J Mater Sci (2018) 53:146–184 REVIEW Review

Review: nanoparticles and nanostructured materials in papermaking Pieter Samyn1,*

¨ hlund4, and Alain Dufresne5 , Ahmed Barhoum2,3, Thomas O

1

Institute for Materials Research (IMO-IMOMEC), Applied and Analytical Chemistry, Hasselt University, 3590 Diepenbeek, Belgium Department of Materials and Chemistry (MACH), Vrije Universiteit Brussel (VUB), 1050 Brussels, Belgium 3 Chemistry Department, Faculty of Science, Helwan University, Helwan, Cairo 11795, Egypt 4 Department of Natural Sciences, Mid Sweden University, 85170 Sundsvall, Sweden 5 CNRS, Grenoble INP, LGP2, Université Grenoble Alpes, 38000 Grenoble, France 2

Received: 19 June 2017

ABSTRACT

Accepted: 29 August 2017

The introduction of nanoparticles (NPs) and nanostructured materials (NSMs) in papermaking originally emerged from the perspective of improving processing operations and reducing material consumption. However, a very broad range of nanomaterials (NMs) can be incorporated into the paper structure and allows creating paper products with novel properties. This review is of interdisciplinary nature, addressing the emerging area of nanotechnology in papermaking focusing on resources, chemical synthesis and processing, colloidal properties, and deposition methods. An overview of different NMs used in papermaking together with their intrinsic properties and a link to possible applications is presented from a chemical point of view. After a brief introduction on NMs classification and papermaking, their role as additives or pigments in the paper structure is described. The different compositions and morphologies of NMs and NSMs are included, based on wood components, inorganic, organic, carbon-based, and composite NPs. In a first approach, nanopaper substrates are made from fibrillary NPs, including cellulose-based or carbon-based NMs. In a second approach, the NPs can be added to a regular wood pulp as nanofillers or used in coating compositions as nanopigments. The most important processing steps for NMs in papermaking are illustrated including the internal filling of fiber lumen, LbL deposition or fiber wall modification, with important advances in the field on the in situ deposition of NPs on the paper fibers. Usually, the manufacture of products with advanced functionality is associated with complex processes and hazardous materials. A key to success is in understanding how the NMs, cellulose matrix, functional additives, and processes all interact to provide the intended paper functionality while reducing materials waste and keeping the processes simple and energy efficient.

Published online: 12 September 2017

Ó

Springer Science+Business

Media, LLC 2017

Address correspondence to E-mail: [email protected]

DOI 10.1007/s10853-017-1525-4

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Abbreviations CMF Cellulose microfiber CNC Cellulose nanocrystal CNF Cellulose nanofiber CNP Cellulose nanoparticle CNT Carbon nanotube CNW Cellulose nanowhisker DP Degree of polymerization GCC Ground calcium carbonate GO Graphene oxide rGO Reduced graphene oxide LbL Layer by layer MCC Microcrystalline cellulose MTM Montmorillonite (plate-shaped clay) NCC Nanocrystalline cellulose NMs Nanomaterials NPs Nanoparticles NSMs Nanostructured materials PCC Precipitated calcium carbonate SWCNT Single-wall carbon nanotube MWCNT Multiwall carbon nanotube

Important definitions and terminologies The European Commission described the term Nanomaterials (NMs) as a natural, incidental or manufactured material containing particles where at least 50% of the particles have one or more dimensions in the size range of 1–100 nm. The British Standards Institution proposed the following definitions. Nanomaterials (NMs): material with any external dimension in the nanoscale or having internal structure or surface structure in the nanoscale. Nanoparticles (NPs): nano-object with all three external dimensions in the nanoscale. Nanofiber: nano-object with two similar external dimensions in the nanoscale and the third significantly larger. If the lengths of the longest and the shortest axes of the nano-object differ significantly (typically by more than three times), they are referred to as nanorods or nanoplates. Nanocomposite: multiphase structure in which at least one of the phases has at least one dimension in the nanoscale. Nanostructured materials (NSMs): materials having internal nanostructure or surface nanostructure. The NPs and NSMs have become materials of active research and expanding commercial importance. The current NMs can be categorized into five material-based categories: (1) carbon-based NMs,

which are composed mostly of carbon, such as carbon nanotubes (CNT), graphene nanosheets, and nanodiamond; (2) inorganic-based NMs, which include non-carbon NPs and NSMs such as metals (Au, Ag, and Cu NPs) and metal oxides (ZnO, TiO2 and Fe2O3 NPs); (3) organic-based NMs mostly containing organic matter, excluding carbon-based or inorganic-based NMs, such as nanostarch, nanocellulose, and polymer NPs; (4) composite-based NMs that combine NPs with other NPs or NPs combined with larger bulktype materials (e.g., composite nanofibers) or more complicated structures, such as a metal organic frameworks and mixed metal oxides [1]; and (5) biobased NMs, which are composed mostly of biomaterials such as enzymes and nanobacteria. The NMs can be also organized into a natural or man-made (engineered) based on their origin. The first classification idea of NMs was given by Gleiter [2] based on the crystalline forms and chemical composition (internal structure) [3]. Pokropivny and Skorokhod reported a new classification for the NMs, based on the dimensionality of the nanostructures and their components. In this classification, the recently developed composite NSMs (i.e., 0D, 1D, 2D, and 3D) are included [4].

Papermaking industry Paper serves as a base substrate with various demanded properties of renewability, biodegradability, recyclability, affordability, mechanical flexibility and offers a broad possibility to modify its surface properties toward specific properties. Paper substrates can be produced from different natural fiber sources offering a wide variety of morphologies. Typical raw materials include softwoods (low-density long fibers), hardwoods (high-density short fibers) besides non-wood based materials (bagasse, bamboo, kenaf, reed, straw, grasses). Broadly, the paper production includes three main steps: 1.

2.

Pulping which involves isolating of the cellulose fiber from the unwanted raw material. Natural sources of fibers, e.g., wood fibers, are a cellulosic–fibrillar composite made of fibrous raw materials (cellulose fibers), hemicelluloses, lignin, and extractives (i.e., oils, waxes, minerals). Beating and refining which involves a process during which the cell walls are fibrillated to

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produce many microfibrils. This process increases the surface area and a number of contact points between neighboring fibers. Wet-end papermaking which involves flowing a very dilute aqueous suspension of the separated cellulose fibers (refined fibers) through a very fine wire mesh so that the water drains through, leaving the fibers to settle together into a felted layer (paper web).

Paper sheets are highly porous and hygroscopic and consequently hardly compatible with most coating colors and printing inks. Therefore, the paper is often externally filled with various additives to control penetration of coating colors and inks as well as to improve its dimensional stability, surface smoothness, and optical properties. The non-fibrous additives to paper stock are classified into two groups: (i)

(ii)

Functional additives such as fillers, coating pigments, sizing agents, retention aids, and wet- and dry-strength additives. These additives are used to modify end-product properties such as increasing opacity, brightness, and printability of paper and coloring of paper with brilliant colors, controlling water penetration in the body of the paper, and increasing wet and dry strength of the paper, respectively. Control additives such as retention aids, drainage aids, pitch control agents, deformers, bacteriocides, and slimicides. These additives are used to affect the performance of stock at the wet end of the paper machine as controlling filler retention, accelerating the water drainage and dirt removal from the paper stock, hindering foam formation, and inhibiting bacterial growth, respectively.

Addition of the fillers at the wet-end section and coating of the paper surface with pigments are commonly used to control the paper surface properties and prevent printing problems. The fillers are added to the base of paper, typically between 10 and 20% filling the voids around fiber areas with the appropriate amount of retention aids. Fillers perform many different functions at the same time and improve sheet formation, dimensional stability and optical properties, reduce gas permeability, increase furnish drainage rate, machine speed, productivities, and enhance the printability in addition to reducing the

production cost by replacing higher cost fiber. The coating color usually comprises an aqueous suspension of pigment and binder with a solid content of 50–70%, containing about 80–90 wt% pigment and 10–20 wt% binder [5]. The binder commingles the pigment particles and provides the required mechanical strength. Supplementary additives such as thickeners, dispersing agents, and pH-control additives, i.e., acids or bases, lubricants or biocides, may be included [6]. The coating colors may be specifically adapted for reception of given functional inks or applications [7]. The coating pigments fill in crevices and create a tight, flat, smooth surface with a better quality than after sizing or perfect fiber blending. The final surface properties and paper performance depend on the shape, size, and size distribution of the fillers and coating pigments [8].

Introduction of nanotechnology in papermaking The introduction of nanotechnology in papermaking initially emerged from the need for improving processing of paper sheets with lower energy consumption and better sheet formation. Starting from the 1990s, nanotechnologies also opened new opportunities for creating new paper types with enhanced performance. The use of nanotechnology in papermaking may serve as a tool to enhance the sustainability of papermaking processes and products, by, e.g., (1) more efficient use of resources: stronger papers can be formed with lower base paper weight, and (2) utilization of side-stream materials: recycled fibers or fibers with inferior properties can be converted into strong nanofibers. The fillers and coating pigments of NPs and NSMs have been systematically developed, yet their exploitation has to be fully explored. The small particle sizes and high surface area are conferring to the paper a high surface quality (very smooth surface) or low gas permeability among other new functionalities. The porous structure and hydrophilic fiber surface allow the paper to absorb suspensions of NPs by capillary forces, yielding a high amount of NP fillers upon drying. The use of NMs in industrial papermaking nowadays mainly focuses on the use of inorganic pigments, minerals, ceramics, and starch, while bio-based nanoadditives are usually not considered. This is mainly due to the lack of production and adequate

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processing techniques for the latter materials. A number of studies were devoted to special functions of paper, including transparency [9], low gas permeability [10], water repellency [11], photocatalytic activity [12], antimicrobial activity [13], electrical and thermal conductivity [14], and magnetic property [15]. A number of possibilities for NPs and NSMs providing specific paper properties are summarized in the scheme of Fig. 1. In this review paper, the intrinsic properties for several NPs and NSMs that can potentially be used in papermaking will be discussed focusing on their chemical synthesis, processing, composition, and intrinsic particle properties. In particular, the inorganic, organic, carbon-based, and composite NMs will be highlighted, which can be converted into functional papers by to two approaches, (1) making nanopaper from their fibrillary structure (e.g., cellulose- and graphite-based nanopaper), or (2) adding the NMs to a regular wood pulp. At the end, techniques to functionalize cellulose fibers will be illustrated.

Cellulose nanostructures Cellulose is a polysaccharide, being a linear polymer comprising repeating units of glucose held together by 1,4 linkages and thus having the general formula (C6H10O5)n. The cellulose fibers are organized in the secondary wall of the wood cells and serve as a mechanical reinforcement providing stiffness to the wood structure. Figure 2 shows a schematic representation of the cross section of an aggregate of cellulose microfibrils. The structural formula has hydroxyl (–OH) groups present in the glucose unit, which are responsible for the hydrogen bonding capacity of cellulose fibers and give them the ability to bond into a strong structure. On the other hand, they also contribute to the hygroscopic nature of cellulose. Intermolecular hydrogen bonds give rise to bundles of cellulose that form partly crystalline microfibrils, their sizes varying from species to species. The cellulose microfibril is not an entirely crystalline material but has been found to consist of a range of more or less ordered states. Hemicelluloses

Figure 1 Opportunities for NPs and NSMs providing specific paper properties by (i) adding the NMs to a regular wood pulp and (ii) making nanopapers from their fibrillary structure.

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Figure 2 a Hierarchical structure of wood fibers and the characteristic of cellulose fibrils. b Atomic structure of a cellulose chain repeat unit. Note the hydroxyl groups (red circles) in each repeat unit [16].

comprise a group of heteropolysaccharides that have polymeric repeat units comprising sugars other than glucose, or with more than one sugar repeat unit including those of mannose and galactose. The latter serves as interface compatibilizer between the cellulose fibers and lignin matrix within the word structure. The repeat units of hemicellulose are far fewer than in cellulose, numbering only 100–200 compared with up to 5000 for cellulose. However, their structures are often more complicated, being partly due to the presence of more than one saccharide type, due to the presence of side chains, and due to the pressure of esterified or oxidized alcohol groups. Lignin is an aromatic polymer serving as matrix or glue of the wood providing cohesion of the wood fibers and good compressibility of the wood. The chemical pulping process generally serves to remove the hemicelluloses, lignin, and extractives, but some may be retained in the final paper sheet. The proportion and structures of cellulose, hemicellulose, lignin, and extractives vary in hardwoods and softwoods and also between species. Almost all of its properties are undesirable for papermaking applications, and the highest qualities of paper are usually made from pulps from which most of the hemicelluloses and lignin have been removed. Lignin causes the paper to become brittle, and it is also oxidized photochemically to form colored by-products which give rise to yellowing and discoloration. Newsprint is the best

example of this, but all mechanical pulps in which the lignin is still largely present display this effect. Due to the hierarchical arrangement of microfibrils into the cell wall of native cellulose fibers, the fibrous structure can be disintegrated into nanoscale morphologies of single or bundles of elementary fibrils. The nanocellulosic materials have distinct dimensions and morphologies depending on the source and processing routes as illustrated in Fig. 3 and Table 1 [17], including cellulose microfibrils (CMFs), cellulose nanofibrils (CNFs), and cellulose nanowhiskers (CNWs) or cellulose nanocrystals (CNCs).

Cellulose microfibers (CMFs) Cellulose microfibers (CMFs) are characterized by their high aspect ratio and consist of bundles of 10–50 cellulose microfibrils with a diameter of 20–60 nm and a length of several micrometers. CMFs form a weblike structure with both amorphous and crystalline cellulose parts. In parallel with the manufacturing process of CMF through a microfibrillation technique pioneered by Turbak et al. [19], the obtained fiber morphology was referred to as microfibrillated cellulose. At present, various production techniques for CMF include either top-down methods breaking down the natural cellulose fibers by enzymatic treatment or mechanical grinding and bottom-up methods to grow bacterial cellulose from glucose [20].

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Figure 3 TEM images of nanocelluloses extracted from bleached corncob residue, an underutilized lignocellulose waste from furfural industry a CNC prepared by sulfuric acid hydrolysis, b cellulose nanocrystals isolated by formic acid hydrolysis, c CNF prepared by 2,2,6,6tetramethylpiperidine-1-oxyl (TEMPO)-mediated oxidation, and d cellulose nanofibrils fabricated by pulp refining [18].

Table 1 Nomenclature of nanocellulose derivatives and their dimensions [17]

Nanocellulose derivative

Diameter (nm)

Length (nm)

Aspect ratio (L/d)

MCC CMF Microfibril CNC TEMPO-oxidized nanocellulose

[1000 10–40 2–10 2–20 3–4

[1000 [1000 [1000 100–600 [1000

1 100–150 [1000 10–100 200-100

Cellulose nanofibers (CNFs) Cellulose nanofibers (CNFs) can be prepared in different ways including nanofibrillation of cellulose plant cell fibers, electrospinning, or formation of bacterial cellulose. The fibrillation of cellulose fibers requires the mechanical isolation of individual fibrils by grinding, cryo-crushing, or high-pressure homogenization [21]. The degree of polymerization (DP) of the original cellulose microfibrils is only slightly reduced, and their structure contains both amorphous and crystalline domains. During grinding

of native wood fiber suspensions, the rotating and stationary disks induce repeated cyclic stresses that cause the disintegration of the fibers. The disks are made of resin with hard silicon carbide materials and designed with different groove configurations to control the flow of the fibers and their resulting morphology. The microgrinding technique is advantageous as it does not require pre-treatments for fiber shortening as used with other processing techniques. Alternatively, the cryo-crushing technique enables to freeze the water within the pulp suspension and liberation of the single fibrils under a high-impact

152 load. The cryo-crushing cannot be used to make very fine fibrils as it remains limited to cellulose fibrils liberated from the primary cell walls [21]. The fibrillated cellulose suspensions are most commonly processed by homogenization, where high internal shear in the fiber structure is induced by a sharp pressure drop and impact forces within the processing chamber. The processed suspensions often contain a heterogeneous mixture of CMF and CNF that are characterized by different diameters and aspect ratios (length/diameter). Depending on the number of processing steps, the geometry of fibers in a CMF suspension remains less homogeneous than CNF suspensions. The non-uniformities in the suspension media are built by the high tendency of aggregation of single cellulose microfibrils and/or the flocculation with larger fibers. The differences between both fiber geometries may not be sharply marked out, but CMFs are generally characterized by a smaller aspect ratio than CNF. Depending on the route of manufacturing, CMF is generally produced by a sole mechanical treatment of native pulp, while CNF is produced by mechanical treatment after chemical pre-treatment of the original pulp fibers [22]. The main disadvantage of mechanical treatments is a high-energy consumption, which varies according to different devices used for fibrillation. Another disadvantage of homogenization is the control over clogging of the channels caused by the disentanglement of the fibers, which require elaborative disassembly and cleaning of the system. The homogenization can be used for a large-scale production with smaller energy demand when it is coupled with different pulp pre-treatments. The immense energy consumption amounting to over 25000 kWh per ton in the production of CNF relates to the shear stresses during multiple passes through the homogenizer [20]. In order to facilitate the fibrillation process, pulp pre-treatments based on enzymes, TEMPO-mediated oxidation, carboxymethylation, and acetylation are also being used [23]. Such pre-treatments reduce the size of the fibers by some pre-fibrillation and remove the primary cell wall where the microfibrils are organized randomly, which more efficiently exposes the more organized fibrils in the secondary cell wall [24]. The oxidized pulp fibers were used for the in situ generation of fibrillated fibers in combination with untreated pulp fibers, and formation of paper sheets with enhanced functionality from the presence of carboxylate groups

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along the fibril surfaces: this simple process allowed the in situ production of CNP in paper handsheets [25].

Cellulose nanocrystals (CNCs) Cellulose nanowhiskers (CNWs) or nanocrystals (CNCs) have a relatively low aspect ratio and possesses extremely good mechanical properties due to their high crystallinity. CNCs are produced from pulp fibers or microcrystalline cellulose (MCC) under chemical acid hydrolysis using sulfuric, hydrochloric, or phosphoric acid to dissolve the amorphous domains. Fiber source and its degree of crystallinity strongly influence the size and geometry of the product: cotton and wood yield highly crystalline CNC (90% crystallinity, width: 5–10 nm, length: 100–300 nm), whereas other sources such as bacteria and algae generate CNC with larger dimensions (width: 5–60 nm, length: 100 nm to several lm) [20]. Furthermore, the geometry depends on the acid concentration (20, 40, and 60 wt%), hydrolysis temperature (20, 40, and 60 °C), and hydrolysis time (2, 4, and 6 h): after the combined treatment with a highpressure homogenization, CNCs with diameters of 11–33 nm and lengths of 199–344 nm were obtained [26]. The surface functionalities (surface charge) of CNC depend on the acid being used: e.g., HCl induces weakly negatively charges, while H2SO4 creates more negative charges in parallel with the functionalization of glucose units with sulfate ester groups. After hydrolysis with HCl, the dispersibility in water remains limited, whereas H2SO4 hydrolysis provides better stability over a wide range of pH [20]. A good monitoring of the process allows optimizing the process to ensure maximum yield and purity characterized by crystallinity and size distribution [27]. The concentrated acids allow production of CNC in yields ranging from 20 to 40%, while the use of diluted acids provides lower yields. While using mildly acidic aqueous ionic liquids, relatively high yields of about 48% could be achieved, as the reduced solvating power of aqueous ionic liquids compared to that of sulfuric acid provides higher hydrolysis efficiency [28]. The two-step hydrolysis has been optimized by using a mildly acidic ionic liquid (IL) 1-buty1-3-methylimidazolium hydrogen sulfate, in order to produce CNC in nearly theoretical yield levels from bleached softwood kraft pulp, bleached hardwood kraft pulp, and microcrystalline cellulose

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[29]. The ionic liquids could also be applied for extraction of CNCs directly from lignocellulosic biomass such as wood, due to coincident delignification, defibrillation, hydrolysis, and derivatization of cellulose [30]. The lignocellulose accessibility during treatment with IL can be enhanced when pre-treating the biomass with a steam explosion process, where 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 [31]. The extraction of CNC with dimensions close to their native state in wood benefits from easy processing without the need for purification/dialysis.

Microcrystalline cellulose (MCC) Microcrystalline cellulose (MCC) can be characterized as a white powder of fibrous particles with sizes of about 40 lm. The partial depolymerization of cellulose into MCC is usually done by an acid hydrolysis treatment (e.g., in the presence of HCl) until the degree of polymerization (DP) levels off toward a constant value [32]. In a first step, the amorphous cellulose parts are hydrolyzed toward a dense cellulose with DP 100–200 and about 80% crystallinity; in a second step, the dense cellulose domains are further hydrolyzed under more severe conditions [33]. As a result, the hydrolysis at low acid concentrations provides MCC that is more resistant to degradation and thermal treatment compared to the hydrolysis at high acid concentrations [34]. The alkalization in sodium hydroxide is applied as a pre-treatment to enhance the accessibility of cellulose by soaking [35]. Other methods for MCC production involve hydrolytic treatment of technical-grade (unbleached Kraft) wood pulp, followed by oxidative delignification of the resulting lignocellulose powdered material with chlorine dioxide [36]. Environmentally friendly processes have been developed by using a two-step depolymerization process of cellulose pulp, including the controlled degradation by electron beam radiation and enzymatic hydrolysis [37].

Cellulose nanoparticles (CNPs) A different shape of spherical CNP can be formed under weak acid hydrolysis and sonication treatments [38]. The crystallinity of the particles slightly increases as the particle size decreases, while they remain in a cellulose II polymorph state.

Hydrophobic NPs of cellulose derivatives, such as cellulose stearoyl esters with given a degree of substitution, have been obtained via a nanoprecipitation process, while the size could be changed depending on the molecular weight and concentration [39]. In general, cellulose acetates with varying degree of substitution (DS) and their derivatives with propionate, butyrate, and phthalate can self-assemble into globular NPs with diameters of 86–368 nm, by using dialysis or nanoprecipitation [40]. Alternatively, CNPs were prepared through enzymatic hydrolysis and ultrasonication treatment of pulp fibers [41]. With the lowering of DP, however, the crystallinity and thermal stability of the particles did not significantly change. CNPs were prepared from regeneration processes by treatment with 1-butyl-3methylimidazolium chloride and high-pressure homogenization [42], forming either elongated fiber or spherical structures. A method for the preparation of amorphous CNP was described by regenerating the dissolved amorphous cellulose from a sulfuric acid solution in cold water [43]. Alternatively, functional NPs are obtained from cellulose derivatives such as 6-carboxycellulose [44], or cellulose 10-undecenoyl ester [45].

Nanopaper substrates Cellulose-based nanopaper Nanopapers fabricated from CNF exhibit unique characteristics in terms of high strength, high optical transparency, very good thermal stability, low thermal expansion, high smoothness, and enhanced barrier properties, which could not be achieved using traditional microsized pulp papers. The films of CNF and CNW provide good oxygen gas barrier resistance due to the strongly reduced penetration of oxygen molecules through the highly entangled fibrillary structure [46]. However, the barrier properties of nanocelullose may highly deteriorate in the presence of moisture due to its hydrophilic nature and high water absorption. Nanocelluloses provide high water vapor transmission rates due to their hydrophilicity, which can be improved after surface modification [47], acetylation [48], or in the presence of lignin [49]. As the size of the nanofibrils is much smaller than the wavelength of visible light, the nanopapers are highly transparent with large light scattering in the

154 forward direction [50], in parallel with lower opacity and brightness [51]. Transparent papers were made from TEMPO-oxidized CNFs (Fig. 4), where the 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)/ NaBr/NaClO was used to modify the surface properties of the pristine wood fibers by selectively oxidizing the C6 hydroxyl groups of glucose. The regular paper is a porous structure composed of untreated wood-pulp fibers with band-shaped geometries where the lumen has collapsed due to beating and pressing operations (Fig. 4a). The paper from TEMPO-oxidized CNF displays a more densely packed configuration where the repulsive force resulting from negative charges at the nanofiber surface loosens the interfibrillar hydrogen bonds between the CNF (Fig. 4b). The high surface area and abundant presence of hydroxyl groups provide nanocellulose papers with high tensile strength up to 214 MPa and modulus of 14 GPa [52]. Due to the high number of hydrogen bonds created between the

Figure 4 a Top left regular paper; bottom left the molecular structure of cellulose. b Top right transparent paper made of TEMPO-oxidized wood fibers; bottom right TEMPOoxidized cellulose with carboxyl groups in the C6 position. SEM images of unzipped TEMPO-oxidized wood fibers (c). d A digital image of transparent paper produced from TEMPOoxidized wood fibers [54].

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CNF, they are able to create fiber networks with high strength: e.g., the addition of 10% CNF to native pulp has increased the tensile strength of a 60 g/m2 base paper by 70%. Based on those properties, nanocellulose papers are candidates for novel applications of high strength and lightweight materials, such as display substrates, porous magnetic aerogels, food packaging, sensors, and catalysts [53]. Nanopapers fabricated from CNC may benefit from the excellent mechanical properties. However, the full mechanical potential of CNC in papers cannot be achieved in the case of random-in-the-plane or random-in-space orientation distributions. Therefore, a magnetic or electric field can be used to orient the CNC in a liquid state, followed by the drying in oriented films [55, 56]. The wet spinning, hot drawing, and electrospinning can similarly be used for fibers with oriented CNC, where the spinning rate and nature of the starting dispersion result in different fiber structures (hollow or porous fibers). The

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chiral liquid crystal self-assembly behavior of CNC allows to design functional nanocomposites with advanced optical properties (Fig. 5) [57]: the critical concentration of the colloidal suspension, which is the lowest concentration where the CNC self-organize, depends on particle size, acidic treatment, preparation conditions, aspect ratio, and ionic strength. As such, an iridescent nanopaper was made of graphene and CNC with water response and high electrical conductivity. The nanopaper exhibits uniformly metallic iridescence, which can be reversibly changed by the hydration or dehydration process [57]. In general, the surfaces of cellulose nanopapers are smoother than papers from traditional wood pulp and they can be applied as a substrate for ink-jet printing [58]. However, there are several disadvantages of using CNF and CNC in papermaking industry, such as [59] (1) poor retention in the fibrous materials; (2) relatively high cost compared with other wet and dry strengthening agents; and (3) negative impact on the drainage properties that directly affect the papermaking process, i.e., energy consumption and machine speed. Figure 5 SEM cross-section photographs of self-assembled CNC nanopaper, with chiral nematic axis at an angle to (a) and parallel to (b) the cut surface. Planar structural models (c) and spatial structural model (d) of (a, b), respectively [57].

Carbon-based nanopaper Carbon-based nanopapers are characterized by a combination of desirable thermal and electrical properties that can help to achieve multifunctional properties [60]. As a common technique, nonionic surfactants are used to aid the dispersion of carbon NMs into an aqueous or organic solvent. The dispersion is sonicated and filtered through a membrane to form the carbon nanopaper. After paper formation, the remaining water and surfactant are removed by drying at 120 °C for 2 h [60]. Carbon nanofiber paper has been prepared under various processing conditions with different types of carbon nanofibers, solvents, dispersants, and acid treatments. The density and network structure of the carbon nanofiber paper correlated with the dispersion quality of carbon nanofibers within the paper, which was significantly affected by papermaking process conditions [61]. Both SWCNT and MWCNT can be used for the creation of so-called bucky paper, with possible applications as molecular sieve membranes [62]. SWCNTs are usually formed into a bucky paper by a multistep dispersion and microfiltration of the CNTs

156 suspension [63]. Typically, SWCNTs have a diameter of 0.8–1.2 nm and a length of 100–1000 nm; MWCNTs have diameters of 20–30 nm and lengths of 1–10 lm. Carbon nanofibers can be obtained with a diameter in the range of 100–200 nm and length of 50–100 mm. The bucky papers from SWCNT or MWCNT are self-supporting mats, appearing as uniform, smooth, and crack-free paper-like sheets [64]. The bucky papers made from vertically aligned CNTs exhibit a high mechanical strength and flexibility owing to the high aspect ratio of CNTs [65]. However, the different qualities of CNTs imply various strength and electronic properties: bucky papers with high strength up to 15.36 MPa and electrical conductivity of 61.17 S/cm were obtained under supercritical fluid (SCF) drying [66]. The hydrophobicity was improved by UV/ozone treatment creating hydroxyl groups and substituting them with alkoxysilane groups [67]. The hydrophilicity was improved by glow discharge plasma treatments to generate functional groups like alcohol or carboxylic groups on CNT sheets to increase their wettability [68]. rGO/Fe3O4 nanodisk hybrid paper was fabricated by a filtration-assisted self-assembly (Fig. 6), where a homogenous suspension of graphene oxide (GO) and Fe3O4 nanodisks was prepared first.

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inorganic nanofillers generally reduce paper strength when filled at high concentrations, due to a reduction in fiber interactions (i.e., lower contact area and occupation of chemical groups at the fiber surface). The functionality of the NPs usually depends on their morphology, which may be classified as 0D structures (e.g., metal and metal oxides NPs), 1D structures (e.g., CNFs, CNCs, CNTs), 2D structures (e.g., nanoclay sheets, graphene), or 3D structures: e.g., metal NPs (Au, Ag, Pd, Ni, etc.), minerals (Cax(PO4)y, CaCO3, and montmorillonite), and carbon NMs (CNTs and graphene) have been incorporated into nanocellulose substrates, resulting in improved electrical, optical, and catalytic properties of the nanopapers. The combination of nanofillers and nanopigments with appropriate processing additives, and deposition techniques may allow creating papers with enhanced functionalities (Fig. 7). The current developments of nanofillers and nanocoating pigments for papermaking wet-end applications fall into the following categories [70]: 1. 2.

3.

Nanofillers and nanopigments in papermaking General

4.

In terms of quantity and consumption in papermaking, fillers and coating pigments represent the second most important material added in a paper manufacture either under the form of micro- or nanostructures. The synthesis of nanofillers (typical range of 1–100 nm, or even in the broader size range of 1–500 nm) has become of interest in latest years, and they have been used in the wet-end applications to enlarge their content in the paper [70]. However, the amount of the fillers has been usually limited to 15–20% because higher levels cause loss of sheet strength and ‘‘dusting.’’ Nanofillers and coating pigments are generally classified depending on their origin being either inorganic, organic, carbon-based, or a composite. Inorganic nanofillers with high aspect ratio may improve properties such as porosity, smoothness, and optical properties [70]. However,

5.

Nanofillers are directly added to paper furnish (pulp) before the wet web formation of papers. Nanofiller/fiber is formed by in situ precipitation, fiber coating, or internal filling of the fiber lumen. Surface functionalization of nanofillers and nanocoating pigments by chemical modification and the modified nanofiller is subsequently added into the paper. Surface coating or encapsulation of nanofillers for protection. Development of high aspect ratio nanofillers and coating pigments as a substituent for traditionally applied NMS.

At present, there remain specific issues associated with NMs to become fully exploitable as fillers in papermaking. From the synthesis point of view, the precise control of shape and properties of NMs remains difficult. This may be resolved by gaining more fundamental knowledge on the nanomaterial structures. The paper machine parts and printing cylinders are worn by paper fillers, where the hardness, particle size, and particle shape of the mineral filler are all important. The use of nanosized fillers can reduce the detrimental effects, but even a very small amount of microsized particles can be harmful. From a processing point of view, however, the dispersion and retention of nanofillers are rather poor

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Figure 6 a Schematic illustration for the fabrication of magnetic rGO/Fe3O4 nanopaper and chemical interactions within both components. Left-bottom inset shows the morphology of pristine a-Fe2O3 nanodisks. b Demonstration of flexibility, c good

magnetic property, and d large-scale fabrication for the asprepared hybrid film with 10 cm diameter. e top-view, f crosssectional SEM images, and g FIB image of the hybrid film. The inset of g shows the FIB notch [69].

and the interactions between nanofillers with some wet-end additives can negatively affect the paper strength. Solutions from the literature include different steps of surface modification to make them more compatible and reactive with the paper fibers. Finally, the price for most NMs is not yet commercially acceptable and only a few types are already industrially acceptable in relation to their significant increase in performance. The increased interest at the academic and industrial level and involvement of interdisciplinary scientific research may indicate a solid base for advancements in the field to be expected in near future. The properties and potential use of different NMs for papermaking are discussed below, including inorganic, organic (polymeric),

carbon-based, and composite NPs that can be used as fillers or pigments.

Inorganic nanofiller and nanocoating pigments The mineral fillers may have lower costs than wood fibers while enhancing paper properties such as brightness, opacity, and print quality. The most common types of inorganic nanofillers widely used in papermaking are as follows: nano-GCC, nanoPCC, natural nanoclay (mined, refined, and modified), synthetic nanoclay, nanokaolin, nano-SiO2, nanotalc, nano-TiO2, and natural nanfibrous (silicates). Different pigments influence the light

158

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Cellulose Nanostrucutres

Nanocoang Pigments

Nanofillers

Funconal Nanopaper Micro/Nano Engineering

Nanodeposion Techniques

Processing Addives

Figure 7 Role of NMs and processing on the development for nanopapers.

scattering in different ways and are important for opacity, brightness, and good printing properties. In particular, the particle size of around half the wavelength of light and narrow particle size distribution is desirable. However, the mechanical properties of paper sheets do not only depend on the particle size, but they are largely determined by the shape and geometry of the fillers [71]. Ground calcium carbonate (GCC) is a relatively bright rhombohedral pigment, while clay possesses platelike structure and lower brightness. Much work has been done on the optimization of particle size of GCC and clay by grinding and fractionation, to maximize light scattering effects. The challenges in using nanopigments on the commercial scale are their poor retention and their cost [72]. Nano-CaCO3 nanofillers may overcome these challenges because of the availability of raw materials as well as the ease and cheap production costs [73]. An overview of different methods for synthesis of micro- and nanosized CaCO3 with specific sizes, polymorphs, and morphologies has been described, depending on experimental parameters such as additive types and concentration, pH, temperature, [Ca2?]:[CO32-] ratio, solvent ratio, mixing mode, and agitation time on the properties of the particles [74]. Precipitated nanoCaCO3 has been efficiently synthesized by carbonation of the precursors Ca(OH)2 and CaO in a closed loop reactor, where the particle size is affected by parameters such as Ca(OH)2 concentration, CO2 flow rate, and reaction time [75]. However, little is known

about its application as filler in papermaking, but it is assumed that the high porosity and high surface area might contribute to strengthening the mechanical properties of paper. The formation of a superhydrophobic paper by employing coating color made of nano-CaCO3 and polymer [76] describes these principles (Fig. 8). PCC fillers coated with Al–Mg–silicate and ZnS2 nanostructures showed an enhancement in the light scattering from a pigment coating of a paper using NPs [77]. Hollow-spherical CaCO3 nanofiller/coating pigment has been synthesized for highly specific light scattering paper [78]. When CaCO3 NPs are used as coating pigments, it produces an ultrathin and high-quality coat paper [79]. Clay particles such as montmorillonte (MTM) or kaolinite are also favored in paper filler and coatings in order to improve barrier resistance, see Fig. 9. The performance of clay particles can be improved by intercalation and exfoliation during modification with organic molecules. An alternative way for modification of kaolinite clay has been presented by the intercalation of low molecular weight polymers, resulting in composite organic/inorganic nanoclay particles obtained by precipitation of hydrophobic organic NPs on the exfoliated clay surfaces after imdization see Fig. 10 [81]. When applied as a paper coating, the modified kaolinite provides the advantage of better dispersion of the platelets within the coating and a better localization of the hydrophobic moieties at the paper surface. Coating color made of nanoclay/CaCO3 was used to reduce air permeability and improve the surface smoothness and printing characteristics of paper [79]. The processing of clay nanopaper is environmentally friendly and very straightforward as it is water based and primarily relies on physical mixing/assembly of NPs. Nano-SiO2 NPs have properties such as a high specific surface area, high gas absorbability, and high oil absorption and can be prepared following traditional sol–gel techniques [83]. The application of SiO2 NPs materials, for instance as nanofiller in the paper industry, may contribute to the fiber-to-fiber bonding when coated with silica, thus improving paper strength [84]. The production of silica NPs from mining side streams has been evaluated as a pigment for ink-jet paper coatings, providing mat papers with very low gloss and improved absorption speed and optimum pore structure for the inks [85]. Core–shell SiO2@CaCO3 nanocomposite fillers show a high degree of retention comparing with microsized PCC,

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159

Figure 8 Schematic diagram of superhydrophobic modification principle and using a simple dip-coating procedure. CaCO3 particles, hydrophobic stearic acid, and polymer latex particles were used for surface roughness control, surface hydrophobic modification agent, and polymer binder [80].

Figure 9 a Schematic representation of clay nanopaper preparation and the oriented structure; b mechanical properties of clay nanopaper containing 50% MTM in comparison with other clay

nanocomposites; and c oxygen barrier properties measured in humid conditions (23 °C 50% R.H.) of clay nanopaper in comparison with currently employed technique [82].

160

Figure 10 Exfoliated and surface-modified kaolinite nanocomposite platelets by deposition of poly(styrene-co-maleimide) or SMI NPs [81].

which can be attributed to the negatively charged surface caused by depositing SiO2 on the CaCO3 surface. The mechanical properties were not significantly affected, while the sheet brightness, whiteness, and opacity were improved compared to microsized PCC [84]. Nanostructured silica with an open network structure used as filler in newsprint and achieved print through reductions of 30% for newsprint and 40% for the yellow directory-grade paper was achieved [86]. The hydrophobic SiO2 NPs were spray-coated on the paper from an alcohol dispersion to create a hydrophobic paper surface [11]. TiO2 NPs have been used as a highly qualified white pigment in papermaking for their advantages such as outstanding refractive index, excellent brightness, a high degree of whiteness, preferable hiding power, and insolubility in alkaline and acidic solutions. The conditions for sol–gel preparation of the TiO2 aqueous dispersion to obtain a homogeneous coating on cellulose strongly depend on the uniformity of the particle size distribution and pH of the dispersion to control the surface charge and possible agglomeration [87]: the formation of monodisperse NPs on the fiber surface resulted in self-cleaning properties and enhanced photocatalytic activity. After preparation of TiO2 NPs by hydrothermal methods, they can be immobilized onto cellulose fibers during papermaking through a nucleation and growth mechanism with different concentrations of TiO2 up to 21 wt% [88]. Superhydrophobic papers (water contact angle of 126.5°– 154.2°) were made with the addition of TiO2 NPs modified with a coupling agent, 3-(trimethoxysilyl)

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propyl methacrylate (MPS), to cellulosic fibers [89]. Cellulosic-TiO2 nanocomposite was prepared and used as a protective coating for old manuscript papers [90]. Photocatalytic and antibacterial papers were developed by surface functionalization of paper with only a very small concentration of TiO2, Ag, and Au NPs. Depending on the ratio of anatase (TiO2) and rutile crystalline phases, the better optical properties and photocatalytic activity can be obtained at optimized values for brightness, opacity, and photocatalytic activity [91]. However, only the weak UV light illumination is required in order to create TiO2 NPs with efficient photocatalytic activity [92]. The encapsulation of TiO2 NPs in hollow silica spheres and subsequent combination with cellulose fibers during papermaking exhibited a higher activity in the photocatalytic decomposition of volatile organic compounds compared to the unsupported NPs: the paper containing naked TiO2 NPs showed a marked deterioration in strength under UV light irradiation, while the direct contact between TiO2 and organic fibers was prevented in the presence of a silica shell [93]. An alternative immobilization method for TiO2 NPs onto paper could be established through the bioconjugation with a fusion protein that remained stable during longer UV exposure times [94]. Although the TiO2 NPs are most extensively used in photocatalytic papers, it is only active under UV light conditions that accounted for \5% of the solar light energy. ZnO NPs possess particular properties as they can be structurally modified for visible light absorption and consequent photocatalytic activity for visible light. The stabilization of ZnO NPs within a paper coating by means of carboxymethyl starch enhances the whiteness and brightness of coated paper and is more stable to UV radiation. The ZnO/starch nanocomposites can be made directly by dissolving starch in the ZnCl2 solution and adjusting the pH by addition of NaOH, in order to make paper coatings with improved surface strength and smoothness. Furthermore, the slow dissolution of ZnO in moist environments results in Zn2? ions for immobilization of microbes. The optical properties and printability of nano-ZnO-coated papers showed excellent antifungal and UV-protecting properties, compared to bulkZnO-coated paper, essential in enhancing paper life. Under an alternative form, ZnO nanorods can be grown under hydrothermal conditions at low temperature on paper supports, resulting in enhanced

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photocatalytic efficiency (e.g., photodegradation of organic dyes) and photocatalytic immobilization of bacteria [95]. ZnO nanowires show efficient antibacterial activity under visible light exposure and can be immobilized onto the paper substrate by a single-step hydrothermal method [96]. Moreover, ZnO has no detrimental effects on the inherent paper properties such as ink-retention capability, while the brightness increased due to light scattering by ZnO nanorods [12]. Indeed, the selection of ZnO nanorods shape influences the photocatalytic activity, as structures with higher aspect ratio and surface defects show significantly higher photocatalytic performances [97]. Alternatively, Mg(OH)2 NPs can be synthesized under electrolytic conditions with a membrane between the anode and the cathode and can be added to the pulp suspension in combination with a retention agent during papermaking, which contribute to the better antibacterial efficiency [98]. The flame-retardant properties of Mg(OH)2 NPs have been exploited in the fabrication of hybrid nanopapers in combination with CNTs [99]. MgO NPs can also be used for the more efficient deacidification of paper [100], where an increase in the pH of paper after deacidification with MgO NPs is to be associated with better penetration of the reagent into the structure of paper in comparison with MgO microparticles. Therefore, Mg(OH)2 NPs have been successfully used for conservation of paper and protection of cellulose against aging [101], because of the following reasons: (1) NPs present a higher efficacy in the deacidification treatment since they are much more reactive; (2) NPs are less aggressive since they are easily converted into the carbonate form; (3) they present minor disadvantages related to the chemical nature of the solvent; (4) papers/books can be treated with very simple procedures and do not require any special apparatus; and (5) NPs modification has substantial economic benefits. The formation of novel nanobelt structures of Mg(OH)2 with very high aspect ratio has opened new perspectives, including low density, high brightness, no need for retention aids, and less detrimental effects on paper strength while adding novel functionalities such as flame-retardant properties to papers [102]. Magnetite (Fe2O3) NPs were used as nanofiller to produce magnetic cellulose fibers and papers. The Fe3O4 NPs have irregular spheres and can be homogeneously dispersed within a cellulose matrix, while their diameter and content can be increased based on

161 the mixture solution of Fe2?/Fe3? precursors [103]. After the surface coating of cellulose fibers with magnetite NPs, the particles remained bonded to the fiber surface after successive washings and sonication, and the fibers could be formed into a paper sheet [15]. The cellulose fibers with magnetite particles can be readily aligned to fabricate unidirectional magnetic papers during papermaking from suspension by controlling the flow rate and concentration of magnetic cellulose fibers [104]. Noble metal NPs such as Ag and Au NPs are commonly used as coating pigments for antibacterial activity and can be prepared by the chemical reduction method using a reducing agent. The cellulose papers can be coated with Ag and Au NPs by a chemical reduction method using two different ratios of sodium borohydride and salt solution, serving as antibacterial water filter [105]. Alternatively, the method of impregnation followed by in situ reduction in Ag salt solution resulted in the deposition of Ag NPs on the cellulose fibers [106]. As such, the paper substrates are immersed into Ag salt solution, together with ultrasonic radiation, to form Ag NPs on the substrate [107]. The combination of Ag NPs with nanofibrillated cellulose was prepared via the electrostatic assembly in water of Ag NP onto CNF. The antibacterial of Ag NPs-nanofibrillated cellulose nanocomposite was used as fillers in starch-based coating formulations for Eucalyptus globulus-based paper sheets [13]. By mixing colloidal Ag NPs and CNFs, a hybrid coating can be deposited on paper for simultaneous improvements in antimicrobial activity, water vapor transmission, oil resistance, and strength [108]. Ag NPs have been used to create porous cellulose fibers in order to serve as a chromatographic column for the separation of small molecules in aqueous solution, as well as sensing substrate for highly sensitive nanoplasmonic detection [109]. Transparent and luminescent nanopapers were developed from CNF and functionalized by rareearth up-converting luminescent Ag NPs, grafted to the CNF matrix. Nanopapers could potentially be used for multimodal anti-counterfeiting, sensors, and ion probes applications [110]. The use of Ag2O NPs has been more favorable than TiO2 for photocatalytic purposes, as it is active under the full light spectrum: therefore, photocatalytic paper was fabricated by incorporating cellulose fibers with graphite fibers which were pre-filled with Ag2O NPs [111].

162 Inorganic nanofillers with high aspect ratio have been successfully developed in recent years for papermaking, reporting improvement in properties such as sheet bulk, porosity, smoothness, optical properties, and strength [70]. The incorporation of fillers as nanofibrous structures or nanowires with high aspect ratio compared to globular structures may allow for higher filler contents up to 50% as they are homogeneously dispersed. The nanofibrous fillers will lead to better retention of fillers, additives, and pulp fines, resulting in significantly reduced biological and chemical oxygen demands in the mill process water, in comparison with spherical particles. Some of the used nanofiber papers make use of silicate and carbon nanofibers. Silicate nanofibers have shown the ability to replace TiO2 as an opacifying pigment used in paper coatings, while they could not be prepared at the same high solids content of TiO2 and produce coatings that are less dense. The refractive indices of the nanofibers fall within a range of 1.6–1.7 comparable to other pigments (talc, CaCO3, and clay) and have an average brightness of 95% [112]. Silicatebased nanofibers have been studied to increase the opacity in some base sheets by replacing pigment fillers [113]. 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 non-isotropic, rodlike nanofiber particles is rather non-uniform. 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 diameter 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 L/D from 10:1 to 50:1 [114]. Another type of silver-based nanowires has been prepared from a solution of silver chloride and silver nitrate into polyvinylpyrrolidone and ethylene glycol [115]: the incorporation of silver nanowires into the paper structure allows to create high conductivity for use of papers as energy storage devices or conducting electrodes [116]. Conductive hybrid nanopapers with excellent mechanical flexibility were synthesized by the assembly of CNF and silver nanowires using a pressured extrusion papermaking technique [117]. The coating formation of a conductive nanofiber network of Ag nanowires

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and CNF onto papers can be obtained by a filtration technique, resulting in uniformly connected conductive networks because of drainage in the perpendicular direction through paper-specific nanopores. The latter is preferred above conventional coating processes that inevitably cause self-aggregation and uneven distribution of the conductive NMS because of the hard-to-control drying process [118].

Organic nanofillers and nanocoating pigments Organic nanofillers have been attracting more interest in recent years and have been introduced in papermaking and coating processes with favorable effects on paper strength and barrier properties. Polyacrylamide NPs have been synthesized by inverse emulsion polymerization of acrylamide in supercritical carbon dioxide and can be used in papermaking industry [119]. As an example of a biodegradable natural polymer, starch nanocrystals/nanoparticles have good potential for use in papermaking surface sizing, paper coating, and paperboard for substitution of petroleum-based fillers, adhesives, and binders. The inclusion of starch NPs with sizes of 50–300 nm in paper coating compositions enhanced the ink-jet printing quality by reducing bleed and shortened drying times through a higher absorption rate [119]. In comparison with traditional cooked cationic or anionic starches, the NPs provide unique properties with low viscosities of the suspensions even at a very high solid concentration (up to 30 wt%) and high bonding strength. Starch granules have been transformed into NPs by a co-extrusion process of starch feedstock together with glycerol as a plasticizer and glyoxal as a cross-linker [120]. Through the reactive extrusion process, a very high solid starch paste can be converted into a thermoplastic melt phase, followed by cross-linking and sizing of the solubilized starch molecules into NPs [121]. The starch NPs could also be grafted on cellulose fibers through 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 [122], especially improvement of water retention value was also demonstrated. The synthesis routes and properties of starch NPs have been fully reviewed before and can be achieved through enzymatic hydrolysis, regeneration, and

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mechanical treatments [123, 124]. After hydrolysis of the amorphous parts of native granular starch in aqueous sulfuric acid solution, starch hydrolyzates could be recovered as precipitates after centrifugation and re-dispersion in water with an ultrasonic treatment: the resulting particles could be prepared with high yield (78%) and crystallinity, having spherical shapes with diameters from 50 to 90 nm [125]. The platelet-like starch nanocrystals obtained after acid hydrolysis have typical lengths of about 20–40 nm and thickness of 4–7 nm. However, acid hydrolysis is often difficult to implement for industrial applications as it often provides low yield and requires long treatment periods. Starch particles can be regenerated by adding organic solvents such as ethanol as a precipitant into precooked native starch solutions [126]. The microemulsion method offers the advantages of very low interfacial tension, large interfacial area, and is thermodynamically stable and affords monodispersed NPs. The synthesis parameters such as stirring rates, ratios of oil/co-surfactant, oil phases, co-surfactants, and ratios of water/oil were found to affect the mean particle size of starch NPs [127]: in particular, the controlled precipitation through dropwise addition of dissolved native starch solution into excess absolute ethanol provides particle sizes of about 150 nm in the presence of surfactants [128]. By using a combination of complex formation with n-butanol and enzymatic hydrolysis, starch NPs with a size of 10–20 nm were prepared [129]. A novel method of ionic liquid/water microemulsion, surfactant, and co-surfactant epichlorohydrin as crosslinker was used to prepare starch with a smooth surface and fine dispersibility [130]. Starch NPs with a mean diameter of 91.4 nm were synthesized with epichlorohydrin as cross-linker through W/IL microemulsion cross-linking reaction at 50 °C for 4 h [131]. Also, the structure of ionic liquid/oil microemulsions is determined by a pseudo-ternary phase diagram and was used for the preparation of NPs with cross-linked starch molecules, having a mean diameter of about 80 nm [132]. Another route involves the self-assembly of waxy corn starch NPs, where surfactants such as sodium dodecyl sulfate significantly increased the dispersion and thermal stability of NPs [133]. Alternatively, nanosized particles of 30–100 nm were prepared using a purely physical method of high-intensity ultrasonication without further need for any chemical treatment [134]. The combination of an ultrasonic treatment and

163 several oxidation steps gradually reduces the crystallinity and particle size with the severity of the oxidation treatment [135]. High-pressure homogenization of a 5% starch slurry in a microfluidizer under high pressure of 207 MPa and up to 20 passes was used to reduce the starch particle sizes to about 10–20 nm [136]. The viscosity of the colloids increased with an increased number of homogenization passes: after microfluidization of starch– water suspensions, the starch granules were partially gelatinized and a gel-like structure was formed on the granular surface [137]. The combination of highpressure homogenization and a water-in-oil (w/o) miniemulsion cross-linking technique was used to form stable NPs depending on the surfactant content, water/oil ratio, starch concentration, homogenization pressure, and cycles [138]. Starch NPs prepared via high-energy ball milling offered sizes of approximately 120 nm in combination with a more rough particle surface with enhanced absorption properties [139]. New technologies of reactive extrusion have been optimized, where particle sizes of 300 nm were obtained after extrusion at 100 °C without cross-linker, while the particle size could be further reduced to around 160 nm with the addition of appropriate cross-linkers and at a lower extrusion temperature of 75 °C [140]. Starch NPs 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 [141]. Starch NPs may potentially be used in papermaking, surface sizing, or paper coating as a biodegradable adhesive for substitution of petroleum-based adhesives. Starch NPs have many advantages over traditional cooked cationic starches or anionic starches, due to their unique properties such as lower suspension viscosity even at relatively high solid contents (up to 30 wt%), and higher bonding strength [120]. Chitosan is well known for its antifungal and antibacterial properties [142], with excellent grease and oxygen barrier properties [143, 144]. Because of similarities in chemical structure, chitosan NPs are fully compatible with cellulosic paper fibers forming

164 hydrogen bonds between molecular chains, and it will improve physical paper properties such as higher strength, lower water absorption, and better smoothness. Chitosan NPs have been prepared following the emulsion method, ionic gelation method [145], reverse micellar method, and self-assembling methods [146]. The particle size, particle formation, and aggregation are directly affected by the molecular weight and degree of deacetylation [147]. By tuning the parameters of ultrasonic emulsification and coalescence, the chitosan micro- and nanospheres could be prepared by precipitation from a W/O emulsion. The ultrasonication for a longer duration or higher amplitude decreased the mean diameter and polydispersity of the NPs in parallel with a greater disarray of chain alignment in the nanoparticle matrix [148]. Usually, chitosan NPs 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 (Fithriyah and Nurul 2015). The deposition of chitosan NPs as a paper coating material from a solution (1–5 wt%) enhanced mechanical properties, while the impregnation rates were depending on the solution viscosity and affinity toward paper [149]. Depending on the pH of the solution, concentration, ratio of components, and mixing method, the chemical interactions can be controlled through the charge density of TPP and chitosan. As such, the relationships between free amino groups on the surface and the characteristics of chitosan NPs prepared by ionic gelation were studied [150]. Alternatively, ultrafine NPs 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 [151]. Smooth chitosan NPs with a diameter of approximately 36 nm were obtained by peroxide degradation of chitosan in HAc solutions applied in the paper bulk for antibacterial properties [152]. The combination of chitosan NPs and nanofibrillated

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cellulose enhanced tensile properties, antimicrobial, and grease-proof resistance of coated papers [153]. A novel generation of poly(styrene-co-maleimide) NPs has been introduced as hydrophobizing agent that can be used as internal sizing or coating pigments. The organic NPs have been synthesized by an imidization reaction of the poly(styrene-co-maleic anhydride) copolymer in the presence of ammonium hydroxide, resulting in a stable aqueous suspension at pH [ 4, with 35 wt% solid content and appropriate viscosity. The NPs are formed by self-organization of the copolymer into spherical units of about 100 nm diameter [154], and their properties can be tuned by appropriate selection of the synthesis conditions. Advantageously, they possess a high glass transition temperature of 170–190 °C and therefore do not have a 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 to good ink reception [155]. Organic nanocapsules could also be synthesized in the presence of vegetable oils, including soy oil, sunflower oil, corn oil, castor oil, rapeseed oil, or hydrogenated oil [156] 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 NPs and the solid content in aqueous suspensions could be increased toward 65 wt%. In particular, the reactivity and interactions between the imidization of NPs and the oil encapsulation highly depend on the type of oils that are used, where polyunsaturated oils such as soy oil [157] have higher reactivity compared to saturated types of oil such as palm oil [158]. As such, the morphology of the nanocapsules 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 to reservoirs for the controlled thermal release of the encapsulated oil [159], which can be used as an active release coating on paper. In general, the polymer micro- or nanocapsules can be synthesized by selfassembly techniques and applied to create intelligent functional papers with bioactivity, self-healing properties, responsive properties to environmental changes, etc. [160]. Other biopolymers have been converted into NPs for use as an additive in papermaking. Hollow polymeric spheres of casein were fabricated through emulsifier-free polymerization coupled with alkali

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swelling approach and proved to exhibit superior opaque characteristics with tunable visible light transmittance and anti-ultraviolet property for coatings in papermaking [161]. Poly(hydroxybutyrate) or PHB was deposited as particles with a micro- to nanostructure on the surface of cellulose fibers by using a phase-separation process including dedicated solvents [162]: 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 of microscale particles with internal nanoscale morphology and submicron sized particles of PHB has been further optimized by the selection of other solvents and they have been applied onto paper by a simple dip-coating process [163]. Due to thermodynamic instability, the non-solvent diffused into the PHB-solvent system and PHB transformed into micro- to nanostructures, resulting in favorable coating structures with the enhanced hydrophobicity of the paper. The generation of such particle structures during a one-step process has also been implemented for the synthesis of structured poly(Llactic acid) (PLLA) particles with the particular topography of micro- to nanostructures [164]. As a valorization route for residues obtained from 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 [165]. Lignin NPs have been synthesized by various techniques including, e.g., sonication, precipitation, CO2 saturation, continuous solvent exchange, dialysis, and water-in-oil microemulsion-based methods. After sonication of the lignin aqueous suspensions, the average particle diameter was reduced from 1 to 10 lm into 10 to 50 nm by a combination of two main reaction patterns causing side chain cleavage/depolymerization and oxidative coupling/polymerization, respectively [166]. Using nanoprecipitation method, dioxane lignin NPs and alkali lignin NPs were fabricated in a spherical shape with a mean size of 80–104 nm [167]. The flash precipitation of dissolved kraft lignin and organosolv lignin enabled the formation of NPs in the size range of 45–250 nm, which can be further modified by coating their surface with a cationic polyelectrolyte [168]: the precipitation was done by either (1) adding nitric acid into an ethylene glycol solution of lignin or (2) adding water into an acetone solution of lignin, which

165 resulted in phase separation in the form of lignin NPs. Depending on the precipitation conditions, the stability of the NPs at a given pH range can be tuned and density or porosity of the lignin domains can be varied [169]. Spherical lignin NPs 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 NPs were produced by adsorption of poly(diallyldimethylammonium chloride) [170]. NPs 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 [171]. The utilization of other non-cellulosic biomass such as hemicellulose offers further opportunities for better yield of pulp and paper products and for the production of tailor-made pulps with improved properties. The xylan fractions obtained by hydrolysis and mild alkaline extraction or obtained by delignification formed nanoscale particles through self-assembly and aggregation [172]. The further conversion of hemicellulose into micro- and nanoparticle structures has been achieved by a simple coacervation based on neutralization of an alkaline solution with an acid solution of HCl or acetic acid as non-solvent. NPs from corncob xylan have been reported with sizes of 120–1790 nm, where particle size stability and morphology were influenced by the surfactant concentration [173]. In particular, the microparticles formed at high xylan concentrations and NPs were produced at a lower concentration of the polymer solution. The hemicellulose fraction of corncobs containing acetylated xylan is relatively high so that the depolymerization and cleavage resulted in NPs of around 20 nm [174]. After esterification, xylan derivatives could selfassemble into spherical NPs with mean diameters ranging from 162 to 472 nm and be used for release [175]. The structure of traditional sizing agents, such as alkylketene dimer (AKD), has been modified into spherical NPs deposited onto cellulose fibers: after extraction with chloroform, hot water, and dioxane/ water of AKD-treated cellulose and nanocellulose films, the AKD molecules were melted and transformed into spherical NPs 37 nm in diameter, forming a sea/island structure [176]. Also, the expansion

166 of AKD in a supercritical CO2 solution resulted in the formation of flake-like particles with sizes of about 1–2 lm and porous structure that could be controlled by the pre-expansion pressure and temperature. In comparison, AKD could also be crystallized during evaporation of acetone solvent while forming rather nanoscale crystalline needle structures [177]. Crystallizing wax was also obtained from evaporation of AKD from a cyclohexane solution to prepare fine NPs [178]. Nanocellulose can be used as an additive/filler in papermaking for mechanical reinforcement, improved barrier properties or rheological modification of fiber or coating dispersions. The applications and benefits of nanocellulose added into a paper formulation were acknowledged in several reviews [179], creating interesting properties such as smoothness, optical transparency, good mechanical properties, low coefficient of thermal expansion, and possibilities for further functionalization [21]. The use of nanofibrils in combination with a cationic polyelectrolyte such as poly(amide-amine) epichlorohydrin (PAE) enhanced the wet and dry strength of paper [180]. The type of mixing nanocellulose additives into the paper composition plays an important role in the efficiency of the strength additive: (1) it can be directly mixed into the pulp, (2) it can be premixed with a certain component such as the filler or long fiber fraction and deposited on their surfaces by retention aids, or (3) it can be pre-flocculated with a retention polymer before addition into the sheet and adding the retention aid to the fiber furnish. These mechanisms will determine whether the nanocellulose will be loosely or not bonded to the larger particles before dewatering, or it is retained as a coating layer on one or several components. In general, the papers containing CNFs as filler substantially improved the binding of inorganic fillers with the paper fibers: in particular, the simultaneous grinding of inorganic filler particles together with CNF provided favorable inclusion of the fillers into the fibrillated fiber web rather than being only deposited on the surface of the paper fibers. In one particular example, the filler matrix was selectively reinforced by adding nanocellulose to the suspensions of kaolin and calcium carbonate and coupled with a highly cationic starch: in this way, it was possible to achieve higher filler content while maintaining the same paper strength. The composite fillers of CMF produced by co-grinding cellulose fibers with GCC,

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PCC, or kaolin minerals are commercially available for several new applications in fiber-based packaging. They offer benefits in (1) net cost saving from filler increase without sacrificing paper properties or runnability, (2) higher initial wet web strength, opacity, and smoothness, (3) closing up the paper structure for benefits in coating and fiber/filler optimization, and (4) only marginal impact on wet-end chemistry. Recently, a method for preparing dried colloidal PS particles was developed by addition of a small amount of CNFs into an aqueous dispersion of colloidal PS particles (Fig. 11) [181]. This finding is promising for the fabrication of printed electronics, where CNFs will serve as a flexible coating material for devices. It is also more environmentally friendly than the use of typical molecular surfactants [181].

Carbon-based nanofillers and coating pigments Papers for use in paper electronics have attracted much research attention recently. One of the most useful paper functionalities is electrical conductivity. However, cellulose-based papers are not electrically conductive. Adding a highly conductive carbon NM such as CNTs, graphene, or carbon nanofibers, as either a thin film, or incorporated internally, increases the functionality and versatility of the paper substrate [182]. The addition of CNFs into paper formulation generally creates interesting properties such as smoothness, optical transparency, good mechanical properties, low coefficient of thermal expansion, and possibilities for further functionalization [21]. CNTs have particularly been used as a nanofiller to produce electrically conductive paper [14]. In addition, electrically and thermally conductive papers have been made from graphene NPs and CNCs: the low-cost, environmentally friendly, flexible graphene/CNC nanohybrid papers have a set of properties making them suitable for many potential applications. The mixing of MWCNT with CNF in aqueous suspension and filtration provided tough nanopaper structures with up to 17 wt% MWCNT commingled with CNF, in contrast to the rather brittle nature of other CNT composites [183]. In another study, highly ordered homogeneous CNF/ CNT papers were fabricated using a facile vacuumassisted self-assembly technique, resulting in flexible papers with improved mechanical strength [184]. The

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Figure 11 Microscope image of dried colloidal films dried colloidal mixtures of polystyrene particles (diameter 1.4 lm) and cellulose nanofibers (diameter ca. 20 nm, length ca. 1 lm). The

polystyrene concentration is fixed at 0.1 wt% and that of cellulose is 0 (a), 0.01 (b), and 0.1 wt% (c) [181].

choice of the dispersant is critical in preventing agglomeration of CNT in the paper sheet: therefore, a hybrid paper of CNT and pulp fibers was prepared by using an anionic surfactant, dodecyl itaconate, as an effective dispersant to prevent agglomeration of CNTs in water dispersion. After that, a novel pH adjustment process was used to facilitate the deposition of CNTs on the cellulose fibers [185]. Apart from the dispersing route, a paper-like carbon nanotube sheet was also fabricated by a ‘‘dry’’ route consisting of mechanically pressing/rolling an array of highly oriented nanotubes with controllable structure: the deposited thin films of silica filled with 22 wt% CNT were deposited on paper substrates using an air brush via the sol–gel route, providing a conductive film nearly continuous film substrate with good interconnection, despite the fibrous structure of

the substrate. The scheme of a process for incorporating graphene nanosheets and cellulose fibers is presented in Fig. 12 [186]. A porous and absorbent cotton paper was coated with SWNTs by using the acid treatment dispersion method: this way, originally hydrophobic SWNTs became hydrophilic because carboxyl groups (– COOH) were attached to SWNT ends. The acidtreated SWNTs bonded to the cotton fibers very well because the cotton fibers have hydroxyl groups that can form strong chemical bonding with the carboxyl groups on the acid-treated CNTs [187]. Alternatively, carbon nanotubes can be deposited on cellulose pulp microfibers that are first coated by the layer-by-layer nanoassembly technique using alternating polyethyleneimine/polystyrenesulfonate layers and

Figure 12 a Scheme of the process for the fabrication of conductive paper using cellulose fibers and graphene nanosheets (GNSs): dispersion of GNSs in water (i) and mixing with cellulose pulp (ii) to form a homogeneous GNS and cellulose dispersion (iii), followed by infiltration to induce the deposition of GNSs onto cellulose fibers (iv), resulting in the formation of GNS-coated

interconnected cellulose composite paper (v). b Photographs of the GNS/cellulose composite paper (black) and a pure cellulose paper (white) as a comparison (top); the bent composite paper, showing the flexibility of the paper (bottom left); a flexible supercapacitor made with a composite paper adhered to a copper foil as the electrical conductor (bottom right) [186].

168 subsequently processed by papermaking techniques [188].

Composite nanofillers and nanocoating pigments The modification of paper-grade fillers with surfacenanostructuring has become a valid alternative to traditional surface modification [189]. Modification of CaCO3 NPs with SiO2 has been proposed to obtain modified filler suspension with a high degree of stability and acid-resistant properties, e.g., using fluosilicic acid. Amorphous silica and calcium fluoride were precipitated on the surface of the CaCO3. The surface compounds formed by surface modification contributed to improving the acid resistance of unmodified CaCO3 [190]. The light scattering from CaCO3 particles can be increased by coating the granules with a thin continuous layer of higher refractive index material [84]: as such, the light scattering properties of precipitated calcium carbonate (PCC) have been improved by additional coating the fillers with aluminum–magnesium–silicate and zinc sulfide NPs [77]. For similar levels of filler content, it was found that the strength properties of the handsheets (tensile, tear, and burst indexes) produced with the silica-coated PCC were always significantly better than those obtained with the unmodified PCC, while also the filler content could be increased [191]. The higher strength suggests that a thin film of silica coating the calcium carbonate crystals has a positive effect on the filler-to-fiber-to-filler bonding [192]. In parallel, the modification of PCC with low amounts of cellulose acetate, cellulose acetate butyrate, and ethylcellulose allowed to establish hydrogen bonding with the fibers through the carbonyl groups of the ester bonds. Both techniques allowed overcoming one of the drawbacks of incorporating mineral fillers in papermaking, namely the limitation on the usage of high filler amounts. The platelike shape is beneficial for opacity. In coatings, ground CaCO3 is often blended with clay. Recently, zinc-based nanostructures with host CaCO3 material increase the differences in refractive index between filler–fiber and filler–air interfaces, yielding increased light scattering, i.e., better opacity [193]. The combinations of precipitated CaCO3 with starch and CNF yield nanocomposite that improved the filler retention and increased the bursting and tensile strength of papers compared to conventional PCC, in parallel with a

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higher density at increasing filler content [194]. Vermiculite exchanged with Ag- or Cu-antibacterial ions, multifunctional materials derived from clay minerals and other inorganic materials based on cation exchange reaction and surface modification, and specifically intercalated montmorillonite, saponite, and goethite, can potentially be used as fillers for the antibacterial purpose.

Nanofabrication techniques toward functional papers Surface functionalization of cellulose fiber wall In order to directly tune the surface properties of cellulose fibers or paper substrates, recent trends have been developed for the in situ modification and synthesis of NPs. The nanoparticle deposits are mostly required in small amounts to provide the desired surface properties. They can either be synthesized separately with controlled properties [195], or directly be deposited onto the cellulose fibers during synthesis (see Fig. 13). Following the deposition of particles by traditional methods such as dipcoating or sonication, the NPs often poorly adhere to the substrate or the distribution of NPs is difficult to control because of the morphology, porosity, and fibrillary sizes of the paper. Therefore, the in situ growth and deposition of NPs can enhance the chemical interactions between the formulated NPs and paper. Through the deposition of metallic NPs on cellulose fibers, unique optical properties such as localized surface plasmon resonance could provide fibers with bright colors and higher brightness intensity upon heating [196]. Porous cellulose fibers could be modified through in situ synthesis of noble metal NPs (e.g., Ag, Au, Pt, Pd), resulting in the deposition of particles smaller than 10 nm. The metal NPs were thought to be randomly deposited on the porous NCC matrix, and the surface coverage of NPs on CNFs was low. A cationic surfactant (CTAB) was used during the in situ synthesis of metal NPs on CNFs surface [200]: through the cationic groups adsorbed to the metal, NPs surface can interact with electron-rich HO– groups and anionic charged groups on the CNF surface. The metal NPs synthesized in the absence of CTAB tended to form on the TEM grid substrate instead of the

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Figure 13 a Facile fabrication of transparent superhydrophobic surfaces by spray deposition. SEM images of the spray-coated surfaces of b poly(MMA) and c poly(SiMA). SEM images of the spray-coated surfaces of poly(SiMA-co-MMA) surfaces at d low and e high magnification, optical images of the water contact angles are inserted in the top-right corner of the SEM images [195].

Figure 14 TEM images of the Ag NPs synthesis on tunicate NCC: a without CTAB and b with CTAB [197].

CNF surface, while the metal NPs synthesized in the presence of CTAB oriented along the CNFs (Fig. 14). Noble metals were also used for coating the paper surface. Au, Ag, Pd, and Pt NPs in situ deposited on cellulose paper substrates by immersing the paper

sheets into solutions containing metal ion precursors and reducing agents (Fig. 15) [107]. Deposition of metal NPs can guide through electrostatic interactions between the positively charged cellulose (e.g., after grafting with ammonium ions)

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Figure 15 a Experiment setup for the ultrasonication reaction. The paper was held immersed in the solution with a Teflon disk to prevent it from floating; b changing appearance of the coated papers: a uncoated paper; b 25 mM/30-min-coated paper; c 25 mM/60-mincoated paper; d 100 mM/30min-coated paper [107].

and the either negatively charged NPs or negative metal complex ions. Several of the used methods include the electrostatic assembly of citrate-stabilized metal NPs directly onto the cationic surfaces of cellulose, or the adsorption of negative metal complex ions onto the cationic cellulose followed by a reduction reaction [198]. As such, Ag NPs have been deposited from a solution of Ag? in distilled water mixed with agarose at 80 °C: a visible transformation of silver ions into Ag NPs formed on the agar-coated filter paper was noted from the color change and can be attributed to the reducing nature of agar [199]. In another research, in situ generation of Ag NPs onto bacterial cellulose nanopapers was achieved through the reduction in adsorbed silver ions by the hydroxyl groups of cellulose nanofibers, acting as the reducing agent producing a nanocomposite with embedded NPs [200]. After deposition of the Ag NPs onto bacterial cellulose from an ammonia solution, the uniform spherical NPs (10–30 nm) were generated and self-assembled on the surface of BC nanofibers, forming a stable and evenly distributed Ag NPscoated cellulose nanofiber: it is envisaged that guest molecules of Tollens’ reagent diffused into the inner spaces of cellulose nanopores and 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 [201]. The deposition of silver chloride NPs was also realized by soaking the cellulose membranes in a solution of silver nitrate and sodium chloride: the

silver ions were readily impregnated onto the cellulose fibers after immersion into AgNO3 and bound probably via electrostatic interactions, while the AgCl complexes after immersion into NaCl initiated the formation and further growth of new particles [202]. The simultaneous production of bio-cellulose from a cell-free Gluconacetobacter system together with TiO2 NPs resulted in the direct impregnation of TiO2 within the cellulose matrix, with improved thermal and mechanical properties: the nanoparticle uptake gradually increased with time, and 40% of the initially added NPs were incorporated into the after 15 days of incubation [203]. The in situ growth of TiO2 NPs under hydrothermal treatment of cellulose fibers at 120–200 °C provided flowerlike hierarchical micro- to nanoparticle structures [204]. Using the hydrothermal method, paper fibers with decorated TiO2 were transformed into handsheets that showed a good combination of photocatalytic and antibacterial activity [205]. Similarly, Bi2O3 NPs have been immobilized on the cellulose fibers of the paper matrices by a facile single-step in situ method by hydrothermally treating the cellulose fibers in an alkaline solution [206]. The Au NPs could be deposited onto unbleached softwood kraft pulp without any reducing agent or external linker molecules, suggesting that Au3? ions are efficiently bio-reduced to Au, attributed to the electron-rich lignin component, and consequently bind to the fiber surface [207]. CdS NPs with about 30 nm diameter have been formed on bacterial cellulose from an aqueous solution of Cd(NO3)2
5H2O forming Cd2? ions deposited through ion–dipole interactions, followed by immersion into a Na2S solution to form the

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complexes [208]. The ZnO particles were deposited onto water-wet freshly prepared amidoximated bacterial cellulose from a zinc acetate aqueous solution mixed with a polyol: while ZnO is nucleated via successive hydrolysis and condensation reactions followed by fast particle growth on the surface of growing particles, they have the tendency to grow into relatively spherical particles and to be anchored at the exabsorpted sites, preventing the NPs from aggregating with the help of Am-BC [209]. ZnO NPs were coated on the cellulose fibers by an ultrasonic treatment from a dispersion of NH4OH while immersing the cellulose fibers, followed by heating in order to form a network of ZnO NPs with antibacterial properties [210]. The in situ growth of silica NPs on cotton fabrics was controlled through a simple two-step method with hydrolysis (methyltrimethoxysilane in a nitric acid solution) and condensation (addition of ammonia): as a result, silica particles with size ranging from 100 to 600 nm created a nanoscale roughness feature to the cellulose fibers [211]. The in situ synthesis of platelike Fe2O3 NPs with size about 48 nm and thickness about 9 nm incorporated into a cellulose matrix has been performed with FeCl2 and FeCl3 solution: the concentration of the precursor solution did not influence the particle size and morphology, but the number of deposited NPs increased with higher precursor concentrations [212]. A different method compared to the in situ deposition of NPs is the simple coating of the outer fiber surfaces, where NPs are prepared separately, mixed with pulp fibers and washed: as such, the magnetite NP (Fe3O4) can be synthesized in colloidal suspension and deposited on cellulose fibers to yield papers with magnetically responsive properties comparable to the native Fe3O4, as the NPs are firmly bound to the fiber surface and fully encapsulate it [15]. Figure 16 shows different approaches for deposition of metal NPs onto cellulose fibers. Also, organic NPs could be formed and deposited directly in the presence of cellulose fibers. The in situ generated CNPs can be made from oxidized pulp fibers prepared by 2,2,6,6-tetramethylpiperidinyl-1oxyl-mediated oxidation of Kraft fiber with sodium hypochlorite and sodium bromide [26]. Another production technique where cellulose fibers are modified by an in situ reaction with styrene maleic anhydride, ammonium hydroxide, and plant wax under aqueous environment has been presented, resulting in the deposition of oxidized NPs with

encapsulated wax onto the fiber surface [214]: specifically, the influences of fibrillation on nanoparticle formation and permanent nanoparticle deposition on the fiber surface were investigated, in parallel with the occurrence of fiber fibrillation in the presence of ammonia. The in situ deposition of organic NPs of poly(styrene-co-maleimide) onto micro- and nanofibrillated cellulose has also be realized in order to tune the hydrophobicity of the cellulose fibers (Fig. 17). Depending on the original morphology of the fibrillated cellulose fibers, the reaction conditions should be adapted in order to achieve permanent bonding between the NPs and fibers: the surface modification was mainly governed by the fiber diameter, surface charge, and amount of wax [215]. Only a few more studies have been reported where cellulose nanofibers were directly modified by in situ deposition of organic NPs formed from acrylic monomers [216], or lignin [217].

Internal filling of cellulose fiber lumen with nanoparticles For internal filling of the fiber lumen, sufficient mixing of the aqueous mixture containing pulp fibers and nanofiller particles should be achieved to load the NPs into the lumens of the cellulose fibers while leaving most of the external surface of the fibers free from these filler particles, resulting in no effects on interfiber bonding during papermaking. Compared with the direct wet-end addition of magnetic filler particles, the filling of fiber lumen has the advantage of decreasing the detrimental effects of filler addition on paper strength. The use of cationic starch as a drystrength agent in papermaking of fibers with their lumens filled with NPs can further improve the paper strength; however, the cationic starch may disturb the location and distribution of the nanofiller. The high concentration of fillers in the lumen was found to be associated with conditions leading to a positive charge on the filler, e.g., under acidic conditions for TiO2 pigments. A low concentration of weakly bonded filler was associated with a negative charge on the filler, e.g., under alkaline conditions [218]. Fe2O3 NPs were used for filling in the lumen of pulp fibers, but required the addition of aluminum sulfate and polyethyleneimine as retention aids up to a maximum content of 2% for better filling [219]. Fe3O4 NPs have been introduced as magnetic particles by direct deposition inside the fiber lumen and in situ

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Figure 16 Approaches for synthesis of metal NP-nanocellulose hybrid composite: a reduction using an external agent, b reduction via modified nanocellulose surface, and c reduction using nanocelluloses [213]. Figure 17 In-situ deposition of poly(styrene-co-maleimide) NPs onto fibrillated cellulose, a unmodified CMF, b modified CMF, c unmodified CNF, d modified CNF [215].

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Figure 18 SEM images showing magnetic fibers a magnetite NPs introduced into the lumen by mixing the pulp and magnetic NPs; b in situ synthesis magnetite NPs inside the lumen [220].

precipitation in the presence of pulp fibers, which is more efficient than that of direct filling of the fiber lumen (Fig. 18). It has been reported that cationic polyelectrolytes, such as polyethyleneimine (PEI), can be used as an excellent retention aid to improve the retention of filler particles inside the lumen of the fibers. As such, the in situ deposition of Fe2O3 NPs into the fiber lumen was realized from a suspension containing ferrous chloride (FeCl2
4H2O) and ferric chloride (FeCl3) allowing Fe2? and Fe3? salts to homogeneously dissolved, followed by the addition of NaOH and oxidation [220]: the oxidation rate and formation of magnetite NP depend on pH, temperature, time, and rate of oxygen flow, while other iron oxides could form if conditions were not properly controlled.

Layer-by-layer (LbL) deposition of nanoparticles Layer-by-layer (LbL) assembly is a recently developed technique that has been used primarily to fabricate multilayer films on fibers. A series of materials, including polyelectrolytes, inorganic NPs, carbon nanotubes, and biomolecules, can be consecutively coated onto various types of substrates by using electrostatic interaction, hydrogen bonding, dipole– dipole interaction, or hydrophobic interaction. An alternative way for in situ modification of cellulose and nanocellulose materials includes the deposition of functional multilayers by a layer-by-layer deposition technique. The deposition and growth of Au NPs have been realized by the electrostatic assembly on wood cellulosic fibers that were alternatingly dipped in polyelectrolytes and in a colloidal nanoparticle solution [221]. TiO2 NPs were deposited on cellulose

fiber surfaces by composing multilayers from an anionic solution of lignosulfonates and a cationic suspension of the NPs, after pre-treatment of the cellulosic fibers by a cationic solution [222]. In another approach, the oppositely charged TiO2 NPs (anatase) and poly(acrylic acid) (PAA) were alternately deposited on the surface of negatively charged cellulose acetate nanofibers, while the crystalline phase of TiO2 (anatase) remained unchanged in the resultant films [223]. Conductive papers were developed by LbL assembly of conductive polymer onto wood fibers followed by paper sheet formation [224]. In both research reports, the conductive polyanion, PEDOT: PSS, was employed as the conductive element. However, different polycations (PEI [224] or PAH [225] were used, and the conductivities of the resultant papers were 0.25 and 3.5 9 10-6 S cm-1, respectively. Whether this huge discrepancy is due to the electrical properties of the polycations or the physical structure is still not clear. In addition, the chemical stability of the conductive polymer is always problematic, as the doped conductive polymers tend to lose their conductivity above room temperature. Recently, a conductive paper by LbL coating of wood fibers was made with multilayers of conductive indium tin oxide (ITO) NPs and poly(sodium 4-styrenesulfonate) (polyelectrolyte). These treated fibers were manufactured into paper handsheets through conventional papermaking methods, and the conductivity of the resultant papers was 5.2 9 10-6 S cm-1 in the in-plane (IP) direction. The LbL self-assembly technique was also applied to deposit organized multilayers of TiO2 or SiO2 NPs of 30–80 nm diameter and 50-nm-diameter halloysite clay nanotubes on softwood fibers [226]. The absorption on wood fibers or LbL deposition of SiO2

174 NPs and polyelectrolytes allows tailoring the dielectric and mechanical properties of kraft papers, depending on the pH dependent of the charge density of both layers [227]. Peng and his coworkers reported a facile method for fabricating a superhydrophobic paper by LbL deposition of poly(allylamine hydrochloride) (PAH) and lignosulfonatesamine (LSA) on cellulose fiber surfaces, followed by a heat treatment at 160 °C for 30 min. Figure 19 shows the schematic illustration of the preparation process of superhydrophobic cellulose fiber surfaces via LbL multilayers modification.

Nanomaterials risks and regulations The NMs are abundant in nature as they can be produced in natural processes. NPs can penetrate the skin and reach the blood and then other target sites in human body such as the liver, heart, or blood cells. Indeed, not all NPs are toxic and many of them seem to be nontoxic. However, some of NPs can penetrate into the cells, bind with cellular components, and cause cell death. According to toxicological data, the toxicity of NPs depends on various factors such as dose and exposure time, aggregation and concentration, particle size, particle shape, surface area effect, crystal structure effect, surface functionalization, and pre-exposure effect [229]. Many organizations have developed definitions, classifications, and legislation to identify NMs and nanoproducts. The policies on use for NMs vary among the markets used in and are application specific, e.g., for use as chemical, cosmetics, food,

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pesticides, biocides. However, there is yet no specific universal and international regulation for these materials and regulations are mostly used in a caseby-case approach. Nor are there any agreed legal definitions for nanoproducts, no internationally agreed protocols for production, handling or labeling, testing toxicity, and evaluating the environmental impacts of NPs. The European Commission has developed several pieces of EU legislation and technical guidance for NMs. In the USA, regulatory agencies such as the Food and Drug Administration (FDA), the United States Environmental Protection Agency (USEPA), and the Institute for Food and Agricultural Standards (IFAS) have developed several pieces for defining the potential risks of NMs and nanoproducts. The manufacture, import, and use of NMs are regulated under the USEPA Toxic Substances Control Act (TSCA), stating that NMs are not intrinsically hazardous per se and many of them seem to be nontoxic, while others may have beneficial health effects. Apart from that, the workplace exposure is regulated in the Occupational Safety and Health Act (OSHA), stating that the occupational exposure limits for cellulose also apply to cellulose-based NMs. Based on some studies that have been published, it remains unclear whether nanoforms of cellulose have a different hazard profile than conventional cellulose if inhaled. Where classical risk assessment takes into account the rule ‘‘risk = hazard x exposure,’’ more extensive approaches will be needed in future to cover the safety of NMs. At present, a standard risk assessment protocol for NMs has not been established. However, the risk assessment in the future will determine

Figure 19 Schematic diagram illustrating the preparation process of superhydrophobic cellulose fiber surfaces via LbL deposition of poly(allylamine hydrochloride) (PAH) and lignosulfonates-amine (LSA) on cellulose fiber surfaces [228].

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whether the NMs and their nanoproducts are hazardous and whether or not further action is justified in terms of the material itself, converting steps, intended use, and final treatment or disposal. Therefore, the best approach at present is to minimize the potential direct exposure and to work under best available practices. The potential toxicity of metal NPs to human is still a matter of considerable debate. A strong and permanent binding between NPs and paper substrate is required to reduce possible exposure.

Conclusion and outlook Usually, the manufacture of products with advanced functionality is associated with complex processes and hazardous materials. However, incorporating NMs in papermaking brings possibilities to enhance existing products and develop new low-cost, biocompatible products with various functionalities. Employing the NMs in papermaking allows to develop paper nanoproducts with novel functionalities including low-gas-permeable nanopapers, transparent nanopapers, fire-retardant nanopapers, superhydrophobic nanopapers, photocatalytic nanopapers, antimicrobial nanopapers, magnetic nanopapers, electically and thermally conductive nanopapers, sensor nanopapers, printed electronic papers, and papers for energy harvesting and energy storage. The use of nanotechnology in papermaking enhances the sustainability of papermaking processes and products, by, e.g., (1) more efficient use of resources: high strength papers can be formed with lower base paper weight, and (2) efficient utilization of side-stream materials: recycled fibers or fibers with inferior properties can be converted into strong nanofibers. A key to success is in understanding how the NMs, cellulose matrix, functional additives, and processes all interact to provide the intended functionality while reducing materials waste and keeping the processes simple and energy efficient. Therefore, good knowledge on the intrinsic properties and synthesis routes for various NMs is required as their structure or morphology, interaction with the paper substrate, and effects on the final paper properties can be changed according to the synthesis conditions. The properties of the functionalized paper rely on a combination of selected raw NMs, nanocoating pigments, nanofillers, and deposition technique. Some

novel approaches such as grafting of cellulose fibers, filling of the fiber lumen, or LbL assembly of NPs on cellulose fibers may open perspectives for more efficient functionalization of paper fibers toward given properties. While at present huge attention has been paid to the deposition of inorganic and carbon-based NMs, the potential of organic and bio-based NMs has been explored more recently and opens plenty of possibilities. The utilization of NMs in papermaking most certainly has interesting possibilities for the future, and this promising area can offer improvements in cost-effectiveness, energy efficiency, and biocompatibility, as well as giving rise to novel products with functionality that is not available today.

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