Abstract
Cellulose microfiber (MF) formation by electrospinning is affected by several factors. In this paper, fabrication of MF from oil palm mesocarp fiber (OPMF), a biomass residue abundantly available at the palm oil mill, was conducted by electrospinning. The effect of OPMF-cellulose solution properties on the spinnability of the solution was determined. Extracted cellulose from OPMF was dissolved in four different formulations of ionic liquids: (i) ([EMIM]Cl), (ii) ([EMIM][Cl):DMF, (iii) ([EMIM]Cl):([C10MIM][Cl]), and (iv) ([EMIM]Cl):([C10MIM][Cl]):DMF at cellulose concentrations of 1% to 9% (w/v). Scanning electron microscopy (SEM) analysis showed that MF formed had diameter sizes ranging from 200 to 500 nm. MF was formed only at 6% (w/v) cellulose concentration, when DMF was mixed in the solution. The results showed that cellulose concentration and viscosity played major roles in the spinnability of cellulose solution, in which too high viscosity of the cellulose solution caused failure of the electrospinning process and eventually affected the formation of MF. The characteristics of MF obtained herein suggest the potential of OPMF cellulose as a starting material for the production of MF.
Download PDF
Full Article
Factors Affecting Spinnability of Oil Palm Mesocarp Fiber Cellulose Solution for the Production of Microfiber
Tengku Arisyah Tengku Yasim-Anuar,a Hidayah Ariffin,a,b * Mohd Nor Faiz Norrrahim,b and Mohd. Ali Hassan b
Cellulose microfiber (MF) formation by electrospinning is affected by several factors. In this paper, fabrication of MF from oil palm mesocarp fiber (OPMF), a biomass residue abundantly available at the palm oil mill, was conducted by electrospinning. The effect of OPMF-cellulose solution properties on the spinnability of the solution was determined. Extracted cellulose from OPMF was dissolved in four different formulations of ionic liquids: (i) ([EMIM]Cl), (ii) ([EMIM][Cl):DMF, (iii) ([EMIM]Cl):([C10MIM][Cl]), and (iv) ([EMIM]Cl):([C10MIM][Cl]):DMF at cellulose concentrations of 1% to 9% (w/v). Scanning electron microscopy (SEM) analysis showed that MF formed had diameter sizes ranging from 200 to 500 nm. MF was formed only at 6% (w/v) cellulose concentration, when DMF was mixed in the solution. The results showed that cellulose concentration and viscosity played major roles in the spinnability of cellulose solution, in which too high viscosity of the cellulose solution caused failure of the electrospinning process and eventually affected the formation of MF. The characteristics of MF obtained herein suggest the potential of OPMF cellulose as a starting material for the production of MF.
Keywords: Oil palm mesocarp fibers; Microfibrillated cellulose; Spinnability; Electrospinning; Ionic liquids
Contact information: a: Laboratory of Biopolymer and Derivatives, Institute of Tropical Forestry and Forest Products, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia; b: Faculty of Biotechnology and Biomolecular Sciences, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia;
* Corresponding author: hidayah@upm.edu.my
INTRODUCTION
Electrospinning has gained much interest from researchers due to the growing demand for micro- and nano-sized fibers in various industries, such as in the production of composites (Zhu et al.2011), energy devices (Shi et al. 2015), and tissue scaffolds for biomedical purposes (Yoshimoto et al. 2003). This method produces high surface area (Zhu et al. 2011), permeability (Cengiz et al.2009), thermal resistance (Bordes et al. 2009), and crystallinity, as well as finer and continuous fiber materials with sizes ranging from microns to several nanometers (Khajavi and Abbasipour 2012). Due to its capability, electrospinning has been widely used for the production of cellulose microfiber (MF); however, most of the previous studies focused on using commercialized cellulose as a starting material (Quan et al. 2010; Freire et al. 2011; Chen et al. 2015). Because of the difficulty in finding the most appropriate conditions suitable for natural cellulose to be electrospun, it is a less attractive option, so researchers tend to use commercialized cellulose for electrospinning. Appropriate conditions are required to ensure the formation of stable and continuous fiber spinning (Freire et al. 2011), and generally there are three main factors that affect fiber formation by electrospinning: solution parameters, process parameters, and ambient conditions (Shi et al. 2015). Of these factors, solution parameters are the most crucial to control, as electrospinning cannot be conducted without a solution. Solution parameters such as concentration, surface tension, and conductivity greatly affect electrospinning, and all of these factors depend on the starting material used.
In terms of raw material, instead of continuously depending on commercialized cellulose, the potential of lignocellulose as a starting material for MF production should be investigated. In Malaysia, oil palm biomass has great potential due to its abundant availability and high cellulose content. Oil palm mesocarp fiber (OPMF) is a residue obtained from oil palm fruits after oil extraction. In 2013 it was reported that the global production of palm oil was 56 million metric tons. For every metric ton of palm oil produced, 4 tons of dry biomass is generated, of which 12% are OPMF (Geng 2014). Therefore, it is estimated that the global OPMF production in 2013 was 26 million tons. In Malaysia alone, the generation of OPMF is about 11.5 million tons (Hoe 2014). Unlike oil palm empty fruit bunch (OPEFB), which has been extensively studied as a raw material for the production of various bioproducts such as biocomposite, biochar, biofuel, and biocompost, OPMF is usually used in palm oil mills as boiler fuel or burnt as waste, which is wasteful and indirectly contributes to environmental pollution (Then et al. 2013). As a lignocellulosic material, OPMF has potential as an alternative raw material for various bio-based products manufacturing, as it contains approximately 60% cellulose, 11% lignin, and 3% ash content (Sreekala et al. 1997). OPMF also offers high mechanical strength; for example, it has a tensile strength, Young’s modulus, and elongation at break of 80 MPa, 500 MPa, and 17%, respectively (Sreekala et al. 1997). These unique characteristics indicate OPMF for MF production, as it offers good chemical and mechanical properties.
The use of OPMF as a resource for MF production is considered here for the first time. The goal is to identify the potential of this material for nanofiber application. In this study, cellulose was extracted from OPMF, and the OPMF-cellulose was later dissolved in different ionic liquid formulations to determine the effect of the solvents on the properties of the liquid, which eventually affected the spinnability and hence the production of MF by electrospinning.
EXPERIMENTAL
Materials
Oil palm mesocarp fiber (OPMF) was obtained from Seri Ulu Langat Palm Oil Mill, Dengkil, Selangor, Malaysia. The samples were cleaned, dried, and stored in a sealed plastic bag at room temperature prior to further study (Nordin et al. 2013). Potassium hydroxide (KOH), sodium chlorite (NaClO2), and ionic liquids (namely 1-ethyl-3- methylimidazolium chloride ([EMIM]Cl), 1-decyl-3-methylimidazolium chloride ([C10MIM][Cl]), and N,N-Dimethylformamide (DMF)) were purchased from Sigma-Aldrich (St. Louis, MO, USA).
Methods
Chemical pretreatment of oil palm mesocarp fiber (OPMF) for cellulose extraction
The OPMF was pretreated using 5% (w/v) sodium chlorite (NaClO2) at pH 4 to 5, 70 °C, for 90 min, followed by 6% (w/v) potassium hydroxide (KOH) at room temperature for 24 h to remove both hemicellulose and lignin (Iwamoto et al. 2008). The materials were ground with a hammer mill (Miki Pulley Co., Ltd., Japan), and the OPMF-cellulose obtained had an approximate size of 0.5 to 1 mm. The OPMF-cellulose powder was stored in a sealed plastic bag prior to use. All reagents were used as received.
Ionic liquid formulations for determination of liquid properties prior to electrospinning
Prior to electrospinning, the OPMF-cellulose powder was dissolved in ionic liquid. Imidazolium-based ionic liquids, namely 1-ethyl-3-methylimidazolium chloride ([EMIM]Cl) and 1-decyl-3-methylimidazolium chloride ([C10MIM][Cl]), were used. In addition to ([EMIM]Cl), which acted as the main solvent to dissolve OPMF-cellulose, and ([C10MIM][Cl]), which acted as the second ionic liquid to reduce the solution surface tension, N,N-Dimethylformamide (DMF) was also used as a co-solvent to improve spinnability and enhance the uniform MF formation (Karimi et al.2014). The four ionic liquid formulations are modified from Hӓrdelin et al. (2012) and are presented in Table 1.
Table 1. Different Formulations of Ionic Liquid for Dissolution of OPMF-Cellulose
To prepare the solution, an appropriate amount of DMF was added to the OPMF-cellulose, followed by ([C10MIM][Cl]) and ([EMIM]Cl). The diffusion rate of ionic liquid into cellulose is enhanced by adding the co-solvent first (Hӓrdelin et al. 2012). Each mixture was heated at 80 °C and stirred at 200 rpm for 12 h to produce a homogenous cellulose solution, which was subjected to electrospinning and analyses.
Electrospinning
MF prepared by electrospinning were spun using a voltage of 25 kV, working distance of 12 cm, and a rate 10 mL/min onto a water bath type collector (modified from Hӓrdelin et al. 2012). After completing the electrospinning process, the ionic liquids remaining in the electrospun MF were removed using ethanol and they were washed in a flow deionized water. The MF then were oven-dried prior to analyses.
Characterization
Chemical composition analysis
Chemical composition analysis was conducted as reported by Nordin et al. (2013). The dry basis content of cellulose, hemicellulose, and lignin in the starting material was calculated gravimetrically. The OPMF was soaked in a 5% (w/v) sodium chlorite (NaClO2) solution of pH 4 to 5 at 70 °C for 90 min. The samples were oven-dried for 24 h and cooled in a desiccator until a constant weight was recorded. Weight loss after this treatment was considered lignin composition, and the remaining weight was considered holocellulose, which consists of hemicellulose and cellulose composition. The holocellulose was rinsed with deionized water prior to further chemical treatment using 6% (w/v) potassium hydroxide (KOH) at 25 °C for 24 h to remove hemicellulose and retain cellulose. The treated samples were oven-dried for 24 h and cooled in a desiccator prior to analysis. The final weight of the samples was considered cellulose content.
Surface tension analysis
Surface tension analysis was conducted as reported by Seo et al. (2009). The surface tension of each mixture was measured using an EZ-Pi Surface Tensiometer (Kibron Inc, Helsinki, Finland). Deionized water was used to calibrate the samples. For analysis, the tip of the probe was immersed in 3 to 3.5 mL of sample.
Viscosity analysis
Viscosity was measured to determine the flow resistance of solution as described in Wahab et al. (2013). The viscosity of OPMF-cellulose solutions was measured using a Brookfield viscometer model 02072 (Stoughton, MA, USA) at room temperature using a SC43 (31) spindle, and the spindle speed was selected according to the viscosity of the ionic liquids solution.
Degree of polymerization measurement
The degree of polymerization of OPMF-cellulose was measured using a viscometer according to TAPPI T230 om-94 (1994) and ISO 5351-1 as reported by Chauve et al. (2013). The OPMF-cellulose was diluted in solutions containing distilled water and copper (II) ethylenediamine (CED) solution with a ratio 0.01:1:1 (OPMF-cellulose: distilled water: CED). The fiber solution was shaken until all fibers were completely dissolved. The OPMF-cellulose solution was then poured into the Ubbelohde viscometer tube (Type 231, PTA Laboratory Equipment, Vorchdorf, Austria), and the process was carried out at 25 °C for 10 to 15 min. The molecular weight of OPMF-cellulose was determined using the Mark-Houwink approach, which was using Eq. 1,
(1)
where [ƞ] is the intrinsic viscosity and M is the molecular weight. K = 0.42 and α = 1 are constant values of CED solvent (Chung and Um 2014).
Morphological analysis
To observe cellulose dissolution in different ionic liquid solutions, the morphology of the OPMF-cellulose solutions was analyzed using a light microscope (Motic, Model BA310, Richmond, BC, CA) at 40x magnification. The morphology of the electrospun MF was analyzed using field-emission scanning electron microscopy (FE-SEM) (Sirion 200, FEI, Eindhoven, Netherlands) with accelerating voltage of 5 kV. The MF were coated with platinum using a vacuum sputter coater prior to observation by FE-SEM.
X-ray diffraction (XRD) analysis
XRD was performed on a Philips PW3040/00 X’Pert MPD system (Eindhoven, Netherlands) to determine the crystallinity index of MF. The X-ray pattern was recorded in the 2θ range of 5 to 50°, and the crystallinity was calculated using Eq. 2,
(2)
where I002 was evaluated at an angle of 2θ = 23, and it refers to the peak of the crystalline portion of biomass (i.e., cellulose). Iam refers to the peak at about 2θ = 18, which corresponds to the amorphous region (Nordin et al. 2013)
Thermogravimetric analysis
The thermogravimetric (TG) analysis was performed using a TG analyzer (TG 400, Perkin Elmer, Waltham, MA, USA) to identify the thermal stability and decomposition temperature of the raw OPMF, OPMF-cellulose, and OPMF-MF. Samples (about 10 mg) were placed in aluminum cups, and the analysis was carried out from 50 to 550 °C at a heating rate 10 °C/min under nitrogen flow of 20 mL/min.
RESULTS AND DISCUSSION
Characteristics of OPMF Cellulose
Chemical composition analysis showed that the raw OPMF contained 43% cellulose, 33% hemicellulose, and 22% lignin. The hemicellulose and lignin components of OPMF were reduced after chemical pretreatment, leaving approximately 99% cellulose (Table 2). This result is attributed to the use of NaClO2 and KOH during pretreatment.
Table 2. Chemical Composition of the Oil Palm Mesocarp Fibers
Note: Results are means of 3 replicates ± S.D.
The viscosity-average molecular weight and degree of polymerization of the OPMF-cellulose were determined using an intrinsic viscosity measurement. From Table 3, it is shown that molecular weight of the OPMF-cellulose was 157,000 g/mol, while the degree of polymerization was 967. It has been reported that molecular weight and degree of polymerization may affect cellulose properties such as cellulose solubility, mechanical properties of cellulose-based materials, and spinnability (Audrey et al. 2006). From the same table, it is seen that the molecular weight and degree of polymerization of OPMF-cellulose was comparable to cellulose from other lignocellulosic materials. Degree of polymerization of other cellulose samples ranged from approximately 400 to 3000; while the molecular weight ranged from approximately 90,000 to 300,000 g/mol.
X-ray diffraction patterns of the raw and OPMF-cellulose are shown in Fig. 1. Both raw and OPMF-cellulose had diffraction patterns at around 2θ = 23 and 2θ = 18, which represented the crystalline and amorphous regions, respectively (Nordin et al. 2013). The range of crystallinity index increased after chemical pretreatment from 15-18% to 51-55%. The increment of fiber crystallinity index is related to the removal of hemicellulose and lignin from OPMF (Jonoobi et al. 2009), which led to cellulose realignment (Chen et al. 2011). The crystallinity index of OPMF cellulose is comparable to that of other celluloses, as indicated in Table 3. Spectra c and d which represent MF will be discussed later in this paper.
Table 3. Properties of Cellulose from Various Sources
Fig. 1. X-ray diffraction patterns of (a) raw OPMF, (b) OPMF-cellulose, and MF from (c) 6% (w/v) ([EMIM]Cl):([C10MIM][Cl]):DMF and (d) 6% (w/v) ([EMIM]Cl):DMF solutions
The thermal stability of OPMF is presented in Fig. 2. The decomposition temperature for 10% degradation (Td10%) of the raw OPMF and OPMF-cellulose was 230 °C and 300 °C, respectively. The increment of fiber thermal stability was mainly related to the removal of thermally unstable hemicellulose. Hemicellulose has a thermal degradation temperature in the range of 190 to 230 °C, but cellulose starts to degrade at 210 °C and higher (Nordin et al. 2013). Because OPMF-cellulose contained almost exclusively cellulose, it had better thermal stability than the raw OPMF. Additionally, the higher thermal stability for OPMF-cellulose could be explained by the higher crystallinity of the sample, which was due to the removal of amorphous regions from the raw OPMF as a result of hemicellulose removal. This is in agreement with the report by Si et al.(2014). Residue remaining after heating to 550 °C indicates the presence of carbonaceous materials in both raw OPMF and OPMF-cellulose (Nordin et al. 2013).
Fig. 2. (a) TGA and (b) DTG thermograms of raw OPMF, OPMF-cellulose, MF from 6% (w/v) of ([EMIM]Cl):DMF solution and ([EMIM]Cl):([C10MIM][Cl]):DMF solution
Spinnability of OPMF-Cellulose
The spinnability of the OPMF-cellulose in four different ionic liquid formulations was evaluated. ([EMIM]Cl) was selected as the main solvent because it has low viscosity, low toxicity (ED50 = 2860 mg dm-3), low corrosiveness, low melting temperature (< 253 K), and high ability to dissolve cellulose (Sun et al. 2009; Quan et al. 2010). An additional ionic liquid ([C10MIM][Cl]) was chosen, as it can act as a surface active ionic liquid that reduces the surface tension (Freire et al. 2011) of the OPMF-cellulose solutions. DMF acted as a co-solvent to enhance the spinnability (Ferrer et al. 2012).
The spinnability properties of the OPMF-cellulose solutions are summarized in Table 4. The presence of co-solvent, i.e., DMF, assisted in the spinnability of OPMF-cellulose solutions. All samples dissolved in ionic liquids without DMF were unable to be electrospun, and hence, there was no formation of MF. DMF has relatively high dielectric constant properties; it enhances the conductivity of OPMF-cellulose solutions, which indirectly enhances fiber formation (Bhardwaj and Kundu 2010). The presence of DMF in cellulose solutions aids in the formation of bead-free and smaller diameter MF (Bhardwaj and Kundu 2010; Gholipour and Bahrami 2011).
Table 4. Spinnability of OPMF-Cellulose in Different Ionic Liquid Solutions for Microfibrillated Cellulose Formation
Notes: -, cannot be electrospun; +, can be electrospun, beads formation with size approximately >1 μm; ++, can be electrospun with discontinuous MF formation; +++, can be electrospun with continuous MF formation
It is interesting to note that MF formation from the OPMF-cellulose solutions in the presence of DMF varied with the variation in cellulose concentration in the solvents. In both ionic liquids with DMF, only solutions with 6% (w/v) OPMF-cellulose were able to form MF continuously. At 3% (w/v) cellulose concentration, MF was formed discontinuously. There was no MF formation for 1 and 9% (w/v) OPMF-cellulose solutions. This is an interesting finding because MF formation did not have a linear relationship with the OPMF-cellulose concentrations. To explain this finding, cellulose dissolution, the viscosity of cellulose solution, and its surface tension were examined.
OPMF Cellulose Dissolution
Figure 3 shows microscopic images of the OPMF-cellulose solutions. Complete dissolution was achieved for all four ionic liquid formulations at concentrations of 1, 3, and 6% (w/v). ([EMIM]Cl), ([C10MIM][Cl]), and DMF dissolved cellulose, which is the crucial step in electrospinning. Thus, the selection of solvent is important, as it may influence the dissolution of cellulose and subsequently, spinnability.
Fig. 3. Dissolution of OPMF-cellulose in different ionic liquid solutions as observed under light microscope (40x magnification).
A small amount of non-dissolved OPMF-cellulose was seen when 9% (w/v) OPMF-cellulose was used, in all ionic liquid solutions. This result indicates that incomplete dissolution occurred at high concentration solutions. Cellulose tends to agglomerate with ionic liquids in 9% (w/v) OPMF-cellulose solutions and solidified as they were unable to mix well with the ionic liquids. This condition might affect the degree of chain entanglement afterwards, which is a key factor in the formation of continuous fibers.
OPMF-Cellulose Solution Viscosity
Different formulations influenced the solution viscosity values (Fig. 4). OPMF-cellulose in ionic liquid formulations without DMF exhibited higher viscosity than formulations with DMF. The increment in cellulose concentration in the solution caused the solution to have higher viscosity, as expected. The highest viscosity was recorded by 9 wt.% OPMF-cellulose in ([EMIM]Cl) : ([C10MIM][Cl]), at 29,897 mPa.s, followed by 9 wt.% OPMF-cellulose in ([EMIM]Cl), ([EMIM]Cl) : ([C10MIM][Cl]) : DMF, and ([EMIM]Cl) : DMF at 28,720, 22,671, and 9,879 mPa.s, respectively.
This result showed that ([C10MIM][Cl]) enhances solution viscosity. Furthermore, intermolecular forces between both ([EMIM]Cl) and ([C10MIM][Cl] were strong, as it recorded higher viscosity than the OPMF-cellulose solutions without ([C10MIM][Cl]). This condition might be due to the entanglement between long alkyl side chain of ([C10MIM][Cl] to ([EMIM]Cl). The viscosity of solutions containing solely ionic liquid was relatively high. The use of high viscosity ionic liquids in electrospinning is challenging because it may affect the polymer concentration, which affects the degree of chain entanglement and the formation of continuous sprayed fibers (Gholipour and Bahrami 2011). Therefore, instead of solely depending on ionic liquids, the addition of co-solvent reduces solution viscosity, and this effect was successfully achieved in this study.
Rapid viscosity reduction was observed after DMF was added in OPMF-cellulose solutions. DMF affected the solutions properties, as it reduces friction between solution molecules, which indirectly reduces the solutions resistance (Yakymovych et al. 2016). This explains the function of DMF in reducing viscosity.
Fig. 4. The effect of DMF addition on the viscosity of the OPMF-cellulose solutions at different cellulose concentrations
Viscosity increased regularly with increased OPMF-cellulose concentration regardless of the ionic liquid formulations, and this result is comparable to that reported by Si et al. (2014). In electrospinning, an optimum solution concentration is required because it influences fiber formation. If the solution concentration is too low, high surface tension of the polymer solution may cause interruption during spraying which consequently causes the fibers to turn into beads (Haider et al. 2015). A highly concentrated solution is preferred for electrospinning, as it increases viscosity, which indirectly increases the chain entanglement and overcomes the surface tension to form beadless MF. However, if the polymer concentration and viscosity are too high, fibers with larger diameters are formed because the solution cannot be sprayed continuously (Si et al. 2014).
In this study, the viscosity of the solution was affected by cellulose concentration and the use of co-solvent. It is important to have these two parameters balanced in order to achieve the appropriate viscosity for electrospinning of the cellulose solution.
Surface Tension of OPMF-Cellulose Solutions
Ionic liquid formulations also influenced the surface tension of OPMF-cellulose solutions (Fig. 5). The use of ([EMIM]Cl) alone contributed to higher surface tension than combined ionic liquids. DMF addition resulted in higher surface tension. Statistical analysis showed that there was significant difference between different OPMF-cellulose concentrations as well as between different ionic liquid formulations.
Fig. 5. The effect of DMF addition on the surface tension of the OPMF-cellulose solutions at different cellulose concentrations. Capital letters indicate differences (P < 0.05) for OPMF-cellulose concentration, while small letters indicate differences (P < 0.05) for ionic liquid formulations. All data are means of 3 replicates ± S.D. (Different alphabets indicate significant difference, and vice-versa).
Although there was no significant difference in surface tension for all the solutions tested at similar concentration, the addition of ([C10MIM][Cl]) as a second ionic liquid reduced the overall surface tension of the cellulose solution. This result reflects that ([C10MIM][Cl]) has a tendency to migrate to the vapour-liquid interface in polar medium and causes reduction in the surface tension of the solution (Freire et al. 2011). In contrast, the addition of DMF in OPMF-cellulose solutions containing ionic liquid contributed to higher surface tension, which is attributed high surface tension of the DMF (36.73 mN/m) (Hӓrdelin et al. 2012).
Surface tension of the OPMF-cellulose solution was also dependent on solution concentration. The surface tension was reduced with increased OPMF-cellulose concentration regardless of solution formulations. Surface tension of all the solutions without the addition of OPMF-cellulose (0%) ranged between 45 and 49 mN/m. This value declined after OPMF-cellulose was added; for example, the surface tension of 1% (w/v) OPMF-cellulose solution was 35 to 44 mN/m. The lowest surface tension range was recorded at 9% (w/v) OPMF-cellulose solution, at 25 to 26 mN/m. This data shows that surface tension can be lowered in viscous and concentrated solutions.
Factors Affecting Spinnability and MF Formation
Table 5 summarizes the liquid characteristics and their effect on spinnability and MF formation. OPMF-cellulose solution properties such as concentration, viscosity, solubility, and surface tension remarkably affected the spinnability and MF formation. The desired morphology and structure of MF can be obtained by manipulating these parameters during electrospinning.
It is interesting to note that MF was formed only when 6% (w/v) of OPMF-cellulose was used and when the solutions contained DMF. To clarify this result, it is important to relate the solution concentration with viscosity. Figure 4 shows that the OPMF-cellulose solution viscosity increased simultaneously with OPMF-cellulose solution concentration. Nevertheless, the viscosity range was lower when DMF was used. For instance, 6 wt.% OPMF-cellulose in ([EMIM]Cl) had viscosity value of 27,123 mPa.s, while the viscosity decreased drastically to 3,986 mPa.s in the presence of DMF. A marked difference in OPMF-cellulose solution viscosity affected the spinnability, despite the same concentration of OPMF-cellulose being used, i.e., 6 wt.%. The same result was observed for OPMF-cellulose in ([EMIM]Cl) : ([C10MIM][Cl]) solution, where the presence of DMF reduced the viscosity. Similarly, only cellulose concentration at 6 wt% was electrospun for the production of MF.
As discussed earlier, surface tension was not significantly affected by cellulose concentration and type of ionic liquid formulation. Table 5 shows that surface tension for all ionic liquid solutions containing 6 wt.% OPMF-cellulose was in the narrow range of 26 to 28 mN/m. It is therefore difficult to justify the effect of surface tension on spinnability of OPMF-cellulose solution.
Based on these results, both solution concentration and viscosity determined the spinnability of cellulose solution in the formation of MF. Too high or too low concentration and viscosity caused failure during electrospinning. Reduced cellulose solution viscosity due to the addition of DMF seemed to assist the solution to be electrospun into MF for 6 wt.% OPMF-cellulose solution, which may indicate that the solution having lower viscosity may have better spinnability. However, for low concentration of cellulose, for example at 1 wt.%, no fiber formation was observed despite the low solution viscosity. It is believed that fiber deformation has occurred due to charges instability and insufficient amount of cellulose. As the result, beads were mainly formed. There have been research on the formation of beads from low concentration solutions; these results are probably due to the insufficient viscosity to continuously generate fibers (Chung and Um 2014; Okutan et al. 2014; Nitanana et al. 2015).
The results showed that viscosity alone was not the reason for the formation of MF. Cellulose concentration in the solution and the addition of DMF, which eventually stabilized the solution charges, also played important roles. Sufficient charges are needed to stretch the solution from the tip to collector; otherwise continuous MF cannot be obtained (Dufresne 2012). The formation of MF in OPMF-cellulose solutions containing DMF could also be attributed to its properties, i.e., high dielectric constant, high electric conductivity, and low vapor pressure (Dufresne 2012).
Table 5. Summary of OPMF-Cellulose Solutions Concentration, Viscosity, and Surface Tension and their Relationship with OPMF-Cellulose Spinnability
Notes: -, cannot be electrospun; +, can be electrospun, beads formation with size approximately >1 μm; ++, can be electrospun with discontinuous MF formation; +++, can be electrospun with continuous MF formation
Hӓrdelin et al. (2012) reported that cellulose solutions with surface tension greater than 42 mN/m could be electrospun. The results reported here do not support that conclusion, as it was shown that MF formation was best at a surface tension around 28 mN/m. However, there was no marked difference in the effect of surface tension on spinnability, as the spinnability depends on cellulose concentration and viscosity, not only surface tension. Si et al. (2014) mentioned that solutions with very high surface tension may inhibit electrospinning due to instability of the charged jet towards the cellulose solution. The electrical charges need to be high enough to overcome the high surface tension of the solutions. Contrary to solutions that recorded high surface tension, MF with high uniformity can be generated from solutions with low surface tension. The contradiction between the two authors might be due to the properties of the cellulose used, selection of solvents and co-solvent, and other factors such as applied voltage and distance between tips to collector.
Characteristics of MF from OPMF-Cellulose
FE-SEM micrographs of electrospun MF from OPMF-cellulose solutions are shown in Fig. 6. There were no beads and droplets found on the electrospun MF. The average diameter of fibers produced from 6% (w/v) ([EMIM]Cl): DMF solution was 300 to 600 nm, while the diameter of fibers produced from 6% (w/v) ([EMIM]Cl): ([C10MIM][Cl]):DMF solution was 200 to 500 nm. This result indicated that electrospun MF produced from ([EMIM]Cl):DMF had a wider diameter range than that from ([EMIM]Cl):([C10MIM][Cl]):DMF.
The range of crystallinity index of the raw OPMF, OPMF-cellulose, and OPMF-MF was obtained from XRD analysis. Table 6 shows that there was increased crystallinity range after electrospinning, which was 55 to 57% for MF produced from 6% (w/v) ([EMIM]Cl):DMF solution and 57 to 60% for MF produced from 6% (w/v) ([EMIM]Cl:([C10MIM][Cl]):DMF solution, compared with OPMF-cellulose at 51 to 55% (Table 3). The pattern of XRD diagram is shown in Fig. 1. The increased crystallinity index confirmed that electrospinning improved the crystallinity value of OPMF fiber. Increased crystallinity may increase the tensile strength (Alemdar and Sain 2008). Hence, the MF from the OPMF-cellulose may be suitable as filler for the manufacturing of nanocomposites, as they enhance the thermal, tensile, and mechanical properties of the nanocomposite (Alemdar and Sain 2008)
Table 6. Comparison of Electrospun Microfibrillated Cellulose from Commercialized Cellulose
Thermal stability of MF from both ionic liquid formulations was analyzed, and the TG and DTG curves are shown in Fig. 2. The MF produced from 6% (w/v) ([EMIM]Cl) and DMF solution started to decompose above 302 °C, while MF from 6% (w/v) ([EMIM]Cl, ([C10MIM][Cl]) and DMF solution started to decompose above 304 °C. Residues were formed after heating to 550 °C, which indicates the presence of carbonaceous materials in the fibers (Nordin et al. 2013). This result clearly illustrates that the thermal stability of OPMF-MF increased after electrospinning, and this could be attributed to the reduction of amorphous region, which happened due to high degree of crystallinity of the MF after electrospinning (Jonoobi et al. 2009; Nordin et al. 2013).
Given the morphology, average diameter, crystallinity, and thermal stability of MF produced from the OPMF-cellulose, OPMF-cellulose can be used as a feedstock for MF production. The analyses showed that the properties of MF produced from the OPMF-cellulose were comparable to those from commercialized cellulose, as shown in Table 6. The average diameter of MF produced from commercialized cellulose was approximately 120 to 2,000 nm, while this study shows that MF of less than 500 nm were produced from OPMF-cellulose. The crystallinity and thermal decomposition of these MF was also not significantly different than MF from other sources. This result suggests that the OPMF-cellulose can be used as an alternative feedstock for large-scale MF production especially in Malaysia, as it can be obtained easily from palm oil mills throughout the year.
CONCLUSIONS
- The spinnability of OPMF-cellulose solution for the production of MF was greatly affected by the cellulose concentration and viscosity of the solution. The use of DMF in ionic liquid affected the viscosity of cellulose solution very much, despite the same concentration of cellulose used in the ionic liquid without DMF. This condition assisted in spinnability of the cellulose solution. This study shows that MF can only be formed from 6% (w/v) OPMF-cellulose solutions in the presence of DMF.
- Surface tension was mainly affected by the cellulose concentration. The use of DMF and co-solvent did not vary the surface tension value significantly. MF formation did not rely solely on the surface tension value.
- MF from OPMF-cellulose dissolved in ([EMIM]Cl:([C10MIM][Cl]):DMF had a diameter of 200 to 500 nm, which is comparable to that produced from commercialized cellulose. The characteristics of MF produced in this study indicate that OPMF is a promising candidate for MF production.
ACKNOWLEDGMENTS
The authors are grateful for financial support from the Ministry of Higher Education (MOHE), Malaysia through SATREPS research grant (vote no: 6300156). The first author of this paper is sponsored by Universiti Putra Malaysia (UPM) under GRF scheme and MOHE under MyBrain15.
REFERENCES CITED
Alemdar, A., and Sain, M. (2008). “Isolation and characterization of nanofibers from agricultural residues – Wheat straw and soy hulls,” Bioresource Technol. 99(6), 1664-1671. DOI: 10.1016/j.biortech.2007.04.029
Audrey, F., Maria, W. H., and Pernilla, W. (2006). “Electrospinning of cellulose-based nanofiber,” J. Appl. Polym. Sci. 103(3), 1473-1482. DOI: 10.1002/app.24912
Bhardwaj, N., and Kundu, S. C. (2010). “Electrospinning: A fascinating fiber fabrication technique,” Biotechnol. Adv. 28(3), 325-347. DOI: 10.1016/j.biotechadv.2010.01.004
Bordes, P., Pollet, E., and Averous, L. (2009). “Nano-biocomposites: Biodegradable polyester/nanoclay systems,” Prog. Polym. Sci. 34 (2), 125-155. DOI: 10.1016/j.progpolymsci.2008.10.002
Cengiz, F., Krucińska, I., Gliścińska, E., Chrzanowski, M., and Göktepe, F. (2009). “Comparative analysis of various electrospinning methods of nanofibre formation,” Fibres Text. East. Eur. 72 (1), 13-19.
Chauve, M., Barre, L., Tapin-Lingua, S., Perez, D. S. S., Decottignies D., Perez, S., and Ferreira, N. L. (2013). “Evolution and impact of cellulose architecture during enzymatic hydrolysis by fungal cellulase,” Adv. Biosci. Technol. 4, 1095-1109. DOI: 10.4236/abb.2013.412146
Chen, H., Ni, J., Chen, J., Xue, W., Wang, J., Na, H., and Zhu, J. (2015). “Activation of corn cellulose with alcohols to improve its dissolvability in fabricating ultrafine fibers via electrospinning,” Carbohydr. Polym. 123, 174-179. DOI: 10.1016/j.carbpol.2015.01.023
Chen, W., Yu, H., Liu, Y., Chen, P., Zhang, M., and Hai, Y. (2011). “Individualization of cellulose nanofibers from wood using high-intensity ultrasonication combined with chemical pretreatments,” Carbohydr. Polym. 83(4), 1804-1811. DOI: 10.1016/j.carbpol.2010.10.040
Chung, D. E., and Um, I. C. (2014). “Effect of molecular weight and concentration on crystallinity and post drawing of wet spun silk fibroin fiber,” Fibers Polym. 15(1), 153-160. DOI: 10.1007/s12221-014-0153-8
Dufresne, A. (2012). Nanocellulose: From Nature to High Preformance Tailored Materials, De Gruyter, Berlin, Germany.
Ferrer, A., Filpponen, I., Rodríguez, A., Laine, J., and Rojas, O. J. (2012). “Valorization of residual empty palm fruit bunch fibers (EPFBF) by microfluidization: Production of nanofibrillated cellulose and EPFBF nanopaper,” Bioresource Technol. 125, 249-255. DOI: 10.1016/j.biortech.2012.08.108
Freire, M. G., Teles, A. R. R., Ferreira, R. A. S., Carlos, L. D., Lopes-da-Silva, J. A., and Coutinho, J. A. P. (2011). “Electrospun nanosized cellulose fibers using ionic liquids at room temperature,” Green Chem. 13(11), 3173. DOI: 10.1039/c1gc15930e
Gassan, J., and Bledzki, A. K. (1998). “Alkali treatment of jute fibers: Relationship between structure and mechanical properties,” J. Appl. Polym. Sci. 71(4), 623-629. DOI: 10.1002/(SICI)1097-4628(19990124)71:4<623::AID-APP14>3.0.CO;2-K
Geng, A. (2014). “Upgrading of oil palm biomass to value-added products,” in: Biomass and Bioenergy: Applications, K. R. Hakeem, M. Jawaid, and U. Rashid (eds.), Springer International Publishing, Switzerland.
Gholipour, K. A., and Bahrami, S. H. (2011). “Effect of changing solvents on poly(ε-Caprolactone) nanofibrous webs morphology,” J. Nanomater. 2011 (Article ID: 724153), 1-10. DOI: 10.1155/2011/724153
Haider, A., Haider, S., and Kang, I.-K. (2015). “A comprehensive review summarizing the effect of electrospinning parameters and potential applications of nanofibers in biomedical and biotechnology,” Arab. J. Chem. (In press). DOI: 10.1016/j.arabjc.2015.11.015
Härdelin, L., Thunberg, J., Perzon, E., Westman, G., Walkenström, P., and Gatenholm, P. (2012). “Electrospinning of cellulose nanofibers from ionic liquids: The effect of different cosolvents,” J. Appl. Polym. Sci. 125(3), 1901-1909. DOI: 10.1002/app.36323
Henriksson, M., Henriksson, G., Berglund, L. A., and Lindström, T. (2007). “An environmentally friendly method for enzyme-assisted preparation of microfibrillated cellulose (MF) nanofibers,” Eur. Polym. J. 43(8), 3434-3441. DOI: 10.1016/j.eurpolymj.2007.05.038
Hoe, T. K. (2014). “Utilization of oil palm fruits mesocarp fibres waste as growing media for banana tissue culture seedling in Malaysia,” J. Adv. Agric. Technol. 1(1), 52-55. DOI: 10.12720/joaat.1.1.52-55
ISO 5351-1 (2004). “Pulps – Determination of limiting viscosity number in cupri-ethylenediamine (CED) solution,” International Organization for Standardization, Geneva, Switzerland.
Iwamoto, S., Abe, K., and Yano, H. (2008). “The effect of hemicelluloses on wood pulp nanofibrillation and nanofiber network characteristics,” Biomacromolecules 9(3), 1022-1026. DOI: 10.1021/bm701157n
Jonoobi, M., Harun, J., Shakeri, A., Misra, M., and Oksmand, K. (2009). “Chemical composition, crystallinity and thermal degradation of bleached and unbleached kenaf bast (Hibiscus cannabinus) pulp and nanofibers,” BioResources 4(2), 626-639.
Karimi, S., Tahir, P. M., Karimi, A., Dufresne, A., and Abdulkhani, A. (2014). “Kenaf bast cellulosic fibers hierarchy: A comprehensive approach from micro to nano,” Carbohydr. Polym.101, 878-885. DOI: 10.1016/j.carbpol.2013.09.106
Khajavi, R., and Abbasipour, M. (2012). “Electrospinning as a versatile method for fabricating coreshell, hollow and porous nanofibers,” Sci. Iran. 19(6), 2029-2034. DOI: 10.1016/j.scient.2012.10.037
Nitanana, T., Opanasopitb, P., Akkaramongkolpornc, P., Rojanaratad, T., and Ngawhirunpate, T. (2015). “Effects of solution parameters on morphology and diameter of electrospun polystyrene nanofibers,” Adv. Mater. Res. 194-196, 629-632. DOI: 10.4028/www.scientific.net/AMR.194-196.629
Nordin, N. I. A. A., Ariffin, H., Andou, Y., Hassan, M. A., Shirai, Y., Nishida, H., Yunus, W. M. Z. W., Karuppuchamy, S., and Ibrahim, N. A. (2013). “Modification of oil palm mesocarp fiber characteristics using superheated steam treatment,” Molecules 18(8), 9132-9146. DOI: 10.3390/molecules18089132
Okutan, N., Terzi, P., and Altay, F. (2014). Affecting parameters on electrospinning process and characterization of electrospun gelatin nanofibers,” Food Hydrocoll. 39, 19-26. DOI: 10.1016/j.foodhyd.2013.12.022
Qi, H., Chang, C., and Zhang, L. (2009). “Properties and applications of biodegradable transparent and photoluminescent cellulose films prepared via a green process,” Green Chem. 11(2), 177-184. DOI: 10.1039/B814721C
Quan, S. L., Kang, S.G., and Chin, I. J. (2010). “Characterization of cellulose fibers electrospun using ionic liquid,” Cellulose 17(2), 223-230. DOI: 10.1007/s10570-009-9386-x
Seo, J. M., Arumugam, G. K., Khan, S., and Heiden, P. A. (2009). “Comparison of the effects of an ionic liquid and other salts on the properties of electrospun fibers, 2-poly(vinyl alcohol),” Macromol. Mater. Eng. 294(1), 45-53. DOI: 10.1002/mame.200800199
Shi, X., Zhou, W., Ma, D., Ma, Q., Bridges, D., Ma, Y., and Hu, A. (2015). “Electrospinning of nanofibers and their applications for energy devices,” J. Nanomater 2015(Article ID: 140716), 1-20. DOI: 10.1155/2015/140716
Si, Y., Tang, X., Yu, J., and Ding, B. (2014). Electrospun Nanofibers for Energy and Environmental Applications, Nanostructure Science and Technology, Springer Heidelberg, Berlin, Germany.
Sreekala, M. S., Kumaran, M. G., and Thomas, S. (1997). “Oil palm fibers: Morphology, chemical composition, surface modification, and mechanical properties,” J. Appl. Polym. Sci. 66(5), 821-835. DOI: 10.1002/(SICI)1097-4628(19971031)66:5
<821::AID-APP2>3.0.CO;2-X
Sun, N., Rahman, M., Qin, Y., Maxim, M. L., Rodríguez, H., and Rogers, R. D. (2009). “Complete dissolution and partial delignification of wood in the ionic liquid 1-ethyl-3-methylimidazolium acetate,” Green Chem. 11(5), 646-655. DOI: 10.1039/B822702K
TAPPI T230 om-94 (1994). “Viscosity of pulp (capillary viscometer method),” TAPPI Press, Atlanta, GA.
Then, Y. Y., Ibrahim, N. A., Zainuddin, N., Ariffin, H., and Wan Yunus, W .M. Z. (2013). “Oil palm mesocarp fiber as new lignocellulosic material for fabrication of polymer/fiber biocomposites,” Int. J. Polym. Sci. 2013 (Article ID: 797452), 1-7. DOI: 10.1155/2013/797452
Wahab, A. N. H., Tahir, M. P., Yunus, M. N. Y., Ashaari, Z., Yong, A. C. C., and Ibrahim, N. A. (2013). “Influence of resin molecular weight on curing and thermal degradation of plywood made from phenolic prepreg palm veneers,” J. Adhesion 90(3), 210-229. DOI: 10.1080/00218464.2013.780971
Wang, Y., Wang G., Cheng H., Tian G., Liu Z., Xiao Q. F., Zhou X., Han X., and Gao X. (2010). “Structures of bamboo fibers for textiles,” Text. Res. J. 80(4), 334-343. DOI: 10.1177/0040517509337633
Yakymovych, A., Vus, V., and Mudry, S. (2016). “Viscosity of liquid Cu-In-Sn alloys,” J. Mol. Liq. 219, 845-850. DOI: 10.1016/j.molliq.2016.04.055
Yoshimoto, H., Shin, Y. M., Terai, H., and Vacanti, J. P. (2003). “A biodegradble nanofiber scaffold by electrospinning and its potential for bone tissue engineering,” Biomaterials 24(12), 2077-2082. DOI: 10.1016/S0142-9612(02)00635-X
Zhu, J., Wei, S., Patil, R., Rutman, D., Kucknoor, A. S., Wang, A., and Guo, Z. (2011). “Ionic liquid assisted electrospinning of quantum dots/elastomer composite nanofibers,” Polymer 52(9), 1954-1962. DOI: 10.1016/j.polymer.2011.02.051
Articles submitted: August 27, 2016; Peer review completed: November 4, 2016; Revised version accepted: November 19, 2016; Published: December 1, 2016.
DOI: 10.15376/biores.12.1.715-734