Synthesis of Rice Straw Cellulose Ester for Use as Biodegradable Plastic Film
Usarat Ratanakamnuan
1, aand Yanyong Ninsin
1, b1Department of Chemistry, Faculty of Science, Maejo University, Chiang Mai, 50290, Thailand
a[email protected], b[email protected]
Keywords: Cellulose, Rice Straw, Esterification, Biodegradable film
Abstract. In this research, the feasibility to obtain cellulose film from rice straw was investigated.
After delignification and bleaching of rice straw, the rice straw pulp was treated by acid hydrolysis in order to obtain rice straw cellulose powder. After that, the esterification of rice straw cellulose was carried out by using lauroyl chloride as an esterifying agent, toluene and pyridine as a solvent and a catalyst, respectively. The optimum condition for esterification was examined in terms of temperature and reaction time. Chemical structure and properties of modified cellulose such as morphology, thermal stability, and solubility were investigated. The rice straw cellulose ester film was prepared by casting method and the films obtained were tested for their tensile properties.
Introduction
The disposal of the used plastics has become a major environmental problem. The development and use of biodegradable plastics are one of the solutions to reduce environmental problem. The alternative way to develop the biodegradable plastics is the synthesis of new biodegradable cellulose-based film. Cellulose is a polysaccharide consisting of anhydroglucose units (AGU) linked together by (1-4)-β-D-glucosidic bonds. Cellulose belongs to the family of biodegradable and renewable polymer that provides a broad range of important functional properties, and is widely used in industry today. However, some of the inherent properties of cellulose limit its utility in certain applications. The intrinsic lack of solubility of native cellulose in water and most organic solvent systems due to a large portion of inter- and intramolecular hydrogen bond in its structure constitutes a major obstruction for utilizing cellulose in many industrial applications. In order to overcome this insoluble nature of cellulose and extending the range of applications of the polymer, chemically modified cellulose was developed. The properties of cellulose can be modified by, for example, partial hydrolysis, oxidation, etherification and esterification of hydroxyl group. Several researches on the esterification of cellulose have been published [1-5]. Cellulose esterification with fatty acid chloride in homogeneous system using N,N-dimethylacetamide/lithium chloride (DMAc/LiCl) as solvent has been carried out to obtain biodegradable plastic film [6-8]. However, esterification of cellulose in heterogeneous system using toluene as a cellulose-swelling medium has been studied widely by many researchers [9-12]. In previous works [13,14] esterification of cellulose from waste cotton fabric in DMAc/LiCl solvent system using several types of fatty acid chloride as an esterifying agent and N,N-dimethyl 1-4-aminopyridine as a catalyst was achieved. The preliminary studies also showed that it was possible to achieve cellulose film from waste cotton fabric.
Rice straw is an abundant lignocellulosic agricultural byproduct from rice production in farming in Thailand. It is a renewable and cheap resource for cellulose fiber. The chemical compositions of rice straw fiber are cellulose (28-36%), hemicelluloses (23-28%), lignin (12-14%), and ash (14-20%) [15]. Accordingly, rice straw is of interest as a possible resource for cellulose-based film production. In this study, cellulose from rice straw was esterfied with lauroyl chloride in heterogeneous system using toluene as a medium and using pyridine as a catalyst, in order to save the solvent cost. The optimum condition for esterification of rice straw cellulose was reported. The chemical structure and properties of both cellulose laurate powder and film were investigated in terms of morphology, thermal properties, solubility, and mechanical properties.
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Experimental Materials
Rice straw was obtained from local rice feilds, Chiangmai Thailand. Lauroyl chloride (98.0%, Merck), toluene (99.5%, RCI Labscan), pyridine (99.5%, Merckt), sodium hydroxide (97.0%, RCI Labscan), hydrogen peroxide (35.0%, QReCTM), hydrochloric acid (37.0%, RCI Labscan), ethanol (absolute, RCI Labscan), and chloroform (Labscan) were used as received.
Delignification, Bleaching and Hydrolysis
The rice straw was boiled in water for 1 hour in order to remove water-soluble hemicellulose.
Then, it was heated and stirred with 0.5 M NaOH solution at 60°C for 2 hours in order to remove lignin and remaining hemicellulose from the rice straw pulp. After delignification, the rice straw pulp was filtered and washed with distilled water. The delignined pulp was bleached by 10% (v/v) H2O2 in 0.5 M NaOH at 60°C for 2 hours. After that, the bleached pulp was washed several times with distilled water untill its pH become neutral. The bleached rice straw pulp was hydrolyzed in boiling 2 M HCl for 2 hours. After filtered, washed with distilled water, and dried in an oven at 60°C for 24 hours, the rice straw cellulose powder was obtained.
Esterification, and Film Casting
The cellulose powder (2 g) was esterified in the mixture of toluene (15 mL), pyridine (5 mL), and lauroyl chloride (10 mL). The heating temperature and reaction time were varying from 30-80°C and 5-25 hours, respectively. At the end of the reaction, the cellulose laurate powder was precipitated and washed with ethanol. Finally, the rice straw cellulose laurate film was prepared by casting with chloroform.
Characterization and Testing
The functional group and chemical structure of rice straw cellulose and cellulose laurate were examined by 1H-NMR spectroscopy in CDCl3 using a Varian Inova 500 MHz spectrometer and by FTIR spectroscopy using a Perkin Elmer Spectrum RX, respectively. Thermogravimetic analysis of cellulose laurate was performed with Mettler Toledo TGA/SDTA 851e at a heating rate of 10°C/min from 30°C to 600°C under nitrogen atmospher. Measurements of calorimetric properties of sample were carried out by Differential Scanning Calorimeter (PerkinElmer Pyris Diamond DSC) with a heating rate of 10°C/min from 50°C up to 300°C in a nitrogen atmosphere. The morphology of cellulose and cellulose laurate was investigated using a scanning electron microscope (JEOL:
JSM-5410LV). The solubility test of modified cellulose was determined at 10% (w/v) concentration in acetone, toluene, chloroform, dichlorometnae, and dimethylacetamide. Tensile tests of cellulose films were performed using a crosshead speed of 10 mm/min and a gauge length of 100 mm, according to the ASTM D882 standard method by Universal Testing Machine (LLOYD LR 10K).
Results and Discussion
Rice straw cellulose laurate was obtained by esterification according to Fig. 1. The influence of reaction time and temperature on %weight increase (%WI) of rice straw cellulose laurate is presented in Fig. 2. The result shows the overall trend that %WI of cellulose laurate continuously increased with the increasing of reaction time and temperature up to the maximum value and then the decrement in %weight increase of cellulose laurate was observed. For example, at the constant reaction temperature of 60°C, the %weight increase of cellulose laurate continuously increased with the reaction time until it approached the maximum %weight increase (245.75%) at 10 hours of reaction time, then %weight increase of cellulose laurate gradually decreased. When we consider at the constant reaction time, for instance, at 10 hours, the result revealed that the %weight increase of modified cellulose increased with the increment of reaction temperature until it reached the appropriate value at 60°C (245.75%), after this point the %weight increase was dropped. The enhancement of %WI with the increasing of reaction time and temperature was due to the high temperature and time increase the diffusion rate of acid chloride. However, the decreasing of %WI at the higher temperature and longer reaction time could be represented that the extreme esterification reaction can cause cellulose degradation, as evidenced by a brown color of esterified rice straw cellulose.
From the result, when considering on energy and time saving with the highest %WI (245.75%) of modified cellulose without degradation of cellulose, the esterification of rice straw cellulose with lauroly chloride should be conducted at 60°C for 10 hours.
Rice straw cellulose Lauroyl chloride
Cellulose laurate
O
R = H or CO(CH2) 10CH3
n
O O O
OR
OR
RO O
O OR
OR RO
n
O O O
OH
OH
HO O
O OH
OH
HO H3C (CH2)
10 C Cl
Time (hour)
0 5 10 15 20 25
%Weight increase
0 50 100 150 200 250
300 30o
C 60o
C 80o
C
Fig. 1. Esterification of rice straw cellulose Fig. 2 The relationship between %weight increase of cellulose laurate and reaction time (hour) at different reaction temperatures
Chemical structure of cellulose laurate was investigated by 1H-NMR spectroscopy (spectrum not shown). 1H-NMR analysis of cellulose laurate that shows signals between 0.86-2.3 ppm were corresponding to the alkyl group of lauroyl side chain. The peaks observed in cellulose laurate in the range of 3.38-5.08 ppm were related to the anhydroglucose unit protons. In addition, the unmodified and modified cellulose were also characterized by FTIR spectroscopy The FTIR spectra (Fig. 3) provide an evidence of esterification by showing a decrease in the intensity of the characteristic band of hydroxyl group in cellulose around 3360 cm-1, that arising from the hydroxyl group of cellulose was substituted by alkyl group from lauroyl chloride. This decrease in the intensity occurred concurrently with the presence of the important ester carbonyl band at 1746 cm-1 (C=O) and an increase in the intensity of the methyl band (C-H stretching due to CH2 and CH3) around 2857 cm-1 and 2924 cm-1, respectively. Both 1H-NMR and FTIR results confirm an accomplishment in the modification of rice straw cellulose.
Fig.3. FTIR spectra of rice straw cellulose and Fig.4 TGA curve of rice straw cellulose and rice straw cellulose laurate rice straw cellulose laurate
Thermal transition temperature was characterized by a differential scanning calorimetry (DSC).
The DSC thermograms of unmodified cellulose and cellulose laurate do not show any significant transition temperature (thermogram not shown). Cellulose is a linear high polymer having glucose as
a repeating unit, and it has an opportunity of forming significant hydrogen bonding. The resulting high intermolecular forces, in addition to the regular structure of the polymer, result in its high degree of crystallinity. Consequently, no melting of cellulose can occur at temperatures that cause pyrolysis [16]. In this sense, the cellulose crystallinity in rice straw cellulose prevents them from showing thermoplasticity up to melting. Moreover, rice straw cellulose did not show glass transition temperature due to their high crystalline structure. However, no signs of melting temperature (Tm) of the esterified cellulose were detected in the range of heating scans (50°C-300°C). The Tm above the thermal decomposition temperature might be possible in the temperature close to the decomposition temperature (300°C-Td).
Fig. 5 shows the SEM micrographs of (a) unmodified and (b) modified rice straw cellulose.
After acid hydrolysis of rice straw pulp, it became white powder as seen by naked eyed. However, under microscope, morphology of rice straw cellulose revealed short fiber shape having smooth surface with approximately 5-10 µm in diameter and 40-50 µm in length. The surface morphology of rice straw cellulose before esterification was different from the esterified rice straw cellulose in that the rice straw cellulose laurate showed rough surface and larger dimension. This implied that the substitution of acyl group of lauroyl chloride lead to an aggregation on rice straw cellulose surface.
The solubility of esterified rice straw cellulose was determined at the concentration of 10%
(w/v) in acetone, toluene, chloroform, dichlorometnae, and dimethylacetamide. Introduction of hydrophobic acyl group in the molecular structure of cellulose is expected to alter its solubility properties. The studies showed that the unmodified cellulose is insoluble in any common organic solvents whereas cellulose laurate is soluble in toluene, chloroform, and dichloromethane. The substitution of acyl group leads to an increase in the lipophilicity and the destruction of intra- and intermolecular hydrogen bondings of rice straw cellulose laurate. As a result, rice straw cellulose laurate can completely dissolve in organic solvents.
(a) (b)
Fig. 5. SEM micrographs of (a) rice straw cellulose and (b) rice straw cellulose laurate
After modification, cellulose laurate powder was converted into plastic film by casing with chloroform. The thickness of the film was about 40-70 µm. The tensile strength, elongation at break, and elastic modulud of rice straw cellulose laurate film were approximately 7.29 MPa, 9.04%, and 206.73 MPa, respectively. The tensile properties of some widely used commercial polymers such as low-density polyethylene (LDPE) or polypropylene (PP) [17] are compared with our data. The tensile properties of rice straw cellulose laurate films prepared in this research were far behind those of LDPE and PP film, which means that this rice straw cellulose laurate film is brittle and easily to be deformed.
Conclusion
Rice straw can be transformed into polymer film by esterification with lauroyl chloride. The optimum condition for esterification of rice straw cellulose based on the highest %WI was 60°C for 10 hours. The ultimate %weight increase at this condition was 245.75%. The FTIR spectra provide an evidence of cellulose esterification by the presence of important ester carbonyl band (C=O) at 1746 cm-1 and the methyl band (C-H stretching) at 2857 cm-1 and 2924 cm-1. The morphology of modified
cellulose revealed agglomeration of modified cellulose particles. Rice straw cellulose laurate can be dissolved in toluene, dichloromethane and chloroform. Cellulose ester film was obtained by casting method in chloroform solution. The tensile strength, elongation at break, and elastic modulud of rice straw cellulose laurate film were approximately 7.29 MPa, 9.04%, and 206.73 MPa, respectively.
Acknowledgement
The authors are grateful to the Faculty of Science, Maejo University Chiang Mai, Thailand for financial support. This work is also supported by the Nation Research Concil of Thailand (NRCT).
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Synthesis of Rice Straw Cellulose Ester for Use as Biodegradable Plastic Film 10.4028/www.scientific.net/AMR.488-489.980