Hydrophobization of lignocellulosic materials part II: chemical modification

The most common way to characterize the extent of the modification of lignocellulosic materials is the degree of substitution (DS). The DS represents the number of reacted hydroxyl groups per glucose residue and has a maximum value of 3.

Alkyl ketene dimers (AKDs) are oxetan-2-one derivatives, typically made by combining fatty acid and fatty acid chlorine derivatives (Fig. 2). These molecules have been industrially applied to hydrophobize (or size) paper and cardboard since the 1950s (Downey 1953). AKD reacts with (hemi)cellulose, attaching a hydrophobic ester to the hydroxyl sites. AKD is typically prepared as an emulsion and subsequently added to the sheet-forming pulp before sheet formation. The AKD method has been reviewed by Lindström and Larsson (2008), highlighting the high number of variables and complexities that determine if AKD sizing is successful or not. Important factors that influence the success of AKD hydrophobization are listed below:

The five given factors are codependent. Typically, good retention is achieved by preparing a cationic AKD emulsion, which spreads and distributes well on the anionic cellulose surface. Shorter R-chains (R in Fig. 1) typically improve retention and distribution of AKD, due to their smaller size and less hydrophobic character. On the other hand, long fatty acid chains tend to increase the contact angle of the hydrophobized material. However, increased R length makes the AKD more viscous and hinders its diffusion within the cellulosic material.

Innovation in AKD sizing is a continuous process (Cunha and Gandini 2010). Recently, novel methods of impregnating materials with AKD using supercritical CO₂ have been used (Adenekan and Hutton-Prager 2019). The CO₂ pressure was found to be important at the beginning of impregnation. After 40 days, all contact angle values stabilized at 130°–140° compared to ~ 40° for the untreated sample. Russler et al. (2012) showed it was possible to achieve high AKD loading (> 100%). Interestingly, these films displayed enhanced moisture sorption, indicating that AKD agglomeration and its interaction with cellulose may induce morphological changes facilitating water sorption.

Even though a lot is known about this technique, it seems that the obtained contact angles are highly variable, from about 100° (Li and Neivandt 2019) to 140° (Adenekan and Hutton-Prager 2019). Moreover, it seems to be unclear whether AKD binds cellulose by hydrogen bonding or covalently (Lindström and Larsson 2008; Adenekan and Hutton-Prager 2019; Li and Neivandt 2019). The importance of the type of binding is difficult to discern, as materials where the binding is believed to be electrostatic seemingly have a long lifetime and high contact angle (e. g. as in Adenekan and Hutton-Prager (2019)). The migration of AKD within the material over time indicates that the formation of covalent cellulose-AKD bonds is a slow process (Adenekan and Hutton-Prager 2019). Possibly, this varies depending on the impregnation method. For example, Li and Neivandt (2019) also observed a time-dependent increase in the contact angle. This evolution was temperature-dependent and was accelerated by higher temperature curing. They observed AKD agglomerates and hypothesized that at temperatures slightly above 50 °C (AKD melting temperature), AKD molecules are free to move, and distributed evenly across the cellulose surface. They found that heating below this temperature-induced the slowest evolution of the contact angle, reaching its maximum after 600 min. For curing temperatures above 50 °C, this time was shorter than 60 min. Comparing this with Adenekan and Hutton-Prager (2019), where samples were impregnated and then stored at ambient conditions, it is reasonable that the contact angle would settle after 140 days.

An alternative to AKD is alkenyl succinic anhydride (ASA), which may also be used as a paper sizing agent (Lackinger et al. 2016). Sizing by ASA is considered superior to AKD by some, as the kinetics are faster (Lackinger et al. 2016). Though faster, ASA is seemingly a somewhat poorer hydrophobing agent (CAs < 90°) compared to AKD (CAs < 100°) when compared experimentally on silica surfaces (Lindfors et al. 2007). The reason may be that ASA is easily hydrolyzed by water compared to AKD (Lindfors et al. 2007; Ashish et al. 2019; Arancibia et al. 2021). The water stability of ASA has recently been investigated by Arancibia et al. (2021), improving it by adding CNF to the ASA suspension. Another possibility is preparing ASA and polyacrylamide emulsions, yielding contact angles above 100° for about 200 min in hand sheets prepared from cellulosic pulp (Ashish et al. 2019). An overview of the modifications by AKD and ASA is shown in Table 2.

Esterification is one of the most used methods for cellulose hydrophobization via chemical modification. Esterification reactions occur between the hydroxyl groups of the cellulosic materials and either a carboxylic acid, an acid anhydride or an acyl chloride, introducing an ester group to the cellulose molecule (Fig. 3) (Missoum et al. 2013; Wang et al. 2018).

An important subgroup in the esterification reactions is acetylation reactions, which are used to introduce acetyl functional groups. The acetylation of cellulosic materials (fibers, nanocelluloses) is typically performed by the reaction between the material and a dry mixture of acetic acid and acetic anhydride in presence of an acid catalyst (Habibi 2014). The substrates are either dry cellulosic materials or solvent-exchanged cellulose suspensions. Acetylation reactions can occur in homogeneous and heterogeneous media, as illustrated in Fig. 4. Homogeneous acetylation is performed in the acetylation medium, namely acetic acid or an acetic anhydride/acetic acid mixture, without the need of adding a diluent. As the reaction progresses, the surface cellulose molecules become soluble in the reaction medium as soon as they are sufficiently acetylated. This peeling-off effect leads to changes in the morphology of the (nano)fibers and allows the reaction to progress towards the core of the substrate until the (nano)fibers are completely acetylated and this results in a full dissolution of the substrate. On the other hand, reactions occurring in heterogeneous media, in the so-called “fibrous process”, require the addition of a non-swelling diluent such as toluene, benzene or amyl acetate to the acetylation medium. In this case, the presence of the non-swelling agent allows the modified molecules to remain insoluble in the reaction medium and only the surface of the substrate is modified without affecting its morphology. Acetylation weakens the interchain hydrogen bonds, facilitating the desorption of the modified molecules (Tripathi et al. 2018). The resulting cellulose acetates are recovered by precipitation, usually in water or in aqueous acetic acid followed by washing with water (Sassi and Chanzy 1995; Habibi 2014; Wang et al. 2018). The DS of cellulose acetates has a strong impact on the physical properties of the polymer. Cellulose monoacetates are soluble in water, while diacetates are soluble in acetone and triacetates are insoluble in both acetone and water, but soluble in dichloromethane (Edgar 2004).

Etherification reactions create ether bonds at the hydroxyl groups of cellulosic materials and, when performed in heterogeneous conditions, require the activation of the hydroxyl groups with a base, which weakens the hydrogen bonds between the polymer chains and makes them more accessible when they are in suspension (Fig. 7). The etherifying agents are alkyl/silyl chlorides or bromides, oxiranes, or vinyl compounds (Heinze et al. 2018). Etherification reactions in general are not specific to a certain hydroxyl group (Habibi 2014; Kargarzadeh et al. 2018). Like esterification, etherification can be limited to the surface. An advantage of etherification over esterification is that it is a more regioselective reaction for the substitution of C6 (Fox et al. 2011).

Silylation is the reaction between nucleophilic groups such as alcohols, carboxylic acids and amines, with silyl groups (R₃Si-). Silylation is used to graft molecules to cellulose and its derivatives via covalent bonds (Missoum et al. 2013; Kargarzadeh et al. 2018). The reaction requires the presence of water and involves three steps: (1) hydrolysis of the silane molecules to the corresponding silanols (R₃Si-OH) in the presence of water, (2) adsorption of the silanols onto cellulose surfaces by hydrogen bonding, and (3) self-condensation of the silanols into a siloxane network and condensation of the siloxanes with cellulose by thermal treatment. The grafted silanes can form various structures, depending on the experimental conditions and the number of hydrolyzable substituents in the molecule. Common hydrolyzable groups in silane molecules are -Cl, -OR, -NMe₂ (Fadeev and McCarthy 2000; Andresen et al. 2007). Figure 8 shows three possible architectures of the grafted silanes, which can polymerize in the horizontal direction (self-assembly), covalently react with one or more hydroxyl groups (covalent attachment), or polymerize in the vertical direction (vertical polymerization) (Cunha and Gandini 2010). Monofunctional silanes (with only one hydrolyzable group) can only be grafted to the surface through covalent attachment, while difunctional silanes (two hydrolysable groups) can be grafted by both covalent attachment and vertical polymerization. Lastly, trifunctional silanes (three hydrolyzable groups) can be grafted by the three modalities shown in Fig. 8. The polymerization of trifunctional silanes into 3-D siloxanes is the most common process when there is sufficient water in the system (Fadeev and McCarthy 2000).

Superhydrophobicity is achieved when the water contact angle on a surface is > 150°, and is often the result of lowering the surface energy of the substrate and increasing its roughness (Samyn 2013). The most well-known superhydrophobic surfaces found in nature are lotus leaves, which are self-cleaning and have antifouling properties. Efforts have been made to mimic the surface structure of these leaves, which have nano and microstructures that can form air pockets, resulting in superhydrophobic properties. So far, the most common approach to obtaining superhydrophobicity on cellulosic surfaces consists of the combination of two factors: first, the deposition of particles to increase the surface roughness; and second, the coating of the substrate with a hydrophobic compound. In this section, we summarize some of the existing methods used to obtain superhydrophobic cellulosic surfaces that follow this principle.

Cellulose fibers from filter paper were first coated with five layers of titanium oxide, and then a monolayer of octyltrimethoxysilane (OTMS) (Fig. 14) (Sujing et al. 2010). Before modification, the filter paper absorbed water droplets, but after treatment, the contact angle increased to 154.8°. The super-hydrophobic surfaces displayed very good stability since the contact angles remained unchanged even after storage in air for 18 months. The contact angles did not vary in the pH range from 3 to 11 and remained above 148° after 48 h in these solutions. Paper samples coated with 5, 10 and 20 layers of titania and a monolayer of silanes of varying chain length presented contact angles > 150°. The highest contact angle (160.5°) was obtained for paper coated with 10 layers of titania and a monolayer of OTMS.

Substrates including lignocellulosic materials such as wood, bamboo, cotton and paper were also coated with titanium oxide nanoparticles with a diameter of 25 nm, and then vinyltriethoxysilane (Fig. 15) (Wu et al. 2016). All surfaces presented contact angles above 150° for water, coffee, milk, red wine, cola, and soya sauce. The same procedure was followed to coat paper with silica nanoparticles with diameters between 15 and 100 nm, yielding contact angles above 150°. Further experiments on the effect of the size of titanium oxide particles were performed using particles with diameters between 25 and 300 nm, and showed that superhydrophobic surfaces were obtained regardless of the particle size, although the water contact angles decreased slightly with increasing particle size. The properties of all substrates remained unchanged after storage at 90% RH for 96 h.

Another technique to prepare amphiphobic surfaces consists of etching the surface of cellulose fibers in an alkaline solution, followed by deposition of five titanium oxide layers and monolayers of PFOTMS on top (Jin et al. 2012). The water and hexane contact angles of the modified surface were 158° and 146.5°, respectively. Moreover, such surfaces presented low bacterial adhesion. Zhang et al. (2017) cross-linked CNF microspheres deposited on filter paper, followed by coating with MTMS. The diameter of the microspheres was found to be 1–10 µm. The water contact angle of the coated paper reached 162°, and the samples maintained their superhydrophobic properties while immersed in oil, allowing them to be used for oil filtration from oil/water mixtures. An overview of the methods based on particle deposition and silane coating is given in Table 4.

Carbamylation of cellulose is performed by the reaction of an isocyanate with the hydroxyl groups of cellulose, resulting in the formation of a urethane moiety, as shown in Fig. 16. This type of reaction can be used to graft other types of molecules using isocyanate groups as linkers.

Amidation of cellulosic materials usually consists of a two-step process, where the first one introduces carboxylic groups to the cellulose molecules (via esterification, TEMPO oxidation, periodate oxidation, or other methods), and the second one yields an amide bond from the reaction between these carboxylic groups and amines (Fig. 18). In the cases where TEMPO oxidation is used as a pre-treatment, the modification can only occur at the oxidized OH groups, in this case, the ones in C6. On the other hand, periodate oxidation results in the cleavage of the bond between C2 and C3, forming a dialdehyde that can form other structures (Nypelö et al. 2021).

A “one-pot” amidation reaction of TEMPO-oxidized cellulose nanofibrils (TOCNFs) was reported by Gómez et al. (2017). The reaction requires the presence of an acylating agent, in this case a uranium salt, which reacts with anionic TOCNFs, forming an active ester, which further reacts with primary amines (dodecylamine and octadecylamine). Contact angle measurements confirmed the increase in hydrophobicity after amidation. The substrates modified with dodecylamine displayed a contact angle of 61°, and the ones modified with octadecylamine of 67°, which are significantly higher than the contact angle of the unmodified TOCNFs (17°). In addition to increased hydrophobicity, the modified films also displayed improved thermal properties. Esterification and amidation of TEMPO-oxidized cellulose nanocrystals (TOCNCs) were performed using aliphatic acid chlorides and amines with chain lengths C3–C18 (Bendahou et al. 2015). The DS after esterification (0.36–0.45) was higher than after amidation (0.05–0.26). The contact angles increased with the hydrocarbon chain length and had values of 92°–108° for the esterified CNCs, and 84°–94° for the amidated ones, as shown in Table 6.

Click chemistry is based on a series of orthogonal reactions that are highly selective, result in high yields and are performed in mild conditions. Typically, these reactions do not yield byproducts, or they are easily removable. Click reactions include copper(I) catalyzed azide-alkyne cycloadditions, nucleophilic ring-opening reactions, and thiol-ene reactions, among others (Hoyle and Bowman 2010).

Metal-catalyzed azide-alkyne cycloadditions are the most common click chemistry reactions. The reaction takes place between terminal alkynes and azides, and Cu (I) is often used as a catalyst, although other metals (Ru, Ag, Au, Ir, Ni, Zn, Ln) can also be used (Fig. 19). The products of these reactions are 1,4-disubstituted 1,2,3-triazoles. The advantages of the copper-catalyzed reaction compared with the non-catalyzed reaction are that it is regioselective, and can be performed under milder conditions and in water or alcoholic solvents. Disubstituted alkynes cannot be used in copper-catalyzed reactions because they require deprotonation of the terminal alkyne, but it is possible to perform azide-alkyne cycloadditions with disubstituted alkynes using ruthenium (Wang et al. 2016).

A generic method for the modification of regenerated cellulose, filter paper and CNFs was developed based on azide-alkyne cycloaddition click reactions (Filpponen et al. 2012). Carboxymehtyl cellulose was functionalized with either azide or alkyne groups, and then it was adsorbed onto the different cellulosic substrates, resulting in the functionalization of these substrates, as illustrated in Fig. 20. The following step consisted of the click reaction between the cellulosic films and molecules with the complementary click functionalities. To demonstrate the broad applicability of this synthetic strategy, the click reaction was performed using very diverse molecules, such as a protein (bovine serum albumin), a fluorescent probe (dansyl), and a synthetic polymer (polyethylene glycol). Although hydrophobization of cellulose is not discussed in the article, this method provides the possibility of hydrophobic modification of the substrates.

Similarly, Hettegger et al. (2015, 2016) reported a method for modification of never-dried cellulosic substrates by azide-alkyne click reactions. The substrates (BNC sheets, pulp, MFC, viscose fibers and TENCEL gel) were modified with (3-azidopropyl)triethoxysilane in aqueous media and at mild conditions. As a proof of concept, the azido-modified substrates were successfully reacted with an alkyne-terminated fluorescent dye through a copper-catalyzed alkyl-azide cycloaddition reaction. As demonstrated by the authors of this work, this is a versatile and general approach that applies to many different types of substrates and introduces azide functionalities that can easily react via click reactions with a vast number of chemical compounds.

Fatona et al. (2018) used triazine chemistry to modify the surface of CNCs, which can be used as a platform for further click-chemistry modifications. The authors modified the cyanuric chloride linker with a molecule of choice and then grafted the resulting compound onto the surface of CNCs. The substituents octadecylamine, propargylamine and benzylamine increased the hydrophobicity of the CNCs, which formed stable suspensions in chloroform. The modification did not affect the crystallinity of the CNCs. Propargylamine substituents could also be used to further graft other molecules (e.g. fluorescent dyes) by azide-alkyne click reactions. Octadecylamine was also grafted from filter paper by triazine chemistry. The reaction time increased the contact angle of the modified paper, which reached its maximum (125°) after 3 h.

Click reactions between thiols and enes can occur via a radical reaction (thiol-ene reactions) or via the formation of anionic species (thiol-Michael addition) and yield a thioether (Fig. 21). Thiol-ene reactions are usually photoinitiated. The initiator generates a thyil radical that adds to the alkene generating a carbon centred radical, which in turn abstracts a hydrogen from another thiol. On the other hand, thiol-Michael additions are base/nucleophile-catalyzed reactions that occur by nucleophilic attack of an electron-deficient C=C bond by a thiolate. The product of this reaction is an anion, which abstracts a hydrogen from another thiol or a base, forming the resulting thiolate. Both thiol-ene and thiol-Michael reactions are tolerant to the presence of air and oxygen and can be performed in various solvents, including water (Lowe 2010; Fairbanks et al. 2017).

Thiol-ene click chemistry combined with silylation was used as another approach to modify MFC films (Tingaut et al. 2011). First, an MFC suspension was modified with either vinyltrimethoxysilane or 3-mercaptopropyltrimethoxysilane to introduce alkene and thiol functionalities, respectively. Then, films of these two qualities of MFCs were formed and further modified via a thiol-ene reaction with methylthioglycolate and allylbuyrate, respectively. A different approach consisted in clicking vinyltrimethoxysilane with methylthioglycolate, and then grafting the resulting compound onto MFC films (Fig. 22). The first and third modification pathways presented faster reaction kinetics and were more controllable than the second one. Despite not being reported in the article, these modifications introduce alkyl chains to the film surfaces, increasing the hydrophobicity of the substrates.

CNC aerogels were also modified by a thiol-ene reaction. The aerogels were first functionalized with methacrylate groups via an esterification reaction (Aalbers et al. 2019). The product was further reacted with thiols of varying hydrophobicity: 3-mercaptopropionic acid, 6-sulfonylhexan-1-ol, 1-dodecanethiol, 1H,1H,2H,2H-perfluorodecanethiol, and cysteamine. The modified aerogels were used for xylene absorption, where the most hydrophobic aerogels displayed a better performance.

Diels–Alder cycloaddition and the thiol-Michael reaction were used as a method to graft two different fluorescent dyes from CNFs based on click chemistry (Navarro et al. 2015). The first step of the modification involved modifying the CNFs with 2-furoyl chloride. Then fluorescein diacetate 5-maleimide was grafted via a Diels–Alder cycloaddition, while an additional step (a Diels–Alder cycloaddition between furoate-CNF and a bismaleimide derivative) was required before grafting 7-mercapto-4-methylcoumarin via the thiol-Michael reaction. By combining both types of click reactions, it was possible to obtain CNFs grafted with both fluorescein and coumarin resulting in multicolor labelled CNFs. This modification approach opens the possibility of simultaneously grafting different substituents based on two different click chemistry reactions.

Similarly to carbamylation, click reactions were also used to graft polymers from cellulosic substrates as a method to increase their hydrophobicity. PCL was grafted from cellulose fibers (Krouit et al. 2008), CNCs (Zhou et al. 2018), and CNFs (Pahimanolis et al. 2011) by azide-alkyne cycloaddition reactions, and from filter paper by a thiol-ene reaction (Zhao et al. 2010). Polymer grafting is outside of the scope of this review, and therefore the topic is not discussed in detail here. A section on this topic is included in Hydrophobization of lignocellulosic materials part III: modification with polymers (Rodríguez Fabià et al. 2022b). Table 7 summarizes the examples reported in this review based on click-chemistry reactions.

When modifying lignocellulosic substrates, the extent of the modification is a key parameter because in addition to, for example, increasing the hydrophobicity of the substrate, it can also modify its physical properties. The issue of the extent of the modification was also raised by Eyley and Thielemans (2014) in their review on surface modification of cellulose nanocrystals. The authors question the structural integrity of the cellulose nanocrystals (CNCs) when a surface DS larger than 1 is obtained. The authors also question the suitability of bulk techniques, such as elemental analysis, that are commonly used to estimate the surface modification of CNCs, and conclude that the crystallinity of the substrates should be analyzed before and after the modification to assess the extent of the reaction.

Although often in the literature it is not specified, it is important to differentiate between surface and bulk modification. Surface modifications are of great interest because they have minimal effects on the properties of the (nano)fibers and nanocrystals, allowing to maintain the structural and mechanical properties of these materials, as well as their molar mass (Tardy et al. 2021). Although cellulose has three hydroxyl groups available for modification, steric effects and intrachain hydrogen bonding hinder the access to the C3-OH. Therefore, surface modifications are mostly limited to the C6-OH and the C2-OH. Recent publications (Beaumont et al. 2021a, 2021b) reported highly C6-OH regioselective acetylation reactions that limit the modification to the surface, and thus ensure the preservation of the mechanical properties of the substrates.

Based on the cited studies, utilizing nanocellulose, microfibrils, fiber or film/paper as a substrate, seemingly has no pronounced effect on the obtained DS or contact angle. The same seems to be true in terms of the crystallinity of the substrates (Table 1).

The presence of lignin and hemicellulose is poorly investigated with respect to their effect on the chemical modification of lignocellulosic substrates. A very instructive study by Castellano et al. (2004), showed that siloxanes require humidity in order to graft onto cellulose. This was due to the partial hydrolysis of siloxanes occurring only under humid conditions. However, dry modification of siloxane on lignin was feasible. This was related to the increased reactivity of the phenolic OH group in lignin as opposed to the aliphatic OH groups in cellulose. Concerning hemicellulose, the same modification routes applicable to cellulose typically also work for hemicellulose (Fang et al. 2000; Li and Pan 2018). Several hemicellulose modification methods simply aim at modifying hydroxyl moieties as they do with cellulose. However, hemicelluloses may be partially methylated/carboxylated (Ebringerová 2005). One might then envision that other modification routes are possible.

It is difficult to compare the results presented in this review since the substrates and the experimental methods are so diverse. However, some general trends can be observed, especially in the cases where the authors tested several modifying agents. For instance, adsorption often yields the lowest contact angles (see: Rodríguez-Fabià et al.  2022a),  followed by amidation. Both etherification, esterification and carbamylation yield high contact angles, as well as silylation and particle deposition in combination with silylation. Except for surfactant adsorption, all methods are capable of yielding CAs > 90°. Superhydrophobicity (CAs > 150°) may be obtained by silylation (Table 2-SI), particle deposition + silylation (Table 4), wax adsorption, or plasma etching (see: Rodríguez-Fabià et al. 2022a). These methods often use fluorinated chemicals. However, all other referred methods except for surfactant adsorption and amidation yielded contact angle values close to the 150° threshold.

Covalent modification methods are characterized by the degree of substitution. There was no direct correlation between DS and obtained contact angle, as this also depends on the substituent type and substrate, which varied across the studies. A general observation is that for the same substrate and modification route, the contact angle usually increases with increasing chain length of the modifying agent. Esterification and silylation are the most common modification techniques for hydrophobization of cellulosic materials. These two techniques have been used both for solution and gas-phase modification, and more recently water-based reactions.

One of the most effective methods to obtain (super)hydrophobic materials is the use of fluorinated chemicals often in combination with nanoparticles. Despite providing the substrates with high hydrophobicity, fluorinated chemicals and other hydrophobing agents pose a threat to the health and the environment and are not suitable for use in food packaging and other applications. In the case of nanoparticles, the health and environmental effects of most of these materials are still unknown. Thus, hydrophobized materials necessarily need to be rinsed before use. Furthermore, chemical leaching may occur during use. It is thus extremely important to evaluate this thoroughly. In our opinion, specific material requirements and material testing is an immature field that needs to be further developed. The complexities of this process are described by Neltner et al. (2011). Therefore, it is paramount that environmentally friendly methods for hydrophobization of celluloses are developed.

The term waterproof is a difficult one, as water may be vapour or liquid and have various pressures and temperatures. Seemingly, one aim is to increase the use of lignocellulosic materials in food packaging materials and reduce the amount of plastic in e. g. the carton laminate composite for packaging of liquids. From a material point of view, a material becomes waterproof when:

In this paper, the references utilize different methods to demonstrate water repellency. However, by “waterproof” one typically refers to not allowing the passage of liquid water upon filtration. Typically, cellulosic materials are not waterproof, but may become so by incorporating suitable nanoparticles (Farooq et al. 2019). As of yet, hydrophobized cellulose has only limited waterproof characteristics such as high contact angle and low water vapor transmission rate (WVTR). A recent study (Tedeschi et al. 2021) claims “waterproof” cellulose reaches similar contact angles and WVTRs as nylons and Teflon but does not discuss the material interactions with liquid water.

The general focus on hydrophobization in the literature is to achieve high contact angles or improved water repellency using methods ranging from “simple” (e. g. silylation) to more advanced (e. g. advanced click chemistry). Generally, the aim is to increase contact angles of the lignocellulosic substrates from about 30° to above 90°, which is the usual contact angles for most plastics, e.g. 102° for polyethylene (De Geyter et al. 2008), and 97° for polypropylene (Vlaeva et al. 2012); although some plastics such as polylactic acid may have contact angles slightly below 90° (Guo et al. 2015).

Though we do not criticize these efforts, we believe that the focus should be shifted towards maintaining an adequate water repellency over longer times as opposed to simply achieving high contact angles. From the referenced studies it should be clear that cellulosic materials are as of now still unsatisfactory as liquid water barrier materials.

The 12 principles of Green Chemistry (Anastas and Warner 1998) may be used as a guideline to evaluate the hydrophobization methods. In terms of prevention of waste, the E-factor of most reactions, a parameter that relates the weight of produced waste to the weight of the desired product, is typically above 500, meaning that 500 kg of waste is generated per kg product. For multi-step reactions, this number is estimated to be more than 1000 (Sheldon 2007). The atom economy becomes poorer when halogens are a by-product of substitution (Lee et al. 2020), or when pH adjustment is required (Bendahou et al. 2015). It is here worth noting that a large substituent (OTMS at 374.7 g/mol vs TCMS at 149,5 g/mol) also improves the atom economy. The use of catalysts is also important. Catalysts are preferred over reducing/oxidizing agents, as they improve atom economy. Implementing less hazardous chemical synthesis and safer solvents and auxiliaries is achieved by avoiding organic solvents, and hydrophobing agents that cause harmful effects on the environment. A notable study is the one-pot hydrophobization by Hu et al. (2017), performing hydrophobization of CNC in water by attaching tannic acid and decylamine. Though we note that decylamine is harmful, the entire reaction was performed in water with tannic acid and CNCs as the major constituent. However, we note that the final contact angle was below 90° (74.4°). Protocols for water-based silanization have also been developed, which yielded relatively high grafting densities, about 0.4 mmol/g (Beaumont et al. 2018). However, the water contact angle is not reported. These studies highlight the fact that the substituent supposed to render hydrophobicity, should also be mildly water-soluble if water is to be used as the solvent. In turn, this will influence the degree of obtained hydrophobicity, since as of now, the highest contact angles are obtained in organic solvents (Bendahou et al. 2015; Orsolini et al. 2018; Tursi et al. 2018; Lee et al. 2020). Material structural modification before hydrophobization is perhaps a route that could render materials more hydrophobic without using chemicals. Orsolini et al. (2018) showed that porous materials were slightly more hydrophobic compared to dense materials after hydrophobization, most likely due to improved diffusion of the hydrophobing agent through the porous structure. Others have shown that hornification of lignocellulosic material may give contact angles above 90° by reducing hydroxyl group accessibility (Ding et al. 2019; Yang et al. 2019). Hornification without simultaneous reduction in the degree of polymerization increases material stiffness which might be problematic for some end-user applications.

As discussed in Rodríguez-Fabià et al. (2022a), water swelling is a diverse concept, that is characterized by several different methods. The aim of hydrophobization is typically to reduce swelling kinetics (e. g. the time it takes to sorb water) as well as the maximal amount sorbed. The reason for this in packaging, is that swelling deteriorates the lignocellulosic material either mechanically or by increasing gas (including water) permeation. The most problematic (ambient) gases in food preservation are water, CO₂ and oxygen. A collection of literature values of these permeabilities are found in the supplementary information.

The gas permeation values in the supplementary information (SI-Table 3) should serve as a reminder that lignocellulosic materials may have comparable or even better properties than plastics, when they have not sorbed water. The effect of material swelling was illustrated by Fukuzumi et al. (2011), where the oxygen permeability (oxygen transfer rate, OTR) of TOCNF-Na-film was investigated at 23 °C and at both 0% relative humidity (RH) and 50% RH. The corresponding OTRs were 10^–4 and 2 × 10^–2 mL × cm/(m² × 24 h × atm), indicating a 200-fold increase at 50% RH. Aulin et al. (2010) found that a water content of 15 wt%, was the threshold where water started to heavily influence oxygen permeability. Bardet et al. (2015) prepared CNF films with different nanofillers: montmorillonite (MMT) and TOCNC. After thermal treatment of the CNF/TOCNC films, the oxygen permeability of the film was similar to that of the CNF/MMT film (2.1 and 1.7 cm³ µm m⁻² day⁻¹ kPa⁻¹, respectively) at 85% RH. These studies truly underline the effect of lignocellulosic water sorption on gas permeation and will not be detailed further. Secondly, as with gas permeation, lignocellulosic materials are sensitive to water uptake. This has been demonstrated by Aulin et al. (2010), where the carboxylated MFC film storage modulus declined from 30 to 19 GPa, at ~ 25 wt% swelling. A decline in both Youngs modulus and tensile strength with increasing film water content has also been found by Zhang et al. (2016).

Plastic materials have a very diverse range of pore sizes dependent on the type of polymer, chain length and film fabrication method (Elyashevich et al. 2005; Bernards and Desai 2010). In this context, pore size alone may be equal to or smaller/larger compared to cellulosic films (Guo and Catchmark 2012; Östlund et al. 2013). Gas permeabilities are thus not necessarily lower in plastic compared to cellulosic materials (SI-Table 1). This feature is of course also dependent on the cellulose/polymer chemistry, temperature and water content. In terms of water swelling/penetration, plastic materials are usually superior barrier materials. This is a question of film thickness and porosity, but also a matter of chemistry. In all the referenced studies, several hydrophilic hydroxyl groups in cellulose remain after hydrophobization, as indicated by DS < 3. Hydroxyl groups will eventually interact with water, and the subsequent water swelling increases the pore size and reduces the material density (Sjöstedt et al. 2015). In this context, it is worth noting that the degradation of hydrophobization is not investigated in most of the referenced studies. Such degradation may in addition to making the barrier water permeable, also pose a toxicity risk for the consumer (Garnier et al. 1998). This is particularly important when considering that fluorination is a well-established hydrophobization method.

Several studies that initially reach (super)hydrophobic behaviour focus on a very short time characterization of water repellency parameters (droplet roll-off, WVTR, contact angle) (Reverdy et al. 2018). Such studies do not focus on whether the produced material is suitable for packaging. Regarding packaging, it is fair to assume that water repellency may be limited to water vapour permeability between 0 and 30 °C. Moreover, one might envision that products (liquids) may be stored long term (weeks) inside such materials. In most of the reported examples, the achieved contact angles decline over short times (minutes) and are generally not recorded over longer times (Shang et al. 2018; Yuan and Wen 2018; Huang et al. 2019). We finally note that the modified AKD impregnation (Adenekan and Hutton-Prager 2019) method gave hydrophobic surfaces after 140 days. Even though a new sample was most likely used per measurement (the droplet was not positioned continuously at the same sample area for 140 consecutive days) this shows that the AKD method is most likely superior to all other methods presented in this paper when it comes to the stability of the hydrophobic coating. Other chemical modification methods likely provide equally stable surfaces, but the data is not available. From a resource-saving perspective, it would be favorable to replace AKD methods with chemical-free (e.g. plasma) methods. However, this might not be feasible for most applications.

In the “Chemical hydrophobization techniques by molecule substitution” section, many examples of cellulose aerogels and filter paper for oil/water separation are given. Cellulose aerogels are used as oil absorbents and are typically modified by chemical vapor deposition. These materials have attracted a lot of interest due to their biodegradability, renewability, low density, high porosity and good mechanical properties. Several of the modified aerogels presented in this review demonstrated better oil sorption capacities than commercial absorbents (Sai et al. 2015; Laitinen et al. 2017; Rafieian et al. 2018). In general, the sorption capacity increases with decreasing density of the oil. For instance, Laitinen et al. (2017) developed CNF aerogels that could absorb up to 142.9 g/g of marine diesel oil, while commercial polypropylene absorbents had a capacity of 8.1–24.6 g/g. The authors also tested other types of oils and organic solvents, and in all cases, the modified aerogels performed better than the commercial materials. One of the advantages of using cellulose-based aerogels for oil sorption is that the oil can be recovered by simple squeezing of the aerogels, which allows for the reutilization of the sorbents. A key factor in the development of these materials is their recyclability, meaning how many times they can be reused, and their sorption capacity after each cycle. Some promising results have already been obtained (Laitinen et al. 2017), where the sorption capacity is as high as 70% after 30 cycles, but in other cases the sorption capacity drops after the first cycle (Mulyadi et al. 2016). Similarly, hydrophobic filter paper can be used to separate oil/water mixtures. In this case, the oil flux through the filter and the separation efficiency are the key parameters. Yu et al. (2019) reported hydrophobic filter paper that presented a separation above 99% of heavy organic solvent/water mixtures that was maintained after 30 filtration cycles. This type of paper with both hydrophobic and lipophilic properties can be an energy-efficient alternative to the current oil/water separation methods since it does not require any energy input.

Water-repellent and self-cleaning surfaces are super-hydrophobic surfaces with very low sliding angles. This allows the water droplets to slide on the surface, mimicking the lotus leaves (Wei et al. 2020). This type of cellulose material is often found in the textile industry, where superhydrophobic cotton is used for waterproof, stain-resistant, self-cleaning and anti-biofouling textiles (Teisala et al. 2014). One of the most important requirements for the textile industry is that the modification of cotton fabric must withstand abrasion and several cycles of washing (Yang et al. 2018).

There are many examples mentioned in this review where cellulosic materials are used as reinforcements in composites (Habibi and Dufresne 2008; Li et al. 2019). In these cases, hydrophobization enhances the dispersibility of the cellulosic material within the matrix as well as the compatibility between the two components, often resulting in an improvement of the mechanical and physical properties of the composite. For instance, Li et al. (2019) developed a biocomposite consisting of acetylated CNCs in a PHBH matrix that improved both the tensile strength and Young’s modulus by over 20% with respect to unmodified CNC/PHBH composites. Similarly, the water vapor and oxygen permeability were improved by 76 and 30% compared with the unmodified CNC/PHBH composites.

However, the applications of hydrophobic cellulosic materials are not limited to the ones mentioned above. Other fields of interest are microfluidic devices (Li et al. 2010), printed electronics (Gozutok et al. 2019), and applications closely related to the packaging industry such as printing paper, and water-repellent paper and cardboard (Wu et al. 2016). Although this review mainly focuses on the hydrophobization of CNF/CNC, in industry the hydrophobization of paper or pulp (kraft and mechanical pulp) is more relevant.