Nanocellulose is derived from cellulose and is an emerging biomass nanomaterial. The modification methods that have been reported so far can be roughly divided into three categories: One is physical adsorption, which mainly uses surfactants, oligomers or copolymers and other modifiers to change the surface of nanocellulose by physical adsorption. Hydrophilic and hydrophobic properties, it is a green and efficient modification method; the second is chemical methods, mainly including silylation, alkanoylation, esterification modification, etc.; the third is polymer grafting, which is grafted on the surface of nanocellulose. Branch hydrophobic polymer to improve hydrophobic performance.

Tianjin University of Science and Technology, Sun Lin, etc. reviewed the research status of physical adsorption modification, chemical modification and polymer graft modification, etc., and concluded that hydrophobic nano cellulose and its composites are used in packaging materials, papermaking, The application in the field of water purification, in addition, also put forward the challenges and future development direction of nano-cellulose hydrophobic modification research.

1 Hydrophobic modification of nanocellulose

1.1 Physical adsorption

Physical adsorption is the hydrophobic modification of nanocellulose through the adsorption of nanocellulose to surfactants, quaternary ammonium salts or copolymers. The physical adsorption method has a simple process and can well retain the integrity of nanocellulose. It is a simple and environmentally friendly modification method. However, in the physical adsorption process, the modifier mainly relies on weak binding force such as van der Waals force and hydrogen bond to interact with nanocellulose. Therefore, under a certain external force, the adsorbed hydrophobic substances will fall off, and there will be greater instability.

1.1.1 Adsorption of cationic surfactants Modified cationic surfactants are a class of substances with a hydrophobic structure, which can be adsorbed to the surface of nanocellulose under electrostatic action, so that nanocellulose hydrophobic materials with excellent hydrophobic properties can be prepared, but due to For reasons such as unstable adsorption-desorption balance, high critical micelle concentration and low steric repulsion, there are still free surfactant molecules in the matrix, which will reduce the mechanical properties of the material and affect the application of the material.

Qing et al. added cetyltrimethylammonium bromide to the prepared cellulose nanocrystals (CNC) and reacted for a period of time. After washing, centrifuging, freezing, and drying, the modified CNC was obtained, and the CNC and ten were determined. The dispersion stability of CNC coated with hexaalkyltrimethylammonium bromide in organic solvents; the results show that the CNC coated with hexaalkyltrimethylammonium bromide has good dispersibility and stability. It can be seen that the hydrophobicity of the modified CNC has been improved to a certain extent and can be used for hydrophobic drug delivery. Li Chuan et al. used the method of adsorbing cationic surfactants to load cetyltrimethylammonium bromide onto nanocellulose grafted with succinic anhydride to obtain a new nano-drug carrier (CTAB@NCSA) ; After characterization, it is found that the hydrophobicity of the modified nanocellulose has been improved, which can effectively combine water-insoluble luteolin (LUT) and luteolin (LUS), making it used as a drug carrier, and has excellent Load capacity and control release time performance. This carrier uses the intermolecular force and hydrophobic force to load drugs, which expands the application field of nanocellulose to a certain extent.

1.1.2 Modification of adsorbed quaternary ammonium saltThe method of adsorbing quaternary ammonium salt can be used to hydrophobically modify nanocellulose. The quaternary ammonium salt can be adsorbed on the surface of nanocellulose through the adsorption of positive and negative ions, and can be introduced by different Quaternary ammonium salt to adjust the mechanical properties of nano cellulose.

Yin et al. under alkaline conditions, the TEMPO/NaBr/NaClO oxidized nanocellulose crystals with a substance ratio of 4:1 were respectively combined with stearyl trimethyl ammonium chloride (1) and benzyl trimethyl chlorination. Ammonium (2) and diallyldimethylammonium chloride (3) are mixed and stirred (as shown in Figure 1). Then, after washing, centrifuging, freezing, and drying, modified nanocellulose crystals are obtained. The characterization results showed that the contact angle of CNC modified by stearyltrimethylammonium chloride increased from 35.8° to 54.6°, and the contact angle of CNC modified by diallyldimethylammonium chloride Increased from 35.8° to 48°, the contact angle of CNC modified by benzyltrimethylammonium chloride increased from 35.8° to 46°, and the hydrophobicity of nanocellulose was improved and uniformly dispersed in the polymer matrix Medium; At the same time, the breaking strength and breaking elongation of CNC after adsorption of quaternary ammonium salt are improved compared with CNC before modification. It can be seen that the hydrophobic modification of adsorbed quaternary ammonium salt is an effective way to improve the compatibility of CNC and organic matrix. method. Shimizu et al. prepared dry 2,2,6,6-tetramethylpiperidine-1-oxycellulose nanofibers (TOCN) and quaternary alkyl ammonium (QAs) into hydrophobic nanocellulose membranes by casting method After the modification, the contact angle of the nanocellulose membrane increased from 50° to 100°, which can convert the hydrophilic TOCN surface into hydrophobic simply and effectively.

Salajkovš et al. combined TEMPO-oxidized cellulose nanocrystals with aqueous solutions of quaternary ammonium salts (including stearyl trimethyl ammonium chloride, phenyl trimethyl ammonium chloride, glycidyl trimethyl ammonium chloride and diallyl Dimethyl ammonium chloride) was mixed, and then stirred, dialysis, dried, and dispersed to obtain a modified CNC suspension (as shown in Figure 2); the characterization found that the CNC contact angle after the modification of the quaternary ammonium salt solution was from 17° Increase to 71°, improve hydrophobic performance, and can be redispersed in organic solvents (such as toluene) with good dispersibility. Modified CNC can form well-dispersed nanocomposites with non-polar polymers.

Tahani et al. esterified and quaternized the reactive solubilizer (a statistical copolymer of 2-(dimethylamino)ethyl methacrylate and 2-hydroxy methacrylate) to make the surface of cellulose nanofibers The negative charge adsorbs the quaternary ammonium group in the reactive solubilizer and matches it electrostatically; in addition, the solubilizer "shell" has a methacrylate functional group, which can undergo free radical reactions during processing. Studies have found that the dispersibility of modified cellulose nanofibrils (CNF) in polycaprolactone has been improved, and the hydrophobicity of CNF has been improved after modification. At the same time, the mechanical properties of CNF are also improved to a certain extent due to the adsorption of reactive solubilizers.

Tahani et al. esterified and quaternized the reactive solubilizer (a statistical copolymer of 2-(dimethylamino)ethyl methacrylate and 2-hydroxy methacrylate) to make the surface of cellulose nanofibers The negative charge adsorbs the quaternary ammonium group in the reactive solubilizer and matches it electrostatically; in addition, the solubilizer "shell" has a methacrylate functional group, which can undergo free radical reactions during processing. Studies have found that the dispersibility of modified cellulose nanofibrils (CNF) in polycaprolactone has been improved, and the hydrophobicity of CNF has been improved after modification. At the same time, the mechanical properties of CNF are also improved to a certain extent due to the adsorption of reactive solubilizers.

1.1.3 Modification of adsorption diblock copolymer dispersant The modification of adsorption diblock copolymer dispersant refers to the formation of polymerization on the surface by designing a diblock copolymer structure with hydrophilic

1.2 Chemical modification

Hydrophobic modification of nanocellulose can be achieved by chemically modifying the surface of nanocellulose. The hydroxyl groups in nanocellulose have high reactivity, and the surface properties of nanocellulose can be controlled by chemical modification of hydroxyl groups. Through silylation modification, alkanoylation modification and esterification modification, hydrophobic groups can be introduced on the surface of nanocellulose, thereby effectively improving the hydrophobicity.

1.2.1 Silylation modificationSilylation modification is a commonly used method. Silane has a strong affinity with hydroxyl groups at room temperature and can interact with the hydroxyl groups of nanocellulose. The surface builds a stable -Si-O-C-type hydrophobic three-dimensional network structure. Monosilane has active chemical reaction characteristics, so that the modified nanocellulose has the characteristics of anti-oxidation, non-toxicity, and environmental protection. It is an ideal material for constructing hydrophobic materials; using different silane reagents, the performance of nanocellulose obtained is not The same, such as the use of trimethylchlorosilane (TMCS) modified nano cellulose has excellent mechanical properties. However, silylation modification still has disadvantages such as high cost of modification equipment, harsh reaction conditions, and relatively slow reaction.

Zhou Lijie and others used trimethylchlorosilane (TMCS) to hydrophobically modify the prepared polyvinyl alcohol/cellulose nanofibril (PVA/CNFs) composite aerogel, and then react with reduced graphene oxide (rGO) to obtain Hydrophobic rGO/PVA/CNFs composite aerogel; the results show that after TMCS hydrophobic modification treatment, the surface of the aerogel forms a hydrophobic layer structure, and the contact angle of the rGO/PVA/CNFs composite aerogel increases from 0° to 138°, the hydrophobic performance is significantly improved; in addition, the composite aerogel also has a porous structure, with an oil absorption rate of 78 g/g, which can be used to adsorb a large area of oil or organic solvents. Zhu Zhaodong et al. used chemical vapor deposition (CVD) to modify the spray-dried cellulose nanoparticles with methyltrimethoxysilane (MTMS), and formulated it as a superhydrophobic coating sprayed on qualitative filter paper to make superhydrophobic filter paper. After characterization, it is found that the contact angle of superhydrophobic filter paper is as high as 160°, the surface energy is reduced, and the thermal stability is improved. Compared with the superhydrophobic filter paper prepared by perfluorooctyltriethoxysilane (PFOTES) modified CNC, although the hydrophobic effect is basically the same, methyltrimethoxysilane (MTMS) is cheaper and has better applications effect.

1.2.2 Alkyl acylation modification Alkyl acylation is one of the chemical reactions used for the modification of nanocellulose. Alkyl acylation modification refers to the addition of acetic anhydride and After toluenesulfonyl chloride and other modifiers, the exposed hydroxyl groups on the surface of nanocellulose are converted to -COCH3, which makes the nanocellulose change from hydrophilic to hydrophobic. Alkylation modification can not only protect the central structure of cellulose, but also better control the degree of substitution. The dispersibility of alkanoyl nanocellulose in acetone and ethanol is stable and good, but there is a certain swelling phenomenon. Zhou Jing et al. used bleached bamboo pulp as raw materials, combined mechanical and chemical modification methods, and butyryl chloride as modification reagents to prepare modified cellulose nanofibrils (CNF); through the structure and morphology of CNF Characterization, it is found that the dispersibility of modified nanocellulose in weakly polar solvents has been significantly improved, which shows that its hydrophobic properties have also been improved, which provides a better preparation for hydrophobic polymer bio-based materials. means. Li et al. modified CNFs by attaching 10-undecenoyl chloride (as shown in Figure 4), and then suction filtered to obtain a hydrophobically modified cellulose nanofiber membrane, which improved the hydrophobicity of nanocellulose. Studies have shown that the modified CNFs film has better dispersibility, surface roughness and tensile strength have been significantly improved, and has good moisture resistance, which is very suitable for packaging materials.

1.2.3 Esterification modification Esterification modification is to modify the alkyl chain on the surface through light esterification of nanocellulose. The esterification reaction is a dehydration reaction, so it is usually not feasible in an aqueous medium, but it has been studied Under the action of the catalyst, the esterification modification is carried out in an aqueous medium. The esterified nanocellulose has higher elastic modulus and yield strength, good thermal stability, but poor tensile strength.

Jatin et al. used ultrasonic treatment to esterify the lactic acid in the aqueous medium with the hydroxyl groups on the surface of the cellulose nanofibers under high temperature and pressure (as shown in Figure 5) to obtain modified cellulose nanopaper with strong mechanical properties. By comparing with ordinary nano-paper, it is found that the elastic modulus, yield strength and thermal stability of modified nano-paper have been improved, and it has excellent storage performance under humid conditions.

Spinella et al. obtained hydrophobic CNC in one step through Fischer esterification and acid hydrolysis, and prepared LA-CNC functionalized with lactic acid and CNC by direct melt blending to improve the dispersion of CNC in the polymer matrix. Studies have shown that the contact angle of LA-CNC blends increases, the oxygen permeability decreases, and the storage modulus is significantly improved. The improvement of these properties expands the application range of the material. Liu Xing et al. acetic acid esterified and hydrophobically modified nanocellulose to obtain acetate nanocellulose (ANC), and then nanocomposites were prepared by compounding nanocellulose and acetate nanocellulose with polylactic acid (PLA). Comparing the two, it is found that the dispersibility of acetate-esterified nanocellulose in the polylactic acid matrix is better than that of nanocellulose, and acetate-esterified nanocellulose can increase the crystallinity and crystallization rate of polylactic acid; in addition, the obtained The hydrophobicity of the polylactic acid nanocomposite is significantly higher than that of the composite prepared by unmodified nanocellulose, and the contact angle is increased from 23° to 45°, but the mechanical properties of the polylactic acid nanocomposite are lower than that of the unmodified .

1.3 Polymer graft modification

Polymer grafting is the grafting of long-chain polymers or oligomers to the surface of nanocellulose through covalent bonds, which can not only make nanocellulose hydrophobically modified, but also make nanocellulose targeted Functional modification, and the thermal stability is well improved. Grafting polymer side chains on nanocellulose can not only improve its hydrophobicity, but also form a large number of physical entanglements at the polymer-nanocellulose phase interface, and has a certain degree of compatibility in the target polymer. Zhou et al. grafted polycaprolactone diol onto nanocellulose, and the hydrophobic composite material obtained maintained the size structure of nanocellulose, and at the same time, due to its good dispersibility in chloroform, it aggregates in water and floats on the water surface. It can be seen that its hydrophobic performance has been improved; at the same time, its thermal stability has also been significantly improved. Li et al. prepared flexible CNF aerogels by cross-linking CNF with glycidoxypropyltrimethoxysilane (GPTMS) and branched poly(ethyleneimine) (b-PEI). Subsequently, α-bromoisobutyryl bromide (BiBB) was used as an initiator to introduce it into the surface of the CNF aerogel modified by GPTMS and b-PEI (CNF-PEI) to make the BiBB modified CNF aerogel (CNF-Br ) Can be successfully polymerized and grafted with N,N-dimethylamino-2-methylmethacrylate polymer (PDMAEMA) to prepare nanocellulose aerogels with controllable surface wettability (as shown in Figure 6. Show). The surface of the grafted aerogel presents a certain degree of hydrophobicity, with a contact angle of up to 130°; and in the presence of CO2, the surface of the aerogel can change from the modified hydrophobicity to the original hydrophilicity, realizing aerosolization The reversibility of the wetting of the glue surface.

Table 1 summarizes and summarizes the hydrophobic modification methods of nanocellulose involved in this article and their hydrophobic effects after modification.

2 Application of hydrophobic nano cellulose
2.1 Packaging materials

With the massive consumption of energy and the intensification of environmental pollution, the "white pollution" caused by non-biodegradable plastics is causing widespread concern. In October 2018, the European Union pointed out that it would ban or restrict the use of certain single-use plastic products before 2021 to prevent the continuous spread of plastic pollution into the ocean. Scientists are conducting research on biodegradable packaging materials. Nanocellulose is a biodegradable material of natural origin. It has good film-forming properties. Its hydrophobic modification reduces the water vapor transmission rate and can be well applied toPackaging materials.

Li et al. grafted 2-(dimethylamino) ethyl methacrylate (DMAEMA) onto the CNC surface by atom transfer and free radical polymerization to prepare a covalently bonded CNC-g-PDMAEMA, and then added Alkyl bromides with different carbon chain lengths convert the tertiary amino group of CNC-g-PDMAEMA into a quaternary ammonium group, so that the cellulose nanocrystals are transformed from a hydrophilic material to a hydrophobic material. The contact angle of the material increases from 70° to 140°. It greatly improves the hydrophobicity while reducing the wettability of the material, and it also has a certain antibacterial property. The prepared CNC composite material has the potential to be used as aseptic packaging materials.

Farnoosh et al. studied the effect of adding nanocellulose and polymethyl methacrylate (PMMA) to the whey protein isolate/walnut oil film on the performance of the whey protein isolate/walnut oil film and found that PMMA was grafted to nanocellulose. Above, the surface of nanocellulose can be made hydrophobic. The hydrophobic nanocellulose and whey protein isolate/walnut oil film form a biocomposite membrane. After measuring its contact angle reaches 96°, the hydrophobic performance is improved; compared with unfilled membrane In comparison, the water vapor barrier properties of the composite membrane are improved by 64%. Whey protein isolate itself has good oxygen barrier and mechanical properties, and is degradable. On this basis, its water permeability is reduced, and better water vapor blocking performance can be used in the packaging field; at the same time, walnut oil film It has no effect on human health and can show unique advantages in food packaging.

In addition, Hu et al. immersed the CNF/HNTs-ZnO hybrid film in (1H, 1H, 2H, 2H-heptadecafluorodecyl) trichlorosilane and isopropanol, and used the cationic surface adsorption method to make it hydrophobic. Modified, the modified CNF/HNTs-ZnO hybrid film is obtained, with a contact angle of over 155°, showing excellent thermal stability and UV stability, and has broad application prospects in the field of packaging materials. Song et al. obtained a composite film by grafting the hydrophobic monomer n-butyl acrylate on nanocellulose filaments and incorporating biodegradable polylactic acid, which reduced the water vapor transmission rate and can be applied to degradable green packaging materials In the field.

2.2 Papermaking

The application of hydrophobic nanocellulose in paper-based functional materials is the direction of new materials research, and giving traditional paper new functions is also a hot spot that scientists have been working on.

Li Jing reacted the mixture of TEMPO-oxidized nanocellulose (TONC) and octadecylamine (ODA) at 50℃ for 4h under alkaline conditions to obtain modified TONC. By characterizing the paper prepared by modified TONC, it was found that the contact angle was significantly increased; at the same time, the thermal stability, tearing degree and sizing degree were improved. It can be seen that the performance of paper mixed with hydrophobic nanocellulose has been improved to a certain extent, and the application range is wider. Hu Xuejiao used 2-bromoethyl isobutyrate (EBIB) as initiator, copper wire as catalyst, and pentamethyldiethylenetriamine (PMDETA) as ligand to prepare modifier polymethyl acrylate, and then added activated nano Cellulose, reaction to obtain polymethyl acrylate grafted nano cellulose (CNC-g-PMA). Studies have shown that the thermal stability of nanocellulose after graft modification is improved, and it has better compatibility with hydrophobic systems, and the physical and chemical properties of the prepared paper are improved, which can expand the application of nanocellulose in the field of papermaking. The scope of application.

Compared with traditional paper, hydrophobically modified nanocellulose as an additive can promote various properties of paper. Traditional paper limits its application range due to its hydrophilicity, and the addition of hydrophobic nanocellulose to paper makes the paper have a certain degree of hydrophobicity, thereby expanding the application field of traditional paper to the hydrophobic direction, which can replace some non-degradable , Traditional hydrophobic materials that are harmful to the environment, such as plastics, greatly reduce environmental pollution.

2.3 Water purification

With the development of heavy industry, more than 1.4 million tons of petrochemical products are leaked into the global ocean every year. Therefore, the separation of water-oil/chemical pollutants is essential to the balance of the ecosystem. Hydrophobic nano-cellulose oil-absorbing materials have the advantages of cheap, biodegradable, high porosity and good oil-water selectivity, and are widely used in water purification research.

Xu et al. immersed the prepared reduced graphene oxide-coated nanocellulose (rGO/CNF) aerogel in a trimethylchlorosilane (TMCS) solution, and heated it in an oven for 2 hours to obtain TMCS/rGO/CNF. Aerogel. The study found that the contact angle of TMCS/rGO/CNF aerogel is larger than that of rGO/CNF aerogel, up to 117°, the porosity is improved, and the surface has high oil absorption capacity, which can treat a variety of oils in a short time. Both exhibited good adsorption capacity, the adsorption capacity reached 39 times its own mass, and can effectively remove oil from water. The aerogel material prepared by this method has little pollution, can be reused, does not cause environmental pollution, and has the potential to be developed as a universal, efficient and safe adsorbent.

Shang Qianqian et al. added methyltrimethoxysilane to the nanocellulose crystal suspension, stirred for 2h under acidic room temperature, and freeze-dried to obtain a superhydrophobic cellulose composite aerogel. The results show that the prepared aerogel has the properties of light weight and porosity; the contact angle can reach up to 157°, the hydrophobicity is significantly improved; at the same time, the thermal stability is also improved. Through a series of oil-water separation experiments, it is found that it can realize oil-water separation quickly and effectively, while also exhibiting excellent adsorption stability and high-efficiency circulation, so it has great potential application value in water-oil separation.

In addition, Jeddi et al. prepared hydrophobic spherical nanocellulose aerogels through simple freeze-drying, which showed excellent absorption efficiency for oil and organic solvents, and showed good performance in oil/water solutions. Selectivity, can effectively and continuously remove petroleum and chemical spills. Huang et al. used a two-step spray method to attach a strong superhydrophobic coating to the nanocellulose crystals. The water contact angle reached 163°. It has high mechanical strength and excellent self-cleaning properties, and can efficiently cyclically separate organic solvents and oils in water.
3 Conclusion

Information source: Biomass Chemical Engineering

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