Diisocyanate modifiable commercial filter paper with tunable hydrophobicity, enhanced wet tensile strength and antibacterial activity
Graphical abstract
Introduction
Cellulose, as the most abundant natural resource on earth, is an ideal raw material for producing various functional materials, because of its biodegradability, biocompatibility, non-toxicity, low-cost, environmental friendliness and safety risks. According to their different structures and properties, cellulose-based materials can be utilized on food-packaging, reinforced composite, biomedical, and printed electronic devices (Lavoine & Bergström, 2017; Wei et al., 2020; Zhu et al., 2016).
Usually, cellulose-based materials are present in various forms (1D, 2D and 3D structures), corresponding to nanocellulose with a width below 100 nm, 2D materials (films and membranes), and 3D materials (hydrogels and aerogels) (Bian, Dong et al., 2020; Chen, Li, Abe, & Yano, 2018; Dai et al., 2020; Du et al., 2019; Hou et al., 2019; Wang, Chen, Zhu, & Yang, 2017). Among them, nanocellulose, produced with chemical/biological pretreatments followed by mechanical fibrillation, has outstanding properties including high aspect ratio, large specific surface area, and excellent optical properties (Bian, Chen et al., 2020; Bian, Dong et al., 2019; Du et al., 2020, 2016; Wang et al., 2018). Nanocellulose can be used as an excellent supramolecular template for functionalization in sensor fabrication and electronic boards (Chen, Yu et al., 2018; Wu et al., 2018). 2D materials, such as films and membranes, are prepared by combining nanocellulose with other functional materials to obtain enhanced characteristics, such as optical, mechanical, barrier, anti-ultraviolet, insulating, magnetic performances (Fang, Hou, Chen, & Hu, 2019; Feng et al., 2017; Wang et al., 2020; Yang, Shen, Wang, & Qiu, 2019). And these functional films and membranes are widely emerged in high technology fields, such as optoelectronic devices, biomedicine, smart sensing, and environmental protection (Hosoya, Sakamoto, & Yogo, 2014; Sadasivuni et al., 2015; Shimizu, Saito, & Isogai, 2016; Song et al., 2018; Wang et al., 2020). 3D materials mainly include hydrogels with highly hydrated physically and/or chemically cross-linked 3D networks as well as aerogel with large specific area and uniform pore structures. Cellulose-based hydrogels are usually prepared directly from native cellulose using different appropriate solvents or blended with other polymers by physical/chemical cross-linking, showing potential applications in drug delivery system, tissue engineering, blood purification, and water purification (Chang & Zhang, 2011; Chang, Duan, Cai, & Zhang, 2010; Du et al., 2019; Varaprasad, Jayaramudu, & Sadiku, 2017; Ye, Watanabe, Iwasaki, & Ishihara, 2003). Cellulose-based aerogels are nano/microporous and lightweight materials obtained by sublimating the solvent components from the hydrogels, which has attracted great interests for superabsorbents, insulating materials, supercapacitor, and electromagnetic shielding materials (Jiménez-Saelices, Seantier, Cathala, & Grohens, 2017; Liu et al., 2020; Long, Weng, & Wang, 2018; Zheng, Cai, & Gong, 2014; Zheng, Kvit, Cai, Ma, & Gong, 2017). However, there are still some drawbacks as followed: (1) the high capital cost, long operation duration, and potential environmental concerns during nanocellulose production; (2) the low homogeneity of membranes or films mixed with other polymers and the difficulties to make film smooth due to the evaporation of water; (3) the high energy cost using freeze-drying or critical point drying to produce hydrogel or aerogel, which limit their commercial applications.
Among various cellulosic substrates, cellulose paper, derived from the mature paper-making industry, is considered as an ordinary commodity in daily life. It is used in information recording/transmission and product packaging/storage, and plays a critical role in the history of mankind (Zhang et al., 2018). Nowadays, compared with nanocellulose-based materials, cellulose paper has advantages of scale-up and low-cost production, showing great potential for petroleum-based plastic replacement. However, cellulose paper still faces several challenges. For example, the hydroxyl groups on the surface of cellulose fibers are highly sensitive to moisture, leading to the water instability of cellulose paper and low mechanical properties in wet conditions. Therefore, hydrophobic modification is of great importance for cellulose paper to broaden the scope of their potential applications.
The existing modification method of filter paper included silanization, atom transfer radical polymerization, electro-spraying microspheres, compositing with hydrophobic substances, etc (Gao, Huang, Xue, Tang, & Li, 2017, 2015; Lindqvist & Malmström, 2006; Liu et al., 2018). Zhang et al. used fluorine-free organosilanes (methyltrichlorosilane and octadecyltrichlorosilan) to modify with laboratory filter paper. The uniform and particulate nanostructures endowed the paper with high water contact angle (>150°) (Zhang, Kwok, Li, & Yu, 2017). Gao et al. developed a facile one step method to prepare the hybrid polyvinylidene fluoride (PVDF)/SiO2 microspheres based on electro-spraying for super-hydrophobic coating. The coated filter paper could not only separate the oil with the pure water but also the corrosive solution including the salt, acid and alkali solution (Gao et al., 2017). Some natural binders, such as lignin, was deposited on the surface of filter paper, then hot-pressed to form wood-inspired lignin-cellulose composite. And the resulting paper exhibited an outstanding tensile strength of 200 MPa, significantly higher than that of conventional cellulose paper (40 MPa) and some commercial petroleum-based plastics (Jiang et al., 2019). However, these means usually need inert gas protection, high temperature and pressure, or high energy consumption. Developing facile and efficient technologies is a key aspect to realize the commercial production of functional filter paper.
This work was intended to simultaneously improve the wet strength and hydrophobicity of commercial filter paper by cross-linking the surface hydroxyls of cellulose fiber with rigid, hydrophobic diisocyanate. It is conceivable that the paper production and modification are easily incorporated in the existing roll-to-roll paper fabrication line. Therein, swollen cellulose fiber with abundant hydroxyl groups functioned as polyols to react with isocyanate to form urethane linkages, as shown in Fig. 1. In order to overcome the incompatibility of cellulose fiber with organic solvent for diisocyanate, cellulose paper was solvent-exchanged to acetone (solvent for diisocyanate), allowing cross-linking reaction to proceed. The modified filter papers were investigated in terms of morphologies, chemical structures, dry and wet mechanical properties, hydrophobicity, etc. and were also assessed for antibacterial activity.
Section snippets
Materials
Commercial filter paper (FP, medium) was purchased from GE Bio-technology, Hangzhou. Methylene diphenyl diisocyanate (MDI, 98 %, Aladdin), acetone (AR, 99.5 %, Nanjing Chemical Reagent), triethylamine (AR, 99 %, Aladdin), tert-butanol (AR, 99 %, Nanjing Chemical Reagent), chloroform (AR, 99 %, Nanjing Chemical Reagent), and methylene blue (Indicator, Aladdin) were used as received without further purification.
FP modification with MDI
FP with diameter of 7 cm was soaked in 4% (w/w) sodium hydroxide solution for 2 h to
Modified MDI-FP characterization
In order to improve the reaction efficiency, the original FP was firstly swollen in sodium hydroxide solution to expose more hydroxyl groups, then solvent-exchanged using acetone. Fig. S1 showed the effect of different treatments on the morphology of FP. After the alkaline swelling and solvent exchange, width of the fiber was increased and the surface became rough with some irregular voids. These abundant hydroxyl groups on the surface of FP were functioned as polyols to react with isocyanate
Conclusions
In this work, we developed a facile method to remarkably improve the hydrophobicity and wet tensile strength of commercial filter paper using MDI as cross-linking agent, while also endowing the modified paper with excellent antibacterial activity. The modified paper showed highest water contact angle of 137.0° and wet tensile strength of 4.8 MPa, significantly higher than that of the original paper. The formation of urethane linkages improved the hydrophobicity and wet mechanical properties,
CRediT authorship contribution statement
Xuelian Zhou: Conceptualization, Methodology, Investigation, Writing - original draft, Writing - review & editing. Yanqiao Fu: Investigation, Formal analysis. Lidong Chen: Investigation. Ruibin Wang: Methodology. Xiu Wang: Resources. Yingchun Miao: Resources. Xingxiang Ji: Resources, Supervision. Huiyang Bian: Conceptualization, Methodology, Investigation, Writing - review & editing, Funding acquisition. Hongqi Dai: Conceptualization, Supervision, Funding acquisition.
Acknowledgements
This work was supported by the Foundation (No. KF201917) of State Key Laboratory of Biobased Material and Green Papermaking, Qilu University of Technology, Shandong Academy of Sciences and the National College Students Innovation and Entrepreneurship Training Program (201910298155H). We also would like to thank Guizhen Zhu and Fan Su (Advanced Analysis & Testing Center, Nanjing Forestry University) and Prof. Yimin Fan (Nanjing Forestry University) for providing valuable help of SEM, XRD and
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