Carbon Aerogel Based Waterborne Ultra-Black Coatings with High Light Absorption

coatings

Article Carbon Aerogel Based Waterborne Ultra-Black Coatings with High Light Absorption

Jie Xu 1, Yifan Shi 2 , Jiangling Li 3, Guangzhen Cui 3,* and Guangxin Gu 1,*

1 Academy for Engineering and Technology, Fudan University, Shanghai 200433, China; [email protected] 2 School of Chemistry and Materials Science, Liaoning Shihua University, Dandong Road 1, Fushun 113001, China; [email protected] 3 Key Laboratory of Science and Technology on Electromagnetic Environmental Effects and Electro-Optical Engineering, The Army Engineering University of PLA, Nanjing 210007, China; [email protected] * Correspondence: [email protected] (G.C.); [email protected] (G.G.)

Abstract: In this paper, we wish to report the preparation of ultra-black ﬁlms via spraying coatings composed of waterborne binders and low-cost carbon aerogels on pre-treated tinplate. The CAs were

prepared by annealing resorcinol-formaldehyde resin (RF resin) and the following CO2 activation, of which the reflectance was less than 0.4% in a wide wavelength range. The reflectance of different coatings, which using CAs as functional pigments, ranged from 1.8% to 4.3% in the visible light region (400−760 nm), while it ranged from 1.9% to 4.2% in the near-infrared region (760–1100 nm). Further studies revealed the relationship between the pigment-to-binder ratio and reflectance and found the best ratio to be 0.96, and the minimum reflection was less than 1.8%. Outstanding adhesion to the tinplate substrate was also achieved using a two-component polyurethane binder after the thermal cycling test carried out from −100 ◦C to 100 ◦C. The fabrication process of ultra-black coatings is particularly convenient to remove the constraints of high costs and complex processes, making it instructive guidance for industrial production. Citation: Xu, J.; Shi, Y.; Li, J.; Cui, G.; Gu, G. Carbon Aerogel Based Keywords: carbon aerogels; high light absorption; ultra-black coating; waterborne binder; outstand- Waterborne Ultra-Black Coatings ing adhesion with High Light Absorption. Coatings 2021, 11, 563. https://doi.org/ 10.3390/coatings11050563 1. Introduction Academic Editor: Ivan Jerman Since the ‘black body’ theory was first introduced into thermodynamics by Kirchhoff and the law of black body radiation was proposed by Planck [1,2], the interest in the Received: 17 April 2021 research of ideal electromagnetic absorbers [3] to simulate a black body gradually arose [4]. Accepted: 6 May 2021 Great efforts have been made toward the design, synthesis, and optimization of such Published: 11 May 2021 materials because of the outstanding optical and thermal properties introduced by their high light absorptivity [5,6]. Therefore, various materials based on metal, carbon, silicon, Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in etc., were reported to possess excellent light absorbing property [7–12] and applied to published maps and institutional affil- different fields, such as photothermal conversion [13,14], optical instrumentation [15,16], iations. aerospace applications, as well as satellites [17–19]. As far as we know, the properties of high light absorbing of ultra-black materials originate from the periodic structures endowed by nature or by artificial designing [7,20–23], such as needle-like ‘forest’ or inverse V-type structures. Steglich et al. managed to fabricate an ultra-black silicon absorber with needle- like forest morphology to process an absorptance of more than 99.5% [11]. Through bionics, Copyright: © 2021 by the authors. Zhao et al. pyrolyzed the scale wings of Ornithoptera goliath butterfly at 650 ◦C and Licensee MDPI, Basel, Switzerland. This article is an open access article finally obtained the ultrathin inverse V-type structure anti-reflection materials entirely distributed under the terms and consisting of carbon and having a reflectance of <1% in the visible light region [24]. In recent conditions of the Creative Commons years, carbon materials, for instance, carbon nanotubes (CNTs) [25–27], carbon aerogels Attribution (CC BY) license (https:// (CAs) [28,29], carbon foam [30], carbon fiber composites [31], and carbon spheres (CSs) [32], creativecommons.org/licenses/by/ as research hotspots, were reported to possess not only good capacitive performance [33,34] 4.0/). but also great light absorptivity in both visible and infrared regions, which was attributed

Coatings 2021, 11, 563. https://doi.org/10.3390/coatings11050563 https://www.mdpi.com/journal/coatings Coatings 2021, 11, 563 2 of 12

to the peculiar porous structures, large conjugated system and moderate free-electron density of carbon [35]. Moreover, low density and a wide source of raw materials are excellent advantages of these materials in industrial applications compared with other element-based materials [36]. Therefore, multiple carbon materials have been applied to produce ultra-black coatings with extremely low reflectance. Magdassi et al. reported carbon nanotube black coating using multi-walled carbon nanotubes (MWCNTs) mixed with a silicon-based binder to obtain a reflectance of <4% in the visible light region and <5% in the near-infrared region [26]. Theoretically, a subwavelength structure was demonstrated by Sun et al., in which light absorption resulted from the reduced electron mean free path in subwavelength micropores [37]. This conclusion helped us in designing coatings with subwavelength structures in order to reduce diffuse light reflection. Additionally, by adjusting the ratio of the reactants and catalyst in precursor solutions, various preparation processes have been developed and improved for fabricating artificial CAs with three dimensional porous subwavelength structures since first synthesized by Pekala [38–41]. Therefore, we tried to apply the subwavelength structured CAs as functional fillers to prepare coatings with high light absorption. In this study, an appropriate ratio of resorcinol, formaldehyde, and sodium carbonate in the precursor was determined to fabricate hydrosol. The resorcinol-formaldehyde solution was dissolved in deionized water (DIW) with sodium carbonate as base catalyst. Then, a hydrothermal process was performed to obtain the hydrogel. The final CAs with hierarchical porous nanostructures were derived from the pyrolysis of such hydrogel and a subsequent CO2 activation process. Coatings of different binder types composed of these carbon aerogels show a reflection of <2.1% in the visible region and <2.2% in the NIR region at the best experimental conditions. The mechanical properties of adhesion and thermal cycling tests prove that the coating is reliable enough for applications in extreme situations. Furthermore, the convenience of preparation and commercial availability of raw materials enable the facile realization of industrial production of such ultra-black coatings.

2. Experimental 2.1. Materials Sodium carbonate, formaldehyde (37% in water), resorcinol, and all other reagents were purchased from Sinopharm Chemical Reagent Co., Ltd. (SCR, Shanghai, China) and used as received without further purification. Anionic polyacrylate dispersion along with aliphatic isocyanate (2K-WPU), anionic polycarbonate modified polyurethane dispersion (DLC-F), and aliphatic anionic acid modified polyurethane dispersion (PUD) were purchased from Covestro Polymers (Shanghai, China) Co., Ltd. The two-component ceramic coating Sunactive CD200 (2K-CERA.) was purchased from Shanghai Kinlita Chemical Co., Ltd. (Shanghai, China) Other additives, wetting agents, flatting agents including dispersing agents were purchased from BYK-Chemie GmbH (Tongling, Germany). Both resin and additives were used as received.

2.2. Synthesis of CAs Through the Sol-Gel Process The carbon aerogels were obtained via the carbonization of resorcinol-formaldehyde (RF) gels, which derived from the addition reaction and polycondensation reaction under alkaline condition provided by sodium carbonate. The molar ratio of formaldehyde to resorcinol (F/R) was adjusted to 2, and this mixture was dissolved in a certain amount of deionized water. Subsequently, as a base catalyst, alkaline sodium carbonate (C) was added, with the molar ratio of formaldehyde to the catalyst (F/C) being ﬁxed at 600. This aqueous solution was denoted as the precursor solution, the total weight of resorcinol and formaldehyde (W%) was set at around 5%. Typically, 3 g of resorcinol was added in a round bottom beaker containing 135 mL deionized water (DIW). Then, 4.2 g of formaldehyde was added dropwise under stirring. The resulting solution was stirred for 15 min and subsequently 9.6 mg of sodium carbonate was added. After stirring for another 20 min, the precursor solution was transferred into a polytetraﬂuoroethylene (PTFE) mold and Coatings 2021, 11, 563 3 of 12

sealed in a steel hydrothermal reactor. The reactor was cured in a constant temperature humidity chamber at 85 ◦C for 72 h. The resulting gel was soaked in acetone for 24 h for a few times to completely remove residual solvents and water. After the solvent exchange process, the wet gel was subsequently transferred into a fridge at –16 ◦C, stored for 3 h to keep the structure robust, and then dried in a freeze-dryer for 24 h. The dried sol was carbonized at 900 ◦C under a nitrogen atmosphere for 2 h. The resulting black cylindrical materials were the target product of CAs and named CA-S.

2.3. Activation of CAs with CO2 CA-S were annealed in a tubular furnace at 1000 ◦C for 2 h under the mixture gas −1 −1 environment (N2 flow of 75 mL·min and CO2 flow of 75 mL·min ). The heating rate was set at 10 ◦C per minute from room temperature to the target temperature, and the natural cooling process was followed. Both the heating and cooling processes were performed under a nitrogen atmosphere at a flow rate of 250 mL·min−1. The obtained CAs activated by CO2 were denoted as ACA.

2.4. Preparation of Ultra-Black Coating In the primary formulation, the CA aqueous dispersion was mixed with various ratios of different binders to fabricate different polymer-based ultra-black coatings. For a typical experiment, a certain amount of dispersing agent (10 g) was added to 70 mL deionized water, which was stirred using a laboratory mill for 10 min in a plastic beaker at ambient temperature. With a stirring speed of 400 rpm, 20 g ACA was added gradually to avoid splashing due to the low pile density. On completely adding ACA, the stirring speed was increased to 2000 rpm and maintained for 1 h to acquire the ACA aqueous dispersion. A certain amount of resin (2K-WPU/PUD/2K-CERA./DLC-F) was added in another plastic beaker, followed by the dropwise addition of additives with a stirring speed of 700 rpm. Then, the ACA aqueous dispersion was added, and the resulting solution was dispersed using a laboratory mill at a speed of 700 rpm for 60 min to obtain the ultra-black coatings.

2.5. Characterization The microscopic structures of the carbon aerogels were investigated using scanning electron microscopy SEM (JSM-7800F, Japan Electronics Corporation, Tokyo, Japan). The crystal structure of the sample was examined on an X-ray diffraction instrument (XRD, Bruker D8 Advance, Leipzig, Germany), with Cu target (0.154 nm) as the radiation source, tube voltage of 40 kV, tube current of 40 mA, and scanning speed of 5 ◦min−1. Absorbance was identified indirectly by measuring the reflectance of the film at wavelengths ranging from 1100 nm to 400 nm using a U-4100 Spectrophotometer (HITACHI). Thermogravimetric analysis (TGA) was performed on TA Q500 (TA Instruments, New Castle, CA, USA) under a nitrogen atmosphere. Raman spectra (ThermoFisher Scientific DXR2xi, Waltham, MA, USA) were recorded from 600 cm−1 to 2500 cm−1 under 532 nm excitation. The pore size distribution and specific surface area were measured via the N2 adsorption method at 0 ◦C in a magnetic suspension balance using the Brunauer-Emmett-Teller (BET) nitrogen adsorption/desorption technique (ASAP 2020 V4,0 from Micromeritics, Norcross, GE, USA). Film thickness was measured using a SURFIX B thickness gauge from PHYNIX GmbH & Co. KG, Neuss, Germany. The adhesion properties were measured through an ISO2409/ASTM D3359 [42,43] cross-cut test and evaluated using the above standards.

3. Results The synthesis of the RF aerogels is schematically illustrated in Figure1. Commercially available raw materials of resorcinol and formaldehyde can easily co-dissolve in deionized water and form a semi-opaque hydrosol using sodium carbonate as the base catalyst under alkaline conditions. The obtained precursor solution was sealed in a PTFE mold and subjected to a low-temperature hydrothermal process at 85 ◦C for 72 h. The resulting hydrogel was soaked in acetone for 24 h, the process was replicated 3 times to remove Coatings 2021, 11, x FOR PEER REVIEW 4 of 12

from PHYNIX GmbH & Co. KG, Neuss, Germany. The adhesion properties were measured through an ISO2409/ASTM D3359[42,43] cross-cut test and evaluated using the above standards.

3. Results The synthesis of the RF aerogels is schematically illustrated in Figure 1. Commer- cially available raw materials of resorcinol and formaldehyde can easily co-dissolve in deionized water and form a semi-opaque hydrosol using sodium carbonate as the base catalyst under alkaline conditions. The obtained precursor solution was sealed in a PTFE Coatings 2021, 11, 563 mold and subjected to a low-temperature hydrothermal process at 85 °C for 724 of h. 12 The resulting hydrogel was soaked in acetone for 24 h, the process was replicated 3 times to remove the residual water and solvent. After the freeze-drying process, the cylindrical productthe residual was water pyrolyzed and solvent. at 900 °C After in a thetubular freeze-drying furnace for process, 2 h. In the this cylindrical process, to product protect the wasmorphological pyrolyzed structure at 900 ◦C of in CAs a tubular from collapsing, furnace for a 2heating h. In thisrate process,of 10 °C/min to protect from ambient the ◦ temperaturemorphological to structure 400 °C and of CAs 5 °C from/min collapsing, from 400 a°C heating to 900 rate°C was of 10 engagedC/min fromunder ambient a nitrogen ◦ ◦ ◦ ◦ environment.temperature to During 400 C andthe 5activationC/min from procedure, 400 C to we 900 controlledC was engaged the flow under rate a of nitrogen carbon di- oxideenvironment. and nitrogen During to themaintain activation an appropriate procedure, etching we controlled efficiency the in ﬂow order rate to of stabilize carbon the originaldioxide andthree nitrogen-dimensional to maintain structure. an appropriate etching efﬁciency in order to stabilize the original three-dimensional structure.

Figure 1.1. FabricationFabrication and and morphology morphology of carbonof carbon aerogels. aerogels. (a) Schematic(a) Schematic illustration illustration of the of fabrication the fabrica- tionprocess. process. Resorcinol Resorcinol and formaldehydeand formaldehyde hydrosol hydrosol were were converted converted into hydrogel into hydrogel by hydrothermal by hydrother- malprocess, process, freeze-dried freeze-dried for 24 for h, and24 h, annealed and annealed at 900 at◦C 900 to form°C to the form porous the porous carbon carbon aerogels, aerogels, and the and theactivating activating process process built built more more nanostructure nanostructure pores pores resulting resulting in a lower in a lower reﬂectance. reflectance. SEM images SEM im ofages of(b )( CA-Sb) CA and-S and (c) ACA.(c) ACA. The The scale scale bar isbar 400 is nm. 400 nm.

Subsequently, thethe process process to to prepare prepare CAs CAs with with subwavelength subwavelength structures structures was was per- per- formed, asas illustrated illustrated in in Figure Figure1. According 1. According to its to morphology, its morphology, more pores more and pores microstruc- and micro- structurestures exist exist in ACA in ACA than than in CA-S in CA on-S comparing on comparing the SEM the SEM images images (Figure (Figure1b,c). 1b, Forc). the For the purpose ofof analyzinganalyzing the the type type of of carbon carbon in in these these two two samples samples (CA-S (CA and-S and ACA), ACA), Raman Raman spectra werewere recordedrecorded and and shown shown in in Figure Figure2a. 2a. The The spectrum spectrum of both of both samples samples shows shows two two −1 −1 characteristic peaks peaks located located at at approximately approximately 1355 1355 cm cmand−1 and 1585 1585 cm cm, which−1, which are ascribedare ascribed to the D band and G band, respectively. The D band results from disordered carbon and to the D band and G band, respectively. The D band results from disordered carbon and other defects, whereas the G band is assigned to the zone center E2g mode, which is the other defects, whereas the G band is assigned to the zone center E2g mode, which is the consequence of the ordered sp2 bonded carbon [44]. Moreover, X-ray diffraction analysis was performed, and the results are shown in Figure2b. The XRD pattern of both CA-S and ACA exhibits two diffraction peaks at 2θ of around 24◦ and 44◦, corresponding to the (0 0 2) and (1 0 1) planes, respectively. These data imply that carbon aerogels are composed of both graphite carbon and amorphous carbon [45,46]. Nevertheless, the activation step shows a limited effect on the major structure and the type of carbon due to the similar curves. To further study the difference in physical properties after activation, more measurements were performed. Coatings 2021, 11, x FOR PEER REVIEW 5 of 12

consequence of the ordered sp2 bonded carbon [44]. Moreover, X-ray diffraction analysis was performed, and the results are shown in Figure 2b. The XRD pattern of both CA-S and ACA exhibits two diffraction peaks at 2θ of around 24° and 44°, corresponding to the (0 0 2) and (1 0 1) planes, respectively. These data imply that carbon aerogels are composed of both graphite carbon and amorphous carbon [45,46]. Nevertheless, the activation step shows a limited effect on the major structure and the type of carbon due to the similar curves. To further study the difference in physical properties after activation, more measurements were performed. Coatings 2021, 11, 563 5 of 12

Figure 2. (a) Raman spectra of ACA and CA-S. (b) X-ray diffraction patterns of ACA and CA-S. (c) Isotherms of ACA and Figure 2. (a) RamanCA-S. (d )spectra BJH pore of distribution ACA and of ACACA- andS. (b CA-S.) X-ray diffraction patterns of ACA and CA-S. (c) Isotherms of ACA and CA-S. (d) BJH pore distribution of ACA and CA-S. The pore structures of CA-S and ACA were investigated via low-temperature N2 absorption-desorption measurements. Brunauer-Emmett-Teller specific surface areas as high 2 2 Theas 1544.7pore mstructures/g and 649.7 of m CA/g- wereS and measured ACA forwere ACA investigated and CA-S, respectively. via low Using-temperature the N2 ab- t-plot method, the micropore surface area and mesopore/macropore surface area of CA-S and sorptionACA-desorption were calculated measurements. to be 448.22 m2/g and Brunauer‒Emmett‒Teller 201.42 m2/g and 911.52 m2/g and specific 633.13 m 2 surface/g, areas as high asrespectively. 1544.7 m The2/g and activated 649.7 process m2/g produced were measured twice the micropores for ACA and and thrice CA the mesopores,-S, respectively. Using the t-plotwhich method, was in good the accordance micropore with thesurface isotherms area shown and in Figuremesopore/macropore2c. The isotherm curve ofsurface area of CA-S andACA ACA can be classifiedwere calculated as type IV, while to be that 448.22 of CA-S canm2/g be classifiedand 201.42 as type m II,2/g according and 911.52 to m2/g and the IUPAC classification of adsorption isotherms [47]. The black line in Figure2c for CA-S 2 633.13 rapidlym /g, respectively. elevates at a low The relative activated pressure process (P/P0), indicating produced the formationtwice the of micropores abundant and thrice the mesopores,micropores. which The absorption was in good continues accordance to increase with without the a isotherms plateau near shown P/P0 of in 1.0, Figure 2c. The isothermsuggesting curve theof ACA presence can of mesoporesbe classified and relative as type macropores. IV, while In that contrast, of CA the hysteresis-S can be classified as loop of the red line for ACA in Figure2c is associated with capillary condensation taking type II,place according in its mesopores, to the IUPAC and the initialclassification part of the of isotherm adsorption is attributed isotherms to monolayer- [47]. The black line in Figuremultilayer 2c for adsorptionCA-S rapidly [48]. Furthermore, elevates at the a BJHlow absorption relative pore-sizepressure distribution (P/P0), indicating was the for- mationdetermined, of abundant which micropores. showed the expected The absorption results (Figure continues2d). The corresponding to increase pore-without a plateau size distribution of both the samples before and after activation samples shows a high- near P/Pintensity0 of 1.0, peak suggesting at the abscissa the of 1–5 prese nm,nce especially of mesopores 1–2 nm, and theand slope relative of ACA macropores. is more In contrast, theprecipitous hysteresis than loop that of of CA-S the during red line the microporefor ACA range, in Figure indicating 2c is that associated the samples, with capillary condensationparticularly taking ACA, had place abundant in its micropores mesopores, with diameters and the of 1–2 initial nm on thepart nanostructure of the isotherm is at- framework. Notably, the average pore size decreased significantly in the range of 10 to tributed to monolayer-multilayer adsorption [48]. Furthermore, the BJH absorption pore- size distribution was determined, which showed the expected results (Figure 2d). The corresponding pore-size distribution of both the samples before and after activation samples

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shows a high-intensity peak at the abscissa of 1–5 nm, especially 1–2 nm, and the slope of ACA is more precipitous than that of CA-S during the micropore range, indicating that the samples, particularly ACA, had abundant micropores with diameters of 1–2 nm on the nanostructure framework. Notably, the average pore size decreased significantly in the range of 10 to 100 nm, demonstrating the transformation from micropores to mesopores, which is in accordance with the isotherm curve results and changes in BET specific Coatings 2021, 11, 563 surface area. 6 of 12 The absorption rate was measured indirectly by testing the reflectance of the materi-

100als, nm, and demonstrating the reflectance the transformation of CA-S fromand micropores ACA is toshown mesopores, in Figure which is 3. in The average reflectance of accordanceCA-S ranged with the from isotherm 0.78% curve to results 0.60%, and changes while in that BET specific of ACA surface was area. less than 0.40%, indicating that The absorption rate was measured indirectly by testing the reflectance of the materials, andthe theactivation reflectance process of CA-S and has ACA a positive is shown in effect Figure3 in. The decreasing average reflectance the reflectance of due to the presence CA-Sof more ranged micropores from 0.78% to 0.60%,and mesopores. while that of ACA The was line less thanchart 0.40%, shown indicating almost a relatively flat slope thatranging the activation from process1100 nm has ato positive 400 nm, effect which in decreasing indicates the reflectance uniform due toabsorption the in both visible and presence of more micropores and mesopores. The line chart shown almost a relatively flatNIR slope regions. ranging fromThis 1100 feature nm to 400helps nm, whichin forming indicates uniforma uniform absorption film, in providing both steady light absorb- visibleance andin desired NIR regions. applications. This feature helps in forming a uniform film, providing steady light absorbance in desired applications.

Figure 3. Reflection of CA-S and ACA. Figure 3. Reflection of CA-S and ACA. Herein, we used ACA as the light-absorbing materials of the ultra-black coatings. In our primary formulation of coatings, four different types of binders were mixed well along withHerein, ACA aqueouswe used dispersions, ACA as and the the blacklight coatings-absorbing were formed materials by airbrush of the ultra-black coatings. In sprayingour primary such mixture formulation onto pretreated of Aluminum coatings, plate. four For a typical different preparation, types 12 g of of binders were mixed well ACA aqueous dispersion was added into one-component, two-component, polycarbonate- modifiedalong with waterborne ACA polyurethane aqueous and dispersions, silicon-based binder, and andthe the black pigment-to-binder coatings were formed by airbrush (PB,spraying the weight such of pigment mixture to the onto weight pretreated of resin in the Aluminum dried film) ratio plate was set. For as 0.58. a typical preparation, 12 g of The reflectance of the four types of coatings acquired by spraying was measured at the visible-NIRACA aqueous light region dispersion (400–1100 nm) and was presented added in terms into of wavelength-to-reflectance. one-component, two-component, polycar- Asbonate shown- inmodified Figure4a, the waterborne primary formulations polyurethane of coatings exhibitand silicon some differences-based in binder, and the pigment-to- the range of 400 nm to 1100 nm. Moreover, the reflectance of the 2K-ceramic coating is aboutbinder 1.8%, (PB while, the that weight of DLC-F of is 4.3%, pigment which represented to the weight the best andof resin the worst in result,the dried film) ratio was set as respectively.0.58. The Furthermore,reflectance the of average the reflectancefour types of 2K-WPU of coatings and PUD acquired is 2.3% and 3.6%,by spraying was measured at respectively. We assumed that the overall reflected light of the coating is composed of interfacialthe visible reflection-NIR due light to the region refractive (400 index– mismatch1100 nm) and backwardand presented scattering inside in terms of wavelength-to-re- theflectance. coating film. As To shown figure out in whether Figure the 4a, interfacial the primary reflection plays formulations an important roleof coatings exhibit some dif- in the overall reflectance of different coatings, a pure resin film without any additive orferences pigment wasin the brushed range on a of PET 400 membrane nm to with 1100 a coating nm. rodMoreover, (from RD Specialties, the reflectance of the 2K-ceramic 100coatingµm), baked, is about and subjected 1.8%, to while optical tests.that Fortuitously, of DLC- theF is results 4.3%, reveal which almost represented the the best and the sameworst reflectance result, curve respectively. (Figure4b), and Furthermore, the deviation between the theaverage curves was reflectance less than of 2K-WPU and PUD is 4%; therefore, we can assume that the pure resins are basically similar. Interestingly, the DLC-F2.3% showsand a3.6%, lower reflectivity respectively. in the form We of pure assumed resin but the that highest the value overall was obtained reflected light of the coating is oncomposed applying the ofultra-black interfacial coating, suggesting reflection that due the reflectance to the of refractive the clear coating index itself mismatch and backward has an extremely minor effect on the overall reflectance in the ultra-black coating. Moreover, scattering inside the coating film. To figure out whether the interfacial reflection plays an important role in the overall reflectance of different coatings, a pure resin film without any additive or pigment was brushed on a PET membrane with a coating rod (from RD Specialties, 100µm), baked, and subjected to optical tests. Fortuitously, the results reveal almost the same reflectance curve (Figure 4b), and the deviation between the curves was less than 4%; therefore, we can assume that the pure resins are basically similar. Interest- ingly, the DLC-F shows a lower reflectivity in the form of pure resin but the highest value

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was obtained on applying the ultra-black coating, suggesting that the reflectance of the Coatings 2021, 11, x FOR PEERclear REVIEW coating itself has an extremely minor effect on the overall reflectance7 ofin 12 the ultra-

black coating. Moreover, SEM was performed to investigate the surface morphology of these four types of coatings in order to find out the reason for the difference in optical Coatings 2021, 11, 563 propertieswas obtained(Figure on5). applying Interestingly, the ultra -theblack surface coating, morphologysuggesting that of the both reflectance 2K7- ofceramic 12of the coating and 2Kclear-WPU coating seems itself more has ancomplicated extremely minor than effect that onof the overallother tworeflectance samples in the (PUD ultra -and DLC- black coating. Moreover, SEM was performed to investigate the surface morphology of F) that thesepresented four types lower of coatingsreflectance. in order The to differencesfind out the reasonmay be for due the differenceto the morphology in optical change SEM was performed to investigate the surface morphology of these four types of coatings in resultingproperties from the(Figure pol 5).ymerization Interestingly, ofthe the surface two morphology-component of both system, 2K-ceramic which coating is further en- order to find out the reason for the difference in optical properties (Figure5). Interestingly, and 2K-WPU seems more complicated than that of the other two samples (PUD and DLC- hancedthe with surface the morphology presence of of both pigments. 2K-ceramic Accordingly, coating and 2K-WPU we seemscan moreinfer complicated that a more complex F) that presented lower reflectance. The differences may be due to the morphology change surfacethan morphology that of the other rather two than samples a flat (PUD surface and DLC-F) helps that in presenteddecreasing lower the reflectance. interfacial reflection resulting from the polymerization of the two-component system, which is further en- of the driedThe differences film. may be due to the morphology change resulting from the polymerization hancedof the two-component with the presence system, of pigments. which is further Accordingly, enhanced we with can the infer presence that a of more pigments. complex surfaceAccordingly, morphology we can rather infer that than a a more flat surface complex helps surface in decreasing morphology the rather interfacial than reflection a flat ofsurface the dried helps film. in decreasing the interfacial reflection of the dried film.

FigureFigure 4. (a) ReﬂectanceReflectance ofof primary primary coating coating formulations. formulations. (b )(b Reﬂectance) Reflectance of pureof pure resin. resin. Figure 4. (a) Reflectance of primary coating formulations. (b) Reflectance of pure resin.

Figure 5.5. SEMSEM ofof PUD,PUD, DLC-F, DLC-F, 2K-WPU, 2K-WPU, and and 2K-CERA. 2K-CERA. ﬁlm. film. The The scale scale bar bar represents represents 10 µ m.10 µm.

Considering thethe practical practical applications applications of of the the material material in aerospacein aerospace optical optical auxiliary auxiliary equipment, maintaining maintaining a steadya steady weight weight is particularly is particularly important important because because weight isweight directly is di- proportional to the orbital coefficients, so any change in weight under the weightlessness rectly proportional to the orbital coefficients, so any change in weight under the weight- condition will result in changes in the orbit. Thus, TGA of the dry film was performed to lessness condition will result in changes in the orbit. Thus, TGA of the dry film was per- Figure 5.determine SEM of thePUD, binder DLC with-F, the2K best-WPU, thermal and stability.2K-CERA. To simulate film. The the scale application bar represents situation, 10 µm. formedthe coating to determine film was heated the binder to and with maintained the best atthermal 125 ◦C stability. and 150 ◦ CTo for simulate 30 min the each, application and a situation,heating rate the of coating 10 ◦C per film minute was fromheated 50 to◦C and to 150 maintained◦C was used at during125 °C theand heating 150 °C process. for 30 min ConsideringAs shown in Figurethe practical6, the 2K-WPU applications binder hardly of lostthe anymaterial weight, in with aerospace the weight lossoptical auxiliary equipment, maintaining a steady weight is particularly important because weight is di-

rectly proportional to the orbital coefficients, so any change in weight under the weightlessness condition will result in changes in the orbit. Thus, TGA of the dry film was performed to determine the binder with the best thermal stability. To simulate the application situation, the coating film was heated to and maintained at 125 °C and 150 °C for 30 min

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Coatings 2021, each,11, x FOR and PEER a heating REVIEW rate of 10 °C per minute from 50 °C to 150 °C was used during the 8 of 12 heating process. As shown in Figure 6, the 2K-WPU binder hardly lost any weight, with the weight loss ratio being less than 0.5%, and was hence inferred to be the best. In contrast, the 2K-ceramic coating showed a 1% weight loss ratio, while both PUD and DLC-F have ratios significantlyeach, andlarger a heatingthan 1%. rate The of slight 10 °C increase per minute in weight from during50 °C to the 150 heating °C was- used during the Coatingsup2021 period, 11, 563 is due toheating the adsorption process. As of shownN2 at the in Figuresurface 6, of the the 2K porous-WPU binderCAs. Clearly, hardly8 ofthelost 12 any weight, with thermal stability ofthe two weight two- componentloss ratio being coatings less thanbehaved 0.5%, better and wasthan hencethe single inferred compo- to be the best. In con- nent ones, indicatingtrast, the the better 2K- intermolecularceramic coating combination showed a 1% of weightthe two -losscomponent ratio, while system, both PUD and DLC-F which can lead to betterhaveratio beingratios physical less significantly than and 0.5%, chemical and waslarger hence properties. than inferred 1%. to The be the slight best. In increase contrast, thein 2K-ceramicweight during the heating- coating showed a 1% weight loss ratio, while both PUD and DLC-F have ratios signiﬁcantly up period is due to the adsorption of N2 at the surface of the porous CAs. Clearly, the larger than 1%. The slight increase in weight during the heating-up period is due to the thermal stability of two two-component coatings behaved better than the single compo- adsorption of N2 at the surface of the porous CAs. Clearly, the thermal stability of two nenttwo-component ones, indicating coatings the behaved better better intermolecular than the single component combination ones, of indicating the two the-component system, whichbetter intermolecular can lead to combinationbetter physical of the two-componentand chemical system, properties. which can lead to better physical and chemical properties.

Figure 6. TGA analysis of PUD/DLC-F/2K-WPU/2K-CERA. film.

The adhesion property of the coatings to the substrates was evaluated using the ISO

2409/ASTM D3359 tapeFigure test 6. TGA with analysis cross of PUD/DLC-F/2K-WPU/2K-CERA.-cuts. The adhesion property film. is marked by the fraction of detachmentFigure of the 6. coating. TGA analysis Typically, of PUD/DLC the film-F/2K thickness-WPU/2K should-CERA. be film. less than 60 μm, and the cross-cutting spacingThe adhesion was propertyset to 1 of mm, the coatings using toadhesive the substrates tape was to evaluatedstick the usingcutting the ISO 2409/ASTMThe adhesion D3359 property tape test withof the cross-cuts. coatings The to adhesion the substrates property iswas marked evaluated by using the ISO parts several times theafter fraction cross of-cutting. detachment The of results the coating. were Typically, evaluated the filmvia thicknessnaked-eye should observa- be less tions and classified2409/ASTM thanaccording 60 µm, and toD3359 thea standard cross-cutting tape test scale with spacing (class cross was 0- set cuts.is tothe 1 The mm, best adhesion using with adhesive no propertydetachment, tape to stickis marked by the frac- class 5 is the worstti withtheon cuttingof more detachment parts than several 65% of times detachmentthe aftercoating. cross-cutting. forTypically, ISO The 2409, resultsthe class film were 5Bthickness evaluated is the best viashould naked- with be less than 60 μm, andeye observationsthe cross-cutting and classified spacing according was to set a standard to 1 mm, scale using (class 0adhesive is the best withtape no to stick the cutting no detachment, classdetachment, 0B is the class worst 5 is thewith worst more with than more 65% than 65%detach detachmentment for for ASTM ISO 2409, D3359). class 5B The typical adhesionpartsis theresults bestseveral with are times no presented detachment, after crossin class Figure- 0Bcutting. is 7. the worst The withresults more were than 65% evaluated detachment via for naked-eye observa- tionsASTM and D3359). classified The typical according adhesion results to a standard are presented scale in Figure (class7. 0 is the best with no detachment, class 5 is the worst with more than 65% detachment for ISO 2409, class 5B is the best with no detachment, class 0B is the worst with more than 65% detachment for ASTM D3359). The typical adhesion results are presented in Figure 7.

Figure 7. Typical adhesion images of (a) DLC-F, (b) 2K-CERA., (c) PUD, (d) 2W-WPU. Figure 7. Typical adhesion images of (a) DLC-F, (b) 2K-CERA., (c) PUD, (d) 2W-WPU. The 2K-WPU coating shows the best adhesion as ISO class 0/ASTM class 5B, while both the 2K-ceramic coating and PUD present ISO class 1/ASTM class 4B adhesion. In The 2K-WPU coating shows the best adhesion as ISO class 0/ASTM class 5B, while contrast, DLC-F shows the worst grade with ISO class 3/ASTM class 3B adhesion. We both the 2K-ceramicexpected coating that and the 2K-ceramic PUD present coating wouldISO class exhibit 1/ASTM good adhesion class due 4B to adhesion. its good optical In contrast, DLC-F showsFigureperformance, the 7. Typicalworst but thegrade adhesion result with seems images ISO contrary. classof (a The) 3/ASTMDLC low-F, adhesion (b class) 2K level- CERA.,3B wouldadhesion. (c cause) PUD, theWe (d loose )ex- 2W-WPU. pected that the 2K-ceramic coating would exhibit good adhesion due to its good optical performance, but the resultThe seems2K-WPU contrary. coating The shows low adhesionthe best adhesion level would as ISO cause class the 0/ASTM loose class 5B, while combination of theboth coating the and2K- cesubstrate,ramic coating which and resulted PUD in present a tiny interspaceISO class 1/ASTMbetween classthe 4B adhesion. In contrast, DLC-F shows the worst grade with ISO class 3/ASTM class 3B adhesion. We expected that the 2K-ceramic coating would exhibit good adhesion due to its good optical performance, but the result seems contrary. The low adhesion level would cause the loose

combination of the coating and substrate, which resulted in a tiny interspace between the

Coatings 2021, 11, x FOR PEER REVIEW 9 of 12

film and substrates. This interspace provided room for air bubbles, steam, or a trace amount of water, and the impurities will slowly increase and finally break through the coating film, forming micro-tunnels to let more impurities in, which will further enlarge Coatings 2021, 11, 563 the size of the tunnels. Considering the combined influence of poor adhesion9 of 12 in anti-corrosion, mechanical properties and weather resistance, we decided to use the 2K-WPU resin as the final binder for ultra-black coating. combinationThe samples of the of coatingdifferent and substrate,PB ratios which ranging resulted from in a 0.58 tiny interspaceto 1.15 were between brushed using a the film and substrates. This interspace provided room for air bubbles, steam, or a trace wire amountrod of of60 water, µm andonto the polished impurities alumin will slowlyum increase plate andand finally dried break at room through temperature the for 20 min beforecoating film,being forming placed micro-tunnels in an oven to let at more 50 °C impurities for another in, which 2 willh, to further finally enlarge obtain the the dried film with sizea thickness of the tunnels. of Consideringabout 34 µm. the combined As we influenceexpected, of poor the adhesion sample in of anti-corrosion, the lowest PB ratio (PB mechanical properties and weather resistance, we decided to use the 2K-WPU resin as the 0.58)final shows binder the for highest ultra-black refle coating.ctance of 2.5%, which decreases as the PB ratio increases (Fig- ure 8). However,The samples when of different the PBPB ratios ratio ranging reaches from 0.96, 0.58 to the 1.15 reflectance were brushed reaches using a wire the lowest value (red rodline of in 60 Figureµm onto 8.). polished On aluminumincreasing plate the and PB dried ratio at room(1.06 temperature and 1.15), for negative 20 min results were before being placed in an oven at 50 ◦C for another 2 h, to finally obtain the dried film with acquired,a thickness i.e., the of about reflectance 34 µm. As starts we expected, to increase. the sample Although of the lowest samples PB ratio with (PB a 0.58) high PB ratio (less than shows1) show the highestbetter reflectanceabsorption of 2.5%, properties, which decreases the adhesion as the PB ratioproperty increases decreases (Figure8). as the PB ratio increasesHowever, (Table when the1). PB The ratio samples reaches 0.96, with the reflectance a PB ratio reaches of the 0.96 lowest and value below (red line exhibit ISO class in Figure8.). On increasing the PB ratio (1.06 and 1.15), negative results were acquired, i.e., 0/ASTMthe reflectance class 5B starts adhesion, to increase. while Although those samples over with0.96 a highshow PB ISO ratio (lessclass than 1/ASTM 1) show class 4B adhesion.better In summary, absorption the properties, larger the the adhesion PB ratio, property the worse decreases would as the be PB the ratio adhesion. increases Therefore, in this study,(Table1). a The PB samples ratio of with 0.96 a PB was ratio selected of 0.96 and as below the exhibit optimal ISO classchoice 0/ASTM for both class 5B good reflectance and adhesion.adhesion, while those over 0.96 show ISO class 1/ASTM class 4B adhesion. In summary, the larger the PB ratio, the worse would be the adhesion. Therefore, in this study, a PB ratio of 0.96 was selected as the optimal choice for both good reflectance and adhesion.

FigureFigure 8. Reflectance 8. Reﬂectance of of different PB PB ratio ratio samples. samples.

Table 1. Adhesion classiﬁcation of different PB ratios according to ASTM and ISO standard methods. Table 1. Adhesion classification of different PB ratios according to ASTM and ISO standard meth- Coatings as Prepared After Thermal Cycling Test ods. PB Ratio ISO 2409 ASTM D3359 ISO 2409 ASTM D3359 1.15Coatings Class 1 as Prepared Class 4B Class 2After Thermal Class 3B Cycling Test PB Ratio 1.06 Class 1 Class 4B Class 1 Class 4B 0.96 Class 0 Class 5B Class 0 Class 5B 0.87ISO 2409 Class 0ASTM Class 5B D3359 Class 0ISO 2409 Class 5B ASTM D3359 0.77 Class 0 Class 5B Class 0 Class 5B 1.15 0.67Class Class 1 0 ClassClass 5B 4B Class 0Class 2 Class 5B Class 3B 0.58 Class 0 Class 5B Class 0 Class 5B 1.06 Class 1 Class 4B Class 1 Class 4B Ultra-black coating with high light absorption can be used not only for optical de- 0.96vices but also in aerospaceClass 0 applications suchClass as 5B satellites. For aerospaceClass 0 applications, Class 5B coatings will be exposed to vacuum and starkly bear the extreme environment, where 0.87the temperature mayClass be more0 than 150 ◦CClass for long 5B periods. WhenClass the satellites 0 travel Class 5B around the earth, the surface temperature will rise signiﬁcantly where the equipment is 0.77 Class 0 Class 5B Class 0 Class 5B

0.67 Class 0 Class 5B Class 0 Class 5B

0.58 Class 0 Class 5B Class 0 Class 5B

Coatings 2021, 11, x FOR PEER REVIEW 10 of 12

Ultra-black coating with high light absorption can be used not only for optical devices but also in aerospace applications such as satellites. For aerospace applications, coatings will be exposed to vacuum and starkly bear the extreme environment, where the temperature may be more than 150 °C for long periods. When the satellites travel around the earth, the surface temperature will rise significantly where the equipment is directly exposed to sunlight and will drop significantly when traveling through shadow areas [49]. Thus, the thermal cycling test was performed to simulate the space environment. The Eu- Coatingsropean2021, 11, 563 Space Agency standard (ESA ECSS-Q-70-04A, 2008) and National Military 10stand- of 12 ard of PRC (GJB 2052.8-2006) recommend a test where 100 heating-cooling (100 °C for heating and –100 °C for cooling) cycles are performed with a dwell time of 5 min. We directly exposed to sunlight and will drop significantly when traveling through shadow performed the test areasstrictly [49]. according Thus, the thermal to this cycling standard. test was performed The samples to simulate were the first space placed environment. in an oven at 100 °C for 10The min European and Spacethen Agencytransferred standard into (ESA a ECSS-Q-70-04A,freezing pump 2008) at and –100 National °C within Military 10 s for another 5 min.standard This cycling of PRC (GJB was 2052.8-2006) repeated recommend 100 times. a test After where the 100 thermal heating-cooling cycling (100 test,◦C for heating and –100 ◦C for cooling) cycles are performed with a dwell time of 5 min. We the surface states wereperformed examined the test strictly using according the naked to this eyes standard. for Theflaking samples and were using first placed an optical in an microscope for morphologicaloven at 100 ◦C fordamage. 10 min andAs thenwe transferredexpected, into there a freezing was hardly pump at any –100 ◦changeC within in the morphology compared10 s for another with 5 min. initial This cyclingsamples, was repeatedindicating 100 times. good After stability the thermal of coatings cycling test, in the surface states were examined using the naked eyes for flaking and using an optical extreme environments.microscope Light for morphological absorption damage. propertyAs we (%R) expected, and there adhesion was hardly grade any change were in the also tested. The reflectancemorphology remained compared the withsame, initial while samples, the indicating adhesion good slightly stability ofdecreased coatings in extreme in high PB ratios samples. environments.The samples Light having absorption PB ratios property above (%R) and0.96 adhesion show the grade worse were also adhesion tested. The test reflectance remained the same, while the adhesion slightly decreased in high PB ratios results, while the otherssamples. exhibit The samples the havingclass PB0/ASTM ratios above class 0.96 5B show adhesion the worse (Table adhesion 1, testFigure results, 9). The good adhesionswhile in extreme the others exhibitcold and the class hot 0/ASTM conditi classon may 5B adhesion be due (Table to the1, Figure moderate9). The goodintra- molecular flexibilityadhesions of the inpolyurethane extreme cold and bond hot condition [50]. may be due to the moderate intramolecular flexibility of the polyurethane bond [50].

Figure 9. Typical adhesion test image of thermal cycling test of PB 0.96 samples. (a,c) Morphology and cross-cuts image beforeFigure thermal 9. Typical cycling test.adhesion (b,d) Morphology test image and of cross-cuts thermal image cycling after thermal test of cycling PB 0.96 test. samples. (a,c) Morphology and cross-cuts image before thermal cycling test. (b,d) Morphology and cross-cuts image after 4. Conclusions thermal cycling test. In summary, in this study, we succeeded in preparing waterborne coatings composed of home-made activated CAs, which possess good adhesion and high optical absorption. In 4. Conclusions addition, the formulations for the preparation are commercially available and the process In summary, is in reliable this forstudy, large-scale we production.succeeded The in CAs preparing were derived waterborne from resorcinol-formaldehyde coatings com- resin (RFR) via sol-gel using a hydrothermal method, annealed, and reactivated using CO2. posed of home-madeThe ultra-blackactivated coatingsCAs, which of CAs comprisingpossess good two-component adhesion waterborne and high polyurethane optical absorption. In addition,containing the formulations waterborne additives for the can preparation be applied using are different commercially wet deposition available methods, and the process is reliablefor example, for large spray-scale coating, production. painting, and The coil CAs coating, were which derived allow them from to resorcinol flatten and - form a uniform film. formaldehyde resin (RFR)The adhesion via sol- propertiesgel using and a hydrothermal optical properties method, can be modified annealed, by adjusting and reac- the tivated using CO2.CAs/binder The ultra ratio,-black and wecoatings found the of optimal CAs ratio comprising between absorption two-component and adhesion water- to be borne polyurethane0.96. containing For the sample waterborne with the adhesion additives grade can as class be 0applied after the thermal using cyclingdifferent test, anwet average reflectance of 2.1% within the visible and NIR regions was achieved. deposition methods, for example, spray coating, painting, and coil coating, which allow them to flatten andAuthor form Contributions: a uniform G.G.film. conceived and designed the experiments; J.X. and Y.S. performed the experiments and wrote the paper; G.C. and J.L. review. All authors have read and agreed to the The adhesionpublished properties version and of the optical manuscript. properties can be modified by adjusting the CAs/binder ratio, andFunding: we Thisfound research the was optimal supported ratio by the between National Key absorption Research and Developmentand adhesion Program to of be 0.96. For the sampleChina with (No.2017YFA0204600). the adhesion grade as class 0 after the thermal cycling test, an average reflectanceInstitutional of 2.1% within Review Board the Statement:visible andNot applicable.NIR regions was achieved. Informed Consent Statement: Not applicable. Author Contributions: G.G. conceived and designed the experiments; J.X. and Y.S. performed the experiments and wrote the paper; G.C. and J.L. review. All authors have read and agreed to the published version of the manuscript.

Coatings 2021, 11, 563 11 of 12

Data Availability Statement: The data presented in this study are available in this article. Acknowledgments: The assistance of laboratory members is greatly appreciated. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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