Article pubs.acs.org/Langmuir
Spray-Assisted Nanocoating of the Biobased Material Urushiol Hirohmi Watanabe,*,†,‡,§ Aya Fujimoto,† and Atsushi Takahara*,†,‡,§ †
JST, ERATO Takahara Soft Interfaces Project, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan Institute for Materials Chemistry and Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
‡
S Supporting Information *
ABSTRACT: We have demonstrated the spray-assisted coating of the catechol derivative, urushiol. Spraying a mixture of urushiol and iron(II) acetate formed a uniform coating about 10 nm thick, as confirmed by AFM observations. XPS measurements revealed that various substrates, including polyolefins and thermosetting resins, were successfully coated with urushiol. The coating showed good solvent tolerance and coating adhesion after baking at 100 °C for 10 min or after aerobic oxidation for several days. Interestingly, quartz crystal microbalance (QCM) measurements and strain-induced elastic buckling instability for mechanical measurements (SIEBIMM) revealed that density and Young’s modulus of the spray-assisted nanocoatings were higher than those of spray-coated samples. Moreover, the coating was uninvolved in physical properties except surface properties, as demonstrated by several experiments. Because urushiol is a promising biobased material, our unique spray-assisted coating technique could provide a general approach for material-independent surface modification techniques that are environmentally sustainable.
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chains accelerate the curing,3 and thermal curing is complete within minutes, whereas conventional oxidative polymerization of phenolic and catechol derivatives means they require more than 12 h to cure. Moreover, the flexibility of the side chain imparts robustness to the cured material.3 These properties make urushiol a promising coating material among the phenolic and catechol derivatives. In this work, we demonstrated a simple surface modification method using urushiol. A sprayassisted nanometer-thick coating was applied to the material surface, and the physical properties, such as the mechanical properties of the material, were examined. Moreover, the surface antioxidant properties were investigated. Because urushi is suitable for coating a variety of substrates, our method could be used as a material-independent approach for surface modification.
INTRODUCTION Urushi (oriental lacquer) is a traditional natural resin that is often used as a coating for wood or paper substrates. The resin is tapped simply and permanently from the lacquer tree (Toxicodendron vernicifluum); urushi is regarded as a renewable resource because it can be cultivated. Products coated with urushi, such as Japanese lacquerware (or urushiware), are highly prized for both their artistic value and their superior physical and chemical properties for practical use.1,2 The main component in urushi, called urushiol, is a catechol derivative with a long unsaturated hydrocarbon side chain, and the chemical structure imparts various properties to the coatings.3 One of the impressive characteristics of urushi coatings is their antioxidant properties. “Urushigami monjo” are ancient paper documents that were impregnated with urushi by putting paper on the surface of urushi sap in a container. Because the documents have remained intact for more than 1500 years, they are valuable primary source materials for studying Japanese history. However, impregnation techniques similar to those used for lacquer-impregnated documents produce thick coatings that significantly change the physical properties of the material. For example, lacquer-impregnated paper loses its flexibility. Therefore, achieving a coating thinner than 1 μm is important for retaining the physical properties of the material.4 Many phenolic and catechol derivatives are found in nature besides urushiol.5−8 For example, 3,4-dihydroxyphenylalanine residues form an adhesive protein in mussels that is now attracting attention as a biomimetic natural adhesive and coating material.7 Plant tissues also contain various phenols and polyphenols, such as gallic acid and tannic acid.8 The advantage of urushiol over other phenolic and catechol derivatives arises from the unique side chain. Thermal Diels−Alder reactions and thermal addition reactions of the unsaturated hydrocarbon side © XXXX American Chemical Society
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EXPERIMENTAL SECTION
Spray-assisted urushiol coating was performed as follows. Iron(II) acetate was first dissolved in a 5:57 (v/v) mixture of ethanol and propylene glycol monomethyl ether acetate. Next, urushiol was added to the solution and the solution changed from light brown to black. Caution: Uncured urushiol can cause an allergic skin rash on contact and should be handled carefully. The iron(II) acetate concentration was 0.5 equiv with respect to urushiol. The typical concentration of urushiol was 10 mM. The solution was homogenized with an ultrasonic homogenizer (Branson Sonifier ultrasonic cell disruptors; Emerson Electric Co., U.S.), and then was transferred to an air pump spray can (Cuisipro #83 7530; Browne & Co., Canada) for the spray deposition. The reservoir of the can was made of polypropylene with a capacity of 8 fl oz (236.6 mL). The size of the nozzle was 500 μm in Received: October 27, 2014 Revised: February 8, 2015
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DOI: 10.1021/acs.langmuir.5b00131 Langmuir XXXX, XXX, XXX−XXX
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Langmuir
Figure 1. Change in water contact angle value of urushiol-Fe coatings with time (a) without baking and (b) with baking at 100 °C. diameter. The can was filled with the solution and pressurized by pumping cycles so that the spray rate remained approximately constant during the entire deposition process. The spray rate was about 1.05 mL/s as directly determined by the volume measurements. Spraying was carried out perpendicular to the substrate surfaces, which were fixed vertically. This allowed drainage of the liquid along the surface.9 Typical spraying time was about 2 s. The wet sample was kept in air for a while to dry the surface, and was then baked at 100 °C for 10 min as necessary. Poly(dopamine) (PDA) nanocoating was performed by soaking the substrates in a dilute aqueous solution of 3,4-dihydroxyphenethylamine hydrochloride (DA) containing 10 mM tris(hydroxymethyl)aminomethane (TRIS buffer, pH 8.5) for 24 h.7 After being rinsed with pure water three times, the sample was dried in air.
mode of the unsaturated double bonds in the side chain, were observed when the material was measured immediately after spraying on an Au substrate (Figure S1(a), Supporting Information). The peaks decreased in size over several days, indicating the aerobic oxidation of the unsaturated bonds in the side chains to form a cross-linked structure. The peaks decreased in size immediately after baking at 100 °C because of the thermal reaction of the unsaturated bonds (Figure S1(b), Supporting Information). XPS measurements were conducted to determine the chemical species present at the surface. When 10 mM urushiol-Fe was sprayed on the VUV-treated Si substrate, only C1s, O1s, and Fe2p3 peaks were observed, at 285, 532, and 712 eV, respectively (Figure S2(a), Supporting Information). However, Si2p and Si2s peaks at 98.5 and 150 eV were observed when 1 mM urushiol-Fe was sprayed (Figure S2(b), Supporting Information). The Si peak was observed regardless of the spraying time up to 10 s, indicating that the Si substrate was partially coated. Therefore, a urushiol concentration of 10 mM was used in further experiments. The atomic ratio was calculated from the narrow-scan spectra. The C/O atomic ratio of the 10 mM urushiol-Fe on the Si substrate was in good agreement with the theoretically calculated value (Table 1).
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RESULTS AND DISCUSSION Spray-Assisted Urushiol Coating. The coating was qualitatively evaluated by static water contact angle (CA) measurements. When the mixture of 10 mM urushiol and iron(II) acetate (abbreviated as “urushiol-Fe”) was sprayed on vacuum UV (VUV)-treated Si substrates (CA = 23°), the CA of the substrate was changed to 85−90°, demonstrating that the coating method was successful. A change in CA was also observed when the concentration of the solution was reduced to 1 mM. In contrast, the CA value was not changed when urushiol alone was sprayed without iron(II) acetate. Adding iron(II) acetate causes the oxidative reaction of the catechol moiety of urushiol to form oligomers,3 and a relatively highmolecular-weight precursor could be required for spray-assisted coatings. Interestingly, spraying the mixture of DA with iron(II) acetate or with TRIS aqueous solution also did not work. The CA of the samples did not change after the coating. This suggests that the long alkyl side chain of urushiol is important in spray-assisted coatings. The CA value of the coated Si substrates gradually decreased with time (Figure 1a) and reached a constant value of 75−77° several days after spraying. The change in CA arose from the aerobic oxidative reaction of the unsaturated bonds in the urushiol side chains.1 The change in CA was accelerated by baking at 100 °C (Figure 1b), and the CA reached a constant value of around 72−73° within 1 h. The unsaturated bonds in the side chain were rapidly consumed by the thermal Diels− Alder cycloaddition and other addition reactions during the baking.3,10 The saturated CA value was in good agreement with that of a thermally cross-linked, spin-coated sample (CA = 67− 69°).11 The molecular structure of the material was analyzed by IR measurements. The absorption peaks at 3015 and 945 cm−1, which are the stretching vibration and out-of-plane bending
Table 1. Atomic Ratio of Urushiol-Coated Si Substratesa spray (10 mM) spray (10 mM) + baking spray (1 mM) + baking spin-coating + baking theoretical
C (%)
O (%)
Fe (%)
Si (%)
80.7 84.5 49.4 74.1 83.6
17.6 14.6 30.1 25.0 14.5
1.5 0.9 0.4 0.7 1.8
