Synthesis and characterization of carboxymethyl cellulose from palm kernel cake.Introduction
Cellulose is a naturally occurring polysaccharide and is the most abundant renewable resource available. It is a glucose polymer photosynthesized by solar energy in various plants and act as the structural basis of the plant cell wall (Richardson and Gorton, 2003). Cellulose from various sources has been found used in industry nowadays such as sugar beet pulp, lantana camara, and water hycianth and sago waste. Cellulose is a linear polymer of anhydroglucose units linked at C-1 and C-4 by b-glycosidic bonds. This is confirmed by the presence of three hydroxyl groups with different acidity/reactivity, secondary OH at the C-2, secondary OH at the C-3, and primary OH at the C-6 position, and, accordingly, by the formation of strong various intermolecular and intramolecular hydrogen bonds. Despite its simple molecular structure, cellulose shows a large complexity and variability in its supermolecular arrangement in cellulose fibrils. It is organized into fibrils, which are surrounded by a matrix of lignin and hemicelluloses.
Cellulose is commonly converted into useful derivatives by etherification. Among these, Carboxymethyl cellulose (CMC) is the most important water soluble derivative, with many applications in the food, cosmetics, pharmaceutical and detergents industries, etc. The modified cellulose is a linear, long chain, water-soluble, anionic, man-modified polysaccharide. Purified CMC is a white to cream colored, tasteless, odorless, free-flowing powder(Mario et al., 2005). CMC is useful in systems where hydrophilic colloids are involved, and they show ability to suspend solids in aqueous media, stabilize emulsions, absorb moisture from the atmosphere, solubilize proteins (milk proteins, egg proteins), thicken solutions (sugar solutions, paints) and form films. CMC gives good enteric coatings for powders and tablets (Dapia et al., 2003).
Production of cellulose derivatives is done by reacting the free hydroxyl groups in the anhydroglucose units(AGU) with various chemical substitution groups. The introduction of the substituent disturb the inter- and intramolecular hydrogen bonds in cellulose, which leads to liberation of the hydrophilic character of the numerous hydroxyl groups and restriction of the chains to closely associate (Togrul and Arslan, 2003). However, substitution with alkyl groups reduces the number of free hydroxyl groups.
Physical and chemical properties of CMC are mainly determined by the degree of substitution (DS), distribution and degree of polymerization. Among these properties, DS has the greatest influence on the solubility, shearing stability and rheological behavior of CMC solution and its stability against temperature and low molecular additives. DS represents the number of the carboxymethyl groups in the molecular unit of the anhydroglucose units. In principal, all hydroxyl groups (HO-2, HO-3, and HO-6) in the anhydroglucose unit can be substituted, and the maximum degree of substitution (DS) is being 3 (Salmi, et al., 1994).
In this investigation, the production of CMC using palm kernel cake extracted cellulose was investigated. The cellulose was converted to CMC through carboxymethylation process using William Etherification technique in heterogeneous system. CMC was characterized in terms of DS and viscosity.
Materials and Methods
2.1 Oil Extraction of PKC
Palm kernel was obtained from a local palm oil factory (Beaufort, Sabah). It is first dried in sunlight and then ground in blender to pass through a 600 [micro]n size screen. The dried powder was first extracted with isopropanol in a Soxhlet apparatus for 18 hours which were divided into 3 cycles. The de-oiled palm kernel powder was then allowed to dry in an oven at 60[degrees]C for 18-24 hours.
2.2 Isolation of Cellulose
Acidified sodium chlorite process to extract cellulose was used in this research. Alkali treatment is the pretreatment step of cellulose extraction. 3.3 g of oil removed palm kernel cake was mixed with 100ml of 7.5% aqueous NaOH solution. The mixture was stirred for 1 hour at room temperature. The pretreated PKC was then filtered and washed with 95% ethanol and distilled water repeatedly twice to remove the base. The hemicellulose-free palm kernel cake was then dried in oven at 60[degrees]C for 24 hours.
For acidified sodium chlorite process, 2 g of hemicelluloses free palm kernel was mixed with 100 mL distilled water, 15 mL glacial acetic acid and 2 g of sodium chlorite. The mixture is stirred for 2 hours at 75[degrees]C. The residue was filtered and washed with 95% ethanol and distilled water. This step was repeated. Similarly, the washed palm kernel is dried in oven at 60[degrees]C for 24 hours. The scheme of extraction stages for cellulose from palm kernel cake is shown in Fig. 1.
2.3 Synthesis of Carboxymethyl Cellulose (CMC)
The extracted cellulose from PKC was then converted to CMC in two steps, alkalization and etherification of cellulose under heterogeneous conditions. In alkalization pretreatment, 5g of palm kernel cellulose was weighed and added to 250 ml Schott bottle followed by 100ml of isopropanol. 20 mL of 17.5% aqueous sodium hydroxide was added drop-wise while it is stirred for an hour at 30[degrees]C. After alkali treatment, etherification reaction was continued by adding 6g of sodium monochloroacetate (SMCA) in reaction mixture and placed in a water bath with horizontal shaker. The reaction mixture was heated up to reaction temperature of 50[degrees]C and shaken for 2 hours of reaction time. Longer time of reaction will increase degradation of polymer and will reduce the DS value as well. The slurry was then soaked in methanol for overnight. On the next day, the slurry was neutralized with 90% of acetic acid until reach pH 6-8 and then filtered. CMC was purified by washing with 70% ethanol for five times to remove undesired byproduct. Then the CMC was filtered and dried at 60[degrees]C in an oven for 24 hours.
[FIGURE 1 OMITTED]
2.4 Fourier Transform Infrared Spectroscopy
The extracted cellulose and carboxymethyl cellulose product were calibrated by using Fourier Transforms IR (FTIR) instrument. To get the spectra, a pellet made from sample was ground with KBr. Transmission was measured at the wave number range of 4000-4400[cm.sup.-1].
2.5 Yield Measurement
Product yield was measured based on dry weight basis. The net dry weight of carboxymethyl cellulose was divided by 5g of cellulose to get the yield value.
Product Yield = weight of dried CMC/dry weight of cellulose
2.6 Determination of Degree of Substitution
The degree of substitution (DS) of the sample CMC was determined by the standard method (ASTM. 2005). 4 g of sample and 75ml. of 95% ethyl alcohol was agitated in 250ml. beaker for 5 min. Then 5 ml of nitric acid was added. A hotplate was used to boil the solution. The solution was then removed from hotplate and further stirred for 10 minutes. By using vacuum pump, liquid solution was decanted and washed with 80% ethyl alcohol (60[degrees]C) for 5 times. Then the precipitate was washed with a small quantity of anhydrous methanol and apply vacuum to remove the alcohol. Lastly, the filter was dried at 105[degrees]C for 3 hours and cool in desiccators for half an hour. 1 to 1.5g of dry carboxymethylcellulo s e was added to 100ml of water and 25 ml of hydroxide 0.3N with agitation. The solution was heated to boil for 15 to 20 minutes. After the products dissolved, the mixture was titrated by 0.3N HCI. Phenolphthalein indicator was added to observe the color change from Mexican pink (dark pink) to colorless.
To calculate the degree of substitution, the equations (1) and (2) were used:
a = BC-DE/F
Degree of substitution = 0.162 x A/1-(0.058 x A) (2)
A = milli-equivalents of consumed acid per gram of specimen;
B = volume of Sodium hydroxide added;
C = concentration in normality of sodium hydroxide added;
D = volume of consumed chloric acid;
E = concentration in normality of chloric acid used;
F = specimen grams used;
162 is the molecular weight of the anhydrous glucose unit and 58 is the net increment in the anhydrous glucose unit for every substituted carboxymethyl group.
2.7 Determination of Viscosity
2.7 g of carboxymethyl cellulose from palm kernel cake was weighted and dissolved in 90 mL of distilled water in a 100 mL beaker. Then Brookfield Rotational Viscometer was used to measure the viscosity of the dissolved CMC. The conditions in measuring viscosity are listed in Table 1.
Results and discussion
3.1 Extraction oil from PKC
After oil extraction, the remaining weight of palm kernel is 62.86% of the original weight. This represents that the amount of oil extracted is 37.14%.
Isopropanol was chosen to extract oil from palm kernel because it is a polar solvent and is suitable for oil extraction because of the anti oxidant property of palm kernel cake. The temperature used for extracting oil from palm kernel cake was maintained around 135[degrees]C to minimize degradation of heat-sensitive bioactive compounds present.
3.2 Yield of Cellulose
From the measurement, acidified sodium chlorite process recovered 65.66% from 20-30% composition cellulose in palm kernel cake. That's mean around 13-20% of cellulose was recovered from 100% of palm kernel cake. Recovered cellulose is white with milky color. With the environmental concern, hydrogen peroxide is getting attention as an alternative for chlorine-based treatment (Sun et al., 2000). Fenton reagent can be added to strengthen its capability in cellulose extraction that will be subjected in future study (Kazuchiro et al., 2000). Hydrogen peroxide is widely used for bleaching of mechanical pulps under alkaline conditions.
Traditionally, acidified sodium chlorite is commonly used to delignify wood as an initial step in the isolation of pure cellulose, and its reaction with lignin is fast compared with its reaction with cellulose. Sodium chlorite is a moderately strong oxidizing agent whose use does not introduce the possibility of heavy metal contamination. The solid sodium chlorite is stable and very soluble (Ford, 1986). Besides, acetic acid actively takes part in the hydrolysis of hemicelluloses. The strong oxidizing property of sodium chlorite yields pure cellulose with no lignin remaining inside. This can be proven by the FTIR spectra of cellulose in Fig. 2. However, its rate of reaction with the cellulose is still happened and the reaction damages the fibers during the usual conditions of bleaching (Sun et al., 2004). An advantage of delignification with acetic acid is that it can be followed immediately by bleaching, with sodium chlorite.
Cellulose prepared from palm kernel represents a milky white powder. Fig. 2 shows the FTIR spectrum of palm kernel cellulose. Fig. 2 represents the cellulose spectrum using acidified sodium chlorite extraction method. Absorption band at 3473.59 [cm.sup.-1] is due to the stretching frequency of the -OH group as well as intramolecular and intermolecular hydrogen bonds in a cellulose(Pushpamalar et al., 2006). Peaks at wave number of 1084.70 [cm.sup.-1] is due to >CH2-O-C[H.sub.2]. Spectrum shows C-H stretching vibration at peak with wave number of 2925.20 [cm.sup.-1]. While another proof of cellulose were the band 1420.89 [cm.sup.-1] and 1319.05 [cm.sup.- 1] that assigned to -C[H.sub.2] scissoring and -OH bending vibration.
Fig. 3 shows the spectrum of carboxymethyl cellulose from palm kernel cake. The presence of a new and strong absorption band at 1604.16 [cm.sup.-1] is due to the CO[O.sup.-] group. It is an evidence that hydroxyl group of cellulose was replaced with carboxyl group when carboxymethylation reaction occur. Despites the cellulose backbone that describe in previous paragraph, spectrum in Fig. 3 also proves that the sample is CMC because it has a fingerprint region for CMC bonds. Mario et al., 2005 have found the carboxyl groups and its salts wave numbers 1600-1640[cm.sup.-1] and 1400-1450[cm.sup.-1] respectively.
3.3 Degree of Substitution of CMC
The degree of substitution of carboxyl group in carboxymethyl cellulose can be determined from both the IR spectra and potentiometric titration. Values obtained from potentiometric titration correspond to the absolute values of the degree of substitution (Rosnah et al., 2004). CMC which obtained with alkalization of cellulose then followed by carboxymethylation process using Sodium Monochloroacetate (SMCA) was in the range 0.4-1.3. When the DS is below 0.4, the CMC is swellable but insoluble, while above this value, CMC is fully soluble with its hydro affinity increasing with increasing DS(Varshney et al., 2006). Since the DS value for CMC from palm kernel cake is 0.67, obviously, it is fully soluble in water and its solubility will increase with temperature.
The use of isopropanol as solvent in carboxymethylation reaction is to provide miscibility and accessibility of the etherifying reagent to the reaction center of the cellulose chain rather than glycolate formation (Pushpamalar et al., 2006). The carboxymethylation reaction efficiency is higher by using the high polarity solvent such as isopropanol. In this work, two hour reaction period gives sufficient times for diffusion and absorption of the reactants. This provides a better contact between the etherifying agent and cellulose. While, investigation for others reaction times in future is needed for better quality of CMC product from PKC.
A comparison between DS value of carboxymethyl palm kernel cellulose with other sources cellulose is shown in Table 2. Significant differences were observed in the DS values. However the ranges of the DS value ([approximately equal to] 0.2-1.2) almost the same and it is fulfill DS requirement as CMC product (DS=0.3). DS value in this work is 0.67 that within the range. Different experimental conditions and chemicals used is the caused of the difference.
3.4 Yield of CMC
The yield of CMC product in this work is 1.6574g/g. According to Silva et al., 2004, the yield is certainly a function of the amount of material lost during dialysis step. More degradation occurred and larger amount of low molecular weight material were released when more drastic reaction conditions (higher temperature, NaOH and SMCA concentration) is applied. This will results in smaller amount of carboxymethylated polymeric product remained to be recovered by drying.
3.5 Viscosity of CMC
Viscosity of CMC is an important parameter for the industrial use. It provided information for flow characteristics of the fluid flow involved in processing operations and products using different concentrations of CMC.
The carboxymethyl palm kernel is in the category of low viscosity. The commercial range for low viscosity CMC is 20-50 cP in 2% concentration of solution. The viscosity for palm kernel cake CMC is only 66.6 cP. The percentage of carboxymethyl palm kernel dissolved in distilled water is just 3% dry weight. There is some suspended sediment in the prepared solution which affects the viscosity. During hemicelluloses removal step, NaOH was used. However, during the washing process with ethanol, there is some remaining NaOH which could not be washed away thoroughly. This results in extra NaOH contained inside the final product of carboxymethylation and thus decreased the viscosity. The low viscosity may be also due to degradation of polysaccharide. Table 3 represents the CMC product parameters obtained in this work.
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
In this work, CMC is successfully extracted from palm kernel cake. The DS value is 0.67 with viscosity of 66.6 cP at 25[degrees]C and yield of 1.6574g/g. Common bleaching and delignification technique involving sodium hydroxide and sodium chlorite was used to extract the cellulose from PKC. However, new method is yet to be developed in extracting cellulose as the over usage of acetic acid and sodium chlorite can be toxic and harmful. Optimization of reaction conditions for preparing carboxymethylation from palm kernel cake should be carried out to determine the optimum conditions which will give the highest yield, DS value and viscosity.
The authors are grateful to Dr. Chu Chi Ming for providing financial support and the facilities to this work.
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A. Bono, P.H. Ying, F.Y. Yan, C.L. Muei, R. Sarbatly, D. Krishnaiah
Chemical Engineering Program, School of Engineering and Information Technology, University, Malaysia Sabah, Locked Bag No. 2073, 88999 Kota Kinabalu, Sabah, Malaysia
Corresponding Author: D. Krishnaiah, Chemical Engineering Program, School of Engineering and Information Technology, University, Malaysia Sabah, Locked Bag No. 2073, 88999 Kota Kinabalu, Sabah, Malaysia ;Tel: +60-8-832-0000 x 3059, fax: +60-832-0348, E-mail: firstname.lastname@example.org
Table 1: Conditions in viscosity measurement Conditions Value Percent dry weight of CMC 3% Torque 20.7% Temperature 25 [+ or -] 1[degrees]C Spindle 02 RPM 250 Table 2: DS value of CMC from different sources of cellulose. Degree of Substitution Sources of Cellulose Reference (DS) Water hyacinth Barai et al., 1996 0.24-0.73 Sago waste Pushpamalar et al., 2006 0.33-0.82 Sugar beet pulp cellulose Togrul and Arslan, 2003 0.11- 0.67 Lantana camara Varshney et al., 2006 0.20-1.22 Palm Kernel Cake This work 0.67 Table 3: CMC product parameters. [C.sub.6][H.sub.7][O.sub.2][(OH).sub.2] Formula COONa]n Degree of substitution 0.67 Color Light Brown Form Powder Yield 1.6574g/g Viscosity at 25[degrees]C 66.6 cP pH 6.5-8.0 Solubility in water S oluble in water Film Formability Able to form film