Engineered nanomaterials for papermaking industry

Chapter

9

Engineered nanomaterials for papermaking industry Pieter Samyn* , Ahmed Barhoum**,† *Institute for Materials Research (IMO), Hasselt University, Diepenbeek, Belgium; **Vrije Universiteit Brussel (VUB), Brussels, Belgium; †Helwan University, Helwan, Cairo, Egypt

1 INTRODUCTION Paper is a network structure of cellulosic fibers formed when a very dilute aqueous suspension of the separated fibers flows on to a very fine wire mesh so that the water drains through, leaving the fibers to settle together into a felted layer. Most papers also contain a number of nonfibrous materials, such as china clay and rosin size to impart new properties, or to improve the existing properties of paper. Paper has played a vital role in the cultural development of mankind. It still has a key role in communication and needed in many other areas of our society. Paper includes a wide range of products with very different applications such as communication, cultural, educational, artistic, hygienic, sanitary, as well as storage and transportation of all kinds of goods. It is almost impossible to imagine a life without paper. Paper has been an essential part of our civilization for at least two thousand years. In recent years, papers have been transformed into multifunctional materials that can serve a variety of technological sectors: for example, microfluidic papers, flexible electronics, diagnostic sensors, energy devices, medical care… The base material is derived from natural fibers, which are the structural cells of plants originating principally from a wide variety of sources that have morphological, as well as physical and chemical complexities. Wood provides about 93% of the world’s virgin fiber requirement, while the other sources (nonwood) contribute the remainder. Papermaking is a vast, multidisciplinary technology that has expanded tremendously made in all areas of papermaking, including raw materials, production technology, process control and end products. Broadly, there are two fundamental steps in paper production. The first step is isolating

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the cellulose fiber from the raw material source, removing any unwanted materials (lignin and hemicellulose) and converting the fibers into suitable form for papermaking. This involves beating or refining the fiber, a process during which the cell walls are fibrillated to produce many microfibrils. These increase the number of contact points between neighboring fibers and enhances paper strength. The second step is interweaving the fibers into a finished mat of material, which constitutes paper. Overall, commercial pulping processes include: (1) mechanical (ground wood); (2) chemical (bleached or unbleached); (3) semichemical (combining mechanical and chemical treatments); and (4) thermomechanical (wood preheated with stream). Each is used alone or in combination to manufacture the vast numbers of papers and paperboards available. Filler materials are additionally used in papermaking in order to enhance to processing or enhance specific properties of the paper sheet. The nonfibrous additives to paper stock can be classified into two groups: 1. Functional additives: these additives are components of paper and modify its properties, such as: a. Fillers or pigments for increasing opacity, brightness, and printability of paper. b. Sizing Agents used for controlling water penetration in the body of paper. c. Dry-strength additives, as starches, gums, and resins for increasing strength. d. Wet strength resins for increasing strength when wet. 2. Control additives: these additives are used to affect the performance of stock at the wet-end of the paper machine as: a. Retention aids for controlling the retention of the fillers with the paper sheet. b. Drainage aids for accelerating the water drainage and drying of paper. c. Pitch control agents for controlling the pitch and assist their removal. d. Deformers for hindering the foam formation in pulping process. e. Bacteriocides and slimicides or microbicides for inhibiting bacterial growth. The fillers in the paper sheet partly adhere to the fiber surface and are partly held by electrostatic forces, but it is also retained mechanically in the open space within the fiber network. The relative magnitude of these forces will vary based on the type of pulp, other chemicals used and numerous other factors. However, these forces are not strong enough to hold the filler immobilized in the sheet when it is exposed to the hydraulic forces required to

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give a high drainage rate. Most papers containing fillers are two-sided in the sense that thereby contains more pigment on the felt or topside than on the wire side. This may be a serious disadvantage for the user, especially about the printing properties that are dependent on the amount of filler located at or near the paper surface. At wet-end machine, retention aids are chemical additives letting tiny particles aggregate through electrostatic forces and form lager units (flocculation), which are then bound uniformly to the fibers. Cationic polyacrylamide (C-PAM) is the dominant polymer used in retention aid systems. Such systems take advantage of bridging flocculation mechanism. In order to have a good retention, high solid content suspensions, high sheet grammage, and high degree of beating also improve the filler retention. The interest in nanotechnology for papermaking industry has strongly increased in the latest decade in order to improve the properties of existing paper products or create new functionalities to papers. The use of nanotechnology in papermaking may serve as a tool for further enhancing the sustainability of papermaking processes and products by, for example (1) more efficient use of resources: stronger papers can be formed with lower base paper weight, (2) valorization of side-stream materials: recycled fibers or fibers with inferior properties can be converted into strong nanofibers. The application and production of nanofibers, nanofilleres, nanopigments, nanoadditives to the paper industry allow for the improvement to the properties of the produced papers, encompassing mechanical, printability, glossiness, and gas barrier properties. The fabrication of nanostructured paper-based materials may be achieved by top-down approaches (e.g., fibrillation) or bottom-up approaches (electrospinning). Another range of nanomaterials for papermaking include nanoparticles (NPs) and nanostructured materials (NSMs) used as nanofillers in order to create some unique functions, such as magnetic, photocatalytic, flame retardant, antibacterial, deodorizing, electrically conductive, and thermal buffering properties, is highlighted. In this chapter, a broad overview of nanomaterials derived from paper products or nanofillers used during papermaking is given. The recent advances in producing nanocellulose as microfibrillated cellulose or cellulose nanowhiskers are discussed, together with the preparation routes for a range of inorganic and organic nanofillers. The electrospinning of paper constituents is an alternative way for the creation of nanostructured materials. Some novel trends include the in situ modification of nanofibers and nanofillers in order to tune their surface properties. However, most prominent challenges remain associated with the definition of suitable processing conditions, which are evaluated in terms of rheology and retention

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properties of nanomaterials. The benefits of nanofillers and nanopigments to create several functionalities to the paper bulk or surface are shortly discussed.

2 NANOCELLULOSE 2.1  Micro- and nanofibrillated cellulose The hierarchical structure of native cellulose fibers with an arrangement of fibrils into the cell wall allows disintegrating the fibrous structure into nanoscale morphologies including either single or bundles of elementary fibrils. Production of cellulose microfibrils (CMF) or cellulose nanofibrils (CNF) by fibrillation of cellulose plant cell fibers requires the mechanical isolation of individual fibrils by processes of grinding, cryocrushing or high-pressure homogenization. During processing of native pulp fibers (Fig. 9.1A), the longitudinal section of cellulose fibers is subjected to microfibrillation by refining, where some fibril bundles are partially released onto the internal and external fiber surfaces (Fig. 9.1B). Otherwise, the fibers are fully fibrillated into a fine mesh of nanoscale fibers during the nanofibrillation by grinding, resulted in fiber diameters of 5–100 nm (Fig. 9.1C). [1].

■■FIGURE 9.1  Microfibrillation and nanofibrillation of cellulose fibers via mechanical treatment: (A) bleached cellulose pulp fiber, (B) microfibrillation using refiner; and (C) nanofibrillation using disk-grinding process. Source: E. Afra, H. Yousefi, M.M. Hadilam and T. Nishino, Comparative effect of mechanical beating and nanofibrillation of cellulose on paper properties made from bagasse and softwood pulps, Carbohydr. Polym. 97, 2013, 725–730.

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During mechanical treatment of a wood fiber suspension, the repeated cyclic stresses induced by a rotating and stationary disc result in the disintegration of the fibers into the microfibrillar components. The discs contain different groove configurations to guide the flow of the fibers and are made of resin with hard silicon carbide materials. The main advantage of processing with the microgrinder is that the mechanical fiber shortening pretreatment utilized with other processing techniques may not be required. In cryo-crushing, the water contained in the pulp suspension is frozen in liquid nitrogen and the fibrils are consequently liberated by a high-impact load. However, cryo-crushing is not suited to produce very fine fibrils and remains limited to cellulose fibrils from primary cell walls [2]. The homogenization is most commonly applied for processing of the cellulose fibers, which are subjected to sharp pressure drops and impact forces inducing high internal shear in the fiber structure. The processing over 10–20 cycles with a pressure drop of about 55 MPa typically results in microfibrillated cellulose fibers (MFC). The more severe conditions under 5–20 numbers of passes combined with pressures of 55–210 MPa typically results into the production of nanofibrillated cellulose fibers (NFC) with diameters in the range of 10–100 nm and the length of few hundred nanometers to some microns.

2.2  Cellulose nanowhiskers and nanoparticles Another form of cellulose consists of microcrystalline cellulose (MCC) as a white particulate powder of fibrous material with particle size of approximately 40 µm, obtained by partial depolymerization of native cellulose by acid hydrolysis, for example, using HCl [3]. The hydrolysis is usually carried out until the leveling-off degree of polymerization (DP) is reached [4]. The MCC can be further transformed into nanoscale fibers under more severe acid hydrolysis conditions. The cellulose nanowhiskers (CNW) or cellulose nanocrystals (CNC) contain highly crystalline and stiff needle-like fibers with relatively low aspect ratio, typically having a diameter of 2–20 nm and length of 100–600 nm [5]. The concentrated acids allow production of CNW in yields ranging from 20% to 40 %, while the use of diluted acids provides lower yields. While using the mildly acidic aqueous ionic liquids, relatively high yields of about 48 % could be achieved [6]. The lower solvating power of the aqueous ionic liquid compared to that of concentrated sulfuric acid likely contributes to the greater hydrolysis efficiency. After optimization of a two-step hydrolysis with mildly acidic ionic liquid (IL) 1-buty1-3-methylimidazolium hydrogen sulfate, CNWs were produced in near theoretical yield levels from bleached softwood kraft pulp, bleached hardwood kraft pulp, and microcrystalline cellulose [7]. The latter technique also allows extracting CNWs directly

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■■FIGURE 9.2  Example of morphologies for cellulose nanostructures [5], (A) cellulose nanocrystals, (B) cellulose nanoparticles [10].

from lignocellulosic biomass (Figure 9.1B) due to the capability of the ionic liquid to for simultaneous delignification, defibrillation, hydrolysis, and derivatization of cellulose from wood [8]. This technique was also used in combination with a steam explosion as a pretreatment to enhance lignocellulose accessibility [9]: in a typical steam explosion process, the lignocellulose is first soaked in a dilute aqueous alkaline or acidic solution and then steamed under pressure until the sudden release of the pressure. This route for extraction of CNW in dimensions close to their native state in wood benefits from easy processing without the need for purification/dialysis compared to traditional routes. The cellulose nanoparticles (CNPs) with spherical shapes can be produced by weak acid hydrolysis and sonication [10]: the obtained cellulose particles were of cellulose II polymorph, there was also a tendency for the cellulose crystallinity index to increase as the particle sizes became smaller (Fig. 9.2B). The CNPs can also be prepared through enzymatic hydrolysis and ultrasonication treatment of fibers [11]. The crystallinity and thermal stability of the particles did not significantly change while the degree of polymerization decreased. The regenerated cellulose nanoparticles (RCNs) including both elongated fiber and spherical structures were prepared from 1-butyl-3-methylimidazolium chloride followed by high-pressure homogenization [12].

3  MICRO- AND NANOFILLERS FOR PAPERMAKING The addition of internal fillers at the wet-end section (in the paper bulk) and pigments in the coating section (at the paper surface) is now a very common practice. Fillers that are dominantly applied in the paper industry are

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mostly minerals. Fillers are added to paper in various percentages, typically between 10% and 20%, to filling in the voids around fiber areas crossing. The paper fillers are fine, white pigment powders and most important nonfibrous materials. They are manufactured from natural minerals or synthetically from various raw materials. The purposes of using fillers are: 1. to fill out spaces between fibers, thus causes smoothening the surface and improving, e.g., evenness of formation, printability, paper opacity, dimensional stability, and gloss. The brightness is also usually improved. Their use also leads to better printability. 2. to cut manufacturing costs because most fillers are cheaper than fibers. The use of fillers saves up to 40% fiber material, enhancing the economic position of the paper industry. The addition of fillers facilitates drainage and drying of the formed sheets. Exceptions are specialty pigments, which are expensive and added only in small amounts to achieve certain paper properties. In addition, they improve surface smoothness and gloss of paper. Fillers are generally white pigments that can be divided into two major categories, including: (1) regular fillers: these have wide application and cost lower than that of cellulosic fiber, for example, clay, talc, ground calcium carbonate (GCC), precipitated calcium carbonate (PCC); (2) specialty fillers: these usually have lower volume applications and costs are sometimes comparable with or even higher than cellulosic fiber as TiO2, ZnO, and composite pigments, for example, clay, titanium oxide, and synthetic silica– silica oxides. The application of fillers also improves dimensional stability, paper’s appearance, and so on. However, there are disadvantages due to the use of too much filler about 20%. It is generally considered that the use of inorganic fillers, especially at high loading levels, has the following disadvantages or limitations: (1) Reduction of paper strength: the replacement of the fibers by inorganic fillers reduces the area of contact and bonding between the remaining fibers, thus increasing the dusting tendency. (2) Reduction of sizing efficiency and filler retention: the high filler loading results in higher solids content of the circulating system and reduction of sizing efficiency and filler retention; and (3) Increase of abrasion and dusting: the abrasive characteristic of mineral fillers increase wear to the wire on the paper machine and to the printing plates in the printing press, which leads to higher fine material content in the circulating water system. Dispersing of fillers in papermaking systems needs to be improved under certain conditions, and can be done by surface modification [13]. The microsized fillers have unwanted effects on paper and papermaking. These drawbacks relate to the loss of bonding between fibers. In practice,

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this is evidenced by low stiffness, linting in printing. Additionally, abrasion on the paper machine can increase. Although the importance of filler in improving the paper properties, paper machine parts (wire, foils, rolls, ceramics) and printing cylinders are worn by paper fillers. Typically, not only the hardness of the mineral itself is significant, but also the particle size and particle shape. Furthermore, even a very small amount of large (>20 microns) particles are detrimental. Sharp particle edges, of course, are abrasive. Even a low content of hard impurities, such as quartz is very abrasive. Therefore, the need for having fillers in nanosize is so important to avoid the detrimental effects. As nanofillers in at least one dimension could provide better performances than traditional fillers in applications such as adsorption, which can benefit from a high surface area; in such applications they may confer certain unique attributes and functionalities to papers, which is also one of the major concerns of papermakers when considering the use of nanofillers. The nanofilled paper could also have such attributes as high smoothness, good appearance, or even “smart” characteristics, which includes possible new security functions, superior potential for information storage, or other previously unimagined functions. However, certain challenges are associated with nanofillers, such as high cost, difficulty in structure and performance control, poor dispersibility and retention, possible severe negative effects on paper strength, possible detrimental interactions between nanofillers with some wet end additives, and the industry-related limitations. Also, it is fairly evident that the industrial use of nanofillers in papermaking is now only limited to a certain degree. However, the benefits associated with the use of nanofillers should encourage worldwide papermakers to pay attention to this potentially promising research area, and make every effort to create positive breakthroughs [14]. The nanofillers enhance paper properties and enable using lower filler without the negative effect on the tensile strength. However, little is known in the literature about the application of nanofillers in papermaking. Commercially available fillers (clay, talk, GCC, and PCC) with specifically desired properties are directly added to furnish before the wet web formation of papers, with the goal of achieving paper properties, such as light scattering for opacity and brightness. The microfillers also can be combined with nanostructures for the fabrication of nanostructured composite fillers. For example, the development of nanostructured PCC fillers (consisting of zinc-based nanostructures combined with scalenohedral CaCO3 particles) and the novel fillers have been claimed to be able to confer excellent optical properties to papers. The most common types of nanofillers widely expected used in papermaking are as follows: nanocelluloses (nanofibers, nanowiskers,

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nanoparticles), nanotalc, nanoclay, nanokaolin, nanoCaCO3, nanoSiO2, nanoTiO2, and nanoZnO. The incorporation of fillers as nanofibrous structures with high aspect ratio (2D and 3D) may allow increasing the filler content up to 50% as they are homogeneously dispersed. The refractive indices of the TiO2 nanofibers fall within a range of 1.6 to 1.7 comparable to other pigments (Talc, CaCO3, and Clay) and have an average brightness of 95% [15]. The silicate-based nanofibers have been studied to increase the opacity in some base sheets by replacing pigment fillers [16]. Depending on cooking conditions, the size, shape, and structure of the silicate particles can be modified. Therefore, the silicate particles not only affect opacity but can be used to increase the bulking or glossing properties of the base sheet. The packing of nonisotropic, rod-like nanofiber particles is rather nonuniform. The type of silicate particle that affects opacity the most is a long, fibrous, nanoparticle. The production of this type of silicate nanofiber involves the hydrothermal reaction of silica and lime under high temperature and pressure conditions. The diameters of these types of pigment particles can range from 50 to 200 nm and the length can range from 1 to 4 microns, resulting in aspect ratio from 1:10 to 1:50 [17]. Nowadays, several of the used methods for incorporation of nanofillers in paper products include the electrostatic assembly of metal nanoparticles directly onto the cationic surfaces of cellulose, or the adsorption of negative metal complex ions onto the cationic cellulose followed by a reduction reaction [18]. The electrostatic self-assembly allows to create composite papers by subsequent absorption of oppositely charged layers consisting of, for example, quaternized chitosan (positively charged) and graphene oxide sheets that are loaded with gold nanoparticles (negatively charged) (Fig. 9.3) [19]. Similarly, the silver nanoparticles have been deposited from a solution of Ag+ in distilled water mixed with agarose at 80°C: a visible transformation of silver ions into silver nanoparticles formed on the agar coated filter paper was noted from the color change and can be attributed to the reducing nature of agar [20]. In another research, the in situ generation of silver nanoparticles onto bacterial cellulose nanopapers was achieved through the reduction of adsorbed silver ions by the hydroxyl groups of cellulose nanofibers, acting as the reducing agent producing a nanocomposite with embedded nanoparticles [21]. After deposition of the silver nanoparticles onto bacterial cellulose from an ammonia solution, the uniform spherical nanoparticles (10–30 nm) were generated and self-assembled on the surface of cellulose nanofibers, forming a stable and evenly distributed Ag nanoparticles coated cellulose nanofiber: it is envisaged that guest molecules of Tollens’ reagent diffused into the inner spaces of cellulose nanopores and

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■■FIGURE 9.3  Electrostatic assembly of oppositely charged layers onto cellulose fibers and preparation of cellulose composite paper [19].

reacted with the fibers, while the nanofiber network played a role of reactive template. During particle deposition, Ag(NH3)2OH, as a weak oxidant, was likely to react with the hydroxyl groups and CH2OH was firstly oxidized to aldehyde group (CH=O), which could further react with Ag(NH3)2OH [22]. Another type of Fe2O3 nanofillers are incorporated in a nanofibrillated cellulose network and are immobilized through surface interactions between hydroxyl groups (Fig. 9.4) [23].

4  ELECTROSPINNING OF PAPER-BASED NANOFIBERS In parallel with the processing of synthetic nanofibers, the electrospinning technique has been frequently used to produce biopolymer nanofibers that can be applied as functional papers with enhanced properties such as barrier properties or filtration. Different types of biopolymers used for electrospinning have been reviewed [24], including proteins [25], polysaccharides

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■■FIGURE 9.4  Immobilization of Fe3O4 NPs in the 3D nanofibrillated cellulose network through interactions between carboxyl groups/hydroxyl groups of NFC and surface hydroxyl groups of Fe3O4 NPs [23].

[26], lignin [27], and DNA [28]. However, most biopolymers are difficult to dissolve in water and/or organic solvents and tend to agglomerate because of their complex molecular structures. In order to obtain uniform nanofiber structures without beads, the creation of a suitable electrospinning solution with appropriate viscosity is a most challenging task and requires the addition of a coadjutant polymer to improve the mechanical properties of the nanofibers or supplementary addition of salts to increase the conductivity or surfactants to reduce the surface tension. The processes with several solvent systems such as NMMO/H2O, LiCL/DMAc, trifluoroacetic acid and ionic liquids have been developed starting from pure cellulose. However, the removal of solvents is often a main difficulty and has resulted in the more favorable processing of cellulose derivate. A short review on the processing of polysaccharides, cellulose, and chitosan, and their derivatives, including cellulose acetate, ethyl cellulose, and hydroxypropyl cellulose has been published before [29]. Starting from a solution of kraft pulp and N-methylmorpholine-N-oxide (NMMO), regenerated micro- and nanofibers of cellulose with high strength were produced [30]: advantageously, the lower concentration, higher nozzle temperature and higher voltage would result in smaller fiber diameters with a

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lower probability of including defects and smaller flaw sizes, thereby increasing the mechanical properties. The nanofibers have also been produced from a combination of wood pulp, acetylated wood pulp and polyethylene oxide, indicating that the structure of electrospun composite fibers became less crystalline after spinning while the acetylated wood was well dispersed and oriented along the length of composite fibers [31]. Other sources for electrospun cellulose nanofibers include recycled cotton waste that has been converted into cotton balls, yarns and cotton batting using trifluoroacetic acid as the solvent [32]. The lignocellulosic sisal fibers and sisal pulp could be processed at room temperature from solutions in trifluoroacetic acid (TFA): despite the ability of TFA to esterify the hydroxyl groups, the spectroscopic observations indicated the absence of trifluoroacetyl groups in the electrospun samples [33]. In general, nanofibers could be obtained from various biomass in combination with alkali-treatment and ionic liquid: the lower lignin content resulted in better spinnability, fine and uniform fiber diameter, higher crystallinity, and decrease of thermal stability [34]. Traditionally, the electrospinning can be performed by using spinning solutions with a cellulose concentration of 3 wt. % and 6 wt. % in NMMO [35]. For a given solvent, the spinning procedure starting from pure cellulose can be performed under ambient conditions at room temperature without post-spun treatment [36]. The properties and microstructure of nanofibers obtained by electrospinning of cellulose solutions depend on effects of solvent system, the degree of polymerization of cellulose, spinning conditions, and postspinning treatment such as coagulation with water: for example, cellulose fibers obtained from LiCl/DMAc are mostly amorphous, whereas the degree of crystallinity of cellulose fibers from NMMO/water can be controlled by various process conditions including spinning temperature, flow rate, and distance between the nozzle and collector [37]. The nonwoven fibers of cellulose could be created by electrospinning of cellulose in a nonvolatile ionic liquid, 1-butyl-3-methylimidazolium chloride (BMIMCl), using a syringe contained in a constant-temperature chamber because of the high melting point of the ionic liquid [38]. The ionic liquid precursor N-methylimidazole proved to be a promising cosolvent candidate and was thus selected for further studies together with the ionic liquid 1-ethyl-3-methylimidazolium acetate [39]. Alternatively, the electrospinnability of native cellulose in room temperature ionic liquids was achieved by using in a highly efficient 1-allyl3-methylimidazolium chloride [40]. The electrospinning of cellulose from ionic liquids has been further optimized by adding various cosolvents, such as dimethylacetamide (DMAc), dimethyl formamide (DMF), and dimethyl sulfoxide (DMSO), in order to decreased the surface tension, viscosity and entanglement density of the network and increase the conductivity of the spinning dope, which contributed to a continuous jet [41]. Comparing to

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■■FIGURE 9.5  Cellulose nanofibers obtained by electrospinning from different solvent systems with ionic liquids: (A) IL/DMF; (B) IL/DMAc [42].

DMAc, DMF showed more significant influence on the fiber diameter and the crystallinity [42]. The solubility of cellulose in ionic liquids is highly affected by changes in solvent properties on the molecular level in the binary solvent systems. The cellulose fibers spun from binary solvents exhibited significantly higher crystallinity than the fibers from neat ionic liquid, where good the average and the standard deviation of the fiber diameter in the fiberweb prepared from the solution with DMF were significantly small comparing to the one with DMAc (Fig. 9.5). The electrospinning of cellulose can be facilitated starting from derivates, such as cellulose acetate, hydroxypropyl cellulose, hydroxypropyl methyl cellulose, or ethyl-cyanoethyl cellulose, which can be dissolved in suitable volatile solvents [43]. The cellulose esters can be dissolved in nonpolar solvents such as acetone, dichloromethane, chloroform and methyl acetate. A new solvent system for the electrospinning of cellulose acetate nanofibers, a mixed solvent of acetic acid/water (75/25) was developed, yielding long uniform nanofibers with an average diameter of 180 nm from a 17 wt. % CA solution [44]. The quality of electospun fibers can be controlled by changing the collector plate, indicating that cellulose acetate fibers electrospun with trifluoroacetic acid onto a polypropylene nonwoven material had poorer properties compared to an aluminum foil as collector plate [45]. By using ethyl-cellulose, the electrospinning was not enhanced due to a low viscosity below a 6 wt. % concentration in 2,2,2-trifluoroethanol, while the morphology was changed from a beaded fiber to a uniform fiber structure with larger fiber diameter at the higher concentrations [46]. After derivatization of trimethylsilyl cellulose with given degree of substitution, needleless electrospinning was used to create different cellulose structures depending

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on the source, e.g., the cellulose fibrous networks were made from spruce pulp or hollow half spheres were produced from viscose pulp [47]. The electrospun cellulose substrates can be used as a model substrate for further functionalization or enzymatic hydrolysis [48]. However, the poor mechanical performance and selection of solvent systems for electrospun cellulose fibers are often a limitation. Alternatively to the electrospinning, porous nonwoven membranes based on nanofibrillated cellulose (NFC) modified by a hydroxyethyl cellulose (HEC) polymer coating were prepared from an aqueous suspension that is subjected to simple vacuum filtration in a paper-making fashion, followed by supercritical drying. After postdrawing of the random mats, the modulus and strength are strongly increased into stretchable and strong cellulose nanopaper structures with good mechanical properties [49]. Other constituents of paper-based materials such as hemicelluloses have been used for electrospinning after free radical polymerization with acrylic acid [50]. The xylan fibers have been electrospun in combination with polyvinyl alcohol and could be crosslinked to further control its properties, such as wettability, presence of functional groups and mechanical properties [51]. The electrospinning of pectin and pullulan blends could be performed without use of a synthetic carrier polymer or nonaqueous solvents, resulting in molecular entanglement between both components and improved spinnability (Liu et al., 2016). The electrospinning of lignin has received considerable interest in order to create alternatives for synthetic carbon fibers. Depending on the type of lignin used, different concentrations and types of solvents such as ethanol, dimethylformamide, or water are required to prepare the electrospinning solution with optimum viscosity. The anionically charged sodium carbonate lignin was mixed with polyethylene oxide (PEO) to enhance the electrospinning performance by improving the viscoelastic properties [52]. The interactions between lignin and PEO in alkaline aqueous solutions create association complexes, which increases the viscosity of the solution. The effect of polymer concentration, PEO molecular weight, and storage time of the solution before spinning on the morphology of the fibers was studied: it showed that after one day the viscosity dropped and fiber diameter decreased [53]. The sulfur-free softwood lignin behaves as rigid spheres, instead of free draining macromolecules in DMF and generate uniform fibers only at the higher volume fractions in order to create the required viscoelasticity [54]. In general, a clear transition from electrospray or beaded fibers to uniform fibers was observed upon addition of poly(ethylene oxide) for all of technical lignin [55]. The preparation of lignocellulosic nanofibers from biomass other than pulp or cotton

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has been demonstrated by using akali-treatment and electrospinning in presence of an ionic liquid, where presence of lignin often reduced spinnability [34]. It remains challenging to blend lignin with other polymers due to its brittle nature and poor dispersion in many composites. In order to improve the miscibility and compatibility of lignin with other plastics, a series of poly(methyl methacrylate) (PMMA) grafted lignin copolymers were prepared from atom transfer radical polymerization. The lignin mass fractions in the copolymers varied from 5.6% to 46.1%. These lignin–PMMA copolymers were further blended with poly(ε-caprolactone) (PCL) and engineered into nanofibrous composites by electrospinning [56]. Other reinforcement of the lignin nanofibers was created by adding up to 15% cellulose nanocrystals, resulting in better thermal stability due to the strong interaction of the lignin-PVA matrix with the dispersed CNCs, mainly via hydrogen bonding [57]. Using nanoscale thermal analyses, the continuous phase was determined to be lignin-rich and the discontinuous phase had a lignin/PVA dispersion. Importantly, the size of the phase-separated domains was reduced by the addition of CNCs [58]. After electrospinning the solutions of lignin/ethanol/platinum acetyl acetonate and lignin/ethanol, the carbonization process decreased the oxygen content of the fibers and increased the carbon content to produce a well-developed microporous carbon nanofiber structure [59]. Alternatively to electrospinning, the lignin nanfibers were produced by a process involving the rapid freezing of an aqueous lignin solution, followed by sublimation of the resultant ice (Fig. 9.6B), to form a uniform network comprised of individual interconnected lignin nanofibers, followed by carbonization [60].

■■FIGURE 9.6  Lignin nanofibers obtained by different techniques, including (A) electrospinning [53], (B) freezing from aqueous solution [60].

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For several applications, the electrospinning allows to create paper-like materials from a very diverse range of materials. The direct electrospinning of fibers into controllable three-dimensional (3D) architectures deposited onto paper allows the design and controlled fabrication of electrospun 3D structures such as grids, walls, hollow cylinders, or three-dimensional logos: an element central to the success of 3D electrospinning is the use of a printing paper placed on the grounded conductive plate and acting as a fiber collector: once deposited on the paper, residual solvents from nearfield electrospun fibers can infiltrate the paper substrate, enhancing the charge transfer between the deposited fibers and the ground plate via the fibrous network within the paper [61]. The paper-based materials prepared from aromatic polyamide fiber by traditional paper making methods have problems that the fiber dispersing performance is poor, while electrospun aromatic polyamide fiber paper-based material has excellent mechanical performance and is more stable [62]. Other electrospun paper-like materials composed of sponge-like silicon nanofibers coated with a carbon layer have been used for lithium-ion batteries [63]. The hybrid poly(vinylidene fluoride)-titanium dioxide nanofiber mats have been prepared by a combination of electrospinning and electrospraying and can be used as photocatalytic paper [64]. The electrospinning of PHB fibers in combination with a polymethacylate coating offers the potential for fabrication of paper-based biosensors, which can be employed for immobilization of the dengue antibody and subsequent detection of dengue enveloped virus in enzyme-linked immunosorbent assay [65]. In general, the electrospinning technique can be used for the deposition of nanoscale coatings on paper or single cellulosic fibers [66].

5  ORGANIC NANOADDITIVES FOR PAPERMAKING Besides traditional organic materials that are used as additives in papermaking, a specific range of bio-based additives has gained increasing interest in recent years, in particular the nanoparticles of starch (Fig. 9.7A) and chitosan (Fig. 9.7B). The biodegradable natural polymer, such as starch nanocrystals or nanoparticles have good potential for use in papermaking surface sizing, paper coating, and paperboard for substitution of petroleum based fillers, adhesives, and binders. Starch granules have been transformed into nanoparticles by a coextrusion process of starch feedstock together with glycerol as a plasticizer, and glyoxal as a crosslinker, and can be used as a paper binder [69]. Traditionally, the starch-based spherical nanoparticles (Fig. 9.7A) could be obtained via nanoprecipitation [67], microemulsion [70], or using a dropwise solvent exchange method controlling the morphology by selection of various solvent and nonsolvent systems [71], or. The starch nanoparticles

5 Organic nanoadditives for papermaking 261

■■FIGURE 9.7  Biopolymers used under nanoparticle form as organic nanoadditive, (A) starch nanoparticles [67], (B) chitosan nanoparticles [68].

can be formed through a reactive extrusion process, where a very highsolids starch paste is converted into a thermoplastic melt phase by solubilization, followed by the crosslinking and sizing of the solubilized starch molecules into nanoparticles [72]. The synthesis routes and properties of starch nanoparticles have been fully reviewed before [73,74], and can be achieved though enzymatic hydrolysis, regeneration and mechanical treatments. The starch could also be grafted on cellulose fibers through the hydrogen bond formation among cellulose, starch, and ammonium zirconium (IV) carbonate, which affects water drainage, water removal in the wet web press and drying section, and paper properties [75], especially the improvements of water retention value were also demonstrated. The starch nanoparticles prepared from cooked cornstarch gel with ethanol and reacted with diethylenetriamine pentaacetic acid, were complexed with chitosan as part of a general chemical strategy to improve their incorporation into a corrugated containerboard matrix and increase interfiber bonding: this approach provides a uniquely renewable and useful approach to enhance dry strength of pulp while maintaining environmental compatibility, industrial compatibility, and paper quality [76]. The chitosan is another bio-based additive known for its antifungal and antibacterial properties [77], with excellent grease and oxygen barrier properties [78,79]. The chitosan nanoparticles have been prepared following the emulsion method, ionic gelation method [80], reverse micellar method and self-assembling methods [81]. The particle size, particle formation and aggregation is directly affected by molecular weight and degree of deacetylation [82]. By tuning the parameters of ultrasonic emulsification and coalescence, the chitosan micro- and nanospheres could be prepared by precipitation from a W/O emulsion [83]. The ultrasonication for a longer

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duration or higher amplitude decreased the mean diameter and polydispersity of the nanoparticles in parallel with a greater disarray of chain alignment in the nanoparticle matrix [84]. Usually, chitosan nanoparticles are prepared through an ionic gelation between positively charged chitosan dissolved in acetic acid and negatively charged sodium tripolyphosphate anions in water, providing a better mechanical strength when applied as a paper coating due to the increasing interfibrillar bonding by diffusion of the particles into the pores of the paper [85]. Depending on the pH of the solution, concentration, ratios of components and mixing method, the interactions can be controlled through the charge density of tripolyphosphate and chitosan. As such, the relationships between free amino groups on the surface and the characteristics of chitosan nanoparticles prepared ionic gelation were studied [86]. Alternatively, ultrafine nanoparticles with a narrow size distribution can be prepared as reverse micelles. The latter are formulated by dissolving a surfactant in an organic solvent, while adding an aqueous solution of chitosan. The control over particle size and size distribution is enhanced for the lower molar mass of chitosan, probably as a result of either a reduction in the viscosity of the internal aqueous phase or an increase in the disentanglement of the polymer chains by reversed emulsification [87]. The smooth chitosan nanoparticles with diameter of approximately 36 nm were obtained by peroxide degradation of chitosan in HAc solutions applied in the paper bulk for antibacterial properties [88]. The combination of chitosan nanoparticles (Fig. 9.7B) with nanofibrillated cellulose formed a nanocomposite coating on paper, which enhances barrier properties and particularly higher strength due to presence of chitosan [68]. Other biopolymers have been converted into nanoparticles for use as additive in papermaking. The poly(hydroxybutyrate) or PHB was deposited as particles with a micro- to nanoscale structure on the surface of cellulose fibers by using a phase-separation process including dedicated solvents [89]: as a disadvantage, the properties of the paper fibers gradually reduced due to the requirements for long immersion times in an ethanol/water coagulation bath. Therefore, the synthesis microscale particles with internal nanoscale morphologies and submicron-sized particles of PHB have been further optimized by the selection of other solvents and they have applied onto paper by a simple dip-coating process [90]. Due to the thermodynamic instability, the nonsolvent diffused into the PHB-solvent system and PHB transformed into microparticles with an internal nanoscale structure (Fig. 9.8A) or PHB nanoparticles (Fig. 9.8B), which were applied as hydrophobic paper coating. The generation of such particle structures during a one-step process, has also been implemented for synthesis of structured poly(l-lactic acid) (PLLA) particles with particular topography of micro- to nanoscale binary structures [91].

5 Organic nanoadditives for papermaking 263

■■FIGURE 9.8  Polyhydroxybutyrate (PHB) particles with micro- to nanoscale structures [90], (A) internally-structured PHB microparticles, (B) nonstructured PHB nanoparticles.

As a valorization route for residues obtained the pulping process, nanostructures of lignin have been created. The solution structures of lignin were proposed as one of the key elements in controlling lignin nano-/ microparticle preparation [92]. The lignin nanoparticles have been synthesized by various techniques including, for example, sonication, precipitation, CO2 saturation, continuous solvent exchange, dialysis, and water-in-oil microemulsion-based methods. After sonication an aqueous suspensions of lignin, the average particle diameter was reduced from 1–10 µm into 10–50 nm by a combination of two main reaction patterns causing side chain cleavage/depolymerization and oxidative coupling/ polymerization, respectively [93]. Using nanoprecipitation method, dioxane lignin nanoparticles (DLNP) and alkali lignin nanoparticles (ALNP) were fabricated in spherical shape with mean size of 80 to 104 nm [94]. The flash precipitation of dissolved Kraft lignin and organosolv lignin enabled the formation of nanoparticles in the size range of 45–250 nm, which can be further modified by coating their surface with a cationic polyelectrolyte [95]: the precipitation was done either by (1) adding the nitric acid into an ethylene glycol solution of lignin, or (2) adding water into an acetone solution of lignin, which resulted into phase separation in the form of lignin nanoparticles. Depending on the precipitation conditions, the stability of the nanoparticles in a given pH range can be tunes and density or porosity of the lignin domains can be varied [96]. The spherical lignin nanoparticles were also obtained by dissolving softwood kraft lignin in tetrahydrofuran and subsequently introducing water into the system through dialysis, where the surface charge of the particles could be reversed and stable cationic lignin nanoparticles were produced

264 CHAPTER 9  Engineered nanomaterials for papermaking industry

by adsorption of poly(diallyldimethylammonium chloride) [97]. The nanoparticles of lignin derivatives containing high hydroxyl group content were obtained by a chemical method directly modifying the lignin by hydroxymethylation under different conditions of lignin/aldehyde ratio, pH, and temperature [98]. A novel generation of poly(styrene-co-maleimide) nanoparticles has been introduced as hydophobizing agents that can be used as internal sizing or coating pigments. The organic nanoparticles have been synthesized by an imidization reaction of the poly(styrene-co-maleic anhydride) copolymers in presence of ammonium hydroxide, resulting in a stable aqueous suspension at pH > 4, with 35 wt. % solid contents and appropriate viscosity. The nanoparticles are formed by self-organization of the copolymer into spherical units of about 100 nm diameter [99], and their properties can be tuned by appropriate selection of the synthesis conditions. Advantageously, they possess high glass transition temperatures of 170–190°C and therefore do not have tendency for softening during further processing. These materials were applied as a coating onto paper and paperboard surfaces to study the chemical and morphological properties in relation with good ink reception [100]. The nanoparticles could also be synthesized in presence of vegetable oils, including soy oil, sunflower oil, corn oil, castor oil, rapeseed oil, or hydrogenated oil [101] and then form smaller particles of 20–50 nm diameter: at a maximum limit, up to 70 wt. % vegetable oils could be incorporated into the nanoparticles and the solid content in aqueous suspensions could be increased towards 65 wt. %. In particular, the reactivity and interactions between the imidization of nanoparticles and the oil encapsulation highly depends on the type of oils that is used, where poly-unsaturated oils such as soy oil [102], have higher reactivity compared to the saturated types of oil such as palm oil [103]. As such, the morphology of the nanoparticles also depends on the type of oil and they can either be a pure core-shell structure or they can form rather porous nanostructures. After synthesis, the nanoparticle structures have been applied as reservoirs for controlled thermal release of the encapsulated oil [104]. The in-situ deposition of organic nanoparticles of poly(styrene-comaleimide) onto micro- and nanofibrillated cellulose has also be realized in order to tune the hydrophobicity of the cellulose fibers (Fig. 9.9). Depending on the original morphology of the fibrillated cellulose fibers, the reaction conditions should be adapted in order to achieve permanent bonding between the nanoparticles and fibers: the surface modification was mainly governed by the fiber diameter, surface charges and amount of wax [105].

6 Processing of nanomaterials in papermaking 265

■■FIGURE 9.9  In-situ deposition of poly(styrene-co-maleimide) nanoparticles on MFC and NFC, (A) unmodified MFC, (B) modified MFC, (C) unmodified NFC, (D) modified NFC [105].

6  PROCESSING OF NANOMATERIALS IN PAPERMAKING The knowledge on rheological properties of nanoparticles as papermaking additives and their flow behavior in process equipment is crucial to optimize the coating, pumping or mixing of the nanomaterials in the papermaking process. The processing and rheological properties of fibrillated cellulose suspensions have been largely studied and can be quite complex, as reviewed before [106]. The composition of the fibrils and the homogeneity of the fibril size distribution will largely affect the rheological properties of MFC/NFC suspensions: in particular, the conditions of homogenization and degree of fibrillation will affect the viscosity [107]. Different types of pretreatments for the MFC by either beating under enzymatic conditions or carboxymethylation influence the visco-elastic properties and creep behavior [108]: the latter mainly influence the degree of

266 CHAPTER 9  Engineered nanomaterials for papermaking industry

fibrillation and eventually induce surface charges. The rheological behavior and the dynamic oscillatory rheological behavior of paper coatings, an increase in solid content of paper coatings reinforces the interaction between particles and the inner structure of the suspensions, in turn resulting in a considerable increase in yield stress, thus making the coatings difficult to flow [109]. The steady shear rheological results indicate that the paper coatings with nanoTiO2 typically exhibited a pseudoplastic fluid behavior as characterized by obvious shear thinning. The viscosity of the nanoTiO2 with 30 wt. % solid content could be reduced after surface modification with carboxyl zircoaluminates, resulting in better coating properties [110]. Compared to the hydrophilic unmodified nano TiO2, modified nanoTiO2 could contribute more to the viscosity of paper coatings leading to a higher dynamic elastic storage modulus and viscous loss modulus (Liu et al., 2016). Furthermore, the paper coatings with nanoparticle pigments displayed a solid-like elastic behavior with significant increase in storage and loss moduli at the higher levels of nanoparticle pigments, which implies that the incorporation of nanoparticle pigments can impart a great amount of elastic and viscous energy to paper coatings. In combination with chitosan as paper coating, the presence of TiO2 nanoparticles led to the decreased state viscosity and dynamic viscoelasticity, which may facilitate the production and industrial application of high-solid content coating. The effect of the TiO2 nanoparticles is more pronounced at low shear rates and the relative effect diminishes with increasing shear rates due to shear thinning [111]. However, the maximum solid content of inorganic colloids is often limited by aggregation of the nanoparticles induced by flocculation and collapse of the dispersed state [112]. The rheology of nanoclay (montmorillonite) suspensions indicated that the strength and dynamic parameters of the coagulation structure highly depend on the nature of the introduced surfactants [113]: the nonionic surfactants did not result in significant change in the rheological properties, while the anion surfactants strongly interacted with the clay particles resulting in a strong decrease of the shear viscosity. [114]. Within a dispersion of organic solvents, the nanoclay showed a gel-like behavior with a final structure depending on the applied shear stress [115], while a reduction in the gel stability occurred at higher inorganic content. The retention of fine particles in the wet section of the papermaking process is a result of the aggregation and adsorption on the fiber surfaces in order to get bound into the paperweb. The utilization of nanosilica in combination with cationic polyelectrolytes can have positive effects on improving the retention of fines and the rate of drainage. The anionic colloidal silica particles have been used traditionally to enhance the dewatering

6 Processing of nanomaterials in papermaking 267

and retention properties [116], although the interaction mechanisms of nanoparticles responsible for retention and drainage in pulp processing are not yet fully understood and can be attributed to several effects. The anionic nanosilica may possibly neutralize the starch cationic charges by penetrating in the amorphous structure, thereby compressing the double layer. The very strong association of fine materials with the fibers occurring through microflocculation (i.e., aggregation of fillers through a high-molecular weight cationic polyelectrolyte) provide improvements in drainage and retention. The other proposed mechanisms consider an interchange of the weak hydrogen bonds with strong ionic bonds leading to the contraction and collapse of flocs [117]. The interactions between nanoparticles and cationic polyelectrolyte molecules in the wet-end may be considered as a bridging mechanism that involves the complexation between the polyelectrolyte and the nanoparticles, leading to semireversible bridging and contraction. It has been proposed that the incorporation of nanoparticles into a network of cationic starch/fines/fibers converts the wet-end fiber mat into a new porous structure with better retention and drainage [118]. The concentrations in the dual system of cationic starch and nanosilica have been tuned [119]: however, the interactions between cationic starch and nanosilica and resultant zeta potential changes do not have an easily predictable effect on drainage and retention. Because by adding more nanoparticles, the retention and discharge capacities increase, while zeta potential deviates from the iso-electric point. The incorporation of bio-based by-products originating from sugar, i.e. molasses that contain nanoadditives (sucrose) and gums (starch), was proven to act as a strength promoting retention aid for inorganic fillers such as kaolinite [120]: it is assumed that the nanoscale structure of sucrose-swollen fibers is manipulated to avoid collapse, which provides smaller pore sizes during paper formation and better retention of fillers. Due to the high specific surface area and hydrophilicity of MFC, it has a strong water retention property and addition into a given pulp will reduce dewatering by increasing the hydraulic pressure. The water retaining behaviour and swelling into a gel structure has recently been studied in paper coating formulations as a replacement for the traditional water retention aid/thickener function of polymers, such as carboxymethyl cellulose or polyacrylic salts [121]. The dewatering of a new type of MFC based composite with 70 wt. % precipitated calcium carbonate was shown to be better than traditional softwood based fibers, as the high amount of filler in the structure does not contribute to the binding of water and helps to provide channels for water removal [122]. A key element in the manufacturing paradigm is the use of high consistency suspensions to improve retention and minimize the need for water removal after forming [123].

268 CHAPTER 9  Engineered nanomaterials for papermaking industry

7  NANOFUNCTIONAL PAPER When the paper was fabricated using cellulose nanofibers, the nanofiber paper produced some unique performance characteristics, including high optical transparency, low thermal expansion, high smoothness, and enhanced barrier properties, which could not be achieved using traditional microsized pulp papers. A significant number of studies were devoted to the creation of specialty papers by the application of nanopigments in papermaking, including protected paper [124], low gas permeable paper [114], transparent paper [125], hydrophobic paper [126], photocatalytic paper [127], antimicrobial paper [128] conductive paper [129], magnetic paper [130], and printed electronic paper [131]. Nanofillers and nanocoating pigments are important to control the paper surface properties to prevent problems occurring within the printed layer. The paper makers use the nonfibrous raw materials (fillers) to impart new properties and to modify and improve the existing properties of paper, such as: j

j

j

j

j

j

Paper whiteness and opacity: the nanofillers and nanocoating pigments, such as nanoPCC, nanoTiO2, and nanoZnO have high refractive indices and reduce the transmission of light by refraction. Paper transparency: the nanofibrillated cellulose and cellulose nanowhiskers has been used for making transparent nanopaper with good mechanical properties and possibilities for functionalization [132] Gas permeability: the nanocellulose and mineral fillers, such as nanocaly provide good oxygen gas barrier resistance, where the oxygen molecules penetrated much more slowly within nanofibrillated cellulose films due to the higher fibril entanglements [133]. Paper smoothness: the coatings with nanocelluloses (CNWs) and mineral nanofillers (nanoclay, and nanoPCC) will affect the surface characteristics after calendaring the paper surface and improve paper smoothness. Paper strength: when coated with nanoSiO2, the fiber-to-fiber bonding and thus paper strength improves [134]. Nanofibrillated cellulose nanofibers with a small size and high surface area and flexibility may naturally increase the strength of the network analogous to what can be achieved by using cellulosic fines. Electrical conductivity: carbon based nanomaterials such as graphene and carbon nanotubes in addition to metal nanoparticles such as (Ag and Cu NPs) have been used to develop electrically conductive papers for printed electronics [135].

8 Conclusions and outlook 269

j

j

j

Magnetic papers: Fe2O3 nanofillers or nanocoating pigments can be used to produce magnetic papers [130]. Photocatalytic activity and antibacterial activity: functions such as photocatalytic and antibacterial papers can be developed by surface functionalization of paper with only a very small concentration of nanoTiO2, nanoZnO, nanoAg, and Au NPs [127,128]. Other ingredients such as nanosizing agents and colouring nanomaterials (nanoprinting inks) contribute towards the quality, printability and general characteristics of the resulting papers and paperboards.

8  CONCLUSIONS AND OUTLOOK A broad variety of engineered nanomaterials has been developed in recent years to serve the needs of paper industry in terms of improving resource utilization, sustainability, better control on paper processing and creating novel functionalities to paper substrates. The cellulosic nanomaterials may be produced directly from lignocellulosic biomass and are available under different morphologies (micro- and nanofibrillated cellulose, cellulose nanowhiskers, cellulose nanoparticles), or they may be created artificially by electrospinning offering possibilities for the processing of cellulose, lignin and hemicelluloses. The nanosized materials have been applied as internal fillers comprising both inorganic and organic nanoparticles. In particular, advances in the synthesis of bio-based nanomaterials and their applicability in either bulk or surface coating of papers processes have been highlighted. In parallel with the synthesis of nanoparticles, dedicated application methods have been developed to ensure good interactions between the nanoparticles and the paperweb structure, including, for example, electrostatic assembly and in situ deposition. However, the processes for favorable dispersion of nanomaterials and interactions between nanoparticles and paper fibers should be carefully controlled and needs good understanding of the physico-chemical and rheological characteristics. While the opportunities for creation of new functionalities in paper products have been demonstrated, first applications of engineered nanomaterials in different stages of the papermaking become accepted on industrial scale. Some of the main issues to be resolved for further stimulating the applicability of nanomaterials in papermaking include control close control on the particle geometries, homogeneous dispersibility, possible interactions with the papermaking process and balancing the required functionalities with paper strength. Therefore, future efforts should mainly be directed towards the identification of novel techniques for incorporation of nanomaterials in the paper web structure.

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