Due to the low-carbon environmental protection concept, a large proportion of paper and paperboard produced are recycled every year. In 2014, approximately 60% of the raw materials for pulp and paper industries were fibers recycled from waste paper and paperboard globally [1
]. However, paper or paperboard made of recycled fibers generally had inferior physical properties compared to those made of virgin fibers [2
]. Consequently, effective reinforcing agents are used for processing recycled fibers into quality paper products. In addition, paper with excessive inorganic fillers for special surface properties, e.g., fire retardant, also requires reinforcement [5
]. Nano-cellulose, due to their structural similarity, compatibility and affinity to pulp fibers, as well as their high mechanical strength, was used to reinforce paper made from recycled fibers [3
] or paper with excessive inorganic fillers [5
Bacterial cellulose (BC) is a special type of nano-cellulose secreted in vitro by bacteria, of which the most studied species is Gluconacetobacter xylinus
]. BC has the same chemical structure as plant-based cellulose and has a higher crystallinity and degree of polymerization [10
]. The microstructure of BC is also different from plant-based cellulose. BC does not have a macrofibril structure. The BC microfibrils are 10–100 nm in diameter and interlace with each other forming a fine net structure [12
], leading to a high specific surface area. When BC pellicles are disintegrated into small fragments or fibers, abundant free hydroxyls will be released. Being used as reinforcing agents during paper-making, the BC fibers can bridge between fibers and improve the physical properties of papers. Our previous studies have already shown that the addition of BC lower than 1% of paper dry weight can effectively improve the tensile strength of paper produced from some high-quality fibers, such as softwood pulp, hardwood pulp, sugarcane bagasse pulp, and bamboo pulp; these fibers are more fibrillated and, thus, have more surface hydrogen bonding sites for BC fibers bridging between [12
]. However, the reinforcing effect of BC on recycled fiber paper was limited [12
]. Recycled fibers suffer from fiber hornification causing less surface hydrogen bonding sites [3
]. Nevertheless, BC fibers with abundant free hydroxyls should have been able to increase the total numbers of hydrogen bonding sites within the recycled fiber matrix and thus the paper strength [3
]. However, BC fibers aggregate with themselves instead of being evenly distributed within the fiber matrix, causing little improvements in the tensile strength of paper produced from recycled fibers [12
]. The agglomeration was also a problem when using nano-cellulose, e.g., cellulose nanofibers (CNF) and nanocrystals (CNC), as reinforcing agents for recycled fiber paper [3
In addition to the mixing of nano-cellulose with plant fibers during sheet forming, the coating of inorganic nanoparticles onto paper surface may also be an effective method to reinforce paper. The coating of Mg(OH)2
] and Halloysite (Al2
O) nanotubes [17
] onto paper surface can improve paper tensile strength by 40–50%. The dispersion and stabilization of those inorganic nanoparticles were vital to their reinforcing effects; e.g., the Mg(OH)2
nanoparticles were stabilized trimethylsilyl cellulose [16
] and the halloysite nanotubes were stabilized by hydroxypropylcellulose [17
Consequently, the dispersion or stabilization of nano-materials was one of the key factors in determining their effects in paper reinforcement. This study is to explore the effective dispersion or stabilization of BC fibers so that their reinforcing effects on paper made from recycled fibers could be improved. Two types of stabilization mechanisms, i.e., steric and electrostatic repulsions, are often used to explain the dispersion or the colloidal stability of cellulose nanofibers or nanocrystals suspended in water [2
]. A good dispersion or colloidal stability can be reached by non-covalently adsorbing or covalently grafting macromolecules to the fiber surface, which prevents the fibers from aggregating due to steric hindrance [19
]. Many natural polymers or macromolecules are water soluble and have good adhesion to cellulose fibers such as xyloglucan, β-d
-glucan, carboxymethyl cellulose (CMC), etc. [19
]. Having good adhesion to cellulose fibers, xyloglucan had been proved to disperse cellulose nanocrystals well [19
]. Adding CMC to cellulose fiber suspension led to better dispersion than adding xyloglucan, which may be due to the surface charges of CMC [19
]. Bulky macromolecules or polymer chains, for instance, poly(ethylene oxide) or poly(phenylene oxide), can also be grafted onto cellulose fiber (nanocrystalline cellulose) surfaces leading to the formation of well-dispersed and stable colloidal aqueous suspension [22
]. The hydroxyls on cellulose nanofiber surface may be chemically modified forming charged groups on surface so that the electrostatic repulsion effects prevent such fibers from aggregating [24
]. Cellulose nanofibers or nanocrystals produced by TEMPO-mediated oxidation or concentrated H2
treatment would have their surfaces modified with negative charges, which may naturally be good for their dispersion [2
]. However, for general cellulose nanofibers, surface modification with anionic agents is usually required to obtain better dispersions. Meanwhile, cationic surface modification can also improve the dispersion of cellulose fibers [25
]. Cellulose nanofibers can be grafted with glycidyltrimethylammonium chloride (GTMAC) [25
] or epoxypropyltrimethylammonium chloride [26
] to improve their dispersions.
In addition to effective dispersion, the increased retention of nano-cellulose during paper forming may further improve the paper strength. With addition of cationic macromolecules, e.g., chitosan and cationic polyacrylamide, CNC and CNF can more effectively improve the strength of paper produced from recycled fibers [3
]. The cationic modification of BC fibers increased their retention in paper and, thus, further improved the strength of paper made of sugarcane bagasse pulp [13
]. However, BC fiber retention is not a problem for recycled fiber. Our previous study has shown that the BC fiber retention rate can reach 95% in recycled fibers and was much higher than in other high quality fibers, due to smaller fiber size and larger numbers of fiber fines for recycled fibers [12
]. It states again that BC fiber aggregation is a key factor preventing its effective reinforcement to paper made of recycled fibers.
In this study, natural or modified polysaccharides such as xylan, carboxymethyl cellulose, glucomannan, and polyethylene oxide were used as additives in order to improve the dispersion of BC fibers through steric repulsions. BC fibers were also oxidized through TEMPO-mediated oxidation in order to generate negative surface charges so that BC fibers dispersion would be improved through electrostatic repulsion. At the last, the dispersed BC fibers were used to reinforce paper produced from recycled fibers and mechanisms involved in various methods for dispersing BC fibers were also investigated.
2. Materials and Methods
Recycled fiber pulp (RFP), made from old newspaper (ONG) and old magazine (OMG), was obtained from Guangzhou Paper Co., Ltd. (Guangzhou, China) with a Canadian Standard Freeness (CSF) of 270 mL; the pulps were kept at moisture content of 90% at 4 °C and were used within 30 days of preparation. CMC-I (Mw = 90,000, DS = 0.7), CMC-II (Mw = 250,000, DS = 1.2), CMC-III (Mw = 250,000, DS = 0.7), nonionic polyethylene oxide (PEO, average Mw ~4,000,000) and sodium hypochlorite solution (6–14% active chlorine basis) were purchased from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). Xylan (from sugarcane bagasse, Mw = 30,000) was purchased from Shanghai Yuanye Biochemical Co., Ltd. (Shanghai, China). Glucomannan (from konjac) was purchased from Hubei Yizhi Konjac Biotechnology Co., Ltd. (Hubei, China). Cationized starch (DS = 0.025), which was produced from (3-chloro-2-hydroxypropyl)trimethylammonium chloride modified corn starch, were provided by Guangzhou Paper Co., Ltd. (Guangzhou, China). All other chemicals used are of analytical grade.
2.2. Preparation of Bacterial Cellulose (BC)
ATCC23767 was obtained from Nanjing High Tech University Biological Technology Research Institute Co., Ltd. (Nanjing, China) and used to produce the bacterial cellulose (BC) pellicles. Static fermentation method was used to produce the BC pellicles. Detailed preparation methods and characterizations of the BC pellicles were shown in Xiang et al. [12
2.3. BC Fiber Dispersion Evaluation
Stock solutions (2 g/L) of CMC-I, xylan, cationic starch, glucomannan, and nonionic PEO were prepared. Approximately 5 g (corresponding to 0.075 g dry weight) of wet BC pellicles (MC = 98.5%) or oxidized-BC were cut into small pieces and placed into a lab blender (SKG-1246, Foshan, China). Given amounts (corresponding to give the weight ratio of macromolecules/BC of 0, 0.05, 0.1, 0.25, and 0.5) of the macromolecule stock solutions were added. Deionized (DI) water was then added to make the mixture volume to 250 mL. The mixture was blended by the lab blender for 2 min to prepare the BC fiber suspension with a BC fiber concentration of 0.3 g/L (dry matter). The BC fiber suspension was poured immediately into a 250 mL-cylinder. The freshly prepared suspensions with BC fiber well dispersed was white and non-transparent in the 250 mL-cylinder. With time, the aggregation of BC fibers resulted in the shortening of the non-transparent portion, which were recorded with time.
For zeta potential measurement, particle concentration in the suspension should be kept between 10−5
volume fraction [27
]. The freshly prepared BC fiber suspensions were diluted 10 times with DI water to a BC fiber concentration of 0.03 g/L (dry matter). The pH value was ~7.0 for all the solutions. Approximately 1.5 mL of the diluted fiber suspensions were then poured into the sample cell and the zeta potential was measured through a nanoparticle analyzer (Horiba SZ-100Z, Kyoto, Japan). The dynamic (absolute) viscosities of the solutions of different dispersants were determined at 25 °C by using a rotational viscometer (Brookfield DV-II HE, Middleboro, MA, USA) at a rotational speed of 250 rpm; the solution concentration was set to 0.15 g/L (dry matter), which was the same as the concentration of the dispersants in BC fiber suspensions when weight ratio of macromolecules/BC was 0.5.
2.4. Adsorption of Macromolecules onto BC Fibers
Stock solutions of CMC-I, xylan, cationic starch, glucomannan, and nonionic PEO with various concentrations (200 mg/L, 600 mg/L, and 1000 mg/L) were prepared. Approximately 1 g of wet BC pellicle (MC = 98.5%) was disintegrated in 250 mL DI water by the lab blender in instant mode three times. The disintegrated BC and 30 mL of the stock solution was added into a conical flask. The mixture has a pH of ~7.0 and was stirred at 350 rpm for 2 h. The BC was then filtered and washed through a sand-cored funnel (pore size 30–50 μm). The filtrate was oven dried and the increment of weight was the macromolecules that did not adsorbed on BC.
2.5. The TEMPO-Mediated Oxidation of BC Fibers
The TEMPO-mediated oxidation of BC fibers was conducted according to Saito et al. [28
] with minor modifications. Approximately 20 g of wet BC pellicle (MC = 98.5%) was disintegrated by the lab blender in water in instant mode for three times and the water was filtered out through a sand-cored funnel (pore size 30–50 μm). The disintegrated BC fibers were suspended in water (30 mL) containing TEMPO (0.008 g, 0.05 mmol) and sodium bromide (0.05 g, 0.5 mmol). Into the BC fiber suspension, 0.5 mL, 1 mL, and 2 mL of the NaClO solution (6–14% active chlorine basis) were added. The pH was adjusted 10 by the addition of 0.1 M HCl. The TEMPO-mediated oxidation was continued at room temperature by stirring at 350 rpm for 3 h. During the reaction, the pH was maintained at 10 by adding 0.5 M NaOH. The oxidized BC fibers were filtered through a sand-cored funnel (pore size 30–50 μm) and washed with DI water until pH neutral. The samples were denoted OBC-0.5, OBC-1, and OBC-2, respectively, according to the volume of NaClO solution added to the oxidation process, e.g., 0.5 mL, 1 mL, and 2 mL.
2.6. Paper Handsheet Preparation and Characterization
BC membranes were mixed with stock solutions of CMC, xylan, cationic starch, glucomannan, or nonionic PEO and mechanically disintegrated by a lab blender (SKG-1246, Foshan, China) for 5 min. The recycled fiber pulps and the disintegrated BC fibers were mixed with 1% ratio (proportion of BC based on the total fiber dry weight) and dispersed with a standard pulp-disintegrator at a consistency of 1% (m/m) for 15,000 r. The hand sheets were made from the mixed pulp through a standard sheet former (Labtech 200-1, Laval, QC, Canada). The grammage of each sheet was maintained at 70 g/m2 (dry weight) level. The sheets were dried and equilibrated for 24 h at 23 °C and 50% humidity prior to physical tests. The dry tensile index and air permeability of the sheets were tested according to TAPPI standards (T494 and T251 wd-96, respectively). For wet tensile strength measurement, the middle part of the specimens was soaked in water for 8 s and the specimens were tested immediately for the tensile index. Paper sheet surface morphologies were evaluated using SEM (Zeiss Evo-18, Munich, Germany).
In this study, natural or modified polysaccharides, such as xylan, CMC, glucomannan, and cationized starch were used as additives to improve the dispersion of BC fibers or its reinforcing effects to recycled fiber paper. Good dispersion of BC fiber is helpful to its reinforcement to the dry tensile strength of paper made from recycled fiber, even though the improvement was not very high, with up to ~12% caused by macromolecule-dispersed BC. This showed that the quality of pulp fibers is still very important to the reinforcement effects of BC fibers. Proper dispersion of BC fibers can compensate for the pulp quality to some extent.
The adsorption of the macromolecules onto BC fibers is one of the key factors for their ability to disperse BC fibers. Some macromolecules, such as cationized starch, can evidently improve the paper strength alone without adding BC due to their fiber retention ability, but its adsorption to BC fibers can still improve BC dispersion and, thus, further improve the paper strength. The dispersing effects of the macromolecules can be correlated with the tensile strength of paper sheets. Among the macromolecules studied, glucomannan had the best dispersing ability to BC fibers and led to the best paper tensile strength improvement.
TEMPO-mediated oxidation is effective in improving the colloidal stability of BC fiber suspensions, but not effective in improving the BC fiber ability to enhance paper dry tensile index, probably because BC was degraded by the oxidation process. However, the incomplete oxidation of hydroxyls to aldehyde on BC fibers helps to improve the paper wet tensile index.