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Extraction of cellulose nanowhiskers: natural fibers source, methodology and application

C. Chaves Hernandez1 and D. dos Santos Rosa1 1Federal University of ABC, Av. dos Estados, 5001, Brazil

The application of cellulose nanowhiskers in polymeric composites is growing due to its good properties, which expands the applicability in the industrial sector, although many studies are still in the laboratory stage. Silva, R. et al. cites many segments for application of natural cellulosic fibers, wide applicability detaching, from textile industry to some studies for the use of fibers such as heavy metals absorbent in industrial waste treatment.[1]

For a good understanding of the methodologies used to obtain cellulose nanowhiskers as well as their sources and their final applications, this chapter will propose a small literature review about this topic. Initially, will be approached the obtaining sources present in the literature, emphasizing the existence of different properties from the variation of raw materials. In a second step, will be discussed the methodologies employed, their advantages and disadvantages, highlighting the best results obtained in each case. In the final topic, will be shown the importance of the field research through the final application of cellulose nanocrystals.

Keywords: nanowhiskers; nanocrystals; cellulose; natural fibers; methodology.

1. Introduction

When we think of cellulose, the first thing that comes to mind is the paper, in its various forms processing. However, by studying their characteristics, we found that their chemical characteristics, obtaining and application, go beyond what is familiar. Cellulose is a natural polymer, abundantly found around the world, as the component in greater quantities in plants, and the wood is its most commonly source exploited by the cellulose industry. Cellulose chemical composition is basically carbon, hydrogen and oxygen and the composition gives you the carbohydrate rating. In its native form, it is known as cellulose I, which is divided into two classifications, cellulose I and Iβ.[2] Due to its complex system of hydrogen bonds, it has the tendency to crystallization and is formed by crystalline and amorphous microstructures, being also formed of crystalline microfibrils. Moreover, it has a strong resistance to alkaline treatments, but can easily be hydrolysed by acids, being relatively resistant to oxidizing agents. According to the literature for producing cellulose nanowhiskers from plants, it is necessary that the structure be deconstructed.[2,3] At this point, we come to the focus of this chapter, the cellulose nanowhiskers. The nanowhiskers are extracted nanoparticles pulp fibers, most often through acid hydrolysis process. This name received nanowhiskers, is due to its physical characteristics of rigidity, thickness and length. Are structures that have dimensions on the nanometer scale, also composed of crystalline and amorphous regions. In the literature you can find other classifications related to that material, such as nanocrystals, crystallites, nanofibrils or cellulose whiskers.[4,5] According Silva, D. J. and D’Almeida, M. L. O. apud Samir et al., the growth of individual high purity crystals is allowed by virtue of the cellulose whiskers be regions that grow under controlled conditions. The high ordaining of its structure may confer high tensile strength, and significant changes in other features such as electrical, optical, magnetic, ferromagnetic, dielectric and conductivity.[6] Other features that have favored the use of nanowhisker as reinforcing agent is the fact that it has a large specific surface area (estimated at hundreds of m2.g-1), higher modulus (about 150 GPa), high ratio appearance (diameter/length) and large ability to act as reinforcement with meaningful results, even at low application percentages, also possess low density, non-abrasive nature, providing less wear to the equipment at the time of processing, non-toxic character, biocompatibility and biodegradability. Despite showing several positive aspects in this material, there are some points that make it disadvantageous, as such as high hydrophilicity and low thermal stability, thereby limiting its processability along some polymeric matrices.[5,7] Nascimento, D. M. et al. exposes factors such as low production cost, easy obtaining process , low density, non-abrasive nature, biocompatibility and biodegradability as advantages to be considered to encourage the use of cellulose nanowhiskers instead of carbon nanotubes as fillers or reinforcing agents.[5] All the features listed above drive forward the cellulose nanocrystals research and gain space in laboratories around the world. This research field has shown promising and is positively contributing to science and technological aspects, inasmuch as it generates a large volume of publications. Some of these will serve as basis for the next topics to be presented in this book chapter.

Polymer science: research advances, practical applications and educational aspects (A. Méndez-Vilas; A. Solano, Eds.) _______________________________________________________________________________________________

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2. Natural fibers source

Many plant sources are used for the cellulose extraction in the studies of nanowhiskers as aforesaid, although its composition differ from one to another as can be seen in Table 1. In most cases, the plants sources used for cellulose nanowhiskers extraction are leavings that does not have an exact destination or are considered unsuitable for use by industry, such as corn straw waste, banana crop residues, wood chips, wheat straw, among others. The use of waste considered non-reusable is expanding, so that environmental issues and the need for biomaterials development is the focus of current science. Table 1. List of examples of natural cellulose fibers and their percentages in the composition.

Fiber Source Origin % Cellulose Reference Banana Leaf 60,0 – 65,0 [8,9]

Coir Fruit 32,0 – 43,0 [10] Cork bark Leaf 12,0 – 25,0 [11] Corn cob Stalk 33,7 – 41,2 [11] Cotton Seed 82,7 – 95,0 [11] Curaua Leaf 63,4 – 73,6 [9,12]

Flax Stem 64,0 – 84,0 [9,11] Hardwood Stem 39,0 – 50,0 [11]

Hemp Stem 67,0 – 78,0 [9,11] Jute Bast 51,0 – 78,0 [9]

Kenaf Bast 44,0 – 72,0 [9] Maize Straw Straw 28,0 – 44,0 [13]

Nettle Bast 53,0 – 86,0 [9] Ramie Bast 67,0 – 99,0 [9]

Rice Husk Straw 25,0 – 35,0 [14] Softwood Stem 42,0 – 50,0 [11]

Sugar cane bagasse Stem 32,9 – 50,0 [11] Sisal Leaf 60,0 – 73,0 [9,11]

Wheat Straw Stalk 30,0 – 35,0 [11,15] The fibers chemical composition varies according to its type, but it’s basically composed of cellulose, hemicellulose, lignin and pectin, each one with different characteristics, as will be shown below to be treatise on each component. Due to this composition, natural fibers are regarded as natural composite wherein the cellulose fibrils are wrapped in a lignin matrix.[3,16] Each of these components has its own characteristics, cellulose has a structure formed by D-anhydroglucose (C6H11O5), repeated 1,4-β-D-glucoside links linked at C1 and C4 positions and presents about 10000 polymerization degree . Each unit repeated in its structure contains three hydroxyl groups, wherein these groups and the hydrogen bonds capability to be responsible for the crystal packing direction and the physical properties of cellulose.[2,3] The hemicellulose is not a type of cellulose, although its name do this kind of analogy. It comprises polysaccharides groups with a sugars and carbon rings C5 and C6 combination. Hemicellulose differs from cellulose in certain aspects, this one contains several different sugar units whereas cellulose unit presents only 1,4-β-D-glucopyranose. Another aspect which differs is that it has a considerable degree of branching with pendant side chains containing groups that will be responsible for a non-crystalline nature, whereas cellulose is a linear polymer. Finally, another issue of considerable importance is that the native cellulose has degree of polymerization between 10-100 times higher than hemicellulose (degree of polymerization: 50-300). The function of hemicellulose is to form a support matrix for the cellulose microfibrils.[2,3,9] Lignin is considered a complex hydrocarbon polymer, aliphatic and aromatic and which has generally a high molecular weight. It has a hydrophobic nature and is completely amorphous, being totally insoluble in most solvents, and cannot be discriminated as a monomeric unit. It is responsible for the stiffness characteristic of plants. The lignin can be considered as a thermoplastic polymer having a glass transition temperature (Tg) at approximately 90 ° C and melting temperature about 170 ° C. When submitted to hot alkali treatment it can solubilize, but it’s not hydrolyzed by acids.[3,9] The last natural fiber component in importance is pectin which is a collective name given to the heteropolysaccharides, being responsible for the flexibility of plants.[3,9] There’s two type of plants classification to be chosen as a source for natural fibers extraction , being considered as primary plants those ones are grown due to its high fiber content in the composition, for example, jute, hemp and sisal, while the secondary ones are produced as a by-product, like pineapple, oil palm and coconut crop residues. Yonder this plants classification , there’s a fibers types classification founded, divided into six types. The inner bark fibers are found in plants such as jute, flax, ramie and hemp; the leaf fibers can be obtained from sisal, pineapple, while the seed fibers


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are extracted from coconut and cotton fiber. Already the core fibers are found in hemp and jute, the reed fibers, wheat, corn and rice and lastly, there is a general name for all other types found in wood and roots. As can be seen in Table 1, there is a vast amount of suitable fibers to be worked, expanding the range of options cellulose nanowhiskers sources to be used by researchers and this classification comes to assist them in this choice.[16,17]

3. Methodologies

It is known that the natural fibers are incompatible with the hydrophobic polymer matrices, tending to agglomerate, because these fibers are hydrophilic and, due to this reason, exhibit a poor resistance to the mix. To correct this problem of high water absorption, it is necessary to treat the fibers by aliphatic and cyclic hydrophobic structures, since these structures have reactive functional groups that are capable of forming hydrogen bonds between these reactive groups and the matrix polymeric and through these treatments there is a tendency to make the fibers hydrophobic, thereby providing improved adhesion between fiber and matrix.[16] Due to the large number of studies conducted to date, there are several methods used for treatment and extraction of natural fiber cellulose nanowhiskers.[16–20] The pulp forming structure allows the isolation of nanofibrils by methods which facilitate the breaking of glycoside bonds in nanodomains disordered. For this extraction, they can be employed mechanical or chemical processing methods and even both processes at different stages.

3.1 Alkali Treatement

The alkali treatments are used for cleaning and surface modification of natural fibers, thereby reducing surface tension and improving adhesion between the fiber and the polymer matrix. Liu, W. et al. show the effect of alkaline treatment on the structure, morphology and thermal properties of native grass fibers, which were treated with sodium hydroxide solutions with concentrations of 5 and 10% by weight in water, remaining in these solutions for varying times. Due to the grass structure, that is very sensitive to alkali treatment, how much more the concentration of the solution and also residence time in contact with the same increases, the fibrous region becomes more pronounced as it is removed from the area intrafibrilar. This indicates that larger amounts of hemicellulose and lignin are removed from the fiber structure, and reduction of the amount of hemicellulose, the interaction between the fibers is reduced, which facilitates their separation. The maximum fibers degradation temperature improves gradually with increasing concentration and residence time in contact with the solutions, showing that the treatment improves the thermal properties of the fibers. This change in thermal behavior is also due to lower amounts of hemicellulose and lignin in its structure. It is possible to establish a correlation between increased concentration of the solutions, the treatment time and the thermal stability of the fiber.[18] The fibers alkali treatment can be done in more than one step, initially can be done a treatment to removal of organic substances with low molar mass and greases commonly present in the plant structure, and this first step is called mercerizing. The second step is bleaching, much used by the paper industry, where the main goal is to remove lignin and hemicellulose from the fiber composition, in addition to providing the removal of characteristic color, leaving white. For a good result of bleaching, in some cases, it was was necessary to repeat this step a few times, varying according to the result expected by the researcher.[19–21] As Xue, L. et al. describes in his work, the alkali treatment is one of the most common on fiber for subsequent application for thermoplastics and thermosets reinforcement , because its action is on the hydrogen bonds, generating an improvement in the contact surface, increasing surface roughness. The author reports that by this treatment are removed substances such as lignin, oils and greases that cover the outer surface of the fiber cell wall , in addition to depolymerize cellulose, exposing the crystallites of short duration. Thus, it is reported that this treatment has two effects on the fiber: the surface roughness increases, resulting in better mechanical lock; and increases the amount of cellulose exposed on the fiber surface, thereby increasing the number of possible reaction sites.[21] It can be argued that the alkali treatment causes the improvement in mechanical properties of the fiber such as increased tensile strength, modulus, impact and stiffness, but must be paid attention to the solution concentration used in the treatment because in large concentrations the opposite effect may occur, worsening the mechanical properties of the fiber. This loss of properties at higher concentrations is due to the fact that there is an excess of natural fiber delignification, resulting in a weak or damaged fiber. The tensile strength of the composite decreased dramatically after exceeding the optimum concentration of NaOH.[16,21] The delignification (dewaxing) is generally performed with alcohol or benzene, and the NaOH treatment, followed by drying at room temperature. Many oxidizing bleaching agents such as sodium hypochlorite and hydrogen peroxide are used commercially. Bleaching usually results in weight loss and tensile strength. These losses are mainly attributed to the action of a bleaching agent or an alkali or alkaline reactant in the non-cellulosic components of the fiber such as lignin and hemicellulose.[16] The alkali treatment is considered as a pretreatment for subsequent treatment with acid, where the most commonly used reagent for the treatment is sodium hydroxide (NaOH), could be used alone as well as in solution with hydrogen peroxide (H2O2), but there are also examples of using potassium hydroxide (KOH), calcium hypochlorite (Ca(ClO2))


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and sodium chlorite (NaClO2). This treatment is a very important step for the fibers preparation to be retrieved later nanofibers.[4,16,19,21,22]

Fig.1. Examples of photomicrographs of maize straw waste treated by alkali: a) mercerizing solution of Ca(ClO)2 + AcOH. b) bleaching with H2O2 + NaOH solution.

3.2 Acid Hydrolysis

Martins, D. F. et al. reports that the acidic hydrolysis is the method most known and common used for the extraction of cellulose nanowhiskers, could be performed using sulfuric acid (H2SO4) or hydrochloric acid (HCl). This method is based on the fact that the crystalline regions are insoluble in acid, under the conditions used for extraction. The extraction process and the cellulose source are extremely influent on the morphology and other properties of nanowhiskers thus choosing the hydrolysis process to be used becomes an important step in the successful obtainment of the nanocrystals. It is known that the use of different acidic solutions may cause differences in the stability of the colloidal solution, due to the presence of different loads on the fiber surface. The use of H2SO4 for extraction leads negative sulphate groups introduced on the outer crystals surface during the hydrolysis process, and it is considered responsible for the stabilization of crystals in the resulting colloidal solution, although the presence of sulfate groups cause the reduction thermal stability, since a large amount of sulfate group on the cellulose leads to decreased thermal degradation of the cellulose. If hydrochloric acid is used instead of sulfuric acid to hydrolyze the native cellulose, the thermal stability of the prepared nanocrystals is improved, but the nanocrystals are likely to agglomerate due to a lack of electrostatic repulsion force between the particles crystal, resulting in an unstable solution.[23] The use of acid hydrolysis treatment for cellulose nanowhiskers extraction cause digestion of the pretreated fiber structure amorphous region, resulting in crystalline nanoparticles.[24] Siqueira, G. apud De Souza Lima and Borsali, describes the principle of the cellulose amorphous regions disruption , to produce cellulose nanocrystals. The hydronium ions can penetrate the material in these amorphous domains that promote hydrolytic cleavage of glycosidic bonds releasing individual crystallites.[25] Still in his work, Siqueira, G. et al. mention some studies where was assessed the stability of colloidal suspensions of cellulose nanocrystals. Dufresne, A. report that the stability of the nanocrystal suspensions depends on the dimensions of their dispersed particles, the size of its polydispersity and its surface charge. Yet in that direction Araki et al., also cited by Siqueira, G., compared the effects of the sulfuric acid use or hydrochloric acid to produce stable suspensions of cellulose nanocrystals. As could be seen, the sulfuric acid could provide more stable aqueous suspensions than the hydrochloric acid because hydrochloric acid produces cellulose nanocrystals with minimal loading area and the prepared nanocrystals by hydrolysis in sulfuric acid have a negatively charged surface due to esterifying the hydroxyl groups of the surface, for generating sulfate groups.[25] Another important issue of using acid hydrolysis for the extraction of natural fibers, cellulose nanowhiskers is the size particle reduction. Johar, N. et al. commented on the results that the treatment should eventually reduce the size of the micro fibers to the nanoscale, the results for distribution in diameter and aspect ratio of the fibers showed that many of the nanoparticles are contained in the range of 15 - 20nm and 10 - 15nm, respectively.[26] Variations in the hydrolysis process may influence the particles size obtained, and the variation of the acid treatment time may be a factor of changing the diameter of the fibers, by increasing the contact time of the fibers with the treatment yields particles with smaller diameter, in addition to reducing the size variation, narrowing its range.[6]


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Fig.2. Example of photomicrograph of maize straw waste treated by acid hydrolysis with H2SO4 solution.

3.3 Oxidation mediated by 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)

The nanofibriled cellulose can be produced through oxidation method 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and subsequent mechanical dispersal in water. This method has some important features: the final material can be dispersed in water, with almost all completely individual fibrils having homogeneous widths of 3-4 nm; there are abundant presence of carboxylate groups on the surfaces of crystalline cellulose fibrils (≈ 1.7 nm-2 carboxylate groups), by which the electrostatic repulsion and/or working of the osmotic behavior, is effective between the fibrils obtained by this method anionically charged water.[27–29] Iwamoto, S. et al. reported in their study that the native wood pulp oxidation using radical TEMPO as a catalyst in an aqueous medium with sodium hypochlorite (NaClO) and sodium bromide (NaBr) at pH 10 may cause the formation of C6 carboxylate groups present at the surfaces microfibrils, maintaining the original crystallinity of cellulose I and the crystal width.[28] The films prepared by TEMPO-oxidation of cellulose nanofibers dispersed in water have good characteristic properties such as good transparency and resistance to stretch, low thermal expansion and very low values of oxygen permeability.[28,30] These characteristic properties of the films produced by this method is evaluated by Fukuzumi, H. and colleagues, where cellulose nanofibrils were produced with an average width ≈ 4 nm, but with different lengths 200, 680 and 1100 nm. By analyzing the viscosity average degree of polymerization (DPV) for each of the nanofibrils produced, the values were found of 250, 350 and 400, respectively. As can be seen, the nanofibrils shorter length produce DPV values but have high light transmittance, both for nanofibrils dispersed in water, as for the films. In contrast, neurofibrils more length show better results for tensile strength and elongation at break for the film. The barrier properties found for all three films with different nanofibrils lengths vary, wherein the nanofibrils with greater length have better barrier to oxygen. However, for the barrier properties to water vapor, the length of nanofibrils provided no significant effect and were mainly influenced by the water vapor transmission rates hydrophobic base film.[27] In another work, Fukuzumi, H. et al. analyzes the thermal properties of the nanofibrils obtained from bleached kraft pulp produced by TEMPO-oxidation and also subsequent treatment with calcium chloride solution (CaCl2), calcium acetate (Ca(OAc)2), calcium nitrate (Ca(NO3)2) and calcium iodide (CaI2) to ion exchange, converting sodium carboxylate groups into carboxylate groups calcium. The thermal decomposition (Td) point of the nanofiber obtained by TEMPO-oxidation was 222°C, while for the original pulp was 275°C, the derivative peak temperature of thermogravimetry (DTG) from both samples were 273°C and 314°C respectively. These decreases in Td and DTG values are due to the decarbonation anhydrous glucuronate units, which are formed by the cellulose TEMPO-oxidation. Td fibers for alkali treatment decreases to 264°C due to the removal of 86% of the carboxyl groups present in nanofibrils and subsequent conversion of residual sodium caboxilatos groups to free carboxyl groups. When nanofibrils are subjected to treatments with solutions of CaCl2, Ca(OAc)2, Ca(NO3)2 and CaI2, there is increased DTG peaks, reaching more than 300°C, it is possible to improve the thermal stability of nanofibrils obtained by this method.[30]

3.4 Mechanical

The mechanical methodology of cellulose nanofiber extraction presents some advantages over chemical treatment processes being an environmentally ecological method that does not require the use of solvents or chemical reagents. Moreover, the characteristic properties of the obtained material presented puts it as a viable material for application as reinforcement in polymer matrices.[20,31] The mechanical process consumes energy during its performance, however, allows the use of all waste for production of nanofiber, while the chemical process, approximately half the residue is dissolved during the treatment.[10]


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Mitbe, A. et al. presents a comparative study of two forms of extraction corn stover cellulose nanofibers, by acid hydrolysis and by mechanical means. Initially, the waste was treated by basic means and after the cellulose pulp was extracted , it was submited to these two processes. The used mechanical process consists of two steps: initially the natural fiber was processed into a mechanic mixer and subsequently passed through a mechanical grinder. By comparing the results of the nanofibers obtained by this method with nanowhiskers obtained by chemical treatment, it is possible to check that the dimensions of nanofibers were 4-10 nm in diameter and long in some microns, while nanowhiskers chemically treated have a diameter between 3-7 nm and a length of 150-450 nm. As for crystallinity results, the nanofibers showed degree of crystallinity of 66.4%, while for the nanowhiskers, found the result of 72.6%, this degree of crystallinity lower is because the mechanical process to break the crystalline domains the cellulosic fibers. Regarding the mechanical properties, the nanofibers obtained by mechanical process show improvement of stress transfer fiber-to-fiber, which means improvement in mechanical properties, and thermal properties of this material were also improved, with greater stability.[20] Another author who used the mechanical processing in their study was Oksman, K. et al. that made a comparison between nanowhiskers obtained by ultrassonification, homogenization and acid hydrolysis. The degree of crystallinity of the material introduced were 73% after ultrassonification, 77% after homogenization, and 75% after acid hydrolysis. through this comparative study, the author was able to show that the nanowhiskers obtained by mechanical means have better thermal stability than those obtained by chemical treatment. However, this stability is not higher than the thermal stability of native cellulose due to the cellulose chains are shorter and have lower degree of polymerization. Regarding the dimensional residues obtained by sonication and homogenization had sizes of about 10 nm.[32] Although has been presented ultrassonification methodology, the homogenization processes and grinding fibers gained greater focus on the production of micro and nanofibrils.[9]

4. Aplications

A material having good properties as cellulose nanowhiskers may be designed for various applications, ranging from applications by the textile industry to applications in materials that require high performance. In the automotive industry for example, natural fibers is used to replace non-renewable sources, which also enables the production of lighter and secure parts, since these materials do not generate sharp edges when broken; besides, it possessing excellent mechanical properties, are less abrasive, causing less wear to the processing equipment and being biodegradable after exhausted its lifetime , may be discarded. In addition, the lignocellulosic fibers are excellent for use in the polymers chemistry and composites as reinforcement in polymer materials, because it is a sector in great economic expansion, scientific and technological knowledge partially transferred to the productive sector. What deserves more attention from the scientific community is the intense use for polymers and composites development to avail fully the unique characteristics of the wide variety of existing lignocellulosic matrices and to obtain particles with nanoscale dimensions of the fields highlighted in progress studies, as is the case of nanowhiskers.[1] As noted by Rosa, M. F. et al.[19], different approaches have been used for the preparation of nanowhiskers, all leading to obtaining different nanofibrils, depending on where the pulp is extracted, its pretreatment and the actual disintegration process. During their study, it was used as the lignocellulose fiber coconut shell, which has high strength and durability because of the high lignin content, when compared to other natural fibers. In recent studies we have demonstrated positive effects on the thermal stability and mechanical properties of the composites and blends with lignin. Some blends presented in these studies had composed formulations, for example, polylactic acid + Thermoplastic starch (TPS) + cassava bagasse; Polyvinyl Alcohol + fibers from banana; Polyethylene + extracted cellulose whiskers fiber ramie; also showing the variety of polymer matrices that can be applied lignocellulosic fibers as reinforcement. Another factor to consider is that in polymer composites the lignin may have a dispersing agent role as for improving dispersion of cellulose whiskers.[19,33–35] Eichhorn, S. et al. presents other applications for nanocomposites with cellulose nanowhiskers. Adhesives used by the timber industry may be improved through the incorporation of cellulose nanofibers as reinforcing agents. The epoxy reinforcement study with fibrous filler, an adhesive that is not commonly used by the timber industry has shown that it is possible to improve the hardness of the adhesive mixtures. Thus, when comparing these materials with various types of adhesives used by the timber industry, it is observed that there is a strong correlation between the hardness and adhesive strength of the bond between the adhesive and wood. The wide use of urea-formaldehyde (UF) as wood glue, because of its low cost, is hampered by its fragility and tendency to present microcracks, limiting its mechanical performance. In a study cited by Eichhorn, S. apud Wolfgang Gindl and Josef Keckes about adhesive with nanocelulose reinforcement, we obtained a great result that indicated that UF adhesive was possibly hardened after the incorporation of cellulose nanofibrils. This hypothesis is confirmed by the fact not exhibit microcracks in the surface of this modified adhesive, furthermore, the shear strength was found for enhanced adhesive cellulose is 13.8 MPa, whereas the value found for structural adhesives is 10MPa, indicating that the presence of pulp considerably improves the mechanical performance of the bonds between adhesive-wood.[2] There is also the possibility to incorporating nanocrystals pulp for the production of coatings, as the Borsoi, C. and collaborators study , which were made composites of polyaniline (PANI), aminopropyltriethoxysilane (APS), epoxy


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and nanowhiskers, applied to carbon steel with a zirconium conversion coating (Zr). It was found that the nanowhiskers incorporation in PANI improves its flexibility properties for cracks reduction and improve adhesion because were not seen detachments of the polymeric coating. Through morphological study has verified that the nanowhisker functionalized with APS has better dispersion results in epoxy, showing no clumps. Through this study, it was viable the use of this composite as carbon steel coating.[36] Another interesting application for cellulose nanowhiskers is in electronic displays and solar cells where is sought good optical properties such as transparency. Eichhorn, S. apud Hiroyuki Yano showed a study of reinforced composites cellulose nanofiber, using various polymer matrices and achieving good transparency on it, combined with excellent thermal expansion coefficient results (CTE). These composites intended for use in electronic displays and solar cells also require flexibility, and they need to be suitable for the processes of roll-to-roll production, that allowed continuous deposition of functional materials such as wiring metal, transparent film, conductive films and barrier to gases in an optically transparent, flexible plastic roll, thus permitting the use of a production process at low cost, relatively simple and suitable for the production of flexible electronic devices. One difficulty faced by the manufacturing sector of this type of device is that many of the polymers have a high CTE in the order of 50 ppm.K-1 and this value is even greater when it comes to flexible polymers exceeding 200 ppm.K-1. When done the deposition of functional materials on polymeric substrates, it can have consequences as cracks and other damage caused by varying temperatures employed in assembly processes due to the mismatch of thermal expansion coefficients of the different materials. Due to present these points of difficulty in developing a composite with transparency and low CTE (similar to glass, 8 ppm.K-1), it is extremely important to the completion of the roll-to-roll process. Thus, the addition of cellulose nanowhiskers is shown to be ideal, since components with a diameter less than one tenth of the wavelength of visible light are free light mirror, contributing to good optical results.[2] Still in his work, Eichhorn, S. apud presents the application of the cellulose nanowhiskers in a combination with deoxyribonucleic acid (DNA), showing the wide application for this material, it can be used even in biomedical devices. In the studies presented by the author, DNA oligomers were used to control connections with cellulose nanowhiskers showing be possible to combine the low cost of such materials with its versatile chemistry for the production of nanoscale devices. This material after junction DNA/nanowhiskers, still can not be regarded as a composite, but this material opens doors for the future are created nanocomposites based on the structuring of biomaterials by self-assembly methods.[2] The cellulose nanocrystals applications in the field of biomedicine do not stop there, recent studies have shown the importance of their use for the production of materials for drug delivery. In their study, Villanova, J. C. O. et al. showed the importance of improve the development of suitable excipients for producing pills by direct compression technique with a good response for controlled release formulations. So, we evaluate the physico-chemical and flow properties for a new polymeric excipient ethyl acrylate, methyl methacrylate and butyl methacrylate, synthesized by suspension polymerization using nanowhiskers cellulose as co-stabilizers to be used as direct compression modified release pills. The carrier granules obtained have a spherical shape are called cellulose nanowhiskers beads (CNWB). Analysis of the particle size of the granules found that the inclusion of cellulose nanowhiskers lead to reduction of the particle size and distribution of particle size in a narrow range. By in vitro testing, it was found that the obtained material is non-toxic, and, by the parameters Hausner ratio Carr index and cotangent angle, it was found that the material has good flow properties. Furthermore, the granules were stable during the compression process and when in contact with water. Therefore, the granules CNWB have been designed to be biocompatible for pharmaceutical applications and, the dissolution profiles achieved with CNWB, as carrier of propranolol hydrochloride pills showed that the proposed excipient formed matrices that are capable of releasing the drug for 12 hours. However, the matrix formation can be influenced by the presence of other components and parameters involved in the compression process, which requires further study.[37] In the same context, Mauricio, M. R. and collaborators investigated the synthesis of micro starch hydrogel composite with cellulose nanowhiskers for drug delivery system. The introduction of vinyl bonds in the starch and cellulose nanowhiskers, played a role binder forming crosslinked, forming a micro hydrogel. The cellulose nanowhiskers can act as an emulsifying agent for the emulsion, while improving the sphericity and uniformity of the microparticles. The drug release was regulated in response to changes in CNW quantities because it was through the kinetic of modeling of the release, that it was found that the drug release is trigged by an anomalous mechanism and that, through the addition of whiskers in microparticles starch, it changes this mechanism, making the release rate 2.9 times slower when the addition occurs.When combined with starch, cellulose nanowhiskers played a role in retarding the release of the drug.[38] Another interesting application of controlled drug delivery system was presented by Kolakovic, R. et al. who developed a cellulose nanofiber film as a matrix for the material, with prolonged duration function (within 3 months) and support for drug administration. The film was produced by filtration of produced nanofibers and loaded with the drug, being a easy process to run, with only three steps and 90% effectiveness, and the final drug loading in the range 20 - 40%. The films have excellent mechanical properties and can be easily handled after the preparation and in drug release studies was observed a sustained release drug along periods of up to three months, with drug release kinetics depending on the drug used. The cellulose nanofiber films have shown promise for use in extended-release


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drugindicated for use with sparingly soluble drugs in water in order to sustain release for up to 90 days. Considering the slow rate of release and small amounts of drug released within 24 h, this system has not proved suitable for oral application. However, it can be used in applications requiring sustained release over a period of time the drug in other ways, for example, parenterally (implant), topical (transdermal patch), or ocular administration.[39] But not only in the advanced materials is that cellulose nanowhiskers are applied, the search for improvements in mechanical properties of most common materials industry, contribute to the increase in numbers researchs in this area. Menezes, A. J. et al. shows a study where is reinforced polyethylene (PE) with cellulose nanowhiskers, by extrusion to form a nanocomposite. This study shows the possibility of cellulose nanocrystal processing by an completely industrial process means without damaging the properties desired for the material. The homogeneity presented by nanocomposites increased with the length of the grafted chains. a significant improvement in elongation at break was observed when sufficiently long chains are grafted on the surface of the nanoparticles.[35] Still in search of improvements in polyethylene properties, Castro, D. O. et al. in his research reinforced biopolyethylene (green polyethylene) of high density (HDBPE) with cellulose nanowhiskers extracted from curauá fiber, using castor oil, soy and linseed epoxidized as compatibilizers. The whole process was conducted by extrusion and hot pressing, aiming to evaluate the dispersion of nanowhiskers in the polymeric matrix. Through the analysis of TGA/DTG was possible to verify that the films showed an increase in temperature degradation and narrowing the temperature range in which it occurs, probably for protection generated by oil on the surface of cellulose nanowhiskers. When using oils, it is also observed improvement in tensile strength and Young's modulus and also has improved dispersion of the nanocrystals in the film.[12] The biopolymers may also be reinforced with cellulose nanowhiskers, because due to their organic nature, tend to degrade when exposed to favorable environmental conditions. Furthermore, it is known that these polymers do not have good mechanical properties and with the strengthening from cellulosic materials, they tend to improve. Robles, E. et al. studied the changes of lactic acid poly behavior (PLA) after application of cellulose nanocrystals extracted from blue agave, observing the improvement in mechanical composite properties, as well as evaluating the PLA nanofiber obtained by mechanical process and nanocrystals obtained by acid hydrolysis, noting that the different ways of obtaining cellulose nanoscale can provide different characteristics to the final composite, such as improved mechanical properties and hydrophobicity (because of the non-polar covalent bond formation between hydroxyl agents and free coupling) also improves dispersion within the matrix, which is important in creating materials with barrier property to water.[40] Another study by Hossain, K. M. Z. et al. showed the possibility of incorporating cellulose nanowhiskers to PLA, poly forming a self lactic acid (PLA SR) composite reinforced. This composite was produced by a PLA fibers oriented coating process with a mixture of polyvinyl acetate (PVAc) and cellulose nanowhiskers as a binder prior to hot compression at 95°C. Upon cross analysis of the PLA SR composite after hot pressing, it confirmed that the PLA fibers had maintained their morphology. The incorporation of 8% by weight of cellulose nanowhiskers within the PLA SR composites showed an increase in flexural strength (48%) and modulus (39%) compared with the control compound (flexural ≈ 82 MPa and module ≈ 3.9 GPa). Furthermore, while the composite control PLA SR was found to be quite brittle, and the addition of nanowhiskers PVAc gave the self reinforced composite a more ductile behavior. Another interesting feature is that the control composite showed a complete breakdown of the consolidated fibers, while the self reinforced composite showed a certain alignment in fiber portions, even after full flexion of the samples, which reduced the chances of occurrence of sudden failures during bending test.[41] In a recent study, Haafiz, M. K. M. et al. also reinforced PLA with cellulose nanowhiskers extracted residues from of palm oil production. the mechanical and thermal properties of the composites produced were verified. For the result of tensile strength, the composite with 3 phr (per hundred rubber) of nanowhiskers showed an improvement of 61% compared to pure PLA and also showed a significant change in the PLA Young's modulus, but declined the elongation at break due to the loss of mobility of the polymer chains. The nanowhiskers dispertion was better for composites with small charges than the big charge composites (5 phr). The Tg, Tc and Tm temperatures were improved with the addition of nanowhiskers, although the Thermal stability (T10, T50 and Tmax) analyzes points to a linear decrease according the nanowhiskers load is increased. The author sets the composite developed as a potential material for the production of coating membranes, packaging for foods and agricultural products as well as applications in the automotive industry, parts where high temperature stability is not required.[42] Pereira, A. L. S. et al. also studied the improvement of nanocrystalline cellulose incorporation into a biopolymer, and polyvinyl alcohol (PVOH) used as template. The use of cellulose nanowhiskers in biodegradable polymers is to increase the versatility of the polymer can be used in unusual final products. By studying this merger, the author checks the application feasibility of PVOH packaging. Thus, found that the composite with 3% concentration of nanowhiskers was improvement in tensile strength, the maximum elastic tensile modulus, and the barrier properties of PVOH films, with small effects on color and brightness, not compromising the transparency of the films. This overall benefit of the films of nanowhiskers PVOH has an important application of nanocelulose packaging material.[33] The use of biodegradable polymers for the development of packaging materials have gained prominence because is very high the volume generated in the post-consumption, aggravated by be a short life cycle material. Moreover, the use of environmentally friendly material has been shown promising in replacing petroleum polymers, as well as enhancing


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the agricultural sector, with allocations to some industry waste. The use of starch from tapioca and/or corn shows promise for this purpose, because it combines its wide availability at a relatively low price. The films produced using thermoplastic starch (TPS) are described as: isotropic, odorless, tasteless, colorless, nontoxic and biodegradable characteristics favor their use. But in contrast to this, they have poor tensile properties and high permeability to water vapor compared to conventional petroleum-derived films due to its hydrophilic nature and its sensitivity to moisture and difficult factors to be controlled. Savadekar, N. R. and Mhaske, S. T. showed that the incorporation of cellulose nanofibers in thermoplastic starch, even at very low ions, have improved properties. The concentration of 0.4 wt% nanofiber TPS films show 46.10% improvement in tensile strength compared to the base polymer film, while at 0.5% by weight, the tensile strength worsens. Permeability testing water vapor (WVTR) and oxygen transmission rate (OTR) indicate improvements in vapor barrier properties of TPS water matrix. These results are due to factors such as the nanometer size effect of nanofiber (high L / D), the high content of crystalline regions of cellulose, homogeneous dispersion of nanofibers within TPS and the strong interaction between matrix/nanofiber.[43] Nasri-Nasrabadi, B. and colleagues also showed satisfactory results on analyzes TPS reinforced cellulose nanofibers, it was found that the yield strength and Young's modulus of the nanocomposites have improved when compared with pure TPS film. The glass transition temperature (Tg) of the films was shifted to higher temperatures as the concentration of nanofibers had been increased. The resistance to moisture absorption of the film improve with the concentration of 10% by weight of cellulose nanofibers, although the transparency of nanocomposites has reduced compared to pure TPS films.[44] In the same context, Alves, J. S. et al. studied the thermoplastic starch enhanced with cellulose nanocrystals and gelatin, where the objective was to determine the mechanical properties of the composite, and check if there was improvement in water permeability of the films. The mechanical properties of the films studied increased significantly with the addition of gelatin and cellulose nanocrystals, another satisfactory results for the films with low gelatin content and cellulose nanocrystals was that showed maximum degradation temperature, although the results for water permeability have shown improving only on the film produced with the addition of only gelatin.[45] The applications for cellulose nanowhiskers are in increasing advancement, proving to be a promising research field and with great relevance since short life cycle products such as packaging, even products that require greater care and technologies, such as solar panels, electronic systems and even in biomedicine, or even as controlled drug delivery systems. The researchs should follow the most diverse ways, until this material get the production lines and could be sold in an economically viable way.

5. Conclusion

The agro-industrial waste gaining valueallied with the wide variety of lignocellulosic plants, turn feasible economically and environmentally to obtain cellulose nanocrystals, contributing to the reduction of waste improperly discarded by industry and also provide the production of biodegradable products, environmentally friendly. The processes used to obtain either chemical or mechanical, prove to be adequate and effective. It can be seen that the extraction of lignin and hemicellulose is of utmost importance for the successful treatment of the vegetable fibers and subsequently obtain cellulose nanowhiskers. The alkali treatment is the basis for the final treatments to obtain cellulose nanowhiskers wherever it is the mean: by acid, by TEMPO or mechanical means. Because it is a material with good mechanical, optical, and electrical properties, and prove compatible with various other materials such as polymer matrices, there is a wide range of applications, becoming an important material in engineering polymer.

Acknowledgements The support by UFABC, CNPq (Process no 447180/2014-2 and 306401/2013-4) and CAPES is gratefully acknowledged.

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