Papermachine runnability of never dried, dried, and enzymatically treated dried pulp

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1 Proceedings of the 6th International Conference on Biotechnology in the Pulp and Paper Industry: Advances in Applied and Fundamental Research Papermachine runnability of never dried, dried, and enzymatically treated dried pulp S. Abubakr *, K. Rutledge-Cropsey, and J.H. Klungness USDA Forest Products Laboratory, One Gifford Pinochot Drive, Madison, WI , USA * Corresponding author SUMMARY This study investigated the effect of drying on the runnability of pulps. In addition, an enzyme was used to enhance drainage of the once-dried fiber. Three papermachine runs were made. The enzyme treatment was done to reduce the degree of horrification of the frees and improve drainage. The enzyme treatment consisted of adding a cellulase with a high filter paper activity to the pulp slurry after fiberizing. After the treatment, the pulp was refined to the original freeness. In each run, papermachine drainage, wet-web strength, pulp viscosity, and handsheet burst index were measured. INTRODUCTION The main difference between once-dried and never-dried fiber is that the dried fibers are hornified; that is, irreversible fiber bonding within the fiber wall has occurred that resists reswelling (1-7). Recycling depolymerizes cellulose (5,6), generates fines, and shortens the fiber length, thus a lower freeness is observed (1,2,4,7). Specifically, the drying process has been reported to decrease the specific volume and surface areas of pulp (8). The dried fibers are also more difficult to wet because of their glassy crystalline nature compared with the highly amorphous, hydrophilic virgin fiber (5,7,8). In addition the redistribution of olefinic compounds during drying results in self-sizing (5,9). The increased crystallinity of dried fiber also renders the fibers more brittle and difficult to bond with other fibers (5). As a result of these changes in the fiber, the papermaker experiences problems with drainage and wet-web strength. An increase in the drainage and wet-web strength would allow the papermaker to increase production rates and recycled fiber content levels. This study investigated the application of enzymes, specifically cellulases, to assist in increasing drainage and wet-web strength on the papermachine. The application of cellulases on pulp for the purpose of enhancing the freeness has been researched and patented by several research groups (10-16). Sarkar et al. (11) showed that cellulases with a polymer treatment enhanced the freeness of OCC (old corrugated containers) and linerboard/newspaper furnishes without a significant loss in tensile or burst strength. Pommier et al. (14) also reported that cellulase-treated mixed papers containing OCC had better drainage properties while maintaining strength properties. Bhat et al. (16) had similar results with bleached and unbleached recycled softwood pulp. In addition. Pommier et al. (13) reported that on an industrial scale. the cellulase action increased the freeness to the point 151

2 where the headbox consistency could be decreased or the machine speed could be increased. Stork et al. (17) explained that the drainage enhancement by cellulase action was a result of the hydrolysis of amorphous cellulose on the surface of the fibers, and not a selective hydrolysis of fines. However, Jackson et al. (12) found a selective hydrolysis of the fines fraction that resulted in a larger, longer fiber content of cellulase-treated wastepaper compared with the control. In the study reported here, we focused on papermachine runnability enhancement in the production of bleached kraft papers. The cellulose substrate used in our study was a mixture of bleached hardwood and softwood kraft pulps, typical of what would be found in printing and writing papers. We compared pilot papermachine runnability, specifically drainage and wet-web strength, of never-dried, once-dried, and once-dried cellulase-treated furnishes. We also measured the effects of drying and enzyme treatment on burst strength and viscosity. MATERIALS AND METHODS A commercial enzyme preparation consisting of cellulases was used in the laboratory experiments and papermachine run. The enzyme mixture had both cellulase and filter paper activity. Falter paper activity is said to be a measure of total or true cellulase activity (18) and a measure of the effect on the crystalline portion of the cellulose. In our previous publication (19), this enzyme was labeled Enzyme C and reported to have a filter paper activity significantly greater than several other commercial cellulases. Machine chest pulp from the once-dried fiber run was used in experiments to determine the optimum addition level of enzyme. Each experiment consisted of treating 20-g ovendried (OD) fiber at various levels of enzyme. The 3.0% consistency slurry was heated and maintained at 50 C in a water bath and adjusted to ph 5.0. Agitation was constant at 125 rpm. The commercial enzyme was added at the rates of 0% 0.2% 0.4%, 0.6%, and 0.8% (ml of crude enzyme/g OD fiber). In each experiment, the enzyme was diluted to 30 ml prior to adding it to the stock slurry. The final consistency of the experiments was 2.9%. The enzyme was reacted with the pulp for 20 min. At the end of the 20 min the reaction was stopped by adding NaOH to obtain a ph of 7.5. The enzyme-treated pulp and control pulp were tested for freeness and viscosity. Handsheets were made according to TAPPI T-205 and tested for burst index using TAPPI T-220. Detailed information is found in (33). RESULTS AND DISCUSSION As expected the freeness increased with increasing enzyme addition rates, while the burst index decreased. This loss in strength with increasing enzyme is similar to the findings of Oltus et al. (20). The freeness began to level out near 600 CSF at 0.6% enzyme addition. Figure 3 shows that the largest decrease in viscosity occured at the 0.1% level. Viscosity continued to decline with additional enzyme concentration. Table I summarizes the machine chest freenesses and fiber viscosity obtained in all three runs. Machine chest freenesses were 390,340,445, and 310 ml for the never-dried, once-dried, once-dried enzyme-treated, and once-dried enzyne-treated and refined samples, respectively. The freeness decreased more than was expected when the enzyme-treated pulp was lightly refined. Pommier et al. (13) reported that other researchers (21-27) found significant energy 152

3 savings by refining biologically treated pulps. Also, Kim et al. (28) found it took less time to fiberize wastepaper when it was enzyrnatically pretreated. In our study, the enzyme treatment also decreased the fiber viscosity (Table I). However, the viscosity decrease was not as large as predicted by the preliminary laboratory work and may be a sign of enzyme inhibition by the higher shear rate experienced in the hydrapulper. The freeness did not reach the level achieved in the laboratory experiments probably for the same reason. Kaja et al (29) reported that the binding of the enzyme to the cellulose is inhibited by a high shear rate, and excessive and prolonged shear will denature the cellulase. The drainage profile of the enzyme-treated run again appears to be more uniform than the other runs. It appears that the overall drainage rate for the enzyme run was greater than the once-dried control run but less than the never-dried run. However, when the drainage rates were normalized by the headbox flow rates for each run, the drainage of the enzyme-treated run was 50.8% compared with 50.0% for the never-dried run and 44.1% for the once-dried control run (Table I). This means that the enzyme treatment increased the drainage rate to the level of the never-dried fiber, even though the freeness of the enzyme-treated fiber was 80 ml less than the never-dried fiber. It is not understood how a higher drainage rate can exist when the measured freeness is low. It may be that the enzyme treatment disintegrates the small particles that have a high affinity for water (13) and high surface area (12) and that refining does not regenerate them. If this is true, then an enzymatic treatment would increase the freeness and drainage and subsequent refining would defibrillate the fibers, thereby decreasing the freeness without affecting the drainage. As mentioned previously, wet-web tensile strength was normalized to the solids level of each sample location of the never-dried run. The solids level at each press section sample location is summarized in Table II. Table III illustrates that the most significant loss in wet-web tensile strength resulted from drying the never-dried fiber. It also shows that the enzyme-treated once-dried fiber is slightly stronger in wet-web tensile strength than is the untreated oncedried fiber after the second and third presses. It is unclear whether this difference is a result of the light refining that the enzyme-treated fiber received, the enzyme action alone, or a combination of the two. Wet-web stretch was tested at the couch and each press. There was no significant difference in wet-web stretch between the never-dried once-dried, and once-dried enzyme-treated runs as shown in Table IV. Table V summarizes the results of the burst index that was performed on handsheets made of machine chest pulp from the never-dried, once-dried, and enzyme-treated runs. An additional sample of pulp was collected after treating the pulp with enzyme prior to refining. The burst index for this sample is also included in Table V. As expected, the burst index decreased from the never-dried run to the once-dried run. This is an example of the effect of hornification that occurs during drying. Calendering could contribute to some of the loss in strength. It was reported that calendering flattens the fibers and reduces the fiber length (1,5). However, it is probably not a key factor in the strength loss, because the calendering was kept constant and to a minimum in all papermachine runs. The burst index of the enzyme-treated pulp before refining was significantly less than the once-dried control and less than that predicted by the laboratory experiments. When refined, the enzyme-treated pulp was refibrillated to a 310 ml CSF, and the burst index returned to the level equal to the burst index of the once-dried pulp. Therefore, it appears that no permanent damage was done to the fibers by the enzyme. 153

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5 Our study confirmed that the first drying of the fiber is the most detrimental to fiber strength and recycle potential. The irreversible hornification that occurs during the first drying is seen in the loss of bonding potential, specifically tensile strength of the wet-web andburst index, from the never-dried run to the once-dried run. Horrification was reflected by a decrease in viscosity and burst index from the never-dried fiber run to the once-dried fiber run. The enzyme action caused a further decrease in viscosity; however, burst index was not affected. The enzyme-treated papermachine run did not show any dramatic effects; however, it did confirm that enzymes have the potential for making wastepaper a viable resource to the papermaker. The enzyme treatment prior to refining enhanced the freeness of the wastepaper and may have contributed to a decrease in vacuum requirement. It has been reported that cellulases break down the cellulose that has a high affinity to water but does not contribute to the overall hydrogen bonding potential of the fibers (13). We achieved better drainage without a significant effect on strength properties when we treated the wastepaper with a cellulase prior to refining. It has also been repotted that refining prior to enzyme treatment results in strength enhancement with little change in freeness (13). Process flexibility at the point of enzyme addition would give the papermaker more control over drainage and strength properties. A small observation but a very important one in this preliminary study was the fact that the enzyme-treated fiber seemed easier to refine; i.e., a larger decrease in freeness was produced with decreased amounts of energy. A decrease in pulp disintegration time was also reported in the studies by Kim et al. (28). They attributed this action to an attack on interfiber bonding areas through partial enzymatic hydrolysis or partial depolymerization of carbohydrates on the fiber surface. A decrease in disintegration time would significantly decrease the energy requirements in the hydrapulping and refining processes. 155

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