Development of bio-based hybrid resin, from natural lacquer

Progress in Organic Coatings 77 (2014) 24–29

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Development of bio-based hybrid resin, from natural lacquer Shinji Kanehashi, Hiroki Oyagi, Rong Lu, Tetsuo Miyakoshi ∗ Department of Applied Chemistry, Meiji University, 1-1-1 Higashi-mita, Tama-ku, Kawasaki 214-8571, Japan

a r t i c l e

i n f o

Article history: Received 11 May 2013 Received in revised form 26 June 2013 Accepted 15 July 2013 Available online 29 August 2013 Keywords: Natural lacquer Composite Urushiol Resin Coating Hybrid

a b s t r a c t Preparation and structure analysis of a bio-based hybrid material composed of natural lacquer, epoxy, and organic silane compounds were investigated using liquid and solid-state nuclear magnetic resonance. The good composition of additives in the hybrid was determined by the drying, hardness, and resin-molding properties. Although natural lacquer alone cannot form thick resins, this bio-based hybrid material showed good resin formation at room temperature without thermal treatment. This result could be based on the enhancement of curing by the sol–gel reaction between natural lacquer and the organic silane compound, and a crosslink reaction between organic silane and epoxy groups. At the same time, oxidative polymerization at the unsaturated side chains in the urushiol was enhanced by the sol–gel reaction because the catechol hydroxyl groups, which have an antioxidative property, reacted with the organic silane. In addition, this bio-based resin possesses a thermoset property because curing of the hybrid was improved by thermal treatment. Based on the structure analyses, the sol–gel reaction between urushiol and organic silane compound proceeded immediately, indicating the high reactivity of this sol–gel reaction. On the other hand, the reaction between bisphenol A-type epoxy resin and the organic silane seems to progress slowly after the epoxy ring opening. In addition, a sol–gel reaction occurred between the amine group in the organic silane and the hydroxyl group formed after the crosslink reaction of the epoxy group. These results suggested that the improvement in drying and molding properties of the hybrid was based on the chemical reactions among all components (i.e., natural lacquer, epoxy, and organic silane). © 2013 Elsevier B.V. All rights reserved.

1. Introduction Many functional polymer materials such as film and resin have been synthesized to make more comfortable and convenient in our daily life. Most of these polymer materials are derived from oil and require large amounts of energy to produce. Therefore, an alternative to oil-based industrial products using renewable resources is desired due to recent environmental problems, global warming, and depletion of fossil fuels. Development of polymers from natural products, such as plant oils and non-food materials, is one of the very attractive ways to solve such environmental problems economically and ecologically. Natural lacquer (urushi) is one of the traditional natural polymers in Japan that has a beautiful glossy appearance and high durability [1]. Lacquer sap consists of urushiol, which has C15 unsaturated hydrocarbons at 3 or 4 catechols, water, a gummy substance, a nitrogenous material, and laccase [2,3]. One advantage of natural lacquer is that it is polymerized enzymatically in the presence of moisture and laccase without any organic solvent,

∗ Corresponding author. Tel.: +81 44 934 7203; fax: +81 44 934 7906. E-mail address: [email protected] (T. Miyakoshi). 0300-9440/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.porgcoat.2013.07.013

making natural lacquer, bio-based coating material. Therefore, polymer materials using natural lacquer have been investigated for use as resins [4,5], hybrids [6–8], and composites [9,10]. The laccase in lacquer sap plays an important role in this polymerization. The autoxidation of natural lacquer occurs at unsaturated side chains of urushiol [1]. However, because these reactions progress slowly, the curing of natural lacquer generally takes longer than that of conventional organic coatings. The drying of natural lacquer is strongly affected by the environmental conditions such as humidity and temperature. This enzymatic oxidation reaction requires a highly moist environment (70–80% RH) for activation of laccase [11]. Furthermore, it is difficult to fabricate thick resin products from natural lacquer alone. Therefore, fabrication of thick resin material using natural lacquer is also important to expand the use of natural lacquer application as effective utilization of renewable resources. We previously developed hybrid lacquers composed of natural lacquer and amine-functionalized organic silane compounds that show good drying property at low relative humidity [8,12,13]. Hybridization by the sol–gel reaction between OH groups in the lacquer and organic silane compounds considerably improves the film properties such as drying speed and hardness [12,14]. In addition, we recently reported hybrid microwave-adsorption materials

S. Kanehashi et al. / Progress in Organic Coatings 77 (2014) 24–29 O

OH

O O

O

OH C15H25-31

Urushiol

H N H2N

O O Si O

N-(2-aminoethyl)-3-aminopropyl trimethoxysilane (AATMS)

Bisphenol-A epoxy (BPAE)

H N H2N

O O Si O

N-(2-aminoethyl)-3-aminopropyl triethoxysilane (AATES)

H2N

O O Si O

3-aminopropyl trimethoxysilane (APTMS)

Fig. 1. Chemical structure of urushiol, bisphenol-A epoxy (BPAE), and silanes, N-(2-aminomethyl)-3-aminopropyl trimethoxysilane organic (AATMS), N-(2-aminomethyl)-3-aminopropyltriethoxysilane (AATES), and 3-aminopropyltrimethoxysilane (APTMS).

prepared from natural lacquer, epoxy, and organic silane compounds [9]. However, the chemical structure of these hybrids has not been described. Therefore, the chemical structure of a biobased hybrid resin fabricated by chemical reactions among natural lacquer, epoxy, and organic silane compounds and optimization of additive components were investigated in terms of the drying, hardness, and molding properties of the hybrids. 2. Experimental 2.1. Chemicals The natural lacquer used in this study was purchased from Doityu Shoten, Co. Ltd., Osaka, Japan. Urushiol and lipid component of the lacquer were extracted from raw lacquer sap according to the previous study [14]. Bisphenol A-type epoxy resin (BPAE, Epoxyclear 305114) was purchased from I-Resin Co., Ltd., Tokyo, Japan, and used without further purification. Organic silane compounds, N-(2-aminomethyl)-3-aminopropyl trimethoxysilane (AATMS), and 3-aminopropyltrimethoxysilane (APTMS) were purchased from Tokyo Chemical Industry Co., Ltd., Tokyo, Japan. N-(2-aminomethyl)-3-aminopropyltriethoxysilane (AATES) was kindly supplied by Shin-Etsu Polymer Co., Ltd., Tokyo, Japan. These chemical structures of compounds used in this study are presented in Fig. 1. 2.2. Preparation of hybrid lacquer and resin Ten grams of natural lacquer and given ratios of additives such as BPAE and organosilicon compounds were mixed for 5 min at room temperature. After fabrication of a hybrid lacquer, the mixture was uniformly coated on a square glass plate (70 mm × 70 mm × 1.3 mm) at 23 ◦ C ± 1 ◦ C using a 76 ␮m thickness applicator (Yoshimitsu Seiki, Tokyo, Japan) which has ± 10 ␮m deviation. This lacquer was stored in the dark at 25 ◦ C and 50% RH to evaluate the drying and hardness properties. For preparation of the thick resin, the mixture of natural lacquer, BPAE, and organosilicon compounds was poured into a fluororesin tube (outside diameter: 16 mm, inside: 14 mm, and thickness: 20 mm) at room temperature. After three days, formed thick resin was taken from the tube. The effect of thermal treatment on the curing property of hybrids was also investigated. 2.3. Characterization The molecular weight of the hybrid was determined at 40 ◦ C by aqueous phase gel permeation chromatography (GPC; TSK-gel column ␣-3000, ␣-4000 and ␣-M, ␾7.8 mm × 300 mm × 3, Tosoh Co. Ltd., Tokyo, Japan) using dimethylformamide (DMF) as an eluent with 0.01 mol LiBr on a high-performance liquid chromatography

25

system with an RI-8012 refractive-index detector with polystyrene standards. The elution rate was 0.8 ml/min. The drying process of the epoxy coatings at 23 ◦ C ± 1 ◦ C can be divided into three stages: dust-free dry, touch-free dry, and harden dry (HD). Each stage was measured using an automatic drying time recorder (RC auto-recorder of painting drying time, TaiYu Co. Ltd., Osaka, Japan) at 23 ◦ C ± 1 ◦ C and 60% relative humidity. The pencil hardness is performed based on the current national standard of GB/T6739-1996. H and B indicate the hardness and softness, respectively, of tested coatings, and higher numbers express the relative hardness or softness of the tested coatings. F and HB indicate medium hardness. However, F is a slightly harder coating than HB. In the present study, pencil lead hardness was determined using a C-221 (Yoshimitsu Seiki, Tokyo, Japan) at 23 ◦ C ± 1 ◦ C. The gel content of the epoxy coating was determined. Coatings were immersed in acetone at 23 ◦ C ± 1 ◦ C for 24 h, and the nonsoluble parts were filtered and dried in a Taiyo muffle furnace (Isuzu, Tokyo, Japan) for 1 h at 50 ◦ C, cooled, and subsequently examined at room temperature to remove residual solvent before weighing. The gel content was calculated using the following equation: gel content(%) =

M1 × 100 M0

(1)

where M1 and M0 are the weight of the insoluble fraction and the original weight of the completely dried epoxy coating, respectively. The structural analysis was conducted via liquid-state proton and carbon nuclear magnetic resonance (1 H- and 13 C-NMR) spectroscopies using a JNM-ECA500 spectrometer (JEOL Ltd., Tokyo, Japan). Samples were dissolved in deuterated dimethyl sulfoxide (DMSO) solution with chemical shifts referenced from tetramethylsilane (TMS). Cross-polarization/magic angle spinning (CPMAS) solid-state 13 C-, 29 Si-, and 15 N-NMR experiments were performed on a JNM-ECA400 NMR spectrometer (JEOL Ltd., Tokyo, Japan) using a zirconium sample tube (␾6 mm). 3. Results and discussion 3.1. Molecular weight distribution GPC was conducted to determine the molecular weight distribution of hybrids, and the results are summarized in Table 1. As the organic silane content increased, the molecular weight and molecular weight distribution tended to increase. This result could be based on the sol–gel reaction between organic silane and urushiol, as previously reported [12]. On the other hand, the increase in the molecular weight for hybrids using AATES seemed to be lower than that of other silane compounds such as AATMS and APTMS. This result indicates that the organic silane reactivity of the sol–gel reaction in the methoxy group seems to be higher than that in ethoxy group. Therefore, the molecular weight of hybrids was affected more by the use of organic silane compounds other than BPAE. 3.2. Drying and hardness properties To determine the suitable addition ratios of organic silane and BPAE, we measured the drying and hardness properties of various bio-based hybrid lacquers. The drying property of the lacquers is summarized in Table 2. The addition of BPAE and silane compound to the natural lacquer significantly improved the drying property at room temperature. The hybrid lacquers prepared with 20 wt% of BPAE (Entries 3, 6, and 9) dried more rapidly than natural lacquer (Entry 1). On the other hand, an BPAE at higher concentration (i.e., 30 wt%) decreased the drying property, indicating that the excess BPAE could prevent the sol–gel reaction between urushiol and the

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S. Kanehashi et al. / Progress in Organic Coatings 77 (2014) 24–29

Table 1 Molecular weight of bio-based hybrid lacquers. Entry

1 2 3 4 5 6 7 8 9 10 a

Silane

None AATMS

AATES

APTMS

Additive content (wt%)

Content ratio (%)a

Silane

BPAE

Monomer

Oligomer

Polymer

Mn

Mw

Mw /Mn

0 10 20 30 10 20 30 10 20 30

0 30 20 10 30 20 10 30 20 10

49.1 47.3 24.4 21.0 57.4 47.3 44.1 44.5 37.2 17.2

45.7 35.9 48.4 50.6 34.7 40.3 42.9 38.9 42.0 52.0

5.2 16.8 27.2 28.4 7.9 12.4 13.0 16.6 20.8 30.8

510 520 1070 1030 430 530 610 540 1260 1120

2140 3840 7560 8080 1670 3190 3360 3960 9240 9320

4.2 7.4 7.1 7.8 3.9 6.0 5.5 7.3 7.3 8.3

Molecular weight

Oligomer: dimer 5 molecular weight < 10,000 g/mol, polymer: molecular weight = 10,000 g/mol.

Table 2 Drying property and hardness of baio-based hybrid lacquers. Entry

Silane

1 2 3 4 5 6 7 8 9 10

None

a b

AATMS

AATES

APTMS

Additive content (wt%)

Drying property (h)a

Silane

BPAE

DF

TF

HD

1 day

2 days

3 days

7 days

14 days

0 10 20 30 10 20 30 10 20 30

0 30 20 10 30 20 10 30 20 10

18.5 6.9 0.9 0.5 17.7 1.4 1.2 7.5 0.2 1.5

23.1 20.0 4.6 1.9 24< 12.3 4.6 17.2 6.9 7.7

24< 24< 13.1 14.6 24< 23.9 16.9 23.6 13.1 20.0

TF TF HD DF HD HD HD HD HD HD

HD HD 6B HD DF HD HD HD HD HD

HD HD 6B HD TF HD HD HD HD HD

HD HD 6B HD HD HD HD HD 6B HD

6B HD 4B 5B HD HD HD HD 6B 6B

DF: dust free dry, TF: touch free dry, and HD: harden dry (based on JIS-K-5400). Pencil hardness: HD  6B < B < HB < F < H  9H.

organic silane compound. There is no obvious difference in the hardness of bio-based hybrids, regardless of the kind of organic silane compound and their ratios. Among them, the hybrids using AATMS seem to be hard compared with the natural lacquer. Based on these results, the better condition for preparation of hybrids using BPAE and organic silane compound seems to be 20 wt% of AATMS and 20 wt% of BPAE (Entry 3) in this study. 3.3. Molding property The molding property of bio-based hybrids at room temperature is summarized in Table 3. The natural lacquer cannot cure thick resin materials because of the slow oxidization reaction. The hybrids composed of 20 wt% of organic silanes, AATMS and APTMS, and 20 wt% of BPAE showed good molding property. This result is in good agreement with the drying and hardness properties of the hybrids. This is because the reactions among urushiol, BPAE, and organic silane improved their molding property. However, the Table 3 Molding property of bio-based hybrid resins. Entry

Silane

1 2 3 4 5 6 7 8 9 10

None

a

Hardnessb

AATMS

AATES

APTMS

Additive content (wt%) Silane

BPAE

0 10 20 30 10 20 30 10 20 30

0 30 20 10 30 20 10 30 20 10

: cured, : cured but brittle, and ×: uncured.

Molding propertya

×         

hybrids using AATES cured but were very brittle. This result was consistent with the GPC measurement. Therefore, the methoxy or ethoxy groups in organic silane compounds affect the sol–gel reaction with urushiol or BPAE. In addition, the effect of excess organic silane or BPAE on curing at room temperature could prevent the enhancement of drying for thick resin and therefore reduce the molding property. Basically, thermal treatment enhances the curing time of thermosetting resin materials. The effects of temperature on curing of the bio-based hybrids using AATMS and BPAE are summarized in Table 4. The cured hybrid resins were obtained with more than 30 min at 70 ◦ C. As the temperature increased, the resins tended to cure but were brittle. As mentioned before, the sol–gel reaction of this system proceeded easily at room temperature. The rapid solgel reaction between urushiol and AATMS at high temperature (i.e., over 80 ◦ C) could make more rigid structure, indicating that this rigidity could reduce the molding property. In addition, methanol as a by-product in the sol-gel reaction between could erupt explosively from the resins at high temperature, suggesting that boiling of the methanol in the resins may affect the brittleness. Therefore, these results suggest that this bio-based hybrid can be used in thermosetting resin materials because the thermal treatment of this hybrid enhanced the curing property.

3.4. Gel content Fig. 2 presents the gel content of the natural and bio-based hybrid lacquers. As expected from the drying and hardness properties of hybrids, the gel content of all hybrids were higher than that of the natural lacquer, indicating that the hybrid lacquers prepared from BPAE and organic silane had a higher crosslink density than the natural lacquer. The detailed structure of the hybrid was analyzed using hybrids prepared from BPAE and AATMS because they

S. Kanehashi et al. / Progress in Organic Coatings 77 (2014) 24–29

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Table 4 Effect of curing temperature and time in molding property of hybrid lacquer resins. Entry

Silane

Silane 1 2 3 4 5 a

Temperature (◦ C)

Additive content (wt%) BPAE

20

20

AATMS

60 70 80 90 100

Time (min) 10

20

30

40

×a × ×  

× ×   

    

    

: cured, : cured but brittle, and ×: uncured.

100 AATMS 20wt% BPAE 20 wt%

Gel content (wt%)

80

AATES 20wt% BPAE 20 wt%

60 None

APTMS 20wt% BPAE 20 wt%

40

20

0

0

2

4 6 Time (days)

8

10

Fig. 2. Gel content of natural lacquer and bio-based hybrid lacquers. Fig. 4.

13

C-NMR spectra of BPAE, urushiol, and bio-based hybrid (solvent: DMSO).

seem to be good components to prepare the hybrid resin based on the GPC, drying, and hardness properties. 3.5. Structure analysis 3.5.1. Liquid-state NMR Fig. 3 presents liquid-state 1 H-NMR spectra of BPAE, urushiol, and hybrids prepared from BPAE and AATMS. A decrease in the peak at 7.3–6.8 ppm, which corresponds to the aromatic proton of BPAE, was observed, whereas the peak at 6.7–6.5 ppm which corresponds to the aromatic proton of urushiol, decreased and was shifted and broadened by the addition of AATMS [14]. In addition, the peaks at 2.69 and 2.81 ppm corresponding to the epoxy group of BPAE decreased in the hybrid. On the other hand, a new peak at 3.19 ppm could be based on the methanol produced by

Fig. 3.

1

H-NMR spectra of BPAE, urushiol, and bio-based hybrid (solvent: DMSO).

alcoholysis and the sol–gel reaction between urushiol and AATMS. Based on these results, the sol–gel reaction between urushiol and organic silane compound proceeded, and the epoxy crosslink reaction between BPAE and amine group occurred in the AATMS. Fig. 4 presents liquid-state 13 C-NMR spectra of BPAE, urushiol, and the hybrid. Similarly, the peaks at 41 and 44 ppm corresponding to the epoxy group of BPAE were shifted. Furthermore, the urushiol aromatic carbons in hybrid detected at 145–141, 129–120, and 112 ppm were broadened. In addition, the aromatics of BPAE at 158 and 143 ppm decreased. Based on these results, the reaction proceeded among urushiol, BPAE, and silane. On the other hand, a new broad peak at 150–145 ppm was detected in the hybrid. This peak was assigned to silyloxy ( O Si ) units [14,15].

Fig. 5.

13

C CPMAS spectra of urushiol, urushiol + silane, and bio-based hybrid.

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S. Kanehashi et al. / Progress in Organic Coatings 77 (2014) 24–29

Fig. 6.

29

Si CPMAS spectra of urushiol + silane and bio-based hybrid.

3.5.2. Solid-state NMR Fig. 5 presents the solid-state 13 C-NMR spectra of urushiol, urushiol + silane, and a hybrid. The hydroxyl-substituted carbon of urushiol were detected at 142 ppm, while the peaks of these carbons were slightly shifted to the low magnetic field and appeared at 148 ppm for urushiol-silane and the hybrid, implying that the hydroxyl groups of urushiol have been replaced by the silyloxy ( O Si ) units [14]. The aromatic carbon of BPAE detected at 158 ppm as presented Fig. 4, disappeared in the hybrid, suggesting that the reaction among these components proceeded. Fig. 6 presents the solid-state 29 Si-NMR spectra of urushiol + silane and the hybrid. The peaks from −55 to −70 ppm which belonging to highly condensed-siloxane ( Si O Si ) units were detected in all hybrids [14,16,17]. This is because the sol–gel reaction among silane, urushiol, moisture in the natural lacquer, and OH groups was produced by the epoxy reaction. It is considered that predominantly the sol–gel reaction occurred and that a condensed inorganic network was formed in the hybrids [14,18,19]. Furthermore, from the disappearance of the methoxy carbon peak seen at 50 ppm in the 13 C-NMR spectra, it can be surmised that the other linkage units were connected to the urushiol and showed as phenoxysilane [12,14]. 3.5.3. Possible reaction The 1 H- and 13 C-NMR analyses and GPC measurement of hybrids showed the sol–gel reaction between urushiol and the organic silane compound proceeded immediately, indicating the high

reactivity of this sol–gel reaction. On the other hand, the reaction between BPAE and the organic silane progressed slowly after the epoxy ring opening. These spectra also showed that the epoxy in BPAE and amine group in organic silane had reacted almost completely after curing. Based on these results, the possible reaction route is presented in Fig. 7. Firstly, the sol–gel reaction between urushiol and organic silane compound proceeded, and then the crosslink reaction between BPAE and organic silane occurred. A decrease in the hydroxyl groups in the urushiol promoted the autoxidation of urushiol. According to the solid-state NMR, almost all primary and secondary amine groups reacted with the epoxy of BPAE. Furthermore, the methoxy group of organic silane reacted with not only urushiol but also with the hydroxyl group of BPAE produced by a crosslink reaction. Therefore, this bio-based hybrid could be based on the chemical reactions among all components (i.e., natural lacquer, epoxy, and organic silane). In addition, this bio-based hybrid material can be applied in industrial applications, not only as coating materials but also resin molding. 4. Conclusions Bio-based hybrid materials were prepared from natural lacquer, epoxy, and organic silane compound. According to structure analysis, the sol–gel reaction between natural lacquer and organic silane compound, and a crosslink reaction between organic silane and epoxy groups proceeded. An excess of BPAE or organic silane prevented each chemical reaction (i.e., sol–gel and crosslink) based on the GPC and the drying, hardness, and resin molding properties. At the same time, the oxidative polymerization at the unsaturated chains in the urushiol was enhanced by the sol–gel reaction because the catechol hydroxyl group, which has an antioxidative property, reacted with the organic silane. In addition, the curing time of the hybrid was improved by thermal treatment, indicating that this resin was thermosetting property. NMR spectra showed that almost all primary and secondary amine groups reacted with BPAE. Furthermore, the methoxy group of organic silane reacted not only with urushiol but also with the hydroxyl group of BPAE formed after the crosslink reaction. These results suggest that the enhancement of drying, hardness, and molding properties of the hybrid was based on chemical reactions among all components (i.e., natural lacquer, epoxy, and organic silane). Therefore, this bio-based hybrid material can be applied to industrial applications as not only a coating but also to produce thick resin materials.

Fig. 7. Possible reactions among urushiol, BPAE, and organic silane compound.

S. Kanehashi et al. / Progress in Organic Coatings 77 (2014) 24–29

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