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Hashemzehi, M., Mesic, B., Sjöstrand, B., and Naqvi, M. (2022). "A comprehensive review of nanocellulose modification and applications in papermaking and packaging: Challenges, technical solutions, and perspectives," BioResources 17(2), 3718-3780.


The increasing usage of petroleum-based compounds has prompted numerous environmental concerns. Consequently, there has been a steady rise in research on the synthesis of useful materials from natural sources. Paper technologists are seeking environmentally acceptable dry end and wet end additives. Among the bio-based resources available, nanocellulose is a popular sustainable nanomaterial additive in the paper industry because of its high strength, high oxygen barrier performance, low density, great mechanical properties, and biocompatibility. NC’s extensive hydroxyl groups provide a unique possibility to dramatically modify the hydrophilicity and charge of the surface in order to improve their potential applications in the paper industry. The current paper reviews two series of surface modifications, each with various subcategories, depending on why modified nanocellulose is added in the paper production: to improve barrier properties or to improve mechanical properties of packaging materials. The methods presented in this study use the minimum amount of chemically hazardous solvents to have the least impact on the environment. This review focuses on modifications of nanocellulose and their subsequent application in the papermaking. The knowledge and the discussion presented in this review will form a literature source for future use by various stakeholders and the sustainable paper manufacturers.

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A Comprehensive Review of Nanocellulose Modification and Applications in Papermaking and Packaging: Challenges, Technical Solutions, and Perspectives

Mozhgan Hashemzehi,a Beko Mesic,a Björn Sjöstrand,a,* and Muhammad Naqvi a,b

The increasing usage of petroleum-based compounds has prompted numerous environmental concerns. Consequently, there has been a steady rise in research on the synthesis of useful materials from natural sources. Paper technologists are seeking environmentally acceptable dry end and wet end additives. Among the bio-based resources available, nanocellulose is a popular sustainable nanomaterial additive in the paper industry because of its high strength, high oxygen barrier performance, low density, great mechanical properties, and biocompatibility. NC’s extensive hydroxyl groups provide a unique possibility to dramatically modify the hydrophilicity and charge of the surface in order to improve their potential applications in the paper industry. The current paper reviews two series of surface modifications, each with various subcategories, depending on why modified nanocellulose is added in the paper production: to improve barrier properties or to improve mechanical properties of packaging materials. The methods presented in this study use the minimum amount of chemically hazardous solvents to have the least impact on the environment. This review focuses on modifications of nanocellulose and their subsequent application in the papermaking. The knowledge and the discussion presented in this review will form a literature source for future use by various stakeholders and the sustainable paper manufacturers.

DOI: 10.15376/biores.17.2.Hashemzehi

Keywords: Nanocellulose; Modification of nanocellulose; Papermaking; Coating; Barrier; Mechanical strength

Contact information: a: Department of Engineering and Chemical Sciences, Karlstad University, Sweden; b: 2Department of Chemical Engineering, American University of Middle East, Kuwait;

* Corresponding author: Tel: +46705519303; E-mail:



Background and Motivation

Packaging is an important component, in the view of customers, to maintain the quality and safety of the goods, provide information about the product, facilitate transportation, reduce food waste during transportation and storage, allow safe storage, prevent product damage, and reduce economic losses. To achieve the above goals, packaging systems must provide a physical barrier to microbial and physicochemical damages, environmental conditions, and external stimuli (Verghese et al. 2013; Mohammadian et al. 2020). As a result, the packaging industry currently relies upon paper, aluminum, conventional petroleum-based derivatives such as polyethylene, polyvinyl chloride, polypropylene, polystyrene, waxes, and/or fluorine-based derivatives as coating that serve a variety of functions, in order to manufacture competitive packages (Ferrer et al. 2017). For instance, paper is a significant packaging material in both developed and developing countries. Paper can take that role because it is a non-toxic, non-polluting, non-radiation-emitting material for versatile use that performs exceptionally well in terms of environmental friendliness (Huang 2017).

Wood fibers in paper packaging create a porous structure that allows oil, water, and gas molecules to pass through, leading to inadequate barrier properties. Additionally, paper packaging readily absorbs moisture from the environment and/or packaged food, resulting in deterioration of its mechanical strength, which is detrimental to the integrity and quality of packaged goods (Zhang et al. 2021a). Therefore, various materials can be used to address such issue, including aluminium foil and other metalized films, which are extensively used in food packaging. Aluminum foil provides a more complete barrier against moisture, oxygen, and other gases, volatile aromas, and impact of light than that of any plastic laminate material (Lamberti and Escher 2007). However, as reported by Wang et al. (2018), the packaging systems that contain an aluminum foil layer are difficult to recycle, and therefore they have a significant environmental footprint (Wang et al. 2018).

Another way to improve the barrier properties of the paper/paperboard would be the application of synthetic polymers derived from petrochemicals, since some of them have superior mechanical properties, heat stability, and certain barrier properties against a variety of compounds such as oxygen, carbon dioxide, water vapor, and aroma compounds (Cazon and Vázquez 2020). However, plastics production and processing are energy-intensive processes; they result in increased greenhouse gas emissions of enormous magnitude, which contribute to global warming. Moreover, plastic waste has been a pressing issue for many years due to plastics’ resistance to degradation (Yogalakshmi and Singh 2020). Indeed, these fossil-based plastics are usually non-biodegradable, non-compostable, and contribute to environmental pollution (Dissanayake et al. 2021). Furthermore, when they are burned, fossil carbon disoxide as well as toxic pollutants such as carbon monoxide, chlorine, hydrochloric acid, dioxin, furans, amines, benzene, 1,3-butadiene, and acetaldehyde are released, posing a risk to the environment and human health (Dissanayake et al. 2021).

Recently, the consumption of this kind of synthetic polymer has increased massively due to the COVID-19 situation (Gorrasi et al. 2020). This pandemic has resulted in an increase in demand for certain paper products, as reflected in Fig. 1 (Liu et al. 2020).


Fig. 1. Potential increasing demand for paper products amid the COVID-19 pandemic, redrawn from (Liu et al. 2020)

Unfortunately, the total waste generated has recently accelerated globally due to ineffective waste management and inadequate segregation systems. Only about 173 million tonnes waste is collected for recycling and landfill purposes after consumer use, with the remainder being improperly discarded (Rai et al. 2021). In addition, consumer preferences and health requirements are contributing to shifts in preferences toward more natural, high-quality products (Petkoska et al. 2021). Renewable carbon resources, such as starch, cellulose, hemicellulose, lignin, and plant oil, are derived from plant and wood biomass via photosynthesis from atmospheric carbon dioxide (Iwata 2015). They are being introduced into the market as a true alternative to conventional plastic formulations in packaging, reflecting additional benefits in their commercial use, including biocompatibility, barrier properties, nontoxicity, and non-polluting characteristics (Mellinas et al. 2016). Among the carbon-renewable sources, the properties of nanocellulose (NC) are critical to its significance: it is biodegradable and renewable, has high stiffness and reinforcing capacity, and has a large specific surface area (Skočaj 2019). NCs have a high elastic modulus (110 to 220 GPa) and tensile strength (7.5 to 7.7 GPa), as well as a low density and a high concentration of hydroxyl groups on their chains, which expand their application for a variety of desired purposes (Samadani et al. 2019).

Nanocellulose (NC) has significant potential to be used in papermaking process as a sustainable packaging material. The concept of sustainable packaging involves analysis and documentation to assess the design, material selection, processing, and lifecycle of the package (Abdul Khalil et al. 2016). The Sustainable Packaging Alliance’s definition of sustainable packaging identified four principles that must be considered concurrently when evaluating or designing packaging. Packaging must meet the following criteria (Lewis et al. 2010): I) be fit for purpose (effective): NC due to the durability and flexibility, can maximize the product-to-packaging ratio and in turn, meet this requirement (Abdul Khalil et al. 2016); II) consume as little material, energy, and water as possible (efficient): cellulose-based material is expensive in terms of price, but there is a cost-effective isolation method, which results in greater availability for developing relatively inexpensive polymer-based products. For instance, manufacturing biodegradable plastics from agricultural waste may pave the way for researchers and scientists seeking cost-effective, environmentally friendly materials (Yaradoddi et al. 2020); III) generate as little waste as possible (cyclic); IV) pose the fewest possible health and safety concerns to people and ecosystems (safe): The incorporation of NC into the packaging to substitute fossil fuel-based materials reduces the impact on the environment. In particular, recycled cellulose is utilized to make transparent films for pharmaceutical and food packaging, which reduces the amount of raw materials required during the manufacturing process while also degrading in the environment (Oun et al. 2020). Based on this information, cellulose offers promising prospects for use as a sustainable packaging material, but there are some issues that will need further investigation when utilized at higher production levels. For instance, studies have reported a reduction in dewatering rates following the addition of different types of NC to papermaking feed (Rantanen and Maloney 2013). Furthermore, in humid conditions, the barrier performance is significantly reduced because of NC’s inherent hygroscopic nature (Hubbe et al. 2017; Koppolu et al. 2019). Such obstacles imply a strong need for continued research in the coming years to maximize the potential of NC in a wide variety of packaging applications. Currently, the only cellulose-based film-like material that is routinely used in packaging is cellophane. It is used as a packaging film due to its effectiveness as a gas barrier in dry conditions. However, the production of cellophane is environmentally damaging, as it produces harmful by-products and utilizes sulfur-based chemicals (Reshmy et al. 2020). To overcome these drawbacks, the NC needs to be somehow modified or combined with biopolymers or other environmentally friendly NC. Depending on the purpose of the addition of NC in papermaking, whether as coating material or wet end additive, it may require a particular type of modification. This review is particularly focused on research publications that shed light on well-known obstacles to successfully implementing modified NC products to enhance packaging performance. The review will build knowledge and discuss the potential for modified NC to be used in papermaking to address the aforementioned concerns regarding the development of gas, oil, and water-resistant coatings and enhancement of the mechanical strength while minimizing the dewatering problems. A better understanding of different NC modification processes and the parameters that must be considered will potentially provide solutions to improve the paper quality, optimize the runnability of the papermaking process, enhance barrier properties, and in turn, increase the use of this technology in packaging. Indeed, the effectiveness of packaging materials is directly related to mechanical and barrier properties. Therefore, whenever in the manuscript we discuss the positive effects of application modified nanocellulose on their mechanical or barrier properties, it means their effectiveness on the packaging material. With this objective in mind, the following outline for this review paper is provided.

Objectives, Scope, and Limitations

As shown in Table 1, numerous review articles and chapters have been published on NC modification and the application of NC in the papermaking processes. The main purpose of this article is to expand the knowledge on the use of modified NC in paper manufacturing, with an emphasis on more environmentally friendly modification methods.

A brief overview of sustainable cellulose-based packaging is outlined in Section 1. Section 2 summarizes NC for packaging applications and provides a comprehensive overview of the NC, types and characteristics, sources of NC, barrier and strength agents. In Section 3, a key overview of the challenges associated with the use of NC in papermaking is provided, including I) Technical challenges, ii) Processability and runnability challenges including dewatering retention and formation challenges, iii) Economic challenges. Section 4 contains techniques for NC modification that are divided into two parts: hydrophobization and charge functionalization. Section 5 presents modified NC which have been used in papermaking and the possibilities of using modified NC in the papermaking process are highlighted together with the discussions on future directions of research and development in the studied area.

Table 1. Review Papers Focusing on the Modification of NC and Application of NC in Packaging



Biofibers are derived from biological sources and can be divided into non-wood fibers and wood fibers (Gholampour and Ozbakkaloglu 2020). Asyraf et al. (2020) defined natural fibers by origin (animals, minerals, plants, and microorganisms) and they are classified further in Fig. 2.

Fig. 2. Classification of the origin of different biofibers, redrawn from Campilho (2015)

Certain non-wood fibers outperform wood fibers in terms of physical and mechanical properties, cellulose content, crystallinity, and weight. As a result, they are suitable for engineering applications (Gholampour and Ozbakkaloglu 2020). The fiber’s characteristics are affected by its dimensions and chemical composition. They are principally composed of cellulose, hemicellulose, starch, wax, lignin, and other substances (Vijay et al. 2019). Among these substances, cellulose is the most critical component discussed in this review due to its high mechanical strength properties, large surface area, intriguing optical and rheological properties, and ease of surface modification. In theory, the tensile strength of cellulose crystalline areas is 50 to 100 GPa, which makes them stronger than E-glass. Additionally, NC exhibits shear-thinning and anomalous behaviour such as time-dependent increase in viscosity, which are advantageous for rheology properties (Jasmania and Thielemans 2018). It’s interesting to note that cellulose is the most abundant natural polymer on the globe, since it is created by photosynthesizing green plants, which cover 30% of the planet’s land area (Oberlintner et al. 2021). Additionally, algae, bacteria, fungi, grasses, and aquatic plants, and animals can be sources of cellulose (Dhali et al. 2021). Furthermore, Amara et al. (2021) outlined the isolation of cellulose from available agro-industrial by-products and wastes, which offers numerous economic and environmental advantages (Amara et al. 2021). One has to keep in mind that the amount of cellulose, its degree of crystallinity, and crystalline size might vary between different sources of cellulose (Dhali et al. 2021).

Cellulose fibers can be converted into fibrils with a lower diameter that is eventually made up of organized linear cellulose molecular chains. Due to this hierarchical structure, as shown in Fig. 3, fibrillated cellulose has a great deal of morphological and fibril size variability, resulting in mechanical, optical, thermal, fluidic, and ionic capabilities that can be significantly superior to those of parent cellulose fibers (Li et al. 2021d).


Fig. 3. Schematic illustration of the hierarchical structure of cellulose in plant-based biomass, redrawn from Rytioja et al. (2014); Green=Cellulose, blue=hemicellulose, purple=lignin and red= pectin

Very highly fibrillated cellulose is also known as nanofibrillar cellulose, cellulose nanofiber, and cellulose nanofibril (Abdul Khalil et al. 2014). Nanocellulose (NC) types can be classified based on three major factors: i) source material for cellulose (e.g., trees, plants, algae, bacteria, tunicate), ii) method of isolation (i.e. refinement by mechanical shear or acid hydrolysis which define two classifications of NC, nanofibrillated cellulose, and cellulose nanocrystals), and iii) surface chemistry (i.e. surface chemistry might be as a result of a particular nanocellulose production method or the result of subsequent chemical modification) (Li et al. 2021c).

Nanocellulose (NC) can be isolated in a variety of ways from a wide range of lignocellulosic sources. Usually, the lignocellulosic source first undergoes some pretreatment in order to achieve superior properties for the final material (Jonoobi et al. 2015). Then, a variety of strategies have been used to produce NC, most notably acid hydrolysis and mechanical treatment, either alone or in combination with other methods to achieve the desired nanoparticles characteristics (Abdul Khalil et al. 2014). These two approaches result in obtaining particle with large differences in morphology that can be classified into two groups: nanofibrillated cellulose (NFC, also known as cellulose nanofibril) and cellulose nanocrystals (CNC). The major distinction between CNC and NFC would be the number of amorphous regions and their dimensions (Oberlintner et al. 2021; Sacui et al. 2014). CNC consists of rod-shaped nanoparticles with a diameter of 10 to 20 nm and a length as high as several hundred nanometers (Fig. 4a), depending on the biological source and isolation protocol. CNC is formed by chemically hydrolyzing the amorphous regions of cellulose (Rol et al. 2019a), whereas NFC consists of fibril networks with a fibril diameter of 10 to 20 nm. The nano-scale fibrils within typical NFC are long and flexible, as can be seen in Fig. 4b. They are formed through a process called fibrillation, which involves the separation of cellulose fibrils. There are multiple procedures for obtaining NFC, including high-pressure homogenization, TEMPO-mediated oxidation, enzymatic hydrolysis, mechanical fibrillation/grinding followed by high-intensity ultrasonication, steam explosion, and cryo-crushing (Oberlintner et al. 2021). It is worth mentioning here that NFC is highly popular among scientists, with at least 65% of publications on the subject of NC focusing on NFC, according to Rol et al. (2019a).


Fig. 4. (a) Distributions of CNC and (b) is NFC, redrawn from Kumar et al. (2021)

The two main forms of NC, namely NFC, and CNC, exhibit distinct characteristics, and these have sparked increased interest among researchers. In terms of barrier properties, as shown in Fig. 5, the web structure of NFC has a complicated diffusion path. CNC particles, on the other hand, tend to form a more organized layered structure with fewer voids and a higher density (Wang et al. 2020b).

Fig. 5. Scheme of a gas passing through CNC (left) and NFC (right) films via cross sections, redrawn from Wang et al. (2020b)

The NFC suspensions can display hard solid-like viscoelastic characteristics even at relatively low concentrations, due to the densely entangled network. The viscosity, storage modulus, and loss modulus rise progressively with the increase of NFC concentration. The rheological behaviour of the CNC suspensions has been found to be strongly influenced by the concentration and aspect ratio of CNC. Elastic gel-like behaviors are observed at high concentration levels, whereas viscous liquid-like materials are observed at low concentrations (Li et al. 2015c). This has piqued scientists’ curiosity in the rheological features of NC. In general, the percolation networks, chemical interactions, and entanglement of NC enable the achievement of highly viscous suspensions in aqueous solutions at sufficient concentrations. On the other hand, continuous shearing aligns NC with the direction of flow, leading to decrease in viscosity as the shear rate increases. Such reductions are reversible when the systems come to rest. The particular high-low shear flow behavior of the NC make it attractive to be used as rheological modifiers in a wide variety of applications (Li et al. 2021c).

Both CNC and NFC, exhibit extraordinary mechanical properties in the dry state as a result of their high crystallinity and intermolecular interactions such as hydrogen bonding between cellulose chains (Saba et al. 2017; Hivechi et al. 2019). While NC has excellent mechanical properties in the dry state, its weak wet strength is a limiting factor in many applications. This issue is caused by the intrinsic hygroscopic properties of NC because their surface is covered with hydroxyl groups (Seddiqi et al. 2021) which also cause adsorption of moist air and liquid water by NC, resulting in considerable swelling of NC (Walther et al. 2020).

Nanocellulose in Packaging

As a result of the above-mentioned issues, numerous scholars have demonstrated that NC products can be used in papermaking applications, since they are advantageous in the production of strong paper and barrier-coated packaging paper products (Ehman et al. 2020; Hu et al. 2021). They also offer significant potential due to their low weight, non-toxicity, low thermal expansion, improved mechanical properties, gas barrier properties, electrical conductivity and environmental friendliness (Kumar et al. 2021). The use of NC to enhance barrier properties and mechanical strength of paper for packaging applications is detailed in the following sections.

Barriers (gas barrier and water vapor barrier)

Packaging with high barrier properties extends the shelf-life of foods while providing safety and protection when they are transported and stored. The industry and academia are both aiming to develop high-performance packaging. They are also concerned with having high barrier layers for biodegradable materials, which will require more research. Many strategies have been proposed for improving biodegradable polymers for use in packaging barriers such as different biopolymer blends, Chemical vapor deposition (CVD) of inorganic (like metal or metal oxide), incorporation of nanoparticles crystallization/orientation and chain architecture structures (Wu et al. 2021). Due to the high aspect ratio of NFCs, they are well suited for use in the aforementioned ways to enhance gas barrier capabilities (Kwon et al. 2020). The relatively high crystallinity of NFC could be part of the reason for these barrier qualities. It could also be due to the strong hydrogen bonds between cellulose and the matrix, as well as the longer path necessary for gas molecules to diffuse (Hubbe et al. 2017).

NFC as barriers can be applied to the surface of the paper in a variety of ways, including spray, bar, roll, and size press coating, resulting in a range of coat weights (Brodin et al. 2014). Coating paper and paperboard with CNF has no discernible effect on tensile strength but significantly improves bending stiffness. This is because bending stiffness of paper and paperboard is proportional to the third power of their thickness. Covering paper with CNF thickens the papers, resulting in a stiffer product (Mazhari Mousavi et al. 2018; Ham et al. 2020). Chemin et al. (2019) fabricated multi-layered hybrid thin films with more tortuous paths by using CNC and gibbsite nanoplatelets to enhance the oxygen barrier properties (Chemin et al. 2019). Mazhari Mousavi et al. (2018) also combined CNF and carboxymethyl cellulose (CMC) to coat paper, claiming that there was a significant improvement in the barrier properties of paperboard, which was directly related to the coat weight and surface coverage (Mazhari Mousavi et al. 2018). Recently, nanofiller-NC composites have developed as a novel form of composite material with strong strength and gas barrier properties (Mirmehdi et al. 2018). Seyedmohammad Mirmehdi et al. (2018) proposed a coating layer of a hybrid composite of NFC (matrix) and nano-clay (mineral filler) on paper substrate. Indeed, the use of nano-clay alone lowered tensile strength as well as the rate of oxygen transmission rate (Cairns et al. 2019). The application of NFC, on the other hand, increased the thickness of the films, affecting their strength and barrier properties. As a result, the combination of NFC (matrix) and nano-clay can result in interesting strength and gas barrier qualities (Mirmehdi et al. 2018). NC can also be applied as a filler (NC-filler), contributing to improvement of the gas barrier properties of packaging material since (i) NC-filler material may act as physical obstacles and makes for a more tortuous journey (ii) the addition of NC-fillers also decreases the polymer’s free volume (Rigotti et al. 2020). Xu et al. (2019a) reported the great capability of NFC and nanofillers chitin whisker (CHW) as nanofillers for improving the mechanical and barrier properties of polybutylene succinate-based films for biodegradable food packaging (Xu et al. 2019a). Dhar et al. (2015) also demonstrated the production of nano-biocomposite films based on poly (3-hydroxybutyrate)/CNC with superior gas barrier properties for food packaging applications. However, there is an issue with obtaining homogeneous NC-filler dispersion and strong NC-filler-polymer interactions (Wu et al. 2021). It should be added that these bio-barriers exhibit comparable properties to those of engineering polymers, with the exception of the water vapor barrier (Chowdhury et al. 2019). Indeed, the water vapor permeability of unmodified NC does not follow the same trends due to the hydrophilicity of cellulose (Kwon et al. 2020). In order to improve the water vapor barrier property of NFC-based substances, inorganic fillers or cross-linking agents, as well as chemical modification of NFC has been suggested by Yook et al. (2020).

Strength agents

Paper’s strength is affected by three aspects: (a) fiber strength, (b) fiber bonding capability, and (c) fiber entanglement properties. The usage of strength agents mostly reflects an enhancement in fiber bonding strength due to greater hydrogen bond formation (Li et al. 2021a). The NCs as strength agents have the potential to enhance the mechanical strength, density, barrier properties/air resistance, filler retention, and flocculation (He et al. 2016). It has been demonstrated that addition modified cellulose nanofibrils to the fiber matrix improves filler retention, lead to significantly enhanced dry and wet strength of the paper (Lourenço et al. 2019).

The mechanical strength and modulus of NFC is particularly impressive, and the tensile strength of single fibers is nearly twice that of some metals (such as Carbon Steels ASTM A36, ASTM A529, ASTM 709 due to these 3 nm-wide fibrils (Zhao et al. 2018). Furthermore, NCs benefit from Young’s modulus values between 20 and 50 GPa, which enables it to be used in a wide variety of fields such as nanopapers, NC films, aerogels, and hydrogels (Usov et al. 2015; Thomas et al. 2018). In papermaking, NFC could be applied in the wet section of the papermaking process to increase the strength of paper. The strength improvement is partially accomplished by filling in voids and pores at the edges of each fiber bond, thereby increasing the available bonded area (Brodin et al. 2014). He et al. (2017) demonstrated enhancement of strength properties by filling inter-fiber spaces of the sheet with higher surface area particles, and then the contact area at the fiber to fiber joint edge is increased, allowing for more hydrogen bonding sites (He et al. 2017). Additionally, NFC can enhance the paper’s strength capabilities by establishing strong fiber-to-fiber bonds as a result of their nanostructure and huge surface area, which leads to more hydrogen bonds formed in a strong network (Das et al. 2020). Lv et al. (2017) investigated the effect of using the MFC-ground calcium carbonate (GCC) complex on the strength and optical properties of handsheets. They used an MFC dosage of 5%, based on the GCC weight. The results indicated that the modified GCC acted like fibers and had improved retention due to the effective adsorption between GCC and MFC. Better paper strength qualities were attained at the same filler quantity (Lv et al. 2017). Su et al. (2020) also premixed MFC with GCC and discovered that the MFC-modified GCC increased the opacity of the filled paper while also improving its tensile strength. It is vital to note that high NC addition does not always result in favorable reinforcement effects for paper (Yuan et al. 2016) and optimization is essential in determining the ideal amount of additive.

The second parameter to optimize is the degree of fibrillation, which has a significant effect on the mechanical properties of NCs and, more specifically, on the paper sheet. This means that MFC with a broader particle size distribution shows less improvement in mechanical properties but more efficient retention in the fiber web. Even before sheet formation, NFC with a high degree of fibrillation can be distributed more effortlessly in the bulk suspension. As a result, a more homogeneous distribution occurs, which improves the nanostructure’s strength (González et al. 2014). However, a high degree of fibrillation does not always result in a beneficial reinforced effect, owing to their shorter length and lower aspect ratio. Indeed, short fibrils are more difficult to retain on fibers, so they contribute less to the strengthening of interfibrous bonding (Hu et al. 2021).

Challenges of Nanocellulose in Papermaking

Runnability challenges

In the paper manufacturing process, runnability refers to a machine’s ability to produce high-quality paper at maximum capacity. Therefore, the maintenance of “good” operation plays a vital role in paper production (Bondoc 2018). Some researchers have investigated the influence of NC application on the runnability of the papermaking process (Kajanto and Kosonen 2012; Lu et al. 2019; Charani and Moradian 2019). Kajanto and Kosonen (2012) used a high-speed pilot paper machine to test the use of NFC as a reinforcement agent in the paper. During the trial, overall runnability was satisfactory. No web cracks are found related to the issue with paper strength or dewatering. The total retention level stayed steady and the formation remained satisfactory without the need for any further changes. Dewatering in the press and former sections was acceptable (Kajanto and Kosonen 2012). Lu et al. (2019) investigated the properties of base paper’s wet-web strength and pressability, as well as their effect on the frequency of sheet breaks and machine runnability. They reported that after applying 5% NFC to the sample, the tensile energy absorption increased from 6.32 to 10.93 J/m2. As a result, they stated that adding CNFs to the paper-making process increases wet-web strength and improves paper machine runnability (Lu et al. 2019).

Charani et al. (2019) found that combining less cationic starch (CS) (0.5%) with 3 percent NFC greatly boosted paper breaking length while lowering CS-related process challenges (Charani and Moradian 2019). Another study found that adding 3, 6, and 9 wt% NFC to handsheets increased linearly the tensile index to around 55 N m gL at 35SR, which was judged to be suitable for paper machine runnability (Vallejos et al. 2016). Ankerfors et al. (2014) used the FEX pilot paper machine to examine the impact of MFC on the runnability and paper strength of a fine-paper. During the pilot paper machine trial, no serious runnability difficulties were reported. They also demonstrated that dewatering was affected by the MFC (NC), particularly in the wire portion (Ankerfors et al. 2014). As a result of the decreased drainage rate, the machine’s speed must be decreased as well, and there would need to be strategies in place to compensate for the decreased drainage rate caused by the NC application. This means that, for example, by using retention and drainage aids, paper machine running speeds can be greatly boosted; also, the retention of fine and NC particles increases and contributes to the better runnability of paper machines (Jin et al. 2014).

Many studies have looked at the effects of NC as a coating of paper (Aulin and Ström 2013; Chowdhury et al. 2019; Lengowski et al. 2019; He et al. 2021). Regarding their runnability as the coating can be mentioned that resistance to dewatering into the substrate is a critical process parameter. Indeed, both wet film formation and process runnability are influenced by water retention or the rate at which water is discharged into the substrate. A rapid release of significant amounts of water from MFC suspensions into paper can induce fiber swelling and de-bonding, resulting in runnability issues such as web breaks (Lengowski et al. 2019). Thereby, understanding and controlling the water retention characteristics of NC suspensions is critical for a successful coating process (Kumar 2018).

Additionally, high water content in NFC coatings results in greater drying costs, which are undesirable except if outweighed by enhanced product properties (Brodin et al. 2014). Drying also contributes to increasing the concentration of NC and then would affect drying dynamics and assembly operations (Wang et al. 2019).

The next challenge with runnability is that NC suspensions are difficult to coat onto substrates using conventional coating and metering mechanisms. This is due to blockage caused by NC fiber aggregation or the NC suspensions’ extraordinarily high viscosity (Kumar 2018). NFC suspensions are well-known for their high viscosity, even when the solids content is low, contributing to the limitation of the size of the operation window (Brodin et al. 2014). It should be added that improvement and controlling the rheological features of NFC-containing coating formulations is a significant challenge for lowering water content while maintaining a uniform coating layer (Brodin et al. 2014). As a result of changes in the rheology, papermakers must vary the nip load, doctor blade angle, or pressure when introducing NFC into the composition (Sharma et al. 2020).

Processability Challenges


Nanocellulose (NC) has not yet been widely applied in the papermaking industry, and this has been attributed to difficulties in using it (Balea et al. 2020). Nanofibrillated cellulose (NFC) usually increases the time needed for drainage dewatering, and a short draining time is needed in the papermaking process to minimize production costs and energy (Hu et al. 2021). Numerous causes to the delayed dewatering efficiency can be summarized as:

  • Dense layers are created,
  • Cellulose particles clog drainage channels in the wet web,
  • The reaction to vacuum is inefficient because of flocculation,
  • The healing mechanism allows for the repair of thin areas of paper,
  • A thin membrane forms on the surface of the paper when it comes into contact with a wet-press felt, due to rewetting of the sheet (Hubbe et al. 2020).

Besides, NFC creates a highly entangled network in aqueous dispersion, exhibiting a gel-like network at 1% solid levels. NFC surface charge appears to influence the rheological and dewatering properties of furnishes through its influence on swelling and effective binding of water. In fact, swelling of the fiber walls results in fewer fiber contacts, which reduces floc size and therefore lowers frictional forces between fibers, affecting rheology and dewatering of the fiber suspension network under flow (Dimic-Misic et al. 2013). Additionally, dewatering is closely linked to the shear-thinning behaviour of furnish. When the shear field disrupts the suspension’s microstructure, then the process can spread across the pad’s flow channels. As a result, the application of repetitive shear cycles can help the dewatering process (Dimic-Misic et al. 2013). According to some researchers, the presence of cationic NFC (CNFC) can also aid dewatering while improving the wet-web strength of paper sheets (Lu et al. 2020b). Diab et al. (2015) reported that cationic microfibrillated cellulose (CMFC) was unable to increase the pulp’s drainage. The author recommends that more trials using CMFC with a higher degree of cationization should be conducted to confirm that CMFC cannot be employed as drainage agents (Diab et al. 2015).

Lu et al. (2020b) assumed that the increased fines retention and density of paper sheets facilitated by CNFC addition are due to the factors including the following:

(1) The CNFC nanoscale dimensions provide a large specific surface area, which helps in adsorbing and “catching” the fines to create the CNFC-fines complex in the pulp;

(2) The high aspect ratio of CNFC would make it easier to twine and “lock” the fines together in order to create the CNFC-fines complex.

(3) The high surface charge density of CNFC’s quaternary ammonium ions enhances their integration with fines to create the CNFC-fines complex (Lu et al. 2020b).

Aside from the surface charge, other factors influencing the dewatering rate of NC used in paper production include the location of the NC addition and the type of retention additive used in paper manufacturing. Indeed, some researchers believe that by adding relatively small amounts of NFC directly to the pulp with appropriate retention aids, dewatering issues could be avoided (Li et al. 2021a). Ottesen et al. (2016) premixed NFC and retention chemicals with filler and predicted that the retention chemicals would enable NFC or fillers to adsorb preferentially to accessible surfaces. They reported large reductions in dewatering times through the use of retention chemicals that bind the NFC to the various components of the paper furnish. These benefits can be amplified even more for higher NFC doses by placing NFC in the filler fraction before adding it to the furnish (Ottesen et al. 2016). In general, the complicated interactions between cellulose pulp, NFC, mineral fillers, and regularly used additives should be taken into account to address this issue (Li et al. 2021a).


While a high aspect ratio of NC is critical for producing high-quality nanopapers, it is also one of the primary causes of agglomeration and flocculation problems. From the standpoint of colloid science, the most basic solution is to increase the electrostatic charge on the nanofibrils’ surface, which will cause like charges to repel each other, preventing aggregation and flocculation (Li et al. 2015c). However, from the perspective of particle retention, hydrodynamic shear and turbulence can easily wash the comparatively small negative charge particles off the web and reduce retention in the forming phase. The increase in retention appears to be due to the retention agent’s charge neutralization and aggregation properties (Ahadian et al. 2021). Therefore, there would be a trade-off between retention and flocculation in this case that must be considered.

In terms of retention, polyelectrolytes are often used by researchers to improve the retention efficiency of fillers, fines, and small particles and to reduce dewatering times in the papermaking process. These polyelectrolytes have the ability to bind fines so that they remain in wet sheets. This happens due to a flocculation effect, such that the fines are electrostatically connected to fibers. These kinds of polyelectrolytes enhance the drainage rate and retention of small particles while influencing the reinforced effect of NFC (Hu et al. 2021). The presence of retention aids was found to affect the position and association of fibrils during adsorption, and hence it affected the final handsheet characteristics. For instance, the densities of sheets formed in the presence of polyvinylamine as retention aids are lower than those formed in the absence of a retention aid. This could be explained by the fact that when retention agents are present, fibrils are more likely to attach to the fibers surface before dewatering rather than filling the pores between the fibers during dewatering (Hollertz et al. 2017). NFC-retention additive interactions and appropriate dosage of retention additive must be studied in order to select the most beneficial combination (Merayo et al. 2017b). Taipale et al. (2010) demonstrated that by selecting the right materials and operating conditions, the strength qualities could be improved without compromising the drainage. First, a thin MFC (NC) layer can develop on the adsorbed CS, and this nanonetwork can then coat the fibers instead of filling the gaps between them (Taipale et al. 2010). Merayo et al. (2017) also believed that employing the proper retention additive at the right dose could help to alleviate the dewatering issue while using NFC in paper production. So, they investigated the idea of interaction between MFC (NC) and retention agents (polyvinylamine, chitosan (CH), cationic starch (CS), and two type cationic polyacrylamide (C-PAM additive formed by poly-quaternary ammonium chloride and C-PAM-B additive formed by polyamine, PAM and hydrated bentonite clay) in order to increase the strength of recycled paper while minimizing adverse effects on the drainage process. When comparing the behavior of each additive at the lowest doses tested, C-PAM and C-PAM-B came out on top, followed by CH. Polyvinylamine also showed satisfactory drainage time results at the lowest dose; however, such dose was ten times higher than C-PAM, C-PAM-B, and CH. The CS was the least effective additive tested, especially at low and moderate doses (Merayo et al. 2017b). This result also implies that the conformation of fibrillated material onto cellulose fibers is influenced by retention aids; the final configuration indicates the extent to which the degree of strength and drainage rate have been increased (Zambrano et al. 2020). Finally, a single retention additive may not be satisfactory, and the development of two-component reagents has been recommended as a way to improve performances (Lourenço et al. 2019). In the case of NFC as retention agents, it can be mentioned that NFC l with a high specific surface area has the ability to flocculate cationic particles mostly via bridging/networking. Following the application of high shear and flocs breaking, subsequent retention agents on the flocs may help to approach flocs, resulting in reflocculation via patch or networking mechanisms. However, chemical modification of NFC is required to enhance its electrostatic interactions with other particles in the system. (Korhonen and Laine 2014).


The term “formation” (more formally “formation uniformity”) refers to the spatial variation in the density of fibers in the paper (Na and Muhlstein 2019). It can be determined by examining the sheet’s mass distribution in its plane. The goal of the fiber network formation process is to distribute the pulp as evenly as possible on the wire to ensure that the product properties remain consistent across the width of the web (Norman 2000). As shown in Fig. 6, flocculation and dispersion are two competing effects in the papermaking process.

Fig. 6. Right: Random Fiber distribution; Left: Fiber Flocculation distribution, redrawn from Dodson and Serafino (1993)

The importance of paper’s formation uniformity in terms of sheet appearance is widely acknowledged among papermakers, as the strength is controlled by fiber-to-fiber interactions that are developed during the process of paper formation, consolidation, and drying (Su et al. 2012). It has long been recognized that paper’s physical properties are somewhat dependent on its formation, with the effect being significant for tensile strength but small for elastic modulus (Anson et al. 2007). As reported by Motamedian et al. (2019), the paper formation is frequently improved by shortening fibers; this explains why the refining process might have an effect on the formation (Motamedian et al. 2019). From another perspective, increased fiber refining increases bonding formation and can somewhat offset the negative effect of filler on strength qualities (Ankerfors et al. 2014). Nanofibrillated cellulose (NFC) can bridge the filler-induced voids in the fiber-to-fiber bonding area, similar to fines, and affect the fiber network (Ämmälä et al. 2013). Two alternative processes can be used to explain the enhancing effect of NC on base paper: (1) NC promotes fiber–fiber bonding by bridging fibers together, and (2) NCs can create network architectures between relatively coarse fibers, enhancing the load-bearing capacity of papers (Li et al. 2021a). As NCs are derived from natural fibers, possessing inherent characteristics of pulp fibers, the slender filaments can form a densely entangled network structure (Li et al. 2021a). Merayo et al. (2017a) also confirm that the sheet homogeneity (formation index) improves due to a reduction in floc size by using NFC.

However, their small size makes it challenging to retain NFC during sheet formation in order to make use of their bond-forming capacity. As mentioned above, various solutions have been proposed to maintain the NC in the sheet formation process. For example, the use of a retention aid not only assists in the retention of the NC in the furnish, but it also can contribute to the formation of a homogeneous formation (Diab et al. 2015). Ahola et al. (2008) combined paper NFC with a cationic poly(amideamine epichlorohydrin) to improve the wet and dry strength of paper and they showed that when PAE first adsorbed on the fiber surface, it generated a homogeneous and viscous coating of NFC. Salminen (2010) studied the effect of (C-PAM on the short fiber fraction and CS on the long fiber fraction on the formation, retention, and strength of the paper. They claimed that adding C-PAM to short fiber fractions would prevent long fibers flocculation, resulting in the improved formation and, in turn, increased wet web strength.

Economic Challenges

Nanocellulose (NC) applications have a 6.4 million metric ton yearly market potential in the United States, and a 35 million metric ton global market potential. Currently, cellulose nanoparticles have the largest volume potential in paper and packaging applications. The automobile, construction, personal care, and textile sectors are other examples of high-volume applications (Shatkin et al. 2014). According to Nelson et al. (2016), the United States Department of Agriculture’s global NC market size projection will be reached around 2045. Commercialization of technologies for producing NC from kraft or dissolving pulp has occurred recently. Pulp is a costly raw material that necessitates rigorous, often energy-intensive chemical or mechanical processing (Blair and Mabee 2021). Additionally, chemical costs and investment in manufacturing equipment and paper machines, are other factors that influence the cost of NC manufacture (Li et al. 2021a). Due to the confidentiality issue, it is hard to provide a precise estimate of the end-use product’s cost. A rough calculation based on the raw material pricing and manufacturing costs revealed that the cost of NFC will range between USD$ 7 and 12 per kilogram of dry material (Dhali et al. 2021). Estimated CNC production costs differ widely across the literature; de Assis et al. (2017) approximated that manufacturing costs for CNC production ranged from 3632 to 4420 (dry equivalent) USD t-1. Their model process was acid hydrolysis of dissolving pulp (de Assis et al. 2017). There are currently a few pilot projects producing these nanomaterials, such as CelluForce (Canada), USDA’s Forest Products Laboratory, InnoTech Alberta (Canada), FPInnovations (Canada), and Cellulose Lab (Canada) (Rudie 2017). Numerous difficulties and obstacles remain along the path from pilot to large-scale production. For instance, CNC that is produced via acid hydrolysis presents challenges in terms of waste treatment, yield, drying, and redispersion. Numerous efforts have been made to overcome these types of problems such as the fabrication of NC using solid organic acids hydrolysis (Chen et al. 2016; Jia et al. 2017). The recovery of and the reuse of these kinds of acids is easily achievable through a conventional and commercially proven crystallization process at a low or ambient temperature. In fact, since they can be recovered using a conventional and commercially proven crystallization method, organic acids are uniquely excellent candidates for making sustainable and green cellulose nanomaterials (Chen et al. 2016; Jia et al. 2017).

In the case of NFC, the economic problem is related to the usage of mechanical methods, which involve large amounts of energy consumption (Wang and Cha 2019). Eriksen et al. (2008) calculated that a production NC by homogenizer can consume up to 70,000 kWh/t of energy, which is nearly 100 times more than the energy required to make one ton of paper (Eriksen et al. 2008; Chauve and Bras 2014). Therefore, various pre-treatment methods have been developed to significantly reduce the energy consumption associated with NC production. Even with pre-treatments, matching the cost of NC production to specific benefits such as mechanical strength performance remains a considerable difficulty (Ang et al. 2019). The economy of producing NFC materials depends largely on the pre-treatment method (e.g. enzymatic, carboxymethylation, TEMPO-modified NFC, etc.). The least costly process is likely to be the enzymatic pre-treatment process, which produces NFC from pulp integrated with a pulp mill at a cost of 0.4 €/kg and is currently used in large-scale paper manufacturing applications. The cost of non-integrated NFC in papermaking applications should be less than 2.5€/kg (Klemm et al. 2018). Indeed, integrating the production of fibrillated cellulose products with the current forest and paper industries appears to be a synergistic technique for cutting large-scale production costs (Li et al. 2021d). With the advent of low-cost commercial NC sources, there is still space for new applications and improvements to existing ones in a variety of industries that require sophisticated materials (Trache et al. 2020). In general, the economic challenges that remain in the field of NC manufacturing that need to be overcome in order to ensure rapid utilization and commercialization include the following: 1) Cost-effectively drying of NC suspensions while preserving distinct NC particle shape for optimal re-dispersion during end-use 2) costs related to the creation of international standards across the supply chain, and 3) development of low-cost, quick characterization methods for detecting product quality (Nelson et al. 2016). Additionally, in the paper industry, unmodified cellulose has few applications in packaging films. When cellulose is modified chemically, mechanically, or enzymatically, it can be used in a variety of applications (Fotie et al. 2020). Therefore, in addition to the preparation of NC, the cost of treatment should be considered, which is currently an expensive process (Nelson et al. 2016; Blair and Mabee 2021).

There is an effort underway to develop processes that will enable NC to be sold at a lower cost, for instance with increased scale production costs could be significantly reduced. Furthermore, capital costs could be reduced through government investment or infrastructure repurposing. Increased process efficiency or internal energy generation could also help to reduce energy costs (Nelson et al. 2016). Along with the successful extraction of NC via pre-treatments (Ang et al. 2019), some practices such as developing new, environmentally friendly, and inexpensive solvents as pre-treating agents are generating increased interest in academia and industry (Liu et al. 2019b).

Following the decrease in the cost of protection, additional research must be conducted to ensure the final materials’ feasibility and marketability, in particular (1) life-cycle evaluations and degradability studies, as required to demonstrate the environmental impact; (2) continuous cost-cutting optimization of the fibrillated cellulose production process; (3) a careful balance between usability and biodegradability when discarded; and (4) a recycling technique for composites incorporating fibrillated cellulose (Li et al. 2021d).


Modifying NC can help address some of the above limitations, since the hydroxyl groups in the NC chain are critical in enabling NC to be used in a variety of applications. They can be adjusted in a variety of ways, depending on the application, and NC can be impacted based on the chemical and condition utilized (Rol et al. 2019a). These modifications result in the acquisition of desirable properties, which improve their effectiveness for a particular application (Trache et al. 2020).

In this review paper, modification techniques are further classified into two types based on the reason for adding modified NC to the paper, as shown in Fig. 7. The first category is associated with the goal of improving the barrier properties of paper, with a particular emphasis on NC hydrophobization and its usage as a dry end additive. The next category is involved with the modification to impart stable positive or negative electrostatic charges on the surface to improve retention and mechanical properties. The factor that could determine the effectiveness of modification is the degree of substitution (DS), with the maximum value being 3, which occurs when all hydroxyl groups on anhydroglucose units react (Oberlintner et al. 2021).

Fig. 7. Various modification methods that will be discussed in this review article

Hydrophobization of Nanocellulose

The presence of a high hydroxyl group concentration on the surface of NC confers on them inherent hydrophilicity. Self-agglomeration and hydrophilicity have limited their applicability, prompting development of surface functionalization and hydrophobization in order to effectively utilize their potential (Chin et al. 2018). In general, functionalization with chemical grafting is preferred over physical grafting because the covalent bonding of grafted chains to the substrate surface prevents desorption and promotes longer chemical stability (Sun et al. 2020). However, the need for solvent-based or toxic-based systems have limited their application to industrial scale-up. As a result, several European projects have been started, such as the SUNPAP, to discover alternatives to these solvents (Missoum et al. 2013). As a consequence, researchers are increasingly concentrating their efforts on developing more environmentally friendly strategies for chemical surface modification, which can be unquestionably the most important aspect of NFC functionalization (Missoum et al. 2013). With this in mind, in the following text summarizes some of the hydrophobization methods (not totally green but a bit more “environment-friendly” methods) that use the fewest amount of solvent and toxic materials possible.

Hydrophobization: Physical adsorption of surfactants

Surfactant adsorption is a green, industrially scalable, and environmentally friendly method for engineering the NC surface (Chi and Catchmark 2017; Szlek et al. 2022). In fact, the amphiphilic nature of surfactants is ideal for modifying the particles’ surface tension and contact angle. Surfactants can reduce the interfacial tension between air and water, as well as the forces that keep water in the filter cake capillaries (Patra et al. 2016). When they are adsorbed onto the less charged regions of NC surfaces, the net Gibbs free energy is decreased, as shown in Fig. 8 (Tardy et al. 2017).

Fig. 8. Schematic illustration effect of adsorption surfactant on the Gibbs energy. Note that the details of the “hydrophilic cellulose” are not intended to represent a specific chemical structure or tubular nature.

Surfactants are made up of two parts: a hydrophilic head and a hydrophobic tail (Vaziri Hassas et al. 2014). According to the charge of their hydrophilic moiety, there are four types of surfactants: anionic, cationic, non-ionic, and amphoteric (Kurpiers et al. 2021). Two key aspects influence surfactant adsorption: the surfactant’s interaction with an electrostatic force and the surfactant’s hydrophobicity, which will cause it to interact with the surface if it is less polar than water (Aulin et al. 2008).

An anionic surfactant’s affinity for NC surfaces can be explained by hydrophobic interactions between the surfactant’s hydrophobic groups and the intrinsic hydrophobic domains found on cellulosic materials (Tardy et al. 2017). Cationic surfactants are predicted to have a high affinity for NC surfaces due to the different charges. These electrostatic interactions are enhanced by the presence of functional groups on NC. The reason for this is that electrostatic interactions are an order of magnitude more powerful than secondary interactions (Tardy et al. 2017). Ly et al. (2020) demonstrated that the nature of the surfactants used in the CNC modification can be attributed to the increasing of water contact angle (WCA) in the modified samples. All three surfactants considered (cetyltetramethylammoniumbromide with C16 single chain, dimethyldidodecylammonium bromide with C12 double chains, and dimethyldihexadecylammonium with C12 double chains), have the ability to arrange their hydrophobic tails around CNC surfaces, resulting in hydrophobicity, although the longer the surfactant’s aliphatic chain, the greater the hydrophobicity of the CNC (Ly and Mekonnen 2020). It should be added the modification of cellulose is dependent not only on the type of surfactant, but also on the amount of adsorption (Patra et al. 2016). For instance, de Lima et al. (2019) found that the surfactant had no effect on free hydroxyls. They reported that (1) the CNC has a strong physical interaction and steric stabilization, which can be seen as an increase in particle size, and (2) the surfactant does not serve as a stabilizing agent, resulting in no charge differences (de Lima et al. 2019).

It is important to notice that the use of surfactant chains makes the crystalline ordering decrease, and the spacing between the planes consequently will increase. Souza et al. (2020) modified NC during isolation with an anionic surfactant. According to their study, this in situ modification led to a decrease in the NC dimensions because of the electrostatic interactions with the surfactant which may improve the grinding process, making it more productive (Souza et al. 2020).

Hydrophobization: using oil

Vegetable oils can be used to modify the surface of cellulose nanomaterials through an esterification process or by grafting vegetable oil, as shown in Fig. 9. The majority of vegetable oils are triglycerides, which are made up of glycerol molecules and three long-chain fatty acids joined by ester bonds at the hydroxyl groups. The long-chain fatty acids are more hydrophobic than shorter fatty acids and thus more stable at high temperatures (Le and Nguyen 2020; Hashemzehi et al. 2022).


Fig. 9. Transesterification reaction between cellulose and triglycerides

Shang et al. (2013) modified CNC by using castor oil and diisocyanate as a linking agent. They believed that two of the three hydroxyl groups in castor oil should be terminated to leave only one active hydroxyl group available for covalent grafting onto CNC. Following a successful graft, the surface energy decreased and the contact angle increased significantly (Shang et al. 2013). Pourmoazzen et al. (2020) anchored successfully covalently cholesterol to CNC surfaces. It was clear that hydrophobic CNC had been achieved, because the contact angle was increased to above 115 (Pourmoazzen et al. 2020).

Transesterification results in a little increase in the crystalline index, which has been related to hydrolysis or crystallization of the amorphous portion of NC under acidic conditions and high temperatures during esterification reaction (Yoo and Youngblood 2016). Wei et al. (2017) used a transesterification reaction to study the chemical modification of CNC using canola oil fatty acid methyl ester. As compared to unmodified CNC, the trans-esterified CNC had greater thermal stability and hydrophobicity. Besides, glycerol, a by-product of the canola oil fatty acid methyl ester manufacturing process, can be used as emulsions and polymer processing aids (Wei et al. 2017). Gorade et al. (2019) developed the hydrophobic modification of microcrystalline cellulose (MCC) using rice bran oil (RBO). The WCA of AUMCC was 92.2°, with the surface energy dropping to 35.56 mN/m and water absorption of 0.9 mL/mg. AUMCC’s thermal stability and crystallinity deteriorated as treatment time progressed (Gorade et al. 2019). Dhuiege et al. (2018) present a new technique for surface functionalization of CNC in water, by transesterification of vinyl acetate. They believed that developing a water-based technique would considerably reduce the treatment’s environmental impact. As reported by Dhuiege et al. (2018), the increasing hydrophobicity contributes to the progressive change of the dispersive characteristics in various solvents (Dhuiège et al. 2018). Onwukamike et al. (2018) have done research on sustainable cellulose transesterification using the CO2-DBU solvent. They were able to achieve a DS of 1.59 and good thermal stability (95% at 360 °C) as well as high elastic moduli (up to 478 MPa), maximum stress of about 22 MPa, and the maximum elongation (strain) of about 35% (Onwukamike et al. 2018). Mokhena and John (2020) used canola vegetable oil to chemically modify NFC with and without catalyst. When NFC was esterified in the presence of a catalyst, a considerable extent of derivitization occurred, leading to relatively high DS) Modification without the addition of a catalyst resulted in a less significant change in the crystallinity index (Mokhena and John 2020). Concerning the disadvantages of this method, it should be noted that this method had a significant effect on the crystallinity, mechanical, light transmittance, and thermal properties (Mokhena and John 2020).

The next-generation materials containing oil, alkyd resins, and epoxy resin can play a key role as a coating binder because of their great flexibility in terms of construction, property changes, and their low cost (Taylor 2001). They could also be very interesting candidates for commercial application and up-scaling when combined with NC due to their hydrophobicity, biodegradability, and renewability (Aulin and Ström 2013). Their adhesive properties and excellent mechanical performance make them particularly well-suited for use in coating applications (Aziz et al. 2021). Aulin and Ström (2013) evaluated the effects of the different types of alkyd resin oil and varying the length of the oil on the unmodified NC-coated papers. Alkyd resins having longer fatty acid chains allow for the efficient formation of a hydrophobic polymer layer. Alkyd resins based on tall oil and linseed oil at the same oil length were compared. The alkyd resin based on linseed oil was shown to be the most effective in terms of lowering water vapor transmission rate (WVTR) due to the fatty acid composition and the degree of unsaturation (Aulin and Ström 2013). Xu et al. (2019b) also used epoxidized soybean oil (ESO) as a low-cost, sustainable, and environmentally friendly raw material for forming modified cellulose. The WCA of modified cellulose aerogels was 132.6° when the mass fraction of ESO was 30 wt.% (Xu et al. 2019b). Huang et al. (2017) investigated the effect of grafting ESO with varying weight percentages onto properties of the cellulose. When the ESO concentration was raised, the WCA increased to 145.1°. The modified cellulose surface became rougher, which further increased its hydrophobic properties (Huang et al. 2017). Lu et al. (2015) developed a biodegradable agent to coat the NC film with a low WVTR. To reduce the WVTR, a rod coater was used to apply a coating agent composed of AESO and 3-aminopropyltriethoxysilane (APTS). The resulting film has great potential as a green packaging material (Lu et al. 2015).

Auclair et al. (2020) reported the use of coating systems with acrylated epoxidized soybean oil and CNC. They reported that CNC should be hydrophobized to make it more compatible with the hydrophobic AESO matrix due to a polarity mismatch between AESO and CNC. Therefore, the negative or positive electrostatic charges were applied to the CNC surface, resulting in improved polymer matrix dispersion. The enhancement compatibility was also achieved by adjusting the surface energy properties of nonpolar or hydrophobic polymer matrices (Auclair et al. 2020). This means that in order to improve the adhesion between MFC and the epoxy resin polymer matrix, the surface of the MFC should be modified, changing its character from hydrophilic to hydrophobic while keeping the crystalline structure of the MFC intact (Siró and Plackett 2010). Thus, two more steps are required to modify NFC with this type of biomaterial in order to achieve the desired results, which drives up the cost of coating materials. Moreover, these materials have a relatively long cure time and require high temperatures to cure (Tambe et al. 2016). They also typically have low tensile strengths, which limits their use as structural materials. The incorporation of fillers or reinforcements has been demonstrated to be a viable strategy for increasing the mechanical strength of shape-memory polymers (Sain et al. 2020).

Hydrophobization: Silylation

The silylation method is less environmentally damaging than the esterification methods, and it can be carried out via a solid-gas reaction without the use of a solvent (Jarrah et al. 2018). The surface hydroxyl groups on hydrophilic fibers are converted to alkyl silyl ethers via silylation, resulting in hydrophobic surfaces as shown in Fig. 10 (Jarrah et al. 2018). A silylation reaction produces ammonia as a by-product, which is a significant advantage of this method (Jankauskaitė et al. 2020).

Fig. 10. Schematic diagram of the possible products of CH3Si-modified cellulose, redrawn from Jarrah et al. (2018)

Khanjanzadeh et al. (2018) modified NC with 3-aminopropyltriethoxysilane, without using hazardous solvents. The surface morphology of nanocrystals was not considerably affected by functionalization. Nie et al. (2021) also modified the hydroxyl groups of the NFC surface with triethoxy-1H,1H,2H,2H-tridecafluoro-n-octylsilane, which enhanced their triboelectric charge density and hydrophobicity. The modified NFC showed growth in contact angle from 52° to 128° (Nie et al. 2021). Jarrah et al. (2018) used a gas-solid silylation reaction with silyl chlorides (RSiCl3) to attach hydrophobic alkyl groups on the cellulose surface of cotton fibers, thereby converting them into hydrophobic materials and achieving a DS in the range of 0.10 to 0.29 per glucose unit (Jarrah et al. 2018). Yook et al. (2020) prepared various types of NFC, such as cellulose nanofibril (NFC), lignocellulose nanofibril, carboxymethylated cellulose nanofibril, silylated cellulose nanofibril (S-NFC), alkyl ketene dimer-added NFC (AKD-NFC), and coated on linerboard and wood-free paper to investigate the air, oxygen, water and grease barrier properties of these papers. The S-NFC and AKD-NFC were more water-resistant than NFC. The S-NFC had WCAs over 90 degrees. However, because of the short length of the hydrophobic alkyl groups in S-NFC, they may not inhibit effective water vapor transfer (Yook et al. 2020). It should also be emphasized that silane coupling agents have a high affinity for hydroxyl groups, even at room temperature; the NC core chains become silylated to the point of being soluble in the reaction medium, resulting in crystal disintegration and loss of the cellulose’s original morphology (Ghasemlou et al. 2021)

Hydrophobization: Using plant polyphenols

The application of plant polyphenols has recently been introduced as an approach for developing hydrophobic nanoparticles in an environmentally friendly manner, in which low-cost plant polyphenols serve as precursors for the development of multifunctional coatings (Hu et al. 2017). Tannic acid (TA) is a naturally occurring polyphenolic compound with a wide range of bonding abilities. It can complex or cross-link macromolecules at several binding sites using a variety of interactions, including hydrogen and ionic bonding as well as hydrophobic interactions (Fan et al. 2017). Hu et al. (2017) proposed a simple method for producing CNC-TA-DA (decylamine) via covalent binding to increase the water contact angle and the ability to be redispersed in organic solvents. Furthermore, they believed that this simple method is less time-consuming than polymer grafting and produces particles with minimal aggregation and morphological changes (Hu et al. 2017). Missio et al. (2018) used NFC and condensed tannins to create a durable, long-lasting packaging material. The physical and mechanical requirements were met by NFC, while the tannin was added for its antioxidant qualities. This film had a higher density and better surface hydrophobicity, resulting in a 6-fold improvement in air-barrier qualities (Missio et al. 2018). Huang et al. (2019) used layer-by-layer assembly to layer CH and TA on NFC, since TA has adhesive properties as a result of the aromatic components in its composition, allowing it to form coatings on a variety of nonporous and porous substrates. The key force behind the formation of multilayers is the electrostatic interaction between positively charged CH and negatively charged TA. Improved mechanical properties and strong antibacterial properties were found in this layer by layer structured film (Huang et al. 2019). Missio et al. (2020) used a mixture of NFC and condensed TA to fabricate functional film. They also used a non-ionic surfactant to give them even more control over the tannin-cellulose interface and, as a result, over the film properties. These tannin-containing films had a higher hydrophobic character, as shown by increased WCA (Missio et al. 2020).

Hydrophobization: Chemical vapor deposition (CVD)

Solution-based methods are commonly used to modify materials; however, in these procedures, finding an adequate solvent can be a limiting factor due to the fact that it must be compatible with both the substrate and the coating. Conformal coating of micro- or nanoscale structures is also limited by surface tension effects. As a result, methods involving the vapor phase, such as CVD, are preferred for coating complex surfaces (Cheng and Gupta 2018). Chemical vapor deposition (CVD) methods are capable of producing homogenous and conformal organic thin films without using solvents, catalysts, or separation agents (Hilt et al. 2014). CVD benefits from the excellent interface and surface morphology, growth of complex heterostructures with numerous layers, growth on patterned substrates, multiple wafer scale-up, and high layer purity (Pessoa et al. 2015). CVD procedures also are well-controlled techniques that allow the operator to quickly control the amount of material by changing the coating thickness (Hilt et al. 2014). Fluorocarbons, silicones, and organic or inorganic compounds are employed in these tactics to apply to cellulose and reduce surface energy (Leal et al. 2020). Yu et al. (2019) applied a one-step gas-solid reaction for the production of hydrophobic NC and bacterial cellulose (BC) with CVD of perfluorooctyltriethoxysilane. The results showed that perfluorooctyltriethoxysilane was successfully incorporated into the CNC and bacterial cellulose. They likewise believed that the vapor-based technique can be used to produce hydrophobic cellulosic materials on a large scale for industrial purposes (Yu et al. 2019). Rafieian et al. (2018) modified NFC via CVD of hexadecyltrimethoxylan (HDTMS). They believed that HDTMS with the longer the alkyl chain of hexadecyl (16 carbon atoms) compared to methyl (one carbon atom) contribute to making the polymer more hydrophobic, allowing the WCA of NFC to increase to 139. They found that untreated cellulose had a higher crystallinity than those treated with CVD (Rafieian et al. 2018). Cunha et al. (2018) also studied the impacts of CVD post modification on NFC filaments properties. Two organosilanes with varied numbers of methyl substituents were chosen for this purpose. Various surface structures including continuous, uniform coating layers or three-dimensional, hairy-like structures were observed, which served to decrease the surface energy and had a big impact on water interactions (Cunha et al. 2018). The decrease in surface energy was not expected and explained by the influence of the polar component in the organosilanes. A drawback of CVD is that it frequently involves extremely high temperatures and vacuum conditions. In addition, in situ monitoring of the process is restricted to specific analysis. However, Yang et al. (2020b) developed low-temperature-CVD with the goal of modifying cellulose-based to make them hydrophobic. Vacuuming has been used to allow hydrophobic gas to enter and react with the wood (Yang et al. 2020b).

Hydrophobization: Atomic layer deposition (ALD)

Atomic layer deposition (ALD) is a sub-set of the CVD process that uses a series of self-limiting surface adsorption reactions (chemisorption) carried out in a vacuum to control composition and the thickness of the deposed film with a thickness resolution of Angstroms (Wooding et al. 2020). ALD allows for higher uniformity and conformance on complex substrates as compared to standard CVD procedures, since the precursor molecules have a longer lifetime to diffuse to the cavities in complicated three-dimensional substrates (Wooding et al. 2020). Rare-earth oxide ceramics have been recognized as having strong hydrophobic surfaces, so they should find the widespread application (Azimi et al. 2013). A schematic description of the ALD process is shown in Fig. 11 (Oviroh et al. 2019).

  1. The first precursor is exposed in the reactor chamber to form a layer on the substrate.
  2. Remove any remaining initial precursors and by-products.
  3. The second precursor is exposed.
  4. Excess second precursor and by-products are purged

The procedure is repeated until the desired film thickness is obtained.