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Benzodifuranone based color-changing epoxy-polyamine coating
Dyes and Pigments 164 (2019) 198–205
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Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig
Benzodifuranone based color-changing epoxy-polyamine coating a
b
a
Weixiu Zeng , Zhifeng Deng , Haoran Wang , Haichang Zhang
b,c,∗∗
, Qixin Zhou
T
a,∗
a
National Center for Education and Research on Corrosion and Materials Performance, Department of Chemical and Biomolecular Engineering, The University of Akron, Akron, OH 44325, United States b National and Local Joint Engineering Laboratory for Slag Comprehensive Utilization and Environmental Technology, School of Material Science and Engineering, Shaanxi University of Technology (SNUT), Hanzhong, Shaanxi 723001, PR China c Key Laboratory of Rubber-Plastics of Ministry of Education/Shandong Province (QUST), School of Polymer Science & Engineering, Qingdao University of Science & Technology, Qingdao, Shandong 266042, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Benzodifuranone Color-changing Epoxy-polyamine Coating Spectral reflectance
A new color-changing epoxy coating was developed by incorporating benzodifuranone (1,4-BDF) pigments into epoxy-polyamine coating matrix. The 1,4-BDF based epoxy coating exhibited a color change from dark blue to brown responding to environmental conditions, such as temperature, UV light, and pH. The color of the coating was demonstrated by the spectral reflectance and CIELAB values measured by a spectro-guide spectrophotometer. The color change was due to the reaction of 1,4-BDF pigment with the polyamine curing agent to form an unstable product characterized by UV/Vis and FTIR analysis. Moreover, the coating's opacity and light reflectance changed along with the color change. Opacity is an effective parameter to link the UV accelerated testing and sunlight exposure to demonstrate the change of the coating's appearance.
1. Introduction Color-changing materials are those which exhibit changes in their color (reversible or irreversible) as a response to a stimulus in environmental conditions. The stimulus can be the change of temperature, light intensity, pH, electric field, or magnetic field, and the corresponding materials are thermochromic, photochromic, chemichromic, electrochromic, and magnetochromic materials, respectively [1]. Colorchanging materials can be designed as useful instruments to extend the aesthetic, functional, and energy-saving performance of objects. Colorchanging materials have been applied in many aspects, for example, in inks to display the desired color, in lenses to filter the infrared radiation, and in architecture to optimize the energy consumption [2–4]. Recent developed color-changing coatings are mostly thermochromic and photochromic coatings applied in buildings, especially for roofs and windows [5–8]. These thermochromic coatings response to the environment thermally and change their color from darker to lighter as the temperature rises. These color-changing coatings were developed by adding thermochromic pigments. However, some thermochromic pigments were sensitive to the components of the coatings and lost their properties after blending with the coating matrix. Hence, these pigments were microencapsulated before incorporating into the
coatings to retain their thermochromic property [4,9]. The photochromic coatings change color while undergoing UV irradiation. Similar to thermochromic coatings, they experience the color change because of the incorporated photochromic colorants. Previously, our group developed a one-coat epoxy system with bright orange color, UV stability, and corrosion resistance by incorporating diketopyrrolopyrrole [10]. Due to the promising properties of benzodifuranones (BDFs), such as high photo- and thermal stability, good brightness, and etc., it is desirable to study the BDF based one-coat epoxy coating. BDFs belong to a class of high performance pigments, which were first reported by Greenhalgh et al. in 1980 [11]. BDFs exhibit red to blue colors depending on the substitution pattern. Only a few BDF derivatives were synthesized and used as functional blocks in the past 30 years [12–16]. In recent years, BDF or its analogs have been attracting a great attention owing to their potential to make conjugated polymers and copolymers for developing organic photovoltaic devices [17–25]. BDFs exhibit advantages in deep color value, high tinctorial strength, and good brightness. On the basis of these advantages, BDFs had been used as dispersing dyes for textiles, especially for polyesters [26]. However, to the best of the authors’ knowledge, BDFs have not be reported in organic coating applications. Based on this work, 3,7-diphenyl-benzo[1,2-b:4,5-b’]-difuran-2,6-
∗
Corresponding author. Corresponding author. National and Local Joint Engineering Laboratory for Slag Comprehensive Utilization and Environmental Technology, School of Material Science and Engineering, Shaanxi University of Technology (SNUT), Hanzhong, Shaanxi, 723001, PR China. E-mail addresses: [email protected] (H. Zhang), [email protected] (Q. Zhou). ∗∗
https://doi.org/10.1016/j.dyepig.2019.01.016 Received 25 August 2018; Received in revised form 27 December 2018; Accepted 12 January 2019 Available online 17 January 2019 0143-7208/ © 2019 Elsevier Ltd. All rights reserved.
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dione (1,4-BDF) was synthesized and first-time incorporated in an epoxy-polyamine coating matrix. An interesting phenomenon of the 1,4-BDF based epoxy coating was found that the coating experienced a series of color change during the coating preparation. The color change of the coating was studied and quantified by using a spectrophotometer. The influence of environmental conditions on the color change, such as temperature, UV light, and pH were comprehensively investigated. The reason for the color change was understood by UV/Vis and FTIR characterizations. Besides color change, the opacity and light reflectance of the pigmented coatings were also evaluated. The opacity of the coatings under the UV accelerated testing and sunlight exposure was compared.
epoxy resin-EPON™ Resin 828 was added and magnetically stirred for 10 min. Subsequently, the curing agent-EPIKURE™ 3164 was added into the mixture. After the mixture was homogenized under stirring for 10 min, it was de-gassed in an ultrasonic bath for 5 min. The coating was applied by a film applicator onto the steel panels with a wet film thickness of 120 μm. Before applying the coating, the panels had been rinsed with acetone and deionized water. All the coating samples were cured at room temperature and kept away from sunlight for one week. The final dry film thickness was 49 ± 4 μm. All the coating samples contain 2 wt % (solid weight) of the 1,4-BDF pigments.
2. Experimental
2.4.1. Color measurement The spectral reflectance, CIELAB values, and opacity under different environmental conditions were measured by using a spectro-guide spectrophotometer (BYK). The spectral reflectance was obtained in the wavelength range from 400 to 700 nm. The CIELAB values were acquired with a D65 illuminant and 10° standard observer. The opacity was recorded before and after exposing coatings under UV light or sunlight.
2.4. Measurements
2.1. Materials Epoxy resin (EPON™ Resin 828) and curing agent (EPIKURE™ Curing Agent 3164) were kindly provided by Hexion Specialty Chemicals. The weight ratio of EPIKURE™ Curing Agent 3164 to EPON™ Resin 828 is 1.36. Curing agent D230 (Poly(propylene glycol) bis(2aminopropyl ether)), 1,4-dihydroxybenzene, mandelic acid, acetone in ACS grade, 1,2,4-trichlorobenzene, and nitrobenzene were purchased from Sigma-Aldrich. Deionized water was produced by Smart2Pure water purification system (Thermo Scientific). Steel test panels (QD-39) (75 cm × 225 cm) were purchased from Q-lab. All the chemicals were used without further purification in this study.
2.4.2. 1H NMR & 13C NMR 1 H NMR spectra and 13C NMR spectra were obtained using a Mercury-300 spectrometer (Varian) in DMSO‑d6. Chemical shifts were reported as δ values (ppm). 2.4.3. UV/Vis spectra The UV/Vis spectra of all samples were collected in solid states by using a UV-1800 UV/Vis spectrophotometer (Shimadzu). The spectra were acquired in the wavelength range from 200 to 800 nm.
2.2. Synthesis of 1,4-BDF 3,7-diphenyl-benzo[1,2-b:4,5-b’]-difuran-2,6-dione (1,4-BDF) was synthesized in the lab. Using a Dean-Stark apparatus, 1,4-dihydroxybenzene (1.1 g, 10 mmol), and mandelic acid (4.5 g, 30 mmol) were dissolved in 1,2,4-trichlorobenzene (20 mL). The reaction mixture was stirred for 4 h at 200 °C allowing formed water to distill off before being cooled down to room temperature. Then nitrobenzene (4.9 g, 40 mmol) was added. The mixture was stirred for another 30 min at 200 °C before it was cooled down, poured into methanol (100 mL), and filtered to recover the crude product. The crude product was refluxed in methanol. After cooling, a red solid compound with a wine color in appearance (2.7 g, yield: 81%) was obtained. 1H NMR (300 MHz, DMSO‑d6) δ ppm: 7.83 (d, J = 15 Hz, 4H), 7.66 (d, J = 51 Hz, 4H), 7.47 (d, J = 15 Hz, 2H), 7.46 (d, J = 15 Hz, 2H). 13C NMR, (300 MHz, CDCl3):135.71, 133.00, 132.76, 132.21, 130.67, 130.62, 130.29, 129.36. Microanalysis found C, 77.62%; H, 3.61% (C, 77.64%; H, 3.55%). UV/Vis (thin film): 396 nm and 500 nm.
2.4.4. Fourier transform infrared spectroscopy Nicolet iS10 FTIR spectrometer (resolution: 4 cm−1; scan number: 32) was used to analyze the chemical composition of the coatings. The FTIR spectra of all coating samples were recorded with the attenuated total reflectance (ATR) model. The range of scanning wavenumber was from 400 to 4000 cm−1. The background was taken before each sample testing. All the FTIR spectra were subtracted the background. 3. Results and discussion 3.1. Property of 1,4-BDF We began our studies with the synthesis of the 1,4-BDF pigment. The synthesis of the 1,4-BDF required the condensation of 1 eq. of 1,4dihydroxybenzene and 2 eq. mandelic acid leading to a double cyclization of derivative, followed by oxidation to the conjugated BDF in a yield of 81%. The chemical structure of the 1,4-BDF is shown in Fig. 1a. The 1H NMR spectra of 1,4-BDF exhibited all of the expected resonances with no discernible peaks corresponding to impurities (See Fig. S1). The chemical shift between 7.47 and 7.83 ppm were typical for the protons
2.3. Preparation of 1,4-BDF based epoxy coating The 1,4-BDF pigments were partially dissolved and dispersed in acetone by using magnetic stirring for 5 min at 1000 rpm. Then, the solution was treated in an ultrasonic bath for 30 min. After that, the
Fig. 1. (a) Chemical structure of 1,4-BDF. (b) Digital image of the synthesized 1,4-BDF pigment. (c) Morphology of the 1,4-BDF pigment. 199
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Fig. 2. Digital images of the color-changing phenomenon during the preparation of the 1,4-BDF based epoxy coating. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
after heating for 2 h at 40 °C and 60 °C, while it changed to brown after heating to 100 °C. A transition color between dark blue and brown appeared at 80 °C. The color change can be quantified by measuring the spectral reflectance, because color is the quality of an object with respect to the light reflected by the object. For example, violet objects are the objects that reflect violet light. To quantify the color change, the spectral reflectance over the visible region (400–700 nm) were recorded by a spectrophotometer. As indicated in Fig. 3b, the spectral reflectance curves of the coatings after heating at 40 °C and 60 °C are close to that of the control sample, indicating the color difference is very negligible. The spectral reflectance increased when the coating heated at 80 °C and reached to the highest value when the coating heated at 100 °C. The change of the reflectance curves for the coating samples at different temperature presented the exact same trend as the color change shown in Fig. 3a. For further depicting the color change, the color was described in the CIELAB color space system, as shown in Fig. 3c. In the CIELAB color space, L∗ is the lightness axis (black (0) to white (100)); a∗ is the green (< 0) to red (> 0) axis; and b∗ is the blue (< 0) to yellow (> 0) axis. Fig. 3c shows that all the CIELAB values (L∗, a∗, and b∗) increased as the increase of the temperature. The increase of b∗ indicates the color evolves from blue to yellow, while the increase of a∗ demonstrates the color changes from green to red. The change of a* and b* presents the color change from dark blue to brown (Fig. 3a). As the L∗ value getting higher, the coating becomes translucent. Both the spectral reflectance curves and CIELAB data quantitatively evaluated the color change of the coatings at different temperatures, which verified that the temperature affected the color of the 1,4-BDF based epoxy coating.
of the phenyl ring of BDF, while the core of BDF's protons were at 7.46 ppm. The synthesized 1,4-BDF pigment exhibits a wine color appearance (Fig. 1b) and flake shape under the observation of SEM (Fig. 1c). 3.2. Color change A series of color change was found during the process of preparing 1,4-BDF based epoxy coatings. As shown in Fig. 2, the solid 1,4-BDF has a wine color appearance. A light brown color was found as the pigment was partially dissolved and dispersed in acetone. A wine-colored pastry was formed as adding the epoxy resin (EPON 828) into the pigment solution. However, the color changed to dark blue immediately once the curing agent was added into the mixture. After well mixing, the coating was applied on the steel panel. The coatings maintained dark blue color when they were cured at room temperature, while the coatings' color changed to brown if they were cured at 100 °C. To the authors’ knowledge, the color change of 1,4-BDF during coating preparation has yet been reported. 3.2.1. The effect of temperature Based on the color change during the coating preparation, it is proposed that the 1,4-BDF based epoxy coating will change its color as the change of the temperature. To test this hypothesis, the coating samples were put in the oven at different temperatures (40, 60, 80, and 100 °C). After 2 h in the oven, the samples were taken out for testing. To avoid other environmental influences, the coating samples were cured at room temperature away from sunlight for one week before putting into the oven. The coating sample cured at room temperature was used as the control. As shown in the digital images of Fig. 3a, there is a gradual color change as increasing the temperature. At room temperature, the coating exhibited a dark blue color and the color maintained almost the same
3.2.2. The effect of UV From our preliminary research, we found that the 1,4-BDF based epoxy coating changed color when it was exposed to the sunlight. Since Fig. 3. Color change of the 1,4-BDF based epoxy coating samples after heating at different temperatures. (a) Digital images of the coating samples after heating at different temperatures for 2 h. (b) Spectral reflectance of the coating samples after heating at different temperatures for 2 h. (c) The change of L∗, a∗, and b∗ values of the coating samples as a function of the heating temperature. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
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Fig. 4. Color change of the 1,4-BDF based epoxy coating samples before and after UV exposure (UVA-340 of 0.68 W/m2 exposure). (a) Digital images of the coating samples with different UV exposure time periods. (b) Spectral reflectance of the coating samples with different UV exposure time periods. (c) The change of L*, a*, and b* values of the coating samples as a function of UV exposure time. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
reflectance in the range of 550–650 nm should present a brown color (yellow + orange) appearance, which was observed for the samples after > 6 h exposure. We also explored how the CIELAB values (L∗, a∗, and b∗) changed with the UV exposure time. As can be seen in Fig. 4c, all the values increased after UV exposure. The increase of a∗ and b∗ presented the coating color changed to brown, while the increase of L∗ demonstrated the coating became more transparent. Similar to the effect of temperature, the color of the 1,4-BDF based epoxy coating was influenced by the UV light as measured by the spectral reflectance and CIELAB color system. The coating became browner and more transparent with the longer UV exposure.
sunlight consists of UV light, visible light, and infrared light, and material change or degradation were mostly caused by the UV light, it is proposed that the color change of the coating under the sunlight mainly comes from the influence of the UV light. To test this hypothesis, the coating samples were exposed to UVA (340 nm) radiation at 45 °C with a 0.68 W/m2 irradiance. The samples used for UV testing were cured at room temperature with a dark blue color. Fig. 4a shows that the color evolution of the 1,4-BDF based coating as the UV exposure time increases. Before UV exposure, the color of the coating was dark blue, while it changed to brown after exposing 6 h. As the exposure time continuously increasing, the color became browner. When the exposure time increased to 20 h, the coating not only showed a brown color but also looked translucent, which means the hiding power of the coating was dropped. The color change was also quantified by spectral reflectance curves (Fig. 4b). During the 20 h UV exposure, the reflectance in the entire spectral range increased with the UV exposure time. The lowest reflectance was observed for the coating without exposure, while the highest reflectance was the one with the longest exposure time. It is worth mentioning that the coatings with > 6 h exposure exhibited high broad reflection bands in the range of 550–650 nm. Since the reflectance around 570–590 nm produces a yellow color and the reflectance around 590–620 nm produces an orange color, the high
3.2.3. The effect of pH Besides the color change responding to the temperature and UV light, the effect of pH was also investigated. The coating samples were prepared according to the same procedure in the experimental section, which means the coatings were cured at room temperature without the sunlight. The color of the coating samples before testing was dark blue. Five coating samples were immersed in different pH solutions (pH from 1.05 to 12.49), respectively, as shown in Fig. 5. At certain time periods (1, 3, 6, 9, 12, 24, and 48 h), the coated panels were removed from the setup, cleaned by deionized water, and gently swiped by swipes for
Fig. 5. The coating samples were immersed in the solutions with different pH. 201
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Fig. 6. Color change of the 1,4-BDF based epoxy coating samples after immersion in different pH solutions. (a) Digital images of the coating sample after immersion in pH = 1.05 solution. (b) Digital images of the coating samples after immersion in the other pH solutions (from pH = 3.93 to pH = 12.49). (c) Spectral reflectance of the coating sample after the immersion in pH 1.05 solution. (d) Color difference (ΔE*) change of the coating samples as a function of immersion time in different pH solutions. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
influenced by the very strong acidic environment while it was free from affecting by the acidic or alkaline environment.
color testing. The images in Fig. 6a shows a series of color change for the coating sample under the immersion of pH 1.05 solution for 48 h. The color changed from dark blue to brown after 3 h and then became browner as increasing the immersion time. On the contrast, as shown in Fig. 6b, there was no obvious visible color change for the coatings immersed in the other solutions (pH 3.93 to pH 12.49) up to 48 h. The coating presented a slight light color in the other solutions (pH 3.93 to pH 12.49) may ascribe to the influence of the sunlight since the coating samples did not undertake special treatment to avoid sunlight during the 48 h exposure in different pH solutions. Spectral reflectance of the 1,4-BDF based epoxy coatings before and after immersing in different pH solutions were collected by the same spectrophotometer and procedure as described in the previous sections. Clearly, for coatings immersed in pH 3.93, DI water, pH 9.22, and pH 12.49 solution, the reflectance curves remained the same during the exposure (Fig. S3). Only the coating immersed in pH 1.05 solution obtained an increased reflectance with the immersion time, especially at the range of 550–650 nm. The change of the spectral reflectance presented the same trend as the visible color change (Fig. 6a and b). In the CIELAB color space, the color change can also be described as the color difference (ΔE*). As another method to interpret the color change, ΔE* combines the effect of L*, a*, and b* as defined below,
ΔE ∗ =
3.3. The reason for color change As discussed above, the 1,4-BDF based epoxy coating will change color with the increased temperature, under the UV exposure, or under the low pH solution. Even under the sunlight, the color of the coating will undergo a slight change. The unstable color performance may contribute to a chemical reaction in the coating to form a new product. To test this hypothesis and understand the color change, the coatings under UV exposure were investigated by UV/Vis and FTIR analysis as discussed below. 3.3.1. UV/Vis A set of UV/Vis experiment was applied to analyze the spectral absorption of the 1,4-BDF based epoxy coatings. Fig. 7a shows the UV/ Vis spectra of the 1,4-BDF based epoxy coating before and under UV exposure with different time periods. As the UV exposure time increased, the absorption peaks maintained at the same wavelength (400 nm and 600 nm), but their intensity decreased. The change indicated that the coating's color was fading. To investigate the possible reason for the color change of 1,4-BDF based epoxy coating, different combinations of the coating components were placed under the UV exposure: 1,4-BDF + curing agent, epoxy + curing agent, curing agent itself, and 1,4-BDF itself. The UV/ Vis spectra of these combinations before and after UV exposure were shown in Fig. 7b. There were no characteristic peaks at 400 nm and 600 nm for all the components except for the combination of the 1,4BDF and curing agent. Moreover, after exposing under the UV light, the 1,4-BDF + curing agent exhibited a decrease in the absorption intensity, while the other combinations retained the same. The location of characteristic peaks and evolution trend of the 1,4-BDF + curing agent were the same as the 1,4-BDF based epoxy coating (Fig. S5). This
(ΔL∗)2 + (Δa∗)2 + (Δb∗)2
ΔL∗ = L∗1 − L∗2 ; Δa∗ = a∗1 − a∗2 ; Δb∗ = b∗1 − b∗2 , where 1, 2 represents the value at the different time period, respectively. The larger the ΔE* value, the bigger the color change. The ΔE* values maintained stable after immersing in these pH solutions (pH 3.93, DI water, pH 9.22, and pH 12.49). However, the ΔE* value increased by around 39 after 48 h immersion in the pH 1.05 solution. For the coating in the pH 1.05 solution, the ΔE* values were almost proportional to the immersion time. Thus, based on the results of the spectral reflectance and CIELAB color values, the color of the 1,4-BDF based epoxy coatings was 202
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Fig. 7. (a) UV/Vis spectra of the 1,4-BDF based epoxy coating samples under different UV exposure time periods. (b) UV/Vis spectra of the combinations of different components in 1,4-BDF based epoxy coating systems before and after UV exposure. (c) UV/Vis spectra of the 1,4-BDF pigment, the curing agent, and the mixture of the two components. (d) Digital images of the color-changing phenomenon of the combination of 1,4-BDF and the curing agent before and after UV exposure. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
the π-conjugation extension and the donor (amide unit)-acceptor (1,4BDF unit) system formed, the cross-linked materials resulted in the optical absorption red-shift compared to the 1,4-BDF. To test the stability of the new product, the curing agent was directly added to the 1,4-BDF solution (1,4-BDF and solvent). Subsequently, the mixture was applied on the steel panel and exposed to the UV light. As shown in Fig. 7d, the color of the mixture changed after the UV exposure.
indicated that the change of the UV/Vis spectra of the 1,4-BDF based epoxy coating is due to the change of the 1,4-BDF + curing agent. It is hypothesized that the color change is because the 1,4-BDF pigment can react with the curing agent to form a new hydrogenbonded cross-linked product and the new product is unstable (Scheme S1) [27–29]. To test this hypothesis, the UV/Vis absorption spectra of the 1,4-BDF, the curing agent, and the mixture of the two components were measured. As shown in Fig. 7c, the absorption peaks of the 1,4BDF pigment located at 376 nm and 495 nm, while the curing agent almost exhibited no absorption in the visible light. However, once the two materials mixed, the UV/Vis absorption presented a strong bathochromic shift over 100 nm comparing to the 1,4-BDF, which indicated that a new product formed. The formed new product might be the hydrogen-bonded cross-linked materials (Scheme S1) [27–29]. Due to
3.3.2. FTIR Based on UV/Vis measurement, it concluded that the color change of the 1,4-BDF based epoxy coating might be caused by the reaction of the 1,4-BDF pigment and the curing agent which formed a new hydrogen-bonded cross-linked product and the new product is unstable under the UV light. To understand the chemical structure change of the 203
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Fig. 8. FTIR spectra of the 1,4-BDF, the curing agent D230, and the combination of the 1,4-BDF and the curing agent D230 before and after UV exposure.
Fig. 9. Opacity of the 1,4-BDF based epoxy coating samples as a function of exposure (UV exposure/sunlight exposure) time. UV exposure condition: UVA-340 with 0.68 W/m2 radiance at 45 °C.
Fig. 10. (a) Opacity fitting of the coatings under UV exposure. (b) Opacity fitting of the coatings under sunlight exposure. (c) The opacity relationship between samples under UV exposure and sunlight exposure.
product under the UV-light, while D230 units are quite stable.
new product under the UV light, the new formed product was characterized by FTIR analysis. The curing agent D230 was used in the FTIR study because D230 has a similar chemical structure as to the curing agent 3164, but it is a pure polymer that can easier be analyzed and identified in FTIR while the 3164 is a mixture. Detailed information of D230 and 3164 are described in the supplementary material. Fig. S6 shows the FTIR spectra of the pure 1,4-BDF and pure D230 before and after UV-light exposing, which indicates that both materials are photostable under the UV-light. However, once the 1,4-BDF and D230 were mixed, the product was unstable under the UV-light (Fig. 8). After exposing this mixture to the UV-light for 20 h, most peaks do not change except that: i) the peak between 3039 cm−1 and 3102 cm−1 is decreased, which could be attributed to the decomposition of benzene ring of the 1,4-BDF; ii) the peak at 1734 cm−1 is shifted to 1656 cm−1; this could be ascribed to the decomposition of the core of the 1,4-BDF, which causes the signal of carbonyl units shift. It seems that the 1,4-BDF units are decomposed in the new hydrogen-bonded cross-linked
3.4. Opacity change As mentioned above, during the color change of the 1,4-BDF based epoxy coating, it was also found that the coating became more and more transparent demonstrated by the increase of the L* value. It indicated that besides the color change of the coating from dark blue to brown, the coating also experienced an opacity change that may lose its hiding power. To further study this phenomenon, the 1,4-BDF based epoxy coating was applied on opacity drawdown charts (BYK), and the charts were exposed under the UV light. During the UV exposure, a spectro-guide spectrophotometer was used to measure the opacity. The opacity changes of the coating chart under the indoor sunlight exposure (from November 20th, 2017 to January 29th, 2018 in Akron, Ohio, USA; the average day length is 9.5 h) was recorded to compare the opacity change under the UV light (0.68 W/m2 irradiance). All the 204
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opacity charts were cured at room temperature, away from sunlight before testing. A single opacity chart comprises of a black area and a white area. As defined in ASTM D2805, the contrast ratio is the ratio of the reflectance (Y-tristimulus value) of a coating on the black surface to that of the identical coating on the white surface. Opacity is the contrast ratio multiplied by 100, where 100% is complete hiding and 0% is no hiding. The opacity values were plotted as a function of exposure (UV exposure/sunlight exposure) time in the same figure. As can be seen in Fig. 9, the opacity of the coatings decreased with the increase of the exposure time, either in UV light or indoor sunlight. The original opacity was about 75% before exposure, which is very high considering only 2% (wt %) of the 1,4-BDF added. After 4 h UV exposure, the opacity decreased into the half of the original opacity. Meanwhile, it took more than one week of indoor sunlight exposure to reach the same point. As the exposure continued, the opacity kept decreasing. In the end, the opacity of both coating samples fell into a platform around 10% after 20 h UV exposure or 11 week indoor sunlight exposure. The coatings looked transparent on the charts. The trend of the opacity change for the coatings under UV exposure and indoor sunlight exposure is similar. To depict the relationship between UV exposure time and sunlight exposure time for reaching the same opacity, a model fitting was attempted in this study. As shown in Fig. 10a and b, an asymptotic regression model was applied for the opacity change under UV or sunlight exposure. The opacity relationship between the samples under UV exposure time and sunlight exposure time was generated, as shown in Fig. 10c. Therefore, UV exposure can be used as an accelerated weathering method to predict the sunlight exposure. Opacity can be used as an effective and key parameter to link UV accelerated testing and sunlight exposure to demonstrate the change of coatings’ appearance. Since the 1,4-BDF based epoxy coating changes color under sunlight, it has potential to be used in colored mulch to aid plant growth by adjusting the light reflectance [30–33].
References [1] Ferrara M, Bengisu M. Materials that change color: Smart materials, intelligent design. PoliMI SpringerBriefs; 2014. [2] Kulčar R, Friškovec M, Hauptman N, Vesel A, Gunde MK. Colorimetric properties of reversible thermochromic printing inks. Dyes Pigments 2010;86:271–7. [3] Van Gemert B. The commercialization of plastic photochromic lenses: a tribute to John Crano. Mol Cryst Liq Cryst 2000;344:57–62. [4] Azens A, Granqvist CG. Electrochromic smart windows: energy efficiency and device aspects. J Solid State Electrochem 2003;7:64–8. [5] Mennig M, Fries K, Lindenstruth M, Schmidt H. Development of fast switching photochromic coatings on transparent plastics and glass. Thin Solid Films 1999;351:230–4. [6] Ma Y, Zhu B, Wu K. Preparation and solar reflectance spectra of chameleon-type building coatings. Sol Energy 2001;70:417–22. [7] Karlessi T, Santamouris M, Apostolakis K, Synnefa A, Livada I. Development and testing of thermochromic coatings for buildings and urban structures. Sol Energy 2009;83:538–51. [8] La TG, Li X, Kumar A, Fu Y, Yang S, Chung HJ. Highly flexible, multipixelated thermosensitive smart windows made of tough hydrogels. ACS Appl Mater Interfaces 2017;9:33100–6. [9] Di Credico B, Griffini G, Levi M, Turri S. Microencapsulation of a UV-responsive photochromic dye by means of novel uv-screening polyurea-based shells for smart coating applications. ACS Appl Mater Interfaces 2013;5:6628–34. [10] Zeng W, Zhou Q, Zhang H, Qi X. One-coat epoxy coating development for the improvement of UV stability by DPP pigments. Dyes Pigments 2018;151:157–64. [11] Greenhalgh CW, Carey JL, Newton DF. The synthesis of quinodimethanes in the benzodifuranone and benzodipyrrolidone series. Dyes Pigments 1980;1:103. [12] Hallas G, Yoon C. Synthesis and properties of naphthodifuranones and naphthofuranonepyrrolidones. Dyes Pigments 2001;48:121–32. [13] Hallas G, Yoon C. The synthesis and properties of red and blue benzodifuranones. Dyes Pigments 2001;48:107–19. [14] Cai Z, Gao J, Li X, Xiang B. Synthesis and characterization of symmetrical benzodifuranone compounds with femtosecond time-resolved degenerate four-wave mixing technique. Optic Commun 2007;272:503–8. [15] Ulmann PA, Tanaka H, Matsuo Y, Xiao Z, Soga I, Nakamura E. Electric field dependent photocurrent generation in a thin-film organic photovoltaic device with a [70]fullerene-benzodifuranone dyad. Phys Chem Chem Phys 2011;13:21045–9. [16] Cerón-Carrasco JP, Ripoche A, Odobel F, Jacquemin D. Excited-state nature in benzodifuranone dyes: insights from ab initio simulations. Dyes Pigments 2012;92:1144–52. [17] Zhang H, Yang K, Chen YM, Bhatta R, Tsige M, Cheng SZD, et al. Polymers based on benzodipyrrolidone and naphthodipyrrolidone with latent hydrogen-bonding on the main chain. Macromol Chem Phys 2017;218:1–10. [18] Zhang K, Tieke B. Low-bandgap benzodifuranone-based polymers. Macromolecules 2011;44:4596–9. [19] Cui W, Yuen J, Wudl F. Benzodipyrrolidones and their polymers. Macromolecules 2011;44:7869–73. [20] Zhang K, Tieke B, Forgie JC, Vilela F, Skabara PJ. Donor-acceptor conjugated polymers based on p- and o-benzodifuranone and thiophene derivatives: electrochemical preparation and optical and electronic properties. Macromolecules 2012;45:743–50. [21] Zhang H, Neudörfl JM, Tieke B. Naphthodifuranone-based monomers and polymers. Macromolecules 2013;46:5842–9. [22] Yang K, He T, Chen X, Cheng SZD, Zhu Y. Patternable conjugated polymers with latent hydrogen-bonding on the Main Chain. Macromolecules 2014;47:8479–86. [23] Kimoto A, Tajima Y. Donor-acceptor type p-conjugated copolymers based on soluble benzodifuranone. Macromol Symp 2014;346:36–42. [24] Liu B, Chen X, Zou Y, Xiao L, Xu X, He Y, et al. Benzo[1,2-b:4,5-b’]difuran-based donor-acceptor copolymers for polymer solar cells. Macromolecules 2012;45:6898–905. [25] Deng Z, Yang K, Li L, Bao W, Hao X, Ai T, et al. Solution processed air-stable pchannel organic crystal field-effect transistors of Aminobenzodifuranone. Dyes Pigments 2018;151:173–8. [26] Greenhalgh CW, Newton DF. Bemodifwanone chromogen and its application. Jsdc 1994;110:178–84. [27] Coleman MM, Skrovanek DJ, Hu J, Painter PC. Hydrogen bonding in polymer blends. 1. FTIR studies of urethane-ether blends. Macromolecules 1988;21:59–65. [28] Garcia D, Starkweather HW. Hydrogen bonding in nylon 66 and model compounds. J Polym Sci Polym Phys Ed 1985;23:537–55. [29] Zhang H, Deng R, Wang J, Li X, Chen YM, Liu K, et al. Crystalline organic pigmentbased field-effect transistors. ACS Appl Mater Interfaces 2017;9:21891–9. [30] Andino JR, Motsenbocker CE. Colored plastic mulches influence cucumber beetle populations, vine growth, and yield of watermelon. Hortscience 2004;39:1246–9. [31] Mahmoudpour MA, Stapleton JJ. Influence of sprayable mulch colour on yield of eggplant (Solanum melongena L. cv. Millionaire). Sci Hortic (Amst) 1997;70:331–8. [32] Shiukhy S, Raeini-Sarjaz M, Chalavi V. Colored plastic mulch microclimates affect strawberry fruit yield and quality. Int J Biometeorol 2015;59:1061–6. [33] Díaz-Pérez JC, Batal KD. Colored plastic film mulches affect tomato growth and yield via changes in root-zone temperature. J Am Soc Hortic Sci 2002;127:127–35.
4. Conclusions This work first-time introduced the 1,4-BDF organic pigment into one-coat epoxy coating matrix which leads to a color-changing property of the coating responding to varied environmental conditions, such as temperature, UV, and pH. The stimulus to trigger the color change and the reasons for the color change were comprehensively investigated and understood. The 1,4-BDF based epoxy coating exhibited a color change from dark blue to brown at high temperature, under UV exposure, or under low pH. At the same time, the coating experienced a change of the hiding power resulting in a more translucent appearance. Based on the UV/Vis study and FTIR analysis, the reason for the color change was mainly caused by the reaction of the 1,4-BDF with the polyamine curing agent to form an unstable product. In addition, by studying the change of the light reflectance during the color change, UV accelerated testing and sunlight exposure can be linked by the coating's opacity. Acknowledgments The authors acknowledge the support from the faculty start-up funding from The University of Akron, the Natural Science Foundation of China, under Grant 21805151, and the Natural Science Foundation of Shandong Province, China, under Grant ZR2018MB024. The authors appreciated the technical support with SEM from Dr. Lingyan Li at National Center for Education and Research on Corrosion and Materials Performance. Appendix A. Supplementary data Supplementary data to this article can be found online at https://
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Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig
Benzodifuranone based color-changing epoxy-polyamine coating a
b
a
Weixiu Zeng , Zhifeng Deng , Haoran Wang , Haichang Zhang
b,c,∗∗
, Qixin Zhou
T
a,∗
a
National Center for Education and Research on Corrosion and Materials Performance, Department of Chemical and Biomolecular Engineering, The University of Akron, Akron, OH 44325, United States b National and Local Joint Engineering Laboratory for Slag Comprehensive Utilization and Environmental Technology, School of Material Science and Engineering, Shaanxi University of Technology (SNUT), Hanzhong, Shaanxi 723001, PR China c Key Laboratory of Rubber-Plastics of Ministry of Education/Shandong Province (QUST), School of Polymer Science & Engineering, Qingdao University of Science & Technology, Qingdao, Shandong 266042, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Benzodifuranone Color-changing Epoxy-polyamine Coating Spectral reflectance
A new color-changing epoxy coating was developed by incorporating benzodifuranone (1,4-BDF) pigments into epoxy-polyamine coating matrix. The 1,4-BDF based epoxy coating exhibited a color change from dark blue to brown responding to environmental conditions, such as temperature, UV light, and pH. The color of the coating was demonstrated by the spectral reflectance and CIELAB values measured by a spectro-guide spectrophotometer. The color change was due to the reaction of 1,4-BDF pigment with the polyamine curing agent to form an unstable product characterized by UV/Vis and FTIR analysis. Moreover, the coating's opacity and light reflectance changed along with the color change. Opacity is an effective parameter to link the UV accelerated testing and sunlight exposure to demonstrate the change of the coating's appearance.
1. Introduction Color-changing materials are those which exhibit changes in their color (reversible or irreversible) as a response to a stimulus in environmental conditions. The stimulus can be the change of temperature, light intensity, pH, electric field, or magnetic field, and the corresponding materials are thermochromic, photochromic, chemichromic, electrochromic, and magnetochromic materials, respectively [1]. Colorchanging materials can be designed as useful instruments to extend the aesthetic, functional, and energy-saving performance of objects. Colorchanging materials have been applied in many aspects, for example, in inks to display the desired color, in lenses to filter the infrared radiation, and in architecture to optimize the energy consumption [2–4]. Recent developed color-changing coatings are mostly thermochromic and photochromic coatings applied in buildings, especially for roofs and windows [5–8]. These thermochromic coatings response to the environment thermally and change their color from darker to lighter as the temperature rises. These color-changing coatings were developed by adding thermochromic pigments. However, some thermochromic pigments were sensitive to the components of the coatings and lost their properties after blending with the coating matrix. Hence, these pigments were microencapsulated before incorporating into the
coatings to retain their thermochromic property [4,9]. The photochromic coatings change color while undergoing UV irradiation. Similar to thermochromic coatings, they experience the color change because of the incorporated photochromic colorants. Previously, our group developed a one-coat epoxy system with bright orange color, UV stability, and corrosion resistance by incorporating diketopyrrolopyrrole [10]. Due to the promising properties of benzodifuranones (BDFs), such as high photo- and thermal stability, good brightness, and etc., it is desirable to study the BDF based one-coat epoxy coating. BDFs belong to a class of high performance pigments, which were first reported by Greenhalgh et al. in 1980 [11]. BDFs exhibit red to blue colors depending on the substitution pattern. Only a few BDF derivatives were synthesized and used as functional blocks in the past 30 years [12–16]. In recent years, BDF or its analogs have been attracting a great attention owing to their potential to make conjugated polymers and copolymers for developing organic photovoltaic devices [17–25]. BDFs exhibit advantages in deep color value, high tinctorial strength, and good brightness. On the basis of these advantages, BDFs had been used as dispersing dyes for textiles, especially for polyesters [26]. However, to the best of the authors’ knowledge, BDFs have not be reported in organic coating applications. Based on this work, 3,7-diphenyl-benzo[1,2-b:4,5-b’]-difuran-2,6-
∗
Corresponding author. Corresponding author. National and Local Joint Engineering Laboratory for Slag Comprehensive Utilization and Environmental Technology, School of Material Science and Engineering, Shaanxi University of Technology (SNUT), Hanzhong, Shaanxi, 723001, PR China. E-mail addresses: [email protected] (H. Zhang), [email protected] (Q. Zhou). ∗∗
https://doi.org/10.1016/j.dyepig.2019.01.016 Received 25 August 2018; Received in revised form 27 December 2018; Accepted 12 January 2019 Available online 17 January 2019 0143-7208/ © 2019 Elsevier Ltd. All rights reserved.
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dione (1,4-BDF) was synthesized and first-time incorporated in an epoxy-polyamine coating matrix. An interesting phenomenon of the 1,4-BDF based epoxy coating was found that the coating experienced a series of color change during the coating preparation. The color change of the coating was studied and quantified by using a spectrophotometer. The influence of environmental conditions on the color change, such as temperature, UV light, and pH were comprehensively investigated. The reason for the color change was understood by UV/Vis and FTIR characterizations. Besides color change, the opacity and light reflectance of the pigmented coatings were also evaluated. The opacity of the coatings under the UV accelerated testing and sunlight exposure was compared.
epoxy resin-EPON™ Resin 828 was added and magnetically stirred for 10 min. Subsequently, the curing agent-EPIKURE™ 3164 was added into the mixture. After the mixture was homogenized under stirring for 10 min, it was de-gassed in an ultrasonic bath for 5 min. The coating was applied by a film applicator onto the steel panels with a wet film thickness of 120 μm. Before applying the coating, the panels had been rinsed with acetone and deionized water. All the coating samples were cured at room temperature and kept away from sunlight for one week. The final dry film thickness was 49 ± 4 μm. All the coating samples contain 2 wt % (solid weight) of the 1,4-BDF pigments.
2. Experimental
2.4.1. Color measurement The spectral reflectance, CIELAB values, and opacity under different environmental conditions were measured by using a spectro-guide spectrophotometer (BYK). The spectral reflectance was obtained in the wavelength range from 400 to 700 nm. The CIELAB values were acquired with a D65 illuminant and 10° standard observer. The opacity was recorded before and after exposing coatings under UV light or sunlight.
2.4. Measurements
2.1. Materials Epoxy resin (EPON™ Resin 828) and curing agent (EPIKURE™ Curing Agent 3164) were kindly provided by Hexion Specialty Chemicals. The weight ratio of EPIKURE™ Curing Agent 3164 to EPON™ Resin 828 is 1.36. Curing agent D230 (Poly(propylene glycol) bis(2aminopropyl ether)), 1,4-dihydroxybenzene, mandelic acid, acetone in ACS grade, 1,2,4-trichlorobenzene, and nitrobenzene were purchased from Sigma-Aldrich. Deionized water was produced by Smart2Pure water purification system (Thermo Scientific). Steel test panels (QD-39) (75 cm × 225 cm) were purchased from Q-lab. All the chemicals were used without further purification in this study.
2.4.2. 1H NMR & 13C NMR 1 H NMR spectra and 13C NMR spectra were obtained using a Mercury-300 spectrometer (Varian) in DMSO‑d6. Chemical shifts were reported as δ values (ppm). 2.4.3. UV/Vis spectra The UV/Vis spectra of all samples were collected in solid states by using a UV-1800 UV/Vis spectrophotometer (Shimadzu). The spectra were acquired in the wavelength range from 200 to 800 nm.
2.2. Synthesis of 1,4-BDF 3,7-diphenyl-benzo[1,2-b:4,5-b’]-difuran-2,6-dione (1,4-BDF) was synthesized in the lab. Using a Dean-Stark apparatus, 1,4-dihydroxybenzene (1.1 g, 10 mmol), and mandelic acid (4.5 g, 30 mmol) were dissolved in 1,2,4-trichlorobenzene (20 mL). The reaction mixture was stirred for 4 h at 200 °C allowing formed water to distill off before being cooled down to room temperature. Then nitrobenzene (4.9 g, 40 mmol) was added. The mixture was stirred for another 30 min at 200 °C before it was cooled down, poured into methanol (100 mL), and filtered to recover the crude product. The crude product was refluxed in methanol. After cooling, a red solid compound with a wine color in appearance (2.7 g, yield: 81%) was obtained. 1H NMR (300 MHz, DMSO‑d6) δ ppm: 7.83 (d, J = 15 Hz, 4H), 7.66 (d, J = 51 Hz, 4H), 7.47 (d, J = 15 Hz, 2H), 7.46 (d, J = 15 Hz, 2H). 13C NMR, (300 MHz, CDCl3):135.71, 133.00, 132.76, 132.21, 130.67, 130.62, 130.29, 129.36. Microanalysis found C, 77.62%; H, 3.61% (C, 77.64%; H, 3.55%). UV/Vis (thin film): 396 nm and 500 nm.
2.4.4. Fourier transform infrared spectroscopy Nicolet iS10 FTIR spectrometer (resolution: 4 cm−1; scan number: 32) was used to analyze the chemical composition of the coatings. The FTIR spectra of all coating samples were recorded with the attenuated total reflectance (ATR) model. The range of scanning wavenumber was from 400 to 4000 cm−1. The background was taken before each sample testing. All the FTIR spectra were subtracted the background. 3. Results and discussion 3.1. Property of 1,4-BDF We began our studies with the synthesis of the 1,4-BDF pigment. The synthesis of the 1,4-BDF required the condensation of 1 eq. of 1,4dihydroxybenzene and 2 eq. mandelic acid leading to a double cyclization of derivative, followed by oxidation to the conjugated BDF in a yield of 81%. The chemical structure of the 1,4-BDF is shown in Fig. 1a. The 1H NMR spectra of 1,4-BDF exhibited all of the expected resonances with no discernible peaks corresponding to impurities (See Fig. S1). The chemical shift between 7.47 and 7.83 ppm were typical for the protons
2.3. Preparation of 1,4-BDF based epoxy coating The 1,4-BDF pigments were partially dissolved and dispersed in acetone by using magnetic stirring for 5 min at 1000 rpm. Then, the solution was treated in an ultrasonic bath for 30 min. After that, the
Fig. 1. (a) Chemical structure of 1,4-BDF. (b) Digital image of the synthesized 1,4-BDF pigment. (c) Morphology of the 1,4-BDF pigment. 199
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Fig. 2. Digital images of the color-changing phenomenon during the preparation of the 1,4-BDF based epoxy coating. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
after heating for 2 h at 40 °C and 60 °C, while it changed to brown after heating to 100 °C. A transition color between dark blue and brown appeared at 80 °C. The color change can be quantified by measuring the spectral reflectance, because color is the quality of an object with respect to the light reflected by the object. For example, violet objects are the objects that reflect violet light. To quantify the color change, the spectral reflectance over the visible region (400–700 nm) were recorded by a spectrophotometer. As indicated in Fig. 3b, the spectral reflectance curves of the coatings after heating at 40 °C and 60 °C are close to that of the control sample, indicating the color difference is very negligible. The spectral reflectance increased when the coating heated at 80 °C and reached to the highest value when the coating heated at 100 °C. The change of the reflectance curves for the coating samples at different temperature presented the exact same trend as the color change shown in Fig. 3a. For further depicting the color change, the color was described in the CIELAB color space system, as shown in Fig. 3c. In the CIELAB color space, L∗ is the lightness axis (black (0) to white (100)); a∗ is the green (< 0) to red (> 0) axis; and b∗ is the blue (< 0) to yellow (> 0) axis. Fig. 3c shows that all the CIELAB values (L∗, a∗, and b∗) increased as the increase of the temperature. The increase of b∗ indicates the color evolves from blue to yellow, while the increase of a∗ demonstrates the color changes from green to red. The change of a* and b* presents the color change from dark blue to brown (Fig. 3a). As the L∗ value getting higher, the coating becomes translucent. Both the spectral reflectance curves and CIELAB data quantitatively evaluated the color change of the coatings at different temperatures, which verified that the temperature affected the color of the 1,4-BDF based epoxy coating.
of the phenyl ring of BDF, while the core of BDF's protons were at 7.46 ppm. The synthesized 1,4-BDF pigment exhibits a wine color appearance (Fig. 1b) and flake shape under the observation of SEM (Fig. 1c). 3.2. Color change A series of color change was found during the process of preparing 1,4-BDF based epoxy coatings. As shown in Fig. 2, the solid 1,4-BDF has a wine color appearance. A light brown color was found as the pigment was partially dissolved and dispersed in acetone. A wine-colored pastry was formed as adding the epoxy resin (EPON 828) into the pigment solution. However, the color changed to dark blue immediately once the curing agent was added into the mixture. After well mixing, the coating was applied on the steel panel. The coatings maintained dark blue color when they were cured at room temperature, while the coatings' color changed to brown if they were cured at 100 °C. To the authors’ knowledge, the color change of 1,4-BDF during coating preparation has yet been reported. 3.2.1. The effect of temperature Based on the color change during the coating preparation, it is proposed that the 1,4-BDF based epoxy coating will change its color as the change of the temperature. To test this hypothesis, the coating samples were put in the oven at different temperatures (40, 60, 80, and 100 °C). After 2 h in the oven, the samples were taken out for testing. To avoid other environmental influences, the coating samples were cured at room temperature away from sunlight for one week before putting into the oven. The coating sample cured at room temperature was used as the control. As shown in the digital images of Fig. 3a, there is a gradual color change as increasing the temperature. At room temperature, the coating exhibited a dark blue color and the color maintained almost the same
3.2.2. The effect of UV From our preliminary research, we found that the 1,4-BDF based epoxy coating changed color when it was exposed to the sunlight. Since Fig. 3. Color change of the 1,4-BDF based epoxy coating samples after heating at different temperatures. (a) Digital images of the coating samples after heating at different temperatures for 2 h. (b) Spectral reflectance of the coating samples after heating at different temperatures for 2 h. (c) The change of L∗, a∗, and b∗ values of the coating samples as a function of the heating temperature. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
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Fig. 4. Color change of the 1,4-BDF based epoxy coating samples before and after UV exposure (UVA-340 of 0.68 W/m2 exposure). (a) Digital images of the coating samples with different UV exposure time periods. (b) Spectral reflectance of the coating samples with different UV exposure time periods. (c) The change of L*, a*, and b* values of the coating samples as a function of UV exposure time. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
reflectance in the range of 550–650 nm should present a brown color (yellow + orange) appearance, which was observed for the samples after > 6 h exposure. We also explored how the CIELAB values (L∗, a∗, and b∗) changed with the UV exposure time. As can be seen in Fig. 4c, all the values increased after UV exposure. The increase of a∗ and b∗ presented the coating color changed to brown, while the increase of L∗ demonstrated the coating became more transparent. Similar to the effect of temperature, the color of the 1,4-BDF based epoxy coating was influenced by the UV light as measured by the spectral reflectance and CIELAB color system. The coating became browner and more transparent with the longer UV exposure.
sunlight consists of UV light, visible light, and infrared light, and material change or degradation were mostly caused by the UV light, it is proposed that the color change of the coating under the sunlight mainly comes from the influence of the UV light. To test this hypothesis, the coating samples were exposed to UVA (340 nm) radiation at 45 °C with a 0.68 W/m2 irradiance. The samples used for UV testing were cured at room temperature with a dark blue color. Fig. 4a shows that the color evolution of the 1,4-BDF based coating as the UV exposure time increases. Before UV exposure, the color of the coating was dark blue, while it changed to brown after exposing 6 h. As the exposure time continuously increasing, the color became browner. When the exposure time increased to 20 h, the coating not only showed a brown color but also looked translucent, which means the hiding power of the coating was dropped. The color change was also quantified by spectral reflectance curves (Fig. 4b). During the 20 h UV exposure, the reflectance in the entire spectral range increased with the UV exposure time. The lowest reflectance was observed for the coating without exposure, while the highest reflectance was the one with the longest exposure time. It is worth mentioning that the coatings with > 6 h exposure exhibited high broad reflection bands in the range of 550–650 nm. Since the reflectance around 570–590 nm produces a yellow color and the reflectance around 590–620 nm produces an orange color, the high
3.2.3. The effect of pH Besides the color change responding to the temperature and UV light, the effect of pH was also investigated. The coating samples were prepared according to the same procedure in the experimental section, which means the coatings were cured at room temperature without the sunlight. The color of the coating samples before testing was dark blue. Five coating samples were immersed in different pH solutions (pH from 1.05 to 12.49), respectively, as shown in Fig. 5. At certain time periods (1, 3, 6, 9, 12, 24, and 48 h), the coated panels were removed from the setup, cleaned by deionized water, and gently swiped by swipes for
Fig. 5. The coating samples were immersed in the solutions with different pH. 201
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Fig. 6. Color change of the 1,4-BDF based epoxy coating samples after immersion in different pH solutions. (a) Digital images of the coating sample after immersion in pH = 1.05 solution. (b) Digital images of the coating samples after immersion in the other pH solutions (from pH = 3.93 to pH = 12.49). (c) Spectral reflectance of the coating sample after the immersion in pH 1.05 solution. (d) Color difference (ΔE*) change of the coating samples as a function of immersion time in different pH solutions. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
influenced by the very strong acidic environment while it was free from affecting by the acidic or alkaline environment.
color testing. The images in Fig. 6a shows a series of color change for the coating sample under the immersion of pH 1.05 solution for 48 h. The color changed from dark blue to brown after 3 h and then became browner as increasing the immersion time. On the contrast, as shown in Fig. 6b, there was no obvious visible color change for the coatings immersed in the other solutions (pH 3.93 to pH 12.49) up to 48 h. The coating presented a slight light color in the other solutions (pH 3.93 to pH 12.49) may ascribe to the influence of the sunlight since the coating samples did not undertake special treatment to avoid sunlight during the 48 h exposure in different pH solutions. Spectral reflectance of the 1,4-BDF based epoxy coatings before and after immersing in different pH solutions were collected by the same spectrophotometer and procedure as described in the previous sections. Clearly, for coatings immersed in pH 3.93, DI water, pH 9.22, and pH 12.49 solution, the reflectance curves remained the same during the exposure (Fig. S3). Only the coating immersed in pH 1.05 solution obtained an increased reflectance with the immersion time, especially at the range of 550–650 nm. The change of the spectral reflectance presented the same trend as the visible color change (Fig. 6a and b). In the CIELAB color space, the color change can also be described as the color difference (ΔE*). As another method to interpret the color change, ΔE* combines the effect of L*, a*, and b* as defined below,
ΔE ∗ =
3.3. The reason for color change As discussed above, the 1,4-BDF based epoxy coating will change color with the increased temperature, under the UV exposure, or under the low pH solution. Even under the sunlight, the color of the coating will undergo a slight change. The unstable color performance may contribute to a chemical reaction in the coating to form a new product. To test this hypothesis and understand the color change, the coatings under UV exposure were investigated by UV/Vis and FTIR analysis as discussed below. 3.3.1. UV/Vis A set of UV/Vis experiment was applied to analyze the spectral absorption of the 1,4-BDF based epoxy coatings. Fig. 7a shows the UV/ Vis spectra of the 1,4-BDF based epoxy coating before and under UV exposure with different time periods. As the UV exposure time increased, the absorption peaks maintained at the same wavelength (400 nm and 600 nm), but their intensity decreased. The change indicated that the coating's color was fading. To investigate the possible reason for the color change of 1,4-BDF based epoxy coating, different combinations of the coating components were placed under the UV exposure: 1,4-BDF + curing agent, epoxy + curing agent, curing agent itself, and 1,4-BDF itself. The UV/ Vis spectra of these combinations before and after UV exposure were shown in Fig. 7b. There were no characteristic peaks at 400 nm and 600 nm for all the components except for the combination of the 1,4BDF and curing agent. Moreover, after exposing under the UV light, the 1,4-BDF + curing agent exhibited a decrease in the absorption intensity, while the other combinations retained the same. The location of characteristic peaks and evolution trend of the 1,4-BDF + curing agent were the same as the 1,4-BDF based epoxy coating (Fig. S5). This
(ΔL∗)2 + (Δa∗)2 + (Δb∗)2
ΔL∗ = L∗1 − L∗2 ; Δa∗ = a∗1 − a∗2 ; Δb∗ = b∗1 − b∗2 , where 1, 2 represents the value at the different time period, respectively. The larger the ΔE* value, the bigger the color change. The ΔE* values maintained stable after immersing in these pH solutions (pH 3.93, DI water, pH 9.22, and pH 12.49). However, the ΔE* value increased by around 39 after 48 h immersion in the pH 1.05 solution. For the coating in the pH 1.05 solution, the ΔE* values were almost proportional to the immersion time. Thus, based on the results of the spectral reflectance and CIELAB color values, the color of the 1,4-BDF based epoxy coatings was 202
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Fig. 7. (a) UV/Vis spectra of the 1,4-BDF based epoxy coating samples under different UV exposure time periods. (b) UV/Vis spectra of the combinations of different components in 1,4-BDF based epoxy coating systems before and after UV exposure. (c) UV/Vis spectra of the 1,4-BDF pigment, the curing agent, and the mixture of the two components. (d) Digital images of the color-changing phenomenon of the combination of 1,4-BDF and the curing agent before and after UV exposure. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
the π-conjugation extension and the donor (amide unit)-acceptor (1,4BDF unit) system formed, the cross-linked materials resulted in the optical absorption red-shift compared to the 1,4-BDF. To test the stability of the new product, the curing agent was directly added to the 1,4-BDF solution (1,4-BDF and solvent). Subsequently, the mixture was applied on the steel panel and exposed to the UV light. As shown in Fig. 7d, the color of the mixture changed after the UV exposure.
indicated that the change of the UV/Vis spectra of the 1,4-BDF based epoxy coating is due to the change of the 1,4-BDF + curing agent. It is hypothesized that the color change is because the 1,4-BDF pigment can react with the curing agent to form a new hydrogenbonded cross-linked product and the new product is unstable (Scheme S1) [27–29]. To test this hypothesis, the UV/Vis absorption spectra of the 1,4-BDF, the curing agent, and the mixture of the two components were measured. As shown in Fig. 7c, the absorption peaks of the 1,4BDF pigment located at 376 nm and 495 nm, while the curing agent almost exhibited no absorption in the visible light. However, once the two materials mixed, the UV/Vis absorption presented a strong bathochromic shift over 100 nm comparing to the 1,4-BDF, which indicated that a new product formed. The formed new product might be the hydrogen-bonded cross-linked materials (Scheme S1) [27–29]. Due to
3.3.2. FTIR Based on UV/Vis measurement, it concluded that the color change of the 1,4-BDF based epoxy coating might be caused by the reaction of the 1,4-BDF pigment and the curing agent which formed a new hydrogen-bonded cross-linked product and the new product is unstable under the UV light. To understand the chemical structure change of the 203
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Fig. 8. FTIR spectra of the 1,4-BDF, the curing agent D230, and the combination of the 1,4-BDF and the curing agent D230 before and after UV exposure.
Fig. 9. Opacity of the 1,4-BDF based epoxy coating samples as a function of exposure (UV exposure/sunlight exposure) time. UV exposure condition: UVA-340 with 0.68 W/m2 radiance at 45 °C.
Fig. 10. (a) Opacity fitting of the coatings under UV exposure. (b) Opacity fitting of the coatings under sunlight exposure. (c) The opacity relationship between samples under UV exposure and sunlight exposure.
product under the UV-light, while D230 units are quite stable.
new product under the UV light, the new formed product was characterized by FTIR analysis. The curing agent D230 was used in the FTIR study because D230 has a similar chemical structure as to the curing agent 3164, but it is a pure polymer that can easier be analyzed and identified in FTIR while the 3164 is a mixture. Detailed information of D230 and 3164 are described in the supplementary material. Fig. S6 shows the FTIR spectra of the pure 1,4-BDF and pure D230 before and after UV-light exposing, which indicates that both materials are photostable under the UV-light. However, once the 1,4-BDF and D230 were mixed, the product was unstable under the UV-light (Fig. 8). After exposing this mixture to the UV-light for 20 h, most peaks do not change except that: i) the peak between 3039 cm−1 and 3102 cm−1 is decreased, which could be attributed to the decomposition of benzene ring of the 1,4-BDF; ii) the peak at 1734 cm−1 is shifted to 1656 cm−1; this could be ascribed to the decomposition of the core of the 1,4-BDF, which causes the signal of carbonyl units shift. It seems that the 1,4-BDF units are decomposed in the new hydrogen-bonded cross-linked
3.4. Opacity change As mentioned above, during the color change of the 1,4-BDF based epoxy coating, it was also found that the coating became more and more transparent demonstrated by the increase of the L* value. It indicated that besides the color change of the coating from dark blue to brown, the coating also experienced an opacity change that may lose its hiding power. To further study this phenomenon, the 1,4-BDF based epoxy coating was applied on opacity drawdown charts (BYK), and the charts were exposed under the UV light. During the UV exposure, a spectro-guide spectrophotometer was used to measure the opacity. The opacity changes of the coating chart under the indoor sunlight exposure (from November 20th, 2017 to January 29th, 2018 in Akron, Ohio, USA; the average day length is 9.5 h) was recorded to compare the opacity change under the UV light (0.68 W/m2 irradiance). All the 204
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doi.org/10.1016/j.dyepig.2019.01.016.
opacity charts were cured at room temperature, away from sunlight before testing. A single opacity chart comprises of a black area and a white area. As defined in ASTM D2805, the contrast ratio is the ratio of the reflectance (Y-tristimulus value) of a coating on the black surface to that of the identical coating on the white surface. Opacity is the contrast ratio multiplied by 100, where 100% is complete hiding and 0% is no hiding. The opacity values were plotted as a function of exposure (UV exposure/sunlight exposure) time in the same figure. As can be seen in Fig. 9, the opacity of the coatings decreased with the increase of the exposure time, either in UV light or indoor sunlight. The original opacity was about 75% before exposure, which is very high considering only 2% (wt %) of the 1,4-BDF added. After 4 h UV exposure, the opacity decreased into the half of the original opacity. Meanwhile, it took more than one week of indoor sunlight exposure to reach the same point. As the exposure continued, the opacity kept decreasing. In the end, the opacity of both coating samples fell into a platform around 10% after 20 h UV exposure or 11 week indoor sunlight exposure. The coatings looked transparent on the charts. The trend of the opacity change for the coatings under UV exposure and indoor sunlight exposure is similar. To depict the relationship between UV exposure time and sunlight exposure time for reaching the same opacity, a model fitting was attempted in this study. As shown in Fig. 10a and b, an asymptotic regression model was applied for the opacity change under UV or sunlight exposure. The opacity relationship between the samples under UV exposure time and sunlight exposure time was generated, as shown in Fig. 10c. Therefore, UV exposure can be used as an accelerated weathering method to predict the sunlight exposure. Opacity can be used as an effective and key parameter to link UV accelerated testing and sunlight exposure to demonstrate the change of coatings’ appearance. Since the 1,4-BDF based epoxy coating changes color under sunlight, it has potential to be used in colored mulch to aid plant growth by adjusting the light reflectance [30–33].
References [1] Ferrara M, Bengisu M. Materials that change color: Smart materials, intelligent design. PoliMI SpringerBriefs; 2014. [2] Kulčar R, Friškovec M, Hauptman N, Vesel A, Gunde MK. Colorimetric properties of reversible thermochromic printing inks. Dyes Pigments 2010;86:271–7. [3] Van Gemert B. The commercialization of plastic photochromic lenses: a tribute to John Crano. Mol Cryst Liq Cryst 2000;344:57–62. [4] Azens A, Granqvist CG. Electrochromic smart windows: energy efficiency and device aspects. J Solid State Electrochem 2003;7:64–8. [5] Mennig M, Fries K, Lindenstruth M, Schmidt H. Development of fast switching photochromic coatings on transparent plastics and glass. Thin Solid Films 1999;351:230–4. [6] Ma Y, Zhu B, Wu K. Preparation and solar reflectance spectra of chameleon-type building coatings. Sol Energy 2001;70:417–22. [7] Karlessi T, Santamouris M, Apostolakis K, Synnefa A, Livada I. Development and testing of thermochromic coatings for buildings and urban structures. Sol Energy 2009;83:538–51. [8] La TG, Li X, Kumar A, Fu Y, Yang S, Chung HJ. Highly flexible, multipixelated thermosensitive smart windows made of tough hydrogels. ACS Appl Mater Interfaces 2017;9:33100–6. [9] Di Credico B, Griffini G, Levi M, Turri S. Microencapsulation of a UV-responsive photochromic dye by means of novel uv-screening polyurea-based shells for smart coating applications. ACS Appl Mater Interfaces 2013;5:6628–34. [10] Zeng W, Zhou Q, Zhang H, Qi X. One-coat epoxy coating development for the improvement of UV stability by DPP pigments. Dyes Pigments 2018;151:157–64. [11] Greenhalgh CW, Carey JL, Newton DF. The synthesis of quinodimethanes in the benzodifuranone and benzodipyrrolidone series. Dyes Pigments 1980;1:103. [12] Hallas G, Yoon C. Synthesis and properties of naphthodifuranones and naphthofuranonepyrrolidones. Dyes Pigments 2001;48:121–32. [13] Hallas G, Yoon C. The synthesis and properties of red and blue benzodifuranones. Dyes Pigments 2001;48:107–19. [14] Cai Z, Gao J, Li X, Xiang B. Synthesis and characterization of symmetrical benzodifuranone compounds with femtosecond time-resolved degenerate four-wave mixing technique. Optic Commun 2007;272:503–8. [15] Ulmann PA, Tanaka H, Matsuo Y, Xiao Z, Soga I, Nakamura E. Electric field dependent photocurrent generation in a thin-film organic photovoltaic device with a [70]fullerene-benzodifuranone dyad. Phys Chem Chem Phys 2011;13:21045–9. [16] Cerón-Carrasco JP, Ripoche A, Odobel F, Jacquemin D. Excited-state nature in benzodifuranone dyes: insights from ab initio simulations. Dyes Pigments 2012;92:1144–52. [17] Zhang H, Yang K, Chen YM, Bhatta R, Tsige M, Cheng SZD, et al. Polymers based on benzodipyrrolidone and naphthodipyrrolidone with latent hydrogen-bonding on the main chain. Macromol Chem Phys 2017;218:1–10. [18] Zhang K, Tieke B. Low-bandgap benzodifuranone-based polymers. Macromolecules 2011;44:4596–9. [19] Cui W, Yuen J, Wudl F. Benzodipyrrolidones and their polymers. Macromolecules 2011;44:7869–73. [20] Zhang K, Tieke B, Forgie JC, Vilela F, Skabara PJ. Donor-acceptor conjugated polymers based on p- and o-benzodifuranone and thiophene derivatives: electrochemical preparation and optical and electronic properties. Macromolecules 2012;45:743–50. [21] Zhang H, Neudörfl JM, Tieke B. Naphthodifuranone-based monomers and polymers. Macromolecules 2013;46:5842–9. [22] Yang K, He T, Chen X, Cheng SZD, Zhu Y. Patternable conjugated polymers with latent hydrogen-bonding on the Main Chain. Macromolecules 2014;47:8479–86. [23] Kimoto A, Tajima Y. Donor-acceptor type p-conjugated copolymers based on soluble benzodifuranone. Macromol Symp 2014;346:36–42. [24] Liu B, Chen X, Zou Y, Xiao L, Xu X, He Y, et al. Benzo[1,2-b:4,5-b’]difuran-based donor-acceptor copolymers for polymer solar cells. Macromolecules 2012;45:6898–905. [25] Deng Z, Yang K, Li L, Bao W, Hao X, Ai T, et al. Solution processed air-stable pchannel organic crystal field-effect transistors of Aminobenzodifuranone. Dyes Pigments 2018;151:173–8. [26] Greenhalgh CW, Newton DF. Bemodifwanone chromogen and its application. Jsdc 1994;110:178–84. [27] Coleman MM, Skrovanek DJ, Hu J, Painter PC. Hydrogen bonding in polymer blends. 1. FTIR studies of urethane-ether blends. Macromolecules 1988;21:59–65. [28] Garcia D, Starkweather HW. Hydrogen bonding in nylon 66 and model compounds. J Polym Sci Polym Phys Ed 1985;23:537–55. [29] Zhang H, Deng R, Wang J, Li X, Chen YM, Liu K, et al. Crystalline organic pigmentbased field-effect transistors. ACS Appl Mater Interfaces 2017;9:21891–9. [30] Andino JR, Motsenbocker CE. Colored plastic mulches influence cucumber beetle populations, vine growth, and yield of watermelon. Hortscience 2004;39:1246–9. [31] Mahmoudpour MA, Stapleton JJ. Influence of sprayable mulch colour on yield of eggplant (Solanum melongena L. cv. Millionaire). Sci Hortic (Amst) 1997;70:331–8. [32] Shiukhy S, Raeini-Sarjaz M, Chalavi V. Colored plastic mulch microclimates affect strawberry fruit yield and quality. Int J Biometeorol 2015;59:1061–6. [33] Díaz-Pérez JC, Batal KD. Colored plastic film mulches affect tomato growth and yield via changes in root-zone temperature. J Am Soc Hortic Sci 2002;127:127–35.
4. Conclusions This work first-time introduced the 1,4-BDF organic pigment into one-coat epoxy coating matrix which leads to a color-changing property of the coating responding to varied environmental conditions, such as temperature, UV, and pH. The stimulus to trigger the color change and the reasons for the color change were comprehensively investigated and understood. The 1,4-BDF based epoxy coating exhibited a color change from dark blue to brown at high temperature, under UV exposure, or under low pH. At the same time, the coating experienced a change of the hiding power resulting in a more translucent appearance. Based on the UV/Vis study and FTIR analysis, the reason for the color change was mainly caused by the reaction of the 1,4-BDF with the polyamine curing agent to form an unstable product. In addition, by studying the change of the light reflectance during the color change, UV accelerated testing and sunlight exposure can be linked by the coating's opacity. Acknowledgments The authors acknowledge the support from the faculty start-up funding from The University of Akron, the Natural Science Foundation of China, under Grant 21805151, and the Natural Science Foundation of Shandong Province, China, under Grant ZR2018MB024. The authors appreciated the technical support with SEM from Dr. Lingyan Li at National Center for Education and Research on Corrosion and Materials Performance. Appendix A. Supplementary data Supplementary data to this article can be found online at https://
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