Preparation of carboxymethyl cellulose (CMC) derived from palm fiber waste: Effect of ozone pretreatment on CMC properties

doi:10.21203/rs.3.rs-1496346/v1

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Preparation of carboxymethyl cellulose (CMC) derived from palm fiber waste: Effect of ozone pretreatment on CMC properties

https://doi.org/10.21203/rs.3.rs-1496346/v1

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In this study, carboxymethyl cellulose (CMC) was prepared using cellulose extracted from palm fiber (CPF). The effect of ozone pretreatment on the CMC properties (degree of substitution (DS) and viscosity) was investigated. Cellulose extracted from CPF was treated with ozone, and then CMC was prepared by alkalization followed by etherification using sodium hydroxide (NaOH) and sodium monochloroacetic acid (NaMCA) in an alcoholic medium. The DS of CMC increased with increasing ozone pretreatment time, and increasing amounts of NaOH and NaMCA added. Furthermore, the solubility increased with increasing DS. Therefore, the solubility increased with ozone pretreatment time. The optimum conditions were ozone pretreatment for 3 h, followed by the addition of 10 mL of 30% NaOH and 4.5 g of NaMCA. Under these conditions, the DS, purity, and solubility were 1.05, 79.6%, and 94.6%, respectively. The viscosity of CMC prepared using cellulose treated with ozone decreased with increasing ozone pretreatment time, which was due to the reduced degree of polymerization. Therefore, using ozone pretreatment for CMC preparation was an effective method for controlling the CMC properties.

carboxymethyl cellulose

ozone

palm fiber

viscosity

degree of substitution

The gradual depletion of fossil resources is a serious crisis in the development of the chemical industry. Furthermore, nonbiodegradable petrochemical products cause environmental pollution. Owing to resource depletion and environmental pollution, renewable resources to replace fossil resources are urgently needed (Chen et al., 2017). Cellulose is a bio-based product that is actively being studied as a renewable source of organic substances. In particular, cellulose has been converted into various products (Seidl and Goulart, 2016) and bio-based energy sources, such as fuel ethanol.

The palm plant is an abundant agricultural product with high economic value, and is widely distributed in tropical and subtropical countries, such as Indonesia, Malaysia, Vietnam, and Myanmar (Awalludin et al., 2015). Oil palm production by Indonesian plantations increased from 24.37 million tons in 2014 to 29.67 million tons in 2018 (Sari et al., 2021).

Products produced from oil palm are important for Indonesian agricultural business (Karya et al., 2019). Oil palm fruit bunches are the largest contributors to waste products derived from the oil palm industry, with the mass of palm fiber waste produced comprising 22–24% of the palm fruit mass (Pujiasih et al., 2018). As palm fiber waste is approx. 50% cellulose, the extraction of cellulose from palm fiber waste should be utilized. In previous reports, we have studied the extraction of cellulose from oil palm fruit bunch waste materials (Pujokaroni et al., 2019; Pujokaroni et al., 2020), and prepared carboxymethyl cellulose (CMC) using cellulose extracted from palm fiber (CPF).

CMC is a major cellulose derivative. The degree of polymerization (DP) of cellulose is generally in the range of 200–2000, resulting in strong inter- and intramolecular hydrogen bonds. Therefore, cellulose generally cannot be dissolved in hot or cold water and most common organic solvents. However, carboxymethylation to produce CMC can make cellulose water-soluble (Pujokaroni et al., 2020; Yu et al., 2019). An increasing number of applications are being identified for water-soluble CMC, including as a stabilizer or rheological control agent in paper making, ceramics, oil drilling, medicine, and food (Casaburi et al., 2018; Yusoff et al., 2021). CMC is a polymer based on b-D-glucose units, in which free hydroxyl groups are partially substituted by carboxymethyl groups (Golbaghi et al., 2017).

The characteristics of CMC solutions depend on the DP and degree of substitution (DS) with carboxymethyl, as well as the uniformity of substitution (Casaburi et al., 2018). The DS represents the average number of carboxymethyl groups per anhydrous glucose unit (Dapı́a et al., 2003). Theoretically, CMC can have a maximum DS of 3. Commercial CMC grades are available with a DS of up to 1.5. A DS of less than 0.4 results in CMC being insoluble in water. The DS of commercial CMC grades used in food products is between 0.6 and 1.25 (average DS of 1.0) (Kamthai and Magaraphan, 2017). The reaction of cellulose with sodium monochloroacetic acid (NaMCA) and the resulting structural formula of CMC (with an average DS of 1.0) are shown in Fig. 1.

Cellulose gum with a longer chain length has a higher viscosity. Furthermore, cellulose shows improved reactivity with a decreasing DP because the number of cellulose chain ends affects cellulose reactivity (Willberg-Keyrilainen and Ropponen, 2019; Willberg-Keyriläinen et al., 2019). These factors are important for the preparation of cellulose derivatives, such as CMC. Pretreatments, such as the depolymerization of cellulose to oligomers and reduction of the number of hydrogen bonds between hydroxyl groups in cellulose, will be also a necessary for various applications (Robles Barros et al., 2020; Hivechi et al., 2015; Pujokaroni et al., 2020). Common pretreatment methods include steam explosion, ammonia pretreatment, liquid hot water pretreatment, ball milling, microwave irradiation, and the alkaline method (Heise et al., 2017; Chen et al., 2017; Golbaghi et al., 2017; Hivechi et al., 2015). Furthermore, the viscosity and solubility of CMC are mainly determined by the length of cellulose chains, namely, the DP (Wang et al., 2019; Benchabane and Bekkour, 2008). Viscosity is a particularly important factor for CMC, because the thickening, film formation, water retention, and shape retention of CMC are mainly dependent on its viscosity (Benchabane and Bekkour, 2008; Wang et al., 2019). Therefore, pretreatment is important for controlling the properties of CMC, such as DS and viscosity.

In this study, ozone pretreatment was applied to CMC preparation. Ozone is a green oxidizing agent used to bleach pulp because its high oxidative power effectively removes lignin (Pouyet et al., 2014; Perrin et al., 2014). Ozone oxidation of cellulose changes hydroxy groups to carboxylate groups and decreases the DP (Pujokaroni et al., 2020). This was predicted to improve the reactivity of cellulose, increase the DS and solubility, and control the viscosity of CMC. Therefore, the effects of ozone pretreatment on the purity, DS, solubility, and viscosity were investigated. Furthermore, the effect of preparation conditions on the purity, DS, solubility, and viscosity of CMC was also studied. Optimization of the parameters used for CMC preparation from ozonated cellulose obtained from palm fiber was studied. The effectiveness of ozone pretreatment for controlling the DS, solubility, and viscosity of CMC was also investigated.

Materials

Palm fiber in a chopped form (fiber lengths of approx. 2–5 cm) was obtained from a palm kernel plantation in Samarinda, Indonesia, in July 2017 (Pujokaroni et al., 2019). The palm fiber was pretreated by washing with water to remove sand, soil, and palm oil shell, and then treated with n-hexane to remove oil. The dried palm fiber was stored at room temperature (26 ± 3°C). The contents of holocellulose, a-cellulose, and lignin in the original palm fiber were 68.2% ± 0.66%, 29.5% ± 0.29%, and 35.0% ± 1.27%, respectively. Sodium hydroxide (NaOH), acetic acid, hydrogen chloride (HCl), citric acid, sodium chloride (NaCl), and sodium monochloroacetic acid (NaMCA) were purchased from Fujifilm Wako Pure Chemical Corporation (Osaka, Japan). Sodium chlorite was purchased from Nacalai Tesque, Inc. (Kyoto, Japan). Copper ethylenediamine was purchased from Merck KGaA (Darmstadt, Germany).

Extraction of cellulose

First, holocellulose was extracted from the cellulose palm fiber according to ASTM D114-56, which is a chlorination or bleaching process. Palm fiber (20 g) was soaked with distilled water (650 mL) in a 1000-mL glass beaker heated to 105°C (Pujokaroni et al., 2019). The beaker was then placed in a water bath at 70°C, and acetic acid (4 mL) and sodium chlorite (8 g) were added to the beaker every hour. After 3 h, the cellulose product (holocellulose) was recovered by filtration and rinsed with distilled water.

The recovered holocellulose was treated according to ASTM D113-60 to produce a-cellulose. The holocellulose was soaked in 5% (w/v) NaOH solution (500 mL) for 2 h at 23 ± 2°C. The resulting a-cellulose was isolated by filtration and then immersed in distilled water (500 mL) containing acetic acid (7 mL) to neutralize alkaline cellulose. The mixture was stirred for approximately 30 s before being allowed to settle for 5 min. Residual cellulose was isolated and rinsed with water until no longer acidic, as indicated by a pH meter. The cellulose was then oven-dried overnight at 105°C.

Ozone pretreatment of extracted cellulose

Oxidized cellulose was prepared by ozone pretreatment for 0.5, 1.0, and 2.0 h. Samples (5 g) of cellulose extracted using the method described above were placed in 1000-mL reaction vessels containing 3% (w/v) citric acid solution. The solution was ozonated at 40°C with a continuous feed of ozone gas (1.0 mL/min) (Pujokaroni et al., 2020). The ozonation system consisted of an ozone generator (ED-OG-R6, EcoDesign, Inc., Saitama, Japan) and on-line ozone analyzer (OZM-5000G1, Okitrotec Co., Ltd., Tokyo, Japan). The ozone gas concentration was approx. 45 g/m³. After pretreatment for 1–5 h, the cellulose was recovered. The ozone concentration in the water was 3.67 ppm.

Preparation of carboxymethyl cellulose

First, the original cellulose fiber or ozone-treated cellulose (1 g) was mixed with isopropanol (25 mL). NaOH solution (2.5, 5, or 10 mL; 30%, w/v) was then slowly added to the solution with stirring for 30 min, followed by further stirring at room temperature for 1.5 h to produce alkali cellulose. Next, NaMCA (1.12, 2.25, or 4.5 g) was added to the mixture with stirring for 30 min, followed by reaction at 55 °C for 3.5 h. The mixture was then washed with 80% methanol (200 mL ⋅ 5) to remove undesired salts. The product was then oven-dried at 60 °C for 12 h to afford CMCs. The experimental design is shown in Table 1.

Table 1. CMC preparation conditions

Sample

Ozone treatment time

for extracted cellulose (h)

30% NaOH (mL)

NaMCA (g)

CMC-2.5-(0, 0.5, 1, 3 and 5 h)

0, 0.5, 1, 3 and 5

2.5

1.12

CMC-5-(0, 0.5, 1, 3 and 5 h)

0, 0.5, 1, 3 and 5

2.25

CMC-10-(0, 0.5, 1, 3 and 5 h)

0, 0.5, 1, 3 and 5

4.5

Viscosity

The viscosity of CMC samples was measured using a Brookfield DV-E Viscometer. CMC (1 g) was dissolved in distilled water (100 mL) with rapid stirring. The solution was then allowed to stand for a few minutes to remove air bubbles. All measurements were conducted using a spindle (type 304 s/s UL adapter) at 100 rpm and 27 ± 2 °C.

Solubility

To measure solubility, CMC (1 g) was first dissolved in distilled water (100 mL) at room temperature and stirred for 1 h. The solution was then filtered to separate the undissolved sample, and the remaining sample was dried at 105 °C for 3 h. The solubility of CMC was then calculated as follows:

Solubility (%) = (W₀ − W₁)/(W₀) × 100 (1)

where W₀ is the initial weight of CMC and W₁ is the residual weight after drying (Yu et al., 2019).

Purity

To determine purity, CMC was first washed twice with 80% ethanol and absolute methanol to remove byproducts containing sodium glycolate and NaCl. Next, CMC (2 g) was dissolved in distilled water (50 mL) at 80 °C and stirred for 15 min. After complete dissolution, the solution was centrifuged for 1 min at 4000 rpm and the precipitated solids were removed. Pure acetone (50 mL) was then added to reprecipitate the dissolved CMC. The recovered CMC was then filtered and oven-dried at 70 °C until a constant weight was obtained. The purity of CMC was then calculated as follows:

Purity (%) = W_r/W_i × 100 (2)

where W_r is the residual weight and W_i is the initial weight of the product (Golbaghi et al., 2017).

Degree of substitution

To determine the DS, CMC (1 g) was first added to a 250-mL beaker, followed by 95% ethanol (50 mL) and nitric acid solution (5 mL, 2 M). After stirring for 10 min at room temperature, the mixture was heated to boiling using a magnetic hot plate for 5 min and then stirred for a further 20 min. After filtering the solution, the residue was washed with 95% ethanol (100 mL) until acid and salts were removed. The recovered CMC was washed with methanol, heated until the alcohol was removed, and then oven-dried at 90 °C for 3 h. The resulting CMC (0.5 g) was added to distilled water (100 mL) in a 250-mL Erlenmeyer flask. Sodium hydroxide solution (25 mL, 0.5 M) was added to the flask and the mixture was boiled for about 20 min. The heated solution was titrated with 0.3 M HCl, as observed using phenolphthalein as indicator, which exhibited a color change from Mexican pink (dark pink) to colorless. The DS of CMC can be calculated using the following equations:

A = ((B⋅C) − (D⋅E))/F (3)

DS = (0.162 ⋅ A)/1−(0.058 ⋅ A) (4)

where A is the milliequivalents of acid consumed acid per gram of specimen, B is the volume of NaOH consumed (mL), C is the NaOH concentration (M), D is the volume of HCl consumed (mL), E is the HCl concentration (M), and F is the specimen amount (g). The values of 162 and 58 represent the molecular weight of the anhydrous glucose unit and the net increase in molecular weight of the anhydrous glucose unit for each substituted carboxymethyl cellulose, respectively (Golbaghi et al., 2017).

Sample characterization

FT-IR spectra were measured using an FT-IR-6100 spectrometer (Jasco Inc., Tokyo, Japan) with attenuated total reflection at a resolution of 4 cm⁻¹ throughout the spectral range (4000–500 cm⁻¹) by accumulating 40 scans.

The carboxyl content of the extracted cellulose/oxidized cellulose was determined using the electrical conductivity titration method (Pujokaroni et al., 2020).

The DP of the extracted cellulose/oxidized cellulose was determined from the intrinsic viscosity ([h]), which was measured using an Ostwald viscometer and method ISO-5351-1 (Pujokaroni et al., 2020). The DP was calculated from [h] using the Mark–Houwink–Sakurada equation ([h] = 0.57 ⋅ DP).

XRD patterns were recorded using a Rigaku Type RINT 200 diffractometer fitted with a vertical goniometer and using Cu-Ka irradiation. The X-ray generation conditions were 30 kV and 40 mA. The scan speed was 4° min⁻¹. The crystallinity was calculated from the ratio of crystalline peak areas at 23° (2q) to noncrystalline peak areas at 18.5 (2q).

Table S1 shows the degree of polymerization and carboxylate content of extracted cellulose with ozone pretreatment (Pujokaroni et al., 2020). As the reaction time increased, the DP of extracted cellulose decreased from 1012 to 185 after reacting for 5 h (Table S1). Glycosidic bonds in the cellulose were successfully broken by ozone pretreatment.

The crystallinity of cellulose extracted from palm fiber was 45.2%, while that of cellulose oxidized by ozone was 66.2%. This suggested that residual lignin was decomposed by ozone pretreatment, resulting in an increase in purity of the cellulose extracted from CPF (Pujokaroni et al., 2020). CMC was prepared using the extracted cellulose treated with ozone. Herein, the ozone pretreatment time is the pretreatment time for cellulose extracted from palm fiber.

CMC was prepared using the oxidized cellulose. Cellulose peaks attributed to C–O and C–O–C stretching vibrations were observed at 1200–900 cm⁻¹. The peaks at 1480 and 1620 cm⁻¹ were attributed to C=O stretching in the carboxymethyl COO⁻ group (Unlu, 2013). After carboxymethylation, the intensity of the peaks at 1480 and 1620 cm⁻¹ had increased owing to carboxymethyl group formation (Fig. 2).

Degree of substitution of carboxymethyl cellulose

The DSs of CMC prepared in this study were in the range of 0.56–1.05, as shown in Table 2. The lowest DS of 0.56 was obtained for CMC-2.5-(0 h) (no ozone pretreatment, using 2.5 mL of 30% NaOH and 1.12 g of NaMCA) prepared without ozone pretreatment, while the highest DS of 1.05 was obtained for CMC-10-(3 h) (ozone pretreatment time of 3 h, using 10 mL of 30% NaOH and 4.5 g of NaMCA), as shown in Table 2. The DS increased with an increasing amount of NaOH added. These results suggested that swelling of the cellulose fiber by the high alkali concentration caused defibrillation and cellulose degradation, resulting in accelerated carboxymethylation (Adinugraha et al., 2005). Furthermore, the DS of CMC increased with an increasing amount of NaMCA added, owing to the acetate groups in NaMCA. With a higher concentration of acetate groups, the reactivity of cellulose molecules towards this reagent was increased, facilitating carboxymethylation (Joshi et al., 2015). This finding was supported bythe report of Mondal et al. (Mondal et al., 2015).

Table 2 DSs of CMC prepared using oxidized cellulose.

	Experimental conditions
	30% (w/v) of NaOH volume (mL) : NaMCA weight (g)
	2.5 : 1.12	5 : 2.25	10 : 4.5
Ozone treatment time for extracted cellulose (h)	DS
0	0.56 ± 0.02	0.67 ± 0.04	0.71 ± 0.02
0.5	0.63 ± 0.04	0.64 ± 0.05	0.76 ± 0.03
1	0.71 ± 0.04	0.76 ± 0.03	0.83 ± 0.05
3	0.76 ± 0.04	0.86 ± 0.03	1.05 ± 0.04

Figure 3a shows the relationship between ozone pretreatment time and the DS of CMC-10 (prepared using 10 mL of 30% (w/v) NaOH and 4.5 g of NaMCA). As shown in Table 2 and Fig. 3a, the DS value increased with increasing ozone pretreatment time. The DS of CMC-10-(5 h) (ozone pretreatment time of 5 h, using 10 mL of 30% NaOH and 4.5 g of NaMCA) was the highest in this study, at 1.24. Peaks at 1480 and 1620 cm⁻¹ attributed to C = O stretching of the carboxymethyl COO⁻ group were clearly detected with a long ozone pretreatment time (Fig. 2e).

The DS of CMC prepared in this study increased with a decreasing DP of cellulose (Fig. 3b). Figure 3c shows the relationship between DS and carboxylate content, where the DS increased with increasing carboxylate content. We previously reported that the solubility of cellulose treated with ozone improved with increasing the carboxylate content because carboxylate group formation during ozone pretreatment disrupted hydrogen bonding between the cellulose chains (Pujokaroni et al., 2019). Furthermore, the improvement in solubility was caused by the decrease in DP (Pujokaroni et al., 2020). These results suggested that the DS was related to the solubility of cellulose treated with ozone. The increased reactivity of cellulose was due to the improved cellulose solubility, because cellulose carboxymethylation only partially occurs as a heterogeneous reaction. Therefore, the reactivity between cellulose and reagents was improved.

Ozone pretreatment successfully decreased the DP and increased the carboxylate content of cellulose, as shown in Table S1. The increased DS wascaused by changes in the DP and carboxylate contents of cellulose. The DS of CMC was increased by ozone pretreatment, showing that this was an effective method for CMC preparation.

Purity of carboxymethyl cellulose

Cellulose carboxymethylation is a two-step process accompanied by an undesired side reaction (Komorowska et al., 2017). The first step is an alkalization, where the hydroxyl groups in cellulose chains (Cell–OH) are stimulated and converted to a more reactive alkaline form (Cell–O–).

Cell–OH + NaOH → Cell–ONa + H₂O (4)

This is followed by an etherification step, as shown in Eq. (5), to obtain CMC, and a side reaction, as shown in Eq. (6), which results in the formation of sodium glycolate (Feddersen and Thorp, 2012; Rachtanapun et al., 2012).

Cell–ONa + Cl–CH₂–CO–ONa → Cell–O–CH₂–COONa + NaCl (5)

NaOH + Cl–CH₂–CO–ONa → HO–CH₂–CO–ONa + NaCl (6)

Cellulose isolated from palm fiber was converted to CMC using the two steps described above. After carboxymethylation of the cellulose extracted from palm fiber, CMC was produced (Eq. (5)) with sodium glycolate as a byproduct (Eq. (6)) (Rachtanapun et al., 2012; Golbaghi et al., 2017) (Fig. 1). The purity was the percentage of CMC produced after removing byproduct sodium glycolate. The CMC purity was higher when the amount of CMC remaining after sodium glycolate removal was higher.

The purity of CMC prepared in this study was in the range of 56.1–79.6%, as shown in Table 3. The purity of CMC increased with increasing amounts of NaOH and NaMCA added. These results showed the same trend as the DS results (Table 2). The highest purity of 79.6% was obtained for sample CMC-10-(3 h) (ozone pretreatment time of 3 h, using 10 mL of 30% NaOH and 4.5 g of NaMCA). These results indicated that the carboxymethylation reaction of cellulose effectively progressed with the addition of these chemicals.

When using CMC-10 with ozone pretreatment times of 0, 0.5, 1, 3, and 5 h, the CMC purity obtained was 72.4%, 74.2%, 78.4%, 79.6%, and 79.5%, respectively. As result, the purity of CMC slightly increased with increasing ozone pretreatment time. The change in purity caused by ozone pretreatment was smaller than that obtained using additional amounts of the reagents. Therefore, the purity was not affected by ozone pretreatment. These results suggested that the purity of the prepared CMC was mainly related to the amounts of NaOH and NaMCA added.

Table 3. Purity of CMC prepared using oxidized cellulose

	Experimental conditions
	30% (w/v) of NaOH volume (mL) : NaMCA weight (g)
	2.5 : 1.12	5 : 2.25	10 : 4.5
Ozone treatment time for extracted cellulose (h)	Purity (%)
0	65.8 ± 1.25	71.5 ± 1.05	72.4 ± 2.12
0.5	56.1 ± 1.08	72.5 ± 2.15	74.2 ± 1.48
1	73.4 ± 1.51	75.6 ± 1.24	78.4 ± 1.62
3	68.4 ± 2.01	70.2 ± 1.54	79.6 ± 1.23

Solubility of carboxymethyl cellulose

The effect of adding NaOH and NaMCA on CMC solubility is shown in Table 4. The solubility of CMC is affected by the DS and DP (Yu et al., 2019).

The solubilities of CMC prepared in this study were in the range of 45.3–94.6%, as shown in Table 4. The lowest solubility of 45.3% was obtained for CMC prepared using the extracted cellulose without ozone pretreatment, while the highest solubility of 94.6% was obtained for CMC-10-(3 h) (ozone pretreatment time of 3 h, using 10 mL of 30% NaOH and 4.5 g of NaMCA). The solubility of CMC increased with increasing amounts of NaOH and NaMCA added. Figure 4 shows the relationship between the solubility and DS of the as-prepared CMC. The solubility increased with increasing DS, with a lower DS resulting in poor CMC solubility. For higher DS values, the solubility of CMC was high owing to its increased hydroaffinity resulting from the increased DS (Lakshmi et al., 2017). Therefore, the improvement in solubility was caused by the progression of carboxymethylation.

For CMC-2.5 (prepared using 2.5 mL of 30% NaOH and 1.12 g of NaMCA), the solubility of CMC changed from 45.3–76.4% with increasing ozone pretreatment time. For CMC-5 (prepared using 5 mL of 30% NaOH and 2.25 g of NaMCA), the solubility changed from 64.2–88.5% with increasing ozone pretreatment time. For CMC-10-(0 h) (no ozone pretreatment, using 10 mL of 30% NaOH and 4.5 g of NaMCA) and CMC-10-(3 h), the solubilities were 84.2% and 94.6%, respectively (Table 4). The solubility of CMC increased with an increasing ozone pretreatment time. The solubilities of CMC-2.5-(3 h) (ozone pretreatment time of 3 h, using 2.5 mL of 30% NaOH and 1.12 g of NaMCA) and CMC-10-(3 h) were 76.4% and 94.6%, respectively, representing an increase in CMC solubility of 18.2%. Meanwhile, the increase in solubility from CMC-10-(0 h) to CMC-10-(3 h) was 10.4%. These results indicated that improved CMC solubility was dependent on the amounts of reagents added, with the addition amount of reagents related to the DS of CMC being important.

The optimum conditions for obtaining CMC with a DS, purity, and solubility of 1.05, 79.6%, and 94.6%, respectively, were ozone pretreatment for 3 h, followed by the addition of 10 mL of 30% NaOH and 4.5 g of NaMCA. The crystallinity of the optimized CMC was 30.3%. Under these conditions, the crystalline structure of cellulose was destroyed by NaOH and swelling prior to the carboxymethylation process (Golbaghi et al., 2017).

Table 4. Solubility of CMC prepared using oxidized cellulose.

	Experimental conditions
	30% (w/v) of NaOH volume (mL) : NaMCA weight (g)
	2.5 : 1.12	5 : 2.25	10 : 4.5
Ozone treatment time for extracted cellulose (h)	Solubility (%)
0	45.3 ± 2.15	64.2 ± 3.02	84.2 ± 2.21
0.5	56.1 ± 3.12	75.24 ± 3.01	88.56 ± 1.54
1	75.6 ± 2.87	76.06 ± 2.54	86.12 ± 1.87
3	76.4 ± 1.84	88.5 ± 2.84	94.6 ± 2.54

Viscosity of carboxymethyl cellulose

The viscosity of CMC is shown in Table 5. Viscosity is an important parameter for the characterization of CMC, especially for its use as a stabilizing agent for food suspensions, cosmetics, and industrial products.

The viscosities of CMC-2.5-(0 h), CMC-5-(0 h) (no ozone pretreatment, using 5 mL of 30% NaOH and 2.25 g of NaMCA), and CMC-10-(0 h) ranged from 480 to 2120 mPa·s. The viscosity of CMC-2.5-(0 h) was lowest because carboxymethylation did not occur under these conditions. The highest viscosities were 2120 mPa·s (CMC-10-(0 h)) and 2150 mPa·s (CMC-5-(0.5 h)), suggesting that the carboxymethylation of cellulose had progressed in these samples. For CMC prepared using cellulose without ozone pretreatment, the viscosity increased with increasing amounts of NaOH and NaMCA added. Therefore, the viscosity of as-prepared CMC increased with increasing DS. These results suggested that the viscosity was increased by the carboxymethylation of cellulose.

When cellulose treated with ozone for 0.5 h was used for CMC preparation, the resulting viscosities of CMC-2.5-(0.5 h) (ozone pretreatment time of 0.5 h, using 2.5 mL of 30% NaOH and 1.12 g of NaMCA), CMC-5-(0.5 h) (ozone pretreatment time of 0.5 h, using 5 mL of 30% NaOH and 2.25 g of NaMCA), and CMC-10-(0.5 h) (ozone pretreatment time of 0.5 h, using 10 mL of 30% NaOH and 4.5 g of NaMCA) were 1210, 2120, and 1980 mPa·s, respectively. The viscosity increased with increasing amounts of reagents added and increasing DS. However, for cellulose treated with ozone for over 1 h, the amounts of NaOH and NaMCA added, and DS, had no influence on the CMC viscosity. The viscosities of CMC-2.5-(1 h) (ozone pretreatment time of 1 h, using 2.5 mL of 30% NaOH and 1.12 g of NaMCA), CMC-5-(1 h) (ozone pretreatment time of 1 h, using 5 mL of 30% NaOH and 2.25 g of NaMCA), and CMC-10-(1 h) (ozone pretreatment time of 1 h, using 10 mL of 30% NaOH and 4.5 g of NaMCA)were 1720, 1490, and 1240 mPa·s, respectively. With an ozone pretreatment time of 3 h, the viscosities obtained for CMC-2.5-(3 h), CMC-5-(3 h) (ozone pretreatment time of 3 h, using 5 mL of 30% NaOH and 2.5 g of NaMCA), and CMC-10-(3 h) were 1430, 1290, and 1210 mPa·s, respectively. When the ozone pretreatment time was over 1 h, the viscosity decreased with increasing ozone pretreatment time, despite the amounts of NaOH and NaMCA added, and DS, increasing. Therefore, the effect of ozone pretreatment on the viscosity was larger than that of reagent addition amounts and DS.

The effect of ozone pretreatment for 1–5 h on the viscosity of CMC-10 is shown in Fig. 5. The viscosity and DS of CMC-10-(3 h) were 1210 mPa·s and 1.05, respectively, while those of CMC-10-(5 h) were 530 mPa·s and 1.24, respectively. As mentioned above, carboxymethylation progressed with the addition of NaOH and NaMCA, and the carboxylate content was increased by ozone pretreatment. Therefore, the DS of CMC-10 increased with increasing amounts of reagents added and an increasing ozone pretreatment time (Fig. 3 and Table 2). However, the viscosity of CMC-10 prepared using cellulose treated with ozone decreased with increasing ozone pretreatment time. The DP also decreased with increasing ozone pretreatment time. Therefore, the viscosity of CMC-10 prepared using cellulose treated with ozone was strongly dependent on the DP.

The viscosity of a polymer solution depends mainly on the concentration, chain length, and size (molecular weight) of the dissolved polymer. Therefore, larger molecular weights and larger polymer sizes result in higher polymer viscosity (Mondal et al., 2015). Herein, the viscosity decreased with decreasing DP, and then decreased with increasing ozone pretreatment time. Ozone pretreatment significantly reduced the DP of extracted cellulose and degradation of the polymer chain, which affected the viscosity of CMC.

These results suggested that the viscosity of CMC could be controlled by ozone pretreatment. Furthermore, the DS, which is an important property of CMC, was also improved by pretreating cellulose with ozone. Therefore, ozone pretreatment of cellulose is an effective method for controlling the properties of CMC.

Table 5. Viscosity of CMC prepared using oxidized cellulose

	Experimental conditions
	30% (w/v) of NaOH volume (mL) : NaMCA weight (g)
	2.5 : 1.12	5 : 2.25	10 : 4.5
Ozone treatment time for extracted cellulose (h)	Viscosity (mPa・s)
0	480 ± 95	1960 ± 85	2120 ± 75
0.5	1210 ± 80	2150 ± 100	1980 ± 100
1	1720 ± 100	1490 ± 120	1240 ± 150
3	1430 ± 110	1290 ± 100	1210 ± 75

The results of this study showed that palm fiber waste can be used as a raw material to produce CMC. Ozone pretreatment, and the amounts of NaOH and NaMCA added, significantly affected the properties of CMC, including DS, purity, solubility, and viscosity.

The DS increased with increasing amounts of NaOH and NaMCA added, and was improved by ozone pretreatment. A decrease in DP and increase in carboxylate contents were caused by ozone pretreatment. Therefore, the reactivity between cellulose and the reagents was improved by ozone pretreatment.

The purity and the solubility were mainly related to the amounts of NaOH and NaMCA added. In particular, the solubility increased with an increasing DS of CMC. The optimum conditions to obtain a DS, purity, and solubility of 1.05, 79.6%, and 94.6%, respectively, were ozone pretreatment for 3 h followed by the addition of 10 mL of 30% NaOH and 4.5 g of NaMCA.

With increased ozone pretreatment time, the DP of cellulose was reduced and resulted in differences in the CMC viscosity. As a result, the viscosity decreased with increasing ozone pretreatment time, showing that the viscosity could be controlled by ozone pretreatment.

ACKNOWLEDGEMENTS

The authors would like to thank the Ministry of Research, Technology and Higher Education of the Republic of Indonesia, and Mulawarman University, for granting a scholarship to Mr. Agustu Sholeh Pujokaroni. We thank Simon Partridge, PhD, from Edanz (https://jp.edanz.com/ac) for editing a draft of this manuscript.

Funding: This study was supported by a Management Expenses Grant provided by the United Graduate School of Agricultural Sciences, Ehime University.

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Preparation of carboxymethyl cellulose (CMC) derived from palm fiber waste: Effect of ozone pretreatment on CMC properties

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Abstract

Figures

Introduction

Materials And Methods

Materials

Extraction of cellulose

Ozone pretreatment of extracted cellulose

Preparation of carboxymethyl cellulose

Viscosity

Solubility

Purity

Degree of substitution

Sample characterization

Results And Discussion

Degree of substitution of carboxymethyl cellulose

Purity of carboxymethyl cellulose

Solubility of carboxymethyl cellulose

Viscosity of carboxymethyl cellulose

Conclusion

Declarations

References

Supplementary Files

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