Ind. Eng. Chem. Res. 1997, 36, 2651-2656
2651
Water-Resistant Film from Polyurethane/Nitrocellulose Coating to Regenerated Cellulose Lina Zhang* and Qi Zhou Department of Chemistry, Wuhan University, Wuhan 430072, China
Water-resistant films were obtained from polyurethane (PU)/nitrocellulose coating to regenerated cellulose films, which were prepared by coagulating cellulose cuoxam solution. The PU/ nitrocellulose coating layer was cured at 80 °C for 2 min and formed semi-interpenetrating polymer networks (semi-IPNs) structure. The tensile strength (σb), water resistivity (R), water vapor permeability (P), and light transmittance of the coated films changed with nitrocellulose content in the coating, and the best values (such as σb, 679 kg·cm-2; R, 53%; P, 0.004 g·cm-2·day-1) were attained at 33 wt % nitrocellulose. The TEM, EPMA, DTA, IR, and UV results showed that the coated films have strong interfacial bonding, which is caused by covalent and hydrogen bonds between the cellulose film and the semi-IPNs coating. The biodegradation half-life t1/2 of the coated films in soil at 20-30 °C was given to be 58 days, and after about 6 months the coated films were almost completely decomposed by microorganisms. SEM and the kinetics of decay studied on the biodegradability were discussed. The water-resistant films coated with PU/nitrocellulose have promising application where biodegradation is important. The waste treatment of plastics made from petroleum has become a serious problem because of the difficulty of ensuring reclaimed land and burning in incinerators. Therefore, the development of new plastics that can be degraded by microorganisms in soil and sea water has recently started (Hosokawa et al., 1990). It is well known that cellulose can be broken by microorganisms, and the residues of the degraded products are not harmful to the environment. Thus, it is an earthfriendly material. Recently, cellulose has been reevaluated as a functional material to meet the diverse needs of today’s society because of the unique reactivities and molecular characteristics (Miyamoto, 1995). However, the application of cellulose products is limited by the lack of water-resistivity. Therefore, various waterresistant films and products of cellulose coated with biodegradable coatings such as nitrocellulose (Hagan and Celentano, 1961), chitosan/fine cellulose powder (Hosokawa and Nishiyama, 1990), poly(lactic acid) or its derivatives (Koseki, 1992), etc. have been developed. We have found that after soaking in water for 5 min, the regenerated cellulose film coated with pure nitrocellulose was easily peeled. In the recent work (Zhang et al., 1996b), the regenerated cellulose films coated with polyurethane (PU)/nitrocellulose coating have excellent water-resistivity and could not be peeled after soaking in hot water for more than 24 h. The coating is regarded as semi-interpenetrating polymer networks (semi-IPNs), because only one of the polymers is crosslinked (Hourston and Zia, 1984). In this work, the structure and properties including biodegradability of the coated cellulose films were investigated by using transmission electron microscopy (TEM), electron probe microanalyses (EPMA), scanning electron microscopy (SEM), infrared spectroscopy (IR), ultraviolet spectroscopy (UV), differential thermal analysis (DTA), strength measurement of dry and wet films, and test methods of biodegradation. Experimental Section Preparation of PU/Nitrocellulose Coating. A toluene diisocyanate (TDI) was purchased from Wuhan * Author to whom correspondence is addressed. Phone: +86-27-7882712 Ext. 2455. Fax: +86-27-7882661. S0888-5885(96)00774-9 CCC: $14.00
Chemical Reagent Factory and was redistilled under reduced pressure at 110 °C before use. The PU prepolymer was prepared by reacting trimethylolpropane/ cyclohexanone (1:5 by weight) with the TDI as described in Zhang et al. (1996b). A nitrocellulose (11.5-12.2% of N content) was supplied by Shanghai Dye-chemical Industry Manufactory. An S01-17 solution, which was purchased from Wuhan Double Tigers Coating Co., is composed of castor oil, glycerol, cyclohexanone, etc. The PU/nitrocellulose coating was obtained by mixing the PU prepolymer with nitrocellulose in acetic acid ether and S01-17 solution. Before use, the acetic acid ether as diluting agent, which was 10 times the coating by weight, and dimethylethanolamine as catalyst were added (Zhang et al., 1996b). Film Preparation. The 7% cellulose solution in cuoxam was prepared from cotton linter according to our previous work (Zhang et al., 1994). The solution was spread over a glass plate to give a thickness of 0.3 mm and then placed in coagulation baths of 10% NaOH and of 4% H2SO4 aqueous solution for 2 min, respectively. The regenerated cellulose films obtained were washed in running water, then plasticized with 5% glycerol aqueous solution, and finally dried on a glass plate at room temperature. By using a nonwoven fabric (Bemliese made by Asashi Chemical Industry Co. Ltd.) as a coating tool, the PU/nitrocellulose coating was made on the surface of the dried regenerated cellulose film and then cured at 80 °C for 2 min. A series of the films coded as U10, UNC1, UNC2, UNC3, UNC4, UNC5, UNC6, and UNC7 are corresponding to nitrocellulose content (WNC) in PU/nitrocellulose of 0, 11, 14, 20, 25, 33, 50, and 67 wt %, respectively. The uncoated regenerated cellulose film was coded as RC0. Characterization of Films. IR spectra of the films were recorded with a Nicolet 170 SX FT-IR spectrometer. TEM was performed by using a JEM 100-XT electron microscope operated at 100 kV. Prior to examination, the films were hardened and stained in the solution of osmium tetroxide for 72 h and then embedded in Epon 812. Ultrathin sections were obtained by sectioning on LKB-2088V Ultratome and then stained with uranyl acetate to enhance contrast. EPMA was done by using Electron Probe Microanalyzer JXA8800R (JEOL Superprobe). The film with electrocon© 1997 American Chemical Society
2652 Ind. Eng. Chem. Res., Vol. 36, No. 7, 1997
ductive adhesive was put between two steel sheets; then the section, which was cut from the film perpendicularly to its surface, was coated with carbon. The twodimensional distributions of nitrogen on the section of the film were analyzed by using two different detectors with an electron probe X-ray wave spectrograph and X-ray energy spectrograph. DTA was performed using the Thermal Analyzer DT-30B (Shimadzu Co.). The film was cut to 1 mm length and 1 mm width, then analyzed under a nitrogen atmosphere from 0 to 500 °C at a heating rate of 15 °C/min. Measurements of Properties. The tensile strength (σb) and breaking elongation (b) of the films in dry and wet (soaking in water for 1 h) states were measured on an electronic strength tester XLD-0.1 (The Second Tensile Testing Machine Manufacture of Changchun, China) according to the Chinese standard method (GB 4456-84). The water-resistivity (R) of the films was evaluated from σb(dry) and b(wet) by the following equation:
R ) (σb(wet)/σb(dry)) × 100%
Figure 1. DTA curves of the films of RC0, U10, UNC5, and nitrocellulose.
(1)
The water vapor permeability (P) of the dry films was measured under certain conditions, where the temperature, pressure, and relative humidity are 25 °C, 1 atm, and 80%, respectively, and evaluated as follows:
P ) (Wt - W0)/10S
(2)
where W0 and Wt(g) are the weight of the 500 mL beaker having water sealed by the film at the beginning and on the 11th day, respectively; S is the effective area of the film (cm2). Percent light transmittance (Tr) of the films in the wavelengths of 400 and 800 nm was measured by using a Shimadzu UV-160A spectroscope. Test Methods of Biodegradation. Natural soil was used as the biodegradation environment. Ten test films (15 × 15 cm2) enclosed separately in nylon mesh netting (2 × 2 mm2 mesh size) were buried about 10 cm beneath the soil on our campus. The average values of temperature, moisture, and the pH of the soil measured are 25 °C, 30%, and 6.8, respectively. From 10 to 180 days after burying, the degraded films and fragments were taken out one after another, then rinsed with water, and dried in vacuum at room temperature for at least 1 day before the physico-chemical characterization. The tensile strength (σb) and weight loss Wloss (%) of the degraded films were measured according to previous methods (Zhang et al., 1996a). The degradation rate constant k and half-life t1/2 were estimated from double-logarithm plots of weight loss against burying period (t) in the soil. Scanning electron micrographs of the degraded films were taken with a Hitachi S-570 SEM. The films were coated with gold, and subsequently their surfaces were observed and photographed. Results and Discussion Analysis of Interfacial Structure. In the FT-IR spectrum of the coated film UNC5, the absorption band at 2856 cm-1 (symmetrical stretching of the methylene group in the cured coating) has disappeared, and the intensity of the band at 2272 cm-1 (-NdCdO) markedly decreased. Moreover, a strong absorption band has appeared at 1709 cm-1, which is absent in the regenerated cellulose (Ritcey and Gray, 1988; Zhang et al., 1991). It is regarded that the band at 1709 cm-1
Figure 2. TEM of the coated films of U10 (A) and UNC5 (B).
resulted from superposition of a new band (1700 cm-1) of the ester bonds formed by cellulose with the PU prepolymer in the coating and the band at 1723 cm-1 for the coating layer. It suggests that the PU prepolymer molecules penetrated into the cellulose, and the esterification took place. Comparing with the uncoated cellulose film, the -OH stretching band at 3100-3500 cm-1 for the film UNC5 has broadened, indicating formation of the hydrogen bonds between the groups of cellulose and the coating. The DTA curves of the nitrocellulose and films of RC0, U10, and UNC5 are shown in Figure 1. In the curve of the film RC0, an endothermic process at 342 °C was observed, exhibiting that cellulose degrades completely by the loss of -OH groups and by the breakdown of the pyranosic rings (Shukla et al., 1995). The DTA curve of the film UNC5 showed that the endothermic peak corresponding to the temperature of decomposition of cellulose was shifted toward a higher temperature (350 °C), imparting the increase of thermal stability. Moreover, an additional exothermic peak (209 °C) attributed to nitrocellulose in the coating was observed for the film UNC5. The enhancement in thermal stability of the coated film arose from the covalent bonds between cellulose and the coating, similar to results of thermal properties of glycidyl methacrylate grafted cellulose (Shukla et al., 1995). Figure 2 shows TEM photographs of the section of the films U10 (A) and UNC5 (B). The dark regions in the middle are the PU/nitrocellulose coating layer 1.7 µm deep, and the bottom is cellulose. From the film UNC5, it was observed that the interface between the coating layer and epon (top in Figure 2B) is distinct, but the interface between the coating and cellulose is indistinct. By the way, the interface between the coating and cellulose of the film UNC5 is more indistinct than that of the U10. The indistinct and broad interface of the
Ind. Eng. Chem. Res., Vol. 36, No. 7, 1997 2653
Figure 3. Two-dimensional distribution of N on the section of the film UNC5 by using an electron probe X-ray wave spectrograph.
Figure 4. Two-dimensional distribution of N on the section of the film UNC5 by using an X-ray energy spectrograph: SE, secondary electron images; N, nitrogen; K, KR.
film UNC5 is regarded as the penetration of the PU/ nitrocellulose coating across the interface to cellulose due to good penetrability and miscibility of the coating with cellulose. The two-dimensional distributions of nitrogen and the secondary electron images on the cross section of the film UNC5 analyzed by the electron probe X-ray wave spectrograph and X-ray energy spectrograph are shown in Figures 3 and 4, respectively. As shown from the top figure in Figure 3 and the right in Figure 4, the nitrogen from the coating appeared everywhere in the section, suggesting the diffusion of the coating to the cellulose. The bottom figure in Figure 3 and the left in Figure 4 are the secondary electron images. The bright bands on both sides of the film are protuberances of the surface layers and reflect a fixed structure in the coated film due to the coating of more than the middle section, in which the concave structure is attributed to the cellulose contraction. The thickness of the coated
Figure 5. Nitrocellulose content (WNC) dependence of the tensile strength (σb) of the films in the dry (O) and wet (b) states.
film is ca. 32 µm. Each coating layer of the coated films was 1.7 µm deep according to TEM results, and it was remarkably narrower than the protuberance region from EPMA. It further indicated that diffusion of the PU/ nitrocellulose coating to the cellulose took place and, in addition that the nitrocellulose and cellulose are very miscible and wettable. These results imply that while semi-interpenetrating polymer networks form in the coating, simultaneously the PU prepolymer molecules penetrate into the regenerated cellulose film and give rise to a shared network (Jia et al., 1994) crosslinked with the cellulose and nitrocellulose. In view of this structure, the films coated with PU/nitrocellulose have strong interfacial bonding and excellent water-resistivity.
2654 Ind. Eng. Chem. Res., Vol. 36, No. 7, 1997
Figure 6. Nitrocellulose content (WNC) dependence of the breaking elongation (b) of the coated films in the dry (O) and wet (b) states.
Figure 9. Plot of degradation time (t) against weight loss (Wloss) of the film UNC5 in soil at 20-30 °C.
Table 1. Mechanical Properties (σb, Eb), Water Resistivity (R), Water Vapor Permeability (P), and Biodegradarion Half-Life (t1/2) of the Films film RC0 U10 UNC4 UNC5 a
σb (kg‚cm-2) dry wet
b (%) dry wet
R (%)
102P (g‚cm-2‚day-1)
t1/2 (days)
551 676 667 679
24 12 33 34
32 37 57 53
2.84 0.86 0.55 0.40
30a
179 248 377 357
75 43 45 48
58
From Zhang et al., 1996a.
Figure 10. Degradation time (t) dependence of tensile strength (σb) of the film UNC5 in soil at 20-30 °C.
Figure 7. Nitrocellulose content (WNC) dependence of the water vapour permeability (P) of the coated films at 25 °C.
Figure 8. Nitrocellulose content (WNC) dependence of the optical transmittance of the coated films: 400 nm (O); 800 nm (b).
Effect of Nitrocellulose Content on Mechanical Properties. Figure 5 shows the nitrocellulose content (WNC) dependence of the tensile strength (σb) of the
coated films in the dry and wet states. The strengths of the dry films essentially did not change with the nitrocellulose content until 33 wt % of WNC and then decrease slowly. The strengths of the wet films display a maximum point (380 kg‚cm-2) for a nitrocellulose content of 25-33 wt %, suggesting that the nitrocellulose plays a role in increasing water-resistivity of the film. However, in the case of low PU prepolymer content, the strengths of both the dry and wet films decreased obviously. Figure 6 shows that the breaking elongations (b) of the coated films in the dry and wet states increase with the nitrocellulose content until 33 wt % of WNC and then decrease slowly. Interestingly, the nitrocellulose is more rigid than PU, but the toughness of the films coated with PU/nitrocellulose is higher than one of the films coated with only PU prepolymer. It can be explained by the formation of the semi-IPNs structure between PU and nitrocellulose and of the shared network between PU, nitrocellulose, and cellulose. In addition, Table 1 indicates that the mechanical properties of the coated films are significantly superior to that of regenerated cellulose film uncoated. Effect of Nitrocellulose Content on Water Resistivity. Figure 7 shows the nitrocellulose content (WNC) dependence of water vapour permeabilities (P) of the coated films. With increasing nitrocellulose content, the P decreased until 33 wt % of WNC and then increased. It was observed that the films of UNC4 and UNC5 coated with 25 and 33 wt % of WNC have excellent water resistivity. However, the excess of the nitrocellulose in the coating, such as film UNC7, has resulted in a microphase-separated structure of the coating layer to give great permeability (Zhang et al., 1995). Furthermore, the water-resistivity (R) of the coated films
Ind. Eng. Chem. Res., Vol. 36, No. 7, 1997 2655
Figure 11. SEM of coated film UNC5 degraded in soil for 24 days (B) and 55 days (C) and the original (A).
UNC4 and UNC5 (in Table 1) are far higher than those of the uncoated film RC0 and the U10 coated with pure PU. These results indicated that a certain content of the nitrocellulose in the coating can significantly improve the water resistivity and toughness of the coated film. Effect of Nitrocellulose Content on Light Transmittance. Figure 8 shows the percent light transmittance (Tr) of the coated films. The Tr values keep constant almost with the increase of nitrocellulose content until 33% of WNC and then decrease. Normally, the interface between two materials will cause losses in optical transmission because of the quantity of light, which is scattered and reflected at the interface between dissimilar solid materials (Waever et al., 1995). The good optical transmission of the coated films suggests the strongest interfacial bonding between the cellulose film and coating. The result is in good agreement with the conclusion from analysis of interfacial structure. However, when the nitrocellulose content is more than 40 wt %, the mechanical properties, water-resistivity, and optical transmission of the coated films decreased, suggesting poor networks due to the lack of PU prepolymer. Biodegradation of Coated Films. Biodegradation rate constant k and the half-life t1/2 of the coated film UNC5 in soil were obtained from double-logarithm plots of weight loss (Wloss) against burying period (t) in the soil (Figure 9) as follows:
Conclusions The regenerated cellulose films coated with PU/ nitrocellulose have excellent dry and wet mechanical properties, water-resistivity, light transmittance, and biodegradability. The certain content (25-33 wt % of WNC) of nitrocellulose in the coating is beneficial not only to the enhancement of the strength, toughness, and water-resistivity but also to the environmental protection due to its biodegradability. The PU prepolymer in the coating plays an important role in forming the semiIPN structure of the coating layer and the shared network crosslinked with the cellulose in the film and nitrocellulose in the coating, which results in the strong interfacial bonding between the regenerated cellulose film and the coating. The water-resistant films or cellulose container coated with the PU/nitrocellulose have promising application where biodegradation is important. Acknowledgment This subject was supported by State Economy and Trade Commission of China. We thank Dr. F. Chen of Wuhan Steel and Ion Research Institute, Dr. Y. Chen of Hubei Medical University, as well as Prof. Y. Hu and Miss L. Zhu from College of Life Science at our University for their assistance with EPMA, SEM, and TEM, respectively.
log Wloss ) 0.73 log t + 0.4135 where k ) 0.73 and t1/2 was calculated to be 58 days. From the extrapolation of the plot, the complete decomposing time is about 6 months. The t1/2 of the coated film UNC5 is ca. 28 days longer than that of pure regenerated cellulose films RC0, as shown in Table 1, and the k is smaller than that of RC0 (Zhang et al., 1996a). The degradation time dependence of σb of the coated film UNC5 in soil is shown in Figure 10. The σb of the coated film decreased sharply with the progress of degradation time, similarly to that of the pure regenerated cellulose film (Zhang et al., 1996a). SEM photographs of the film UNC5 biodegraded in the soil for 24 days (B) and 55 days (C) and the original (A) are shown in Figure 11. A decayed structure with fungal mycelia on the surface of the films (B and C) was observed. These results indicate that the regenerated cellulose films coated with PU/nitrocellulose coating are biodegradable, and after about 6 months the coated films were almost completely decomposed by microorganisms in soil.
Literature Cited Hagan, L.; Celentano, V. D. Method of anchoring a nitrocellulose base surface coating to a cellulose sheet. U.S. Partent 3011910, Dec. 5, 1961. Hosokawa, J.; Nishiyama, M. Biodegradable water-resistant coating materials containing cellulosic fibers. JP 02,127,486, 1990. Hosokawa, J.; Nishiyama, M.; Yoshihara, K.; Kubo, T. Biodegradable film derived from chitosan and cellulose. Ind. Eng. Chem. Res. 1990, 28, 800. Hourston, D. J.; Zia, Y. Semi- and fully interpenetrating polymer networks based on Polyurethane-polyacrylate systems. IV. Grafted semi-1-Interpenetrating Polymer networks. J. Appl. Polym. Sci. 1984, 29, 629. Jia, D.; Pang, Y.; Liang, X. Mechanism of adhesion of polyurethane/polymethacrylate simultaneous interpenetrating networks adhesives to polymer substrates. J. Polym. Sci., Part B: Polym. Phys. 1994, 32, 817. Koseki, H. Biologically degradable water- and oil-resistant paper for packaging. JP 04,334,448, 1992. Miyamoto, T. Future development of cellulose chemistry and technology. Kinoshi Kenkyu Kaishi 1995, 34, 2.
2656 Ind. Eng. Chem. Res., Vol. 36, No. 7, 1997 Ritcey, A. M.; Gray, D. G. Cholesteric order in gels and films of regenerated cellulose. Biopolymers 1988, 27, 1363. Shukla, S. R.; Athalye, A. R. Mechanical and thermal properties of glycidyl methacrylate graffed cotton cellulose. J. Appl. Polym. Sci. 1995, 57, 983. Waever, K. D.; Stoffer, J. O.; Day, D. E. Interfacial bonding and optical transmission for transparent fiberglass/poly(methyl methaerylate) composites. Polym. Compos. 1995, 16, 161. Zhang, L.; Yang, G.; Fang, W. Regenerated cellulose membrane from cuoxam/Zincoxene blend. J. Membr. Sci. 1991, 56, 207. Zhang, L.; Yang, G.; Yan, S. Manufacture of biodegradable regenerated cellulose films with high water resistance and high strength. Chinese Patent CN 1,091,144A (Cl.C08L97/02), 1994. Zhang, L.; Yang, G.; Xiao, L. Blend membranes of cellulose cuoxam/casein. J. Membr. Sci. 1995, 103, 65.
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Received for review December 5, 1996 Revised manuscript received March 27, 1997 Resubmitted for review April 2, 1997X IE960774H X Abstract published in Advance ACS Abstracts, June 1, 1997.
