 g/l at the end of the lower cook stage results in decreasing the bleached pulp tensile The pulp viscosity of mill bleached pulp also shows a good correlation with the bleached pulp tensile strength (Figure 19). CONCLUSIONS A higher bleached pulp tensile index is obtained from higher kappa unbleached pulp. Xylan content in pulp does not affect tensile index. cellulose yield shows a better correlation with bleached pulp tensile index than pulp viscosity. Selection of cooking conditions that produce a higher unbleached pulp cellulose yield and pulp viscosity is the key factor to improve bleached pulp tensile index. The cooking chemical charge is a more critical parameter to be controlled than the cooking temperature. Higher tear index is obtained from pulp that has a lower pulp viscosity and cellulose yield. As compared to unbleached pulps, bleached pulps show a lower xylan content and higher tear index but this higher tear index can not be explained by lowering the xylan content due to a large variation of tear index at a given xylan content. In the mill, lowering black liquor EA concentration at the end of lower cook is the key factor for improving the bleached pulp tensile

9 REFERENCES 1. Francisco N. Tamolang, Frederick F. Wangaard, Robert M. Kellogg, Tappi Journal, 51:1, (1968). 2. N. Gurnagul, D.H. Page, R.S. Seth, Journal of pulp and paper science, 16:1, J36-J41 (19). 3. R.S. Seth, D.H. Page, Tappi Journal, 71:1, (1988). 4. Nam Hee Shin, Bertil Stromberg, International Colloquium on Eucalyptus Kraft Pulp, September 4 and , Vicosa, Brazil. 5. Norayr Gurnagul, D.H. Page, Michael G. Paice,, Nordic Pulp and Paper Research Journal no. 3/1992, Jian Li, Geoffery Moeser, Laura Roen, Ind. Eng. Chem. Res. 2000, 39,