The degree of polymerization of the pulp was reduced and its crystallinity was increased after enzymatic pretreatment

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(1)Journal of Korea TAPPI Vol. 53. No. 5, 2021, 5-15p ISSN (Print): 0253-3200 Printed in Korea. http://dx.doi.org/10.7584/JKTAPPI.2021.10.53.5.5. Combined Enzymatic Pretreatment of Pulp for Production of CNF Shin Young Park1, Seokho Lee1,2, Wanhee Im1,3, Hak Lae Lee4,5, Hye Jung Youn4,5,† Received August 17, 2021; Received in revised form September 16, 2021; Accepted September 23, 2021. ABSTRACT In this study, the effects of mixing treatment of endoglucanase and xylanase on characteristics of pulp and cellulose nanofibrils (CNF) were investigated. The degree of polymerization of the pulp was reduced and its crystallinity was increased after enzymatic pretreatment. The suspension viscosity of CNF prepared from enzymatic pretreated pulps was lower than that of untreated CNF. Compared with untreated CNF, more uniform and smaller fibrils could be obtained through combined pretreatment of endoglucanase and xylanase, but higher endoglucanase dosage led to wider fibrils width distribution. The aspect ratio of enzyme-treated CNF was lower than that of untreated CNF. The use of both endoglucanase and xylanase increased the tensile strength and elastic modulus of the sheet, whereas just endoglucanase-treated CNF sheet exhibited lower strength. The light transmittances of each CNF sheet were comparable; however, the casted sheet made using 1.0% endoglucanase-treated CNF was less transparent because of its morphological features. The combined use of endoglucanase and xylanase could facilitate the nanofibrillation and produce the sheet without any deterioration of strength.. Keywords: Cellulose nanofibrils, enzyme, pretreatment, tensile strength, transparent film. 1. Introduction. serious environmental problems, interests in sustainable biomaterials have been increased to. Since petroleum-based materials have caused. replace them. Recently, cellulose nanomaterials of. 1 Department of Forest Sciences, College of Agriculture and Life Sciences, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea, Graduate Student 2 Current Address: R&D Center, Kleannara, Cheongju-Si, Chungcheongbuk-do 28174, Republic of Korea, Researcher 3 Current Address: Moorim P&P Co. Ltd., Ulsan 45011, Republic of Korea, Senior Researcher 4 Department of Agriculture, Forestry and Bioresources, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea, Professor 5 Research Institute of Agriculture and Life Sciences, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea, Professor † Corresponding Author: E-mail: [email protected] (Address: Department of Agriculture, Forestry and Bioresources, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea). J. of Korea TAPPI Vol.53 No.5 Sep.-Oct. 2021. 5.

(2) Combined Enzymatic Pretreatment of Pulp for Production of CNF. which a width is less than 100 nm have received a. cost for CNF production. It was also shown that. great attention owing to its abundance, sustain-. enzymatic pretreatment affects the degree of. ability, and biodegradability. Among them, cellu-. polymerization, crystallinity, and mechanical. lose nanofibril (CNF) is produced through mechan-. properties of CNF. [24,27,30,31]. ical treatment of pulp fibers, e.g. grinding,. Although several studies of enzymatic pretreat-. homogenization, and microfluidization. [1-5] In. ment for CNF production have been performed,. addition to the eco-friendliness of cellulose, CNF. most of those studies used endoglucanase or cello-. has advantages such as high mechanical properties. biohydrolase for pretreatment. Since bleached pulp. and flexibility due to its high aspect ratio. Owing. fibers contain some amount of hemicelluloses in. to these merits, CNF has a potential to be applied. their structure, the role of hemicellulose in CNF. in various fields, including papermaking, [6,7]. production process should also be considered.. packaging, [8,9] composites, [10-12] electronics,. Nonetheless, there have been only few studies. [13,14] medicines, [15] and so on.. which investigated the effect of hemicellulase on. Although CNF has been noticed as an eco-. CNF production. Therefore, in this study, hard-. friendly material, there are some obstacles for. wood bleached kraft pulp was treated with endog-. commercial application of CNF. One of the prob-. lucanase and xylanase with various mixing ratios. lems is its high price because of large energy con-. as a pretreatment for CNF production. Then,. sumption for CNF production. As CNF is produced. properties of CNF and CNF sheet including mor-. by severe mechanical treatment, high amount of. phology, mechanical strength, and optical proper-. energy is consumed during the process, which. ties were measured. From this study, it was aimed to. leads to increase in the production cost. [16,17]. reveal the combined effect of endoglucanase and. Several pretreatment methods of pulp have been. xylanase on the properties of CNF and the CNF sheet.. introduced to reduce the CNF production cost, including mechanical, [18,19] chemical, [20-23] and biological pretreatment. [24-26] Among them,. 2. Materials and Methods. biological pretreatment using enzymes does not need any harmful chemicals, and it is performed in a mild condition, thus it recognized as eco-friendly pretreatment method.. 2.1 Materials Never-dried hardwood bleached kraft pulp supplied by Moorim P&P Co. (Korea) was used as a. Enzymes have been studied for pulping and. raw material which contains about 79% of cellu-. refining process for a long time. Several kinds of. lose, 20% of xylose and 1% of lignin and ash. For. enzymes including endoglucanase, cellobiohydro-. enzymatic pretreatment, endoglucanase (Fibercare. lase, and hemicellulase have been used for these. R, 4500 CNU-CA/g, Novozymes, Denmark) and. processes. Recently, these enzymes have been. endoxylanase (Pulpzyme HC 2500, 2500 AXU/g,. studied for CNF production. Endoglucanase and. Novozymes, Denmark) were used.. cellobiohydrolase which can degrade the structure of cellulose chain have mainly used for pretreat-. 2.2 Enzymatic pretreatment of pulp. ment for CNF production. [27,28] It revealed that. 100 g pulp was disintegrated to 2.5% consistency. pulp treated with enzymes showed lower clogging. with deionized (DI) water. Enzymatic pretreatment. trouble during homogenization. [24,29] In addition,. condition was determined by a preliminary test in. enzymatic pretreatment could reduce the energy. the recommended conditions suggested by the. 6. 펄프·종이기술 53(5) 2021.

(3) Shin Young Park, Seokho Lee, Wanhee Im, Hak Lae Lee, Hye Jung Youn. enzyme supplier. Enzymes and pH 7 buffer solution. CNF production, low shear viscosity of fiber sus-. (10 mL) were added to the pulp suspension. Mixing. pensions was measured using Brookfield viscome-. ratio and total dosage of endoglucanase (G) and. ter to evaluate the extent of fibrillation and vis-. endoxylanase (X) were adjusted to four different. cosity of the CNF suspension. The measurement. conditions as shown in Table 1. Enzymatic treat-. was conducted at 25℃ for 1 min using spindle #64.. ment was conducted at 55°C for 1h, and then the. To observe the morphology of nanofibrils, SEM. suspension was boiled to terminate the enzyme. images of fibers were taken at the different pass-. reaction. After termination of reaction, the sus-. ing number using Field-Emission Scanning Elec-. pension was washed sufficiently with DI water to. tron Microscope (SUPRA 55VP, Carl Zeiss, Ger-. remove the enzymes.. many). In addition, the fibril width of CNFs was. To investigate the effect of enzymatic treatment. measured using Image J program from the SEM. of pulp, cupriethylenediamine (CED) viscosity and. images at magnification of ×50000. More than 300. crystallinity of each treated pulp were measured.. fibrils were measured and the average value and. The CED viscosity measurement was performed in. distribution of the fibril width were obtained.. accordance with TAPPI method (T 230 om-99). A. Crystallinity of CNF was evaluated by XRD with. thin pulp sheet was prepared via vacuum filtration. the same manner in the pulp sample. As like the. and drying, which was used for the determination. analysis of pulp, a thin sheet of CNF was used for. of crystallinity using X-ray diffractometer (XRD,. the analysis.. D8 ADVANCE with DAVINCI, Bruker, Germany). To estimate the aspect ratio of CNF, sedimentation. with Cu Kα radiation at 40 kV and 40 mA. The. behavior of the CNF suspension was evaluated as. diffraction data (2θ) was collected from 3° to 40°. described method by Varanasi et al. [33] CNF sus-. and the crystallinity index was calculated by Segal. pensions with different concentrations from 0.02 wt%. method. [32]. to 0.08 wt% were prepared and left for sedimentation for 48 h. After the sedimentation, a sediment. 2.3 ‌Preparation and characterization of cellulose nanofibrils. height of each suspension was measured. The quadratic regression equations were obtained from a. CNFs were produced from untreated and. plot of suspension consistency versus the ratio of. enzyme-treated pulps, respectively. The pulp sus-. sediment height (hs) to suspension height (ho). From. pension of 1.5% consistency was mechanically. the regression equations, a gel point of CNF sus-. ground using a Supermasscolloider (Masuko. pension and average aspect ratio were obtained.. Sangyo, Japan) at 1500 rpm. The gap of grinding stones was -80 μm and each pulp suspension passed between grinder stones by 24 times. During. 2.4 Preparation of CNF sheet CNF sheet was prepared by two different methods,. Table 1. Enzymatic pretreatment conditions Endoglucanasea. Endoxylanasea. G 0.25%+X 0.25%. 0.25%. 0.25%. G 0.5%. 0.5%. 0. G 0.5%+X 0.5%. 0.5%. 0.5%. G 1.0%. 1.0%. 0. Condition. Total enzyme dosagea 0.5% 1.0%. a. Enzyme dosage was based on the oven dried weight of the pulp. J. of Korea TAPPI Vol.53 No.5 Sep.-Oct. 2021. 7.

(4) Combined Enzymatic Pretreatment of Pulp for Production of CNF. vacuum filtration and casting. In the case of vac-. (a). uum filtration, 0.5 wt% CNF suspension was vacuum-filtrated on the cellulose ester membrane filter (0.2 μm pore Advantec, US) to form 40 g/m2 CNF sheet. Then it was wet pressed at 30 bar for 5 min. After wet pressing, the sheet was separated from membrane filter and then hot pressed at 100°C for 15 min. For casting, 0.5 wt% CNF suspension was poured into a petri dish in a grammage of 40 g/m2, and it was dried at 50℃ oven for 24 h. (b). 2.5 Characterization of CNF sheet Tensile properties of the CNF sheet were investigated using Universal Testing Machine (Instron, USA) with a 500 N load cell. CNF sheet was cut into rectangular specimen with 6 cm length×1.5 cm width for the tensile test. Test was performed at 3 cm span length and the cross-head speed of 10 mm/min. For each CNF sheet sample, the test was conducted more than four times. Tensile strength and strain at break of the sheet were measured, and the elastic. Fig. 1. ‌CED viscosity (a) and crystallinity (b) of untreated and enzymatic pretreated pulps with different enzyme dosage.. modulus was calculated from the strain-stress. cleavage of cellulose chain by enzymes. [24,26,30]. curve.. Among the pulps pretreated with different enzyme. In addition, light transmittance of the CNF sheet. mixing ratio, pulp treated with 0.5% endoglucanase. was measured at the wavelength range from 200 nm. had quite lower viscosity than that treated with. to 800 nm using Cary 100 UV-vis spectrophotometer. 0.25% endoglucanase and 0.25% xylanase, which. (Agilent, USA). Each sample was analyzed in. means DP of cellulose decreased more at higher. duplicate.. dosage of endoglucanase. However, there was no more decrease in the CED viscosity when the endoglucanase dosage was higher than 0.5%. In. 3. Results and Discussion. the case of crystallinity, because endoglucanase attack the amorphous region of the cellulose and. 3.1 ‌Effect of enzymatic pretreatment on pulp properties. hemicellulose, the crystallinity of enzymatic pretreated pulps was higher than that of untreated. To examine the effect of enzymatic pretreatment. pulp. [26,30,34] On the contrary, the treatment of. on the pulp properties, CED viscosity and crystal-. xylanase did not have significant influence on the. linity of the untreated and treated pulps were. DP and crystallinity of the pulp as described in the. evaluated (Fig. 1). CED viscosity of pulp decreased. previous work. [35] From these results, it was. by enzymatic pretreatment. This indicated that. confirmed that the enzymatic pretreatment of pulp. enzyme-treated pulps had lower degree of polym-. with endoglucanase decreased the DP and. erization (DP) than untreated pulp due to the. increased the crystallinity of pulp, which may. 8. 펄프·종이기술 53(5) 2021.

(5) Shin Young Park, Seokho Lee, Wanhee Im, Hak Lae Lee, Hye Jung Youn. affect the process of CNF production and proper-. the morphological properties of CNF are likely to. ties of CNF.. influence the viscosity of the suspension.. 3.2 ‌Effect of enzymatic pretreatment on cellulose nanofibrils. ent enzymatic treatment conditions were examined. Morphologies of cellulose nanofibrils with differusing SEM (Fig. 3). The average fibril width and. Cellulose nanofibrils were prepared from. the fibril width distribution were obtained from. untreated and enzyme-treated pulps, resepctively.. these SEM images (Fig. 4 and Table 2). SEM. Fig. 2 shows a change in the low shear viscosity of. images showed that most fibrils were nanofibril-. the fiber suspension during CNF production. As. lated under 100 nm width for all conditions. How-. the pass number through a grinder increased, the. ever, the average fibril width and the width distri-. viscosity of the suspension increased regardless of. bution were affected by the pretreatment condi-. enzymatic treatment. It means that fibers were. tions. As shown in Table 2, CNFs treated with total. deconstructed into smaller fibrils, which led to an. enzyme dosage of 0.5% showed narrower fibril. increase in surface area. However, an increment in. width (27.7 nm for G 0.25%+X 0.25%, and 29.1 nm. the suspension viscosity was different depending. for G 0.5%) and more uniform distribution com-. on the enzymatic treatment conditions. The enzy-. pared to untreated CNF (average width of 34.4 nm).. matic treated fiber suspensions exhibited lower. This showed that enzymatic pretreatment could. viscosity than the untreated fiber suspension. It. facilitate the nanofibrillation of fibers and make. means that the enzymatic pretreatment can relieve. fibrils more uniform. [37] However, as total. some problems related to the high viscosity of CNF. enzyme dosage increased up to 1.0%, the average. suspension which may occur in mill production and. width and the width distribution of CNFs got wider.. application processes. Especially the suspension. CNF treated with 1.0% endoglucanase had rela-. treated with 1.0% endoglucanase exhibited much. tively larger fibril width of approximately 40 nm. lower viscosity than other enzyme-treated sus-. and wider distribution than untreated CNF. It. pensions. The rheological properties of the pulp. indicated that the appropriate dosage of endoglu-. and CNF suspension are mainly affected by the. canase and xylanase resulted in relatively homo-. consistency of the suspension and the morphologi-. geneous nanofibrils, whereas the higher amount of. cal properties of the fibers. [36] Since the consis-. enzyme could decrease the efficiency of mechanical. tency of each suspension was same, differences in. treatment and led to wider and heterogeneous distribution of fibril widths. [25] To evaluate the aspect ratio of CNF, the sedimentation behavior of CNF suspension was examined. The aspect ratio of each CNF was calculated by plotting the sediment height of the suspension versus the consistency of the suspension. [33] As shown in the Fig. 5, the aspect ratio of the CNF decreased when the dosage of endoglucanase increased. Considering with the result of the CNF width, it indicates that higher endoglucanase dos-. Fig. 2. ‌Change in low shear viscosity of the suspension during CNF production.. age led to production of fibrils with relatively short and wide shape. [30,37] The decrease in DP of. J. of Korea TAPPI Vol.53 No.5 Sep.-Oct. 2021. 9.

(6) Combined Enzymatic Pretreatment of Pulp for Production of CNF. (a). (b). (c). (d). (e). Fig. 3. ‌SEM images of untreated and enzymatic pretreated cellulose nanofibrils (24 times passes): Untreated CNF (a), CNF treated with G 0.25%+X 0.25% (b), G 0.5% (c), G 0.5%+X 0.5% (d), and G 1.0% (e).. Table 2. ‌Average fibril widths of untreated and enzyme-treated CNFs calculated from SEM images (24 times passes) Enzyme condition. Average fibril width (nm). Standard deviation of fibril width (nm). Untreated. 34.4. 20.8. G 0.25%+X 0.25%. 27.7. 13.9. G 0.5%. 29.1. 15.3. G 0.5%+X 0.5%. 31.1. 19.7. G 1.0%. 39.2. 20.9. 10. 펄프·종이기술 53(5) 2021.

(7) Shin Young Park, Seokho Lee, Wanhee Im, Hak Lae Lee, Hye Jung Youn. CNF still exhibited higher crystallinity compared to untreated CNF, while the difference between them was similar (Fig. 6). Among enzymatic treated CNFs, CNF treated with 0.5% endoglucanase showed the highest crystallinity. As the total enzyme dosage increased, the crystallinity of CNF decreased due to the more severe cleavage of cellulose chain. [24,30]. Fig. 4. ‌Fibril width distribution of untreated and enzyme-treated cellulose nanofibrils.. 3.3 ‌Effect of enzymatic pretreatment on properties of CNF sheet CNF sheets were prepared using untreated and enzymatic pretreated CNFs by two different methods, and their tensile properties were evaluated. CNF sheets had different tensile properties depending on the enzymatic treatment condition and sheet prepartion method. The tensile stregth, elastic modulus, and strain at break of each sheet are presented in Fig. 7. Compared to untreated CNF sheet, pretreatment of only endoglucanase decreased the tensile strength of both vacuum filtrated and casted sheets (Fig. 7a). It seemed. Fig. 5. ‌Estimated aspect ratio of untreated and enzyme-treated cellulose nanofibrils.. because endoglucanase-pretreated CNF formed a weaker network owing to shorter length of fibrils than other CNFs, which was expected from the. fibers seemed to result in shorter fibrils by grind-. aspect ratio as shown in Fig. 5. In addition, a. ing. These morphological properties would also. decrease in the DP by enzymatic treatment might. influence the viscosity of CNF suspension. CNF. also led to the lower tensile strength of the sheet,. suspensions treated with enzymes had lower vis-. which agreed with the results in the previous. cosity due to less entangled network between the fibrils because of the lower aspect ratio of enzymetreated fibrils than untreated fibrils. On the contrary, compared to endoglucanase, xylanase did not affect the aspect ratio of CNF significantly. Although the crystallinity of pulp increased by enzymatic pretreatment, it might be further changed during CNF production by the mechanical treatment. Grinding treatment of pulp decreased the crystallinity of cellulose fibers. After grinding, CNF had the crystallinity of 60-70%, which was 12-14% lower than that of pulp. Enzyme treated. Fig. 6. ‌Crystallinity of CNFs depending on the enzymatic treatment conditions.. J. of Korea TAPPI Vol.53 No.5 Sep.-Oct. 2021. 11.

(8) Combined Enzymatic Pretreatment of Pulp for Production of CNF. by the combined effect of endoglucanase and. (a). xylanase. [31] In the case of strain at break, as total dosage of endoglucanase increased, enzyme treated CNF sheets exhibited lower strain values (Fig. 7b). Since enzyme treated fibers had lower aspect ratio and higher crystallinity than untreated fibers, their elongation ability was poor. [26,30,38] The elastic modulus of the CNF sheets showed similar tendency with tensile strength (Fig. 7c). The sheets treated with both endoglucanase and. (b). xylanase had higher modulus than untreated CNF sheets, whereas the treatment of only endoglucanase resulted in lower modulus vaule. Based on these results, it is expected that treatment of xylanase can prevent the loss of the mechanical strength of CNF sheet caused by endoglucanase. The light transmittance of each CNF sheet is shown in Fig. 8. Transmittance of the filtrated CNF sheet was around 90%, while those of casted. (c). sheets ranged from 80% to 85%. It was due to a denser network of the filtrated sheets by pressing during sheet preparation. Between untreated and enzymatic pretreated CNF sheets, there was no noticable difference for filtrated sheets because the compaction by pressing process reduced the effect of CNF morphology on the optical property of the. Fig. 7. ‌Tensile properties of vacuum filtrated and casted CNF sheets: tensile strength (a), strain at break (b), and elastic modulus (c).. sheet. For casted sheets, sheet treated with 1.0% endoglucanase showed the lowest transmittances.. studies. [26,30,37,38] On the contrary, the addition of xylanase in enzymatic pretreatment led to the CNF sheet with higher mechanical properties than the sole glucanase addtion. It is known that prescence of hemicellulose is beneficial to the strength of paper or sheet. [39-41] However, the xylanase-treated CNF sheets in this study did not show such a decrease in strength. It appeared that some xylan still remained on the fibrils and more hyrdogen bonding between the fibrils were created. 12. 펄프·종이기술 53(5) 2021. Fig. 8. ‌Light transmittances of vacuum-filtrated and casted CNF sheets at 600 nm wavelength..

(9) Shin Young Park, Seokho Lee, Wanhee Im, Hak Lae Lee, Hye Jung Youn. Literature Cited. Due to low aspect ratio and heterogeneous fibril width distribution of 1.0% endoglucanse-treated CNF, less packed structure between nanofibrils. 1. Abitbol, T., Rivkin, A., Cao, Y., Nevo, Y.,. was formed during drying, which resulted in the. Abraham, E., Ben-Shalom, T., Lapidot, S.,. low transmittance. [30,42]. and Shoseyov, O., Nanocellulose, a tiny fiber with huge applications, Current Opinion in Biotechnology 39:76-88 (2016).. 4. Conclusions. 2. Rebouillat, S. and Pla, F., State of the art manufacturing and engineering of nanocellu-. In this study, the combined effect of endogluca-. lose: A review of available data and industrial. nase and xylanase was investigated for the pro-. applications, Journal of Biomaterials and. duction of CNF in terms of the properties of CNF. Nanobiotechnology 4:165 (2013).. and CNF sheets. After enzymatic pretreatment,. 3. Kim, J. -H., Shim, B. S., Kim, H. S., Lee, Y.. pulp had lower CED viscosity and higher crystal-. -J., Min, S. -K., Jang, D., Abas, Z., and. linity than untreated pulp due to the cleavage. Kim, J., Review of nanocellulose for sustain-. action of β-1,4-glucosidic chain by endoglucanase.. able future materials, International Journal of. CNF treated with both endoglucanase and xylanase. Precision Engineering and Manufacturing-Green. showed narrower and more unifrom morphology. Technology 2:197-213 (2015).. than that of untreated or only endogluca-. 4. Dufresne, A., Nanocellulose: from nature to. nase-treated CNF. High dosage of endoglucanase. high performance tailored materials, Walter de. led to the wider fibril width. Enzymatic pretreated. Gruyter GmbH & Co KG (2017).. CNF had smaller aspect ratio compared to. 5. Dufresne, A., Nanocellulose: a new ageless. untreated CNF. CNF sheets treated with only. bionanomaterial, Materials Today 16:220-227. endoglucanse exhibited lower tensile strength and. (2013).. elastic modulus, whereas CNF sheets treated with. 6. Brodin, F. W., Gregersen, Ø. W., and Syverud,. mixture of endoglucanase and xylanase did not. K., Cellulose nanofibrils: Challenges and pos-. show noticable decresase in their mechanical prop-. sibilities as a paper additive or coating mate-. erties. Consequently, compared to untreated and. rial–A review, Nord Pulp Pap Res J 29(1):156-. only endoglucanase treatment, the combined use of. 166 (2014).. endoglucanse and xylanase led to the production of. 7. Osong, S. H., Norgren, S., and Engstrand, P.,. homogeneous CNFs and the prevented a redcution. Processing of wood-based microfibrillated. in the mechanical strength of the CNF sheet.. cellulose and nanofibrillated cellulose, and applications relating to papermaking: a review, Cellulose 23:93-123 (2016).. Acknowledgement. 8. Rodionova, G., Lenes, M., Eriksen, Ø., and Gregersen, Ø., Surface chemical modification. This work was supported by the Technological. of microfibrillated cellulose: improvement of. Innovation Program funded by the Ministry of. barrier properties for packaging applications,. Trade, Industry & Energy (project No. 10062717).. Cellulose 18:127-134 (2011).. We appreciate Buckman Laboratories, Korea for providing enzymes.. 9. Lavoine, N., Desloges, I., and Bras, J. Microfibrillated cellulose coatings as new release. J. of Korea TAPPI Vol.53 No.5 Sep.-Oct. 2021. 13.

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