Boosting the NIR reflective properties of perylene organic coatings with thermoplastic hollow microspheres: Optical and structural properties by a multi-technique approach

Boosting the NIR reflective properties of

perylene organic coatings with thermoplastic

hollow microspheres: optical and structural

properties by a multi-technique approach

Ruggeri, 1 _{Marco Geppi,} 1 _{Fabio Bellina,} 1 _{Andrea Pucci}1,*

1_{Dipartimento di Chimica e Chimica Industriale, Università di Pisa, Via Giuseppe Moruzzi 13,}

56124 Pisa, Italy

2_{Cromology Italia S.P.A. Via 4 Novembre 4, 55016 Porcari, Lucca, Italy}

3_{Istituto di Chimica dei Composti Organo Metallici (ICCOM), Consiglio Nazionale delle Ricerche,}

SS Pisa, Via G. Moruzzi 1, 56124 Pisa, Italy Corresponding author:

Prof. Andrea Pucci, Department of Chemistry and Industrial Chemistry, University of Pisa, Via Giuseppe Moruzzi 13, 56124 Pisa, Italy. Email: andrea.pucci@unipi.it

Abstract

This study reports for the first time the use of thermoplastic hollow microspheres (THM) in combination with a perylene bisimide pigment (Paliogen® _{Black L0086 (P-black)) for the}

preparation of acrylic coatings with near-infrared (NIR) reflecting and cooling characteristics. VIS-NIR spectra showed that P-black solid dispersions provide reflectance values in agreement with the P-black content and its crystalline nature. The introduction of THM with average diameter of 25±5 μm with the same weight content of P-black did not substantially affect the crystalline nature of the pigment within the organic coating even if a decreasing of the long range order in the molecular packing of the perylene chromophores emerged from XRD and solid-state NMR investigations. Noteworthy, the addition of the THM successfully contributed in the exponential raising of the total solar reflectance (TSR) of coated black substrates, i.e. reaching up to the 22% at the highest organic layer thickness of 255 μm. The effective combination between THM and P-black was well reflected on the average temperature of the coated surface after IR irradiation, i.e. with a worthwhile cooling effect of about 13 °C already for the lowest layer thickness of 80 μm.

1. Introduction

Human activities are nowadays reported to be partly responsible for the urban heat island (UHI) effect, thus rendering city areas warmer than neighboring rural zones.(Chakraborty and Lee, 2019; Muscio, 2018; Sarrat et al., 2006) Notably, urban regions are densely populated thereby affecting average and peak energy needs particularly all over summer time and depleting energy resources mostly based on fossil fuel so far. Moreover, in EU countries, buildings for residential, industrial and service sectors account for 40% of energy demand and release the 32% of CO2 emissions.

(2014) Notwithstanding the severe requirements for rising the energy efficiency and downsizing greenhouse gas emission, EU renovation progress (about 1.2%) is slower than that planned by the Kyoto and Paris agreement targets.

An available approach to attenuate the UHI effect to satisfy the EU regulation in the 2010/31/EU Directive, is painting the building facades with cool pigments, that is imitating the NIR transparent and reflective properties of certain green plants.(Akbari et al., 2001; Llado et al., 2017) When sunlight illuminates an object, this radiation is partly absorbed, thus warming its surface. This is due to the 52% of the Near Infrared (NIR) component of electromagnetic spectrum of light (700 - 2500 nm) that activates chemical bonds vibration, thus triggering temperature raising at the surface. The heat is then conveyed into the material by conduction, raising the temperature of the internal side by convection. Coatings containing cool pigments are reported as accessible solutions to mitigate the warming outcome provided by the sunlight exposure, and to provide a corresponding cooling effect higher than 10 °C.(Cozza et al., 2015) This approach has been recently reported to reduce the energy demand and therefore beneficial for downgrading the UHI phenomenon.(Jose et al., 2019; Levinson et al., 2005a; Synnefa et al., 2007)

Solar reflectance is an important parameter that determines the amount of NIR radiation reflected by a surface, whereas the thermal emittance estimates how rapidly and efficiently the heat is then dissipated.(Yaghoobian and Kleissl, 2012) Notably, the most important parameter to determine the

cooling effect of a coating is represented by the Total Solar Reflectance (TSR) that is a measure of the amount of incident solar radiation that is reflected by a surface.

Nowadays, NIR reflective coatings are mostly composed by high refractive index inorganic pigments like rare-earth oxides, but their toxicity issues often limit their use and large scale applications.(Jose et al., 2019; Levinson et al., 2005b) Also, titanium oxide is utilized as a good VIS-NIR reflector, but coating degradation possibly occurs due to the strong UV light absorption. Therefore, organic cool pigments represent the most accessible solution owing to their harmless features, low cost and very good compatibility with most polymer coatings. There is now a substantial body of research on perylene bisimide derivatives as cool pigments of organic coatings, thanks to their photo, thermal and colour stability. Moreover, colour modulation in the visible is also provided by selecting different functional moieties at the peripheral units of the perylene nucleus.(Carlotti et al., 2015; Donati et al., 2008; Kaur et al., 2012; Mazhar et al., 2016a; Raj et al., 2017; Würthner, 2004) Notably, perylene derivatives are reported to display NIR transparent and reflective properties thanks to their crystallinity degree and dipole moment of the chromophoric assemplies.(Kaur et al., 2013; Mahmoudi Meymand et al., 2019; Muniz-Miranda et al., 2019) Nevertheless, to date NIR reflectances of cool perylene coatings on black samples are reported to be lower than 40% and efforts are still required to effectively mitigate temperature raising of dark surfaces.

In this work, the use of NIR transparent and reflective perylene derivative, i.e. Paliogen® _Black

L0086 (P-black, Figure 1) was proposed for the first time in combination with thermoplastic hollow microspheres to prepare dark surface coatings with cooling effect and energy savings potential.

Figure 1. Chemical structure of Paliogen® _{Black L0086 (P-black)}

Thermoplastic hollow microspheres are well-known blowing agents and lightweight fillers but are also optimal for developing flexible cool roof coatings.(Sandin et al., 2017) Such characteristics are caused by their hollow nature, their inner concave surface and a large difference in refractive index between the wall and void. This last feature induces Vis-NIR light diffusion and facilitates its scattering on the outer surface of the microsphere. Different organic coatings were prepared as a function of the pigment content and hollow microspheres nature. A transparent acrylic water dispersion polymer utilized in clear varnishes or paints for concrete roof tiles was employed as pigment dispersant in combination with a wetting additive. The NIR reflecting and cooling characteristics of solid dispersions were studied by UV-Vis-NIR spectroscopy and thermal imaging and combined with solid-state NMR and X-ray diffraction investigations to provide an effective structural characterization.

2. Experimental part

Materials

Paliogen® _{Black L0086 (P-black) was provided by BASF and used without further purification.}

Disperbyk 2015 was received from BYK USA Inc. The acrylic water dispersion PRIMAL™ E-822K (50% solid content and glass transition temperature (Tg) of 18 °C) was received from the Dow Chemical Company. Thermoplastic hollow microspheres Expancel 461 WE 20 d36 were obtained from AkzoNobel.

Coating preparation

2 g of PRIMAL™ E-822K were added at room temperature to a desired amount of P-black and Disperbyk 2015 (1:1 by weight), and the mixture mechanically mixed to achieve a homogeneous dispersion. Then, the required amount of thermoplastic hollow microspheres (i.e., from 0 to 10 wt. % with respect to the polymer dispersion) was added with 500 μL of deionized water and the resultant blend was coated over Leneta® _{checkerboard charts by means of an universal applicator}

(ZUA 2000 Zehntner Testing Instruments). Coating films of 60–70 μm of thickness were obtained after drying.

Methods

The solid state NMR study (ssNMR) was carried out on coatings containing the P-black pigment with or without the presence of the thermoplastic hollow microspheres. 1_H-13_C

Cross-Polarization/Magic Angle Spinning (CP/MAS) spectra were recorded on a Varian InfinityPlus 400 spectrometer working at Larmor frequencies of 400.35 and 100.67 MHz for 1_{H and} 13_{C nuclei,}

respectively. All the experiments were performed on a 3.2 mm CP/MAS probehead, equipped with rotors of outer diameter of 3.2 mm, using a MAS rate of 15 kHz. The 1_{H 90° pulse duration was 1.8}

μs.The spectra were recorded using a linear ramp for CP, accumulating 6000-12000 transients with a recycle delay between consecutive scans of 5 s. Different CP contact times (ct) in the range 0.050-2 ms were used, as specified in the text. A SPINAL high-power decoupling pulse scheme was applied on the 1_{H channel during acquisition of} 13_{C signal, using a} 1_{H decoupling field of 87 kHz.}

All the spectra were recorded at room temperature using air as spinning gas. The 13_{C chemical shift}

scale was referred to tetramethyl silane and hexamethylbenzene as primary and secondary references, respectively. UV–Vis–NIR reflectance of organic coatings were recorded according to a previously reported procedure.(Muniz-Miranda et al., 2019) The total solar reflectance (TSR) index was computed by integrating the measured spectral data weighted with the air mass 1.5 beam-hemispherical solar spectral irradiance for 37° sun-facing tilted surface, as described in ASTM G173-03. Typical values of TSR are 0.9 for white surfaces and 0.04 for black coatings. X-ray

powder diffraction (XRD) was performed using a Bruker D2 Phaser diffractometer operating in Bragg-Brentano geometry and equipped with a 1-dimensional Lynxeye detector. Experimental details are reported in the literature.(Zanchetta et al., 2017) The coatings thickness was measured with a CM1S dial indicator (Borletti, Milan, Italy).

A 100W IR lamp (Kerbl, Buchbach, Germany) was used to irradiate the organic coatings. A FLIR™ E6 infrared thermo-camera (FLIR, Wilsonville, OR, USA) was utilized to measure the temperature of the surface of the coatings.

3. Results and discussion

Influence of P-black content on coating reflectivity

Pigment water dispersions were coated at different thickness (from 75 to 255 μm) over the white and black substrates of Leneta® _{checkboards.(Muniz-Miranda et al., 2019)}

100 80 60 40 20 0

R

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wavelength (nm)

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50 40 30 20 10 0

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Figure 2. UV-VIS-NIR reflectance spectra of P-black coatings on white (a) and black (b) charts. Thickness of the coating layer = 160 μm.

UV-VIS-NIR reflectance spectra recorded up to 2500 nm on the white substrate (Figure 2a) show a reflectance behavior that results influenced by the pigment concentration, i.e. particularly pronounced up to 1300 nm, and in accordance with the optical features of the P-black chromophoric unit.(Mazhar et al., 2016b) Since white substrates with reflectance values as high as 80% are usually associated to NIR transparent pigments,(Meymand et al., 2019) the 10 wt.% of P-black concentration represent a good trade-off between substrate coverage and optical characteristics. The NIR reflective properties of P-black emerged by examining the reflectance behaviour coated black substrates (Figure 2b). Notably, reflectance values progressively increased in the 800-1200 nm wavelength interval up to about 48% on passing from 5 wt.% to the maximum of 20 wt.% of P-black content. This result is particularly worthwhile since the interested NIR region represents about 50% of the energy of the NIR component and 25% of the total solar energy.(Kaur et al., 2012) NIR reflectivity has been effectively attributed in the literature by the crystalline nature of symmetrically substituted perylene bisimides derivatives.(Kaur et al., 2012) The X-ray diffractogram reported in figure S1 and 3c displays sharp and intense peaks revealing the crystalline phase of P-black, flanked by baseline deviations in the higher theta regions that are generally attributed to the amorphous phase pigment content. This contribution is also flanked by the

presence of broad peaks at high 2θ angles in the XRD patterns (20-28°) that are addressed to a certain degree of local disorder in the molecular packing of the perylene chromophores.(Raj et al., 2017)

Influence of thermoplastic hollow microspheres on the coating structure and reflectivity

With the aim to boost the reflective properties and the cooling characteristics of P-black coatings on black substrates without exceeding in pigment substrate coverage, thermoplastic hollow microspheres (THM) were added to the 10 wt.% of P-black formulations. Inspections of the optical micrographs of the P-black coatings containing the 10 wt.% of THM (i.e., 1:1 by weight with respect to the pigment) revealed a more textured surface (Figure 3b) with respect to the smooth one that characterizes the bare pigment coating (Figure 3a). Moreover, the presence of the THM rendered the black coating thicker of about 10 μm and particularly brighter (Figure 3b), thus suggesting their effective role in reflectivity enhancement.

Figure 3. Optical micrographs of coatings composed by a) 10 wt.% of black and b) 10 wt.% of P-black + 10 wt.% of THM. c) FE-SEM of 10 wt.% of THM dispersion. d) XRD spectra of P-P-black and P-black + THM dispersions (1:1 by weight)

The scanning electron micrograph of the THM-based coating (Figure 3c) revealed an homogeneous distribution of the hollow microspheres with average diameter of 25±5 μm, without the presence of highly interacting particles or large scale aggregates. Notably, this result definitely indicates a very good compatibility of the THM with the selected acrylic polymer dispersion that effectively coated the microspheres being the thickness enhancement limited to 10 μm only. The P-black coating doped by the THM was again investigated by X-ray diffraction (Figure 3d). The spectrum revealed the same sharp and intense reflections at low theta regions of the bare pigment coating (P-black), whereas an increase of the broad and less defined peaks between 20 and 28 degrees occurred after the addition of THM (P-black+THM). Apparently, the addition of the THM did not affect the overall crystalline fingerprint of P-black. Nevertheless, the concomitant increase of the reflections at high theta regions and the reduction of that below 5° could be possibly addressed to a decreased long range order in the molecular packing of the perylene chromophores.

Further investigation on this last aspect were performed by means of 13_{C ssNMR high-resolution}

experiments. Indeed, ssNMR spectroscopy can be extremely helpful, and complementary to other techniques, for obtaining detailed information on the structure and dynamics of solid materials. (Geppi et al., 2008) However, to the best of our knowledge, its application in the field of NIR-reflective pigments has been very limited so far. In solid systems, 13_{C isotropic chemical shift are}

extremely sensitive not only to the chemical environment related to the molecular structure, but also to conformational properties and inter-molecular interaction, within a spatial length of a few Angstroms, related to the supra-molecular packing. Therefore, crystalline and amorphous phases, as well as different crystal structures, should give rise to different high-resolution 13_{C ssNMR spectra.}

In Figure 4 the 13_{C CP/MAS spectra of the pure P-black pigment and of two NIR reflective coatings}

that due to the characteristics of the CP pulse sequence, which is based on a magnetization transfer from 1_{H to} 13_{C nuclei,} 13_{C CP spectra are not quantitative, and only relative comparisons among}

different samples can be done.

Figure 4. (a) 13_{C CP/MAS spectra of, from bottom to top, the pure P-black pigment and the}

coatings containing 10 wt.% of P-black in the absence (10 wt.% P-black coating) and in the presence of THM (P-black +10 wt.% THM coating). (b) Expansions of the aromatic spectral region of the 13_{C CP/MAS spectra shown in (a). The assignment of the main carbon signals of the acrylic}

polymer and of the P-black chromophore is also reported. The labeling of carbon atoms is shown on top of the figure.

In the spectra of coatings the most intense signals are those arising from the acrylic polymer carbons, for which a possible assignment is reported (Figure 4a). Although the exact composition of the acrylic polymer dispersion is not known, based on previous literature(Ishida et al., 1999; Souto-Maior et al., 2005; Wilhelm et al., 1999) typical signals of poly(butyl acrylate) and poly(methyl methacrylate) seem to be present. After the addition of 10 wt.% of THM, the position, linewidth and relative intensity of these signals remain the same as those observed for the coating without

THM, indicating that the presence of the hollow microspheres does not influence the phase properties of the final polymer film. It can be noticed that the signals of THM are not clearly observable in the spectrum of the coating, probably because they are too weak or lie underneath those of P-black or acrylic polymer carbons. Very broad signals ascribable to THM can be observed around 27, 36 and 46, 90 and 175 ppm in the spectrum of a 1:1 (wt:wt) physical mixture of P-black and THM (Figure S2). These signals are those expected for an acrylonitrile-vinylidene chloride methyl-methacrylate copolymer, which is known to be the main constituent of the THM shell.(Liu et al., 2015; Liu et al., 2013)

However, our main interest is focused on the structural and phase behavior of the P-black pigment, for which a 13_{C solid-state NMR spectrum has never been reported in the literature. In the} 13_C

CP/MAS spectrum of the pure pigment (Figure 4a and b) it is possible to recognize the signals of the perylene bisimide core and of the 4-methoxybenzyl substituents at the bay positions. The aromatic spectral region, an expansion of which is shown in Figure 4b, is characterized by a complex overlapping of signals, arising from the aromatic carbons of the perylene core and of the benzyl groups. A full assignment of this region would require additional selective 13_{C experiments,}

and it is not the aim of this work. Here it is sufficient to say that the number and the position of the different peaks are not explainable with the sole molecular structure, indicating that the supra-molecular packing and inter-supra-molecular interactions must play an important role in determining the

13_{C spectral features. For example, at least two distinct signals at about 111 and 117 ppm are}

observed for the 14, 14a, 19 and 19a carbons, while a single peak would be expected on the basis of the molecular structure. The two signals could be due to carbons belonging to the same substituent or to two distinct substituents, bonded to the opposite halves of the molecule, feeling different chemical and structural environment. Indeed, both the cases are possible on the basis of the crystalline structure.(Hädicke and Graser, 1986; Klebe et al., 1989) In the same spectral region a weaker and broader peak at the intermediate chemical shift of 114 ppm is also visible, which could be due to molecules in a more disordered environment, probably at the edges of the stacked

structures. An additional consideration to be done by looking at the 13_{C CP/MAS spectrum of the}

pure pigment is that there is not a clear evidence of the presence of two distinct amorphous and crystalline phases. Instead, it can be noticed that the spectral lines are not narrow as those expected for a perfectly ordered crystalline phase, but a distribution of isotropic chemical shift seem to be present. This is attributable to a certain degree of local disorder in the molecular packing of the perylene chromophores, which could also explain the presence of the broad peaks at high 2θ angles in the XRD patterns. Passing to the spectra of the coatings the 13_{C signals of P-black remain}

practically unchanged with respect to those observed for the pure pigment (Figure 4a and b); this indicates that the mixing procedure and the film formation process do not induce modifications in the supra-molecular packing and phase properties of the perylene bisimide chromophores, neither in the presence nor in the absence of THM.

The reflectivity features of the P-black coatings doped by THM were then studied. Notably, UV-VIS-NIR spectra of 10 wt.% of P-black coatings over the white substrates revealed an increased reflectivity caused by the addition of the THM, that is more pronounced in the 800-1200 nm wavelength region for the 10 wt.% content (Figure 5a). This result agreed with the NIR transparent features of the THM (Figure S3) and confirmed the brightening effect of the pigment coatings as shown by optical microscopies (Figure 2a-b).

Strong reflectivity enhancement was also observed in the UV-VIS-NIR spectra of THM-doped coatings over black substrates (Figure 5b). It was worth noting that the addition of 5-10 wt.% of THM to the 10 wt.% of P-black coatings boosted the reflectance of more than 40%. Moreover, in the case of P-black coatings doped with the highest amount of THM (i.e., 10 wt.%), reflectance values higher than 30 % were recorded for all the investigated NIR region, possibly thanks to the effective and synergic combination between the P-black and THM optical features (Figure 2b and S3).

a)

b)

Figure 5. UV-VIS-NIR reflectance spectra of 10 wt.% P-black coatings filled with THM at different content (5 and 10 wt.%) on white (a) and black (b) charts. Thickness of the coating layers = 160 μm and 170 μm for the bare P-black and the P-black+THM, respectively.

Aimed at quantifying the cooling effect provided by the pigment coatings over black substrates, total solar reflectances (TSR) were measured according to the ASTM G173-03 (Standard Tables for Reference Solar Spectral Irradiances: Direct Normal and Hemispherical on 37° Tilted Surface), as detailed reported in the experimental part. Normally, TSR of about 6-7 % are characteristics of black or dark pigmented surfaces.(Levinson et al., 2005b) Coating black surfaces with the 10 wt.% P-black dispersion, TSR values around 10% were calculated, i.e. in agreement with the performance

declared by the producer.(Company) It was worth noting that the addition of 10 wt.% of THM contributed in a striking increase of the TSR up to values of 22%. The cooling characteristics provided by the 10 wt.% P-black dispersions over black substrates were then determined as a function of the coating thickness by measuring the average temperature of the surface (Table 1) after irradiation with a 100 W IR lamp for 15 min.

Table 1. TSR values of 10 wt.% P-black coatings on black substrates and average temperatures measured after irradiation with a 100 W IR lamp for 15 min.

Entry Thickness (μm) TSR (%) Average temperature (°C)

Black substrate - 5 61.8 P-black 75 9 56.6 130 10 51.1 160 11 50.1 235 13 49.3 P-black + 10 wt. % THM 80 13 49.2 150 18 46.3 170 19 46.1 255 22 45.7

*the different coating layers between P-black and P-black + 10 wt.% THM samples were due to the presence of THM, only

Notably, the 10 wt.% P-black dispersion cooled down the average temperature of the bare black substrate from 61.8 °C to 49.3 °C for the thickest coating layer (235 μm, Figure S5e). This temperature gap of 12.5 °C agrees well with TSR variations of 8%, possibly caused by the increased number of scattering pigments in the coating layer that maximized its reflectance to about 38% (Figure S4c). This behavior was confirmed by examining the same parameters from the coating layers doped by the 10 wt.% of THM. Notably, maximum TSR of 22% and temperature decreasing of 16.1 °C were recorded for the thickest coating layer (255 μm, Figure S6d) with a record reflectivity of about 50% (Figure S4d).

This peculiar trend appeared more evident by inspecting the plots reported in Figure 6. The addition of the THM strongly contributed in the TSR exponential raising with coating thickness, i.e. with a synergic effect in combination with the P-black pigment (Figure 6a). This effective combination was reflected on the average temperature of the coating after IR irradiation, i.e. with a worthwhile cooling effect already at the lowest layer thickness of 80 μm.

Figure 6. (a) TSR and (b) average temperature variations as a function of the thickness of the 10 wt. % P-black coatings filled with and without 10 wt.% of THM. The experimental data were fitted with an exponential function. The temperature of the coating surface were measured after IR irradiation with for 15 min.

Conclusions

This work demonstrated the potentiality offered by the effective combination of P-black and THM in providing black surfaces with cooling features thanks to the NIR reflective properties of the organic acrylic layer. P-black confirmed its NIR transparent and NIR reflective properties when dispersed in solid acrylic coatings with maximum TSR values of 13% for the thickest layer of 235 μm thanks to its crystalline nature. This feature was not substantially perturbed after the introduction of equiponderal amounts of THM with respect to P-black pigment, as evidenced by structural studies combining XRD and solid state NMR spectroscopies. A striking raising of the overall reflectance of the black solid substrate occurred with increasing the thickness of the organic layer composed by the P-black/THM mixture in the acrylic dispersion. Maximum reflectivity of about 50% was gathered from the thickest coating of 255 μm over black substrates. This substantial value boosted TSR up to 22%, while providing a cooling effect of about 16 °C.

This work provides new insights on the development of modern paints and coatings that can offer a dark and colored coverage to several substrates without contributing to their heating during light exposure.

Acknowledgements

This work was supported by the Tuscany region POR FESR 2014-2020 COOLSUN - New NIR-reflective “cool” organic pigments.

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Supporting information of the manuscript “Boosting the NIR reflective properties of perylene

organic coatings with thermoplastic hollow microspheres: optical and structural properties by a multi-technique approach” by Pierpaolo Minei,1 _{Marco Lessi,}1 _{Luca Contiero,}2 _{Francesca Martini,}1

Silvia Borsacchi,3 _{Giacomo Ruggeri,} 1 _{Marco Geppi,} 1 _{Fabio Bellina,} 1 _{Andrea Pucci}1,*

1_{Dipartimento di Chimica e Chimica Industriale, Università di Pisa, Via Giuseppe Moruzzi 13,}

56124 Pisa, Italy; 2_{Cromology Italia S.P.A. Via 4 Novembre 4, 55016 Porcari, Lucca, Italy;} 3_Istituto

di Chimica dei Composti Organo Metallici (ICCOM), Consiglio Nazionale delle Ricerche, SS Pisa, Via G. Moruzzi 1, 56124 Pisa, Italy

Figure S2. 13_{C CP/MAS spectra of the pure P-black pigment (bottom) and of a 1:1 (wt:wt) physical}

mixture of the P-black pigment and THM (top). The asterisks mark the rotational sidebands due to MAS, while the arrows indicate the signals arising from THM. It can be noticed that these signals are very broad in agreement with the amorphous character of THM.

Figure S3. UV-VIS-NIR reflectance spectra of THM dispersions over white (solid lines) and black (dashed lines) Leneta® _{checkerboard charts. Thickness of the coating layers = 170 μm.}

(a)

(b)

(c)

(d)

Figure S4. UV-VIS-NIR reflectance spectra of respectively 10 wt.% P-black coatings with or without the 10 wt.% THM over white (a,b) and black (c,d) Leneta® _{checkerboard charts as a}

(a)

(b)

(c)

(d)

(e)

Figure S5. Thermographs recorded on 10 wt.% P-black coatings on black substrates as a function of the coating thickness: (a) bare black checkboard (b) 75 μm; (c) 130 μm; (d) = 160 μm; (e) = 235

(a)

(b)

(c)

(d)

Figure S6. Thermographs recorded on 10 wt.% P-black+10 wt.% THM coatings on black substrates as a function of the coating thickness: (a) 80 μm; (b) 150 μm; (c) = 170 μm; (d) = 255 μm.