Jinglei Xie,Shijie Liu

Department of Chemical Engineering, State University of New York—College of Environmental Science and Forestry,Syracuse,NY,13210,USA

Abstract: Nanocellulose is of great interest in various areas nowadays as a natural nanostructured biomaterial. However, in many applications, the high hydrophilicity due to a large number of hydroxyl groups is not desired. The hydrophobic modification of nanocellulose can thus increase its application.This work reviewed recent developments of methods for nanocellulose hydrophobic modification, through physical adsorption and chemical grafting.The applications of hydrophobic nanocellulose were also reviewed.

Keywords: nanocellulose; hydrophobic modification; physical adsorption;chemical grafting; applications

1 lntroduction

Nanocellulose, a product of the most abundant natural material, is widely used in different areas, including papermaking industry[1-4], food industry, emulsion and dispersion[5-6],and many others.Nanocellulose is nanostructured cellulose made from cellulose by using mechanical, chemical and/or biological method. According to the production process and characteristics, nanocellulose can be divided into three categories, namely, cellulose nanocrystals (CNCs), cellulose nanofibrils (CNFs) or nanofibrillated cellulose(NFC),and bacterial cellulose(BC). CNCs is taken from the crystalline region of cellulose by cutting the amorphous regions off through different methods. As a result, the crystallinity of CNCs is extremely high. By comparison,NFC does not have that high crystallinity since it keeps both crystalline and amorphous regions of cellulose and thus has a longer length compared with CNCs,which is in micro scale. With a width ranging from 5 to 20 nm, the aspect ratio of NFC is also higher than that of CNCs. BC, as its name indicates, is produced by bacterial such asAcetobacter,Agrobacterium,Rhizobium, andSarcina. All kinds of nanocellulose are sharing a high aspect ratio, a large specific surface area and thus a strong adsorption ability and relatively high reactivity and many other advantages,which are mainly contributing to their nanoscale structure and make them attractive for researchers.

To make a better use of this natural biomaterial, the surface modification of nanocellulose has received more and more attention in recent years[7]. In different modifications, the hydrophobic modification is extremely important for nanocellulose due to its relatively high hydrophilicity, which is not desired in some cases. For instance, in the production of nanocellulose-based composites, the hydrophilicity leads to a weak compatibility between nanocellulose and hydrophobic polymers. In addition, the formation of hydrogen bonding between high content of hydroxyl groups on the surface results in irreversible aggregation of nanocellulose and a low dispersion ability in aqueous solution[8-9]. As a result, hydrophobic modifications of nanocellulose have become a hotspot for researchers.

A large number of techniques have been used for the surface hydrophobicity modification of nanocellulose,including surface adsorption[10], esterification[11], etc.This work reviewed different surface modifications for the preparation of hydrophobic nanocellulose and its applications in recent years.

2 Nanocellulose surface modification

The high content of surface hydroxyl groups is the main reason for the surface hydrophilicity of nanocellulose. Therefore, to hydrophobically modify the nanocellulose, changing those hydrophilic groups into less hydrophilic ones or even hydrophobic groups or introducing hydrophobic groups directly could function well. To achieve this goal, various methods have been designed and applied in different areas using nanocellulose[12-15]. Those methods can be divided into two groups, including physical adsorption and chemical grafting.

2.1 Surfactant adsorption

Surfactants are molecules with one end hydrophilic and the other end hydrophobic, which are widely used to decrease the surface tension of the target solution. The adsorption of surfactants has been utilized for nanocellulose hydrophobic modification. Bindings are formed between the hydrophilic functional groups on one side of surfactants and hydroxyl groups of nanocellulose, leading to hydrophobic functional groups of the surfactants exposing to the outside of the surfactant-coated nanocellulose. Mariano et al[16]used three kinds of cationic surfactants (C12TAB, C14TAB,and C16TAB)to modify CNFs and achieved surfactantcovered CNFs. It was found that the modified CNFs tended to flocculate because of the hydrophobic interaction. Another reason for the flocculation is the decreased electric repulsing force between CNFs due to the neutralization by cationic surfactants. Similar effects of hydrophobic and electrostatic interactions resulting from the coating of surfactants onto CNC have been published earlier[17-18]. The aggregation or flocculation of modified nanocellulose was reported to occur when the concentration of the surfactants was high, in which case the hydrophobicity of the nanocellulose may be unexpectedly decreased[19]. As a result, hydrophobically modified nanocellulose by surfactants adsorption should be achieved under a low concentration of surfactants.Although cationic and nonionic surfactants are used in most situations for hydrophobic modification of nanocellulose[20], anionic surfactants have been utilized in some researches, in which anionic sulfate ester groups (—OSO3-) are always involved. The anionic sulfate ester groups may form an electric layer on nanocellulose surface, leading to the improvement of its dispersion[21].This is because the adsorbed negatively charged surfactants could increase the surface charge of nanocellulose, contrary to the effects of cationic surfactants. The increased charges improve the repulsing force between nanocellulose and thus improve the dispersion ability.This is also beneficial for the hydrophobicity improvement with more hydrophobic functional groups exposed instead of forming hydrophobic interactions.As a result, a certain amount of literatures have been published focusing on the adsorption of anionic surfactants onto nanocellulose[22-24]. The improvements of thermo stability and mechanical properties after modification using anionic surfactants have also been published[25].

2.2 Grafting

Grafting refers to the reaction of a large molecular chain binding to an appropriate branch chain or functional side-base by chemical bonds. Adding or introducing hydrophobic functional groups onto nanocellulose structure is the most direct and commonly used method for nanocellulose hydrophobic modification.

2.2.1 Esterification

The esterification reaction between nanocellulose structure and carboxylic acid, acid anhydrides or acyl chlorides has been used to introduce hydrophobic functional groups onto nanocellulose surface[26].

The mechanism of nanocellulose esterification by using lauroyl chloride can be seen in Fig.1[27].

As shown in Fig. 1, the hydroxyl group on CNCs was replaced by an ester group, introducing a long carbon chain with high hydrophobicity. Lauroyl chloride is a fatty acid and the reaction is a one-step process. Both a higher hydrophobicity and a better dispersion ability were achieved.

However, the relatively high cost of the chemicals and some toxic chemicals doing harm to the environment make it necessary to find new chemicals for nanocellulose esterification. Alkyl ketene dimer(AKD), one kind of neutral surface sizing agent in papermaking that can be achieved in large scale cheaply, has been used for the hydrophobic modification of NFC by grafting[28].The mechanism of the hydrophobic modification can be seen in Fig.2.

As can be seen in Fig.2,AKD was grafted onto NFC through the reaction between the hydroxyl group and lactone ring on AKD and aβ-keto ester linkage was formed,introducing highly hydrophobic alkyl chains.It has also been found that after modification, the contact angle of nanocellulose increased to a great extent, i.e.,the hydrophobicity was improved greatly. The grafting yield of the reaction can be calculated based on the following equation.

wherem0is AKD weight at the beginning,mrthe weight of AKD in the toluene at the end, andmcthe weight of nanocellulose.

The hydrophobicity of grafted NFC was improved successfully and the final grafting yield was calculated as 40%. The thermo stability was also enhanced after the grafting of AKD.

2.2.2 Graft-copolymerization

Monomers with double bonds are usually used for nanocellulose hydrophobic modification through graftcopolymerization.An example of long-chain fatty acids modified CNC is shown in Fig.3[29].

Fig.1 The reaction between CNCs and lauroyl chloride(adapted from Trinh et al [27])

Fig.2 Hydrophobic modification of NFC through AKD grafting(adapted from Yuan et al [28])

It can be seen in Fig. 3 that the hydroxyl group of nanocellulose was changed into an ester group with a long chain (R=—C11H23) on the other side highly hydrophobic. The contact angle, also as the characterization of the surface hydrophobicity,increased with the increase of the carbon chain length.This indicated that the hydrophobicity was increasing with the carbon chain growing in accordance with a previous study[30]. The grafted-nanocellulose could be both hydrophilic and lipophilic, resulting in the possibility of using it as a surfactant, which would be quite promising since the raw material and the production process are both environmentally friendly.

Fig.3 CNC modification by using fatty acid(adapted from Lu et al [29])

2.2.3 Coupling reaction

Coupling reactions are processes to obtain an organic molecule from two organic chemical units under the help of a metal catalyst. The coupling reaction is a special kind of grafting for nanocellulose hydrophobic modification, in which the hydrophobic chemical is grafted onto the nanocellulose through a linker known as coupling agent. The coupling agent has both hydrophobic and hydrophilic functional groups,leading to the possibility of binding nanocellulose and a hydrophobic chemical together. The coupling agent includes chrome coupling agent, silane coupling agent,titanium ester coupling agent, and other kinds of coupling agents. The silane coupling agent and titanium ester coupling agent are most commonly used for nanocellulose hydrophobic modification, among which 3-aminopropyltriethoxysilane, 3-glycidoxypropyltrimethoxysilane,and titanate are especially popular.The coupling modification using silane coupling agent is also known as the silanation method,commonly used for CNC hydrophobic modification[31-33]. The formation of the silanol group is the key of the silanation method, which is resulting from the hydrolysis of alkoxy groups. The silanol groups will form a polysiloxane chain through self-condensation and adsorb onto nanocellulose by hydrogen bonding.In some cases, with the silanol groups of the silane modifier and —C—OH groups on nanocellulose surface dehydrating, the Si—O—C bonds may be formed between the two[33]. However, the Si—O—C bond is not so stable as the C—O—C or Si—O—Si bond and may transfer into the other two.An example could be seen in a CNC modification by using isocyanatepropyltriethoxysilane (IPTS) as silane agent(shown in Fig.4)[34].

Fig.4 The chemical reaction between CNC and IPTS(adapted from De Oliveira Taipina et al [34])

It can be seen in Fig. 4 that the Si—O—Si structure was formed between two IPTS molecules due to the instability of Si—O—C bonds.

To solve the problem and achieve a better bonding between silane agent and CNC, Lin et al[35]designed a new method by grafting dodecyltrimethoxysilane(DTMOS)onto CNC using KH560(γ-(2,3-epoxyproxy)propytrimethoxysilane) as the coupling agent with a two-step process. Compared with C—O—Si bonds,more stable bonds were formed, including C—O—C bonds between KH560 and CNC and Si—O—Si bonds between DTMOS and KH560 modified CNC. A more stable grafting of a long carbon chain(—C12H25groups)with a very high hydrophobicity was thus obtained.

Different from the mechanism of silane modification, the coupling reaction using the titanate is mainly based on the alcoholysis, surface chelation or coordination exchange. Lica 38, one kind of titanium ester coupling agent, has been utilized for the hydrophobic modification of cellulose microfibrils and the modified cellulose microfibrils were reported to be the most hydrophobic one compared with that modified by other kinds of coupling agents[36].

3 Applications of hydrophobic nanocellulose

With all the advantages of nanocellulose (high aspect ratio, high crystallinity, high elastic modulus, etc.) and high hydrophobicity, hydrophobic nanocellulose is widely used in many different areas, including nanocellulose-based composites[37], films[38], emulsion stabilizers[39], foam lamella stabilizers[40], and many others.

3.1 Hydrophobic nanocellulose based composites

With a high strength and stiffness even compared with spider silk[41], nanocellulose has been applied together with polymers to produce composites for the purpose of increasing the strength properties, such as starchbased matrixes, rubber latex, thermosetting resins, etc.However,nanocellulose is so hydrophilic that it usually appears to be not compatible with hydrophobic polymers during the production of composites.Hydrophobic nanocellulose can solve such problems and becomes popular reasonably.

The hydrophobically modified nanocellulose has been used to produce one kind of biodegradable composite together with a biodegradable polylactic(PLA). The hydrophobic modification was accomplished by a free radical polymerization with hydrophobic monomers grafted onto nanocellulose.PLA pellets were dissolved into tetrahydroguran in a concentration of 10% before being mixed with the modified nanocellulose. A stirring of 2 h was conducted before an overnight evaporation of the solvent to get the composite film prepared. The film used for handsheet surface coating was then characterized by the water vapor transmission rate(WVTR) of the coated paper with the lowest value of 34 g/(m2·d).

3.2 Films

Although nanocellulose has been utilized for the preparation of films and membranes for their renewability[42-43], the restriction brought by the hydrophilicity has led to an increasing popularity of hydrophobic nanocellulose in this area[44-47]. The condensation of different kinds of silanes[48-50]has been utilized for nanocellulose hydrophobic modification before the production of films.

Le et al[38]produced a hydrophobic nanocellulosesilica film by using hydrophobically modified nanocellulose through grafting hydrophobic nanoparticles (dodecyl-terminated silica). The high hydrophobicity of the nanocellulose also provides the final film a great hydrophobicity, which could be widely used in many areas, such as packaging and oilwater separation.

3.3 Emulsions stabilizers

An emulsion is a mixture with two or more kinds of liquids insoluble with each other. The stability of an emulsion is defined as the ability to keep the properties of the system unchanged with time[51-52]. Emulsions are widely applied in different areas and the stability of emulsions is of great importance. The instability of emulsions can be divided into four categories,including flocculation, creaming or sedimentation,coalescence, and Ostwald ripening. Surfactants have been used for the stabilization of emulsions, which,however, are highly costing and even toxic[53-54].Different solid particles have been used to replace the surfactants and the stabilized emulsions in that way are known as Pickering emulsions[55-56]. Although nanocellulose has been utilized as a stabilizer for Pickering emulsions[57-61], the low contact angle of nanocellulose to the interface makes it easy to be wetted by the droplets or the other kind of liquid especially water[62]instead of surrounding the droplets and separating them apart. By comparison, a more stable separation could be formed by hydrophobic nanocellulose surrounding the emulsion droplet,which has also been applied in the stabilization of emulsions[63]. The electrostatic force between nanocellulose will also prevent coalescence of droplets.For instance, vinyl acetate esterified CNC has been proved to be able to stabilize three kinds of emulsions composed of one organic liquid (ethyl acetate, toluene,and cyclohexane)and water[64].

Tang et al[39]used esterified CNFs with cinnamoyl chloride and butyryl chloride as a stabilizer for Pickering emulsions and studied the effects of different characteristics of modified CNFs on the stabilization,including size, hydrophobicity, and concentration. It was proved that hydrophobic CNFs of different sizes can all stabilize the emulsions.With the increase of the concentration of modified CNFs, the size of droplets was decreasing and the creaming rate was also proved to be decreasing, in other words, the emulsions were more stable.

4 Conclusions

With high hydrophilicity, nanocellulose has restricted applications especially concerning about the incompatibility of nanocellulose with hydrophobic polymers. Different kinds of modifications can be used to improve the hydrophobicity of nanocellulose,including physical adsorption using surfactants and chemical grafting (esterification, graftcopolymerization,and the coupling reaction).

The modified nanocellulose possesses high hydrophobicity, making its wide use in nanocellulosebased composites, films, emulsions stabilization and many other areas, possible. As a renewable and sustainable natural nano-scale material, hydrophobic nanocellulose is expected to become increasingly popular in the future.