- Email: [email protected]
Comparison for color change between benzodifuranone and benzodipyrrolidone based epoxy coating
Dyes and Pigments 175 (2020) 108171
Contents lists available at ScienceDirect
Dyes and Pigments journal homepage: http://www.elsevier.com/locate/dyepig
Comparison for color change between benzodifuranone and benzodipyrrolidone based epoxy coating Haichang Zhang a, b, 1, Weixiu Zeng c, 1, Huiling Du d, Yiting Ma b, Zhuoting Ji b, Zhifeng Deng b, **, Qixin Zhou c, * a
Key Laboratory of Rubber-Plastics of Ministry of Education/Shandong Province, School of Polymer Science & Engineering, Qingdao University of Science & Technology, Qingdao, Shangdong, 266042, PR China National and Local Joint Engineering Laboratory for Slag Comprehensive Utilization and Environmental Technology, School of Material Science and Engineering, Shaanxi University of Technology, Hanzhong, Shaanxi, 723001, PR China c National Center for Education and Research on Corrosion and Materials Performance, Department of Chemical, Biomolecular, and Corrosion Engineering, The University of Akron, Akron, OH, 44325, United States d School of Materials Science and Engineering, Xi’an University of Science and Technology, Xi’an, Shaanxi, 710054, PR China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Color-changing Coating Benzodifuranone Benzodipyrrolidone Hydrogen bonding
Compared to traditional coatings, chromogenic coatings—the color change responds to external environment stimulus—have received lots of attention and being widely explored for applications. Due to their advanced features, the development of promising chromogenic coating systems is highly needed. In this work, a system atical and comprehensive comparison was studied regarding the benzodifuranone and benzodipyrrolidone based epoxy coating with different curing agents. The color change of the coatings was investigated and quantified by using a spectrophotometer and the reason for the color change was understood by UV/Vis characterization. The introduced pigment can react with the amide units of the curing agent to form hydrogen-bonded (NH⋯O) mediates, which leads to chromogenic coatings. The more free NH units (primaquine groups and secondary ammonia units) are contained in the curing agent, the color change is easier and stronger. This work provides a new approach to design chromogenic coatings through forming a hydrogen bonding between a pigment and a curing agent in the coating.
1. Introduction In recent years, chromogenic coatings, as a class of smart materials, have received lots of attention and been widely explored for applications in architecture, aircraft, ophthalmic products, sunglasses, buildings, vehicles, smart windows, etc. [1–5]. This is because of their color-changing property or optical characteristics (transparency or light diffraction) in response to external stimuli. The most studied chromo genic materials are divided into four types including thermochromic, photochromic, electrochromic, and gasochromic [5–14]. Until now, most developed chromogenic materials change color only responding to one external stimulus. It is expected that more interesting and promising chromogenic materials should combine the above mentioned two or three characteristics.
Chromogenic coatings are usually achieved by incorporating chro mogenic compounds, such as pigments and dyes, into the coating system which could endow the coating with chromogenic properties [15,16]. The chromogenic coatings can be inorganic, organic, or hybrid systems. Compared to the inorganic or hybrid system, the pure organic one is favorable due to its flexible properties [17,18]. Previously, our group developed a benzodifuranone-based (BDFbased) epoxy organic coating that exhibited a color change from dark blue to yellow responding to the change of temperature, UV light, and pH [19]. Recently epoxy composites attracted lots of attention and used in coating or electronic packing [20–25]. Based on the preliminary study on the interaction between BDF and the curing agent D230 (two pri maquine groups (NH2)), it was found that the color change was ascribed to the interaction between BDF and D230 to form a hydrogen-bonded
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Z. Deng), [email protected] (Q. Zhou). 1 Haichang Zhang and Weixiu Zeng contributed equally. https://doi.org/10.1016/j.dyepig.2019.108171 Received 6 September 2019; Received in revised form 14 December 2019; Accepted 24 December 2019 Available online 26 December 2019 0143-7208/© 2019 Elsevier Ltd. All rights reserved.
H. Zhang et al.
Dyes and Pigments 175 (2020) 108171
cross-linked product and this product is unstable under UV light. In order to develop other color-changing coating systems with organic pigments and to deeply understand the color-changing mechanism, systematical and comprehensive studies on the color change are needed, especially the interaction between the pigment and various curing agents with primaquine, secondary ammonia, or tertiary ammonia units. It is also worthwhile to study different organic pigments to learn the color-changing mechanism. The chemical structure of benzodipyrroli done (BDP) is similar to BDF, while BDP has the nitrogen atom in the core of the pigment instead of the oxygen atom in BDF, as shown in Fig. 1 [26–32]. Both pigments belong to high thermal- and photo-stable and brightness pigments [33,34]. It is expected that the intermolecular hydrogen bonding (NH…OC) will be formed between the carbonyl – O) and the amide (NH) in the BDP core as shown in Fig. 2, before (C– mixing the BDP pigments in the coating system [35–40]. While the BDF pigments cannot generate the intermolecular hydrogen bonding. The objective of this work is to understand the color-changing mechanisms by studying two pigmented epoxy coating systems. The coatings with different organic pigments and different amine curing agents were investigated under UV exposure and characterized for their coating properties including color, morphology, gloss, corrosion resis tance, and adhesion. The color change of the coating was studied and quantified by using a spectrophotometer and the reason for the color change was understood by UV/Vis characterization.
containing 2 wt% and 4 wt% (solid weight) of the BDP pigments, respectively. 2.3. Coating evaluation 2.3.1. UV aging test The UV aging test was performed by exposing the coating samples in a QUV aging chamber from Q-Lab Corporation. In the QUV chamber, the panels were continuously exposed to UVA radiation at 45 � C with 0.68 W/m2 irradiance at 340 nm. 2.3.2. Color, gloss, and thickness measurement The CIELAB values were measured by using a spectro-guide spec trophotometer (BYK) with a D65 illuminant and 10� standard observer. Gloss was measured at 20� by using a Micro-TRI-gloss from BYK. The dried film thickness of samples was recorded with a byko-test 8500 (BYK). At least 15 locations on a coating surface were measured to obtain an average thickness value. 2.3.3. Electrochemical impedance spectroscopy (EIS) EIS measurements were carried out on coating samples to evaluate the coatings’ corrosion resistance. The measuring frequency range was 10 2 to 105 Hz with 10 mV perturbation. All samples were immersed in 3.5 wt% NaCl solution at room temperature during the EIS test. Three electrodes used for EIS measurement were the steel substrate as the working electrode, a saturated calomel electrode as the reference elec trode, and a platinum mesh as the counter electrode. The area of the working electrode was 14.6 cm2.
2. Experimental 2.1. Materials
2.3.4. Mechanical test The adhesion strength values of different formulation samples were measured by a PosiTest pull-off adhesion tester (DeFelsko).
Epoxy resin (EPON™ Resin 828) and curing agent (EPIKURE™ Curing Agent 3164) were kindly provided by Hexion Specialty Chem icals. The weight ratio of the curing agent to epoxy resin is 1.36. The BDF and BDP pigments were synthesized following reference [19,28]. The SEM images of the synthesized BDF and BDP pigments are provided in the supplemental document as Fig. S1. Steel 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. Characterization methods 2.4.1. Morphology of pigments The SEM (Tescan Lyra3) was used to characterize the particle shape and the size of the pigments. The SEM images were taken at 10.0 kV accelerating voltage.
2.2. Preparation of BDF/BDP based epoxy coating
2.4.2. UV/vis spectra The UV/Vis spectra were collected in the solid state by using a UV1800 UV/Vis spectrophotometer (Shimadzu). The spectra were ac quired in the wavelength range from 200 nm to 800 nm.
The BDF/BDP pigments were dispersed in acetone by using D 160 high-speed homogenizer (SCILOGEX) for 5 min at 18,000 rpm. Then, the epoxy resin was added and magnetically stirred for 10 min. Subse quently, the curing agent was added into the mixture. After the mixture was homogenized under stirring for 10 min, it was de-gassed in an ul trasonic bath for 5 min. The coating was applied 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 coating samples were dried overnight at room temperature followed by 2 h of curing in the oven at 100 � C. The final dry film thickness was 50 � 2 μm. E0 represents an epoxy coating without any pigments. EF2 and EF4 represent coating samples containing 2 wt% and 4 wt% (solid weight) of the BDF pigments, respectively. EP2 and EP4 represent coating samples
3. Results and discussion 3.1. Color change comparison between two pigmented coating systems under UV exposure Based on our previous founding that BDF-based epoxy coating will change color under UV exposure, to further discover the color changing, the coating samples with different concentrations of BDF and BDP pig ments were exposed to UVA (340 nm) radiation at 45 � C with a 0.68 W/ m2 irradiance. The blank epoxy coating without any pigment was used as a control. The color stability of BDF-based epoxy coatings and BDP-based epoxy coatings were quantified by color difference (ΔE). Fig. 3 pre sents the color difference of coating samples (E0, EF2, EF4, EP2, and EP4) during the 24 days of the UV exposure. From the trend of E0, it can be seen that the ΔE values of the blank epoxy coating increased during the time of the UV exposure. The same trend can be found for BDP-based epoxy coatings (EP2 and EP4). Also, the ΔE values of EP2 and EP4 were lower than those of E0, which demonstrated that BDP-based epoxy coating had better color stability than the blank epoxy coating. The difference of the ΔE values among E0, EP2, and EP4 became larger as the
Fig. 1. Chemical structure of BDF and BDP. 2
H. Zhang et al.
Dyes and Pigments 175 (2020) 108171
Fig. 2. Intermolecular hydrogen-bonded BDP molecules.
Fig. 3. The change of ΔE of the coating samples as a function of UV exposure time. E0, pure epoxy coating; EF2, 2 wt% BDF in epoxy coating; EF4, 4 wt% BDF in epoxy coating; EP2, 2 wt% BDP in epoxy coating; EP4, 4 wt% BDP in epoxy coating.
increase of UV exposure time. As for BDF-based epoxy coatings (EF2 and EF4), the trend was quite different during the 24 days of UV exposure. The ΔE values were very high after 1-day UV exposure and then decreased with the increase of UV exposure time. The large ΔE value after 1-day exposure is because the BDF pigment undergoes a color change responding to the UV light which has been studied previously [19]. The decrease of the ΔE values is the combined effect of the color-changing property of the BDF pigment and the yellowing of epoxy coating under UV light.
Fig. 4. (a) UV/Vis spectra of the BDF pigment, the curing agent 3164, and the mixture of the two components (reprinted with permission from “Benzodifur anone based color-changing epoxy-polyamine coating” by W. Zeng, Z. Deng, and et al., 2019, Dyes and Pigments, 164, 198–205.). (b) UV/Vis spectra of the BDP pigment, the curing agent 3164, and the mixture of the two components. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
3.2. UV/vis comparison between the two pigments
concluded that UV/Vis shift of BDF and no shift of BDP (Fig. 4) is not related to different pigment concentrations. It is the characteristics of BDF and BDP itself. The color change was further investigated by FTIR for coatings before and after UV exposure. As shown in Fig. S3, the FTIR spectra between the curing agent and pigment þ curing agent is not obvious due to the small amount of BDF or BDP introduced in the curing agent. However, there is a difference of the FTIR spectra of the BDF þ curing agent before and after UV exposure. The peak at 1734 cm 1 is shifted to 1656 cm 1, which could be ascribed to the decomposition of the core BDF. While for BDP þ curing agent, there is almost no change in the absorption between 1700 cm 1 and 1800 cm 1 before and after UV exposure. The shift of BDF þ curing agent demonstrates that the hydrogen-bonded cross-linked product of BDF-based coating is unstable under UV light, while BDP-based coating exhibits stable UV exposure because of the limited hydrogen bonds formed between the pigment and the curing agent.
To understand the difference of the color change between BDF-based epoxy coating and BDP- based epoxy coating, the UV/Vis absorption spectra of BDP, the curing agent 3164, and the mixture of the two components were measured (Fig. 4). As shown in Fig. 4 b, the absorption peak of the BDP pigment locates at 453 nm, while the curing agent 3164 almost exhibits no absorption in the visible light. Once mixing the BDP with the curing agent, there is a slightly red-shifted of 3 nm in the optical absorption and a slightly increased absorption intensity in the band between 550 nm and 700 nm. However, the BDF-based coating system exhibits a much larger bathochromic shift around 24 nm (from 376 nm to 400 nm) and 105 nm (from 495 nm to 600 nm) in the UV/Vis ab sorption comparing to the pure BDF (Fig. 4 a). Our previous study demonstrated that this shift is because the BDF pigment can react with the curing agent to form a new hydrogen-bonded crosslinked product [19]. The difference of this shift indicates that more hydrogen-bonded mediates were formed between BDF and the curing agent, while a few of them were formed between BDP and the curing agent. To study the concentration-dependent absorption of BDF and BDP in the epoxy coating, different concentrations of BDF in the curing agent and BDP in the curing agent were investigated by UV/Vis. As shown in Fig. S2, the absorption peaks present almost no change for different BDF concentrations, only the intensity change. This is the same for BDP in the curing agent with different concentrations. Therefore, it can be
3.3. Color change comparison among different curing agents and pigments To further understand the mechanism on the color change of BDP and BDF pigment, a series of different curing agents were investigated, including curing agent 3164, ethanediamine, diethanol amine, and 3
H. Zhang et al.
Dyes and Pigments 175 (2020) 108171
triethylamine. The chemical structures of the curing agents are list in Fig. 5 a. Ethanediamine and 3164 contain two primaquine groups (NH2) at the end of the chain, while diethanol amine contains one secondary ammonia (NH) unit in the middle of the chain. The primaquine and – O) of the BDP or secondary ammonia could react with carbonyl unit (C– BDF core and form hydrogen bonding (NH…OC), which could alter the materials’ optical and physical properties [35–41]. However, triethyl amine with tertiary ammonia could not form hydrogen bonding with carbonyl groups. As shown in Fig. 5 b, the images of the BDF mixtures show that the color turned to blue when BDF was mixed with the ethanediamine or 3164, while the color turned to brown when BDF was added into diethanol amine (Fig. 5 b, before UV). However, the color was yellow for the triethylamine, as the same as that of BDF with epoxy. The different colors with different curing agents are due to the formation of hydrogen bonding between the BDF with ethanediamine, 3164 or diethanol amine, but no hydrogen bonding was formed with triethylamine. Also, the two primaquine groups in the curing agent 3164 and ethanediamine are easier to form hydrogen bonds with BDF than the secondary ammonia unit in the diethanol amine. The hydrogen bonding does not only form a donor-acceptor system—NH unit as donor moieties and BDF core as acceptor moieties, but also enlarges the π-conjugation. That’s why an obvious color change was observed. In contrast, for the BDP based combinations (Fig. 5 c, before UV), there was a slight color change that cannot be detectable by naked eyes, even the BDP with the curing agent of two NH2 units (ethanediamine or 3164). This could be ascribed to the fact that BDP contains secondary ammonia and carbonyl unit in the core, in which hydrogen bonds could be formed between the neighboring molecules (Fig. 2) [35]. Before mixing BDP with the curing agent, most of the BDP molecules have already bonded with the adjacent molecules by hydrogen bonding. Thus, once the BDP is mixed with ethanediamine or 3164, the limited amount of the non-bonded BDP molecules can react with the NH unit resulting in a slightly bathochromic shift in the UV/Vis absorption as mentioned above in Fig. 4, hence no detectable color change was
observed by naked eyes. To further investigate the color change of the two pigments-based coatings responding to UV, the coatings were exposed to UV irritation for 1 h and coating surfaces with different combinations of the pigments and curing agents were studied. From Fig. 5 b before and after UV, it shows that the color changed from dark blue to light yellow and even close to transparent for the mixture of BDF and the curing agent of two primaquine groups, respectively; the color changed from brown to yel low for the mixture of BDF and the curing agent of secondary ammonia unit, while the color was almost the same for the mixture of BDF with the curing agent of tertiary ammonia and the BDF with epoxy itself. Comparing to the mixture of BDF and diethanol amine, the obvious color change for the mixture of BDF with 3164 or ethanediamine might be ascribed to the more formations of hydrogen-bonded similar crosslinked polymers. Our previous results indicated that the formed crosslinked polymers should exhibit non-stable and the chromophore unit of BDF core will be destroyed easily under UV light in the coating system [19]. This means if more hydrogen bonds are formed between the pigment and the curing agent, it will experience a more obvious color change under UV light. This also explained that the mixture of BDF with triethylamine and the mixtures of BDP-based coatings (Fig. 5 c) exhibited a stable color under UV exposure because of the limited hydrogen bonds formed between the pigment and the curing agent. 3.4. Surface morphology comparison between two pigmented coating systems Hydrogen bonding interaction results in BDP molecules with similar linear-shaped macromolecules (Fig. 2) which causes the BDP almost non-soluble in common organic solvents; in contrast, BDF without intermolecular hydrogen bonding interaction shows good solubility in organic solvents such as acetone. In order to investigate the effect of the hydrogen bonding on the morphology, the coating surfaces were investigated for different concentrations of BDF or BDP pigment, such as 0 wt%, 2 wt%, or 4 wt%, in the epoxy coating. As can be seen from Fig. 6,
Fig. 5. (a) Chemical structures of curing agents; (b) Images of the mixture of BDF with different curing agents before and after UV exposure; (c) Images of the mixture of BDP with different curing agents before and after UV exposure. 4
H. Zhang et al.
Dyes and Pigments 175 (2020) 108171
all the surface of BDF-based epoxy coating exhibit a smooth and almost homogeneous phase even with 4 wt% of BDF. The morphology of the BDF pigmented coating surface is similar to the pure epoxy coating without pigments. However, the surface of BDP-based epoxy coating exhibits unsmooth demonstrated by around 3 μm to 18 μm dark points on the surface. These dark points are the aggregates of BDP particles. It is expected to have a more uneven surface with increasing the BDP por tions. Therefore, the solubility of the organic pigment caused by hydrogen bonding can significantly influence the surface morphology of the coating. A well soluble organic pigment should be soluble in the coating system before spreading the pristine and lead to a smooth sur face. Moderate soluble or non-soluble organic pigments are dispersed in the coating system. Once they spread on the substrate, some particles will reside on the surface resulting in an unsmooth surface. Although the chemical structures of BDF and BDP are quite similar, only one atom is different as shown above in Fig. 1; the intermolecular hydrogen bonding interactions affect the pigment solubility, thus further influence coat ings’ surface morphology.
and after 24 days of UV exposure. The result is presented in gloss retention, which is defined as the gloss value after UV exposure divided by the gloss value before UV exposure. As shown in Fig. 7 b, all samples maintained their gloss including epoxy control, BDF-based epoxy coating, and BDP-based epoxy coating. There is no significant difference among different coating formulations, which demonstrates that the gloss of epoxy coating is not influenced by adding 2 wt% or 4 wt% of BDF or BDP pigment. The adhesion property by pull-off test and corrosion resistance evaluated from the salt fog are included in the supplementary document. 4. Conclusions In this work, a systematical color change comparison between BDF and BDP based epoxy coating with different curing agents, including 3164 and ethanediamine with two primaquine groups, diethanol amine with secondary ammonia unit, and triethylamine with tertiary amine, were comprehensively studied. The UV/Vis spectra demonstrated that the BDF can react with the curing agent to form a hydrogen bonding and result in a significantly bathochromic shift, while there is a slight red shift for BDP with the curing agent. The formed hydrogen bonding mediates are not stable under UV light that caused the color change of the coating under UV exposure, as being studied and verified by a series of coating formulations of two different pigments and four different curing agents. The color of BDF-based coating changed from dark blue to light yellow/transparent or from brown to yellow response to UV-light if the curing agent has primary or secondary ammonia unit, because BDF can react with NH2 or NH unit in the curing agent to form hydrogen bonding. However, there was no obvious color change of BDP-based coatings and BDF coating with triethylamine units because no or less hydrogen bonding mediates were formed between the introduced pigment and the curing agent. The intermolecular hydrogen bonding influenced the solubility of pigments, thus different surface morphol ogies were exhibited among BDF and BDP based coatings. In addition, the surface morphology contributed to the corrosion resistance of the two pigmented coatings, as a result, BDF-based coatings presented better corrosion resistance than that of BDP-based coatings, although all the pigmented coatings maintained a good corrosion resistance. This work provides a new and simple approach to design chromogenic coatings by introducing organic pigments that contain free-carbonyl units to react with the amide groups of the curing agent backbone, to form a
3.5. Corrosion resistance and coating gloss comparison between two pigmented coating systems A series of coating evaluations were studied on the two pigmented coating systems to investigate the effect of pigments on coating prop erty. Fig. 7 a presents the trends of the impedance modulus at low fre quency (0.01 Hz) as a function of immersion time. The change of the impedance modulus demonstrates the change of corrosion resistance of the coating. As can be seen that the low-frequency impedance modulus of all samples decreased slightly as the immersion time increased until 14 days. Further, EP4 presented a big drop while other samples kept almost the same impedance. This can be attributed to the unsmooth coating surface of 4 wt% BDP that would generate some defects on the coating surface which promotes water penetration and further coating degradation. The low-frequency impedance for E0, EF2, EF4, and EP2 are close with each other, which can be ascribed to the inherent barrier property of the epoxy coatings. The EIS results showed that the BDFbased epoxy coating maintained good corrosion resistance while the BDP-based epoxy coating might experience a lower corrosion resistance at higher pigment concentration because of the pigment aggregation caused by the intermolecular hydrogen bonding. Gloss was recorded with the geometry of 20� for all samples before
Fig. 6. Optical images of BDF or BDP in epoxy coating. (a) pure epoxy coating; (b) 2 wt% BDF in epoxy coating; (c) 4 wt% BDF in epoxy coating; (d) 2 wt% BDP in epoxy coating; (e) 4 wt% BDP in epoxy coating. 5
H. Zhang et al.
Dyes and Pigments 175 (2020) 108171
Fig. 7. (a) Impedance modulus at 0.01 Hz of coating samples as a function of immersion time. (b) The gloss retention of coating samples after the UV exposure. E0, pure epoxy coating; EF2, 2 wt% BDF in epoxy coating; EF4, 4 wt% BDF in epoxy coating; EP2, 2 wt% BDP in epoxy coating; EP4, 4 wt% BDP in epoxy coating.
hydrogen-bonded network system between the pigment and curing agent. The hydrogen-bonded product is sensitive to ambient conditions, which results in easy color change during the UV-light exposure or thermal treatment. It should be noticed that more free hydrogens on the amide units could result in a more obvious color change.
[3] Ferrara M, Bengisu M. Materials that change color: smart materials, intelligent design. SpringerBriefs in Applied Sciences and Technology; 2013. [4] Deng J, Xiao Y, Li Q, Lu J, Wen H. Experimental studies of spontaneous combustion and anaerobic cooling of coal. Fuel 2015;157:261–9. https://doi.org/10.1016/j. fuel.2015.04.063. [5] Wu LYL, Zhao Q, Huang H, Lim RJ. Sol-gel based photochromic coating for solar responsive smart window. Surf Coat Technol 2017;320:601–7. https://doi.org/ 10.1016/j.surfcoat.2016.10.074. [6] Garcia G, Buonsanti R, Llordes A, Runnerstrom EL, Bergerud A, Milliron DJ. Nearinfrared spectrally selective plasmonic electrochromic thin films. Adv Opt Mater 2013;1:215–20. https://doi.org/10.1002/adom.201200051. [7] Hu CW, Yamada Y, Yoshimura K. A new type of gasochromic material: conducting polymers with catalytic nanoparticles. Chem Commun 2017;53:3242–5. https:// doi.org/10.1039/c7cc00077d. [8] Parkin IP, Manning TD. Products of chemistry intelligent thermochromic windows. Chem Everyone 2006;83:393–400. [9] Carlotti M, Gullo G, Battisti A, Martini F, Borsacchi S, Geppi M, Ruggeri G, Pucci A. Thermochromic polyethylene films doped with perylene chromophores: experimental evidence and methods for characterization of phase behavior. Polym Chem 2015;6:4003–12. https://doi.org/10.1039/C5PY00486A. [10] Ciardelli F, Ruggeri G, Pucci A. Dye-containing polymers: methods for preparation of preparation of mechanochromic materials. Chem Soc Rev 2013;42:857–70. https://doi.org/10.1039/C2CS35414D. [11] Du H, Ma C, Ma W, Wang H. Microstructure evolution and dielectric properties of Ce-doped SrBi4TiO15 ceramics synthesized via glycine-nitrate process. Process Appl Ceram 2018;12:303–12. [12] Kim D, Kim J, Lee S. Photoswitchable Chromic behavior of conjugated polymer films for reversible patterning and construction of a logic gate. Polym Chem 2017; 8:5539–45. https://doi.org/10.1039/C7PY01145H. [13] Zhang H, Welgterlich L, Neudorfl JM, Tieke B, Yang C, Chen X, Yang W. Synthesis and characterization of 1,3,4,6-tetraarylpyrrolo[3,2-b]-pyrrole-2,5-dione (isoDPP)based donor–acceptor polymers with low band gap. Polym Chem 2013;4:4682–9. https://doi.org/10.1039/C3PY00570D. [14] Harvey CP, Tovar J. Main-chain photochromic conducting polymers. Polym Chem 2011;2:2699–706. https://doi.org/10.1039/C1PY00304F. [15] Pardo R, Zayat M, Levy D. Photochromic organic-inorganic hybrid materials. Chem Soc Rev 2011;40:672–87. https://doi.org/10.1039/c0cs00065e. [16] Bolduc A, Skene WG. Direct preparation of electroactive polymers on electrodes and their use in electrochromic devices. Polym Chem 2014;5:1119–23. https://doi. org/10.1039/C3PY01370G. [17] Zhang H, Tieke B. Conjugated polymers containing benzo- and naphthodione units in the main chain. Polym Chem 2014;5:6391–406. https://doi.org/10.1039/ C4PY00695J. [18] Oh JY, R-Gagne SR, Chiu YC, Chortos A, Lissel F, Wang GJN, Schroeder BC, Kurosawa T, Lopez J, Katsumata T, Xu J, Zhu C, Gu X, Bae WG, Kim Y, Jin L, Chung JW, Tok JBH, Bao Z. Intrinsically stretchable and healable semiconducting polymer for organic transistors. Nature 2016;17:411–6. https://doi.org/10.1038/ nature20102. [19] Zeng W, Deng Z, Wang H, Zhang H, Zhou Q. Benzodifuranone based colorchanging epoxy-polyamine coating. Dyes Pigments 2019;164:198–205. https:// doi.org/10.1016/j.dyepig.2019.01.016. [20] Zhou X, Jia Z, Feng A, Wang X, Liu J, Zhang M, Cao H, Wu G. Synthesis of fish skinderived 3D carbon foams with broadened bandwidth and excellent electromagnetic wave absorption performance. Carbon 2019;152:827–36. https://doi.org/ 10.1016/j.carbon.2019.06. 080. [21] Jia ZR, Gao ZG, Lan D, Cheng YH, Wu GL, Wu HJ. Effects of filler loading and surface modification on electrical and thermal properties of epoxy/ montmorillonite composite. Chin. Phys. B 2018;27:117806. https://doi.org/ 10.1088/1674-1056/27/11/117806. [22] Chen S, Cheng Y, Xie Q, Xiao B, Wang Z, Liu J, Wu G. Enhanced breakdown strength of aligned-sodium-titanate-nanowire/epoxy nanocompsites and their
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Haichang Zhang: Conceptualization, Validation, Data curation, Writing - original draft, Visualization, Project administration, Funding acquisition. Weixiu Zeng: Conceptualization, Methodology, Validation, Investigation, Writing - original draft. Huiling Du: Methodology, Vali dation, Investigation. Yiting Ma: Methodology, Validation, Investiga tion. Zhuoting Ji: Methodology, Validation, Investigation. Zhifeng Deng: Conceptualization, Validation, Data curation, Writing - review & editing, Visualization, Supervision, Project administration, Funding acquisition. Qixin Zhou: Conceptualization, Validation, Resources, Writing - review & editing, Supervision, Project administration, Funding acquisition. Acknowledgments The authors acknowledge the support from the National Natural Science Foundation of China, under Grant 21805151, Natural Science Foundation of Shandong Province, China, under Grant ZR2018MB024, and the faculty start-up funding from The University of Akron, United States. The authors appreciated the technical support with SEM from Dr. Lingyan Li at the National Center for Education and Research on Corrosion and Materials Performance at The University of Akron. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.dyepig.2019.108171. References [1] Lampert CM. Chromogenic smart materials. Mater Today 2004;7:28–35. https:// doi.org/10.1016/S1369-7021(04)00123-3. [2] Granqvist CG, Lansåker PC, Mlyuka NR, Niklasson GA, Avenda~ no E. Progress in chromogenics: new results for electrochromic and thermochromic materials and devices. Sol Energy Mater Sol Cells 2009;93:2032–9. https://doi.org/10.1016/j. solmat.2009.02.026.
6
H. Zhang et al.
[23] [24]
[25] [26]
[27] [28] [29] [30] [31]
[32]
Dyes and Pigments 175 (2020) 108171
anisotropic dielectric properties. Compos Part A: Appl Sci Manuf 2019;120:84–94. doi.org/10.1016/j. compositesa.2019.02.024. Xie Q, Cheng Y, Chen S, Wu G, Wang Z, Jia Z. Dielectric and thermal properties of expoxy resins with TiO2 nanoweires. J Mater Sci Mater Electron 2017;28(23): 17871–80. https://doi.org/10.1007/s10854-017-7728-2. Wu G, Cheng Y, Wang K, Wang Y, Feng A. Fabrication and characterization of OMMt/BMI/CE composites with low dielectric properties and high thermal stability for electronic packaging. J Mater Sci Mater Electron 2016;27:5592–9. https://doi.org/10.1007/s10854-016-4464-y. 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. https://doi.org/10.1016/j.dyepig.2017.12.058. Deng Z, Hao X, Zhang P, Li L, Yuan X, Ai T, Bao W, Kou K. Donor-acceptor-donor molecules based on diketopyrrolopyrrole benzodipyrrolidione and naphthodipyrrolidione: organic crystal field-effect transistors. Dyes Pigments 2019; 162:883–7. https://doi.org/10.1016/j.dyepig.2018.11.028. Chen X, Guo K, Li F, Qiao H, Liu T, Ye Y, Li J. Synthesis and properties of benzodipyrrolidone (BDP)-based donor-acceptor copolymers. J Macromol Sci Part A 2015;52:517–22. https://doi.org/10.1080/10601325.2015.1039322. Zhang H, Ghasimi S, Tieke B, Schade A, Spanger S. Aminobenzodione-based polymers with low band gap and solvatochromic behavior. Polym Chem 2014;5: 3817–23. https://doi.org/10.1039/C3PY01702H. Deng P, Liu L, Ren S, Li H, Zhang Q. N-acylation: an effective method for reducing the LUMO energy levels of conjugated polymers containing five-memered lactam units. Chem Commun 2012;48:6960–2. https://doi.org/10.1039/C2CC32184J. Cui W, Yuen J, Wudl F. Benzodipyrrolidones and their polymers. Macromolecules 2011;44:7869–73. https://doi.org/10.1021/ma2017293. Rumer JW, Levick M, Dai S-Y, Rossbauer S, Huang Z, Binik L, Anthopoulos TD, Durrant JR, Procter DJ, McCulloch I. BPTs: thiphene-flanked benzodipyrrolidone conjugated polymers for ambipolar organic transistors. Chem Commun (J Chem Soc Sect D) 2013;49:4465–7. https://doi.org/10.1039/C3CC40811F. Deng Z, Ai T, Li R, Yuan W, Zhang K, Du H, Zhang H. Conjugated polymers containing building blocks 1,3,45-tetraarypyrrolo[3,2-b]pyrrole-2,5-dione
[33] [34] [35] [36]
[37]
[38]
[39] [40]
[41]
7
(isoDPP), benzodipyrrolidone (BDP) or naphthodiyrrolidone (NDP): a review. Polymers 2019;11:1683. https://doi.org/10.3390/polym11101683. Greenhalgh CW, Carey JL, Newton DF. The synthesis of quinodimethanes in the benzodifuranone and benzodipyrrolidone series. Dyes Pigments 1980;1:103. https://doi.org/10.1016/0143-7208(80)80010-6. Hallas G, Yoon C. The synthesis and properties of red and blue benzodifuranones. Dyes Pigments 2001;48:107–19. https://doi.org/10.1016/S0143-7208(00)000929. Zhang H, Deng R, Wang J, Li X, Chen Y, Liu K, Taubert C, Chen SZD, Zhu Y. Crystalline organic pigment-based field-effect transistors. ACS Appl Mater Interfaces 2017;9:21891–9. https://doi.org/10.1021/acsami.7b03170. Hu Y, Motzer H, Etxeberria A, Fernandez-Berridi M, Iruin J, Painter PC, Coleman MM. Conecering the self-association of N-vinyl pyrrolidone and its effect on the determination of equilibrium constants and the thermodynamics of mixing. Macromol Chem Phys 2000;201:705–14. https://doi.org/10.1002/(SICI)15213935(20000301)201:63.0.CO. 2-9. Zhang H, Liu K, Wu KY, Chen YM, Deng R, Li X, Jin H, Li S, Chung SSC, Wang CL, Zhu Y. Hydrogen-bonding-mediated solid-state self-assembled isoepindolidiones (isoEpi) crystal for organic field-effect transistors. J Phys Chem C 2018;122: 2888–5895. https://doi.org/10.1021/acsjpcc.7b11992. Coleman MM, Skrovanek DJ, Hu J, Painter PC. Hydrogen bonding in polymer blends. 1. FTIR studies of urethane-ether blends. Hydrogen bonding in polymer blends. 1. FTIR studies of urethane-ether blends. Macromolecules 1988;21:59–65. https://doi.org/10.1021/ma00179a014. Garcia D, Starkweather HW. Hydrogen bonding in nylon 66 and model compounds. Hydrogen bonding in nylon 66 and model compounds. J Polym Sci Polym Phys Ed 1985;23:537–55. https://doi.org/10.1002/pol.1985.180230310. Macknight WJ, Yang M. Property-structure relationships in poly-urethanes: infrared studies. Property-structure relationship in poly-urethanes: infrared studies. J Polym Sci Part C: Polym Symp 1973;42:817–32. https://doi.org/ 10.1002/polc.5070420231. Kuo SW, Chuang FC. Studies of miscibility behavior and hydrogen bonding in blends of poly(vinylphenol) and poly(vinbylpyrrolidone). Macromolecules 2001; 34:5224–8. https://doi.org/10.1021/ma010517a.
Contents lists available at ScienceDirect
Dyes and Pigments journal homepage: http://www.elsevier.com/locate/dyepig
Comparison for color change between benzodifuranone and benzodipyrrolidone based epoxy coating Haichang Zhang a, b, 1, Weixiu Zeng c, 1, Huiling Du d, Yiting Ma b, Zhuoting Ji b, Zhifeng Deng b, **, Qixin Zhou c, * a
Key Laboratory of Rubber-Plastics of Ministry of Education/Shandong Province, School of Polymer Science & Engineering, Qingdao University of Science & Technology, Qingdao, Shangdong, 266042, PR China National and Local Joint Engineering Laboratory for Slag Comprehensive Utilization and Environmental Technology, School of Material Science and Engineering, Shaanxi University of Technology, Hanzhong, Shaanxi, 723001, PR China c National Center for Education and Research on Corrosion and Materials Performance, Department of Chemical, Biomolecular, and Corrosion Engineering, The University of Akron, Akron, OH, 44325, United States d School of Materials Science and Engineering, Xi’an University of Science and Technology, Xi’an, Shaanxi, 710054, PR China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Color-changing Coating Benzodifuranone Benzodipyrrolidone Hydrogen bonding
Compared to traditional coatings, chromogenic coatings—the color change responds to external environment stimulus—have received lots of attention and being widely explored for applications. Due to their advanced features, the development of promising chromogenic coating systems is highly needed. In this work, a system atical and comprehensive comparison was studied regarding the benzodifuranone and benzodipyrrolidone based epoxy coating with different curing agents. The color change of the coatings was investigated and quantified by using a spectrophotometer and the reason for the color change was understood by UV/Vis characterization. The introduced pigment can react with the amide units of the curing agent to form hydrogen-bonded (NH⋯O) mediates, which leads to chromogenic coatings. The more free NH units (primaquine groups and secondary ammonia units) are contained in the curing agent, the color change is easier and stronger. This work provides a new approach to design chromogenic coatings through forming a hydrogen bonding between a pigment and a curing agent in the coating.
1. Introduction In recent years, chromogenic coatings, as a class of smart materials, have received lots of attention and been widely explored for applications in architecture, aircraft, ophthalmic products, sunglasses, buildings, vehicles, smart windows, etc. [1–5]. This is because of their color-changing property or optical characteristics (transparency or light diffraction) in response to external stimuli. The most studied chromo genic materials are divided into four types including thermochromic, photochromic, electrochromic, and gasochromic [5–14]. Until now, most developed chromogenic materials change color only responding to one external stimulus. It is expected that more interesting and promising chromogenic materials should combine the above mentioned two or three characteristics.
Chromogenic coatings are usually achieved by incorporating chro mogenic compounds, such as pigments and dyes, into the coating system which could endow the coating with chromogenic properties [15,16]. The chromogenic coatings can be inorganic, organic, or hybrid systems. Compared to the inorganic or hybrid system, the pure organic one is favorable due to its flexible properties [17,18]. Previously, our group developed a benzodifuranone-based (BDFbased) epoxy organic coating that exhibited a color change from dark blue to yellow responding to the change of temperature, UV light, and pH [19]. Recently epoxy composites attracted lots of attention and used in coating or electronic packing [20–25]. Based on the preliminary study on the interaction between BDF and the curing agent D230 (two pri maquine groups (NH2)), it was found that the color change was ascribed to the interaction between BDF and D230 to form a hydrogen-bonded
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Z. Deng), [email protected] (Q. Zhou). 1 Haichang Zhang and Weixiu Zeng contributed equally. https://doi.org/10.1016/j.dyepig.2019.108171 Received 6 September 2019; Received in revised form 14 December 2019; Accepted 24 December 2019 Available online 26 December 2019 0143-7208/© 2019 Elsevier Ltd. All rights reserved.
H. Zhang et al.
Dyes and Pigments 175 (2020) 108171
cross-linked product and this product is unstable under UV light. In order to develop other color-changing coating systems with organic pigments and to deeply understand the color-changing mechanism, systematical and comprehensive studies on the color change are needed, especially the interaction between the pigment and various curing agents with primaquine, secondary ammonia, or tertiary ammonia units. It is also worthwhile to study different organic pigments to learn the color-changing mechanism. The chemical structure of benzodipyrroli done (BDP) is similar to BDF, while BDP has the nitrogen atom in the core of the pigment instead of the oxygen atom in BDF, as shown in Fig. 1 [26–32]. Both pigments belong to high thermal- and photo-stable and brightness pigments [33,34]. It is expected that the intermolecular hydrogen bonding (NH…OC) will be formed between the carbonyl – O) and the amide (NH) in the BDP core as shown in Fig. 2, before (C– mixing the BDP pigments in the coating system [35–40]. While the BDF pigments cannot generate the intermolecular hydrogen bonding. The objective of this work is to understand the color-changing mechanisms by studying two pigmented epoxy coating systems. The coatings with different organic pigments and different amine curing agents were investigated under UV exposure and characterized for their coating properties including color, morphology, gloss, corrosion resis tance, and adhesion. The color change of the coating was studied and quantified by using a spectrophotometer and the reason for the color change was understood by UV/Vis characterization.
containing 2 wt% and 4 wt% (solid weight) of the BDP pigments, respectively. 2.3. Coating evaluation 2.3.1. UV aging test The UV aging test was performed by exposing the coating samples in a QUV aging chamber from Q-Lab Corporation. In the QUV chamber, the panels were continuously exposed to UVA radiation at 45 � C with 0.68 W/m2 irradiance at 340 nm. 2.3.2. Color, gloss, and thickness measurement The CIELAB values were measured by using a spectro-guide spec trophotometer (BYK) with a D65 illuminant and 10� standard observer. Gloss was measured at 20� by using a Micro-TRI-gloss from BYK. The dried film thickness of samples was recorded with a byko-test 8500 (BYK). At least 15 locations on a coating surface were measured to obtain an average thickness value. 2.3.3. Electrochemical impedance spectroscopy (EIS) EIS measurements were carried out on coating samples to evaluate the coatings’ corrosion resistance. The measuring frequency range was 10 2 to 105 Hz with 10 mV perturbation. All samples were immersed in 3.5 wt% NaCl solution at room temperature during the EIS test. Three electrodes used for EIS measurement were the steel substrate as the working electrode, a saturated calomel electrode as the reference elec trode, and a platinum mesh as the counter electrode. The area of the working electrode was 14.6 cm2.
2. Experimental 2.1. Materials
2.3.4. Mechanical test The adhesion strength values of different formulation samples were measured by a PosiTest pull-off adhesion tester (DeFelsko).
Epoxy resin (EPON™ Resin 828) and curing agent (EPIKURE™ Curing Agent 3164) were kindly provided by Hexion Specialty Chem icals. The weight ratio of the curing agent to epoxy resin is 1.36. The BDF and BDP pigments were synthesized following reference [19,28]. The SEM images of the synthesized BDF and BDP pigments are provided in the supplemental document as Fig. S1. Steel 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. Characterization methods 2.4.1. Morphology of pigments The SEM (Tescan Lyra3) was used to characterize the particle shape and the size of the pigments. The SEM images were taken at 10.0 kV accelerating voltage.
2.2. Preparation of BDF/BDP based epoxy coating
2.4.2. UV/vis spectra The UV/Vis spectra were collected in the solid state by using a UV1800 UV/Vis spectrophotometer (Shimadzu). The spectra were ac quired in the wavelength range from 200 nm to 800 nm.
The BDF/BDP pigments were dispersed in acetone by using D 160 high-speed homogenizer (SCILOGEX) for 5 min at 18,000 rpm. Then, the epoxy resin was added and magnetically stirred for 10 min. Subse quently, the curing agent was added into the mixture. After the mixture was homogenized under stirring for 10 min, it was de-gassed in an ul trasonic bath for 5 min. The coating was applied 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 coating samples were dried overnight at room temperature followed by 2 h of curing in the oven at 100 � C. The final dry film thickness was 50 � 2 μm. E0 represents an epoxy coating without any pigments. EF2 and EF4 represent coating samples containing 2 wt% and 4 wt% (solid weight) of the BDF pigments, respectively. EP2 and EP4 represent coating samples
3. Results and discussion 3.1. Color change comparison between two pigmented coating systems under UV exposure Based on our previous founding that BDF-based epoxy coating will change color under UV exposure, to further discover the color changing, the coating samples with different concentrations of BDF and BDP pig ments were exposed to UVA (340 nm) radiation at 45 � C with a 0.68 W/ m2 irradiance. The blank epoxy coating without any pigment was used as a control. The color stability of BDF-based epoxy coatings and BDP-based epoxy coatings were quantified by color difference (ΔE). Fig. 3 pre sents the color difference of coating samples (E0, EF2, EF4, EP2, and EP4) during the 24 days of the UV exposure. From the trend of E0, it can be seen that the ΔE values of the blank epoxy coating increased during the time of the UV exposure. The same trend can be found for BDP-based epoxy coatings (EP2 and EP4). Also, the ΔE values of EP2 and EP4 were lower than those of E0, which demonstrated that BDP-based epoxy coating had better color stability than the blank epoxy coating. The difference of the ΔE values among E0, EP2, and EP4 became larger as the
Fig. 1. Chemical structure of BDF and BDP. 2
H. Zhang et al.
Dyes and Pigments 175 (2020) 108171
Fig. 2. Intermolecular hydrogen-bonded BDP molecules.
Fig. 3. The change of ΔE of the coating samples as a function of UV exposure time. E0, pure epoxy coating; EF2, 2 wt% BDF in epoxy coating; EF4, 4 wt% BDF in epoxy coating; EP2, 2 wt% BDP in epoxy coating; EP4, 4 wt% BDP in epoxy coating.
increase of UV exposure time. As for BDF-based epoxy coatings (EF2 and EF4), the trend was quite different during the 24 days of UV exposure. The ΔE values were very high after 1-day UV exposure and then decreased with the increase of UV exposure time. The large ΔE value after 1-day exposure is because the BDF pigment undergoes a color change responding to the UV light which has been studied previously [19]. The decrease of the ΔE values is the combined effect of the color-changing property of the BDF pigment and the yellowing of epoxy coating under UV light.
Fig. 4. (a) UV/Vis spectra of the BDF pigment, the curing agent 3164, and the mixture of the two components (reprinted with permission from “Benzodifur anone based color-changing epoxy-polyamine coating” by W. Zeng, Z. Deng, and et al., 2019, Dyes and Pigments, 164, 198–205.). (b) UV/Vis spectra of the BDP pigment, the curing agent 3164, and the mixture of the two components. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
3.2. UV/vis comparison between the two pigments
concluded that UV/Vis shift of BDF and no shift of BDP (Fig. 4) is not related to different pigment concentrations. It is the characteristics of BDF and BDP itself. The color change was further investigated by FTIR for coatings before and after UV exposure. As shown in Fig. S3, the FTIR spectra between the curing agent and pigment þ curing agent is not obvious due to the small amount of BDF or BDP introduced in the curing agent. However, there is a difference of the FTIR spectra of the BDF þ curing agent before and after UV exposure. The peak at 1734 cm 1 is shifted to 1656 cm 1, which could be ascribed to the decomposition of the core BDF. While for BDP þ curing agent, there is almost no change in the absorption between 1700 cm 1 and 1800 cm 1 before and after UV exposure. The shift of BDF þ curing agent demonstrates that the hydrogen-bonded cross-linked product of BDF-based coating is unstable under UV light, while BDP-based coating exhibits stable UV exposure because of the limited hydrogen bonds formed between the pigment and the curing agent.
To understand the difference of the color change between BDF-based epoxy coating and BDP- based epoxy coating, the UV/Vis absorption spectra of BDP, the curing agent 3164, and the mixture of the two components were measured (Fig. 4). As shown in Fig. 4 b, the absorption peak of the BDP pigment locates at 453 nm, while the curing agent 3164 almost exhibits no absorption in the visible light. Once mixing the BDP with the curing agent, there is a slightly red-shifted of 3 nm in the optical absorption and a slightly increased absorption intensity in the band between 550 nm and 700 nm. However, the BDF-based coating system exhibits a much larger bathochromic shift around 24 nm (from 376 nm to 400 nm) and 105 nm (from 495 nm to 600 nm) in the UV/Vis ab sorption comparing to the pure BDF (Fig. 4 a). Our previous study demonstrated that this shift is because the BDF pigment can react with the curing agent to form a new hydrogen-bonded crosslinked product [19]. The difference of this shift indicates that more hydrogen-bonded mediates were formed between BDF and the curing agent, while a few of them were formed between BDP and the curing agent. To study the concentration-dependent absorption of BDF and BDP in the epoxy coating, different concentrations of BDF in the curing agent and BDP in the curing agent were investigated by UV/Vis. As shown in Fig. S2, the absorption peaks present almost no change for different BDF concentrations, only the intensity change. This is the same for BDP in the curing agent with different concentrations. Therefore, it can be
3.3. Color change comparison among different curing agents and pigments To further understand the mechanism on the color change of BDP and BDF pigment, a series of different curing agents were investigated, including curing agent 3164, ethanediamine, diethanol amine, and 3
H. Zhang et al.
Dyes and Pigments 175 (2020) 108171
triethylamine. The chemical structures of the curing agents are list in Fig. 5 a. Ethanediamine and 3164 contain two primaquine groups (NH2) at the end of the chain, while diethanol amine contains one secondary ammonia (NH) unit in the middle of the chain. The primaquine and – O) of the BDP or secondary ammonia could react with carbonyl unit (C– BDF core and form hydrogen bonding (NH…OC), which could alter the materials’ optical and physical properties [35–41]. However, triethyl amine with tertiary ammonia could not form hydrogen bonding with carbonyl groups. As shown in Fig. 5 b, the images of the BDF mixtures show that the color turned to blue when BDF was mixed with the ethanediamine or 3164, while the color turned to brown when BDF was added into diethanol amine (Fig. 5 b, before UV). However, the color was yellow for the triethylamine, as the same as that of BDF with epoxy. The different colors with different curing agents are due to the formation of hydrogen bonding between the BDF with ethanediamine, 3164 or diethanol amine, but no hydrogen bonding was formed with triethylamine. Also, the two primaquine groups in the curing agent 3164 and ethanediamine are easier to form hydrogen bonds with BDF than the secondary ammonia unit in the diethanol amine. The hydrogen bonding does not only form a donor-acceptor system—NH unit as donor moieties and BDF core as acceptor moieties, but also enlarges the π-conjugation. That’s why an obvious color change was observed. In contrast, for the BDP based combinations (Fig. 5 c, before UV), there was a slight color change that cannot be detectable by naked eyes, even the BDP with the curing agent of two NH2 units (ethanediamine or 3164). This could be ascribed to the fact that BDP contains secondary ammonia and carbonyl unit in the core, in which hydrogen bonds could be formed between the neighboring molecules (Fig. 2) [35]. Before mixing BDP with the curing agent, most of the BDP molecules have already bonded with the adjacent molecules by hydrogen bonding. Thus, once the BDP is mixed with ethanediamine or 3164, the limited amount of the non-bonded BDP molecules can react with the NH unit resulting in a slightly bathochromic shift in the UV/Vis absorption as mentioned above in Fig. 4, hence no detectable color change was
observed by naked eyes. To further investigate the color change of the two pigments-based coatings responding to UV, the coatings were exposed to UV irritation for 1 h and coating surfaces with different combinations of the pigments and curing agents were studied. From Fig. 5 b before and after UV, it shows that the color changed from dark blue to light yellow and even close to transparent for the mixture of BDF and the curing agent of two primaquine groups, respectively; the color changed from brown to yel low for the mixture of BDF and the curing agent of secondary ammonia unit, while the color was almost the same for the mixture of BDF with the curing agent of tertiary ammonia and the BDF with epoxy itself. Comparing to the mixture of BDF and diethanol amine, the obvious color change for the mixture of BDF with 3164 or ethanediamine might be ascribed to the more formations of hydrogen-bonded similar crosslinked polymers. Our previous results indicated that the formed crosslinked polymers should exhibit non-stable and the chromophore unit of BDF core will be destroyed easily under UV light in the coating system [19]. This means if more hydrogen bonds are formed between the pigment and the curing agent, it will experience a more obvious color change under UV light. This also explained that the mixture of BDF with triethylamine and the mixtures of BDP-based coatings (Fig. 5 c) exhibited a stable color under UV exposure because of the limited hydrogen bonds formed between the pigment and the curing agent. 3.4. Surface morphology comparison between two pigmented coating systems Hydrogen bonding interaction results in BDP molecules with similar linear-shaped macromolecules (Fig. 2) which causes the BDP almost non-soluble in common organic solvents; in contrast, BDF without intermolecular hydrogen bonding interaction shows good solubility in organic solvents such as acetone. In order to investigate the effect of the hydrogen bonding on the morphology, the coating surfaces were investigated for different concentrations of BDF or BDP pigment, such as 0 wt%, 2 wt%, or 4 wt%, in the epoxy coating. As can be seen from Fig. 6,
Fig. 5. (a) Chemical structures of curing agents; (b) Images of the mixture of BDF with different curing agents before and after UV exposure; (c) Images of the mixture of BDP with different curing agents before and after UV exposure. 4
H. Zhang et al.
Dyes and Pigments 175 (2020) 108171
all the surface of BDF-based epoxy coating exhibit a smooth and almost homogeneous phase even with 4 wt% of BDF. The morphology of the BDF pigmented coating surface is similar to the pure epoxy coating without pigments. However, the surface of BDP-based epoxy coating exhibits unsmooth demonstrated by around 3 μm to 18 μm dark points on the surface. These dark points are the aggregates of BDP particles. It is expected to have a more uneven surface with increasing the BDP por tions. Therefore, the solubility of the organic pigment caused by hydrogen bonding can significantly influence the surface morphology of the coating. A well soluble organic pigment should be soluble in the coating system before spreading the pristine and lead to a smooth sur face. Moderate soluble or non-soluble organic pigments are dispersed in the coating system. Once they spread on the substrate, some particles will reside on the surface resulting in an unsmooth surface. Although the chemical structures of BDF and BDP are quite similar, only one atom is different as shown above in Fig. 1; the intermolecular hydrogen bonding interactions affect the pigment solubility, thus further influence coat ings’ surface morphology.
and after 24 days of UV exposure. The result is presented in gloss retention, which is defined as the gloss value after UV exposure divided by the gloss value before UV exposure. As shown in Fig. 7 b, all samples maintained their gloss including epoxy control, BDF-based epoxy coating, and BDP-based epoxy coating. There is no significant difference among different coating formulations, which demonstrates that the gloss of epoxy coating is not influenced by adding 2 wt% or 4 wt% of BDF or BDP pigment. The adhesion property by pull-off test and corrosion resistance evaluated from the salt fog are included in the supplementary document. 4. Conclusions In this work, a systematical color change comparison between BDF and BDP based epoxy coating with different curing agents, including 3164 and ethanediamine with two primaquine groups, diethanol amine with secondary ammonia unit, and triethylamine with tertiary amine, were comprehensively studied. The UV/Vis spectra demonstrated that the BDF can react with the curing agent to form a hydrogen bonding and result in a significantly bathochromic shift, while there is a slight red shift for BDP with the curing agent. The formed hydrogen bonding mediates are not stable under UV light that caused the color change of the coating under UV exposure, as being studied and verified by a series of coating formulations of two different pigments and four different curing agents. The color of BDF-based coating changed from dark blue to light yellow/transparent or from brown to yellow response to UV-light if the curing agent has primary or secondary ammonia unit, because BDF can react with NH2 or NH unit in the curing agent to form hydrogen bonding. However, there was no obvious color change of BDP-based coatings and BDF coating with triethylamine units because no or less hydrogen bonding mediates were formed between the introduced pigment and the curing agent. The intermolecular hydrogen bonding influenced the solubility of pigments, thus different surface morphol ogies were exhibited among BDF and BDP based coatings. In addition, the surface morphology contributed to the corrosion resistance of the two pigmented coatings, as a result, BDF-based coatings presented better corrosion resistance than that of BDP-based coatings, although all the pigmented coatings maintained a good corrosion resistance. This work provides a new and simple approach to design chromogenic coatings by introducing organic pigments that contain free-carbonyl units to react with the amide groups of the curing agent backbone, to form a
3.5. Corrosion resistance and coating gloss comparison between two pigmented coating systems A series of coating evaluations were studied on the two pigmented coating systems to investigate the effect of pigments on coating prop erty. Fig. 7 a presents the trends of the impedance modulus at low fre quency (0.01 Hz) as a function of immersion time. The change of the impedance modulus demonstrates the change of corrosion resistance of the coating. As can be seen that the low-frequency impedance modulus of all samples decreased slightly as the immersion time increased until 14 days. Further, EP4 presented a big drop while other samples kept almost the same impedance. This can be attributed to the unsmooth coating surface of 4 wt% BDP that would generate some defects on the coating surface which promotes water penetration and further coating degradation. The low-frequency impedance for E0, EF2, EF4, and EP2 are close with each other, which can be ascribed to the inherent barrier property of the epoxy coatings. The EIS results showed that the BDFbased epoxy coating maintained good corrosion resistance while the BDP-based epoxy coating might experience a lower corrosion resistance at higher pigment concentration because of the pigment aggregation caused by the intermolecular hydrogen bonding. Gloss was recorded with the geometry of 20� for all samples before
Fig. 6. Optical images of BDF or BDP in epoxy coating. (a) pure epoxy coating; (b) 2 wt% BDF in epoxy coating; (c) 4 wt% BDF in epoxy coating; (d) 2 wt% BDP in epoxy coating; (e) 4 wt% BDP in epoxy coating. 5
H. Zhang et al.
Dyes and Pigments 175 (2020) 108171
Fig. 7. (a) Impedance modulus at 0.01 Hz of coating samples as a function of immersion time. (b) The gloss retention of coating samples after the UV exposure. E0, pure epoxy coating; EF2, 2 wt% BDF in epoxy coating; EF4, 4 wt% BDF in epoxy coating; EP2, 2 wt% BDP in epoxy coating; EP4, 4 wt% BDP in epoxy coating.
hydrogen-bonded network system between the pigment and curing agent. The hydrogen-bonded product is sensitive to ambient conditions, which results in easy color change during the UV-light exposure or thermal treatment. It should be noticed that more free hydrogens on the amide units could result in a more obvious color change.
[3] Ferrara M, Bengisu M. Materials that change color: smart materials, intelligent design. SpringerBriefs in Applied Sciences and Technology; 2013. [4] Deng J, Xiao Y, Li Q, Lu J, Wen H. Experimental studies of spontaneous combustion and anaerobic cooling of coal. Fuel 2015;157:261–9. https://doi.org/10.1016/j. fuel.2015.04.063. [5] Wu LYL, Zhao Q, Huang H, Lim RJ. Sol-gel based photochromic coating for solar responsive smart window. Surf Coat Technol 2017;320:601–7. https://doi.org/ 10.1016/j.surfcoat.2016.10.074. [6] Garcia G, Buonsanti R, Llordes A, Runnerstrom EL, Bergerud A, Milliron DJ. Nearinfrared spectrally selective plasmonic electrochromic thin films. Adv Opt Mater 2013;1:215–20. https://doi.org/10.1002/adom.201200051. [7] Hu CW, Yamada Y, Yoshimura K. A new type of gasochromic material: conducting polymers with catalytic nanoparticles. Chem Commun 2017;53:3242–5. https:// doi.org/10.1039/c7cc00077d. [8] Parkin IP, Manning TD. Products of chemistry intelligent thermochromic windows. Chem Everyone 2006;83:393–400. [9] Carlotti M, Gullo G, Battisti A, Martini F, Borsacchi S, Geppi M, Ruggeri G, Pucci A. Thermochromic polyethylene films doped with perylene chromophores: experimental evidence and methods for characterization of phase behavior. Polym Chem 2015;6:4003–12. https://doi.org/10.1039/C5PY00486A. [10] Ciardelli F, Ruggeri G, Pucci A. Dye-containing polymers: methods for preparation of preparation of mechanochromic materials. Chem Soc Rev 2013;42:857–70. https://doi.org/10.1039/C2CS35414D. [11] Du H, Ma C, Ma W, Wang H. Microstructure evolution and dielectric properties of Ce-doped SrBi4TiO15 ceramics synthesized via glycine-nitrate process. Process Appl Ceram 2018;12:303–12. [12] Kim D, Kim J, Lee S. Photoswitchable Chromic behavior of conjugated polymer films for reversible patterning and construction of a logic gate. Polym Chem 2017; 8:5539–45. https://doi.org/10.1039/C7PY01145H. [13] Zhang H, Welgterlich L, Neudorfl JM, Tieke B, Yang C, Chen X, Yang W. Synthesis and characterization of 1,3,4,6-tetraarylpyrrolo[3,2-b]-pyrrole-2,5-dione (isoDPP)based donor–acceptor polymers with low band gap. Polym Chem 2013;4:4682–9. https://doi.org/10.1039/C3PY00570D. [14] Harvey CP, Tovar J. Main-chain photochromic conducting polymers. Polym Chem 2011;2:2699–706. https://doi.org/10.1039/C1PY00304F. [15] Pardo R, Zayat M, Levy D. Photochromic organic-inorganic hybrid materials. Chem Soc Rev 2011;40:672–87. https://doi.org/10.1039/c0cs00065e. [16] Bolduc A, Skene WG. Direct preparation of electroactive polymers on electrodes and their use in electrochromic devices. Polym Chem 2014;5:1119–23. https://doi. org/10.1039/C3PY01370G. [17] Zhang H, Tieke B. Conjugated polymers containing benzo- and naphthodione units in the main chain. Polym Chem 2014;5:6391–406. https://doi.org/10.1039/ C4PY00695J. [18] Oh JY, R-Gagne SR, Chiu YC, Chortos A, Lissel F, Wang GJN, Schroeder BC, Kurosawa T, Lopez J, Katsumata T, Xu J, Zhu C, Gu X, Bae WG, Kim Y, Jin L, Chung JW, Tok JBH, Bao Z. Intrinsically stretchable and healable semiconducting polymer for organic transistors. Nature 2016;17:411–6. https://doi.org/10.1038/ nature20102. [19] Zeng W, Deng Z, Wang H, Zhang H, Zhou Q. Benzodifuranone based colorchanging epoxy-polyamine coating. Dyes Pigments 2019;164:198–205. https:// doi.org/10.1016/j.dyepig.2019.01.016. [20] Zhou X, Jia Z, Feng A, Wang X, Liu J, Zhang M, Cao H, Wu G. Synthesis of fish skinderived 3D carbon foams with broadened bandwidth and excellent electromagnetic wave absorption performance. Carbon 2019;152:827–36. https://doi.org/ 10.1016/j.carbon.2019.06. 080. [21] Jia ZR, Gao ZG, Lan D, Cheng YH, Wu GL, Wu HJ. Effects of filler loading and surface modification on electrical and thermal properties of epoxy/ montmorillonite composite. Chin. Phys. B 2018;27:117806. https://doi.org/ 10.1088/1674-1056/27/11/117806. [22] Chen S, Cheng Y, Xie Q, Xiao B, Wang Z, Liu J, Wu G. Enhanced breakdown strength of aligned-sodium-titanate-nanowire/epoxy nanocompsites and their
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Haichang Zhang: Conceptualization, Validation, Data curation, Writing - original draft, Visualization, Project administration, Funding acquisition. Weixiu Zeng: Conceptualization, Methodology, Validation, Investigation, Writing - original draft. Huiling Du: Methodology, Vali dation, Investigation. Yiting Ma: Methodology, Validation, Investiga tion. Zhuoting Ji: Methodology, Validation, Investigation. Zhifeng Deng: Conceptualization, Validation, Data curation, Writing - review & editing, Visualization, Supervision, Project administration, Funding acquisition. Qixin Zhou: Conceptualization, Validation, Resources, Writing - review & editing, Supervision, Project administration, Funding acquisition. Acknowledgments The authors acknowledge the support from the National Natural Science Foundation of China, under Grant 21805151, Natural Science Foundation of Shandong Province, China, under Grant ZR2018MB024, and the faculty start-up funding from The University of Akron, United States. The authors appreciated the technical support with SEM from Dr. Lingyan Li at the National Center for Education and Research on Corrosion and Materials Performance at The University of Akron. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.dyepig.2019.108171. References [1] Lampert CM. Chromogenic smart materials. Mater Today 2004;7:28–35. https:// doi.org/10.1016/S1369-7021(04)00123-3. [2] Granqvist CG, Lansåker PC, Mlyuka NR, Niklasson GA, Avenda~ no E. Progress in chromogenics: new results for electrochromic and thermochromic materials and devices. Sol Energy Mater Sol Cells 2009;93:2032–9. https://doi.org/10.1016/j. solmat.2009.02.026.
6
H. Zhang et al.
[23] [24]
[25] [26]
[27] [28] [29] [30] [31]
[32]
Dyes and Pigments 175 (2020) 108171
anisotropic dielectric properties. Compos Part A: Appl Sci Manuf 2019;120:84–94. doi.org/10.1016/j. compositesa.2019.02.024. Xie Q, Cheng Y, Chen S, Wu G, Wang Z, Jia Z. Dielectric and thermal properties of expoxy resins with TiO2 nanoweires. J Mater Sci Mater Electron 2017;28(23): 17871–80. https://doi.org/10.1007/s10854-017-7728-2. Wu G, Cheng Y, Wang K, Wang Y, Feng A. Fabrication and characterization of OMMt/BMI/CE composites with low dielectric properties and high thermal stability for electronic packaging. J Mater Sci Mater Electron 2016;27:5592–9. https://doi.org/10.1007/s10854-016-4464-y. 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. https://doi.org/10.1016/j.dyepig.2017.12.058. Deng Z, Hao X, Zhang P, Li L, Yuan X, Ai T, Bao W, Kou K. Donor-acceptor-donor molecules based on diketopyrrolopyrrole benzodipyrrolidione and naphthodipyrrolidione: organic crystal field-effect transistors. Dyes Pigments 2019; 162:883–7. https://doi.org/10.1016/j.dyepig.2018.11.028. Chen X, Guo K, Li F, Qiao H, Liu T, Ye Y, Li J. Synthesis and properties of benzodipyrrolidone (BDP)-based donor-acceptor copolymers. J Macromol Sci Part A 2015;52:517–22. https://doi.org/10.1080/10601325.2015.1039322. Zhang H, Ghasimi S, Tieke B, Schade A, Spanger S. Aminobenzodione-based polymers with low band gap and solvatochromic behavior. Polym Chem 2014;5: 3817–23. https://doi.org/10.1039/C3PY01702H. Deng P, Liu L, Ren S, Li H, Zhang Q. N-acylation: an effective method for reducing the LUMO energy levels of conjugated polymers containing five-memered lactam units. Chem Commun 2012;48:6960–2. https://doi.org/10.1039/C2CC32184J. Cui W, Yuen J, Wudl F. Benzodipyrrolidones and their polymers. Macromolecules 2011;44:7869–73. https://doi.org/10.1021/ma2017293. Rumer JW, Levick M, Dai S-Y, Rossbauer S, Huang Z, Binik L, Anthopoulos TD, Durrant JR, Procter DJ, McCulloch I. BPTs: thiphene-flanked benzodipyrrolidone conjugated polymers for ambipolar organic transistors. Chem Commun (J Chem Soc Sect D) 2013;49:4465–7. https://doi.org/10.1039/C3CC40811F. Deng Z, Ai T, Li R, Yuan W, Zhang K, Du H, Zhang H. Conjugated polymers containing building blocks 1,3,45-tetraarypyrrolo[3,2-b]pyrrole-2,5-dione
[33] [34] [35] [36]
[37]
[38]
[39] [40]
[41]
7
(isoDPP), benzodipyrrolidone (BDP) or naphthodiyrrolidone (NDP): a review. Polymers 2019;11:1683. https://doi.org/10.3390/polym11101683. Greenhalgh CW, Carey JL, Newton DF. The synthesis of quinodimethanes in the benzodifuranone and benzodipyrrolidone series. Dyes Pigments 1980;1:103. https://doi.org/10.1016/0143-7208(80)80010-6. Hallas G, Yoon C. The synthesis and properties of red and blue benzodifuranones. Dyes Pigments 2001;48:107–19. https://doi.org/10.1016/S0143-7208(00)000929. Zhang H, Deng R, Wang J, Li X, Chen Y, Liu K, Taubert C, Chen SZD, Zhu Y. Crystalline organic pigment-based field-effect transistors. ACS Appl Mater Interfaces 2017;9:21891–9. https://doi.org/10.1021/acsami.7b03170. Hu Y, Motzer H, Etxeberria A, Fernandez-Berridi M, Iruin J, Painter PC, Coleman MM. Conecering the self-association of N-vinyl pyrrolidone and its effect on the determination of equilibrium constants and the thermodynamics of mixing. Macromol Chem Phys 2000;201:705–14. https://doi.org/10.1002/(SICI)15213935(20000301)201:63.0.CO. 2-9. Zhang H, Liu K, Wu KY, Chen YM, Deng R, Li X, Jin H, Li S, Chung SSC, Wang CL, Zhu Y. Hydrogen-bonding-mediated solid-state self-assembled isoepindolidiones (isoEpi) crystal for organic field-effect transistors. J Phys Chem C 2018;122: 2888–5895. https://doi.org/10.1021/acsjpcc.7b11992. Coleman MM, Skrovanek DJ, Hu J, Painter PC. Hydrogen bonding in polymer blends. 1. FTIR studies of urethane-ether blends. Hydrogen bonding in polymer blends. 1. FTIR studies of urethane-ether blends. Macromolecules 1988;21:59–65. https://doi.org/10.1021/ma00179a014. Garcia D, Starkweather HW. Hydrogen bonding in nylon 66 and model compounds. Hydrogen bonding in nylon 66 and model compounds. J Polym Sci Polym Phys Ed 1985;23:537–55. https://doi.org/10.1002/pol.1985.180230310. Macknight WJ, Yang M. Property-structure relationships in poly-urethanes: infrared studies. Property-structure relationship in poly-urethanes: infrared studies. J Polym Sci Part C: Polym Symp 1973;42:817–32. https://doi.org/ 10.1002/polc.5070420231. Kuo SW, Chuang FC. Studies of miscibility behavior and hydrogen bonding in blends of poly(vinylphenol) and poly(vinbylpyrrolidone). Macromolecules 2001; 34:5224–8. https://doi.org/10.1021/ma010517a.