Abstract
Commercial cellulose diacetate with a degree of substitution (DS) of 2.45 was partly deacetylated to cellulose acetate (CA) with different DSs by acid-catalyzed hydrolysis and then reacted with 1-naphthoyl chloride (NpCl) to synthesize CA naphthoate (CANp). Fourier transform infrared and 1H-NMR were used to characterize the chemical structure of CANp. The DS of naphthoate moiety (DSCANp) could be varied from 0.18 to 0.98 by adjusting the molar ratio of –OH in CA unit to NpCl, the DS of CA (DSCA), and the reaction time and temperature. When DSCA was 2.01 and the molar ratio was 1:6, the maximum DSCANp of the product was achieved after a reaction at 80°C for 2 h. With the increase of DSCANp, the thermal stability decreased slightly whereas the anti-ultraviolet property was enhanced. Moreover, the obtained films containing CANp exhibited good ultraviolet resistance as well as chemical resistance.
1 Introduction
Cellulose acetate (CA) prepared by acetylating cellulose is one of the most important cellulose derivatives. Because of its wide range of desirable properties (1), (2), (3), it has been used in modern coating, controlled release, optical film, and membrane as well as traditional textile fields in the forms of fiber, film, and plastic (4), (5), (6). However, it is still very essential to modify CA for its wider applications considering its weaknesses such as poor chemical resistance, low mechanical strength, low thermal resistance, compaction phenomena, and poor ultraviolet (UV) resistance (7), (8), (9), (10), (11), (12), (13). As we all know, cellulose acetate is readily dissolved under acid or alkaline environments, thus greatly limiting its applications. In view of this, many scholars devote themselves to overcoming the poor chemical resistance. For example, Liu et al. (14) blended chitosan (CS) into CA to prepare CA-CS membranes for improving the acid-base range to be consistent with the content of chitosan. By contrast, UV irradiation has been known as one of the most frequent abiotic reactions in the natural environment, and it can cause damages to organic materials like plastics, woods, or polymers in some products (15), (16). Direct exposure of the materials to climatic conditions of sunlight may significantly decrease the lifetime of these products. Hence, much effort has been attempted to improve the UV resistance of CA. Typically, metal oxide nanoparticles such as TiO2, ZnO, ZnS, and CeO2 can be blended into CA to obtain hybrid nanocomposites with UV-absorbing property (17), (18). However, these nanoparticles may induce photodegradation because of their photocatalytic activity (16). Jahan et al. (19), considering the noncatalytic property and good absorbance for UV radiation of graphene oxide, embedded graphene oxide into CA matrix to form a CA/graphene oxide nanocomposite film for improving the UV resistance of CA (20). Unfortunately, similar to most nanoparticles blended with polymers, graphene oxide is prone to falling off.
As one of the most important industry polycyclic aromatic hydrocarbons, naphthalene is commonly used in the synthesis of phthalic anhydride, dye intermediates, rubber additives, and pesticides. Naphthalene belongs to a photosensitive group and can be incorporated into polymer chains for good applications in photovoltaic components (21), (22). Meanwhile, naphthalene is a bicyclic aromatic molecule with six-membered ring, and the polymer containing naphthalene can possess good chemical resistance because of its lower ring strain (23), (24), (25). In addition, introducing naphthalene nucleus, which is chromophorics, into cellulose acetate chains is beneficial to the absorption of UV light (26), (27). However, to the best of our knowledge, previous research has seldom focused on improving CA characteristics by incorporating naphthalene moiety.
In this paper, commercial cellulose diacetate (CDA) was partly deacetylated to CA with different degrees of substitution (DSs) by acid-catalyzed hydrolysis. Then cellulose acetate naphthoates (CANps) with different DSCANp were synthesized homogeneously in pyridine. The chemical structures of CANps were characterized, and their thermal property and UV resistance were evaluated. Finally, the films containing CANp with good UV and chemical resistance were successfully prepared.
2 Materials and methods
2.1 Materials
Cellulose diacetate (CDA) with a DS of 2.45, supplied by Nantong Cellulose Fibers Co., Ltd., was dried to less than 1% moisture before use. 1-Naphthoyl chloride (NpCl) (>99%) was purchased from Aladdin Reagent Co., Ltd. Pyridine was distilled under reduced pressure and stored over molecular sieves. N,N-dimethylformamide (DMF), ethanol (EtOH), glacial acetic acid (HAc), concentrated sulfuric acid (H2SO4), anhydrous sodium acetate (CH3COONa), sodium hydroxide (NaOH), and hydrochloric acid (HCl) were obtained from Kermel Chemical Co., Ltd. All reagents were analytically pure and used without further purification.
2.2 Preparation of CA with different DSs through hydrolysis
CDA (3.0 g) and HAc (13.5 g) were mixed in a 100-ml round bottom flask with a reflux condenser. The mixture was heated to 60°C under continuous agitation in an oil bath and preserved for 20 min to dissolve CDA. Then 0.24 g of H2SO4 and 1.2 g of water were successively added dropwise to the flask to avoid degradation of CDA and precipitation phenomenon. After several hours (2, 4, 6, 8, and 12 h), a proper amount of CH3COONa was respectively added to terminate the reaction. The mixtures were separated by precipitating using excess deionized water and then filtered and washed repeatedly until neutral. Afterward, the precipitates were dried under vacuum at 60°C for 24 h. The DSCA was determined according to standard ASTM D 871-96.
2.3 Synthesis of CANp
One gram of CA was dissolved in 15 g of pyridine with constant mechanical stirring at 80°C in an oil bath. Then NpCl was added proportionally to trigger the reaction. After 2 h, the reaction solution was poured into 200 ml of EtOH to cause the formation of precipitates. The precipitates were collected by filtration and then washed sequentially with EtOH (2×100 ml) and distilled water (2×100 ml). After that, the precipitates were purified by Soxhlet extraction with EtOH for 24 h. Finally, the product was dried under vacuum at 80°C for 12 h.
2.4 Fabrication of CDA, CANp, and CANp/CDA films
The films were prepared by solution casting. CANp (4 wt%) (DSCANp=0.42) and CDA (11 wt%) were dissolved in DMF by vigorous stirring at room temperature until a clear and homogeneous solution was obtained. After standing degassing for 4 h, the solution was cast on a dry, smooth glass plate using a cast knife open at 500 μm. The glass plate was then immersed in ice water bath so that the solution was coagulated to form films. The films were peeled off from the substrate and freeze-dried to remove any residual solvent. For comparison, pure CANp and CDA films were also prepared by the same way.
2.5 Chemical resistance test
Chemical resistance of the film was determined by weight loss (%) resulting from the contact between the film and the acid or alkali of a certain concentration. Preweighed CDA, CANp/CDA, and CANp films were immersed in HCl solution of 5 mol/l for 6 h and NaOH solution of 1 mol/l for 1 h sequentially, and then weighed again at room temperature. The weight loss can be calculated using the following equation:
where Wi is the weight (g) of the film before contact with solvents, and Wc is the weight after contact.
2.6 Characterization
The molecular weights were determined on a VISCOTEK270 gel permeation chromatograph (GPC, mobile phase: tetrahydrofuran (THF); concentration: 4 mg/ml).
Fourier transform infrared (FTIR) spectra were recorded with a Bruker TENSOR37 FTIR spectrometer (Bruker Optics, Ettlingen, Germany) from 4000 to 400 cm−1 using the KBr pellet technique.
1H-NMR spectra were obtained from a Bruker-AV 400 spectrometer (Bruker Optics, Ettlingen, Germany) (500 MHz, TMS as internal standard, room temperature, solvent: DMSO-d6). The DSCANp values can be calculated from the following formula:
where INP and IAGU denote the hydrogen peak areas of naphthoate moiety and CA unit, respectively.
Differential scanning calorimetry (DSC) was performed on NETZSH DSC 204 F1 Phoenix® (NETZSCH-Gerätebau GmbH, Selb, Germany) at a heating rate of 10°C/min in a temperature range of 20–300°C under nitrogen atmosphere. Thermogravimetric analysis (TGA) was performed on a TGA Q5000 IR thermogravimetric analyzer (TA Instruments Co. Ltd., USA) at a heating rate of 10°C/min from 20°C to 600°C under nitrogen atmosphere.
UV-Vis spectra were recorded by a Perkin Elmer Lambda35 UV-Vis spectrophotometer (Olympus Optical Co. Ltd., Japan) (200–400 nm; solvent: THF; concentration: 100 mg/l).
Tensile strength of the films was measured with an LLY-06E/PC electronic monofilament strength tester (Lai Zhou Electron Instrument Co. Ltd., Shandong, China) at room temperature. Rectangular samples (20 mm×10 mm) were prepared from each film and then measured with a gauge length of 10 mm at an extension speed of 10 mm/min.
3 Results and discussion
3.1 Hydrolysis
Cellulose and its derivatives can be functionalized by chemical reaction at the –OH groups. However, up to now, products and their property design by chemical modification are limited to certain esters and ethers, especially for CA. It is noted that there are only a low number of –OH groups in CDA so that it is difficult for CDA to react with other reagents. Hence, CDA is often modified through hydrolysis to endow it with higher reactivity and thus realize property improvement.
The main purpose of hydrolysis, including acid hydrolysis and alkaline hydrolysis (28), is to remove acetyl groups from CDA, thereby obtaining CA with different DSs. However, the products through alkaline hydrolysis cannot dissolve in conventional solvents completely because of the heterogeneous reaction that may lead to uneven distribution of substituents, which is adverse to cellulose acetate modification. Fortunately, these defects can be reduced considerably by acid hydrolysis, which is a homogeneous reaction process. In this study, acid hydrolysis process was adopted, and CA with different DSs was prepared by altering hydrolysis time given other conditions unchanged. As shown in Figure 1, the DS of CA (DSCA) in the range from 2.33 to 2.00 can be achieved through the acid hydrolysis process. The DSCA of hydrolyzed CA samples was determined by titration and then plotted against the reaction time. As can be seen, the DSCA decreases from 2.33 to 2.01 during the reaction time 2–8 h and then shows no significant decrease with further extension of reaction time to 12 h. This is probably due to the formation of HAc during the reaction, which reduced the concentration of concentrated sulfuric acid and thus decreased the reaction rate. These results are economically favorable because this process decreases energy cost and saves reaction time. In addition, it is essential to reduce reaction time because low DS values can generate products with more potential for degradation (29).
A relationship between DSCA of hydrolysis time.
The molecular weights of CAs with different DSCA by hydrolysis were measured with GPC. The results given in Table 1 show that Mn and Mw all decrease with the decrease of DSCA values. It’s worth noting that the degradation phenomenon was serious especially for the DSCA value of 2.00 that hydrolyzed for 12 h.
The molecular weights of different DSCA cellulose acetate.
| DSCA | 2.45 | 2.33 | 2.29 | 2.15 | 2.01 | 2.00 |
|---|---|---|---|---|---|---|
| M̅n | 16,222 | 14,625 | 14,315 | 11,753 | 8557 | 5827 |
| M̅w | 79,282 | 73,944 | 68,297 | 45,778 | 33,053 | 18,623 |
| M̅w/M̅n | 4.887 | 5.056 | 4.771 | 3.895 | 3.863 | 3.196 |
3.2 Synthesis
Esterification of CAs (DSCA=2.45~2.01) with NpCl can produce CANp with DSCANp from 0.18 to 0.98. The synthetic route is illustrated in Figure 2, and detailed reaction conditions and results are given in Table 2. For samples A1–A3, the molar ratio of –OH groups in CA unit to NpCl varied from 1:2 to 1:6, and the DSCANp of the products increased from 0.18 to 0.42. For samples A8–A10 and A11–A13, the esterification of CAs with different DSCAvalues was conducted under the same reaction conditions, and their DSCANp showed the similar increase trend. These results indicate that the DSCANp of CANps increases with the decrease of the molar ratio of –OH groups in CA unit to NpCl and finally reaches a maximum DSCANp at the molar ratio of 1:6. This increase is probably due to the greater availability of 1-naphthoyl ions with higher concentrations in the proximity of CAs. In addition, by comparing the DSCANp of A3, A10, and A13, which were synthesized under the same conditions except for the initial DSCA, it can be observed that higher DSCA can further inhibit the esterification and thus lower the DSCANp of the products. This phenomenon can be attributed to the decrease of the amount of active hydroxyl as well as the increase of stereo-hindrance of –OH from the high number of acetate groups.
The synthesis route of CANp.
Conditions and results of homogeneous reaction of CA with NpCl.
| No. | Reaction conditions | ||||
|---|---|---|---|---|---|
| DSCA | Molar ratioa | Time (h) | Temperature (°C) | DSCANp 1H-NMR | |
| A1 | 2.45 | 1:2 | 2 | 80 | 0.18 |
| A2 | 2.45 | 1:4 | 2 | 80 | 0.34 |
| A3 | 2.45 | 1:6 | 2 | 80 | 0.42 |
| A4 | 2.45 | 1:6 | 1 | 80 | 0.30 |
| A5 | 2.45 | 1:6 | 3 | 80 | 0.44 |
| A6 | 2.45 | 1:6 | 2 | 70 | 0.33 |
| A7 | 2.45 | 1:6 | 2 | 90 | 0.39 |
| A8 | 2.15 | 1:2 | 2 | 80 | 0.57 |
| A9 | 2.15 | 1:4 | 2 | 80 | 0.67 |
| A10 | 2.15 | 1:6 | 2 | 80 | 0.70 |
| A11 | 2.01 | 1:2 | 2 | 80 | 0.64 |
| A12 | 2.01 | 1:4 | 2 | 80 | 0.97 |
| A13 | 2.01 | 1:6 | 2 | 80 | 0.98 |
aMolar ratio of –OH of CA unit to NpCl.
The effects of reaction temperature and time on the DSCANp values were also investigated in this study. For samples A4, A3, and A5, with the increase of reaction time, the DSCANp value increased from 0.30 to 0.44 because prolonging reaction time resulted in favorable effect on the diffusion of reactants, thereby inducing better contact between etherifying agent and CA (29), (30). For samples A6, A3, and A7, the DSCANp value firstly increased from 0.33 to 0.42 with the increase of reaction temperature from 70°C to 80°C, and then decreased to 0.39 with further temperature increase to 90°C. The enhancement of esterification by raising temperature is probably due to the favorable effect of temperature on compatibility of reaction ingredients (31). However, owing to the hydrolysis mechanism (32), the observed DSCANp presents a slight decrease at 90°C.
3.3 Structural characterization
Figure 3 shows the FTIR spectra of CA and CANp. As can be seen in the spectra of all CANp samples, the peaks at 3475 and 2954 cm−1 are attributed to the absorption of –OH and C–H stretching vibrations, respectively, and the peak at 1749 cm−1 corresponds to the C=O stretching band of the ester carbonyl group. Because the carbonyl peak of the resultant acyl naphthalene is overlapped with that of original acetyl, there is only one carbonyl absorption peak in the spectra. Moreover, the peak intensity of C=O stretching band increases with increase of DSCANp. Compared with the spectrum of CA, typical signals of naphthalene appear at 1593, 1576, and 1512 cm−1 (24), (33). Furthermore, the absorption band at 783 cm−1 is related to C–H wagging vibrations of naphthalene ring. Other characteristic peaks are quite similar to those of CA (34). These results indicate that NpCl smoothly reacted with CA.
FTIR spectra of CA and CANp.
To further confirm the structure of CANp, 1H-NMR spectroscopy was conducted on a sample dissolved in DMSO-d6, and the results are shown in Figure 4. As can be seen, the chemical shifts in the range between 3.5 and 5.5 ppm are attributed to the protons (H-1~H-6) of cellulose acetate backbone in the spectrum of CANp, which has the same chemical shifts as that of CA, and the peaks in the range between 7.2 and 8.8 ppm can be assigned to the naphthalene protons (H-9~H-15) of CANp. These results further confirm that CA-mixed esters have been successfully synthesized.
1H-NMR spectra of CA and CANp (CANp-0.57).
3.4 Thermal stability
The thermal stabilities of pure CDA and CANp were measured by DSC and TGA. As Figure 5A shows, the melting temperature (Tm) of CANp decreases obviously relative to that of pure CDA, indicating that the introduction of naphthoic acid groups into cellulose acetate backbone weakens the strength of its inter- and intramolecular hydrogen bond. By contrast, the introduction of naphthalene with bulky structures is conducive to improving the thermal property by increasing the crosslinking density of polymers (33), (35). As a result, the Tm of CANp decreases slightly with the increase of DSCANp. By contrast, the introduction of naphthalene has no significant effect on the mechanical property of CANp films, which can be confirmed by the UV resistance test results before exposure to UV light.
DSC curves (A) and TGA curves (B) of CANp.
Figure 5B displays the TGA curves of pure CDA and CANp. It can be found that the onset decomposition temperature of CANp with different DSCANp of 0.42 and 0.98 occurs at 295°C and 312°C, respectively, a little lower than the onset decomposition temperature of CDA (316°C). These results demonstrate that the thermal stability of CANp decreases to some extent compared to that of unmodified CA.
3.5 UV resistance
The anti-UV property of CANp was investigated through UV-Vis absorption spectra, as Figure 6 shows. The absorption peaks at 245, 298, and 324 nm are similar to those of naphthalene (36), (37), (38). The increase of the naphthalene substituent from 0.42 to 0.98 results in a sharp increase of absorbance at 298 nm from 1.39 to 2.67. It can be concluded that the anti-UV property of CANp is significantly improved with the increase of DSCANp.
UV-Vis absorption spectra of CANp (concentration of CANp in THF: 100 mg/l).
Common polymers always undergo losses in mechanical strength when exposed to UV light (39). Hence, the UV resistance of CDA, CANp/CDA, and CANp films can also be evaluated via the variation of tensile strength using UV lamps (400 W, 200−400 nm) equipped in a lab-made box. The samples were fixed at the same position in the box for 0, 2, 4, 6, and 8 days, respectively. After exposure for preset time duration, the samples were removed from UV lamps, and their mechanical properties were tested. Figure 7 shows the effect of UV exposure time on the tensile strength of CDA, CANp/CDA, and CANp films. Obviously, pure CDA film is very vulnerable to UV light, and its tensile strength decreases approximately from 2.4 to 0.8 MPa after an 8-day irradiation. By contrast, the tensile strength of pure CANp film remains almost unchanged while CANp/CDA film also decreases after UV exposure. These results suggest that naphthalene moiety in cellulose acetate molecules is considerably effective for improving the UV resistance of films.
A correlation of tensile strength of CDA, CANp/CDA and CANp films versus the UV irradiation time.
3.6 Chemical resistance
Figure 8 shows the weight loss (%) of CDA, CANp/CDA, and CANp films after immersion in HCl and NaOH solutions for preset time duration. As can be seen, there is no weight loss (%) in HCl solution for CANp film, and its weight loss (%) in NaOH solution is approximately 6.25%; CANp/CDA film shows a measurable weight loss of 5.88% and 47.06% in HCl and NaOH solutions, respectively; by contrast, the weight loss of CDA film reaches up to 25% and 60% in HCl and NaOH solutions, respectively. These results mean that naphthalene ring groups that possess space shielding effect can hinder the attack to ester groups from the acid or alkaline medium (23), (40), thus enhancing the chemical resistance of films. It is hopeful that such good chemical resistance of CANp films can expand its application fields.
Chemical resistance of CDA, CANp/CDA, and CANp films in 5 mol/l HCl and 1 mol/l NaOH solutions.
4 Conclusions
In summary, CAs with different DSs were prepared by partial acid-catalyzed hydrolysis, and CANps were successfully synthesized by homogeneous reaction with NpCl in pyridine. The chemical structures of CA-mixed esters were characterized by FTIR and 1H-NMR, and the DSCANp was controlled by altering the reaction parameters such as the molar ratio of reactants, the DS of CA, and the reaction time and temperature. According to the DSC, TGA, and UV-Vis results, the thermal stability of CANp was just mildly decreased, but the mechanical strength of CANp was not remarkably affected, whereas the anti-UV property was improved significantly. Meanwhile, the prepared films containing CANp were endowed with good UV and chemical resistance. The novel CA-mixed ester synthesized in this study can potentially be used in textiles, medicines, plastic films, and filtration industries, which will greatly broaden the application fields of CA materials.
Acknowledgments
The authors are thankful to the financial support from Nantong Cellulose Fibers Co., Ltd. (No. R-CNC162017).
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