Polymer Composites Based on Cellulosics Nanomaterials Chapter ABSTRACT

Transcription

1 4.1. ABSTRACT In this study, non-spinnable short staple cotton fibers were microfibrillated in disc refiner followed by nanofibrillation in high pressure homogenizer and their properties evaluated. The extent of fibrillation was improved by increased number of repeated passes. Refining as a pretreatment process helps to circumvent the fiber clogging problem in homogenizer nozzle. Microfibrillation by 30 passes in refining produced the fibrils of size 416 ± 177 nm and subsequent nanofibrillation by 15 passes in homogenization resulted in 112 ± 49 nm as revealed by scanning electron microscopy. While refining process does not significantly affect the crystalline index of fibers, homogenization reduced it by 10% due to very high shear and impact forces and hydrodynamic cavitation. The degree of polymerization of cotton fibers was reduced significantly (35.4%) during this two-stage process. Fourier transform infrared spectroscopy proved that there is no change in the molecular structure of cotton fibrils during this two-stage fibrillation process. Thus produced nanofibrils having potential application as fillers in composites which can be used for various application food packaging, much films membranes etc. add value to the cotton fibers that are not suitable for spinning in textile industries due to their very short length. Vilas Karande 101

2 4.2. INTRODUCTION Cellulose is a renewable, biodegradable and the most abundant biopolymer available in the biosphere (Lee et al, 200) and it is produced in nature at an annual rate of tons (Zhao et al, 2007). Cellulose is the main constituent of the plants serving to maintain their structural integrity. It is also found in bacteria, fungi, algae and some animals (Sun et al, 2007). The properties of nano-forms of cellulose like high tensile strength, low density and biodegradability lead to increasing research interest. Cellulose is the structural material of the fibrous cells with high level of strength and stiffness per unit weight and has a straight carbohydrate polymer chain consisting of β-1-4 D-glucopyranose units and varying degree of polymerization as reported by Kamel, The molecules aggregate and are present in the form of microfibrils as explained by Hult et al., The intra- and inter-chain hydrogen bonding in the cellulose structure play a vital role in governing the reactivity and physical property of cellulose. Natural cellulosic fibers are synthesized mainly in plants and cellulose constitutes 40-50% of wood, 80% of flax and more than 90% of cotton fiber. Recently, many researchers / manufacturers use natural fibers to replace man-made / synthetic fibers as reinforcement material and fillers to make environmentally benign products. Cellulose fibers can be mechanically disintegrated to structural micro/nanoscale fibrils (Ahola et al, 2008 and Nakagaito et al, 2005). Chakraborty et al defines microfibrils as the fibers of diameter in the range of μm (Chakraborty et al, 2005), with corresponding length in the range of 5-50 μm and nanofibrils having at least one dimension in the range of nm. The micro/nanofibrils isolated from the natural fibers have much better mechanical and other functional properties as envisaged by Sakurade et al., Therefore, much attention has been given in the last decade to study their use as fillers in various polymer composites to increase their overall performance. Earlier, our group has reported the production of cellulose microfibrils by disc refining process (Karande et al, 2011). Refining is a pulping method in which the fibers are separated from the matrix by means of shear force. The main objective of this process is to loosen and separate the fibrils from the matrix and to fibrillate them to the desired length and diameter. Subsequent nanofibrillation is possible with the Vilas Karande 102

3 help of high pressure homogenizer that works on the principle of the hydrodynamic cavitation. Cavitation is the phenomenon of sequential formation, growth, and collapse of millions of microscopic vapour bubbles (voids) in the liquid. The collapse or implosion of these cavities creates high localized temperatures and pressure that result into short-lived localized hot-spot in cold liquid as explained by Sawant et al., The hydrodynamic cavitation is generated by passing the liquid through geometry of constriction such as orifice plates and venturi (Kumar et al, 1999, Moholkar et al, 1997 and Patil et al, 2007). Cavitation is widely used for the dispersion of the agglomerated solids, facilitating the rates of filtration, crystallization, and degassing. Tuulmets and Raik proposed their use in reaction engineering for homogenous reactions (Tuulmets et al, 1999) while Nie et al. 2008, in emulsion reactions, and Suslick et al. in reactions where catalysts and/or fine powders are involved (Suslick et al, 1996). In this study, involving a two-stage process, we have used the shear force of refining process for microfibrillation of cotton fibers followed by hydrodynamic cavitation of homogenization process for nanofibrillation. In the previous work involving the production of microfibrils by refining process (Karande et al. 2011), 0.5% consistency of cotton fibers was used for better fibrillation. Here, to increase the productivity along with nanofibrillation, 1.0% consistency was used in refining followed by high pressure homogenization process. The cellulose nanofibrils prepared by above process can be utilized for membrane applications especially those can used for separation air (li et al, 1994, Huang et al, 1994 and li et al, 1996) EXPERIMENTAL Materials Short staple cotton fibers were used as the starting material, sodium hydroxide (AR Grade) and hydrogen peroxide was purchased from Fisher Scientific, Sodium silicate (meta) nonhydrate (LR grade) was supplied by S. D. Fine-Chem Ltd. India, Nonyl phenol ethylene oxide required for wetting of cotton fibers into alkali solution was supplied by Amrutlal industrial products, India. Vilas Karande 103

4 Preparation of Nanofibrils The nanofibrils from cotton fiber were prepared by two-stage process from nonspinnable short staple cotton fibers (variety: Bengal Desi, India) in a top down approach. Since the cotton fibers clog the nozzle of homogenizer, they were first microfibrillated in lab disc refiner. The lab disc refiner supplied by Universal Engineering Ltd, India was used for microfibrillation. Out of two discs, one is stationary (stator) and other one is rotating disc (rotor) which is driven by an electrical motor. Distance between the stationary and rotating disc was maintained at 5 thou (127 μm) and the rotor was operated at 1440 rpm. The bleached cotton fibers were added into the hopper of the refiner at 1% consistency and fed into the fibrillation zone by a helical screw. The output (microfibrils) from the fibrillation zone of refiner was collected in a vessel and this entire process completed in 2 minutes and was considered as one pass. It has been found that more number of passes were required for complete and uniform fibrillation. The sample was refined up to 30 passes and characterization was done after every 5 passes. Mini DeBEE laboratory homogenizer was used for nanofibrillation of microfibrils at a pressure of psi and a flow rate of 250 ml/min. The fibrillated sample and the hydraulic system were cooled using a circulating water bath. Samples were characterized after 1, 3, 5, 10 and 15 passes CHARACTERIZATION Microscopy (SEM and AFM) A Phillips Scanning electron microscope operated at 15 kv was used to take images of cotton fiber samples before and after fibrillation. Samples were coated with gold / palladium using vacuum sputter coater to improve their conductivity. The AFM analysis was carried out using a diinnova AFM (Veeco, Santa Barbara, CA, US) equipped with a 90 µm scanner. A drop of fibril suspension was deposited onto a freshly cleaved mica surface and dried under infrared lamp. All images were obtained using tapping mode in air at room temperature. The silicon nitride cantilever with a spring constant of 40 Nm -1 was used. The scan rate of 1.0 Hz and 512 lines per 5 µm was used to optimum contrast. No filtering was used during scanning. Vilas Karande 104

5 X- ray Diffraction X-ray diffraction (XRD) measurements were performed on a Rigaku wide angle system in a 2θ range between 4 to 40. Crystalline index of the sample was determined by using equation (4.1) [Cao et al, 2002). % Crystalline Index = {[Ic-Ia]/[Ic] x100} (4.1) Where Ic is the peak intensity of crystal plane (2θ = 22 ) and Ia is the peak intensity of amorphous phase (2θ = 18 ). The crystal dimensions of three main equatorial reflections of cellulose, (101), (101) and (002) planes were determined using Scherrer s equation, L h,k,l = Kλ / βcos θ (Daniel et al, 2007), where, K = 0.94, λ = Å, θ is diffraction angle corresponding to the crystal plane and β is the full width at half maximum of the peak angle of the same crystal plane Fourier Transform Infrared spectroscopy For Fourier Transform Infrared spectroscopy (FTIR), the freeze dried fibrils were diluted with potassium bromide in the ratio of 1:100 and made into a pellet. This pellet was analyzed using an IRPrestige-21 FTIR in transmission mode. The spectra recorded were the average of 64 scans and the contribution of background was accounted for during analysis. Crystalline index (CrI) was determined as the ratio of absorbance at 1372 and 2900cm -1 (Ciolacu et al, 2011) Degree of Polymerization Degree of polymerization (DP) of the cotton fiber sample was measured with the help of Ubbelohde viscometer. About 50 mg of sample was mixed with 100 ml of cupra ammonium hydroxide. The mixture was vigorously shaken and then kept in water bath at 25 C for complete dissolution. The viscosity was calculated from the efflux time of the cellulose solution and pure solvent. DP was calculated using equation 4.2, (Grobe et al, 1989), DP = [{(2000*ηspec)/(c*(1+0.29* ηspec)}]. (4.2) Vilas Karande 105

6 η spec = (η/ η 0-1); η spec - Specific viscosity; η/ η 0 -Relative viscosity; c- concentration in g/l; η 0 - time required for the solvent to travel from the upper graduated mark to the lower graduated mark; η time required for the sample to travel from the upper graduated mark to the lower graduated mark Thermal Analysis using DSC and TGA Thermal stability of the cellulose nanofibers before and after the refining and homogenization process was analyzed using the thermogravimetric analyzer (TGA) and differential scanning calorimetry (DSC). In case of TGA the sample was heated from 30 C to 500 C with a heating rate of 10 C/min and change in weight losses were observed. In case of DSC analysis the samples were heated from -50 C to 200 C with a heating rate of changes in the glass transition temperatures were observed RESULTS AND DISCUSSION Scanning Electron Microscopy In refiner, the cotton fibers are fed in between the static and rotating discs with sharp ridges kept at a distance of 5 thou (127 microns). The shear force due to rotation of disc in opposite directions helps in microfibrillation of cotton fibers. In the homogenizer, the microfibril suspension is pumped into the nozzle through the inlet by a high pressure pump. The zirconia nozzle with an orifice of 30 µm was used in this system. The microfibril is forced into the nozzle orifice at high pressure which produces a high velocity jet. It was a first step in the fibrillation process. The microfibril then enters into the absorption cell homogenization through complete mixing. The absorption cell geometry is designed in such way to create a second highvelocity stream, which flows in the opposite direction and around the original fluid jet. The hydrodynamic cavitation due to interaction between these two streams in combination with shear and impact forces helps in nanofibrillation of microfibrils. SEM analysis of both refined and homogenized fibrils was carried out to study the morphology and size distribution. Figure 4.1 shows the SEM micrographs of refined fibrils during different passes. Though during the first 5 passes itself few microfibrils are formed, many are yet to be fibrillated. As passes increased, the uniformity of fibrillation increased along with reduction in fibril size. At the end of 30 passes, more Vilas Karande 106

7 number of fibrils were in the micron size range. This is supported by the reduction in standard deviation of average size by image analysis given in table 4.1. (a) (b) (c) (d) (e) (f) (g) (h) Figure 4.1. Scanning electron micrographs of control cotton fibers (a scale bar equals 20 µm; b scale bar equals 2 µm) and microfibrillated after 5 (c), 10 (d), 15 (e), 20 (f), 25 (g) and 30 (h) passes in refiner. Scale bar represents 2 µm Vilas Karande 107

8 (a) (b) (c) (d) (e) (f) Figure 4.2. Scanning electron micrographs of cotton fibrils during homogenization process after 0 (a), 1 (b), 3 (c) 5 (d), 10 (e) and 15 (f) passes. Scale bar represents 2 µm. Vilas Karande 108

9 Table 4.1. Effect Refining and Homogenization on its average diameter Process No of Passes Average size (nm) ± SD ± ± ± 280 Refining Process ± ± ± ± ± ± 149 Homogenization Process ± ± ± 49 Figure 4.2 shows the SEM images of homogenized fibrils during different passes and its corresponding size by image analysis is given in table 4.1. Here also, similar phenomenon happened, where the variation in size reduced during subsequent passes through the homogenizer. After the refining process, the average size of the cotton fiber was reduced to 416 ± 177 nm from an initial value of 21.5 µm. Subsequent homogenization process reduced the size further down to 112 ± 49 nm. Vilas Karande 109

10 Atomic Force Microscopy Figure 4.3. AFM height images of the refined cotton fibrils. (a) 2D image, (b) 3D image and (c) line analysis. Figure 4.4. AFM height images of the homogenized cotton fibrils. (a) 2D image, (b) 3D image and (c) line analysis. Figures 4.3 and 4.4 shows that the surface morphology of the cotton fibers after refining and homogenization processes by atomic force microscope (AFM). The refined fibrils were still entangled and not completely separated from each other even at very low dilution. In case of homogenized sample, nanofibrils were well separated from each other and its complete length could be visualized. The line analysis of AFM image showed that the refined fibrils had height of 418 nm and that of homogenized samples was nm. The average size analyzed from both SEM and Vilas Karande 110

11 AFM does not necessarily mean the diameter of cotton fibrils since the three dimensional structure collapses and got flattened during the sample preparation and drying processes X-ray Diffraction Figure 4.5. X-ray diffractograms of (a) refined samples and (b) homogenized samples Figure 4.5. represents the X- ray diffraction pattern of refined and homogenized samples after different number of passes Vilas Karande 111

12 Table 4.2. Crystal dimensions and crystalllinity of cotton fibers during refining and homogenization Crystal dimension (nm) Process No. of passes Crystallinity (%) Refining Homogenization It has been observed from the figure 4.5 and table 4.2 that there was no significant change in crystalline index of the cellulose fibrils after various passes through the refiner but as the sample was passed through the high pressure homogenizer, a significant change was noticed. The crystalline index was reduced to % after homogenization from an initial value of %. This may be due to exposure of the fibrils to high velocity jet, shear, cavitation and impact forces, which completely break apart the fibrils in the fluids. However, it has been found that there was no Vilas Karande 112

13 significant change in the crystal dimension even after homogenization due to the fact that these dimensions could not be reached in this two-stage process Fourier Transform Infrared spectroscopy Figure 4.6. FTIR spectra of control (top), refined (middle) and homogenized (bottom) cotton fibers FTIR spectra of the control cotton fibers, refined and homogenized fibrils were depicted in figure 4.6. The spectra showed absorption around 1054, 1106 and 3400 cm -1 which was attributed to stretching bands and confirms the presence of R-OH. The absorption band around 1400 cm -1 was due to scissoring vibration of CH. It has been observed that there is no change in the molecular structure of cellulose was noticed during this two-stage mechanical process. Crystalline index (CrI) has been calculated from the FTIR spectra and it was 0.74 for raw cotton fiber. The CrI of refined fibers was 0.62, 0.57, 0.59, 0.52, 0.54 and 0.55 for 5, 10, 15, 20, 25 and 30 passes, respectively and of homogenized fibers was 0.55, 0.58, 0.51, 0.53 and 0.50 for 1, 3, 5, 10 and 15 passes, respectively. While first 5 passes in refiner accounted for 16% reduction in CrI, subsequent passes in refiner and homogenizer resulted in overall reduction of 32% CrI. When a sample is subjected to shear force and hydrodynamic cavitation, crystalline region has been reduced gradually. Vilas Karande 113

14 Degree of Polymerization Degree of polymerization (DP) is defined as the number of repeating units present in a polymer. Mechanical stresses generated due to shear and cavitation forces had great influence on chain scission and hence on DP. Thus the two-stage fibrillation process resulted in significant reduction of degree of polymerization and size of the cellulose fibrils. Table 4.3. Degree of Polymerization of cotton fibers after Refining and Homogenization Process Process No of Passes Degree of Polymerization ± ± ± 49 Refining Process ± ± ± ± ± ± 29 Homogenization Process ± ± ± 29 Vilas Karande 114

15 From the table 4.3, it has been found that the DP of the cotton fibers was decreased by 27.2% after refining and further decreased by 35.4% after homogenization process. The decrease of DP does not provide information about the fiber length, but indicates the breakage along the fiber direction (Iwamoto et al. 2007). Thermal stability of the cellulose fibers before and after the fibrillation was carried out using thermogravimetric analyzer (TGA) and it was measured as the function of weight loss. From the DSC analysis, shift in glass transition temperature was observed Thermal Analysis using DSC and TGA It was observed from the figure 4.7[A] that onset temperatures of control cotton fibers, refined sample and homogenized samples were 355, 334, 334 and endset temperatures were 380, 357 and 347 C respectively. Weight loss of control cotton fibers, refined and homogenized fibers were 97.52%, and 100% respectively. So from shift in offset temperature and % weight loss we can say that control cotton fibers had more thermal stability as compared to refined and homogenized samples. This may be attributed to slightly higher crystalline index of the control fibers as compared to the other counterparts. Therefore we can conclude that stability of the cellulose fibers were changed slightly but the change is not significant. From the figure 4.7[B], it was observed that glass transition temperatures of the control cotton fibers, refined and homogenized samples were C, C and C respectively. So from these observations we can say that as the cotton fibers were fibrillated using refiner and homogenizer, glass transition temperature reduced as compared to control fibers but shift in Tg was not significant. Vilas Karande 115

16 Figure 4.7. Thermal stability of the cellulose fibers using TGA [A]: a-control cotton fibers, fibrillation after refining and after homogenization and DSC [B]: Control cotton fibers, fibrillation after refining and after homogenization Vilas Karande 116

17 4.6. CONCLUSIONS We demonstrated a two-stage process for production of nanofibrils from nonspinnable short staple cotton fibers. The refining (first stage) acts as a prerequisite process of size reduction of cotton fibers for subsequent nanofibrillation by high pressure homogenization (second stage). Thirty passes through refiner resulted in the microfibrils of size 416 ± 177 nm and fifteen passes in homogenizer resulted in nanofibrils of size 112 ± 49 nm. The refining process does not have any significant effect on the cellulose crystalline index while homogenization reduced it from 87.2 % to 81.5 %. The degree of polymerization has been reduced significantly (35.4%) due to this two-stage process. The molecular structure of the cellulose is remained unaltered by both, pretreatment and homogenization process. Control cotton fibers had slightly higher thermal stability as compared to refined and homogenized samples. Nanofibrils prepared from non-spinnable short staple cotton fibers have immense potential for use as reinforcement agents / fillers in polymer composites. Vilas Karande 117