High Swelling Carboxymethyl Cellulose Synthesized from Coconut Fibers

ABSTRACT To attain the zero waste and green chemistry goals, much emphasis has shifted toward the use of cellulose derived from agricultural biomass. In this study, carboxymethylation of microcrystalline cellulose from coconut fiber by use of monochloroacetic acid in the presence of an alcohol medium under alkaline conditions was suitable for the synthesis of carboxymethyl cellulose. The carboxymethyl cellulose was prepared, and the physical properties, degree of substitution, swelling capacity, and characterization were investigated. The results indicated that the yield of carboxymethyl cellulose, degree of substitution, and swelling capacity were 9.45 ± 0.76 g, 1.82 ± 0.12, and 11.23 ± 0.28 g, respectively. The Fourier Transform Infrared spectra of carboxymethyl cellulose displayed a broad OH peak at 3352 cm−1 and a sharp peak at 1600 cm−1 attributed to -COO. Thermal behavior was investigated by Thermal gravimetric analysis, and the phase transition was determined by Differential Scanning Calorimetry, which revealed that alkalization and esterification of cellulose lead to a decrease in the thermal stability of the polymer. In conclusion, zero waste can be achieved in the coconut industry as it is rich in cellulose that can be converted to carboxymethyl cellulose, which can be utilized to produce emulsifiers and superabsorbent polymers.


Introduction
The major goal of any manufacturing process is to find the most cost-effective and economical way in which goods can be produced hence the need to consider the amount of waste generated and how it can be utilized to achieve a clean environment (NEMA 2011).The endpoint of any product is the environment which results in buildup owing to non-biodegradability, degradation, and lethal substance released (Mburu and Kinyanjui 2013).Because Synthetic fibers have been in use for a long time and have detrimental impacts on the manufacturing sector and environmental management, natural fibers are viable options for synthetic fibers in technical fields (Bright et al. 2022).Coconutbased products have, over time, gained some economic significance, leading to several technologies established to enhance the product's quality (Pandiselvam et al. 2019(Pandiselvam et al. , 2020)).The finding of this study showed that the dwarf variety had the highest bulk properties and the coconuts from Nigeria had thin husks (Pandiselvam et al. 2020).The husk comprises lignin and cellulose, and the punching forces vary depending on the coconut fruit's maturity.
Cellulose is a nearly limitless polymer substance found in all plant cell walls.However, the unique structure of cellulose poses a significant barrier to manufacture and processing.To boost its efficacy as a modification, cellulose must be activated or derivatized, but only at the hydroxyl groups at C 2 , C 3, and C 6 (Abdel-Halim 2014).C 2 and C 3 have vicinal hydroxyl groups, whereas C 6 is a primary alcohol group, which can undergo chemical reactions such as esterification, etherification, or oxidation.These reactions add groups to the network, forming derivatives such as carboxymethyl cellulose, hydroxyethyl cellulose, ethyl cellulose, and hydroxypropyl methyl cellulose (Wen et al. 2015).Carboxymethylation of cellulose is a process that involves alkalization and subsequent esterification, which leads to the substitution of the OH group with the carboxymethyl group.
The degree of substitution (DS) of carboxymethyl cellulose (CMC) determines the swelling behavior, as studies have shown that CMC synthesized from agricultural residues has a DS ranging from 0.3 to 0.8 with yields ranging from 3 to 7 g (Chan et al. 2017).The swelling capacity of some selected biomass has been reported in the range of 2-5 g/g in distilled water (Kimani et al. 2016).CMC finds application in the development of hydrogels (Astrini, Anah, and Haryono 2012), cosmetics and detergents, pharmaceutics (Chan et al. 2017), mineral processing (Merwe and Garbers-Craig 2017), and the food industry (Asl, Mousavi, and Labbafi 2017).The classification of coconut fruit is critical to determining the effective dehusking equipment, producing good quality fiber (Pandiselvam et al. 2020).Through modification and chemical processes such as esterification, cellulose-rich fiber can be used to produce CMC.The goal of this study was to create CMC with a high swelling ratio from coconut fiber, a plentiful agricultural biomass rich in cellulose.Furthermore, the functional groups of modified and unmodified cellulose were evaluated using FTIR, and thermal stability was tested using differential scanning calorimetry and thermal gravimetric analysis.

Extraction of cellulose
Coconut fiber was obtained from a local processing mill, Kocos Kenya, Kilifi County.The fibers were sorted, cleaned, and dried in an oven (Electric Thermostatic Drying Oven DHG-90 0) at 105°C, then cut to 1 cm length and stored.The dried biomass was mixed with a 10% aqueous NaOH solution in a ratio of 1:10 m/v and heated in a mechanical stirrer (model CHS-2) for 3 h at 100°C, filtered, and washed with 10% ethanol and distilled water repeatedly (Collazo-Bigliardi, Ortega-Toro, and Chiralt 2018).The hemicellulose-free biomass cake was dried in an oven at 100°C to constant weight, followed by treatment with peracetic acid (1:5 m/v) at 80°C for 2 h.The residual cake was filtered and washed with distilled water repeatedly, then dried in an oven at 105°C to constant weight (Kimani et al. 2016).Cellulose was converted to microcrystalline cellulose (MCC) through treatment with 2.5 N hydrochloric acid at 85°C for 15 minutes, followed by washing with ethanol (10% v/v) and then drying in an oven set at 105°C to constant weight (Gichuki et al. 2020).This was done in triplicate.

Carboxymethyl cellulose synthesis
The modification and widening of cellulose's crystalline structure using NaOH partially lead to the breakdown of hydrogen bonds, thus making anhydroglucose unit accessible to nucleophiles (Massah et al. 2007).The choice of solvent is ideal as organic solvent acts as a swelling-restrictive agent and hence, limits the hydration of the cellulose chain.CMC was synthesized according to a method described by (Haleem et al. 2014).The extracted MCC was converted to CMC in two steps, alkalization and esterification of MCC under heterogeneous conditions.In brief, 5 g of prepared MCC was weighed and added to 100 mL of isopropanol alcohol, then a dropwise addition of 20% (m/v) NaOH solution.The esterification reaction was initiated by adding 0.79 M of sodium monochloroacetic acid to the reaction mixture and heating it at 50°C for 2 hours.After the first esterification process, the sample was washed with 90% ethanol, filtered, dried to constant weight, and the esterification reaction repeated (Chan et al. 2017).This was carried out in triplicate.

Determination of bulk and tapped density
The bulk density was determined by weighing 10 g of the sample, which was poured into a 100 mL glass-measuring cylinder at an angle of 45°C.Before determining the tapped density, the bulk density was first determined by measuring the untapped volume V 0 for triplicate samples.The tapped density was then determined by measuring the final tapped volume V 1 , after several taps to constant mass on a surface (Azubuike, Odulaja, and Okhamafe 2012;Gichuki et al. 2020).

Determination of Carr's Index and Hausner Ratio
Carr's index and Hausner's ratio were calculated from bulk and tapped densities, as shown in equations 3 and 4, respectively (Kimani et al. 2016).

Determination of degree of substitution
The degree of substitution facilitates elimination and substitution mechanisms for electrophiles, often in case of steric hindrance is a limitation (Hadi et al. 2020).It directly correlates to its capacity to hold water, which is one of the most important parameters, especially when dealing with polymers, as it is the overall measure of its performance in the field.The degree of substitution (DS) of CMC was determined by the titration method, for triplicate samples as described by Kimani et al. (2016).All ester linkages were saponified by NaOH, and the amount of remaining NaOH was determined from the titration with HCl.In brief, 0.2 g of CMC was weighed and placed in a 500 mL conical flask containing 50 mL distilled water, and the mixture was stirred overnight at room temperature.Further, 30 mL of 0.1 N NaOH was added.The mixture was heated at 50°C for 3 h, cooled to room temperature, and titrated against 0.15 N hydrochloric acid with phenolphthalein as an indicator.The DS was then calculated as follows (Chan et al. 2017).The percentage of carbon content (CM) was obtained using Equation 5.
Where A is the normality of HCl used, Vo and Vn volume to titrate blank and sample respectively, M mass of the sample, 162 is the molecular weight of anhydroglucose, and 58 is the molecular weight of carbon content group.

Determination swelling capacity
Swelling capacity is one of the fundamental properties of cellulosic fibers, which the functional groups and the environment influence in terms of pH, temperature, and salinity (Saini 2017).The water absorbency of the MCC and CMC was measured by the tea-bag method (Japanese Industrial Standard (JIS) K 7223) (Boruvkova and Wiener 2011).A non-woven tea bag (200 mm in size and 100 mm in length and width was prepared by heat sealing) 1 g of the sample was charged into it.The tea bag was immersed in water at 25°C, and after 1 h, it was removed, and the excess water was removed by blotting with paper towels (Astrini, Anah, and Haryono 2012).The equilibrium measurements were done using an electronic balance (Sartorius SQP, Practum 1102-1CN).This procedure was carried out repeatedly in water and saline conditions, and the swelling capacity of the sample was calculated as follows: Where; W b is the weight of the blank tea bag after the water treatment, (W t ) is the weight of the wet sample, and W p is the weight of the dry sample.

Characterization
To better understand changes that occur when cellulose is subjected to different modifications, such as esterification, there is a need to observe the introduction of new functional groups, thermal changes, and crystallinity of the CMC structure.Changes in the functional groups present in MCC occasioned by its conversion to CMC were determined using a Shimadzu Fourier Transform Infrared spectrophotometer (Shimadzu model FTS-8000, Japan).10 mg of dry, finely ground sample was mixed with 300 mg potassium bromide powder then a mechanical hand press was used to compress it to a thin transparent film.The scanning range was 4000-400 cm −1 , with 20 scans per sample, and the resolution was 4 cm −1 .Thermal stability of MCC and CMC were evaluated using a Mettler Toledo DSC/TGA 3+ system (Mettler-Toledo GmbH, Switzerland).10 g of the sample was heated from 25 to 600°C at a heating rate of 10°C/min and cooled to 25°C (Collazo-Bigliardi, Ortega-Toro, and Chiralt 2018; Madivoli et al. 2019).The STOE STADIP P X-ray Powder Diffraction System (STOE and Cie GmbH, Darmstadt, Germany) determined the crystallinity index.The X-ray generator was equipped with a copper tube operating at 40 kV and 40 mA with Cu Kα radiation with a wavelength of 1.5409 nm and a 2θ range of 2-90° (Madivoli et al. 2019).The crystallinity index (CrI) was calculated using Equation 8. Cr:I:

Physical parameters analysis
The percentage yield, bulk, and tapped density of MCC and CMC were evaluated, and the results are depicted in Table 1.The percent yield of MCC from coconut fiber was 42.06 ± 1.45%, and 28.72% was reported by Rojas-Valencia et al. (2018).This cellulose from coconut fiber produces yarn for the textile industry due to its biodegradation.The bulk and tapped density of MCC obtained from coconut fiber were estimated to be 0.08 ± 0.00 g/cm 3 and 0.12 ± 0.0 g/cm 3 , respectively.At the same time, that of CMC was 0.55 ± 0.00 g/cm 3 and 0.84 ± 0.0 g/cm 3 , respectively, as indicated in Table 1.Bulk density helps to determine how the substance can be condensed and packed down in a limited space (Murigi et al. 2014).Compressibility values of 20% and higher show that the powder is not free-flowing, whilst compressibility values of 40% and above indicate that the material has a very poor flow character.MCC made from coconut fiber showed a compressibility of 32.00% 0.7, whereas CMC had a compressibility of 33.88% 0.26, indicating that CMC had poor flow characteristics.The Hausner ratio values obtained in Table 1 are all greater than 1.25.This can be described as having poor flow qualities (Azubuike, Odulaja, and Okhamafe 2012).The preparation of CMC from MCC increased the percent recovery of CMC; the yield was 9.45 ± 0.76 g.This is attributed to the substitution of the OH group at C 2 , C 3, and C 6 by the carboxymethyl group during the alkalization and esterification reaction, as shown in Figure 1 (Chan et al. 2017)

Degree of substitution
Table 2 shows the degree of substitution (DS) and carbon content (CM) of the CMC.As can be observed from Table 2, the double esterification process resulted in an increased yield of CMC.The DS is the average number of carboxymethyl groups that have replaced OH groups per anhydroglucose unit in the cellulose structure at C 2 , C 3, and C 6 (Hadi et al. 2020).It is a significant factor that directly influences the hydro-affinity, which determines the solubility of the CMC because a DS of 0.4 indicates that the polymer is swellable but insoluble.A DS of > 0.4, on the other hand, shows that the polymer is both swellable and water-soluble.According to the findings, the CMC generated is swellable and soluble in water, making it suitable for polymer synthesis and emulsifiers.The CM values ranged from 24.0 to 40.0% (Table 2), with a higher CM value indicating a higher DS of the specific CMC.In a related study, Chan et al. (2017) evaluated the effect of agricultural biomass on the degree of CMC substitution.He reported that the agricultural residue utilized to create cellulose affects the DS value of CMC, with coconut fiber cellulose having a DS value of 0.3-0.6 and a percentage CM of 10-20%.When the DS value was low, chemical alteration occurred in the amorphous portions of the cellulose.

Swelling studies
The swelling ratio of MCC and CMC in distilled water and saline solution is illustrated in Figures 2 and 3, respectively.Cellulose fibers are characterized by swelling when immersed in saline and non-saline distilled water to assess their potential as water reservoirs.CMC displayed a higher swelling capacity than MCC, as shown in Figure 2, due to the presence of carboxymethyl groups introduced by the esterification process.These groups bind to the water molecules through hydrogen bonds (Kimani et al. 2016).The swelling ratio in 0.9% NaCl had a similar trend as in distilled water; however, the swelling ratio was almost half of the distilled water due to the association between carboxymethyl groups and Na + ions in the solution.The lower swelling capacity found in saline circumstances is due to a decrease in the formation of hydrogen bonds in the solution.The increased water uptake by CMC makes it an effective polymer that can be used for medical and technical uses (Boruvkova and Wiener 2011).Therefore, this CMC with high swelling capacity can be used for hydrogel synthesis, as an emulsifier, and for pharmaceutical applications.Coir MCC was insoluble in water; after swelling, the absorbed water was released on pressing, unlike the CMC, resulting in a gel-like substance.The high DS of the CMC makes it soluble in water and thus a good material for superabsorbent polymers (Chan et al. 2017).
The surface modification alters the properties of the material.Table 3 displays the ANOVA for four MCC and CMC parameters at P = .05.The research shows that they are significantly different, with P cal = 0.5689 and P cal = 0.3716.The swelling ratio and yield vary depending on the nature of the substance.

Functional groups identification
The FTIR spectra of MCC, CMC, and CMC standards are depicted in Figure 4.The peaks observed in regions 3404 and 3422 cm −1 are characteristic of OH stretching vibrations, while those at around 2900 and 2891 cm −1 are associated with C-H stretching vibration for MCC and CMC samples.The absorption band at 1647 cm −1 is associated with -OH bending vibration due to water absorbed (Madivoli et al. 2019).MCC and CMC display a peak at 1059 and 1067 cm −1 , respectively, attributed to -C-O-C-stretching vibrations.According to Murigi et al. (2014), the absorption band at 1059 cm −1 is due to the glycosidic ether band of -C-O-C-and that at 1157 cm −1 is due to the C-C ring breathing band, this is a major cellulose  characteristic peak (Gichuki et al. 2020).The absorption peak around 930-890 cm −1 is attributed to -C-O-C in plane symmetric, the bending vibrations of C-O.Those absorption bands at 1235 cm −1 were associated with out-of-plane C-OH bonds (Kia et al. 2017).The CMC spectra show a peak at 1600 cm −1, showing the presence of the -COO − group and indicating successful carboxymethylation; this peak was absent on the coconut fiber MCC spectra (Haleem et al. 2014).The presence of a C=O peak suggests that through the esterification process, the crafting of OH groups was successful, thus enhancing the swelling of the CMC synthesized.The peak at 1319 cm −1 and 1404 cm −1 were caused by -CH 2 scissoring vibration and -OH bending vibration due to water absorbed, respectively.The OH group is responsible for the hydrogen bonding with water molecules.

Interpretation of DSC thermograms
Figure 5 illustrates the DSC thermogram demonstrating the thermal stability of MCC and CMC from 0 to 500°C.The MCC thermogram displays an endothermic glass transition peak at roughly 80°C and a CMC at around 95°C, indicating that water or moisture loss occurred (Madivoli et al. 2019;Rosa et al. 2012).This peak is found in amorphous cellulose (Ciolacu et al., 2011).The minor exothermic peak at 208°C is caused by MCC crystallization.Another sharp endothermic peak is seen at 335°C due to cellulose melting, whereas CMC shows endothermic melting at 202°C and exothermic crystallization transition temperatures at 150°C.The disintegration or degradation of CMC resulted in an exothermic process at 372°C.MCC was stable between 380 and 460°C, but over 470°C, it increased in temperature due to disintegration (Astrini, Anah, and Haryono 2012).This effect results from the synthesis of CMC, which has a more organized structure than cellulose (Murigi et al. 2014).

Thermal gravimetric analysis interpretation
The thermal gravimetric research provides information on the material's stability when subjected to different temperatures, as shown in Figures 6 and 7.The TGA curves for MCC and CMC are sigmoid, with significant mass loss happening at the steepest region of the graph, which was lower than that of MCC, which decomposes at 316°C whereas CMC is 272°C, as seen in Figure 5 (Collazo-Bigliardi, Ortega-Toro, and Chiralt 2018).For MCC and CMC, the onset degradation temperature was 35°C and 32°C, respectively.The weight loss for CMC was lower than for MCC, which began at 270°C and 320°C, respectively.The decrease in breakdown is due to the structural difference between CMC, which is ordered, and MCC, which is amorphous.
On the primary endothermic peak, there are two peaks and a shoulder peak on the DTGA curve.The peak focused at 47°C is caused by the loss of water bound in the MCC structure (Astrini, Anah, and Haryono 2012).According to Rosa et al. (2012), the endothermic peak centered at 346°C is caused by cellulose backbone degradation.Hemicellulose depolymerization was responsible for the minor disintegration at 211°C (Poletto et al. 2010).Astrini et al. (2012) discovered that the esterification approach reduced CMC's thermal stability, with a peak temperature of 281°C due to weight loss.Thermal stability can be used to derive the presence of a volatile component, making this a vital stage in product development.

X-ray diffraction
The X-ray powder diffractograms of MCC and CMC are shown in Figures 8 and 9, respectively.The X-ray diffractograms show well-defined and sharp peaks for CMC.The CMC characterized by 2θ peaks at 20°, 31°, 45°, 66°, 75°, and 84°.The 2θ value at 20° and 31° resembles that of MCC at 22° and 34° indicating a shift in the peak position; this could be a result of the derivatization     reaction (Madivoli et al. 2019).The shift relates to the changes in the stability of the material displayed by the TGA and DSC curves.Also translates to a change in crystallinity, which explains the difference swelling capacity of CMC and MCC.The peak at 20° suggests CMC crystallinity due to the intermolecular and intramolecular distance between the -OH group creating a hydrogen bond with the -COOH group (Sethi et al. 2019).The 2θ value corresponds to sodium chloride characteristic peaks at 31°, 45°, 66°, 75°, and 84°.This sodium chloride could be a result of utilizing sodium monochloroacetic acid in the derivatization process, as shown in the chemical equation 1.

Conclusion
Coconut husks, a major agricultural waste containing cellulose fibers responsive to chemical and biochemical modification, can be transformed into a good source of raw material for CMC production.The drive to zero waste can be achieved in the coconut industry as it is rich in cellulose that can be used to produce value-added products.In summary, Carboxymethyl cellulose of high DS values of 0.9-1.8 was prepared successfully and efficiently using monochloroacetic acid through alkalization and esterification processes.The functional group showed a COO − peak absent in the MCC spectra.The swelling capacity for MCC material can be increased through surface modification; thus, the modified cellulose may be used for applications such as the synthesizing of superabsorbent polymers, paper, textile, food, paint, pharmaceutical, and cosmetic industries.The DSC curves indicated that the esterification of MCC resulted in a derivative with low thermal stability, as indicated by a decrease in the melting and crystallization temperature.The TGA thermograms display a decrease in decomposition due to the structural change in CMC.CMC had a significant ash concentration due to the presence of a metal group that MCC did not have.CMC is more reactive than native MCC, thus making it the raw material for synthesizing superabsorbent polymers.
• The synthesized CMC displayed a high degree of substitution and indication of a successful esterification process.• Swelling studies were carried out for MCC and CMC which increased by double the amount.
• Characterization of the MCC and CMC was carried out using FTIR, DSC, TGA, and XRD techniques.
• The high swelling CMC is a potential material for use in the production of superabsorbent polymers as an emulsifier since it is soluble forming a gel-like substance.

Figure 2 .
Figure 2. The swelling capacity of MCC and CMC in distilled water.

Figure 3 .
Figure 3.The swelling capacity of MCC and CMC in 0.9% NaCl.

Table 1 .
Physical parameters of coconut fiber microcrystalline cellulose and carboxymethyl cellulose.

Table 2 .
Degree of substitution for carboxymethyl cellulose.

Table 3 .
ANOVA for four parameters for microcrystalline and carboxymethyl cellulose.