6212
Ind. Eng. Chem. Res. 2009, 48, 6212–6215
Cellulose Triacetate Optical Film Preparation from Ramie Fiber Xiushan Fan,†,‡ Zhong-Wen Liu,†,‡ Jian Lu,† and Zhao-Tie Liu*,†,‡ Key Laboratory of Applied Surface and Colloid Chemistry (Shaanxi Normal UniVersity), Ministry of Education, Xi’an 710062, P. R. China, and School of Chemistry & Materials Science, Shaanxi Normal UniVersity, Xian 710062, P. R. China
A new route for the preparation of cellulose triacetate (CTA) optical films from the biomass of ramie fiber has been found with environmental benefits. CTA with a degree of substitution (DS) of 2.81-2.92 was prepared by the reaction of acetic anhydride with ramie fiber catalyzed by sulfuric acid in acetic acid solution at 55 °C. The CTA film was prepared by casting the solution of CTA dissolving in dichloromethane on the culture disk via spreading the solution through a syringe. The structure and properties of CTA and its film were investigated by Fourier transform infrared (FT-IR), ultraviolet (UV), X-ray diffraction (XRD), nuclear magnetic resonance (NMR), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and titration. It was found that the CTA films prepared from ramie fiber shows a high transparency of 89% and excellent mechanical properties with stress measurements of 31.04-47.80 MPa and strain of 3.99-5.22%. The CTA films prepared from ramie fiber are suitable as protective films for the liquid crystal displays (LCD). Introduction Ramie is an abundant resource in China, with a higher cellulose content (65-75 wt %) than other fibers such as hemp, flax, and jute.1 Although there are large reserves of ramie fibers, most of them have been used inefficiently. It is mainly produced in southern China and known as “China grass” elsewhere.2 One drawback in the use of ramie fiber is that the higher cellulose content causes a high degree of crystallinity and orientation. Cellulose acetate has a wide range of applications. It is widely used as a fiber or in many industrial applications.3-7 Cellulose acetate has low toxicity and is almost nonflammable. Cellulose triacetate (CTA) is one of the most important derivatives of the cellulose acetate and is environmentally benign. CTA has recently also found use as a chiral polymeric sorbent for the chromatographic separation of enantiomers.8 The CTA film has been widely used because of its high moisture regain, low birefringence, moderate mechanical strength, and low production cost. CTA has been used as the base of most photographic films for many years, as a protective film for polarizing plates, and is widely used as a support for the polarizing in the making of liquid crystal displays (LCD). Production of LCDs, as apposed to traditional cathode ray tube (CRT) screens,9 has been developing quickly due to their lower low harmful radiation and lower energy usage. A variety of raw materials such as cotton,10 recycled paper,11 crop straw,12 wood pulp,13 corncob,14 corn fiber, rice hulls, and wheat straw15 have been used to make cellulose acetate, which is mainly used in wastewater treatment, sorbents, or films. The cellulose triacetate optical film is primarily prepared from cotton due to its low degree of crystallinity, orientation, and low impurity. The degree of polymerization in ramie fiber is much higher than that in cotton fiber; therefore, the cellulose triacetate synthesis procedure is difficult as compared with that for cotton. Meanwhile, there is no report on the synthesis of cellulose triacetate from ramie fiber. The motivation of the work is to guide the readers to explore the relationship between the * To whom correspondence should be addressed. Tel. and Fax: +86 29 85303682. E-mail address:
[email protected]. † Key Laboratory of Applied Surface and Colloid Chemistry. ‡ School of Chemistry & Materials Science.
properties of cellulose triacetate (CTA) and the treatment conditions for ramie fiber, especially the degree of substitution, dynamic viscosity, transparency, and impurity. From the reported results, we can see that the procedure of pretreatment of materials and the procedure are complex and consumed times. Furthermore, in the traditional procedure, many reagents were used and some are toxic such as NaOH, nitric acid, ethanol, acetone, ether, and dichloromethane. In this study, we have prepared CTA optical films form ramie fibers. The structure and properties of the resulting CTA and its films were analyzed by IR, UV, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), mechanical property testing, and chemical analysis. Experimental Section Materials. The ramie fibers were purchased from Hu’nan Yuanjiang Mingxing Co. Ltd. Acetic anhydride, acetic acid, sulfuric acid, anhydrous magnesium acetate, sodium hydroxide, hydrochloric acid, and dichloromethane were obtained from Xi’an Chemicals Co. Ltd. The anhydrous magnesium acetate was CP grade. All other chemicals were AR grades and were used without further purification. Characterization. Fourier transform infrared (FT-IR) was performed on a Nicolet 870 spectrometer, with 32 scans per sample. 1 H nuclear magnetic resonance (NMR) spectra in CDCl3 were recorded on a Bruker Avance 300 MHz spectrometer. Powder wide-angle X-ray spectra were recorded over a 2θ angle of 3-60° with a Rigakav D/max-1200 diffracted equipped with a graphite monochromator and using Cu KR radiation at λ ) 0.154 nm (40 kV, 40 mA). The crystallinity of the products was calculated by computer from the diffraction figure.16,17 The differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) measurements were performed on a Thermoanalyzer System, model Q600SDT (TA Co. Ltd. USA). Samples of about 5 mg, placed in a DSC pan, were heated from -10 to 550 °C at a scan rate of 20 °C/min under a constant flow of dry nitrogen.
10.1021/ie801703x CCC: $40.75 2009 American Chemical Society Published on Web 05/27/2009
Ind. Eng. Chem. Res., Vol. 48, No. 13, 2009
The transparency of the CTA was tested by a ultraviolet-visible near infrared (UV-vis-NIR) spectrometer, model Lambd 950 (Perkin-Elmer Co. Ltd. USA). The film was cut to the desired shape for the dynamic mechanical analysis (DMA) measurements (Q800 DMA, TA Co. Ltd. USA). Degree of substitution (DS) for CTA was measured by titration method as will be given in detail in the following section. Preparation of CTA and CTA Film. Ramie fibers were first swelled by spraying with acetic acid for 2 h at a room temperature. The fibers (10.00 g) were then reacted with a mixture of acetic anhydride (40 mL) and acetic acid (40 mL) containing sulfuric acid as a catalyst (0.8 wt % of the ramie fibers) at 55 °C. After the reaction, anhydrous magnesium acetate was added to the reaction mixtures to neutralize the sulfuric acid. The acetic acid solution was added to the reaction solution, and after reaction, the solution was poured into distilled water to precipitate the CTA. The resulting CTA was separated by a centrifugal separator and washed with distilled water until the filtrate tested neutral. Finally, the CTA was dried at 50 °C in air. The film casting solutions were obtained by dissolving the CTA in dichloromethane without heating. The solution was cast on the support (culture dish) from a syringe, with the volume used controlling the thickness of the film. After evaporation, the film was immersed into a water bath to form a film. The film was removed from the water bath and washed thoroughly with distilled water to remove any residual dichloromethane. Finally, the film was dried at a room temperature for 24 h in air. Degree of Substitution (DS) for CTA. A dried CTA powder sample (0.50 g) was transferred to a 250 mL conical flask containing 50 mL of 75% alcohol in water. The solution was stirred until homogeneous at 50 °C over 30 min. Subsequently, 50 mL sodium hydroxide solution (0.50 mol/L) was added with a small amount of phenolphthalein as an indicator. The solution was kept at 25 °C for 24 h and then titrated with 50 mL of 0.50 mol/L hydrochloric acid solution until the red color faded to give a volume of hydrochloric acid (D). A dried cellulose powder sample was also tested as a blank experiment, which also gave a volume of hydrochloric acid (C). The mass of acetic acid formed during titration was calculated from the following equation:16,18 weight of acetic acid ) [(D - C) × NHCl + (A - B) × NNaOH] × 6.005/W ) 6000X/(162 + 42X) where A is the volume of NaOH for CTA (mL), B is the volume of NaOH for cellulose (mL), C is the volume of HCl for cellulose (mL), D is the volume of HCl for CA (mL), NNaOH is the concentration of NaOH (mol/L), NHCl is the concentration of HCl (mol/L), W is the weight of sample in grams (g), and X is the DS. Results and Discussion FT-IR Results. The FT-IR analysis of ramie fibers and CTA is shown in Figure 1. Curves a and b represent FT-IR spectra of ramie fiber and CTA, respectively. The spectrum of the CTA shows the new acetyl groups which have been added to the cellulose of the ramie fiber with vibrations at 1758 cm-1 (sCdO) and 1371 cm-1 (sCsCH3). The spectrum of ramie fiber does not show any peak in this region but shows an absorption at 1640 cm-1 which is attributed to the sOsH bending vibration. This is expected as there is no sCdO in
6213
Figure 1. FT-IR spectra of ramie fiber (a) and CTA (b).
Figure 2. XRD of the ramie fiber (a) and CTA (b).
the ramie fiber molecule. After esterification, a great deal of intra- and intermolecular hydrogen bonds within of cellulose molecules are destroyed, thus the area of the hydroxyl peak at 3300-3500 cm-1 is smaller. The IR spectrum clearly confirms that acetyl groups have been attached to the cellulose skeleton to form cellulose triacetate. X-ray Diffraction (XRD) Analysis. The XRD patterns of ramie fiber and CTA are shown in Figure 2. In contrast with the sharp diffraction peaks for the raw fiber, the CTA spectrum shows no sharp peaks indicating a decrease of crystallinity and a transition in the structure of the cellulose from crystalline to amorphous region. The ramie fiber shows diffraction peaks at 14.9°, 16.5°, and 22.5° which is characteristic of the crystalline form of cellulose I. After esterification, broad peaks are seen at about 11° and 16°, which can be assigned to the introduction of acetyl into the ramie fiber and widen the spaces between layers in the crystal facets.16,17 This indicates that the crystalline transformation of ramie cellulose occurred during the esterification. The crystallinity was calculated by integrating the areas of the crystalline and amorphous phases reported by He.19 Moreover, in comparison with the raw ramie fiber, the intensity of the peaks from the acetylated fiber is obviously decreased, indicating a decrease in crystallinity from 80% to 30% on reaction with acetic anhydride. This may be due to the damage to the hydrogen-bond in network during esterification. NMR Analysis. The 1H NMR analysis of CTA was shown in Figure 3. In the linear molecular structure of CTA, there are three kinds of hydrogen molecules which show two clusters of peak signals as seen in the Figure 3. The proton resonance of the glucose ring (δ ) 3.30-5.20 ppm) and the corresponding
6214
Ind. Eng. Chem. Res., Vol. 48, No. 13, 2009
Figure 6. Transparency of CTA films. Figure 3. 1H NMR spectrum of the CTA derived from ramie fiber.
Figure 4. TGA and DSC results of ramie fiber.
Figure 5. TGA and DSC results of the CTA derived from ramie fiber.
resonance for the methyl protons of the acetate group (δ ) 1.50-2.20 ppm) are shown in Figure 3. The NMR results are in good agreement with previous reports.15 DSC and TGA Measurements. The TGA and DSC results from the ramie fiber are shown in Figure 4, and the results from the CTA are given in Figure 5. The TGA curves (curve a and c) show similar trends with small amounts of mass loss at low temperatures until a plateau region leading to rapid mass loss at the final decomposition of materials. However, the rapid mass loss region for CTA is from 230 to 400 °C, while the rapid mass loss region of ramie fiber is from 300 to 400 °C. These differences are due to the different structures in CTA and ramie fiber. The crystallinity of the ramie fiber increases its stability
Table 1. Mechanical Properties of CTA Films sample no.
acetyl content (%)
DS
static force (N)
stress (MPa)
strain (%)
1 2
60.29 61.56
2.81 2.92
10.38 8.84
47.80 31.04
5.22 3.99
with respect to temperature whereas the amorphous structure of the CTA produced here is more susceptible. There were two stages in the analysis of DSC. First, the temperature was increased to 110 °C and maintained there for 30 min in order to eliminate adsorbed water molecule from samples. Second, the temperature was decreased to room temperature before the scan to 550 °C under dry nitrogen. There is a small heat absorption peak at 95 °C due to vaporization of residual water which has been strongly adsorbed into the cellulose, and the ramie fiber is decomposed completely at 381 °C (curve b, Figure 4). However, the CTA started to decompose at 346 °C and was complete at 390 °C (curve d, Figure 5). The difference between the DSC curves of CTA and ramie fiber again results from the higher crystallinity of the ramie fiber. Optical Properties. Transparency plays an important role in the application of CTA to LCD. The two curves in Figure 6 describe the transparency curves of the two samples of CTA from Table 1. In the visible region (380-780 nm), the transparency of the CTA is more than 89%. The high transparency of CTA films demonstrates that the esterification of ramie fiber is complete. The CTA was formed in the reaction system gradually, giving homogeneous acetylation. This yields homogeneous products which show high transparency. Mechanical Analysis. The mechanical properties of the CTA are listed in Table 1. The resistance to stress, strain and static force decreased upon increases in the acetyl content. Hydrogen bonds exist within and between cellulose molecules. When the cellulose exhibits a high degree of crystallization, these bonds are maximized and this leads to the higher mechanical properties of the ramie fiber. When the acetyl groups are introduced to the cellulose in ramie fibers, the hydrogen bonds are destroyed, and an amorphous structure is formed, the crystallinity decreased. Thus with an increase of the acetyl content, the mechanical properties of CTA are decreased. The stress measurement of CTA at 47.80 MPa and an acetyl content of greater than 60% is in accordance with the requirements for LCD applications (required acetyl content in CTA of 60-62%). Conclusion Cellulose triacetate film, which can be used as protective films for liquid crystal displays, was prepared by the reaction of ramie
Ind. Eng. Chem. Res., Vol. 48, No. 13, 2009
fiber with acetic anhydride catalyzed by sulfuric acid in acetic acid solution at 55 °C. The structure and properties of the resulting CTA and films made from it were analyzed by IR, UV, NMR, XRD, TGA and DSC, DMA, and titration. It is found that the CTA film shows high transparency and high mechanical properties. Further application of CTA film to LCD and scaling up of the process is under way. Acknowledgment The authors gratefully acknowledge the financial support from the 973 Program of China. (No. 2009CB226105). Literature Cited (1) Shen, Z. B. Prospects of chemical utilization of forest resources in China. Chem. Ind. Forest Prod. 1999, 19 (4), 75–80. (2) Liu, F. H.; Liang, X. N.; Zhang, N. G.; Huang, Y. S.; Zhang, S. W. Effect of growth regulators on yield and fiber quality in ramie (Boemheria nivea (L.) Guad.), China grass. Field Crop Res. 2001, 69, 41–46. (3) Jandura, P.; Riedl, B.; Kokta, B. V. Thermal degradation behavior of cellulose fibers partially esterified with some long chain organic acids. Polym. Degrad. Stab. 2000, 70, 387–394. (4) Kiso, Y.; Kitao, T.; Nishimura, K. Adsorption properties of cyclic compounds on cellulose acetate. J. Appl. Polym. Sci. 1999, 71, 1657–1663. (5) Sun, X. Y.; Guan, Y. T.; Wen, G. Q.; Zhu, B. Y. Study on the characteristics of hemp fibres and applications. J. Textile Res. 2001, 4, 234– 237. (6) Goda, K.; Sreekala, M. S.; Gomes, A.; Kaji, T.; Ohgi, J. Improvement of plant based natural fibers of toughening green composites-effect of load application during mercerization of ramie fibers. Composites Part A: Appl. Sci. Manuf. 2006, 37, 2213–2220. (7) Ha, C. Y. Chemistry of Natural Products and Application, 1st ed.; Chemical Industry Press: Beijing, 2003; p 283.
6215
(8) Francotte, E.; Wolf, R. M. Preparation of chiral building blocks and auxiliaries by chromatography on cellulose triacetate (CTA I): Indications for the presence of multiple interaction sites in CTA I. Chirality 1990, 2, 16–31. (9) Wang, G. H. Satus quo and prospects of LCD use cellulose triacetate film foil development. Image Technol. 2005, 3-4, 15–18. (10) Wang, W. J.; Li, Y.; Men, S.; Shao, Z. Q. Structure and properties of Cellulose Triacetate. Mater. Eng. 2007, 8, 49–52. (11) Filho, G. R.; Monteiro, D. S.; Meireles, C.; da, S.; Assunc¸a˜o, R. M. N. De.; Cerqueira, D. A.; Barud, H. S.; Ribeiro, S. J. L.; Messadeq, Y. Synthesis and characterization of cellulose acetate produced from recycled newspaper. Carbohydr. Polym. 2008, 73, 74–82. (12) Zhang, J. X.; Chen, H. Z. Preparation of cellulose acetate from crop straw. J. Chem. Ind. Eng. 2007, 58 (10), 2548–2553. (13) Saka, S. Process for producing cellulose acetate. CN Patent 93106682.4, 1993. (14) Matsuo, S.; Takamura, T. Process for manufacturing cellulose acetate. US Patent 20040002598, 2004. (15) Atanu, B.; Badal, C. S.; John, W. L.; Shogren, R. L.; Willett, J. L. Process for obtaining cellulose acetate from agricultural by-products. Carbohydr. Polym. 2006, 64, 134–137. (16) Mei, J.; Ou, Y. F.; Chen, J. N. The relationship of distribution of degree of substitution and properties of cellulose acetate. J. Cellulose Sci. Technol. 2002, 10 (1), 12–19. (17) Mei, J.; Ou, Y. F.; Chen, J. N. Studies on polymerization and the properties of cellulose acetate at high temperature. J. Cellulose Sci. Technol. 2000, 8 (1), 9–16. (18) Liu, R. Q. The foundation of cellulose chemistry; Science Press: Beijing, 1985; p 217. (19) He, J. X.; Cui, S. Z.; Wang, S. Y. Preparation and crystalline analysis of high-grade bamboo dissolving pulp for cellulose acetate. J. Appl. Polym. Sci. 2008, 107, 1029–1038.
ReceiVed for reView November 9, 2008 ReVised manuscript receiVed April 19, 2009 Accepted May 5, 2009 IE801703X
