Physical and chemical modifications of cellulose fibers for food packaging applications

Cellulose fibers have gained considerable interest for use as gas barriers and reinforcing fillers in food packaging materials because of their interesting properties, including biodegradability, low density, nontoxicity, and low cost. Although modifications of cellulose nanomaterials for reinforcing applications have been reviewed in the literature, they suffer from a lack of comparative results, and the modifications related to food packaging applications have not been covered yet. This study reviews physical and chemical methods used to improve oxygen and water barrier properties, water uptake, mechanical strength, and transparency of different food packaging films of cellulose nanomaterials, and provides comparative results. Heat treatment, esterification, and TEMPO-mediated and periodate oxidation are the most common methods used to improve the barrier properties of free-standing, bilayer, and composite films prepared from cellulose nanomaterials. Modification using silane compounds is a well-established method for improving the mechanical properties of cellulose fibers and polymer composite films.


Introduction
Approximately 40% of the food produced every year is spoiled (Wang et al. 2018); however, appropriate packaging can decrease the spoilage rate substantially. Accordingly, to preserve the quality and safety of food during distribution and storage and to provide end-use convenience, both suppliers and distributors increasingly use more food packaging (Vilarinho et al. 2018). The control of the transmittance of oxygen and moisture between food and its environment is one of the most important factors in preserving the quality of food, extending the shelf life, and preventing food spoilage Wang et al. 2018). Glass and metal exhibit high oxygen and moisture barrier properties. However, there are some disadvantages associated with the use of these materials for food packaging applications, namely their high density that increases the transportation expenses and the high cost of the material itself ). Compared to glass and metal, plastics are much lighter and cheaper.
Hence, about 40% of produced plastic is used for food packaging applications (Vilarinho et al. 2018). Currently, petroleum-based plastics are the most used packaging materials because of their versatility and excellent properties for packaging (Qasim et al. 2020). Although most plastics possess high water vapor barrier properties, their poor oxygen barrier properties result in the eventual loss of food quality (Saallah et al. 2020). Thus, decreasing the oxygen permeability of these materials can increase the shelf life of the packaged food and prevent wastage. Although plastics can be laminated and coated with metal foils to decrease their oxygen permeability, these approaches limit the recycling of plastic wastes and hinder the degradation of biodegradable packaging (Wang et al. 2018). Cellulose has a wide range of applications in many fields, such as paper making and coating processes, oil absorption, energy storage devices, biomedical applications, and composite materials (Nguyen et al. 2019;Tayeb et al. 2018). Due to the high intrinsic oxygen permeability of different types of plastics, cellulose fibers have gained much attention as nanofillers or barrier layers to improve their oxygen barrier properties. The use of cellulose fibers as nanofillers improves the mechanical properties and fracture toughness of the resulting nanocomposites (Bach and Vu 2020;Le Hoang et al. 2018;Vu et al. 2018). In the case of biodegradable packaging materials, which usually show high water vapor permeability, cellulose fibers improve moisture barrier properties (Saallah et al. 2020;Wang et al. 2018). When used as nanofillers for food packaging materials, cellulose fibers exhibit several advantages, including safety, lightweightedness, biodegradability, renewability, non-abrasiveness, abundance, and high transparency Mohit & Arul Mozhi Selvan 2018). Hence, in combination with biodegradable plastics, like polylactic acid (PLA) (Espino-Pérez et al. 2013), cellulose fibers can provide a safe and fully biodegradable composite with enhanced barrier and mechanical properties and negligible loss of transparency. Besides, cellulose fibers can be prepared from different sources, including food wastes and agricultural by-products, allowing for the production of a high value-added product from a free or very lowcost source (Seo et al. 2018).
Despite the many advantages of cellulose fibers as fillers and barrier layers for improving the mechanical strength and barrier properties of food packaging materials, there are certain drawbacks, including low compatibility with most plastics (Vilarinho et al. 2018), high hydrophilicity , and low thermal stability (Ben Azouz et al. 2012). Low compatibility between the cellulose fibers and hydrophobic matrix restricts their dispersion in the polymer matrix and decreases the mechanical strength and barrier properties of the resulting composites. The high hydrophilicity of the fibers results in high adsorption of water vapor, which decreases their oxygen barrier properties and limits their use in the presence of moist foods (Ben Azouz et al. 2012;Hubbe et al. 2017;Vilarinho et al. 2018). Therefore, modification of cellulose fibers is recommended for food packaging applications. Appropriate modification of cellulose nanomaterials should improve one or more properties of the final product, including oxygen and water vapor barrier properties, mechanical strength, and thermal stability. In this regard, physical and chemical modifications have been performed to improve the compatibility of cellulose fibers with hydrophobic polymeric matrixes, decrease their hydrophilicity, and increase their thermal stability (Kargarzadeh et al. 2018;Vilarinho et al. 2018). Any improvement in polymer-fiber interfacial adhesion results in better dispersion in the polymer matrix, improves oxygen and water vapor barrier properties, and enhances the mechanical strength of the final food packaging material. Hence, modification of cellulose fibers for use in food packaging applications presents new horizons for developing sustainable, green, and biodegradable materials, and may be a milestone in the journey of building an environment without plastic wastes.
Although the modification of cellulose nanomaterials has been reported in literature, previous reviews have been limited to reinforcing applications of cellulose nanomaterials, but the importance of these modifications for improving barrier properties has not been discussed. Even though most review articles discuss the mechanisms and effects of the physical and chemical modifications on the mechanical properties in detail, they do not provide the comparative results of the mechanical tests. The purpose of this review was to discuss the effects of different physical and chemical treatments of cellulose fibers on the most important parameters for food packaging applications, namely the oxygen and water vapor barrier, water uptake, mechanical strength, and the transparency of the final composite materials. In this review, the structure of plant and cellulose fibers and their preparation methods are explained briefly. This review aimed to briefly explain the principles behind different cellulose modification methods and the mechanisms of the reactions involved in the treatments. The importance of the oxygen and water vapor barrier ability and the mechanical strength of food packaging materials are discussed. The effects of different treatments on the water vapor and oxygen barrier properties and mechanical strength are also compared.

Plant fiber and cellulose nanoparticles
Plant fibers can be classified depending on the part they are obtained from, such as straw fibers (corn, rice, and wheat), bast fibers (flax, hemp, jute, kenaf, and ramie), wood fibers (softwood and hardwood), leaf fibers (abaca and sisal), grass fibers (bagasse and bamboo), and seed fibers (coir, cotton, and kapok) (Azeredo et al. 2017;Kim & Netravali 2010a, Mohit & Arul Mozhi Selvan 2018Zhang et al. 2020). Depending on the plant type, extraction method, and the place in which the plant is cultivated, fibers are composed of varying amounts of cellulose, hemicellulose, lignin, pectin, waxes, and inorganic compounds (ash). The physicochemical properties of plant fibers are influenced by the species, chemical composition, crystallinity, microfibril angle, and presence of defects (Kim & Netravali 2010bLee et al. 2017;Zhang et al. 2020). The hierarchical microstructure of plant fibers and the chemical structure of cellulose is shown in Fig. 1.
Inter-and intra-hydrogen bonding, as a special feature of cellulose, play a key role in most characteristics of this polysaccharide, such as low solubility, high crystallinity, and low compatibility with other non-polar polymers (Habibi 2014;Islam et al. 2018;Vilarinho et al. 2018). Plant fibers contain various amounts of cellulose embedded as microfibrils in a matrix of hemicellulose and pectin. Cellulose microfibrils possess crystalline and amorphous regions. Tight interchain hydrogen bonding between a bundle of cellulose chains creates a crystalline region of microfibrils, which is inaccessible for most solvents and reagents (Lee et al. 2017;Mohit & Arul Mozhi Selvan 2018;Vilarinho et al. 2018) (Fig.1). Different mechanical, chemical, and biological treatments on plant fibers can produce various micro and nanosized cellulosic products, namely microcrystalline cellulose (MCC), cellulose nanofibrils (CNFs), and cellulose nanocrystals (CNCs) (Kargarzadeh et al. 2018;Lee et al. 2017;Vilarinho et al. 2018). Fig. 1 shows the flowchart of the production of MCC, CNCs, and CNFs from plant fibers as well as schematic representations and SEM images of these structures. To avoid ambiguity about the different terms used in cellulose technology, standard terms and appropriate definitions have been proposed by the Technical Association of the Pulp and Paper Industry (TAPPI) ( Fig. 1) (Kargarzadeh et al. 2018).

Oxygen barrier properties
Lipid oxidation is responsible for most foodstuff deterioration (Lu et al. 2018). Further, the presence of oxygen in the food environment increases the growth rate of bacteria and unwanted harmful reactions that accelerate food spoilage. Hence, besides protecting food from physical damage and contact with dust and other pollutants, food packaging should protect food from deterioration reactions by preventing oxygen transfer between the interior part of the packaging and surrounding atmosphere (Arrieta et al. 2017). In terms of oxygen barrier properties, based on a rough classification suggested by Wang et al. (2018), polymeric packaging materials can be classified into five categories, namely poor, low, medium, high, and very high. The most common non-biodegradable and biodegradable polymers have poor oxygen barrier properties that barely meet the requirements for packaging foodstuffs like fresh meat, cheese, and instant coffee. Although some biodegradable polymers like polyvinyl alcohol (PVA) have high oxygen barrier properties (due to their hydrogen bonding) in dry conditions, their high water absorption capacity leads to a significant drop in oxygen barrier properties in moist conditions, which restricts their use for most food applications (Wang et al. 2018). High oxygen barrier cellulose nanomaterials ( Fig. 1) have been used to make composite films using impermeable commercial polymers like PVC (polyvinyl chloride) and PVA (polyvinyl alcohol), and have also been used as coatings for low oxygen barrier films (Fig. 2) Vilarinho et al. 2018;Wang et al. 2018). However, when used as a thin layer, the high hydrophilicity of cellulose nanomaterials restricts their use for dry foods and in dry conditions. Due to the abundance of hydroxyl groups on the surface of cellulose nanomaterials, their homogeneous dispersion in the polymer matrix, which is crucial for effective oxygen barrier action, is a challenge. Low  (Pereira et al. 2015), b cellulose fibrils embedded in hemicellulose and lignin matrix, adapted from (Lee et al. 2017), c chemical structure of cellulose, d inter-and intramolecular hydrogen bonding, adapted from (Islam et al. 2018), and e production of cellulosic reinforcing materials from plant fibers (Kargarzadeh et al. 2018;Trache et al. 2016). SEM images are reprinted with permission from (Abu-Thabit et al. 2020;Calvino et al. 2020;Lee et al. 2017). compatibility with packaging materials results in low interfacial adhesion between polymer and filler and creates cavities for the diffusion of oxygen molecules. Moreover, the adsorption of water results in cellulose fibers losing their oxygen barrier ability. In order to take full advantage of these fibers as fillers by improving the oxygen barrier properties of polymers, the following issues must be addressed: (i) the high tendency to agglomerate (due to their surface hydroxyl groups), (ii) low compatibility with commercial packaging materials, and (iii) high hydrophilicity and water absorption capacity (Dufresne 2017

Water vapor permeability and water uptake
The transfer of water vapor between food and the surrounding environment plays a crucial role in the shelf life of food products. In the case of fresh foods like vegetables and meat, it is important to avoid dehydration of the product, whereas dry foods like bread must be protected from the humidity of the air (Wang et al. 2018). Water vapor permeability (WVP) refers to the amount of water that transfers across the film per unit of area and time. As the thickness of the film and the water vapor pressure difference across the film are important factors in the amount of transmitted water, the water vapor flux must be normalized for these two variables. Hence, WVP can be expressed as where WVTR is water vapor transmission rate (g/ m 2 Áday), x is the thickness of the film (m) and DP is the differential vapor pressure of water across the film (Pa). The improvement of barrier properties associated with the use of cellulose fibers as a filler is ascribed to decreased diffusion of gases by creating a more tortuous path across the film Hubbe et al. 2017;Wang et al. 2018). The use of cellulosic fibers in various matrices from water-soluble polymers, like PVA (Pereira et al. 2014), to hydrophobic commercial polymers, like PLA (Espino-Pérez et al. 2018), has been reported to decrease the WVP. However, the water sensitivity of films, which refers to the resistance of the film against water vapor, preserving its structure and avoiding swelling, can be affected by the presence of cellulose fillers. For most commercial hydrophobic polymers, water uptake can be a reliable test to evaluate the resistance of films against water. In terms of food freshness and shelf life, water vapor permeability is an important characteristic, especially for the packaging of wet food products, such as meat and vegetables (Azeredo et al. 2017). Although cellulose is not a water-soluble polymer and most of its hydroxyl groups are involved in intra-and inter-molecular interactions in the crystalline regions, the surface hydroxyl groups and those groups in the amorphous region are able to hold a large amount of water. As the volume of cellulose or plant fibers increases as water is absorbed, the shape of the produced composites may change. Further, evaporation of adsorbed water at high temperatures or in dry weather leads to the shrinkage of the reinforcing material and detachment between the filler and host polymer, resulting in a loss of mechanical strength. Furthermore, adsorbed water molecules can loosen the inter-molecular interactions between cellulose chains by disassociating the hydrogen bonding among them. Hence, the adsorption of water in a humid environment results in the plasticization of composites containing cellulose fibers and leads to a significant increase in oxygen permeability .
Thus, to preserve the oxygen and water vapor barrier properties as well as the dimensional stability and mechanical strength of food packaging, especially in a humid environment, and to avoid increasing the weight of the composite, modification of cellulose fibers should be carried out. Improvements in the interfacial interaction between the host polymer and fiber and decreased hydrophilicity of the incorporated fibers can enhance the barrier properties (Espino-Pérez et al. 2018) and decrease the swelling and water uptake (Chuayplod and Aht-Ong 2018). Non-bonding modification of fibers with a hydrophobic material to shield the fiber, or reaction of surface hydroxyl groups with a coupling agent to reduce the accessible hydroxyl groups for adsorbing moisture, are approaches to overcome this problem. If a chemical bond between the filler and polymer can be formed through a coupling agent, the oxygen and water vapor barrier and mechanical properties of the prepared composite can be improved in both dry and wet conditions (Mishra et al. 2001). The results of modifications carried out to reduce the water uptake are shown in Table 1.

Mechanical strength
Tensile properties are the most studied aspect of composite films prepared using cellulose fibers. The high strength and stiffness of cellulose fibers and their high aspect ratio make them good candidates for reinforcing purposes. Besides, lightweightedness, transparency, and biodegradability are advantages of cellulose fibers in comparison with their traditional counterparts, such as glass and carbon fibers. Food packaging must have enough mechanical strength to resist handling damages and protect food items from damage during distribution. Hence, mechanical strength is one of the most important aspects of food packaging materials (Yu et al. 2014). Recently the application of cellulose fibers for improving the mechanical properties of food packaging materials, like PLA (Shojaeiarani et al. 2019) and other petroleum-based thermoset polymers, like polyethylene (PE) (Ferreira et al. 2019) and polypropylene (PP) (Kahavita et al. 2019), has gained much attention. Despite the many advantages of cellulosic fibers as a reinforcing filler, they suffer from several serious drawbacks, including low compatibility with commercial polymers, low dispersibility in commonly used non-polar matrices (like PLA), and high moisture absorption, which leads to the loss of mechanical properties with time. The low compatibility of cellulose nanoparticles with most commercial food packaging polymers results in low interfacial adhesion between the filler and the polymer, and diminishes the stress transfer from the polymer to the cellulose filler, weakening the mechanical strength of the prepared composites. The detachment of the cellulose fillers and polymer matrices increases accessibility to surface hydroxyl groups for the adsorption of moisture and leads to the loss of mechanical strength and barrier properties, which may in turn affect food quality. Thus, physical and chemical modifications that improve the compatibility of the cellulose filler and polymer matrix enhance the mechanical and barrier properties of the packaging materials and increase the shelf life of foods (Mohit & Arul Mozhi Selvan 2018;Vilarinho et al. 2018;Zhang et al. 2020). A comparison of the effects of these modifications in terms of tensile strength, Young's modulus, elongation at break, and tensile strain are shown in Table 2.

Physical modification of cellulose fibers
In this review, physical methods refer to those treatments that do not use any chemical substance to induce a change or modification on the surface of the fiber. These methods include plasma and corona discharges, gamma irradiation, and application of UV light. As physical modification techniques do not consume hazardous chemicals, they represent ecofriendlier methods (Ragoubi et al. 2012).

Heat treatment
In this method, cellulosic fibers are heated in a convection oven to remove impurities, increase the oxygen/carbon ratio, and enhance the acidity of the cellulose surface. Heat treatment increases the  crystallinity of the fiber and enhances its mechanical properties (Cao et al. 2007). Heat treatment reportedly decreases porosity and oxygen permeability of a CNF film (Xia et al. 2018). Sharma et al. (2014) used heat treatment to increase both the barrier and mechanical properties of a CNF film.

Plasma treatment
Plasma contains a mixture of reactive species including free radicals, electrons, and heavy particles that could be used for the modification of fibers or the surface of polymers and films. The most commonly used types of plasma for surface modification of fibers are low pressure, low temperature, atmospheric glow discharge (AGD), and atmospheric pressure plasma jet (APPJ) (Kalia et al. 2013). The type of gas, gas pressure and flow rate, and the applied power, and treatment time are variables that can be controlled to induce the desired effect on the surface of the polymer or fiber. However, due to the risk of degradation of the cellulosic material in the plasma environment, it must be applied in a controlled manner (Sarikanat et al. 2016). Due to the high incompatibility between the microfabricated cellulose and PLA, activation of the polymer surface by atmospheric plasma was performed to fabricate a fully biodegradable bilayer system for food packaging applications (Meriçer et al. 2016). Lu and co-workers (Lu et al. 2018) used cold plasma for coating biaxially-oriented polypropylene/ low-density polyethylene (BOPP/LDPE) with a layer of nanofibrillated cellulose to improve oxygen barrier properties of the packaging material.

Corona modification
Applying high voltage to a bundle of sharp tip electrodes produces a corona in which the plasma is formed only on the tips. The corona treatment is one of the most interesting methods for surface oxidation activation of cellulose fibers used in polyolefin composites (Mohit & Arul Mozhi Selvan 2018). In this method, both the cellulose filler and matrix polymer can be modified using the corona treatment, although the modification of the filler generates more pronounced effects. Corona treatment using air increases the oxygen/carbon ratio on the surface of the fibers and enhances their polarity and wettability. Also, the etching effect of the corona treatment enhances the mechanical anchorage and interfacial adhesion between the filler and the matrix polymer and leads to improved mechanical and barrier properties (Ragoubi et al. 2010).
UV modification UV radiation enhances the formation of carbonyl groups on the surface of wood and natural fibers, thus improving the fiber surface polarity (Mohit & Arul Mozhi Selvan 2018). Moreover, UV treatment can also lead to crosslinking between the neighboring cellulose molecules, enhancing the strength and barrier properties of the treated fiber (Zaman et al. 2009).

Gamma irradiation
High-energy gamma irradiation is a suitable treatment to induce physicochemical changes, such as crosslinking, decomposition, and unsaturation, thereby strengthening and hardening the polymeric material (Mohit & Arul Mozhi Selvan 2018). Cellulose can interact with gamma radiation through the Compton scattering phenomenon, in which trapped macrocellulosic radicals are formed due to the rapid localization of energy within cellulose. The creation of active sites on the surface of the cellulose fiber enhances bonding between the fiber and the matrix (Zaman et al. 2010). For instance, Khan et al. (2013) used gamma irradiation to improve both the mechanical and barrier properties of poly(caprolactone)/nanocrystalline cellulose.

Chemical modification of cellulose fibers
Here, chemical modification refers to methods that use one or more chemical substances to change the physicochemical properties of cellulose particles. As shown in Fig. 1, in each cellobiose moiety of cellulose, there are two secondary (C 2 and C 3 ) and one primary alcohol groups, which could be subjected to different chemical reactions, including oxidation and substitution (Kargarzadeh et al. 2018). Treatment of cellulosic fibers using chemicals can be classified into two categories, one that involves the chemical bonding between matrix and filler and one that does not. However, both methods can improve the mechanical and barrier properties of the composites.

TEMPO-mediated oxidation
The goal of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-mediated oxidation of cellulose is to introduce aldehyde and carboxyl functional groups to the surface of cellulose fibers to improve their mechanical properties. In this method, sodium hypochlorite is added to aqueous cellulose suspensions at pH 10-11 in the presence of TEMPO and sodium bromide. The reaction is carried out at room temperature (Fig. 3a). It is believed that the aldehyde groups can form a covalent bond with cationic resin and enhance the final properties of the composite. The formation of a covalent bond between microfibrils through hemiacetal and acetal linkages improves the mechanical strength of fibers and decreases their water absorption capacity. The possible mechanism behind this phenomenon is shown in Fig. 3b (Lal & Mhaske 2019). Besides the improvement of the mechanical properties of cellulose fibers in free-standing CNF and CNC films, the creation of carboxylic groups on the surface of fibers can enhance the mechanical and barrier properties of the composite by reacting with their functional groups (Lai et al. 2014). Sirviö et al. (2014) used TEMPO-mediated oxidized CNCs to prepare free-standing oxygen barrier films.

Periodate oxidation
Oxidation of hydroxyl groups to aldehyde can be carried out using periodate. As in the TEMPOmediated oxidized cellulose, aldehyde groups can induce crosslinking between the cellulose chains and improve the barrier properties of the film. Cellulose was converted to dialdehyde with lithium chloride (LiCl)-assisted sodium meta-periodate (NaIO 4 ) oxidation by Visanko et al. (2015). These researchers used the oxidized cellulose to prepare butylaminofunctionalized cellulose nanocrystal films with excellent oxygen barrier ability at high relative humidity. Periodate was also used to make self-cross-linked fibrillated cellulose to improve the barrier and water uptake properties of CNF films (Larsson et al. 2013). Plappert et al. (2018) used periodate for the synthesis of 2,3-dialdehyde cellulose (DAC) with a high degree of oxidation to prepare a very hydrophobic cellulose film with remarkable oxygen barrier properties even at high relative humidity.

Coupling agents
Coupling agents are defined as materials that can form a covalent bond with the fiber and at the same time can connect to the polymer matrix (Abushammala & Mao 2019). In this way, they can improve the interfacial adhesion between the filler and polymer matrix, enhance stress transfer, and improve both mechanical and barrier properties (Mohit & Arul Mozhi Selvan 2018). Due to the presence of a high density of hydroxyl groups, which can be subjected to various substitution reactions, the use of coupling agents is one of the most adopted and studied approaches for improving polymer-filler interfacial interaction (Dufresne 2017;Habibi 2014). Coupling agents can also improve the service time of composite materials. In fact, without using coupling agents, many highperformance plastic products would not exist. In the case of composites containing cellulose fiber, this modification can reduce the water uptake of the composite and increase the service time in the wet conditions of the food environment, as well as preserve the oxygen barrier activity of cellulose nanomaterials. Although different classes of coupling agents have been introduced, most only provide shortterm improvement. Organo-silanes (R-Si-(OR) 3 ) are the most effective, commercially available, and lowcost coupling agents. They are widely used to modify surface properties of inorganic fillers and their durability has been proven. Silane possesses hydrolysable groups that enable attachment to the surface of glass or cellulose fibers, and an organic portion (R) which is responsible for improving compatibility between the filler and polymer matrix and decreasing hydrophilicity of cellulose fibers. Coupling agents may contain some highly reactive functional groups, such as amine, vinyl, mercapto, methacryloxy, and polysulfide, that can bond with the host polymer for improved stress transfer (Thakur et al. 2014). In the presence of moisture, silane reacts with surface cellulosic hydroxyl groups and attaches to the fiber through a covalent bond (Fig. 3c). Coupling agents also improve the interfacial adhesion between fiber and polymer, and they can impart adequate functional groups onto the surface of fibers to participate in polymer chain growth and provide an active site for polymer grafting reactions (Demir et al. 2006). Selulosa-Polivinilklorida et al. (2015) proposed a reaction (Fig. 3d) between the amino silane-modified cellulose fiber and polyvinyl chloride. Amino silane-modified cellulosic fibers can also react with the carbonyl group of some polymers, like polycarbonates, through an amidation reaction and form an amide covalent bond, as displayed in Fig. 3e (Threepopnatkul et al. 2009). Radical species that are generated on the surface of polyolefins due to the thermal oxidation and subsequent peroxide decomposition can react with the p bond of silane and form a covalent bond with the modified fiber (Fig. 3f) (Abdelmouleh et al. 2007). In an attempt to reduce the WVTR of PLA/bacterial cellulose, Somord et al. modified bacterial cellulose using 3-(trimethoxysilyl) propyl methacrylate (Somord et al. 2018), resulting in enhanced water vapor barrier properties. As previously mentioned, coupling agents are not limited to silane compounds, and different chemicals can provide different functional groups to connect with both the cellulosic filler and polymer matrix (Virtanen et al. 2014).  (Le Gars et al. 2020), reaction of fiber with k maleated PE and l acrylic acid-functionalized PE (Rout et al. 2001), and m benzoylation of fibers (Sreekumar et al. 2009).

Esterification
Acetylation is a well-studied esterification reaction of cellulose fibers. Fibers become more hydrophobic when acetylated due to the involvement of surface hydroxyl groups, thus their resistance against humidity increases. For this reaction, cellulose fibers are treated with acetic anhydride in the presence of a catalyst (Fig. 3g). Acetylation enhances the interfacial adhesion between the fiber and the polymer matrix and decreases water uptake of the prepared composite. This modification improves the mechanical properties and resistance of the final composite against moisture (Shojaeiarani et al. 2019). An esterification reaction was used to modify CNCs with phenylacetic acid and hydrocinnamic acid. The modified CNCs improved the water vapor and oxygen barrier properties of PLA/ CNC films (Espino-Pérez et al. 2018). An esterification reaction with a diacid can lead to the crosslinking of cellulose fibers. Crosslinking a nanocellulose film with citric acid significantly reduced the oxygen and water vapor permeability (Herrera et al. 2017). Sehaqui et al. (2014) used an esterification reaction to decrease the hydrophilicity and moisture uptake of a CNF film.

Modification using isocyanates
Aliphatic and aromatic mono and di-isocyanates have been used for the modification of cellulose for a long time, as they can act as a surface modifier and conjugating material to graft a variety of molecules and polymers to the surface of cellulosic materials. Depending on the type of required modification, mono or di-isocyanate can be employed through one of the scenarios represented in Fig. 3. Mono-isocyanates are generally used to hydrophobize cellulose fibers, improve their compatibility with nonpolar thermoplastics, like PLA, and increase the mechanical and water vapor permeability (WVP) of the resulting films (Espino-Pérez et al. 2013) (Fig. 3h). Di-isocyanates can be used to bind functional groups onto the nanocellulose surface or to protect it during the processing of thermoset matrices. In this case, one of the isocyanates reacts with nanocellulose and the other reacts with a functional polymer (Fig. 3i). Using isocyanates has resulted in substantial improvements in the interfacial adhesion and mechanical properties of composites (Pinheiro et al. 2017).

Polymer grafting
The modification of cellulose and plant fibers using small coupling agents and other methods may not result in the expected improvement in mechanical and barrier properties. Hence, the grafting of a long chain polymer for improving barrier and mechanical properties has been investigated. Two main strategies, called ''grafting from'' and ''grafting to'' have been employed to graft a modifying polymer to the surface of fibers (Saedi et al. 2014;Xu et al. 2014).
In the ''grafting from'' strategy, the surface of fibers is pre-functionalized with an initiator (e.g., radicals) in the presence of a monomer, the polymerization reaction takes place, and the final polymer forms on the surface of the fiber. In the ''grafting to'' strategy, a previously synthesized polymer is grafted to the fiber surface directly (Le Gars et al. 2020). For these approaches, various methods, such as ring-opening polymerization (ROP) (Fang et al. 2019) and atom transfer radical polymerization (ATRP) (Fig. 3j) (Le Gars et al. 2020), have been employed. The ROP method was used by Fang et al. (2019) to graft LA onto the surface of CNCs to improve oxygen resistance and mechanical properties of polyurethane films. Ambrosio-Martín et al. (2015) modified bacterial cellulose nanowhiskers (BCNWs) by grafting lactic acid oligomers to improve the mechanical properties and oxygen and water barrier ability of PLA/BCNW composite films. In another study, the mechanical and oxygen barrier properties of PLA/BCNW composite films were improved by grafting poly(glycidyl methacrylate) (PGMA) onto BCNWs (Martínez-Sanz et al. 2013). The free radical polymerization strategy was used by Song et al. (2014) to graft butyl acrylate onto CNFs to make a PLA/CNF composite film with high water vapor barrier properties. Malleated polyolefins are widely used to modify the surface of cellulosic fibers for use as reinforcement in nonpolar matrices. The reaction between cellulose and malleated (Fig. 3k) and acrylic acid-modified polyolefins (Fig. 3l) like PP and PE is a common way to graft polyolefins onto cellulose fibers (Rout et al. 2001).

Miscellaneous
In addition to the above-mentioned chemicals and methods, improvements of the mechanical, thermal, water uptake, and barrier properties of cellulose fibers using other chemicals, such as surfactants (Fortunati et al. 2012), permanganate (Sreekumar et al. 2009), and amino silicone oil (He et al. 2015), have been reported. Arrieta et al. (2014) and Fortunati et al. (2012) used surfactants to modify cellulose nanocrystals (CNCs) to improve mechanical, and water vapor and oxygen barrier properties of PLA/CNC nanocomposite films. Benzoylation of cellulose fibers can decrease the thickness of fibers, and the introduction of a benzyl ring to the surface of a fiber can also improve the interfacial adhesion. The presence of a benzyl ring on the surface of a fiber increases the ability of the fiber to connect with active sites of the polymer matrix ( Fig. 3m) (Sreekumar et al. 2009).

Modifications for increasing heat resistance
Cellulose fibers show disadvantages, including poor compatibility with most synthetic polymers, high moisture absorption capacity, and low thermal stability. As the molding temperature of most thermoplastics is higher than the decomposition temperature range of cellulosic fibers (170-200°C), the molding process of composites containing cellulose and plant fibers leads to the formation of tar-like pyrolysis products due to the presence of resin acids and hydrolysable acetyl groups of cellulose (Sain et al. 1993). In the case of CNCs, it is believed that the presence of sulfate groups, formed during the preparation of CNCs using sulfuric acid, is responsible for the low thermal stability of these nanoparticles (Dufresne 2017). Hence, there have been many efforts to solve the problem of low thermal stability of CNCs. Treating CNCs with sodium hydroxide or hydrochloric acid can improve their thermal stability. However, acid treatment worsens the dispersion of CNCs in the polymer matrix. However, surface modification of CNCs can shield sulfate groups and improve thermal stability. Coating CNCs with hydrophilic polymers, like polyethylenoxide (PEO) (Fig. 4) (Ben Azouz et al. 2012), modification with coupling agents (De Menezes et al. 2009), and modification with thermostable boron compounds (Sain et al. 1993) can be used effectively to overcome the low thermal stability and at the same time improve the dispersibility of cellulose fillers in hydrophobic matrices, thus enhancing the mechanical and barrier properties of the composites.

Conclusion and future perspective
As more non-biodegradable materials and non-renewable resources are replaced with biodegradable and renewable ones, the use of cellulose fibers as reinforcing and barrier materials increases. These fibers present new horizons of sustainable development, because they can be produced from fruit and vegetable wastes and agricultural by-products. However, the drawbacks, such as low compatibility with most commercial polymers, high water uptake, and low thermal stability, must be addressed. As shown in this review, the oxygen and moisture barrier properties of PLA, PE, and PP, which are extensively used for food packaging applications, have been improved using cellulose nanomaterials. Thus, much effort has been dedicated towards enhancing the compatibility between cellulose nanomaterials and these polymers, and to improve the mechanical and oxygen and water barrier properties of their composites. Various physical and chemical methods have been developed to improve the mechanical and barrier properties of composites prepared using cellulose fibers. As physical treatments do not use hazardous chemicals, they are the eco-friendliest options. However, application of these methods for modification of cellulose fibers is restricted due to their limitations and minor effects on the fiber surface. Heat treatment is the most widely used physical method for improving the barrier properties of free-standing and bilayer films prepared from cellulose nanomaterials. This method can improve the oxygen barrier properties of cellulose nanomaterials up to 80%. Oxidation of surface hydroxyl groups of cellulose nanomaterials using periodate and TEMPO are common chemical modifications that could be used to improve the oxygen barrier and mechanical properties of both free-standing and composite films of cellulose nanomaterials. The surface hydroxyl groups of cellulose nanoparticles provide active sites for various types of reactions and attaching various molecules and polymers. Esterification, polymer grafting, reaction with mono-and di-isocyanates, and reaction with silane compounds have been extensively employed to modify the properties of cellulose nanomaterials, improve their interaction with host polymers, and enhance the barrier and mechanical properties of the obtained composites. Coupling agents that possess two reactive sites and can connect to both cellulose nanoparticles and host polymers result in additional improvements in mechanical and barrier properties.
Although promising improvements in mechanical, water uptake, and barrier properties were reported in many studies, the long-term applicability and cost for industrial-scale applications of these composites have been ignored. From a practical point of view, silanecoupling agents seem to be an appropriate modifying material, as they are commercially available, can connect fibers and polymer matrix using a covalent bond, and guarantee a long-term improvement in the polymer-fiber interfacial adhesion, mechanical strength, and barrier properties. Hence, the effectiveness of any modification technique should be proved not only in the laboratory but also on an industrial scale.
The most studied aspect of cellulose fibers is their use as reinforcing materials. Considering that improvements in mechanical strength result from improvements in the interfacial interaction between the polymer and cellulose fiber, the effectiveness of this strategy on the barrier properties of the fiber and water resistance of the composites should also be evaluated. Cellulose nanomaterials are suitable candidates for improving oxygen barrier properties of biodegradable plastics like PLA and polyhydroxyalkanoates (PHAs) for food packaging applications. However, problems associated with their use, like low compatibility, high hydrophilicity, and low thermal stability, should be addressed using novel costeffective chemical and physical modification methods.

Declarations
Conflict of interest The authors declare no potential conflicts of interest.
Ethical approval No animal or human studies were carried out by the authors.