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Structural study of oriental lacquer films during the hardening process
Talanta 70 (2006) 146–152
Structural study of oriental lacquer films during the hardening process Noriyasu Niimura a,∗ , Tetsuo Miyakoshi b a
International Technical and Training Center, JEOL DATUM Ltd., 1156 Nakagami-cho Akishima-shi, Tokyo 196-0022, Japan b Department of Industrial Chemistry, Meiji University, Higashimita Tama-ku, Kawasaki-shi 214-8571, Japan Received 3 October 2005; received in revised form 14 December 2005; accepted 16 December 2005 Available online 24 January 2006
Abstract Oriental lacquer is the natural resin obtained by tapping lac trees. It hardens into a tough and insoluble film. The extreme hardness and insolubility are some of the most important functions, which are required for industrial coating materials. In this study, two kinds of oriental lacquer films, traditionally named Kiurushi (raw urushi) and Kuromeurushi produced by two different pretreatments, were analyzed during hardening with Fourier transform infrared spectroscopy (FT-IR), thermogravimetry/differential thermal analysis–mass spectrometry (TG/DTA–MS) and pyrolysis–gas chromatography/mass spectrometry (Py–GC/MS) to investigate their functional expression process. Typical functional groups of the lacquer films were detected by FT-IR. The TG/DTA–MS curves clarified that the thermal degradation of the lacquer films gradually began at around 200 ◦ C, and reached the fastest rate at 400–500 ◦ C. Apparently, FT-IR and TG/DTA–MS could not reveal any difference between the films. On the other hand, Py–GC/MS revealed differences between the films in the peak area ratios of 3-pentadecenylcatechol to 3-pentadecylcatechol and 3-pentadecadienylcatechol to 3-pentadecylcatechol. The ratios of Kiurushi lacquer film were higher than those of Kuromeurushi lacquer film. Both ratios, furthermore, decreased during hardening due to polymerization of the alkenylcatechols into an urushiol polymer skeleton comprising nucleus–side chain and side chain–side chain cross-linkages with 3-pentadecylcatechol at the terminal. The present results suggest that the reaction rate of these cross-linkages in Kuromeurushi lacquer film is faster than that in Kiurushi lacquer film. A good correlation was found between the peak area ratios obtained by Py–GC/MS and hardness obtained by pencil hardening testing. Oriental lacquer expresses the functions – an extreme hardness and insolubility – accelerating the nucleus–side chain and side chain–side chain cross-linkages. Furthermore, it has become clear that the traditional treatments called Nayashi and Kurome effectively accelerate the hardening rate by activating the cross-linkages. © 2005 Elsevier B.V. All rights reserved. Keywords: Structural study; Oriental lacquer film; Natural resin; Cross-linkage; Py–GC/MS; FT-IR; TG/DTA–MS
1. Introduction Natural resin has been used as adhesives, coating and painting materials, and so on for a long time. Nowadays, plastics are mostly used instead of the natural resin. However, it has been realized that plastics cause environmental problems such as dioxin, sick house syndrome and so on in this century. Furthermore, it has been warned that petroleum resources could be exhausted. Development of substitutes for plastics, which cause no environmental problem and are made from renewable resources for global sustainability without depletion of scarce resources, has been desired. In response to this desire, the development of biomimetic polymers has been studied. As an example, cardanol polymers have been developed [1]. These
∗
Corresponding author. Tel.: +81 42 542 5502; fax: +81 42 541 9513. E-mail address: [email protected] (N. Niimura).
0039-9140/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2005.12.039
polymers cause no environmental problem and are made from cashew nut shell liquid, which is oozed from cashew nut shells. Along this line, the synthesis of artificial urushis, which are modeled after natural resins, has been studied [2]. In this situation, we have studied the structure and the process of functional expression of natural resin films [3–6]. Based on these studies, we have developed functional coating films produced from biomasses [7–9]. We have applied various analytical methods to these studies with newly developed analytical techniques [10–13]. For example, an angle-resolved measurements using X-ray photoelectron spectroscopy (XPS) has been applied to surface structural analysis of oriental lacquer films to investigate the process of functional expressions [14,15]. The oriental lacquer is the sap obtained by tapping lac trees, specifically Rhus vernicifera (China, Korea and Japan) [16]. The sap is a latex material composed of urushiol (60–65%), water (20–25%), plant gum (5–7%), glycoprotein (2–5%) and laccase enzyme (1%) [17]. The composition of the urushiol has been investigated using
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Table 1 The components of urushiol R
MW
%
C15 H31 8(Z)–C7 H14 CH CHC6 H13 10(Z)–C9 H18 CH CHC4 H9 8(Z),11(E)–C7 H14 CH CHCH2 CH CHC3 H7 8(Z),11(Z)–C7 H14 CH CHCH2 CH CHC3 H7 8(Z),11(E),13(E)–C7 H14 CH CHCH2 CH CHCH CHCH3 8(Z),11(E),13(Z)–C7 H14 CH CHCH2 CH CHCH CHCH3 8(Z),11(E),14–C7 H14 CH CHCH2 CH CHCH2 CH CH2 10(Z)–C9 H18 CH CHC6 H13 8(Z),11(Z)-C7 H14 CH CHCH2 CH CHC5 H11
320 318 318 316 316 314 314 314 346 344
4.5 15.0 1.5 4.4 6.5 1.7 55.4 7.4 1.5 1.8
gel permeation chromatography (GPC), high performance liquid chromatography (HPLC) and nuclear magnetic resonance (NMR) spectroscopy [18,19]. The components of urushiol are shown in Table 1. It was reported that the dimerization of urushiol proceeds through laccase-catalyzed C–C coupling [20,21]. However, many other kinds of reactions are presumed to occur during the film-making process. By angle-resolved measurements using XPS, we have found that the surface of the oriental lacquer films are rich in plant gum, which acts as a barrier against oxygen diffusion, and that the top surface is covered with urushiol, whose catechol rings are oriented to retard the radical chain reactions. These surface structures are responsible for high durability of the lacquer film. Furthermore, using pyrolysis–gas chromatography/mass spectrometry (Py–GC/MS) to investigate the molecular structure of the urushiol polymers, we have clarified that the urushiol polymers consist of nucleus–side chain and side chain–side chain cross-linkages [22–25]. The oriental lacquer is extremely hard and insoluble. These functions must be attributed to these cross-linkages. In this study, we investigated structural changes of two kinds of oriental lacquer films during the hardening process to clarify the expression process of these functions. One lacquer film was treated by the traditional Nayashi and Kurome methods [26], which had been believed to promote the hardening reaction and improve the quality of the lacquer film, but the other film was not treated by these methods. The process of the functional expression of oriental lacquer films has been studied by comparing the Py–GC/MS results of these lacquer films. 2. Experimental 2.1. Sample Two types of lacquer films – Kiurushi (raw urushi) and Kuromeurushi – were prepared as samples in this study. The former was produced by coating the sap of Chinese lacquer trees at Chengkou in the Hubei Province of China on glass plates without any pretreatment, while the latter with the traditional pretreatment called Nayashi and Kurome [26]. The sap was purchased from Tohityu-Urushi-Ten (Osaka, Japan). The water concentration of the sap was 25%. Nayashi and Kurome processing was carried out in our laboratory as follows: the sap of 20 g was
stirred in an open vessel (bottom diameter: 80 mm; capacity: 100 ml) at room temperature for 1.5 h, and the temperature was then increased from 20 to 30 ◦ C for 2 h until the water concentration was reduced to 3%. The both films were kept in a humidity-controlled chamber with relative humidity of 70% at 20 ◦ C for two weeks. They were then removed from the chamber and left to dry in open air at room temperature for various times from 4 to 60 days. The thickness of lacquer films was 76 m. 2.2. Pencil hardness testing Scratch hardness of the sample films was measured with the pencil hardness testing with a pencil hardness tester: C-221 type (Yoshimitsu Seiki Co. Ltd.) with different pencils of various hardness (B-5H) [27]. The hardness obtained by this testing is expressed as 6H, 5H, . . ., 2H, H, F, HB, B, 2B, . . ., 5B, 6B in the order from the hardest to the softest, based on the pencil hardness. 2.3. Fourier transform infrared (FT-IR) spectroscopy FT-IR spectra of sample films were obtained using a KBr pellet technique with an FT-IR spectrometer: JIR-6500 (JEOL Ltd.). To prepare the KBr pellets, each sample film of 2 mg was ground together with FT-IR grade KBr of 200 mg for 2 min. The FT-IR was operated at a resolution of 2 cm−1 and the collection time was 25 s (16 scans). 2.4. Thermogravimetry/differential thermal analysis–mass spectrometry (TG/DTA–MS) TG/DTA–MS measurements were performed with a thermogravimetry/differential thermal analyzer TG/DTA 6300 (Seiko Instruments Inc.) and a mass spectrometer JMS-K9 (JEOL Ltd.). A sample film of 0.5 mg was placed in a furnace of the analyzer which was programmed to raise the temperature at a constant rate of 30 ◦ C/min in a range from 50 to 900 ◦ C. Helium was used for a carrier gas at a flow rate of 100 ml/min in the TG/DT analyzer. A part of the flow with a reduced rate of 2 ml/min at a capillary interface tube by a splitter was introduced into the mass spectrometer. Evolved gases ionized using an electron ionization at 70 eV were analyzed with the mass spectrometer.
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2.5. Pyrolysis–gas chromatograpy/mass spectrometry (Py–GC/MS) Py–GC/MS was carried out with a vertical microfurnacetype pyrolyzer: PY-2010D (Frontier Lab.), a gas chromatograph: Agilent 6890 (Agilent Ltd.) and a mass spectrometer: JMS-K9 (JEOL Ltd.). A sample film of 0.5 mg was placed in a platinum sample cup, which was set at the top of the pyrolyzer kept at room temperature. The sample cup was inserted into a furnace at 400 ◦ C, followed by heating a gas chromatograph oven, which was programmed to raise the temperature at a constant rate of 20 ◦ C/min in a range from 40 to 330 ◦ C. Helium was used as a carrier gas at a flow rate of 50 ml/min in the pyrolyzer, which was reduced to 1 ml/min with a splitter at the capillary column in the gas chromatograph. A stainless steel capillary column with an inner diameter of 0.25 mm and a length of 30 m coated with 0.25 m thick Ultra Alloy PY-2 – methylsilicone 100% – was used for separation. Ionization method of the mass spectrometer was electron ionization with an ionization energy of 70 eV. 3. Results and discussion 3.1. Pencil hardness testing The hardness of the two types of lacquer films is shown as a function of drying time from 4 to 60 days in Table 2. After 4 day drying, the shortest drying time, Kiurushi shows the hardness of F, while Kuromeurushi shows that of 4H. The hardness
Table 2 Results of pencil hardness testing Drying time (days)
4
14
30
60
Kiurushi Kumomeurushi
F 4H
H 4H
2H 4H
5H 5H
of Kiurushi gradually increases with drying time from F to 5H after 60 day drying. In contrast, that of Kuromeurushi shows rather high hardness of 4H after the shortest drying time and slightly increases to 5H after 60 day drying, the longest drying time, which is the same as that of Kiurushi. The observed hardness in this table clearly shows that the traditional pretreatment of Nayashi and Kurome accelerates the hardening rate of the lacquer film. 3.2. FT-IR measurements FT-IR spectra of the two types of lacquer films, Kiurushi and Kuromeurishi, after 4 day drying are shown in Fig. 1. Overall patterns of these two spectra are quite similar to each other. Eight main absorption peaks are observed in these spectra. They are designated as 1–8, as shown in Fig. 1. Based on these peaks, five functional groups are identified as follows—OH: 3407 cm−1 (1) and 1272 cm−1 (7); C–H: 3013 cm−1 (2), 2926 cm−1 (3), 2853 cm−1 (4) and 1455 cm−1 (6); C O: 1621 cm−1 (5); Ph: about 1500 and 1600 cm−1 ; conjugated triene: 993 cm−1 (8). The OH functional group is attributed to urushiol, but C O and
Fig. 1. FT-IR spectra of the lacquer films after 4 day drying. (a) Kiurushi and (b) Kuromeurushi. OH: (1) 3407 cm−1 , (7) 1272 cm−1 , C–H: (2) 3013 cm−1 , (3) 2926 cm−1 , (4) 2853 cm−1 , (6) 1455 cm−1 , C O: (5) 1621 cm−1 , Ph: about 1500 and 1600 cm−1 , conjugated triene, (8) 993 cm−1 .
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Fig. 2. TG, DTA, DTG curves and TIC of the lacquer films after 4 day drying: (1) Kiurushi and (2) Kuromeurushi.
conjugated triene functional groups do not exist in the urushiol. It has been reported that the peak intensity of C O and conjugated triene functional groups increases with the progress of urushiol polymerization [28], thus these functional groups are attributed to the oxidative polymerization of urushiol. Although the pencil hardness of these two types of films are substantially different as shown in Table 2, only a slight difference between these films is observed even in the intensities of peaks 5 and 8 so that a higher peak resolution for FT-IR spectra is needed to correlate the FT-IR spectra with the pencil hardness. 3.3. TG/DTA–MS measurements TG, DTA and DTG curves and TIC of respective lacquer films after 4 day drying are shown in Fig. 2. No distinct difference was observed in the three curves and TIC of the two types of lacquer films. The TG and DTA curves indicate that the thermal degradation of the lacquer films gradually begins at around 200 ◦ C, and the degradation rate becomes fastest at 400–500 ◦ C. The TIC and DTG curves confirm the above results showing peaks at 470 ◦ C. These peaks were attributed to evolved gas during the thermal degradation. Previously, we reported that the pyrolysis products of glycoprotein are detected from the lacquer film by the pyrolysis at 200 ◦ C. Alkylcatechols and alkenylcatechols are observed as the thermally decomposed components from the terminal alkyland alkenylcatechol-side chains of the lacquer film by the pyrolysis at 300 ◦ C. Alkylphenols and alkenylphenols are detected as the pyrolysis products of the nucleus–side chain C O coupling urushiol polymer, which is the main component of the lacquer film, by the pyrolysis at 500 ◦ C [22]. In this study, the gases evolved at around 200, 300 and 500 ◦ C must be attributed to these steps of degradations. We tried to confirm the detection of these components by identifying each mass spectrum, but it was difficult, because the spectra were very complicated. The evolved gases consisted of mixed compounds. Chromatographic methods are needed to identify these gases.
3.4. Py–GC/MS measurements Fig. 3 shows TICs and mass chromatograms (m/z 320, 318, 316, 314) of the two types of lacquer films after 4 day drying. Peaks 1–3 were detected at retention times of 13.98, 13.90 and 13.62 min in the mass chromatograms (m/z 320, 318 and 316, respectively), but no peak was detected at retention time of around 13 min in the mass chromatograms (m/z 314). If 3-pentadecatrienylcatechol (MW 314), which is urushiol monomer, is produced by the pyrolysis, a peak should be detected at retention time of around 13 min in the mass chromatograms (m/z 314) [7]. Peaks 1–3 were identified as 3pentadecylcatechol (MW 320), 3-pentadecenylcatechol (MW 318) and 3-pentadecadienylcatechol (MW 316) based on the mass spectra. The mass spectrum of peak 1 is shown in Fig. 4. The molecular ion peak was detected at m/z 320. The base ion peak at m/z 123 was a typical fragment ion peak of an alkylcatechol. These pyrolyzed products are urushiol monomers, which are produced from the terminal groups of urushiol polymers [6]. Pyrolysis mechanism of the terminal groups is shown in Fig. 5. The reaction rate of the nucleus–side chain C–C coupling depends on the degree of unsaturation of the side chain. The reaction rate of the nucleus–trienyl side chain C–C coupling is the fastest and that of the nucleus–dienyl side chain C–C coupling is the second fastest. That of the nucleus–monoenyl side chain C–C coupling is third, while that of the nucleus–unsaturated side chain C–C coupling is the slowest [3,5]. Therefore, it is assumed that the terminal groups consist of more saturated urushiols and monoenyl urushiols than dienyl urushiols and trienyl urushiols at the end of the hardening process. To confirm the above assumption, the areas of the respective peaks identified as 3-pentadecylcatechol, 3-pentadecenylcatechol, 3-pentadecadienylcatechol and 3-pentadecatrienylcatechol were measured after 4, 17, 38 and 60 day drying. 3-Pentadecatrienylcatechol was not detected after 4 day drying as shown in Fig. 3. Since the reaction rate of the nucleus–trienyl side chain C–C coupling is the fastest, 3-pentadecatrienylcatechol
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Fig. 3. TICs and mass chromatograms of Kiurushi (1) and Kuromeurushi (2) lacquer films after 4 day drying: (a) TIC; (b) mass chromatogram (m/z 320), 1:3-pentadecylcatechol (MW 320); (c) mass chromatogram (m/z 318), 2:3-pentadecenylcatechol (MW 318); (d) mass chromatogram (m/z 316), 3:3pentadecadienylcatechol (MW 316); (e) mass chromatogram (m/z 314).
has already been taken into the skeleton of urushiol polymer after 4 day drying. 3-Pentadecylcatechol, 3-pentadecenylcatechol and 3-pentadecadienylcatechol were detected in the respective analyses. The peak areas of 3-pentadecenylcatechol and 3-pentadecadienylcatechol significantly decreased with the
increase in drying time. These peak areas of the two lacquer films after 60 day drying were the lowest. The rate of decrease in their peak areas was compared between Kiurushi and Kuromeurushi lacquer films. Fig. 6 shows a comparison of the peak area ratios of 3-pentadecenylcatechol to
Fig. 4. Mass spectrum of peak 1 is shown in this figure: 3-pentadecylcatechol.
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Fig. 5. Pyrolysis mechanism of urushiol polymer: (1) urushiol polymer and (2) 3-alkylcatechol.
4. Conclusions
Fig. 6. Peak area ratios of 3-pentadecenylcatechol to 3-pentadecylcatechol and 3-pentadecadienylcatechol to 3-pentadecylcatechol between Kiurushi and Kuromeurushi: () 3-pentadecenylcatechol/3-pentadecylcatechol of Kiurushi; () 3-pentadecenylcatechol/3-pentadecylcatechol of Kuromeurushi; () 3-pentadecadienylcatechol/3-pentadecylcatechol of Kiurushi; () 3pentadecadienylcatechol/3-pentadecylcatechol of Kuromeurushi.
3-pentadecylcatechol and of 3-pentadecadienylcatechol to 3pentadecylcatechol between Kiurushi and Kuromeurushi lacquer films. All the peak area ratios decreased with the increase in drying time. Comparing the two lacquer films, both 3-pentadecenylcatechol/3-pentadecylcatechol ratio and 3pentadecadienylcatechol/3-pentadecylcatechol ratio of Kiurushi lacquer film were higher than those of Kuromeurushi lacquer film. These ratios of Kiurushi lacquer film were, furthermore, much higher than those of Kuromeurushi lacquer film after 4 day drying. However, these ratios of Kiurushi lacquer film were almost the same as those of Kuromeurushi lacquer film after 60 day drying. These results have a good correlation with those of the pencil hardness testing. Kuromeurushi lacquer film with pencil hardness 4H was much harder than Kiurushi lacquer film with pencil hardness F after 4 day drying. The harder lacquer film has the lower 3-pentadecenylcatechol/3-pentadecylcatechol ratio and 3-pentadecadienylcatechol/3-pentadecylcatechol ratio, while the softer lacquer film has higher ratios. These ratios of the two lacquer films were almost the same value after 60 day drying, because their pencil hardness was the same value of 5H. The ratios significantly indicate the hardness of the lacquer films.
Two kinds of oriental lacquer films, which were produced with two different drying methods, were analyzed during the hardening process using FT-IR, TG/MS and Py–GC/MS. FT-IR and TG/MS analyses could not reveal any difference between the films. On the other hand, the peak area ratios of 3-pentadecenylcatechol to 3-pentadecylcatechol and 3pentadecadienylcatechol to 3-pentadecylcatechol, which were obtained by Py–GC/MS, were different between the films. The ratios of Kiurushi lacquer film were higher than those of Kuromeurushi lacquer film. Furthermore, both ratios decreased during the hardening process, because 3-pentadecenylcatechol and 3-pentadecadienylcatechol polymerize into urushiol polymer skeleton producing a nucleus–side chain cross-linkage, and 3-pentadecylcatechol terminates the polymerization. These results suggest that the reaction rate of nucleus–side chain crosslinkage on Kuromeurushi lacquer film is faster than that on Kiurushi lacquer film. These Py–GC/MS results also indicated a good correlation with the hardness obtained by pencil hardening testing. Oriental lacquer expresses some of the most important functions for industrial use, such as an extreme hardness and insolubility, accelerating the nucleus–side chain and side chain–side chain cross-linkages. Furthermore, it has become clear that the traditional treatments called Nayashi and Kurome effectively accelerate the hardening rate by activating the crosslinkages. It is important to investigate the process of functional expression in material science. Studying the functional expression process effectively promotes development of new functional materials. We hope this study leads to the successful development of new functional coating materials – modified oriental lacquer film, which is one of biomimetic polymers – made from renewable resources for future generations.
References [1] R. Ikeda, H. Tanaka, H. Uyama, S. Kobayashi, Polym. J. 32 (2000) 589. [2] R. Ikeda, H. Tanaka, H. Oyabu, H. Uyama, S. Kobayashi, Bull. Chem. Soc. Jpn. 74 (2001) 1067. [3] N. Niimura, T. Miyakoshi, J. Onodera, T. Higuchi, J. Anal. Appl. Pyrolysis 37 (1996) 199. [4] N. Niimura, T. Miyakoshi, J. Onodera, T. Higuchi, Rapid Commun. Mass Spectrom. 10 (1996) 719.
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[5] N. Niimura, N. Kishi, T. Miyakoshi, J. Mass Spectrom. Soc. Jpn. 51 (2003) 229. [6] N. Niimura, T. Miyakoshi, J. Mass Spectrom. Soc. Jpn. 51 (2003) 439. [7] N. Niimura, Y. Kamiya, T. Sato, I. Katano, T. Miyakoshi, J. Jpn. Oil Chem. Soc. 47 (1998) 171. [8] N. Niimura, T. Miyakoshi, Int. J. Polym. Anal. Charact. 8 (2003) 47. [9] N. Niimura, T. Miyakoshi, Tosou Kougaku 33 (1998) 338. [10] N. Niimura, T. Miyakoshi, Tosou Kougaku 33 (1998) 166. [11] N. Niimura, T. Miyakoshi, J. Onodera, T. Higuchi, Int. J. Polym. Anal. Charact. 4 (1998) 309. [12] N. Niimura, T. Miyakoshi, Surf. Interface Anal. 29 (2000) 381. [13] N. Niimura, T. Miyakoshi, Y. Iijima, Anal. Sci. Suppl. 17 (2001) i155. [14] N. Niimura, Y. Iijima, T. Miyakoshi, Bunseki Kagaku 42 (1993) 605. [15] N. Niimura, Y. Iijima, T. Miyakoshi, Surf. Interface Anal. 24 (1996) 237. [16] K. Nagase, Tosou to Toryo 337 (1981) 66.
[17] J. Kumanotani, K. Inoue, M. Achiwa, L.W. Chen, Polym. Sci. Technol. 33 (1986) 163. [18] Y. Yamauchi, R. Oshima, J. Kumanotani, J. Chromatogr. 243 (1982) 71. [19] J. Bartus, W.J. Simonsick, C. Garner, T. Nishiura, T. Kitayama, K. Hatada, O. Vogel, Polym. J. 26 (1994) 67. [20] J. Kumanotani, S. Tanaka, T. Matsui, Proceedings of the 12th International Conference on Organic Coatings Science and Technology, 1986, p. 195. [21] J. Kumanotani, Zairyou Kagaku 19 (1982) 88. [22] N. Niimura, T. Miyakoshi, J. Onodera, T. Higuchi, Nihon Kagaku Kaishi 9 (1995) 724. [23] N. Niimura, T. Miyakoshi, Tosou Kougaku 33 (1998) 204. [24] N. Niimura, T. Miyakoshi, Tosou Kougaku 33 (1998) 252. [25] N. Niimura, T. Miyakoshi, Tosou Kougaku 33 (1998) 296. [26] K. Taneda, Mokuzai Kougyou 40 (1985) 10. [27] P.R. Guevin Jr., J. Coat. Technol. 67 (1995) 61. [28] T. Kenjou, Toso to Toryou 44 (1988) 69.
Structural study of oriental lacquer films during the hardening process Noriyasu Niimura a,∗ , Tetsuo Miyakoshi b a
International Technical and Training Center, JEOL DATUM Ltd., 1156 Nakagami-cho Akishima-shi, Tokyo 196-0022, Japan b Department of Industrial Chemistry, Meiji University, Higashimita Tama-ku, Kawasaki-shi 214-8571, Japan Received 3 October 2005; received in revised form 14 December 2005; accepted 16 December 2005 Available online 24 January 2006
Abstract Oriental lacquer is the natural resin obtained by tapping lac trees. It hardens into a tough and insoluble film. The extreme hardness and insolubility are some of the most important functions, which are required for industrial coating materials. In this study, two kinds of oriental lacquer films, traditionally named Kiurushi (raw urushi) and Kuromeurushi produced by two different pretreatments, were analyzed during hardening with Fourier transform infrared spectroscopy (FT-IR), thermogravimetry/differential thermal analysis–mass spectrometry (TG/DTA–MS) and pyrolysis–gas chromatography/mass spectrometry (Py–GC/MS) to investigate their functional expression process. Typical functional groups of the lacquer films were detected by FT-IR. The TG/DTA–MS curves clarified that the thermal degradation of the lacquer films gradually began at around 200 ◦ C, and reached the fastest rate at 400–500 ◦ C. Apparently, FT-IR and TG/DTA–MS could not reveal any difference between the films. On the other hand, Py–GC/MS revealed differences between the films in the peak area ratios of 3-pentadecenylcatechol to 3-pentadecylcatechol and 3-pentadecadienylcatechol to 3-pentadecylcatechol. The ratios of Kiurushi lacquer film were higher than those of Kuromeurushi lacquer film. Both ratios, furthermore, decreased during hardening due to polymerization of the alkenylcatechols into an urushiol polymer skeleton comprising nucleus–side chain and side chain–side chain cross-linkages with 3-pentadecylcatechol at the terminal. The present results suggest that the reaction rate of these cross-linkages in Kuromeurushi lacquer film is faster than that in Kiurushi lacquer film. A good correlation was found between the peak area ratios obtained by Py–GC/MS and hardness obtained by pencil hardening testing. Oriental lacquer expresses the functions – an extreme hardness and insolubility – accelerating the nucleus–side chain and side chain–side chain cross-linkages. Furthermore, it has become clear that the traditional treatments called Nayashi and Kurome effectively accelerate the hardening rate by activating the cross-linkages. © 2005 Elsevier B.V. All rights reserved. Keywords: Structural study; Oriental lacquer film; Natural resin; Cross-linkage; Py–GC/MS; FT-IR; TG/DTA–MS
1. Introduction Natural resin has been used as adhesives, coating and painting materials, and so on for a long time. Nowadays, plastics are mostly used instead of the natural resin. However, it has been realized that plastics cause environmental problems such as dioxin, sick house syndrome and so on in this century. Furthermore, it has been warned that petroleum resources could be exhausted. Development of substitutes for plastics, which cause no environmental problem and are made from renewable resources for global sustainability without depletion of scarce resources, has been desired. In response to this desire, the development of biomimetic polymers has been studied. As an example, cardanol polymers have been developed [1]. These
∗
Corresponding author. Tel.: +81 42 542 5502; fax: +81 42 541 9513. E-mail address: [email protected] (N. Niimura).
0039-9140/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2005.12.039
polymers cause no environmental problem and are made from cashew nut shell liquid, which is oozed from cashew nut shells. Along this line, the synthesis of artificial urushis, which are modeled after natural resins, has been studied [2]. In this situation, we have studied the structure and the process of functional expression of natural resin films [3–6]. Based on these studies, we have developed functional coating films produced from biomasses [7–9]. We have applied various analytical methods to these studies with newly developed analytical techniques [10–13]. For example, an angle-resolved measurements using X-ray photoelectron spectroscopy (XPS) has been applied to surface structural analysis of oriental lacquer films to investigate the process of functional expressions [14,15]. The oriental lacquer is the sap obtained by tapping lac trees, specifically Rhus vernicifera (China, Korea and Japan) [16]. The sap is a latex material composed of urushiol (60–65%), water (20–25%), plant gum (5–7%), glycoprotein (2–5%) and laccase enzyme (1%) [17]. The composition of the urushiol has been investigated using
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Table 1 The components of urushiol R
MW
%
C15 H31 8(Z)–C7 H14 CH CHC6 H13 10(Z)–C9 H18 CH CHC4 H9 8(Z),11(E)–C7 H14 CH CHCH2 CH CHC3 H7 8(Z),11(Z)–C7 H14 CH CHCH2 CH CHC3 H7 8(Z),11(E),13(E)–C7 H14 CH CHCH2 CH CHCH CHCH3 8(Z),11(E),13(Z)–C7 H14 CH CHCH2 CH CHCH CHCH3 8(Z),11(E),14–C7 H14 CH CHCH2 CH CHCH2 CH CH2 10(Z)–C9 H18 CH CHC6 H13 8(Z),11(Z)-C7 H14 CH CHCH2 CH CHC5 H11
320 318 318 316 316 314 314 314 346 344
4.5 15.0 1.5 4.4 6.5 1.7 55.4 7.4 1.5 1.8
gel permeation chromatography (GPC), high performance liquid chromatography (HPLC) and nuclear magnetic resonance (NMR) spectroscopy [18,19]. The components of urushiol are shown in Table 1. It was reported that the dimerization of urushiol proceeds through laccase-catalyzed C–C coupling [20,21]. However, many other kinds of reactions are presumed to occur during the film-making process. By angle-resolved measurements using XPS, we have found that the surface of the oriental lacquer films are rich in plant gum, which acts as a barrier against oxygen diffusion, and that the top surface is covered with urushiol, whose catechol rings are oriented to retard the radical chain reactions. These surface structures are responsible for high durability of the lacquer film. Furthermore, using pyrolysis–gas chromatography/mass spectrometry (Py–GC/MS) to investigate the molecular structure of the urushiol polymers, we have clarified that the urushiol polymers consist of nucleus–side chain and side chain–side chain cross-linkages [22–25]. The oriental lacquer is extremely hard and insoluble. These functions must be attributed to these cross-linkages. In this study, we investigated structural changes of two kinds of oriental lacquer films during the hardening process to clarify the expression process of these functions. One lacquer film was treated by the traditional Nayashi and Kurome methods [26], which had been believed to promote the hardening reaction and improve the quality of the lacquer film, but the other film was not treated by these methods. The process of the functional expression of oriental lacquer films has been studied by comparing the Py–GC/MS results of these lacquer films. 2. Experimental 2.1. Sample Two types of lacquer films – Kiurushi (raw urushi) and Kuromeurushi – were prepared as samples in this study. The former was produced by coating the sap of Chinese lacquer trees at Chengkou in the Hubei Province of China on glass plates without any pretreatment, while the latter with the traditional pretreatment called Nayashi and Kurome [26]. The sap was purchased from Tohityu-Urushi-Ten (Osaka, Japan). The water concentration of the sap was 25%. Nayashi and Kurome processing was carried out in our laboratory as follows: the sap of 20 g was
stirred in an open vessel (bottom diameter: 80 mm; capacity: 100 ml) at room temperature for 1.5 h, and the temperature was then increased from 20 to 30 ◦ C for 2 h until the water concentration was reduced to 3%. The both films were kept in a humidity-controlled chamber with relative humidity of 70% at 20 ◦ C for two weeks. They were then removed from the chamber and left to dry in open air at room temperature for various times from 4 to 60 days. The thickness of lacquer films was 76 m. 2.2. Pencil hardness testing Scratch hardness of the sample films was measured with the pencil hardness testing with a pencil hardness tester: C-221 type (Yoshimitsu Seiki Co. Ltd.) with different pencils of various hardness (B-5H) [27]. The hardness obtained by this testing is expressed as 6H, 5H, . . ., 2H, H, F, HB, B, 2B, . . ., 5B, 6B in the order from the hardest to the softest, based on the pencil hardness. 2.3. Fourier transform infrared (FT-IR) spectroscopy FT-IR spectra of sample films were obtained using a KBr pellet technique with an FT-IR spectrometer: JIR-6500 (JEOL Ltd.). To prepare the KBr pellets, each sample film of 2 mg was ground together with FT-IR grade KBr of 200 mg for 2 min. The FT-IR was operated at a resolution of 2 cm−1 and the collection time was 25 s (16 scans). 2.4. Thermogravimetry/differential thermal analysis–mass spectrometry (TG/DTA–MS) TG/DTA–MS measurements were performed with a thermogravimetry/differential thermal analyzer TG/DTA 6300 (Seiko Instruments Inc.) and a mass spectrometer JMS-K9 (JEOL Ltd.). A sample film of 0.5 mg was placed in a furnace of the analyzer which was programmed to raise the temperature at a constant rate of 30 ◦ C/min in a range from 50 to 900 ◦ C. Helium was used for a carrier gas at a flow rate of 100 ml/min in the TG/DT analyzer. A part of the flow with a reduced rate of 2 ml/min at a capillary interface tube by a splitter was introduced into the mass spectrometer. Evolved gases ionized using an electron ionization at 70 eV were analyzed with the mass spectrometer.
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2.5. Pyrolysis–gas chromatograpy/mass spectrometry (Py–GC/MS) Py–GC/MS was carried out with a vertical microfurnacetype pyrolyzer: PY-2010D (Frontier Lab.), a gas chromatograph: Agilent 6890 (Agilent Ltd.) and a mass spectrometer: JMS-K9 (JEOL Ltd.). A sample film of 0.5 mg was placed in a platinum sample cup, which was set at the top of the pyrolyzer kept at room temperature. The sample cup was inserted into a furnace at 400 ◦ C, followed by heating a gas chromatograph oven, which was programmed to raise the temperature at a constant rate of 20 ◦ C/min in a range from 40 to 330 ◦ C. Helium was used as a carrier gas at a flow rate of 50 ml/min in the pyrolyzer, which was reduced to 1 ml/min with a splitter at the capillary column in the gas chromatograph. A stainless steel capillary column with an inner diameter of 0.25 mm and a length of 30 m coated with 0.25 m thick Ultra Alloy PY-2 – methylsilicone 100% – was used for separation. Ionization method of the mass spectrometer was electron ionization with an ionization energy of 70 eV. 3. Results and discussion 3.1. Pencil hardness testing The hardness of the two types of lacquer films is shown as a function of drying time from 4 to 60 days in Table 2. After 4 day drying, the shortest drying time, Kiurushi shows the hardness of F, while Kuromeurushi shows that of 4H. The hardness
Table 2 Results of pencil hardness testing Drying time (days)
4
14
30
60
Kiurushi Kumomeurushi
F 4H
H 4H
2H 4H
5H 5H
of Kiurushi gradually increases with drying time from F to 5H after 60 day drying. In contrast, that of Kuromeurushi shows rather high hardness of 4H after the shortest drying time and slightly increases to 5H after 60 day drying, the longest drying time, which is the same as that of Kiurushi. The observed hardness in this table clearly shows that the traditional pretreatment of Nayashi and Kurome accelerates the hardening rate of the lacquer film. 3.2. FT-IR measurements FT-IR spectra of the two types of lacquer films, Kiurushi and Kuromeurishi, after 4 day drying are shown in Fig. 1. Overall patterns of these two spectra are quite similar to each other. Eight main absorption peaks are observed in these spectra. They are designated as 1–8, as shown in Fig. 1. Based on these peaks, five functional groups are identified as follows—OH: 3407 cm−1 (1) and 1272 cm−1 (7); C–H: 3013 cm−1 (2), 2926 cm−1 (3), 2853 cm−1 (4) and 1455 cm−1 (6); C O: 1621 cm−1 (5); Ph: about 1500 and 1600 cm−1 ; conjugated triene: 993 cm−1 (8). The OH functional group is attributed to urushiol, but C O and
Fig. 1. FT-IR spectra of the lacquer films after 4 day drying. (a) Kiurushi and (b) Kuromeurushi. OH: (1) 3407 cm−1 , (7) 1272 cm−1 , C–H: (2) 3013 cm−1 , (3) 2926 cm−1 , (4) 2853 cm−1 , (6) 1455 cm−1 , C O: (5) 1621 cm−1 , Ph: about 1500 and 1600 cm−1 , conjugated triene, (8) 993 cm−1 .
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Fig. 2. TG, DTA, DTG curves and TIC of the lacquer films after 4 day drying: (1) Kiurushi and (2) Kuromeurushi.
conjugated triene functional groups do not exist in the urushiol. It has been reported that the peak intensity of C O and conjugated triene functional groups increases with the progress of urushiol polymerization [28], thus these functional groups are attributed to the oxidative polymerization of urushiol. Although the pencil hardness of these two types of films are substantially different as shown in Table 2, only a slight difference between these films is observed even in the intensities of peaks 5 and 8 so that a higher peak resolution for FT-IR spectra is needed to correlate the FT-IR spectra with the pencil hardness. 3.3. TG/DTA–MS measurements TG, DTA and DTG curves and TIC of respective lacquer films after 4 day drying are shown in Fig. 2. No distinct difference was observed in the three curves and TIC of the two types of lacquer films. The TG and DTA curves indicate that the thermal degradation of the lacquer films gradually begins at around 200 ◦ C, and the degradation rate becomes fastest at 400–500 ◦ C. The TIC and DTG curves confirm the above results showing peaks at 470 ◦ C. These peaks were attributed to evolved gas during the thermal degradation. Previously, we reported that the pyrolysis products of glycoprotein are detected from the lacquer film by the pyrolysis at 200 ◦ C. Alkylcatechols and alkenylcatechols are observed as the thermally decomposed components from the terminal alkyland alkenylcatechol-side chains of the lacquer film by the pyrolysis at 300 ◦ C. Alkylphenols and alkenylphenols are detected as the pyrolysis products of the nucleus–side chain C O coupling urushiol polymer, which is the main component of the lacquer film, by the pyrolysis at 500 ◦ C [22]. In this study, the gases evolved at around 200, 300 and 500 ◦ C must be attributed to these steps of degradations. We tried to confirm the detection of these components by identifying each mass spectrum, but it was difficult, because the spectra were very complicated. The evolved gases consisted of mixed compounds. Chromatographic methods are needed to identify these gases.
3.4. Py–GC/MS measurements Fig. 3 shows TICs and mass chromatograms (m/z 320, 318, 316, 314) of the two types of lacquer films after 4 day drying. Peaks 1–3 were detected at retention times of 13.98, 13.90 and 13.62 min in the mass chromatograms (m/z 320, 318 and 316, respectively), but no peak was detected at retention time of around 13 min in the mass chromatograms (m/z 314). If 3-pentadecatrienylcatechol (MW 314), which is urushiol monomer, is produced by the pyrolysis, a peak should be detected at retention time of around 13 min in the mass chromatograms (m/z 314) [7]. Peaks 1–3 were identified as 3pentadecylcatechol (MW 320), 3-pentadecenylcatechol (MW 318) and 3-pentadecadienylcatechol (MW 316) based on the mass spectra. The mass spectrum of peak 1 is shown in Fig. 4. The molecular ion peak was detected at m/z 320. The base ion peak at m/z 123 was a typical fragment ion peak of an alkylcatechol. These pyrolyzed products are urushiol monomers, which are produced from the terminal groups of urushiol polymers [6]. Pyrolysis mechanism of the terminal groups is shown in Fig. 5. The reaction rate of the nucleus–side chain C–C coupling depends on the degree of unsaturation of the side chain. The reaction rate of the nucleus–trienyl side chain C–C coupling is the fastest and that of the nucleus–dienyl side chain C–C coupling is the second fastest. That of the nucleus–monoenyl side chain C–C coupling is third, while that of the nucleus–unsaturated side chain C–C coupling is the slowest [3,5]. Therefore, it is assumed that the terminal groups consist of more saturated urushiols and monoenyl urushiols than dienyl urushiols and trienyl urushiols at the end of the hardening process. To confirm the above assumption, the areas of the respective peaks identified as 3-pentadecylcatechol, 3-pentadecenylcatechol, 3-pentadecadienylcatechol and 3-pentadecatrienylcatechol were measured after 4, 17, 38 and 60 day drying. 3-Pentadecatrienylcatechol was not detected after 4 day drying as shown in Fig. 3. Since the reaction rate of the nucleus–trienyl side chain C–C coupling is the fastest, 3-pentadecatrienylcatechol
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Fig. 3. TICs and mass chromatograms of Kiurushi (1) and Kuromeurushi (2) lacquer films after 4 day drying: (a) TIC; (b) mass chromatogram (m/z 320), 1:3-pentadecylcatechol (MW 320); (c) mass chromatogram (m/z 318), 2:3-pentadecenylcatechol (MW 318); (d) mass chromatogram (m/z 316), 3:3pentadecadienylcatechol (MW 316); (e) mass chromatogram (m/z 314).
has already been taken into the skeleton of urushiol polymer after 4 day drying. 3-Pentadecylcatechol, 3-pentadecenylcatechol and 3-pentadecadienylcatechol were detected in the respective analyses. The peak areas of 3-pentadecenylcatechol and 3-pentadecadienylcatechol significantly decreased with the
increase in drying time. These peak areas of the two lacquer films after 60 day drying were the lowest. The rate of decrease in their peak areas was compared between Kiurushi and Kuromeurushi lacquer films. Fig. 6 shows a comparison of the peak area ratios of 3-pentadecenylcatechol to
Fig. 4. Mass spectrum of peak 1 is shown in this figure: 3-pentadecylcatechol.
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Fig. 5. Pyrolysis mechanism of urushiol polymer: (1) urushiol polymer and (2) 3-alkylcatechol.
4. Conclusions
Fig. 6. Peak area ratios of 3-pentadecenylcatechol to 3-pentadecylcatechol and 3-pentadecadienylcatechol to 3-pentadecylcatechol between Kiurushi and Kuromeurushi: () 3-pentadecenylcatechol/3-pentadecylcatechol of Kiurushi; () 3-pentadecenylcatechol/3-pentadecylcatechol of Kuromeurushi; () 3-pentadecadienylcatechol/3-pentadecylcatechol of Kiurushi; () 3pentadecadienylcatechol/3-pentadecylcatechol of Kuromeurushi.
3-pentadecylcatechol and of 3-pentadecadienylcatechol to 3pentadecylcatechol between Kiurushi and Kuromeurushi lacquer films. All the peak area ratios decreased with the increase in drying time. Comparing the two lacquer films, both 3-pentadecenylcatechol/3-pentadecylcatechol ratio and 3pentadecadienylcatechol/3-pentadecylcatechol ratio of Kiurushi lacquer film were higher than those of Kuromeurushi lacquer film. These ratios of Kiurushi lacquer film were, furthermore, much higher than those of Kuromeurushi lacquer film after 4 day drying. However, these ratios of Kiurushi lacquer film were almost the same as those of Kuromeurushi lacquer film after 60 day drying. These results have a good correlation with those of the pencil hardness testing. Kuromeurushi lacquer film with pencil hardness 4H was much harder than Kiurushi lacquer film with pencil hardness F after 4 day drying. The harder lacquer film has the lower 3-pentadecenylcatechol/3-pentadecylcatechol ratio and 3-pentadecadienylcatechol/3-pentadecylcatechol ratio, while the softer lacquer film has higher ratios. These ratios of the two lacquer films were almost the same value after 60 day drying, because their pencil hardness was the same value of 5H. The ratios significantly indicate the hardness of the lacquer films.
Two kinds of oriental lacquer films, which were produced with two different drying methods, were analyzed during the hardening process using FT-IR, TG/MS and Py–GC/MS. FT-IR and TG/MS analyses could not reveal any difference between the films. On the other hand, the peak area ratios of 3-pentadecenylcatechol to 3-pentadecylcatechol and 3pentadecadienylcatechol to 3-pentadecylcatechol, which were obtained by Py–GC/MS, were different between the films. The ratios of Kiurushi lacquer film were higher than those of Kuromeurushi lacquer film. Furthermore, both ratios decreased during the hardening process, because 3-pentadecenylcatechol and 3-pentadecadienylcatechol polymerize into urushiol polymer skeleton producing a nucleus–side chain cross-linkage, and 3-pentadecylcatechol terminates the polymerization. These results suggest that the reaction rate of nucleus–side chain crosslinkage on Kuromeurushi lacquer film is faster than that on Kiurushi lacquer film. These Py–GC/MS results also indicated a good correlation with the hardness obtained by pencil hardening testing. Oriental lacquer expresses some of the most important functions for industrial use, such as an extreme hardness and insolubility, accelerating the nucleus–side chain and side chain–side chain cross-linkages. Furthermore, it has become clear that the traditional treatments called Nayashi and Kurome effectively accelerate the hardening rate by activating the crosslinkages. It is important to investigate the process of functional expression in material science. Studying the functional expression process effectively promotes development of new functional materials. We hope this study leads to the successful development of new functional coating materials – modified oriental lacquer film, which is one of biomimetic polymers – made from renewable resources for future generations.
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