Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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TEMPO-Oxidized Waste Cellulose as Reinforcement for Recycled Fiber Networks Lei Dai,†,‡ Jie Chen,§ Bo Yang,‡ Yanqun Su,‡,∥ Le Chen,§ Zhu Long,*,§ and Yonghao Ni*,‡ †
College of Bioresources Chemical and Materials Engineering, Shaanxi University of Science and Technology, Xi’an 710021, China Department of Chemical Engineering, University of New Brunswick, Fredericton, New Brunswick E3B 5A3, Canada § Key Laboratory of Eco-textiles, Ministry of Education, Jiangnan University, Wuxi 214122, China ∥ China National Pulp and Paper Research Institute, Beijing 100102, China ‡
S Supporting Information *
ABSTRACT: Lignocellulosic recyclable packaging material is in high demand, because of the emergence of e-commerce. Old corrugated container (OCC) pulp is the main raw material in China for manufacturing packaging boxes/bags, but its strength needs improvement. In this project, waste cellulose in the form of used printing paper was oxidized using 2,2,6,6tetramethylpiperidine-1-oxyl radical (TEMPO) concept so that more carboxyl groups were introduced. The TEMPO-oxidized waste cellulose (TOWC) having a carboxyl content of 1.451 mmol/g was then used as reinforcement for the OCC fiber networks. The TOWC addition to the OCC fiber networks at different mass ratios of 5%, 10%, and 15% on its strength properties was studied. The results showed that the addition of TOWC remarkably improved the tensile and other strength properties, which can be explained by the improved hydrogen bonding and interpenetrating capacity of fiber networks, thanks to the increased carboxyl groups and fibrillation after the TEMPO oxidation.
1. INTRODUCTION Lignocellulose-based packaging products are becoming more and more popular, because of advantages such as sustainability, recyclability, low cost, good performance, etc.1,2 Corrugated containers represent an important lignocellulose-based packaging material for shipping and handling various products.3−5 The demand for corrugated containers has been largely increased because of the extraordinarily fast growth of ecommerce in the past decade. Old corrugated container (OCC) pulp is the main raw material in China for manufacturing packaging materials. The utilization of waste cellulose has great significance, because of the economic and environmental concerns.6 However, OCC is usually made of recycled pulp; hence, its strength improvement should be addressed.3,7,8 Mechanical refining or commercial dry strength agents, including cationic starch, poly(vinyl alcohol), and polyacrylamide, are primarily used to improve the mechanical strength of OCC paper in the papermaking industry.3,7,9 Nevertheless, these methods not only increase the energy consumption and cost, but also bring some environmental concerns. Natural materials are preferred as the reinforcement, because of their advantage of wide availability, low cost, and environmental friendliness.10,11 For example, nanosized carbohydrate polymer material (chitosan-complexed starch nanoparticles) has been adopted to enhance the properties of OCC paper.3 Mixed office waste (MOW) paper, consisting of laser printer and photocopier waste papers, accounts for a large ratio of © XXXX American Chemical Society
waste paper. MOW paper is a good waste cellulose source, since it is mainly made of bleached chemical pulp. Nonetheless, MOW paper recycling requires deinking prior to other treatment or processing.12,13 Typically, a peroxide bleaching followed after deinking, to increase its optical properties. As reported, different pulps treated with peroxide bleaching contain the carboxyl contents in the range of 0.047−0.083 mmol/g.14,15 As an efficient regioselective oxidation method for cellulose, TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical)-mediated oxidation has been proved to be capable of introducing carboxyl groups onto C-6 sites on cellulose molecules, and only inexpensive NaClO is consumed during the oxidation process.16−19 Moreover, the cellulose fibrillation occurs during the TEMPO-oxidation process.16 It has been demonstrated that the papers prepared with TEMPO-oxidized cellulose possess better strength than those prepared with cellulose without oxidation.17,18 The strength of paper products is highly dependent on interfiber bonding,20−22 and it is now wellestablished that carboxyl groups on pulp fibers can improve the ensuing paper strength, because of the improved swelling and bonding ability of modified pulp fibers.23 Received: Revised: Accepted: Published: A
October 4, 2017 November 18, 2017 December 4, 2017 December 4, 2017 DOI: 10.1021/acs.iecr.7b04135 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
Scheme 1. Schematic Illustration of the OCC Papers Preparation Process and Interactions between TOWC and OCC Pulp Fibersa
a OCC are short fibers, and TOWC are long fibers with more carboxyl groups and more flexibility (and, thus, better fiber conformability). The OCC and TOWC could form an interpenetrating fiber network.
TEMPO-mediated oxidation was carried out as previously reported.24 Five grams (dry mass) of the above-mentioned slurry was suspended in a carbonate buffer solution (500 mL, 0.1 M, pH 10.24) containing TEMPO (0.1 g) and NaBr (1.0 g) under stirring. The oxidation was started once the NaClO was added. After the oxidation, the pH of the system was adjusted to ∼4.0, so that the carboxyl groups of the oxidized cellulose fibers are protonated. The oxidized cellulose was subsequently washed and filtered with deionized water. The as-prepared TEMPO-oxidized waste cellulose (TOWC) was stored at 4 °C before use. 2.3. Carboxyl Content Determination. The carboxyl content of the TOWC was determined following TAPPI method T237 with minor modifications. A quantity of 0.1 g (dry mass) of TOWC was dispersed in 0.1 mol/L hydrochloric acid (HCl) solution for 120 min and then washed. The obtained TOWC was then reacted with 0.01 mol/L sodium bicarbonate (NaHCO3)-sodium chloride (NaCl) solution for 60 min. The remaining excess amount of NaHCO3 was titrated with 0.01 mol/L HCl. The carboxyl content was calculated using the equation
Herein, we proposed to chemically modify the waste cellulose obtained from MOW paper with TEMPO/NaBr/ NaClO to introduce carboxyl groups onto cellulose fibers, and then use the resultant TEMPO-oxidized waste cellulose (TOWC) to enhance the performance of paper made from OCC pulp. This is based on the following hypotheses: (1) more carboxyl groups, thus more hydrogen bonding, leading to better strength of OCC paper; (2) the fibrillation of waste cellulose in the TEMPOoxidation providing more binding sites; (3) better interpenetrating cellulose networks due to better fiber flexibility of waste cellulose from MOW; and (4) no specific requirements on the residual ink of MOW for its use in OCC paper. As far as we know, there is no report of using TOWC to improve the performance of OCC paper. In this work, we mixed the TOWC with OCC pulp in different mass ratios to prepare OCC paper. The performances, in terms of tensile index, stretch, tear index, burst index, ring crush index, and stiffness of the resultant OCC paper, were investigated. This work has important environmental implications, because it provides an alternative way to turn waste into wealth.
carboxyl content (mmol/g) =
2. EXPERIMENTAL SECTION 2.1. Materials. The old corrugated container (OCC) pulp was kindly provided by a local mill. MOW (mixed office waste), A4 copy paper printed double-sided with a laser printer, was selected as the waste cellulose source in this work. The 2,2,6,6tetramethylpiperidine-1-oxyl radical (TEMPO), sodium bromide (NaBr), sodium hypochlorite (NaClO), and other chemicals were all purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) and used as received. 2.2. TEMPO-Mediated Oxidation of Waste Cellulose. Waste office paper was torn into small pieces and soaked in a 3.5 wt % NaOH solution. The obtained slurry was then stirred at 80 °C for 120 min, followed by thorough washing with water.
0.01 × (V1 − V2) × 1000 M
where V2 and V1 are the volumes of the HCl with and without sample, respectively, and M is the sample weight. 2.4. OCC Paper Formation. In this study, each OCC paper has an area of 200 cm2 and a basis weight of 120 g/m2. For paper formation, the addition of TOWC to the OCC pulp varied at 5%, 10%, and 15%. The papers were dried at 105 °C for 10 min under a vacuum of −0.1 MPa. All these papers were conditioned under conditions of 23 ± 1 °C and 50% ± 1% relative humidity (RH) for 48 h before testing. 2.5. Paper Testing. OCC papers were cut and tested per TAPPI methods (2006). Tensile strength, tear resistance, burst, and ring crush index, as well as stiffness were determined B
DOI: 10.1021/acs.iecr.7b04135 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research
Figure 1. Increases in the carboxyl content of the TOWC, as a function of (a) the reaction time (NaClO dosage was set as 4.0 mmol/g) and (b) NaClO dosage (reaction time was set as 3 h).
Figure 2. SEM characterization of (a, b) the original waste cellulose fibers and (c, d) the TOWC samples obtained under the optimized conditions. Images in panels (a) and (c) have scale bars of 100 μm, while panels (b) and (d) have scale bars of 30 μm.
largely dependent on fibers’ capability of forming interfiber bondings that are extraordinarily related to the carboxyl content on the fiber surface.3,9,25 The high quantity of carboxyl groups on the TOWC could contribute to the interfiber bonding. On the other hand, the cellulose fibrillation during the oxidation could increase the number of exposed functional groups and the fiber flexibility, which would also result in more hydrogen bonds among the pulp fibers. The fibrillated TOWC fibers can lodge between OCC pulp fibers and increase the bonding sites and bonding area. Office paper is mostly made of bleached chemical pulp that has relatively longer fiber lengths than those of OCC pulp. Also, the fibrillation of waste cellulose in oxidation would increase the fiber flexibility. Thus, the incorporation of TOWC in OCC papers would result in better interpenetration of fiber networks, which could also contribute to the improved strength of the papers made thereof. In short, the advantages of TOWC in this system are (1) enhanced hydrogen bonding between
according to TAPPI methods T494, T414, T810, T818 and T489, respectively. 2.6. Scanning Electron Microscopy (SEM) Observation. The TOWC morphology and the surface of the asprepared OCC paper were observed with a scanning electron microscopy (SEM) system (Hitachi, Model SU1510) at an accelerating voltage of 5 kV. To observe the TOWC, a small amount of TOWC was dried to form a thin cellulose film and then mounted on a brass specimen holder. For OCC paper morphology observation, small pieces of OCC papers were cut and mounted on the specimen holder, respectively. All specimens were coated with gold before analysis.
3. RESULTS AND DISCUSSION The preparation process of OCC paper involving TOWC is shown in Scheme 1. The addition of TOWC can improve the bonding between fibers, thereby enhancing mechanical properties of the resultant papers. Mechanical properties of papers are C
DOI: 10.1021/acs.iecr.7b04135 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research
Figure 3. Effect of the TOWC contents on the mechanical properties of the OCC papers: (a) tensile index and stretch, (b) tear and burst index, (c) ring crush index, and (d) stiffness.
pulp fibers, (2) improved penetrating fiber networks, (3) good compatibility with pulp fibers, and (4) natural, nontoxic, and cost-efficient properties. The effect of TEMPO-mediated oxidation conditions (reaction time and oxidant dosage) on the carboxyl content of TOWC was studied. To disclose the influence of reaction time, a certain amount of NaClO (4.0 mmol/g waste cellulose) was used for the oxidation. As shown in Figure 1a, the carboxyl content of the TOWC increased with the extended reaction time. As the reaction time increased from 1 h to 3 h, the carboxyl content increased from 0.956 mmol/g to 1.200 mmol/ g. When the reaction time was further prolonged to 4 h, however, the carboxyl content of TOWC slightly decreased. The reason could be the loss of collected oxidized moieties, especially the oxidized hemicelluloses. Compared to cellulose, hemicelluloses have a lower degree of polymerization (DP); thus, they are easier to degrade during the oxidation. As a result, the oxidation time was optimized to be 3 h, considering the carboxyl content of the oxidized product. Different amounts of NaClO were used in the waste cellulose oxidation to clarify the effect of NaClO dosage on the TOWC, and the results are shown in Figure 1b. The carboxyl content significantly increased from 0.712 mmol/g to 1.519 mmol/g as the NaClO consumption varied from 3.0 mmol/g to 6.0 mmol/ g waste cellulose. The amount of carboxyl groups in TEMPOmediated oxidation process is proportional to the NaClO dosage.26,27 As the NaClO dosage increased from 5.0 mmol/g to 6.0 mmol/g, the carboxyl content of the TOWC increased by 0.068 mmol/g, which represented an increase of 4.69%. Again, this could be attributed to the severe degradation of hemicelluloses under harsh oxidation conditions. In this sense, the NaClO dosage was chosen to be 5.0 mmol/g waste cellulose, in which the obtained TOWC had the carboxyl content of 1.451 mmol/g.
SEM was adopted to observe the morphology of the waste cellulose before and after the TEMPO oxidation. Figures 2a and 2b show the pristine waste cellulose, while Figures 2c and 2d show the TOWC obtained under the optimized oxidation conditions (reaction time of 3 h and NaClO dosage of 5 mmol/ g). The results proved that the bonding area of TOWC was much larger than that of original waste cellulose. The number of pores and their sizes were remarkably decreased. As seen in Figure 2d, the surface of TOWC exhibited a terrace structure, probably because of the removal of hemicelluloses. This also supports the above discussion about variations in carboxyl content of the TOWC. To investigate the effect of TOWC on the mechanical properties of OCC papers, different mass ratios of TOWC were incorporated to prepare the papers. Mechanical properties of the OCC papers, including tensile index, stretch, tear index, burst index, ring crush index, and stiffness, were determined, and the results are shown in Figure 3. As expected, the tensile index of the papers obviously increased with the addition of TOWC (see Figure 3a). Specifically, the papers with 5% of TOWC possessed a tensile index of 34.5 N m/g while the control (without TOWC) had a tensile strength of 29.6 N m/g. The improvement in tensile index could be attributed to the increased bonding between pulp fibers caused by the abundant carboxyl groups on TOWC. It has been successfully proved that paper tensile strength is strongly correlated to the carboxyl group content on fiber surfaces.3,9,25 The tensile strength further increased as the mass ratio of TOWC increased, but the growth rate decreased. To further prove the increased bonding after the incorporation of TOWC, the sheet densities of the resultant OCC papers were also determined in this work, and the results are shown in Figure S1 in the Supporting Information. It is evident that the sheet densities increased with the increased mass ratio of TOWC. Because of the high D
DOI: 10.1021/acs.iecr.7b04135 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research
Figure 4. SEM results of (a) the control paper (OCC only without the addition of TOWC and (b−d) OCC papers with different mass ratios of TOWC (5% (panel (b)), 10% (panel (c)), and 15% (panel (d))).
grammage of the resultant OCC papers, their opacity was not obviously affected by the improved bonding (Table S1 in the Supporting Information). As seen in Figure 3a, the incorporation of TOWC slightly increased the stretch of the OCC papers. The stretch increased from 2.02% to 2.41% as the mass ratio of TOWC increased from 0 to 15%. Studies have shown that carboxyl groups on fiber surfaces have a positive effect on the fiber plasticity.25 As shown in Figure 3b, the addition of 5% TOWC increased the tear index of the OCC papers from 6.26 mN m2/g to 6.86 mN m2/g. However, decreasing of the tear index appeared as the mass ratio of TOWC further increased. The tear index decreased from 6.86 mN m2/g to 6.40 mN m2/g when the amount of TOWC increased from 5% to 15%. Several factors affect the tear strength, including interfiber bonding, fiber strength, fiber length, etc., among which the interfiber bonding and fiber strength are the most important. The addition of small amount of TOWC increased the interactions between pulp fibers, thus having a positive effect on the tear index of the ensuing papers. Yang et al.28 also noticed that a small amount of H2O2-oxidized holocellulose could improve the paper tear index. However, the cellulose fibrillation in oxidation would also deteriorate the strength of the fibers. Thus, the negative effect brought by the TOWC appeared when its mass ratio exceeded a certain level. Enhancement in burst index was observed with the addition of TOWC (Figure 3b). It exhibited increases of 25%, 35%, and 40% when the mass ratios of the TOWC were 5%, 10%, and 15%, respectively. The improvement in burst index with the incorporation of TOWC might be assigned to (1) the enhanced hydrogen bonding ability and (2) the incremental bonding area of pulp fibers. The positive effect on paper burst index resulted from the introduction of carboxyl groups was also confirmed by other researchers.27,29
Ring crush strength is one of the most important factors for packaging boxes. The addition of TOWC also conferred the OCC papers with an enhancement in ring crush resistance, as indicated in Figure 3c. The OCC papers with 5% and 10% of TOWC showed ring crush index values of 9.60 N m/g and 10.07 N m/g, respectively, whereas the control had the value of 8.75 N m/g. Again, this improvement should be partly credited to the increased interfiber bonding and bonding sites brought by TOWC. In addition, because of the cellulose fibrillation in oxidation, the TOWC possessed many more functional groups and a much larger surface area improving its film forming ability, which is supported by the SEM images in Figure 2c. Together with OCC pulp, TOWC forms a network-film hybrid structure, which enhanced the toughness of the OCC papers. The ring crush index decreased to 9.37 N m/g as the mass ratio of TOWC additionally increased to 15%, which indicates that 10% of TOWC is appropriate for ring crush strength. The addition of TOWC also resulted in an improvement in stiffness of the OCC papers (Figure 3d). It remarkably increased from 18.33 mN m to 41.01 mN m as the mass ratio of TOWC increased from 0 to 10%, which could also be explained by the improved bonding between pulp fibers and the improved toughness, as the result of the addition of TOWC. Further increases in the mass ratio of TOWC decreased the stiffness of the OCC papers to 31.33 N m, resulting from the compact structure. SEM images of OCC papers containing different mass ratios of TOWC are shown in Figure 4. Figure 4a is an SEM image of the control (without TOWC), which is typical of paper product with a hierarchical pore structure. Some degraded fibers can also be found in the control sample because OCC pulp has previously gone through at least two recycling processes, resulting in shorter and/or thinner fibers than the original one.3 However, with the incorporation of TOWC, the specimens showed obvious improvement in smoothness. The collapse and E
DOI: 10.1021/acs.iecr.7b04135 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research fibrillation of the fibers increased the contact area among fibers, leading to significantly decreased pore number and size. Increased contact area between fibers can improve interfiber bonding, which also supports the strengthening effect brought by the TOWC. Zhao et al.30 also reported the increased interfiber bonded area after TEMPO oxidation of cellulose. With the incremental mass ratio of TOWC, a larger area of the filmlike structure formed, which also supported the discussion about the enhancement in ring crush strength and stiffness resulting from the addition of TOWC.
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4. CONCLUSIONS This study explores the potential of using TEMPO-oxidized waste cellulose (TOWC) as a strength additive to improve the mechanical properties of old corrugated container (OCC) paper. The results indicated the obvious improvement of tensile index, burst index, ring crush index, and stiffness of the OCC papers with the addition of only 5 wt % TOWC, which was attributed to the improved interfiber bonding and bonding sites with the addition of TOWC. Increased mass ratio of TOWC further increased the strength properties of the papers, except the tear index. Therefore, the present study provides a novel method for both improving the mechanical strength of OCC pulp and utilization of waste cellulose. The replacement of traditional reinforcing agents with environmentally friendly modified waste cellulose is beneficial to the papermaking industry and also addresses the solid waste problem.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b04135. Sheet density and opacity of OCC papers containing different TOWC contents (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (Z. Long). *E-mail:
[email protected] (Y. Ni). ORCID
Lei Dai: 0000-0002-9011-2246 Yonghao Ni: 0000-0001-6107-6672 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge the financial support from the Canada Research Chairs Program. REFERENCES
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DOI: 10.1021/acs.iecr.7b04135 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
