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Green technology recently drags major attention that makes the development of natural and renewable based products are on the rise. Native biopolymers matrices such as starch, natural resin, chitosan, bacterial cellulose and others are generally have low perfomance in term of strength, oxygen barriers and water absorption. Many attempts were conducted to enhance the performance of biopolymers. Focusing on green material development, reinforcement material from natural fiber was studied. Natural cellulose nanofiber possessed valuable characteristics such as a renewable and biodegradable with satisfactory mechanical and barrier properties when applied into a polymer or synthetic polymer. As depletion of the synthetic polymer source becomes critical, an alternative such cellulose nanofiber as biopolymer strengthener is found to be promising. Even nanocellulose has a great potential, however its natural behavior gives a drawback in certain ways. Cellulose fiber is sponge-like structure that could absorb water, when chemically, it contains highly hydrophilic surface that binds water. Therefore, several pretreatment and isolation process has been invented to enhance cellulose properties that later could be incorporated properly into polymer. In this study, we will discuss about the morphology and structure of nanofibers, nanofiber isolations, the importance and effect of pretreatment process, and application of nanofiber in biocomposites. Finally, the aim of this review is to study the current trend and exploration of cellulose based nanocomposite.
Keywords: Biodegradable, Biopolymer, Green Technology, Nanocomposite, Nanofiber
Nanotechnology is found to be a potential technology that emerged in wider areas such as medicine, electronics and food technology by manipulating the nanoparticles for various purposes. A nanoparticles is defined as a discrete entity that has at least one of its dimension is 100nm or less. 1 Nanomaterials may include any of the following nano forms: nanoparticles, nanotubes, fullerenes, nanoclays, nanocor, nanofibres, nanowhiskers and nanosheets. The application of nanomaterial is broad. Some of them were used as nano-sensor in smart food packaging technology. 2 It also could provide an antimicrobial mechanism by introducing nano-bulletin active packaging. The most popular purpose of this nanomaterial is widely used as nanoreinforcement in composite polymer in fact, many studies on nanoreinforcement were reported. Nano-reinforcement that's been studied are such as clay and silicates, 3-14 cellulose-based nanoreinforcements, 15-27 carbon nanotubes, 28-39 starch nanocrystal, 40-41 and chitin/Chitosan nanoparticles. 42-45 However, cellulose nanofiber which also known as nano cellulose, is such an emerging source in material development as "green technology", "eco-friendly", "renewable", "recyclable", "sustainable" and "triggered biodegradable" becoming such intense term nowadays. Generally, this particular nanofiber is used as filler or fabricator for material reinforcement and toughness. In addition, futuristic mind of the researcher with nanotechnology background has found that highly crystalline nanofiber invention is interesting as it has unique properties and sizes different from synthetic nanofibers. 46 Cellulose nanofibers have a high potential to be used as reinforcing material in many different areas. Differs from other nanofiller such as nanoclays, the usage of cellulose nanofiber becomes such serious deal because it is natural and the source is readily available yet abundant. Furthermore, this green cellulose nanofiber is not only renewable but also a multifunctional raw material and is expected to be able to replace many non-renewable materials. 47 Natural cellulose nanofibers come from different sources and category. Natural fibers which also known as lignocellulosic fibers are subdivided based on their origins, which are plants, animal or minerals. 48 In general, natural fiber which origin of the plant is usually referred as cellulosic fiber since the fibers usually often also contain a natural polyphenolic polymer, lignin, in their structure. 49 A general classification of natural fiber is represented in fig. 1.
cellulose (a-cellulose), hemicelluloses lignin, pectin, waxes
Fig. 1: General classification of natural fiber.
The utilization and manipulation of natural fiber source is not new. In history, different class of natural fiber was used as source in making paper, silk, cupboard and others. This natural polymer serves humankind for over centuries before low cost petroleum-based synthetic takes place. This synthetic material well establishes and its applications are varied because of their high specific strength and modulus with longer durability. 50 As times goes by, massive production of synthetic polymer was established to supply wide sectors. However, the use of great amount of this synthetic polymer led to ugly consequences as it gives problem on material disposal and depletion of non-renewable source. At early 90's natural polymer was resurfaced as it gives better prospect. 50 There are reasons that leading to this situation. Todays humankind is aware of eco-friendliness that led to growing attention in reducing environmental impact cause by polymers and composites. Another point is, limited petroleum source force people to find another alternative to decrease pressure on petroleum-based product. Thus, the potential of maximizing the use of renewable material suddenly become interesting. Other than that, sudden increase in research study on this particular field which mean cellulose based material makes data on the properties and morphologies of natural fiber generously available thus gives better understanding in the structure and correlation of bio-polymer properties. Such achievements are possible to reach through modern instrument that assist in better understanding of natural biocomposite hence assist the development of new materials.
Natural fibers are naturally-occurring composite consisting mainly of cellulose fibrils embedded in lignin matrix. 48 Cellulose in the other hand is the abundant organic compound that is a structural component of the cell walls of many plants. It is present mainly in the forest with wood as the most important source.In addition, cellulose fiber has unique pecking order. It comprised of nanofibers assemblies with diameter that range from 2 - 20 mm and a length of more than few micrometers. 46 Other cellulose-containing materials include agriculture residues, water plants, grasses, and other plant substances. Other than cellulose, plant cell wall also contains hemicelluloses, lignin, and small amount of extractives. From recent studies, cellulose is found to be the most common organic polymer and considered as an almost infinite source of raw material for the increasing demand for environment-friendly and biocompatible products. It represents about 1.5 billion tons of the total annual biomass production. 51 As lignocellulosic fiber becomes vital, many possible plants were tested. Natural fiber not only can be harvested from cotton, wool and other ancient source, but it covers different variety and different source all over the world. Available natural fiber that has been conducted in previous research are such as wood, 55,56,57 cotton, 58,59 jute fiber, 60,61 potato tuber cells, 62 wheat straw and soy hulls, 63 pineapple fiber, 64 sisal, 65 oil palm empty fruit bunch, 66 coconut husk, 67 banana fiber, 68 lemons and maize, 69 hemp, 70 sugar beet, 71,72 peas hull, 22 cladodes and spines from Opuntia ficus-indica, 73 prickly pear fruits of Opuntia ficus-indica, 74 and rice husk. 74
In fact, scientists now are looking at the various possibilities of combining biofibres such as sisal, flax, hemp, jute, banana, wood and various grasses with polymer matrices from non-renewable and renewable resources to form composite materials to make the revolution of biocomposite a reality.52 In the end, the combination of those various fibers with natural or synthetic polymer is expected to produce material with great properties. Material developments become even better with the combination of nano-technology. Previously, the fiber are composted to make cupboard, disposable plate and all, but with the help of nano-technology, the fiber could be grind chemically, biologically, and mechanically into nanosize. Obviously, nano-size fiber is more suitable to be incorporated into finer product such as thin film and high quality plastics. Hence, there are many research has been conducted in making nano-size fiber. According to previous method, suspension of cellulose microcrystal from pea hull fiber22 filter papers 53 and pulp wood 55 was prepared by sulfuric acid hydrolysis. However there are several extraction method that been developed and integrates such as chemical and mechanical pulping, enzymatic hydrolysis, high pressure homogenization, and high intensity sonication, where each method has disadvantages and drawback in term of the composition and properties of the final product.
Lignocellulosic fiber and cellulose nanofiber
Over decades, a number of researchers have been involved in investigating the exploitation of natural fibers as load bearing constituents in composite materials. The use of such materials in composites has increased due to many advantages and the fact that they can compete well in terms of strength per weight of material. 48 Basically, lignocelluloses contents that exist in natural fibers are cellulose, hemicelluloses, lignin, and other extractives such as waxes, ashes and others. The standard contents of the constituent have been studied by researchers that found that the amount of each content are varied according to its fiber sources. 52 The cellulose fibrils are aligned along the length of the fibre, which render maximum tensile and flexural strengths,and also provides rigidity to plant. The reinforcing efficiency of natural fiber is related to the nature of cellulose and its crystallinity. The primary occurrence of cellulose is existing lignocellulosic materials in the forest and as well as the agricultural residues, water plants and grasses. Previously, study has been done in vessel of vascular bundles of wheat straw that could reveal the finding little more about morphology and orientation of cellulose.76 Cellulose acts as framework in the vascular bundles and with high chain orientation. As the major constituent of cell wall, cellulose is mostly located in the secondary wall of the biofiber. Fig. 2 77 shows structural constitution of natural fiber cell.
Fig. 2 : Structure and contents of biofiber. 77
There are three major constituents in the lignocellulosic fiber which are cellulose, hemicelluloses and lignin . Cellulose is sufficiently purified plant tissues, used the term "cellulose" for what is nowadays called pulp.78 Other researchers continued to use the term "cellulose" in Payen's original definition.79 Table 1 shows the characteristic and properties of the major constituent in lignocelluloses fiber.
Table 1: Characteristic and properties of the major constituent in lignocelluloses fiber.46, 47 ,79, 80, 81,82
Shape and physical structure
Slender rod like crystalline microfibrils made of solid cellulose
High Order: Crystalline
Low order: Amorphous
Random, amorphous structure
Random, Amorphous structure
D-anhydroglucose (C6H11O5) repeating units joined by 1,4-Î²-D-glycosidic linkages at C1 and C4 position. Each repeating unit contains three hydroxyl groups
A group of polysaccharides composed of a of both 5- and 6-carbon ring sugars
Complex hydrocarbon polymer with both aliphatic and aromatic constituents. Contain five hydroxyl and five methoxyl groups per building unit
Degree of Polymerization
Solubility in alkali
Resistant to strong alkali (17.5 wt%)
Soluble in alkali
Soluble in hot alkali
Solubility in acid
Easily hydrolyzed by acid to water-soluble sugars
Easily hydrolyzed in acids.
Not hydrolyzed in acid
Glass transition state
Oxidizing agent Resistance
Biofibres is composites of hollow cellulose fibrils held together by a lignin and hemicelluloses matrix. 83 The cell wall in fiber is not a homogenous membrane. Each fibril has a complex, layered structure consisting of thin primary wall that is the first layer deposited during cell growth encircling a secondary wall. The secondary wall is made up of three layers and the thick middle layer determines the mechanical properties of the fibre. In the primary cell wall, cellulose consists of roughly 6000 glucose units. In vessel thickening, cellulose exists in the form of cellulose crystalline lamellae instead of cellulose microfibrils. Cellulose fibers however, exhibit a unique structural hierarchy derived from their biological origin. Depends on its origin, they are composed of nanofiber assemblies with a diameter that range from 2 to 20 nm, and a length of more than a few micrometers.14,16,45 For a record, each microfibril is formed by aggregation of elementary fibrils, which are made up of crystalline and amorphous parts. The crystalline parts, which can be isolated by several treatments, are the whiskers, also known as nanocrystals, nanorods, or rodlike cellulose microcrystals, or cellulose crystal could be further exploit as reinforcement.84,85
Fig. 3 : (A) four microfibrils held together by hemicellulose and lignin Internal structure of a cellulose microfibril; (B) parallel elementary fibrils; (C) an elementary fibril containing bundles of cellulose chains; (D) a cellulose chain; (Adapted from Ramos, 2003).86
The exploitation can be possibly done by breaking down the hierarchical structure of the plant into individualized nanofibres of high crystallinity, therefore reducing the amount of amorphous material present. The schematic model of the structure of a microfibril are shown in fig. 3 86 and explain about the hierarchy of the structure as well. Since plant fibers are fibrous and it is possible to do this by means yielding a fibrous form of the material (nanowhiskers, nanofibrils), which due to their aspect ratio (length/diameter) and therefore reinforcing capabilities are potentially suitable for composite materials.
CELLULOSE NANOFIBER ISOLATION
Nanofiber material are vary in types that distinguish its isolation method, raw material and pre-treatment. However, the most important is, the product is mostly influence by disintegration process itself. Combination of mechanical and chemical process could improve the isolation process by means increasing the yield of cellulose production. In general, the mechanical processes included disintegration, refining, cryo crushing, and high-pressure homogenization.87 A schematic diagram of the overall isolation process of nanofibers is shown in fig. 4. In extend, there are several extraction methods that commonly used in order to extract nanofiber that will be discussed in this paper.
Mechanical and chemical extraction
Surface Modification of Nanofiber
Fig. 4 : Schematic diagram of nanofiber isolation process
Before isolation process, a series of process is designed, called as pre-treatment. Major purposes of pre-treatments are to remove unwanted particles before the fiber is further process into nanofiber. Pre treatment and washing able to remove ashes, waxes and noncellulosic compound which is crucial to produce a pure and high quality cellulosic product. Pre treatment also has proven could improve mechanical properties of the composite when it fabricated in it. 88 It also proved that it's able to remove a bit of lignin contents which the elimination of lignin contents of fiber strongly influenced on the properties of the composite. The general concept of pretreatment is illustrated in figure 5. 89 During pretreatment process, lignin seal was broken and crystalline structure of cellulose was disrupted which made lignin removal possible.89 There are several pre-treatment that available that been developed by years of study. However, alkali/base treatment are frequently used method.C:\Users\Public\Pictures\pretreatment.png
Fig. 5 : Disruption process of lignocelluloses material by pre-treatment method.89
Alkaline pretreatments can be divided into two major groups: i) Pretreatments that use sodium, potassium, or calcium hydroxide; and ii) Pretreatments that use ammonia. 90 Alkaline treatments are more effective in lignin solubilization exhibiting only minor cellulose and hemicellulose solubilization, excepting ammonia recycling percolation treatment, which yield biomass solids mostly containing cellulose.91,92,93 During alkali treatment, fibers were dipped in 5% sodium hydroxide solution for about 48 h. Based on Wyman et al., 2005,94 high concentration of sodium hydroxide in pretreatment process will give good results due to the ability of lignin removal and hemicelluloses removal at the end of the process.95 However, 2.5 M of NaOH is the suitable ratio in order to give good results in fiber product and as well as its economical value. 94 However, certain research shown that the pretreatment process reaction could be accelerated by using heat. For an examples in previous finding the temperature of the reaction was kept at 75°C 88 and 80°C. 94 After base treatment, the fiber suspension was filtrated using vacuum filtration. It was further washed with water containing a few drops of acetic acid. At final step, the filter cake was washed again with fresh distilled water water until ph 7. The fiber will let to dry at temperature 50 °C. Various pretreatment methods that available are shown in table 2 below.
Table 2 : Pretreatment and its purpose
Fibers were dipped in 2.5M sodium hydroxide (NaOH) solution, 48 hours, temperature at 80°C.
Fibers were filtrated and washed with distilled water + few drop of acetic acid
Fibers were washed again with distilled water until ph 7
Fibers were dried at 50 °C
Lignin removal and hemicelluloses removal.
Wyman et al., (2005)93
After drying the fiber, it then cut and sieved.
Fibers were soaked with hot distilled water at temperature around 65°C- 80°C, 12 hours
Fibers were soaked in 10%-30%(w/v) NaOH maintained in water bath at temperature 75°C, speed 40rpm about 3hr Fibers were washed with distilled for a few time
Fibers were treated with distilled water containing 2% H2O2 in water bath at temperature 45°C, speed 40 rpm for 8 hour.
Fibers were washed with distilled water
Then, it was treated with 10% (v/v) acetic acid (at room temperature) for 30min. present in treated fiber residue. The fibers were washed repeatedly using distilled water
The washings were repeated until cellulose residue is free from acid.
Treated fiber was dried in an oven at 70°C for overnight
To get uniform size and remove ashes.
Eliminate impurities and large particles.
To remove lignin content
To remove NaOH
To remove more lignin content and active the OH group of the cellulose.
To remove excess H2O2
To neutralize the excess NaOH
Remove acid residue from fiber
Carvalheiro et al., (2008)90
Sun et al., (2004)96
Fibers were soaked into acidified sodium chlorite solution (ph 4-5) at 75 â-¦C for an hour. This process is then repeated five times until the product became white.
The fibers were treated in 3 wt% potassium hydroxide at 80 â-¦C for 2 hr,105 or 6wt% potassium hydroxide at 20°C for 24 hr.66
Fibers are filtered and rinsed with distilled water.
To remove lignin
To leach hemicellulose, residual starch, and pectin.
Neutralize the sample.
Abe & Yano (2010), 105
Fahma et al., (2010)66
Pulp can be bleached, to obtain a whiter product with lower amounts of impurities and improved ageing resistance (yellowing and brittleness resistance). These effects are mainly connected to lignin in chemical pulp. In several stages, different chemicals are used for bleaching, e.g. hydrogen peroxide (H2O2), chlorine dioxide (ClO2), ozone (O3) or peracetic acid. 97 98 Sulfite pulps are more readily bleached and are obtained in higher yields. 99 Other bleaching agent that been used is potassium hydroxide (KOH) that reported can remove hemicelluloses. 66
Isolation of nanofiber is complex that it must through tedious processing step by means of purification, bleaching, fibriliatioan, hydrolysis and the low yield of the final dispersion of cellulose whiskers.17 Through the years, different methods for isolation of cellulose nanofibers from cell wall were developed including mechanical, chemical, chemo-mechanical, and enzymatic isolation processes.87 Previous study reported that cellulose nanofiber was isolated using novel isolation process which is high-pressure homogenization.100,101 The developments of technology lead researchers to enhance and invent processing technique in order to obtain fine and pure nanocellulose. Extraction process and method that available until today such as; mechanical treatments, e.g. cryocrushing grinding,102-106 ; high pressure homogenizing,100,101,107-110 ; chemical treatments, e.g. acid hydrolysis,111-113 ; biological treatments, e.g. enzyme-assisted hydrolysis,114-116 ; TEMPO-mediated oxidation on the surface of microfibrils and a subsequent mild mechanical treatment,117-120 ; synthetic and electrospinning methods,121-123 and ultrasonic technique 123-127 .46 In fact, numerous of studies investigated isolation of nanocellulose from various raw materials, mainly of plant origin, using different techniques.
This process is applicable for pulp consist cellulose fiber that mainly acquire from wood. In industrial, this process is called as pulping process. In mechanical way, ground wood pulp is produced by pressing round wood logs against a rotating cylinder made of sandstone, scraping the fibers off. Another type of mechanical pulp is refiner pulp, obtained by feeding wood chips into the center of rotating, refining discs in the presence of water spray. The disks are grooved, the closer the wood material gets to the edge of the disk, the finer the pulp. 97 Apart from fibers released from the wood matrix, mechanical pulp also contains fines. These are smaller particles, such as broken fibers, giving the mechanical pulp its specific optical characteristics. 97,98
In the homogenization process, dilute slurries of cellulose fibers previously treated by refining are pumped at high pressure and fed through a spring high pressure loaded valve assembly. As this valve opens and closes in rapid succession, the fibers are subjected to a large pressure drop with shearing and impact forces. This combination of forces promotes a high degree of microfibrillation of the cellulose fibers, resulting in microfibrillated cellulose.100 The refining process is carried out prior to homogenization due to the fact that refining produces external fibrillation of fibers by gradually peeling off the external cell wall layers (P and S1 layers) and exposing the S2 layer and also causes internal fibrillation that loosens the fiber wall, preparing the pulp fibers for subsequent homogenization treatment. 129 Others researchers studied how the degree of fibrillation of pulp fibers affects the mechanical properties of high strength cellulose composites.109 In the range between 16 and 30 passes through refiner treatments, pulp fibers underwent a degree of fibrillation that resulted in a stepwise increment of mechanical properties, most strikingly in bending strength.109
Chemical pulping is use to isolate fibers from the wood and mainly to remove the matrix substance lignin. Delignification is done by degrading the lignin molecules, bringing them into solution and removing them by washing. However, there are no chemicals being entirely selective toward lignin. Therefore, also a certain amount of the carbohydrates (cellulose and hemicellulose) is lost in this process. In addition, complete removal of lignin is not possible without severely damaging the carbohydrates. After delignification, some lignin is therefore remaining in the pulp and this amount is determined by the pulps kappa number. Of all pulp produced worldwide, almost three quarters are chemical pulp, of which the major part is produced by the kraft process. 97,98 The kraft process (or sulphate process) is the dominant chemical pulping method worldwide. However, more recently developed pulping methods include the use of organic solvents as ethanol, methanol and peracetic acid (CH3CO3H) for delignification.97,98
In chemical refining, nanocrystalline cellulose were prepared by acid hydrolysis. Hydrolysis was carried out with sulphuric acid with constant stirring. Immediately following the acid hydrolysis, the suspension was diluted 10 times with deionized water to stop the reaction. The suspension centrifuges at 6000 rpm for 10 min to concentrate the cellulose and to remove excess aqueous acid. The resultant precipitate should be rinsed, recentrifuged, and dialyzed against water for 5 days until constant neutral pH.130 Other method reported that the cellulose fiber was hydrolyzed with sulphuric acid (96 wt%) under strong agitation.66 The time of reaction is varied from 15-90 minutes and the reaction terminated by adding cold water. Then the diluted suspensions were centrifuged at 11,000 rpm for 10minutes to obtain precipitate. Using strong agitation, the precipitate were re-suspended in water and centrifuged repeatedly until the ph reached 5.
Steam explosion treatment is currently still being extensively studied as a promising pretreatment method. Lignocellulosic biomass materials can actually be fractionated into biopolymer constituents by steam explosion technology. Treating various biomass resources by steam explosion has been studied by many researchers. 131, 132, 133,134,135 The steam explosion techniques allows lignocellulosic matter undergo high pressure of steam, for short periods of time, followed by sudden decompression (explosion). The process represents a simple treatment for biomass that achieves fiberization by combination of chemical and mechanical action.46 During the steam explosion process, the raw material is exposed to pressurized steam followed by rapid reduction in pressure resulting in substantial break down of the lignocellulosic structure, hydrolysis of the hemicellulose fraction, depolymerization of the lignin components and defibrillization.136,137,138 Some mentions that the steam explosion is an auto hydrolysis process.139 Steam explosion could give significant effect on biomass which leads to the cleavage of some accessible glycosidic links, Î²-ether linkages of lignin, lignin-carbohydrate complex bonds and minor chemical modification of lignin and carbohydrates. In extensive studies, it found that aspect ratio and percentage yield of nanocellulose obtained by this technique has been found to be very high as compared to other conventional methods.
This process consists of combination of chemical pretreatment and high-intensity ultrasonication. In the chemical pretreatment stage, the wood fibers are being purified to prepare the cellulose fibers according to general methods. 104,105 To avoid generating strong hydrogen bonding among nanofibers after matrix removal, the samples are kept in a water-swollen state during the whole chemical process. After chemical pretreatment, the purified cellulose fibers are soaked in distilled water (concentration: âˆ¼0.5% in mass). About 120 ml of solution containing chemical-purified cellulose fibers are then placed in a common ultrasonic generator of 20-25 kHz in frequency equipped with a cylindrical titanium alloy probe tip of 1.5 cm in diameter. The subsequent ultrasonication is conducted for 30 min to isolate the nanofibers.
Surface modification of cellulose nanofiber
The contact angle measurements showed that the surface characteristics of nanofibers were changed from hydrophilic to more hydrophobic when acetylated.87 The acetylation process generally provide better surface characteristic of the nanofiber. Due to the hydrophilic nature of cellulose, it cannot be uniformly dispersed in most non-polar polymer media.46 Hence, cellulose modifications become main interest in order to improve compatibility with a wider variety of matrices. Several methods have been proposed for cellulose surface modification, corona or plasma discharges,140 surface derivatization,141 graft copolymerization,142 and application of surfactant.143,144 Reports on surface modification of microfibrillated cellulose nanofiber (MFC) are limited.145,146 Surface modification of nanocellulose fiber basically adapted from previous study on surface fiber modification. Table 4 shows the effect of surface treatment to fiber properties where the effect of that particular treatment to nanocellulose fiber also could be predicted. Those information could give general idea how the treatment going to take effects on micro fibrillated cellulose nanofiber thus preliminary study could be conducted.
Table 4 : Effect of surface treatments on properties of OPF (Oil Palm Fiber) 150,211-216
Effect on EFB
Amorphous waxy cuticle layer leaches out.
Partially masks the pores on the fiber surface.
Partially eliminates the porous structure of the fiber and causes microlevel disintegration. It degrades mechanical
Imparts a coating on fiber surface
Toluene diisocyanate (TDI) treatment
Makes fiber surface irregular as particles are adhered to surface.
Removes waxy layer from the surface and makes the fiber hydrophobic.
Fibrillation is observed due to leaching out of waxes, gums and pectic substances.
Changes the colour and makes fibers soft. Porous structure is observed after treatment.
Imparts a coating on fiber surface and removes pits containing silica bodies and keeps surface irregular. It improves mechanical properties of fibers.
Keeps the fiber surface undulating and improves mechanical properties
Smoothens fiber surface.
Imparts a rough surface to the fibers and makes pores prominent, which helps improving the mechanical interlockingwith matrix resin.
Imparts bright colour to the fiber. Removal of oil layer exposes surface pits and makes surface coarse.
Makes the surface pores wider and fiber become thinner due to dissolution of natural and artificial impurities.
In previous research that conducted, cellulose was partially acetylated to modify its physical properties while preserving the microfibrillar morphology.147 In this case, the degree of acetyl substitution had a crucial influence on material properties. Other finding recorded that acetylation improved the transparency and reduced the hygroscopicity of cellulose/acrylic resin composite materials.148 However, the composites had an optimum degree of substitution (DS) and excessive acetylation reduced their properties. Acetylation has also been reported to improve the thermal degradation resistance of cellulosic fibers.149 However, no report about acetylation of MFC surfaces have been published so far.
Silane is a coupling agent. It has chemical structure (fig. 6) that allows it to react with water to form a silanol and an alcohol. Fibers were dipped in 1% silane solution (tri-ethoxy vinylsilane) in water-ethanol mixture (40 : 60) for about 3 h. During the reaction, the pH of the solution was maintained to 3.5-4. Fibers were washed and then dried
Fig. 6: tri-ehtoxy vinylsilane.150
CH2CHSi(OC2H5)3 + 3H2Oâ†’ CH2CHSi(OH)3 + 3C2H5OH (1)
CH2CHSi(OH)3 + H2O + Fiber-OHâ†’ CH2CHSi(OH)2O-Fiber + H2O
Equation 1: tri-ethoxy vinylsilane reaction with water to produce silanol and followed by silanol reacts wih cellulosic hydroxyl group.150
The reaction will be proceed with silane molecule absorb chemically onto the fiber. The reaction and absorption of chemical depends on free OH group available on the surface of the fiber. On the other hand, with presence of moisture, silanol will reacts with cellulosic hydroxyl group in the fiber to form stable covalent bonds to the cell wall. The purpose of silynylation is to increase hydrophobicity of the fibers that could be possible to attain because of the removal of the site for moisture absorption. It is because the hydrophobic coupling agent forms a protective monolayer on the proton-bearing. Such modification are most effective in surface regions. However, as the concentration and time of the treatment increases, the treatment effect may may penetrate into the fiber. Same with other reaction principles, there will be a saturation point which no further reaction takes place. 152
Isopropyl utilized dimethylchlorosilane for surface silylation of cellulose microfibrils resulting from the homogenization of parenchymal cell walls. These authors claimed that microfibrils retained their morphology under mild silylation conditions and could be dispersed in a nonflocculating manner into organic solvents. Other method hydrophobized MFC via partial surface silylation using the same silylation agent and reported that when silylation conditions were too harsh, partial solubilization of MFC and loss of nanostructure could occur.152 Films prepared from the modified cellulose by solution casting showed a very high water contact angle (117-146°). It is probable that in addition to decreased surface energy, higher surface roughness as a result of modification could contribute to increased hydrophobicity. It has also been reported that hydrophobized MFC could be used for the stabilization of water-in-oil type emulsions.153
BIOCOMPOSITE AND NANO-BIOCOMPOSITE
Throughout decades, a new branch of polymer composite material has been widely investigated. Bionanocomposite for instance, is composite material based on incorporation of natural nanosized fiber or filler into matrix phase. Each fractional component is usually derived from renewable resources such as starch, cellulose and protein.154 Before digging deeper about this topic, it is crucial to know a basic of polymer. Polymer could be categorized into two general types according to its source which are synthetic polymer and bio-polymer. Synthetic polymer mainly originate from petroleum source whether bio/natural polymer come from renewable source which both of them play an important role in polymer industry
However, development of this advantageous biopolymer is really attract major attention which is why this focusing on this type of polymer. In general, biopolymers can be classified into four different types which are shown in fig. 7. Consequently, various source of bio polymer are invented and classified according to its source. Some of the biopolymer is blend with petrochemical-based polymer to produce biodegradable polymer composite. There are many possible ways in making bio-nanocomposite with all these kind of source. In fact, many attempts have been carried on in order to prepare biodegradable composite using other forms of lignocellulosic materials for instance in nano-form. Nanosize reinforcement is one of the important and almost unfamiliar fields. It exploit sources either organic or inorganic and synthetic or from natural origin. As in natural origin, one of potential materials that can be revealed is cellulose nanofibers.
Fig. 7 : Classification of potential biopolymers in composite fabrication.
Cellulose nanofiber reinforced a natural biopolymer that finally results in fully biodegradable nano-composites. There are other nano-reinforcement particles are available where recent report shown that starch-based nanocomposites have been prepared with various nano-sized fillers such as cellulose nanofibers, chitin and chitosan nanoparticles and nanoclays. However,nano cellulosic fiber attracts more attention due to its good reputation and has provide much advantages than other filler.48 Broadly defined, biocomposites are composite materials made from natural/bio fibre and petroleum derived non-biodegradable polymers (Poly propylene, polyethylene) or biodegradable polymers (PLA, PHA). The latter category i.e. biocomposites derived from plant derived fibre (natural/biofibre) and crop/bioderive plastic(biopolymer/bioplastic) are likely to be more eco-friendly and such composites are termed as green composite .48
The relation between composite, nano composite and green composite is shown in diagram 7. Basically, composite is considered as nanocomposite if one of the materials that incorporated reached size of below 100nm. There are two major approach of producing polymer nanomaterials/nanocomposite. It's either produce nano-scale polymer material or introduce nanomaterial into polymer to produce nanocomposite.155 Depending on the geometry and the nature of the nanofillers, nanocomposites may exhibit new and/or substantially improved properties (e.g. mechanical performance, barrier properties, thermal stability and transparency.156 In thin film food packaging aspect, FMC(food packaging materials have been developed with the inclusion of nano sixe filler which able to improved flexibility, gas barrier properties, temperature control and moisture stability. It eventually could reinforce the polymer matrix. In general, researcher approve that the use of nanoscale fillers in composite films represents a radical, promising alternative to conventional polymer composites.10,156,157
APPLICATION OF NANOFIBER IN COMPOSITE
From previous research, applications of cellulose nanofibers are generally for composite reinforcement. The reinforcement using nanofiber is nowadays dispersing in many fields not only in hard composite but in thin film too. Consequently, its application could cover pretty much every industry from material reinforcement for construction to food packaging. It is possible to be achieved by composing the nanofiber into different source of polymer or biopolymer that available (figure 7). In the beginning of composite development, natural fiber was mixed with petroleum based polymer to create composite. Other than that, the petroleum based polymer also was combined with natural bio-derived polymer to create green composite. The purpose of introducing bio-based material into the petroleum based polymer is to enhance biodegradability. As time goes by, more advance material was created by disintegration of natural fiber into nano-size fiber which finally mixed with nanofiber to create nanocomposite. Composite material studies keep evolves and there are so many possible inventions in new material has been found and studied. Fully green nano-composite is then emerging as green technology became serious business and the researches about this particular field is to be studied.
Mechanical and chemical treatments
Fully Green Nano -Composite
Petroleum Based Polymer
Natural bio-derived Polymer
Figure 7 : The connection between composite, green composite
Starch based nano-biocomposites are a new class of composite composed of nano-sized filler (nanofiller) incorporated into a bio-based matrix.159 Such an association between natural biopolymers and nano-objects, with the aim to obtain synergic effects, is one of the most innovating routes to enhance the properties of these bio-matrices.11 Starch is originated from variety of crops such as potato, wheat, rice and corn. The source is abundant, and readily available at low cost.160 In history, they already have been used to produce biodegradable films to partially or entirely replace plastic polymers because of its low cost and renewability, as well as possessing good mechanical properties.161 Starch which contains major components of amylose and amylopectin, are biopolymers, which are attractive raw materials for use as barriers in packaging materials. At the beginning, water and glycols are commonly used as plasticizer in making bioplastic which make the starch behave like plastic.162,163
However, native starch based film properties suffers from poor mechanical properties and high water uptakes [163, 162]. This limitation has led to the development of the improved properties of starch based films by modifying its starch properties and/or incorporating other materials.165 Blending starch with different proteins could decrease the water vapor permeability of the films and to increase their tensile strength. In fact, there are many attempts has been done to enhance starch base plastic by incorporation with other binder/filler. Previous studies shown that starch based composites were fabricated with different type of filler from different sources such as polyhydroxybutyrate PHB,166,167Polylactides PLA, 168,169,170 Polycapro lactone PCL,122,172,173 Chitosan,174,175,161 Clays176,177,178,179 natural Fiber,180,181,182,183 cellulose whisker,184,185 microcrystalline cellulose,163,186,187 and microfibrillated cellulose MFC & biofiber composite BFC.188,189,190
However, these films still did not perform well compared to synthetic polymer based films. Thus, innovative researchers took a step by incorporating the native starch based plastic with nanofiber to create new class of starch nano biocomposite film to obtain greater level of green plastic where it hopefully could meet the satisfactory level as synthetic polymer based film. New bio-based nanofillers coming from starch191 cellulose192 or chitin193 have been more and more studied during last decade and present the advantage to be renewable, present anywhere, have low cost, have low density, be biodegradable and easily destroyed when incinerate at the end of life. Cellulose based nanoparticles are more and more developed
Poly lactic acid (PLA) based nanocellulose composite
Derivation of natural renewable sources such as corn can produce polylactide or poly (lactic acid), one of the biodegradable thermoplastic polyester. It's manufactured by biotechnological process. Naturally, PLA exist as a low value like any other bioplastic. Thus, it makes reinforcement of this particular bioplastic is crucial. Reinforcement of polylactic acid (PLA) is achieved by incorporating microfibrillated cellulose (MFC, mechanically fibrillated pulp, mostly consisting nanofiber).25 The study was carried out to study the potential of reinforcement by a nanofiber network, with the goal of making sustainable 'Green composite'.25
In order to achieve uniform dispersion of MFC in PLA, both component was premixed and kneaded using solvent. This procedure is to attain uniform dispersion of MFC in a PLA. Mechanical and thermo-mechanical properties of the film were studied after hot pressed.46 Nanocellulose incorporation into PLA composite become such a promising reinforcement. It shown the PLA composite could increase the young modulus and tensile strength of PLA by 40 and 25% respectively without a reduction of yield strain of fiber content of 10 wt%.
Poly-Hydroxyy butyrate (PHA) composite
PHAs are polyesters produced naturally by bacteria from renewable sugars or fats, to store carbon and energy in their cells.194 Since the 1970s, a number of companies have pursued industrial processes to optimize fermentation conditions for polymer growth within the microorganism. It's a polymer that first discovered by Lemoigne and was initially describe as lipid inclusion in the bacterium Bacillus megaterium .195 PHA have the simplest family form of biopolymer which is poly-R-3-Hydroxybutyrate (PHB). A few years after, PHB was demonstrated as high molecular weight polymer utilized in carbon and energy storage by variety of microorganism.196 Emerging potential of PHAs brings out large number of study that bound on microstructure, thermal and mechanical properties through to studies on biodegradation.197
Furthermore, PHA's is renewable source and it's physical properties is similar to conventional plastic which lead more research on this topic.198,199 Nanocrystal was prepared through acid hydrolysis of cellulose microfibril.200 On the other hand, composites were made by dispersing either native or silylated crystals in cellulose acetate butyrate matrixes and solution casting of the dispersions. As a results, PLA composite exhibit better reinforcement characteristics. However, there are one restriction on the use of cellulose crystals as reinforcement is their incompatibility with a typically more hydrophobic thermoplastic matrix. To overcome this problem, cellulose nanocrystals from bacterial cellulose were topochemically trimethylsilylated.46
Hydroxypropylmethylcellulose (HPMC) reinforced with cellulose nano-particles
Hydroxypropyl methylcellulose (HPMC) is one of the cellulose ethers (derivation of plant source) which is the most commonly used. It is approved for food uses by the FDA (21 CFR 172.874) and the EU (EC, 1995) and it is used in the food industry as an emulsifier, protective colloid, stabilizer, suspending agent, thickener, or film former.201 The films obtained from HPMC are resistant to oils and fats, flexible, transparent, odorless, and tasteless but tend to have moderate strength.202 Similar to other biopolymer, the HPMC based bioplolymer exhibit limited performance related to brittleness and moisture barrier.201
The application of nanotechnology by developing polymer nanocomposite films with the addition of fillers in the nanometric range may open new possibilities for improving these mechanical and barrier properties. This particular feature provides nanocomposites unique and outstanding properties never found in conventional composites.Reinforcing effect of the cellulose particles was observed when whiskers were used as the filling material; an increase of 22% in tensile strength and 55% in Young's modulus were achieved while the elongation at break of the films was preserved. Addition of whiskers also improved the water barrier properties of the composite films. This effect was attributed to the lower water affinity of the composite films, as compared with the HPMC films, since the water diffusivity values were not affected by the addition of whiskers.
Furthermore, the whiskers only decreased 3-6% the transparency of the HPMC films showing 86-89% visible light transmission values, allowing application as edible barrier and transparent film. It can be concludes that the composite produced by using whiskers as filling material were transparent, flexible and homogeneous; the nanocomposite films exhibited better water barrier and mechanical properties than HPMC films while decreasing only slightly the transparency of the films. These results indicate the great potential of HPMC/cellulose whiskers composite films for sustainable packaging applications.
Nanocomposite films based on sodium caseinate and nanocellulose fibers
Different from HPMC, Sodium caseinate is the biochemical name for casein, which is a type of protein found in the milk from all mammals. Casein, which is Latin for "cheese," is a major component of commercial cheese and its principle source of protein. Caseinate containing protein (the rest being lactose, lipids, attached moisture, and ashes).203 In general, films made from sodium caseinate and nanocellulose were prepared by dispersing the fibrils into film forming solutions, casting and drying. Sodium caseinate aqueous solutions with protein concentrations of 2.5% (w/v) were prepared by dissolving the sodium caseinate powder in distilled water and stirring continuously for 3 h at room temperature. Appropriate amount of glycerol was added to achieve a glycerol/(sodium caseinate + glycerol) weight ratio of 0.21. Nanocellulose fibers were dispersed in distilled water by ultrasonication and then mixed with the film forming solutions to prepare composites containing 1, 2 and 3 wt% cellulose (dry weight). Films were prepared according to the usual casting method:the solutions were poured into Teflon Petri dishes (diameter = 14 cm) and dried at 35 °C for approximately 10 h in a convection oven.
After the excess of water was evaporated, the obtained films were peeled off from the plates and kept in a closed reservoir at 50% relative humidity (RH) and constant temperature (23 ± 2 °C) for 3 days. As a results, composite films were less transparent and had a more hydrophilic surface than neat sodium caseinate ones. However, the global moisture uptake was almost not affected by filler concentration. Addition of nanocellulose to the neat sodium caseinate films produced an initial increase in the barrier properties to water vapor, and then, it decreases as filler content increased. This was explained in terms of additional detrimental changes (cracks and bubble formation) induced in the morphological structure of the film by the reinforcement. The tensile modulus and strength of composite films increased significantly with increasing cellulose concentrations, while the values of elongation decreased. In the same way it was found that the storage modulus increases considerably with filler addition in the low temperature range (<60 °C), though the effect of temperature on the films performance is even more dramatic, as expected in protein-based materials.
EXPLORATION AND DEVELOPMENT OF CELLULOSE NANOCOMPOSITE
As sustainable development becoming a huge tagline, many research intensely studied on the particular field especially development in biopolymer composites. Effects of global warming, increases of non-biodegradable waste, and depletion of petroleum reserved has triggered intense study of cellulose source material. Cellulose nanofiber becomes so important because incorporation of nanoreinforcement has been related to improvement in overall performance of biopolymers. Many findings validates that the addition of nanofiber could enhance mechanical, thermal and barrier properties of biopolymer which originally have many drawbacks.
Over decades, this particular field of study becomes more interesting that lead to the advancement of nanofiber filler values. This is solely because many research found that properties of filler plays important role in biopolymer reinforcement. Better understanding of organic and polymer chemistries makes the current researcher to look deeply on interaction between polymer matrix and filler (nanofiber) hence lead to the advancement of nanofiber values so that it could be used universally into various biomaterials. Hence, many process step, cellulose nanofiber modification and study on application of cellulose and its compatibility on various polymer has been studied. In recent studies, it shown that by applying other nanoparticles into the nanocomposite has important role to improve the feasibility of use of biopolymer for several application including food packaging.For an example, chitosan flakes could be derive into nanoparticles to be incorporated into bioplastic packaging that act as antimicrobial bullets. The same thing goes on other poarticles such as nanosilver and others. Recent exploration and development on nano-scale cellulose and its related applications is shown in table 5.
Table 5: Exploration and development of cellulose nanocomposite.
Bionanocomposites of thermoplastic starch reinforced with bacterial cellulose nanofibers. Effect of enzymatic treatment on mechanical properties.204
Nanocomposites for food packaging applications.205
Progress in nano-biocomposites based on polysaccharides and nanoclays.154
Fabrication and characterisation of chitosan nanoparticles/ plasticised-starch composites.206
Nanoscale particles for polymer degradation and stabilization-Trends and future perspectives.207
Starch-based composites reinforced with novel chitin nanoparticles.160
Isolation, preparation, and characterization of nanofibers
from oil palm empty-fruit-bunch (OPEFB)66
Structure and properties of nanocomposite films based on sodium caseinate
and nanocellulose fibers.203
Characteristics of cellulose nanofibers isolated from rubberwood and empty fruit bunches of oil palm using chemo-mechanical process.208
Reinforcing potential of micro- and nano-sized fibers in the starch-based biocomposites.209
Synthesis of nano cellulose fibers and effect on thermoplastics starch based films.2010
The future of bio based material becomes brighter not only because the source is infinitely available, but it could compete and may replace synthetic material that available today. All these are possible since there are numerous exploration about cellulose material has been conducted. Available information from previous research could clarify and make present researcher to understand more about characteristic and morphology of cellulose nanofiber. Hence, modification of cellulose nanofiber is possible to achieve chemically or physically. In order to extract nanofibers, various methods have been reviewed. The source of nanofiber are varied either from animal or plat sources. However, every extraction method gave different results in term of quality and yields. Disruption of cellulose may occur during treatment and extraction, hence an efforts in order to reduce is to be explore. An effective production should be achieved or else it will be worthless. Effective production should require low amount of energy with high yield of nanofiber. In effort to fulfill this, mixture of chemical, enzymatic and physical method might be combined into extraction process. In fact, researchers found that by adding pre-treatment into process integration, it could significantly reduce energy consumption. Next, excellent interaction between polymer matrix and nanofiber is important, where some might requires chemical modification of cellulose nanofiber. However, there are various polymer matrixes available that ready to be incorporated with nanocellulose. In general, bio-based polymer is hydrophilic where the incorporation of cellulose nanofiber into it would not be a problem. Contrary, nanocellulose need to be modified before incorporation into hydrophobic synthetic polymer that mostly based on petroleum. Consequently, further exploration regarding modification of cellulose with minimum environment impact, and understanding the chemical and physical interaction occurred between polymer matrix and nanofibers are necessary.