Mnnh4p2o7-Based Coating for High Temperature Assessment on the Surfaces of Cement Composites

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Article MnNH4P2O7-Based Coating for High Temperature Assessment on the Surfaces of Cement Composites

Rajagopalan Sam Rajadurai 1, Jong-Han Lee 1,* , Eunsoo Choi 2 and Joo-Won Kang 3 1 Department of Civil Engineering, Inha University, Incheon 22212, Korea; [email protected] 2 Department of Civil Engineering, Hongik University, Seoul 04066, Korea; [email protected] 3 School of Architecture, Yeungnam University, Gyeongsan 38451, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-32-860-7564

Received: 28 February 2020; Accepted: 14 April 2020; Published: 17 April 2020

Abstract: This study examines the implementation of an MnNH4P2O7 (ammonium manganese III pyrophosphate)-based coating on structural elements to obtain temperature information with color changes. Based on the MnNH4P2O7 material, a coating was prepared and deposited on cement mortar surfaces. Heat experiments were then conducted to evaluate the thermochromism on the fabricated samples. The coated samples exhibited a superior irreversible thermochromic property at 400 ◦C with a color change from dark violet to light grayish blue at the heated surface. The color changes were retrieved at each temperature using a digital camera, and the change in color properties was evaluated in the RGB and L*a*b* color spaces using image processing techniques. With increasing temperature from room temperature, the RGB values were almost constant until 200 ◦C. At higher temperatures, the color changes started to accelerate until 400 ◦C. The values showed a 167%, 567%, and 49% increase in R, G, and B values, respectively, at 400 ◦C. In the L*a*b* color space, when the temperature was increased from room temperature to 400 ◦C, the L*a*b* values showed an increase of 211%, a decrease of 94%, and an increase of 78%, respectively.

Keywords: ammonium manganese III pyrophosphate; coatings; inorganic pigment; temperature assessment; color change; imaging process

1. Introduction Thermochromism is a phenomenon where the color changes as a response to variations in temperature. The color changes are based on the transition and/or transformation of the molecules when the materials are heated up or cooled. The materials can exist in different forms, such as metal oxides, polymers, solid-state semiconductors, and leuco-dyes [1,2]. Thermochromic materials can be classified as reversible and irreversible materials. Reversible thermochromic materials exhibit a shift in color when heated and a return to their original colors when cooled down to room temperature. These materials have been applied for solar reflectance in structures to provide a thermally comfortable indoor environment [3–7]. Kamalisarvestani et al. [8] proposed a thermochromic window coating that changes its color to block solar radiation. In addition, a temperature sensor based on a polydiacetylene material was proposed to show reversible color properties in solutions in response to temperatures between 30 and 70 ◦C[9]. Irreversible thermochromic materials experience a permanent color change at a certain temperature. Some studies were performed to utilize these materials as thermal indicating paints that can monitor the temperature change profiles and provide a damage warning on aero-engine components [10–12]. Moreover, Rabhiou et al. [13] proposed a phosphor-based irreversible thermal coating to measure the temperature in the range of 600 to 1000 ◦C. Most of the previous studies focused on temperature indicating systems on combustors used in gas turbine engines [14] and on the hot surfaces of metals [15]. On the other hand, few studies were designed to apply irreversible materials

Coatings 2020, 10, 396; doi:10.3390/coatings10040396 www.mdpi.com/journal/coatings Coatings 2020, 10, 396 2 of 15 in cement-based structures. Only Ma and Zhu [16] attempted to utilize a reversible thermochromic pigment in cement mortar that changes a color at a low temperature of 42 ◦C. In concrete structures, the strength properties, mainly determined from the composition of concrete, change upon heating. When exposed to elevated temperature, concrete experiences physical and chemical changes, such as the evaporation of physically combined water, the dehydration of calcium silicate hydrate (CSH) and calcium hydroxide, and the decomposition of calcium carbonate and aluminates. With a normal concrete structure, a significant strength loss occurs between 300 and 600 ◦C[17]. The decrease in compressive strength commences from 300 ◦C, and the strength decreases by approximately 50% to 60% at 500 ◦C[18]. The tensile strength of a concrete slab exposed to 400 ◦C recovered 45% of its original strength and 40% of the residual bond strength obtained at room temperature [19,20]. Furthermore, the fatigue strength was not affected until the temperature reached 400 ◦C for concrete beams [21]. The color of concrete changes from normal to pink or red at temperatures ranging from 300 to 600 ◦C mainly due to the oxidation of iron components when using siliceous aggregate, but the color change is not obvious, particularly in concrete with calcareous and igneous aggregates [22,23]. In cement mortars, Yuzer et al. [24] evaluated the compressive strength and color change at high temperature. The strength loss started approximately at 300 ◦C, and the loss of the compressive strength accelerated above 300 ◦C. However, the color changes in cement mortars at high temperatures were visually unclear. Therefore, the beginning of this color change is difficult to recognize by the naked eye. The application of thermochromic materials on structural elements can provide temperature information with high resolution. Therefore, this study examined the color changes of an MnNH4P2O7 material with increasing temperature. Ammonium manganese III pyrophosphate, with the empirical formula MnNH4P2O7, is a finely powdered inorganic pigment with earth abundant components. The MnNH4P2O7 material is non-toxic and chemically stable with a typical dark violet, which has been used mainly in cosmetics, toys, and plastics [25]. The color of MnNH4P2O7 has been reported to provide reversible color changes at 120 to 340 ◦C, which leads to an irreversible phase transition at 340 to 460 ◦C[26,27]. In this study, a thermochromic coating based on the MnNH4P2O7 pigment was proposed to visualize high temperature variations accurately on the surface of cement-based materials with high spatial resolution. Temperature measurements based on the above coatings provide a visual interpretation that further leads to an advanced process to measure color from the digital images [28]. The colors from the concrete surfaces can be measured using a range of instruments, such as a spectrophotometer, calibrated flatbed scanner, and digital camera [29,30]. A colorimetric analysis method was performed to take photographs under different light conditions, and the concrete colors were represented in the chromaticity diagram [31]. Furthermore, a study based on optical microscopy combined with color image analysis was conducted to quantify the changes in color for concrete subjected to elevated temperatures [32]. Digital cameras were also used to assess the surface color changes on siliceous concrete specimens [33–35]. Digital cameras are a desirable and suitable tool for data collection because the image quality has been markedly improved in recent years, and they are affordable and available. Digital images are generally recorded as three color (RGB) pixels. All colors are possibly expressed in cubic space [35]. Moreover, RGB values depend mostly on the instrument used to capture the image. Therefore, the RGB values are usually transformed to a standardized color space that is more suited for individual applications. The International Commission on Illumination (CIE) illustrates standardized color spaces. The CIE 1976 (L*a*b*) color space is a standardized, device-independent, non-linear transformation of the RGB color space modelled with the human perception of color. The L*a*b* color space has linear measures of lightness (L*) and two-color dimensions (a* and b*). The a* dimension represents green (negative) to red (positive) intensities, and the b* dimension represents a spectrum from blue (negative) to yellow (positive) [36]. This study utilizes MnNH4P2O7-based coatings applied on the mortar surfaces to investigate the thermochromic color change and provide standardized color values. For this, heat experiments were performed to evaluate the thermochromic effects of MnNH4P2O7 coatings. Digital images were Coatings 2020, 10, x FOR PEER REVIEW 3 of 15 analyzed using the RGB and L*a*b* color spaces. The MnNH4P2O7 coatings showed an irreversible color change at a high critical temperature that could be monitored using the color values in both RGB and L*a*b* spaces.

2. Inorganic Color Changing Pigment Ammonium manganese III pyrophosphate is a finely powdered inorganic material with a chemical formula, MnNH4P2O7. Table 1 shows the characteristics of MnNH4P2O7 pigment manufactured at Kremer Pigmente in Germany, which is composed of earth abundant components, such as manganese dioxide, ammonium dihydrogen phosphate, and phosphoric acid [26,37]. The dark violet color of the inorganic material is due to the presence of phosphate and ammonia; the material is insoluble in organic and most ionic solvents. Figure 1a displays the dark violet color of the pigment at room temperature. When heated to 370 °C from room temperature, the pigment particles changed from dark violet to reversible blue color, as shown in Figure 1b. At the discoloration temperature, the H2O molecules are driven off from the particles, which leads to the change in Mn2P4O13(NH3)2 and blue color as Coatingsexpressed2020 , in10 , 396Equation (1). When exposed to the room atmosphere, Mn2P4O13(NH3)2 reacts 3with of 15 atmospheric moisture and returns to the original dark violet color [27]. At the higher temperature of 410 °C, the particles experience an irreversible color change to recorded from the surface at all temperatures. The color information of their respective images was grayish yellow, as shown in Figure 1c. The application of further heat to the pigment causes the analyzed using the RGB and L*a*b* color spaces. The MnNH4P2O7 coatings showed an irreversible liberation of NH3 from the particles, which is associated with the oxidation of hydroxylamine to color change at a high critical temperature that could be monitored using the color values in both RGB change Mn2P4O13(NH3)2 to Mn2P4O12 (manganous tetra-metaphosphate), as described in Equation (2). and L*a*b* spaces. Then, the oxidized hydroxylamine (NH3OH) is broken into nitrogen (N2) and water (H2O), or the 2.ammonia Inorganic (NH Color3) is directly Changing oxidized Pigment to nitrogen [27]. When cooled to room temperature, the particles retain the grayish yellow color and do not recover the original color. Ammonium manganese III pyrophosphate is a ﬁnely powdered inorganic material with a chemical formula, MnNH P O . Table1 shows the characteristics of MnNH P O pigment manufactured at 4 2 7 Table 1. Typical characteristics of MnNH4P2O7 pigment4 2 7 [26]. Kremer Pigmente in Germany, which is composed of earth abundant components, such as manganese dioxide, ammonium dihydrogenParameter phosphate, and phosphoricCharacteristics acid [26,37]. The dark violet color of the inorganic material is due toColor the presence of phosphate and ammonia;Dark violet the material is insoluble in organic and most ionicChemical solvents. characterization Figure1a displays theAmmo darknium violet manganese color of the III pigment pyrophosphate at room temperature. Density 2.7–2.9 kg/m3 3 BulkTable density 1. Typical characteristics of MnNH 0.604P2O g/cm7 pigment [26]. Average particle size 2.30 µm Parameter Characteristics pH value 2.5–4.7 Thermal decompositionColor >400 Dark °C violet Chemical characterization Ammonium manganese III pyrophosphate

Density 2.7–2.9 kg/m3 𝟐𝐌𝐧𝐍𝐇 𝐏 𝐎 Bulk density 𝟒 𝟐 𝟕 0.60 g/cm3 (1) ⎯⎯⎯⎯⎯ 𝐌𝐧𝟐𝐏𝟒𝐎𝟏𝟑(𝐍𝐇𝟑)𝟐 (AmmoniumAverage manganese particle size (III) pyrophosphate) 2.30 µm → pH value𝐌𝐧𝟐 2.5–4.7𝐏𝟒𝐎𝟏𝟐 (2) 𝐌𝐧 𝐏 𝐎 (𝐍𝐇 ) ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ 𝟐 Thermal𝟒 𝟏𝟑 decomposition𝟑 𝟐 ⎯⎯⎯⎯⎯⎯⎯ (Manganous tetra>400 metaphosphate◦C )

(a) (b) (c)

FigureFigure 1. 1.Color Color changechange of of MnNH MnNH44PP22OO77 pigment:pigment: ( (aa)) room room temperature; temperature; ( (bb)) 370 370 °C;◦C; ( (cc)) 410 410 °C.◦C.

When heated to 370 ◦C from room temperature, the pigment particles changed from dark violet to reversible blue color, as shown in Figure1b. At the discoloration temperature, the H 2O molecules are driven oﬀ from the particles, which leads to the change in Mn2P4O13(NH3)2 and blue color as expressed in Equation (1). When exposed to the room atmosphere, Mn2P4O13(NH3)2 reacts with atmospheric moisture and returns to the original dark violet color [27]. At the higher temperature of 410 ◦C, the particles experience an irreversible color change to grayish yellow, as shown in Figure1c. The application of further heat to the pigment causes the liberation of NH3 from the particles, which is associated with the oxidation of hydroxylamine to change Mn2P4O13(NH3)2 to Mn2P4O12 (manganous tetra-metaphosphate), as described in Equation (2). Then, the oxidized hydroxylamine (NH3OH) is broken into nitrogen (N2) and water (H2O), or the ammonia (NH3) is directly oxidized to nitrogen [27]. When cooled to room temperature, the particles retain the grayish yellow color and do not recover the original color.

2MnNH P O 4 2 7 H O Mn P O (NH ) (1) (Ammonium manganese (III) pyrophosphate) − 2 2 4 13 3 2 −−−−−→ Coatings 2020, 10, 396 4 of 15

NH3OH N2 + H2O Mn P O Coatings 2020, 10, x FOR PEER REVIEW− → 2 4 12 4 of 15 Mn2P4O13(NH3)2 (2) −−−−−−−−−−−−−−−−−−−−−−→−−−−−−−−−→NH (Manganous tetra metaphosphate) − 3 3. ExperimentsCoatings 2020, 10 , x FOR PEER REVIEW 4 of 15 3. Experiments 3.1. 3.Preparation Experiments of Specimens and Test Variables 3.1. Preparation of Specimens and Test Variables 3.1.The Preparation following of procedures Specimens and were Test followed Variables to prepare the specimens, resembling the application The following procedures were followed to prepare the specimens, resembling the application of of coatings in real buildings. Sample preparation consisted of two stages. The first stage involved the coatingsThe in following real buildings. procedures Sample were preparation followed to consisted prepare ofthe two specimens, stages. resembling The first stage the application involved the preparation of coating surfaces, and the second stage included the preparation of coatings and preparationof coatings of in coating real buildings. surfaces, Sample and the preparation second stage consisted included of the two preparation stages. The of first coatings stage andinvolved fabrication. the fabrication. preparationCement mortar of coating samples, surfaces, 50 and50 the50 mmsecond3 in stage size, wereincluded prepared the preparation by mixing water,of coatings silica and sand, Cement mortar samples, 50 ×× 50 × ×50 mm3 in size, were prepared by mixing water, silica sand, andfabrication. ordinary Portland cement (Type I), manufactured at a local company. The ratios of water to cement and ordinary Portland cement (Type I), manufactured3 at a local company. The ratios of water to and sandCement to cement mortar were samples, 0.4 and 50 2, × respectively. 50 × 50 mm Thein size, prepared were prepared specimens bywere mixing left water, under silica cover sand, inside cement and sand to cement were 0.4 and 2, respectively. The prepared specimens were left under theand laboratory ordinary for Portland 24 h. Theycement were (Type then I), removed manufactured from theat a moldslocal company. and cured The in waterratios of in awater constant to covercement inside and the sand laboratory to cement for 24were h. They0.4 and were 2, respectively. then removed The from prepared the molds specimens and cured were inleft water under in temperature and humidity chamber (20 2 ◦C, 60% 10%) until the test. Subsequently, the mortar a constantcover inside temperature the laboratory and humidity for 24 h. chamber They± were (20 then ± 2 °C,removed± 60% ± from 10%) the until molds the test.and Subsequently,cured in water thein specimens were dried for 24 h at 100 C in an oven. mortara constant specimens temperature were dried and forhumidity 24 h ◦at chamber 100 °C in (20 an ± oven.2 °C, 60% ± 10%) until the test. Subsequently, the Figure2 shows the coating procedure on a cross section of the specimens. The mortar surfaces mortarFigure specimens 2 shows the were coating dried forprocedure 24 h at 100 on °C a crossin an sectionoven. of the specimens. The mortar surfaces were cleaned using sandpaper, and the dust and dirt particles were removed to maintain a clean surface were cleanedFigure using 2 shows sandpaper, the coating and procedure the dust on and a cross dirt sectionparticles of werethe specimens. removed Theto maintain mortar surfaces a clean and allow adhesion of the applied coating to the mortar surface, as shown in Figure3a. The ﬁrst surfacewere and cleaned allow using adhesion sandpaper, of the appliedand the coatingdust and to di thert particlesmortar surface,were removed as shown to maintainin Figure a 3a. clean The firstcoating,surface coating, called and called allow the the prime adhesion prime coating, co ofating, the was applied was dropped dropped coating on theon to the surface mortarsurface with surface, with silica-based silica-based as shown binder inbinder Figure and and water3a. water The at a at1:5 a first1:5 ratio. ratio.coating, The The prime called prime coatedthe coated prime samples cosamplesating, were was were placeddropped placed in on in an thean oven ovensurface at at 100 with 100◦C °Csilica-based for for 10 10 min min binder and and dried anddried water for for 6 6 h hat at ambientatambient a 1:5 ratio. temperature,temperature, The prime asas coated shownshown samples in Figure were3 3b.b. placed To prepareprepare in an oven thethe coating at 100 °C solution, for 10 min pigment and dried and and waterfor water 6 werewereh atadded added ambient at at a temperature, a2:3 2:3 ratio ratio and and as stirred stirredshown for in for Figure10 10 min min 3b. at at Toroom room prepare temperature temperature the coating with with solution, the the slow slow pigment addition addition and ofwater of 10% 10% silica-basedsilica-basedwere added binder binder at a under2:3 under ratio stirring stirring. and stirred. Finally, Finally, for 10the the min coating coating at room solution solution temperature was was poured poured with the on on slowthe the specimen addition specimen ofsurface surface 10% andandsilica-based coated coated in in a abindersingle single layer,under layer, as stirring as shown shown. Finally, in in Figure Figure the 3c. 3coatingc. The The coated coatedsolution surfaces surfaces was poured were were lefton left the undisturbed undisturbed specimen surfaceat at room room temperaturetemperatureand coated for for in sevenaseven single daysdays layer, to to as cure cureshown and and achievein Figureachieve complete 3c. completeThe coated color color exposuresurfaces exposure were on the left on surfaces. undisturbed the surfaces. After at curation, room After temperature for seven days to cure and achieve complete color exposure on the surfaces. After curation,the specimens the specimens were placed were in placed a furnace in a chamberfurnace ch andamber heated and toheated the target to the temperatures target temperatures of 100, 200,of curation, the specimens were placed in a furnace chamber and heated to the target temperatures of 100,300, 200, 400, 300, and 400, 450 ◦andC. At 450 each °C. targetAt each temperature, target temp theerature, specimen the specimen was maintained was maintained for around for 30 around min and 100, 200, 300, 400, and 450 °C. At each target temperature, the specimen was maintained for around 30taken min outand to taken photograph out to photograph the surface ofthe the surface specimen of the using specimen a digital using camera. a digital After camera. that, the After specimen that, 30 min and taken out to photograph the surface of the specimen using a digital camera. After that, thewas specimen placed again was placed in the furnaceagain in and the heatedfurnace to an thed heated next target to the temperature. next target temperature. the specimen was placed again in the furnace and heated to the next target temperature.

FigureFigureFigure 2. 2. Cross2.Cross Cross section sectionsection of ofof the the coatings coatings on on on the the the surface.surface. surface.

(a) (b) (c) (a) (b) (c) FigureFigure 3. 3. SpecimensSpecimens and and coatings coatings on on the the surface: surface: (a) (mortara) mortar surface; surface; (b) prime (b) prime coating; coating; (c) Figure 3. Specimens and coatings on the surface: (a) mortar surface; (b) prime coating; (c) MnNH4P2O7-based coating. (c) MnNH4P2O7-based coating. MnNH4P2O7-based coating. 3.2. Image Acquisition and Post Processing 3.2. Image Acquisition and Post Processing

Coatings 2020, 10, x FOR PEER REVIEW 5 of 15

The color data were collected by photographing the surfaces at all temperature intervals. A digital camera (ILCE-TR, SONY, Tokyo, Japan) with a 7360 × 4912-pixel resolution was placed at a fixed position, approximately 30 cm above the specimens. All the images were recorded using the highest pixel count (most pixels per object area) to provide high quality images. The recorded photographs were imported to a computer and analyzed using an image processing technique. The color and brightness of the light source have a significant influence on the color of the surface. Photographs were taken in a consistent illuminance throughout at all temperatures to minimize the variation in color and brightness. Coatings The2020 ,color10, 396 of the images on the surface of the samples was analyzed using MATLAB Version 9.65 of 15 programming to extract the RGB and L*a*b* color information. From the images obtained at different temperatures, the surfaces of the samples were taken from the background as the regions of interest, 3.2. Image Acquisition and Post Processing as shown in Figure 4a. That is, the sample surface of approximately 1000 × 1000 pixels was separated fromThe the color total data image were (7360 collected × 4912 by pixels). photographing Then, the the mean surfaces and standard at all temperature deviation intervals.of the RGB A and digital cameraL*a*b* (ILCE-TR, color values SONY, were Tokyo, obtained Japan) from with the aseparated 7360 4912-pixel sample surface resolution at each was pixel. placed Furthermore, at a fixed position, the × approximatelyseparated surface 30 cm was above divided the specimens. into 4 × 4 regions All the to images present were more recorded accurate using color thevalues, highest as shown pixel count in (mostFigure pixels 4b. perThe object mean area) and tostandard provide deviation high quality of RGB images. and The L*a recorded*b* color photographsvalues were also were calculated imported to a computerusing the separate and analyzed 4 × 4 regions using andefined image as processing approximately technique. 250 × 250 The pixels color for and each brightness region. of the light sourceEach have color a significant in the RGB influence space on is therepresented color of theby surface.a combination Photographs of spectral were components taken in a consistentof red, illuminancegreen, and throughout blue colors. at The all temperaturesvalues of each to RGB minimize component the variation are in the in range color and0 to 255. brightness. The measured RGBThe color color values of the are images related on to the the surface lighting of conditions the samples (color was and analyzed brightness), using colors MATLAB of the Version objects9.6 being recorded, and the sensitivity of the recording image sensor. Thus, the RGB color space used in programming to extract the RGB and L*a*b* color information. From the images obtained at diﬀerent digital cameras is device-dependent. The color values of the pigment can correspond to predefined temperatures, the surfaces of the samples were taken from the background as the regions of interest, as values of the ColorHexa codes. Hence, the captured images were adjusted by changing the shown in Figure4a. That is, the sample surface of approximately 1000 1000 pixels was separated illumination and settings of the camera to match the predefined ColorHexa× codes [38,39]. ColorHexa from the total image (7360 4912 pixels). Then, the mean and standard deviation of the RGB and is a six-digit color code providing× information about the colors obtained from the images [40]. L*aTherefore,*b* color valuesto compensate were obtained for the fromvariations the separated due to the sample color and surface brightness at each of pixel. the light Furthermore, source, the the separated surface was divided into 4 4 regions to present more accurate color values, as shown in RGB images were transformed into standardized,× device-independent CIE1976 (L*a*b*) space using FigureMATLAB4b. The programming. mean and standard This resulted deviation in values of RGB betweenand L* a0* b*andcolor 100 valuesfor the wereL* dimension, also calculated and −128 using the separate 4 4 regions deﬁned as approximately 250 250 pixels for each region. and 128 for a×* and b* dimensions. ×

(a) (b)

FigureFigure 4. 4.Selections Selections of of the the regionregion ofof interestinterest (ROI): ( (aa)) ROI ROI extracted extracted from from the the background; background; (b) ( bROI) ROI divideddivided into into 4 4 ×4 4 regions. regions. × 4. ResultsEach color and in Discussion the RGB space is represented by a combination of spectral components of red, green, and blue colors. The values of each RGB component are in the range 0 to 255. The measured RGB color values4.1. Thermal are related Response to the of lightingCement Composites conditions and (color MnNH and4P brightness),2O7-Based Coatings colors ofwith the Temperatures objects being recorded, and theMnNH sensitivity4P2O7-based of the recordinginorganic coatings image sensor. were applied Thus, to the a cementRGB color mortar space specimen used in using digital pigment cameras iscoating device-dependent. formulations The and color curing values conditions. of the pigmentFigure 5 canpresents correspond the surface to predeﬁned color of the values mortar of the ColorHexaspecimen codes.without Hence, the MnNH the captured4P2O7 coating. images At were room adjusted temperature, by changing the surface the illumination color of the and cement settings of the camera to match the predeﬁned ColorHexa codes [38,39]. ColorHexa is a six-digit color code providing information about the colors obtained from the images [40]. Therefore, to compensate for the variations due to the color and brightness of the light source, the RGB images were transformed into standardized, device-independent CIE1976 (L*a*b*) space using MATLAB programming. This resulted in values between 0 and 100 for the L* dimension, and 128 and 128 for a* and b* dimensions. − 4. Results and Discussion

4.1. Thermal Response of Cement Composites and MnNH4P2O7-Based Coatings with Temperatures

MnNH4P2O7-based inorganic coatings were applied to a cement mortar specimen using pigment coating formulations and curing conditions. Figure5 presents the surface color of the mortar specimen Coatings 2020, 10, x FOR PEER REVIEW 6 of 15 mortar exhibited a dark grayish color, as shown in Figure 5a. With increasing temperature, the surface color was very similar to the dark grayish color presented at room temperature. At high temperatures between 300 and 450 °C, the surface color of the mortar was slightly darkened, as shown in Figure 5d–f, but no significant color changes were observed by naked eye. The thermochromic behavior of the MnNH4P2O7 coating on the mortar is shown in Figure 6. The mortar sample coated with the MnNH4P2O7 coating showed a dark violet color at ambient temperature, as illustrated in Figure 6a. Figure 6b,c show samples at 100 and 200 °C, respectively. At 100 °C, the coating sample displayed the same dark violet color observed at ambient temperature. At 200 °C, the sample exhibited a very dark, desaturated blue color. This was attributed to the evaporation of water particles, due to heating, moving towards the outer surface of the mortar specimen due to the increase in temperature. Figure 6d shows the sample heated to 300 °C, at which the violet-pattern color on the surface disappeared. That is, the color of the coating changed from very dark desaturated blue to grayish violet with the evaporation of water particles. As the temperature was increased to 400 °C, the surface coating displayed excellent thermochromic progress and turned to light grayish blue. The coatings achieved an extensive, irreversible color change at 400 °C, which was 10 °C lower than the temperature observed for the pigment. This might be due to the Coatings 2020, 10, 396 6 of 15 occurrence of pigments dispersed in the coating, which was manufactured by a 2:3 pigment-to-water ratio. Further heating to 450 °C resulted in a stable light grayish blue color. Figure 6e,f show the color withoutchanges of the the MnNH samples4P2 Owith7 coating. a MnNH At4P room2O7 coating temperature, at 400 °C the and surface 450 °C, color respectively. of the cement After cooling mortar exhibitedto room atemperature, dark grayish the color, samples as shown retained in Figure the5a. changed With increasing light grayish temperature, blue color, the surface indicating color wasirreversible very similar thermochromic to the dark behavior. grayish colorWhen presented applied to at the room surface temperature. of the concrete At high structures, temperatures the betweenthermochromic 300 and MnNH 450 ◦C,4P the2O7 surface coatings color developed of the mortar in the waslaboratory slightly can darkened, provide as a showntool to determine in Figure5d–f, the buttemperature, no signiﬁcant withcolor a change changes in color were defined observed as a by function naked eye.of temperature.

(a) (b) (c)

(d) (e) (f)

Figure 5. Surface colors of the mortar specimen without MnNH4P2O7 coating at (a) ambient Figure 5. Surface colors of the mortar specimen without MnNH4P2O7 coating at (a) ambient temperature; (temperature;b) 100 ◦C; (c) ( 200b) 100◦C; °C; (d) ( 300c) 200◦C; °C; (e )(d 400) 300◦C; °C; (f) ( 450e) 400◦C. °C; (f) 450 °C.

The thermochromic behavior of the MnNH4P2O7 coating on the mortar is shown in Figure6. The mortar sample coated with the MnNH4P2O7 coating showed a dark violet color at ambient temperature, as illustrated in Figure6a. Figure6b,c show samples at 100 and 200 ◦C, respectively. At 100 ◦C, the coating sample displayed the same dark violet color observed at ambient temperature. At 200 ◦C, the sample exhibited a very dark, desaturated blue color. This was attributed to the evaporation of water particles, due to heating, moving towards the outer surface of the mortar specimen due to the increase in temperature. Figure6d shows the sample heated to 300 ◦C, at which the violet-pattern color on the surface disappeared. That is, the color of the coating changed from very dark desaturated blue to grayish violet with the evaporation of water particles. As the temperature was increased to 400 ◦C, the surface coating displayed excellent thermochromic progress and turned to light grayish blue. The coatings achieved an extensive, irreversible color change at 400 ◦C, which was 10 ◦C lower than the temperature observed for the pigment. This might be due to the occurrence of pigments dispersed in the coating, which was manufactured by a 2:3 pigment-to-water ratio. Further heating to 450 ◦C resulted in a stable light grayish blue color. Figure6e,f show the color changes of the samples with a MnNH4P2O7 coating at 400 ◦C and 450 ◦C, respectively. After cooling to room temperature, the samples retained the changed light grayish blue color, indicating irreversible thermochromic behavior. When applied to the surface of the concrete structures, the thermochromic MnNH4P2O7 coatings developed in the laboratory can provide a tool to determine the temperature, with a change in color deﬁned as a function of temperature. Coatings 2020, 10, 396 7 of 15 Coatings 20202020,, 1010,, xx FORFOR PEERPEER REVIEWREVIEW 77 ofof 1515

((a)) ((b)) ((c))

((d)) ((e)) ((ff))

Figure 6. Color change process of MnNH4P2O7 coating obtained at (a) ambient temperature; (b) Figure 6. Color change process of MnNH 44PP22OO7 7coatingcoating obtained obtained at at (a ()a )ambient ambient temperature; temperature; (b ()b ) 100 ◦100C; (°C;c) 200 (cc)) 200◦200C; (°C;°C;d) 300((d)) 300◦300C; (°C;°C;e) 400 ((e)) 400400◦C; °C; (°C;f) 450 ((ff)) 450450◦C. °C.°C.

4.2.4.2. Color Changes Obtained in the RGB Color Space from DigitalDigital ImagesImages withwith TemperaturesTemperatures FigureFigure7 7 presentspresents the changes changes in in the the RGBRGB valuesvaluesvalues onon thethe on thesurfacessurfaces surfaces ofof thethe of mortarmortar the mortar specimen.specimen. specimen. WithWith Withincreasingincreasing increasing temperaturetemperature temperature fromfrom fromroomroomroom temperature,temperature, temperature, thethe meanmean the mean RGB RGBvaluesvaluesvalues werewere werearoundaround around (141,(141, (141,136,136, 129)129) 136, 129)indicatingindicating indicating darkdarkdark grayishgrayish grayish orangeorange orange (Hex(Hex (Hex codecode code #8d8881),#8d8881), #8d8881), andand they andthey they werewere were almostalmost almost invariantinvariant invariant untiluntil until 200200 °C.°C. 200 TheThe◦C. TheRGBRGB valuesvaluesvalues decreaseddecreased decreased byby 8%, by8%, 8%, 9%,9%, 9%, andand and 11%11% 11% atat at300300 300 °C°C◦ Ctoto to showshow show thethe the meanmean mean (124,(124, (124, 118,118, 118, 109),109), 109), respectively,respectively, respectively, correspondingcorresponding toto darkdark grayishgrayish orangeorange (Hex(Hex codecode #7c766d).#7c766d).#7c766d). After After that, that, the the RGBRGB valuesvalues exhibitedexhibitedexhibited relativelyrelatively similarsimilar valuesvaluesvalues ofofof (123,(123,(123, 119,119,119, 113)113)113) atatat 400400400 ◦°C°CC andand (123,(123, 118,118, 113)113) atat 450450 ◦°C,°C,C, which which correspond correspond toto darkdark grayishgrayish orangeorange (Hex(Hex codescodes #7b7771#7b7771 andand #7b7671,#7b7671, respectively).respectively). The cementcement mortarmortar withwith somesome porespores onon thethe surfacesurface providedprovided somesome variationsvariations inin thethe RGBRGB values,values, whichwhich ledled to to the the standard standard deviation deviation inin thethe rangerange ofof 2020 tototo 24.24.24.

FigureFigure 7.7. Color changes and their standard deviations of of the the mortar mortar surfaces surfaces obtained in RGB values.

The quantitative difference of colors on the surface of the coating samples at ambient temperaturetemperature andand thosethose atat criticalcritical temperaturetemperature whenwhen thethe colorcolor completelycompletely changedchanged werewere analyzedanalyzed

Coatings 2020, 10, x FOR PEER REVIEW 8 of 15 using the images obtained at each temperature. The variation in the thermochromic color of the coating samples with increasing temperature was expressed in three-dimensional RGB color space. The mean and standard deviation of the RGB values were calculated at each pixel from the sample surfaces. Figure 8 presents the RGB mean values with the standard deviation obtained from the surfaces with increasing temperature. The mean RGB values were interpreted to determine their corresponding ColorHexa codes and color description at various temperatures, as shown in Table 2. The RGB values at ambient temperature were (76, 31, 157), which represent a dark violet color

Coatings(Hex code2020 ,#4b1f9c).10, 396 These values were relatively constant and stable until 200 °C. As the temperature8 of 15 was increased to 300 °C, the RGB values increased to (159, 146, 187), and the color changed to grayish violet (Hex code #9E91BB). At 400 °C, the thermochromic coating lost its chroma and transformed fromThe dark quantitative to light grayish diﬀerence blue (Hex of colors code on #cad0e9) the surface with of an the increase coating in samples all the RGB at ambient values temperatureto (202, 209, and233). those Compared at critical to those temperature at room when temperature, the color completelythe RGB values changed increased were analyzed by approximately using the images 167%, obtained567%, and at 49%, each respectively. temperature. Few The changes variation in the in RGB the thermochromicvalues were observed color of when the coatingthe temperature samples withwas increased increasing to temperatureabove 400 °C. was The expressed standard deviations in three-dimensional of the RGB values,RGB color as shown space. inThe Figure mean 8, were and standardin the range deviation of 13 to of 30 the fromRGB roomvalues temperature were calculated to 200 at °C. each The pixel deviations from the of samplethe RGB surfaces. values revealed Figure8 presentssome variations the RGB atmean each values pixel withof the the image standard mainly deviation due to obtained uneven fromcoatings the surfacesand some with pores increasing on the temperature.surface. After Thethe coating mean RGB surfacevalues changed were interpretedcompletely to determinelight grayish their blue corresponding at 400 °C, the ColorHexadeviations codeswere relatively and color low, description in the range at various of 10 to temperatures, 11. as shown in Table2.

Figure 8. ColorColor changes and their standard deviatio deviationsns of the coating surface obtained in RGB values.

Table 2. Coatings—RGB color change values and their corresponding color charts. Table 2. Coatings—RGB color change values and their corresponding color charts. Color Values Temperature ( C) Color Values Hex Code Color Description Temperature (°C)◦ Hex Code Color Description RedRed ( R(R)) GreenGreen ( G(G)) BlueBlue ( B(B) ) RoomRoom Temperature Temperature 76 76 ±13 13 31 31 ±12 12 157 157 ±13 13 #4b1f9c Dark violetviolet ± ± ± 100 67 17 35 15 144 18 #432390 Dark violet 100 67± ± 17 35± ± 15 144± ± 18 #432390 Dark violet 200 62 18 47 15 109 30 #3d2f6c Dark desaturated blue 200 62± ± 18 47± ± 15 109± ± 30 #3d2f6c Dark desaturated blue 300 159 15 146 15 187 15 #9E91BB Grayish violet ± ± ± 300400 202159 ±11 15 209 146 ±11 15 233 187 ±10 15 #9E91BB #cad0e9 Light Grayish grayish violet blue ± ± ± 400450 205202 ±11 11 214 209 ±11 11 235 233 ±10 10 #cdd6eb #cad0e9 Light Light grayishgrayish blue blue ± ± ± 450 205 ± 11 214 ± 11 235 ± 10 #cdd6eb Light grayish blue The RGB values at ambient temperature were (76, 31, 157), which represent a dark violet color The images were divided into 4 × 4 regions on the surface, and the mean RGB values were (Hex code #4b1f9c). These values were relatively constant and stable until 200 ◦C. As the temperature extracted over the divided regions. The values extracted at 4 × 4 regions were plotted in the three- was increased to 300 ◦C, the RGB values increased to (159, 146, 187), and the color changed to grayish dimensional diagrams shown in Figure 9. Figure 9 represents a surface plot of the RGB values at room violet (Hex code #9E91BB). At 400 ◦C, the thermochromic coating lost its chroma and transformed from darktemperature to light grayishand 400 blue°C. The (Hex mean code R #cad0e9) values obtained with an increase over 4 × in 4 allregions the RGB werevalues between to (202, 60 and 209, 93 233). at Comparedroom temperature to those and at room 200 temperature,and 207 at 400 the °C.RGB Thevalues meanincreased G values byin approximatelythe 4 × 4 regions 167%, were 567%, between and 49%,17 and respectively. 47 at room Fewtemperature changes and in the betweenRGB values 205 and were 211 observed at 400 °C. when The theB values temperature at room was temperature increased in the 4 × 4 regions ranged from 142 to 176, and from 231 to 236 at 400 °C. As discussed with respect to above 400 ◦C. The standard deviations of the RGB values, as shown in Figure8, were in the range of to the standard deviations, the RGB values obtained in 4 × 4 regions had some variations at room 13 to 30 from room temperature to 200 ◦C. The deviations of the RGB values revealed some variations at each pixel of the image mainly due to uneven coatings and some pores on the surface. After the coating surface changed completely to light grayish blue at 400 ◦C, the deviations were relatively low, in the range of 10 to 11. The images were divided into 4 4 regions on the surface, and the mean RGB values were extracted × over the divided regions. The values extracted at 4 4 regions were plotted in the three-dimensional × diagrams shown in Figure9. Figure9 represents a surface plot of the RGB values at room temperature and 400 C. The mean R values obtained over 4 4 regions were between 60 and 93 at room temperature ◦ × and 200 and 207 at 400 C. The mean G values in the 4 4 regions were between 17 and 47 at room ◦ × Coatings 2020, 10, x FOR PEER REVIEW 9 of 15 temperature but similar values at 400 °C. Some changes at room temperature might have been due to variations in the coating density of the dark and light particles on the relatively small surface. At 400 °C, the color change was obtained uniformly throughout the surface, which minimized the variations. In addition, this study analyzed the total color changes of the mean RGB values obtained at room temperature and 400 °C. The total color change is defined as the Euclidean distance (ΔD) in the RGB color space shown in Equations (3)–(6)

∆𝐷 = (∆𝑅 +∆𝐺 +∆𝐵) (3)

∆𝑅 = 𝑅 − 𝑅 (4) Coatings 2020, 10, 396 9 of 15 ∆𝐺 = 𝐺 − 𝐺 (5) temperature and between 205 and 211 at∆𝐵 400 ◦C. = The𝐵 − 𝐵B values at room temperature in the 4 4 regions(6) × ranged from 142 to 176, and from 231 to 236 at 400 ◦C. As discussed with respect to the standard wheredeviations, ΔR, Δ theG, andRGB ΔvaluesB represent obtained the incolor 4 changes,4 regions in had which some R1 variations, G1, and B at1 are room the temperaturemean values but at × roomsimilar temperature, values at 400 and◦C. R Some2, G2, and changes B2 are at the room mean temperature values at 400 might °C. The have total been color due change to variations ΔD showed in the ancoating increase density of intensities of the dark from and room light temperature particles on to 400 the °C, relatively at which small point surface. ΔD reached At 400 approximately◦C, the color 231.change was obtained uniformly throughout the surface, which minimized the variations.

(a) (b)

(c)

FigureFigure 9. ColorColor intensities intensities in in 4 4 × 44 regions regions on on the the surface: surface: (a (a) )RR values;values; (b (b) )GG values;values; ( (cc)) BB values.values. ×

In addition, this study analyzed the total color changes of the mean RGB values obtained at room temperature and 400 ◦C. The total color change is deﬁned as the Euclidean distance (∆D) in the RGB color space shown in Equations (3)–(6) q ∆D = (∆R2 + ∆G2 + ∆B2) (3)

∆R = R R (4) 2 − 1 ∆G = G G (5) 2 − 1 ∆B = B B (6) 2 − 1 Coatings 2020, 10, x FOR PEER REVIEW 10 of 15

4.2. Color Changes Obtained in the L*a*b* Color Space from Digital Images with Temperatures The RGB images obtained were converted to CIE 1976 (L*a*b*) color space to provide more standardized results. The lightness (L*) is given a value of 100 for white and 0 for ideal black. The a* and b* parameters are in the range of −128 and 128; a* indicates greenness and redness, respectively, and b* indicated blueness and yellowness, respectively. Figure 10 presents the changes in the L*a*b* values on the mortar surface with increasing temperature. The L*a*b* values at room temperature were (57, 0.2, 4), which remained almost constant until 200 °C. At 300 °C, a slight decrease and increase in the L* and a* values was obtained to show the L*a*b* values of (49, 0.9, 5). At the temperatures of 400 °C and 450 °C, the L*a*b* values were very similarly (50, 0.6, 4) and (50, 0.7, 4), respectively. Thus, little change was found in the L*a*b* values on the surfaces of the mortar specimen Coatingswith increasing2020, 10, 396 temperature. The deviations obtained in the L* value were in the range of 8 to 109, ofand 15 those in the a* and b* values were in the range of 1 to 2. Figure 11 presents the changes on the coating surface in the L*a*b* values with increasing where ∆R, ∆G, and ∆B represent the color changes, in which R ,G , and B are the mean values at room temperatures. The mean and standard deviation of the L*a*b1* values1 and1 their corresponding color temperature, and R , G , and B are the mean values at 400 C. The total color change ∆D showed an charts are also summarized2 2 in2 Table 3. The mean L* value◦ at room temperature was 26, which increase of intensities from room temperature to 400 C, at which point ∆D reached approximately 231. represents a dark appearance. The value remained◦ relatively constant up to 200 °C. As the 4.3.temperature Color Changes was increased Obtained in further the L*a*b* to 300 Color °C, Space the fromL* value Digital increased Images withto 62. Temperatures At 400 °C, the L* value rapidly increased to 84, and the color completely changed to a light color. Compared to that at room temperature,The RGB theimages L* value obtained at 400 were °C convertedincreased toby CIE 211%. 1976 The (L* amean*b*) color a* and space b* tovalues provide at room more standardizedtemperature were results. 48 and The − lightness60, respectively, (L*) is given which a valuemeans of redness 100 for white(+a*) and and blueness 0 for ideal (−b black.*). From The thea* and b* parameters are in the range of 128 and 128; a* indicates greenness and redness, respectively, temperature of 200 °C, the a* value gradually− decreased with a concomitant increase in b* value. andComparedb* indicated to the bluenessa* and b* andvalues yellowness, at room temperature, respectively. the Figure a* value 10 presents at 400 °C the decreased changes inby the 94%L* ato*b a* valuesvalue of on 2, the and mortar the b surface* value withat 400 increasing °C increased temperature. by 78% Theto −12.L*a *Theb* values standard at room deviations temperature were werealso (57,obtained 0.2, 4), at whichall pixels remained in the images almost at constant all temperatures. until 200 ◦ TheC. At standard 300 ◦C, deviations a slight decrease of the L and*a*b increase* values inbetween the L* room and a *temperature values was obtaineduntil 300 °C to showranged the fromL*a *5b to* values 8, 2 to of9, and (49, 2 0.9, to 5).11, Atrespectively. the temperatures When the of 400temperature◦C and 450 reached◦C, the moreL*a*b than* values 400 were°C, the very deviations similarly of (50, the 0.6, L* 4) values and (50, were 0.7, in 4), the respectively. range of 5 Thus,to 6, littlewhereas change those was of foundthe a* and in the b* Lvalues*a*b* values were in on the the range surfaces of 1 of to the 2. This mortar means specimen that the with color increasing change temperature.obtained at the The critical deviations temperature obtained of in400 the °CL* wavalues uniform were in over the rangethe entire of 8 tocoating 9, and surface, those in which the a* andshowedb* values relatively were small in the deviations. range of 1 to 2.

Figure 10. Color changes on the mortar surfaces obtained in L*a*b* values.

Figure 11 presents the changes on the coating surface in the L*a*b* values with increasing temperatures. The mean and standard deviation of the L*a*b* values and their corresponding color charts are also summarized in Table3. The mean L* value at room temperature was 26, which represents a dark appearance. The value remained relatively constant up to 200 ◦C. As the temperature was increased further to 300 ◦C, the L* value increased to 62. At 400 ◦C, the L* value rapidly increased to 84, and the color completely changed to a light color. Compared to that at room temperature, the L* value at 400 C increased by 211%. The mean a* and b* values at room temperature were 48 and 60, ◦ − respectively, which means redness (+a*) and blueness ( b*). From the temperature of 200 C, the a* − ◦ value gradually decreased with a concomitant increase in b* value. Compared to the a* and b* values at room temperature, the a* value at 400 ◦C decreased by 94% to a value of 2, and the b* value at 400 ◦C increased by 78% to 12. The standard deviations were also obtained at all pixels in the images at all − temperatures. The standard deviations of the L*a*b* values between room temperature until 300 ◦C ranged from 5 to 8, 2 to 9, and 2 to 11, respectively. When the temperature reached more than 400 ◦C, the deviations of the L* values were in the range of 5 to 6, whereas those of the a* and b* values were in the range of 1 to 2. This means that the color change obtained at the critical temperature of 400 ◦C was uniform over the entire coating surface, which showed relatively small deviations. Coatings 2020, 10, 396 11 of 15 Coatings 2020, 10, x FOR PEER REVIEW 11 of 15

FigureFigure 11.11. Color changes on the coating surfaces obtainedobtained inin LL**aa**bb** values.values.

Table 3. Coatings—L*a*b* color change values and their corresponding color charts. Table 3. Coatings—L*a*b* color change values and their corresponding color charts. Color Values Temperature ( C) Color Values Color Description Temperature◦ (°C) Hex CodeCode Color Description L*L* a*a* b*b* RoomRoom TemperatureTemperature 26 265 ± 5 48 48 ±3 3 –60 –60 2± 2 #4b1f9c Dark Dark violet violet ± ± ± 100 25 6 42 3 –54 3 #432390 Dark violet 100 25± ± 6 42± ± 3 –54± ± 3 #432390 Dark violet 200 24 8 22 8 –32 10 #3d2f6c Dark desaturated blue 200 24± ± 8 22± ± 8 –32± ± 10 #3d2f6c Dark desaturated blue 300 62 6 13 1 –19 1 #9E91BB Grayish violet ± ± ± 400300 83625 ± 6 2 13 1± 1 –12 –19 1± 1 #9E91BB #cad0e9 Light Grayish grayish violet blue ± ± ± 450400 85834 ± 5 1 2 ±1 1 –11 –12 1± 1 #cdd6eb#cad0e9 Light Light grayish grayish blue 450 85± ± 4 ± 1 ± 1 –11± ± 1 #cdd6eb Light grayish blue

Moreover, the images were extracted into 4 4 regions, and the L*a*b* information was analyzed, Moreover, the images were extracted into 4 ×× 4 regions, and the L*a*b* information was analyzed, as shown in Figure 12. In Figure 12a, the mean L* values in the 4 4 regions were in the range of 22 as shown in Figure 12. In Figure 12a, the mean L* values in the 4 ×× 4 regions were in the range of 22 to 33 at room temperature and 83 to 84 at 400 C. The mean a* values obtained at room temperature to 33 at room temperature and 83 to 84 at 400 ◦°C. The mean a* values obtained at room temperature were between 47 and 51 in the 4 4 regions, and those at 400 C were between 2 and 3, as shown were between 47 and 51 in the 4 ×× 4 regions, and those at 400 °C◦ were between 2 and 3, as shown in in Figure 12b. The mean b* values obtained in the 4 4 regions ranged from 61 to 58 at room Figure 12b. The mean b* values obtained in the 4 ×× 4 regions ranged from −61 to −58 at room temperature and 13 to 12 at 400 C, as shown in Figure 12c. temperature and −−13 to −12 at 400 °C,◦ as shown in Figure 12c. The total color changes were also calculated based on the L*a*b* values using Equations (7)–(10). The total color changes were also calculated based on the L*a*b* values using Equations (7)– (10). q 2 2 2 ∆D∗ = (∆L∗) + (∆a∗) + (∆b∗) (7) ∆𝐷∗ = (∆𝐿∗) +(∆𝑎∗) +(∆𝑏∗) (7) ∆L∗ = L∗ L∗ (8) 2 − 1 ∆𝐿∗ = 𝐿∗ − 𝐿∗ (8) ∆a∗= a∗ a∗ (9) 2 − 1 ∆𝑎∗ =∆b 𝑎∗∗=−b 𝑎∗ ∗ b∗ (10)(9) 2 − 1 where L1*, a1*, and b1* represent the color∗ values∗ at room∗ temperature, and L2*, a2*, and b2* represent ∆𝑏 = 𝑏 − 𝑏 (10) the values at 400 ◦C. With ∆L* = 57 of the coating surfaces indicating a light color, the change in a*, i.e., ∆a*, was 46, indicating a light shade of green. The change in b*, i.e., ∆b*, was 47, indicating a light where L1*−, a1*, and b1* represent the color values at room temperature, and L2*, a2*, and b2* represent shade of yellow. Thus, the total color change ∆D* of the coating surface was approximately 87. the values at 400 °C. With ΔL* = 57 of the coating surfaces indicating a light color, the change in a*, i.e., Δa*, was −46, indicating a light shade of green. The change in b*, i.e., Δb*, was 47, indicating a light shade of yellow. Thus, the total color change ΔD* of the coating surface was approximately 87.

Coatings 2020, 10, 396 12 of 15 Coatings 2020, 10, x FOR PEER REVIEW 12 of 15

(a) (b)

(c)

FigureFigure 12. 12.Color Color intensities intensities inin 44 × 4 regions on on the the surface: surface: (a ()a L)*L values;* values; (b () ba)* avalues;* values; (c) (bc*) values.b* values. × 5.5. Summary Summary

TheThe fabricated fabricated samples samples with with thethe MnNHMnNH4P22OO77-based-based coating coating were were found found to torespond respond to increasing to increasing temperature.temperature. Up Up to to 100 100◦C, °C, the the coating coating sample sample displayed displayed the samethe same dark dark violet violet color color obtained obtained at ambient at temperature.ambient temperature. As the temperature As the temperature was increased was increased to 200 ◦C, to the 200 sample °C, the showed sample a showed very dark a very desaturated dark bluedesaturated color mainly blue due color to mainly the evaporation due to the of evaporat water particles.ion of water The violet-patternparticles. The colorviolet-pattern disappeared color and changeddisappeared to grayish and changed violet at to 300 grayish◦C. At violet 400 at◦C, 300 the °C. coating At 400 °C, surface the coating completely surface turned completely to light turned grayish blue.to light Further grayish heating blue. to Further higher heating temperatures to higher and temperatures cooling to room and temperaturecooling to room resulted temperature in a stable lightresulted grayish in a blue stable color, light which grayish indicates blue color, irreversible which indicates thermochromic irreversible behavior. thermochromic behavior. TheTheRGB RGBvalues values at ambientat ambient temperature, temperature, which which represent represent a dark a dark violet violet color, color, were almostwere almost constant constant and stable to 200 °C. At 400 °C, when the thermochromic coating lost its darkness and and stable to 200 ◦C. At 400 ◦C, when the thermochromic coating lost its darkness and completely changedcompletely to light changed grayish to blue,light thegrayishRGB blue,values the increased RGB values by167%, increased 567%, by and 167%, 49%, 567%, respectively. and 49%, The respectively. The RGB values also exhibited relatively small deviations at 400 °C, which means that a RGB values also exhibited relatively small deviations at 400 ◦C, which means that a relatively uniform relatively uniform color change was achieved throughout the entire surface. The total change in the color change was achieved throughout the entire surface. The total change in the RGB values, which RGB values, which is defined as the Euclidean distance, showed an increasing trend with increasing is deﬁned as the Euclidean distance, showed an increasing trend with increasing temperature and temperature and reached approximately 231 at a critical temperature of 400 °C. A small change in the reached approximately 231 at a critical temperature of 400 C. A small change in the RGB values was RGB values was obtained when the temperature was increased◦ to above 400 °C. obtained when the temperature was increased to above 400 ◦C.

Coatings 2020, 10, 396 13 of 15

In the L*a*b* color space, the L* value was essentially dark at room temperature but rapidly increased toward the lightness intensity at 400 ◦C. The a* and b* values demonstrated rich redness and blueness, respectively, at room temperature. The L*a*b* values at room temperature remained relatively constant up to 200 ◦C. At 400 ◦C, when the coating surface completely turned to light grayish blue, the L* and b* values at 400 ◦C increased by 211% and 78%, respectively, and the a* decreased by approximately 94%. Similar to those of the RGB values, the standard deviations of the L*a*b* values were also relatively small, indicating that the color change obtained at a critical temperature of 400 ◦C was uniform over the entire coating surface. The total change in the L*a*b* values of the coating from the room temperature to the critical temperature of 400 ◦C was approximately 87.

6. Conclusions This study presented a method for the quick and exact assessment of the temperature proﬁles after an abnormally high temperature was applied to a structure element. A thermochromic coating based on MnNH4P2O7 was fabricated and successfully applied to the mortar surface. The applied coating provided the excellent color changing process from dark violet to light grayish blue when heated to 400 ◦C. In addition to the change in the visual color, the color properties in the RGB and L*a*b* spaces were evaluated as a function of temperatures. This study successfully demonstrated the feasibility and applicability of the thermochromic MnNH4P2O7 coating as a temperature detection technology. When applied to cementitious materials and structures, the thermochromic MnNH4P2O7 coating could provide a tool for determining the temperature information with the color information deﬁned as a function of temperature. In addition, the results of the study indicate that the developed coating holds great potential for applications in electric appliances, jet engines, and automotive sectors in areas of research and development, maintenance, and non-destructive testing.

Author Contributions: R.S.R., J.-H.L., E.C., and J.-W.K. designed and planned the experiments; R.S.R. performed the experiments; J.-H.L., E.C., and J.-W.K. analyzed the data; R.S.R. and J.-H.L. wrote and revised the paper. All authors have read and agreed to the published version of the manuscript. Funding: This research was supported by a grant from the Mid-Career Research Program (NRF-2019R1A2C1006494) through the National Research Foundation (NRF) Korea. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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