Physiochemical treatments such as ultrasonic wave or microwave were applied to the plant fiber materials to obtain high efficiency in virtue of intensification of heat and mass transfer (Lu et al., 2013). By using microwave assisted technique, it will generate heat by direct conversion of electromagnetic energy which will help in reducing energy consuming and reaction time compared to conventional heating (Lu et al., 2013). While by using ultrasonic wave, a strong mechanical oscillating energy will be produced and effectively breakdown interaction force between cellulose (Karim et al., 2015).
Figure 1.1 Schematic diagram for tree hierarchical structure (Moon et al., 2011)
Numerous preparations methods had been evaluated to obtain stable and usable material for NCC. Based on previous work, nano-crystalline cellulose was prepared by sulphuric acid hydrolysis of cellulose raw materials in aqueous solution, and then freeze-dried (Yu et al., 2013). During acid hydrolysis, amorphous regions are removed leaving only crystalline cellulose (Jasmani & Adnan, 2017). In general, the sulphate group will be attached to the surface of the NCC during the hydrolysis that will bring a decrease in thermal stability. NCC dominates more advantages than cellulose fibers. Such as high surface area, specific strength and modulus and having unique optical properties (Peng et al., 2011). NCC has great strength because of its dense and ordered crystalline structure while its elasticity for the perfect crystal of native cellulose had been estimated between 130 and 250 GPa and tensile strength between 0.8 and 10 GPa (Kos et al., 2014).
Demand of products that made from renewable and sustainable resources had been increasingly needed by consumers industry and government. For the past thousand years, our society had been used natural cellulose as based materials and it is still on going. They develop functionality, flexibility and high mechanical strength by developing hierarchical structure that covers nanoscale to macroscopic dimensions (Moon et al., 2011) (Figure 1). Nano-crystalline cellulose (NCC) has unique characteristics and can be used as a reinforcement in polymer nanocomposites to highly enhance mechanical properties (Yu et al., 2013).
The NCC also holds promise in many different applications such as nanopaper, coatings, adhesives, optical sensors, biomedical scaffolds, filtration membranes, electronic devices, foams, aero gels, etc. (Kos et al., 2014). From these remarkable physiochemical properties and wide appliance prospects, eventually it attracts both research scientist and industrialists.
1.2 Short Summary and Problem Statement
Cellulose is the most abundant renewable polymer that is biodegradable that primarily found in plant biomass. Based on older researches, isolation of nanocrystalline cellulose (NCC) is optimum at a sulphuric acid concentration between 50-60% with temperature within 70℃. Under this condition, NCC is utilized in harsh condition because of high temperature and high acid concentration. Elephant grass was used to find the isolated nanocrystalline cellulose. It has a high productivity of biomass and also it is capable to a repossess large amount of carbon from the environment.(Lima et al., 2014) Dried elephant grass is pre-treat using NaOH under microwave radiation to remove the lignin. By using microwave irradiation, the heating element was produced within a short duration caused by the heating between microwave energy and reactant molecules that exist in the reactant mixture (Kappe et al., 2009).The pre-treat is bleach using 30% of H2O2. Bleach sample will undergo acid hydrolysis using two methods; microwave-assisted and ultra-sonication. Ultrasonic will enhance the diffusion of nanocrystalline cellulose in the suspension (Moon et al., 2011). It will provide a more reactive site for the acid to pass through by wearing down the cellulose surface, therefore, producing nanocrystalline cellulose with high crystallinity (Davoudpour et al.,2013). By the previous study, hydrolysis using microwave-assisted was very fast (Kos et al., 2014). Compared with microwave heating, conventional heating is slow and inefficient (Yin 2012).By applying organic and weak acid, orthophosphoric acid to perform acid hydrolysis on cellulose with the combine of microwave-assisted technique; it can produce high NCC with higher crystallinity. It had been investigated that using orthophosphoric acid will obtain higher crystallinity and lower thermal stability compared to other strong acids.(Zhang et al., 2014)
1.3 Research Objective
To study the effect of crystallinity on nanocrystalline cellulose by using two different method; microwave-assisted and ultra-sonication
To investigate and characterize nanocrystalline cellulose; particle size, zeta potential, scanning electron microscope and XRD.
1.4 Scope of Research
Isolation of nanocrystalline cellulose by using orthophosphoric acid
Application of microwave-assisted technique and ultra-sonication to enhance the acid hydrolysis
Yield of reaction depends on the reaction time and concentration of acid used
1.5 Research Significant
The isolation of nanocrystalline cellulose from elephant grass by a using microwave-assisted technique or ultra-sonication is conducted to identify if there is any different in crystallinity.
2.1 Structure of Plant Fibers
Plant fibers or known as lignocellulosic are a natural cellular with complex hierarchical biocomposite. They are made up of hemicellulose, lignin extractive, waxes and trace elements that combine and forming semicrystalline cellulose microfibril-reinforced amorphous (Figure 2.1). They consist of a stalk of elongated honeycomb cells that combined together by an intracellular ligneous material. The unique structure leads to high mechanical properties and strength differently depends on the species of plants. Chemical composition and structure of fiber very depend on the plant species, the part that extracted from the plant, ages of plant, growth area, and climate (Mariano et al.,2014).
Figure 2.1 Scanning electron micrograph of a sisal fiber.
2.2 Nanocrystalline Cellulose
Nanocrystalline cellulose that is also known as whiskers usually is prepared from native cellulose by isolation of its crystalline region during acid hydrolysis (Mohammad et al., 2014). Mineral acid hydrolysis is used to extract nanocrystalline cellulose by decomposing it from the semi-crystalline cellulosic fibers. The chemical process will start with removing the polysaccharides bound at the surface of the fibril. Followed by the destruction of the amorphous region that will give a rigid rod-like crystalline cellulose sections (Mohammad et al., 2014). Usually, NCC will have a diameter in the range between 10 to 20 nm with the lengths of a few hundred nanometres (Figure 2.1) (Peng et al., 2011). It will own different morphologies and characterization depend on the origin, type of acid used, reaction time and temperature (Linnea Nilsson, 2015). One of the important parameters in controlling the properties of the NCC-based material is the relative degree of crystallinity and geometrical aspect ratio (length-to-diameter, L/d) (Table 2.1) (Peng et al., 2011).
Figure 2.2 TEM images of NCC by sulphuric acid hydrolysis from various origins
Table 2.1 Examples of length (L) and diameter (d) of NCCs from various origins via different techniques
2.3 Cellulose Sources
As mention earlier, characterization of nanocellulose also depends on the different cellulose sources. Several studied have been made with different cellulose sources such as bamboo pulp (Liu & Xie, 2012), acacia mangium (Jasmani & Adnan, 2017), sugarcane bagasse (Kumar et al., 2014) and so on. Much other cellulose sources had been studied for producing NCC in larger scale rather than use microcrystalline cellulose (MCC) that are too expensive (Linnea Nilsson, 2015).
During the 1980s, researchers produced microfibrillated cellulose (MFC) from the mechanical extraction of wood by using cyclic mechanical treatment in high- pressure homogenizer. Homogenization process will cause the degeneration of wood pulp and give out their sub-structural microfibrils. The products, MFC are consisting of strongly knotted and disordered arrangement of cellulose nanofibers (Mohammad et al., 2014).
2.3.2 Agricultural crops and by-products
Woods are absolutely the great source for producing cellulosic fibers. However, competition happened with the furniture industries and pulp and paper industry. Therefore another source that can be used is fibers from crops and by-product produced from different plants. Other than being burned, use as animal feed or biofuel production, these agricultural by-products can be the main source in natural nanofibres production (Mohammad et al., 2014).
Other than wood, plants are also a desirable cellulose source. They are abundant and infrastructure in textile industries for harvest, pulp and product processing are already establish.
2.3.4 Bacterial cellulose
Other than plant as the source, cellulose fibers can also be extracted from certain bacteria. Common bacteria used are Acetobacter, Agrobacterium, Pseudomonas, Sarcina, Alcaligenes, and Rhizobium. Acetobacter xylinum is known as the most efficient in producing bacterial cellulose (Klemm et al., 2011).
2.4 Characterization of Nanocellulose
Characterization of nanocellulose is an important issue to discover their physical, thermal, chemical, and morphological properties during various treatments. Properties that discussed here are physical and morphological.
2.4.1 Physical Properties
Study about physical characterization also includes about particle size analysis, contact angle and surface charge. Zeta potential had been used to identify the surface charge by following the moving rate of charged particle either positive or negative across an electric field. Basically, if the value is smaller than -15mV it showed agglomeration start to happen. While if higher than -30mV, it signifies that enough bilateral repulsion and colloidal stability (Davoudpour et al., 2013).
2.4.2 Morphological Properties
Surface characteristic of nanocellulose can be evaluated by using atomic force microscopic (AFM). Figure 2.2 shows an example of the AFM image from MCC and NFC. Figure 2.2(a) shows a network structure that interconnected and twisted form with the width of 20 to 30 nm. While figure 2.2(b) show low agglomeration with a diameter lower than 10 nm. Structure and dimension of nanocellulose can be reviewed using transmission electron microscopy (TEM). Example of images is shown in Figure 2.1.
Figure 2.3 AFM images from (a) softwood pulp and (b) microcrystalline cellulose
By using scanning electron microscopy analysis (SEM), the surface morphology of nanocellulose films can be identified.
Figure 2.4 SEM images of (a) dissolved pulp (b) MCC
2.5 Application of Nanocrystalline Cellulose
Nanocrystalline cellulose becomes one of the important classes of renewable nanomaterials which can be used in many applications. It have an attractive properties such as high surface area since it is in nanoscale dimensions, has a unique morphology, mechanical strength and low density (Habibi et al., 2010). The main application of NCC is for strengthening polymeric matrix in nanocomposite materials (Peng et al., 2011).
2.5.1 Nanocomposite films
The geometrical aspect ratio of nanocomposite films is basically depends on the length-to-diameter (L/d). Eventually, it will control the mechanical properties and percolation threshold value. Filler that obtains the high value for aspect ratio will give the best reinforcing effect. As the ratio is high, it will certify the existence of percolation, resulting in improvement of mechanical and thermal stability at lower fibre loadings. Different processing methods are used in the preparation of nanocomposite films such as electrospinning, casting evaporation and extrusion and impregnation (Peng et al., 2011).
2.5.2 Medical applications
Based on the safety, efficiency and other attractive properties, usage of nanocrystalline cellulose in medical applications is increasing. The biomedical community has started to exploit utilize the nanocrystalline cellulose to develop hydrogels. Hydrogels are used as a support for medical and pharmaceutical application such as drug delivery, actuators, valves and tissue scaffolds (Hubbe et al., 2008). The biomedical system from biological cellulose and xylan have opportunities for growth bigger in safe biostable environment further than human health applications (Linder et al., 2003).
2.5.3 Protein immobilisation
Marchessault et al. (2006) had provided a ‘protein fishing’ phenomenon for magnetic MCC. Methods for preparing magnetic MCC are producing Mag-Oxy-MCC (oxidation first) and Oxy-Mag-MCC (magnetization first). Table 2.2 showed the result of protein binding capacities of magnetic MCC by using bovine serum albumin (BSA).
Table 2.2 Protein binding capacities of magnetic MCC
A nanocomposite consist of nanocrystalline cellulose and a gold nanoparticle had been investigated as a protein immobilization (Mahmoud et al., 2009). The models were tested using cyclodextrin glycosyl transferase (CGTase) and alcohol oxidase and showed the result of high loading rate in the matrix (Peng et al., 2011).
2.6 Principle of Isolation
Nano-cellulose in the form of nanocrystalline cellulose can be obtained by using various methods of isolation. Generally, processes of producing nanocellulose are alkali pre-treatment and acid hydrolysis. Cellulose nanofibers in cell wall are high in tensile strength. It is important to extract cellulose nanofibers from cell wall to measure the nanoscopic fibrous component.
2.6.1 Alkali pretreatment
In the first stage of purification, certain amounts non-cellulose components such as lignin, wax, and oils that covering the external surface of the fiber cell wall will be removed during alkali pre-treatment. The lignin structure is disrupted and the structural linkage between lignin and carbohydrates is separate. Sodium hydroxide (17%-18%) is usually used in this treatment. Purification from this alkali treatment causing the solubility of lignin and leaving pectins and hemicelluloses (Mohammad et al., 2014).
2.6.2 Acid hydrolysis
During acid hydrolysis, polysaccharides will break down into simple sugars (Mohammad et al., 2014). The reaction between cellulosic fibers and acid solution cause the amorphous parts completely removed and only individual crystallites are remained (Davoudpour et al., 2013). Molecular chain in cellulose consists of the crystalline and amorphous region (Figure 2.4). It is easier for chemical reagent approach inside the amorphous region rather than the crystalline region (Karim et al., 2015). This is due to the compact structure of crystalline. In order to obtain nanocellulose, the strong hydrogen bonds that are hydrophilic between nanocellulose must be broken down to disperses well in polymers with hydrophobic nature (Davoudpour et al., 2013). Usually, concentrated sulphuric acid is used to isolate NCC, however, during this process yield of NCC obtain is low which is lower than 50% (Karim et al., 2015). Therefore, it will create a huge amount of liquid slurry as wastage. Parameters that utilized in production of crystalline nanocellulose involves the acid-to-cellulose fibers ratio, temperature, agitation and reaction time (Habibi et al., 2010).
Figure 2.5 Organization of crystalline and amorphous region in cellulose fiber (Mariano et al., 2014)
2.7 Plant Material
The material used in this study is elephant grass.
2.7.1 History on Elephant Grass
Elephant grass is a monocotyledonous flowering plant with the scientific name of Pennisetum purpureum Schumach coming from the family of Poaceace (grass family) and genus Pennisetum. It is also known as Napier grass, merker grass, and Uganda grass. It is a tropical grass coming from humid tropical Africa. Elephant grass can grow well in all subtropical and tropical regions throughout the years. It has been widely grown as animal fodder in Central and South America, Australia, Pacific island, tropical parts of Asia and the Middle East. The most suitable climate for the growth of elephant grass is 30-35℃ and no growth happened below than 10℃ (Obeng et al., 2015). As the potential for fodder and fast growing plant, it had been spread extensively around the world. Other than forage supplier, it can use to make fences as a windbreak, while the dried material can be used as a fuel source and etc. As the global interest in reducing the fossil fuel spending and its impact on climate, the research had led it as one of the promotion biomass plants as next generation biofuel harvests. With its capabilities to grow fast, it is expected to produce a dry matter (DM) up to 78 tons/ha/annum (35-41 tons/ha average) (Negawo et al., 2017).
2.8 History of Microwave
The microwave was firstly invented by accidental as the by-product during war time. Perry Spencer is the first people that help in design and invent the first model of a microwave oven in Raytheon Manufacturing Company in 1946. He realized that microwave that he was working make a candy bar in his pocket melted (Osepchuk, 1984). When he started the experiment on a microwave, it is found that microwaves could cook foods quicker than conventional ovens. Although Percy Spencer is the first inventor, there are many people that also contribute to designing and construction of the first oven (Osepchuk, 2009). The first oven that went on the market was on 1955 by Tappan and later in Westinghouse license provided by Raytheon Company. It is called as 1161 Radarange with power used 1600 watts. During 1975, sales of the microwave ovens had exceeded the sales of gas ranges. Although at the late 1990s the prices for microwave have come down, it is still higher than any other conventional heating equipment. The usages of microwave ovens keep increasing then as one of the practical necessity in the world.
Based on the previous study, it had been observed that by using microwave heating can produce the greater yield of nanocrystalline cellulose compared to the conventional heating (Peng et al., 2011). It also provides shorter reaction time during microwave synthesis that makes it ideal for reaction scouting and optimization of reaction conditions. Thus, create more decision points per unit time. The usage of microwave synthesis has been a very priceless tool for drug discovery and medicinal chemistry since it is used to decrease the reaction time affectedly. During the past, microwave synthesis had been handled by using standard organic solvents under open vessel conditions (Kappe et al.,2009).
2.8.1 Principle of microwave
Conventional thermal heating is comparatively slow and non-efficient method in transferring energy into the system since it depends on convection currents and thermal conductivity. It resulting the temperature of the reaction vessel higher than the reaction mixture.
Figure 2.6 Comparison of (a) conventional and (b) microwave heating
Generally, microwaves are a form of electromagnetic radiation that has frequencies between 0.3 to 300G Hz with corresponding wavelengths of 1mm to 1m (Figure 2.6) (Kappe et al., 2009b). Therefore it lies between infrared and radio frequency.
Figure 2.7 Electromagnetic spectrum
Based on the typical microwave oven, microwaves are generated in magnetron through a waveguide into the cooking chamber. The chamber is made up of a metallic wall; therefore, it acts as a Faraday cage. Most of the microwaves having a rotating turntable or rotating reflector that act as the stirrer. The door is made up of the glass while the bulbs cavities are covered by metal grids. Since the holes between the metal grids are smaller than the wavelength of microwaves, the grids will act as metal plates.
2.9 Principles of Ultrasonication
Application of ultrasonic technology had been widely used in wastewater treatment and environmental remediation areas. It is effective in degrading organic effluent into less toxic compounds. Advantages obtained by using ultrasonic are potential chemical free, simultaneous oxidation, shear degradation, thermolysis and enhanced mass transfer processes. (Wu et al., 2013)
2.9.1 Principle of ultrasonic
Ultrasound is in the sound range of 20 KHz to 10 MHz generated by a transducer that converts electrical or mechanical energy to high-frequency acoustical energy. Ultrasonic waves can be classified similar to the audible sound wave using a speaker. In the sound wave, the diaphragm of the speaker is automatically moved to and fro, therefore producing low pressure and high-pressure points in the air. While in the ultrasonic wave, the diaphragm needs to move to and fro more vigorously than sound wave (Terzic et al., 2013). High-intensity ultrasonication (HIUS) waves can cause physical phenomena changes by strong mechanical oscillating power. The physical phenomena including formation, expansion, and collapse of microscopic gas bubbles happened when molecules in liquid absorb ultrasonic energy. Isolation of fibrils can accomplish using HIUS energy in both a process and produce is a mixture of microscale and nanoscale fibrils. As the power increase, the temperature of fiber suspension also increased, eventually increase the rate of cellulose fibrillation. The temperature of water suspension can be increased until 91℃.
A mechanical method in producing nanofibers cellulose is by using high-intensity ultrasonication and hydrodynamic forces (Davoudpour et al., 2013). The cavitation will cause the ultrasonic waves producing a strong mechanical stress. Therefore the cellulosic fiber is disaggregation to nanofibers (Guo et al., 2013). To increase fibrillation of nanoscale cellulose, the researcher had combined the ultrasonic with other methods such as acid hydrolysis. It had been found that ultrasonication will lead to folding and erosion of cellulose surface. Thus, it provides more reactive site for acid penetration to preparing high crystalline and smaller size nanocellulose (Davoudpour et al., 2013).