Prediction of heat build-up of solar reflecting coatings based on physico-chemical properties of complex inorganic colour pigments (CICPs) (2023)

Progress in Organic Coatings

Volume 72, Issues 1–2,

September–October 2011

, Pages 65-72

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Heat build-up of coated surfaces – such as roofs, facades and other construction elements, as well as fuel tanks, cooler jackets and warehouses – due to solar radiation and the resulting temperature increase of interiors is highly undesirable and can even be dangerous. One of the methods used to decrease the heat build-up of surfaces is the subsequent application of pigmented organic coatings with high IR reflectance properties. Investigations of the morphology, chemical composition and optical properties of complex inorganic colour pigments (CICPs) were performed and the heat build-up of the coatings was measured. The sedimentation method and laser scattering method were used to determine particle sizes and particle size distribution. The composition of pigments and shape of their particles were evaluated by energy dispersive spectroscopy (EDS) and scanning electron microscopy (SEM). Phase analysis was carried out by the X-ray diffraction method (XRD). Spectral characteristics of pigments were measured at wavelengths from 250nm to 2500nm. The total solar reflectance (TSR) was calculated according to ASTM E903. Heat build-up testing was carried out in a box designed and constructed according to ASTM D4803. Mathematical methods were used to describe dependencies between the tested variables. These dependencies may serve as data that allow to predict heat build-up of coated surfaces as a consequence of the absorption of solar energy.


The need to save energy, which comes as a result of the depletion of natural resources and increasing prices, and global warming, has become one of the main topics of discussion for scientists and government agencies as well as in international forums [1], [2], [3]. Much work has been carried out on utilizing renewable energy sources, i.e. solar energy, and reducing energy usage, e.g.: reducing the exterior heat absorption of buildings which leads to lower temperatures inside buildings and lowers the cost of AC use. In both cases, the use of solar selective coatings with high solar reflectance and emittance for “cool materials” and high absorbance and low emittance for solar collectors could be the solution [4], [5], [6], [7], [8], [9], [10], [11].

Terrestrial solar radiation which reaches the ground contains three types of radiation: UV (5%), VIS (42%) and NIR (53%) with respective wavelengths of 280–400nm, 400–700nm and 700–2500nm [6], [12], [13]. The influence of UV high energy radiation is thought to be responsible for the creation of radicals and free electrons, the selective absorption in the VIS range is responsible for the change in chromaticity, and as a result of NIR radiation absorption there is an emission of heat in the far infrared (IRT). One of the methods of preventing heat build-up on the surfaces of structures leading to heat build-up in the interiors is applying a coating with special pigments which reflect infrared radiation [4], [5], [6], [14], [15], [16], [17]. There is already a large selection of pigments which reflect infrared but work is still continuing on expanding their range and on the prediction of solar radiation reflection via the pigments [10], [12], [18], [19], [20], [21], [22], [23], [24], [25].

We can read about many paints with higher reflectivity containing pigments such as:

  • scattering granular pigments—in the case of which it is difficult to achieve a higher reflectivity than 0.3 [20];

  • metal flake pigments with a particle size of 10–50μm—reflectivity is 0.7–0.75 and after modification 0.8–0.85 [26];

  • infrared reflective chromatic pigments obtained by reflective metal flakes and conventional coloured pigments deposited on the flakes or mixed with them; the thickness of the coloured layer must be controlled and cannot be greater than 5μm in order to achieve a reflection factor of >0.7;

  • complex inorganic colour pigments (CICPs), which reflect IR radiation irrespective of the selective light reflection and thus may be in any colour.

CICPs help to achieve an opaque coating and allow other pigments, coloured or transparent, to be mixed in to achieve the desired colour [11]. CICPs also have exceptional colour fastness, resistance to chemicals, organic solvents and temperature; they don’t bleed or migrate in coatings. Chemically, they are synthetic metal oxides with a structure similar to natural minerals such as: rutile (TiO2), hematite/corundum (Fe2O3/Al2O3) and spinel (MgAl2O4). The deciding factor in its structure is the ratio between oxygen and metal: rutile 2.00 (TiO2), hematite/corundum 1.5 (Me2O3) and spinel 1.33 (Me3O4) [18], when metal ions have a comparable size. The type of elements which appear in CICPs responsible for their colour are transition metal ions with a complete orbital-d (V, Cr, Mn, Fe, Co, Ni, Cu). Other metal ions which appear in pigments are colourless and are introduced to balance the charge of a crystal structure or the slight modification of a shade (e.g. Li, Mg, Ti, Ca, Ba, Nb, Mo, W, Zn, Al, Sb). Compounds of this type absorb light relatively poorly which results in their limited, in comparison to other pigments, colour strength. Pigments from the CICPs group which reflect IR are designed to prevent heating of the surface regardless of the colour, thus their growing range of utilization, ranging from their well known use in masking products, interior car paint, materials for spectator areas of stadiums and parking areas to fire-resistant paints. They are also used specifically for the coatings for “cool roofs” and the exterior walls of warehouses.

Describing the influence of IR pigments on the heat build-up on an object surface and the subsequent passing of the heat into the interior, measured by the rise of temperature in comparison to the surroundings, requires quantifying their effectiveness. Solar radiation incident on any structure may be absorbed (a), reflected (ρ) or transmitted (τ). The energy from these three is equal to that of incident radiation [12]:a+ρ+τ=1

Absorption a transforms into heat energy inside the building which appears as a rise in temperature. Measurable values in Eq. (1) are reflectance and transmittance, and in the case of an opaque coating (τ=0) it can be simplified to a+ρ=1. Having the spectral reflectivity measurement ρλ and knowing the spectral distribution of solar radiation energy allows us to obtain weighted-average values of reflectivity ρλ, which is commonly known as the total solar reflectance (TSR). TSR value quantifies solar radiation not absorbed by a surface and is a significant indicator of heat build-up in structures affected by the sun.

The final change in the temperature of the structure in relation to its surroundings is defined by absorption, which is the measurement of energy transferred to the structure, as well as cooling thanks to transmission, convection and radiation. Applying special coatings has little influence on the first two ways of cooling dependent on temperature, geometry and thermal constants. Radiation, described as a movement of heat from a black body to the surroundings, is a process of cooling in which the amount of energy transferred is expressed by the Stefan–Boltzman law and is a function of the fourth power of absolute temperature T4, geometry and emissivity ɛ [6]. With regard to the linear dependency of heat loss and emissivity, materials with a high ɛ value undergo lower heat build-up in comparison to those with low emissivity. Polymers/resins are characterized by a high ɛ value, and heat build-up of coated structures can therefore be controlled only by selecting pigmentation with a high reflectance.

The interaction of pigments with radiation in the range of VIS and NIR, especially reflectance, absorbance and transmittance, is a critical factor when formulating pigmented coatings with specific properties. Optical properties of pigments were characterized using the Kubelka–Munk theory in the VIS and NIR ranges [9], [10], [15], [16]. Brady and Wake [16] classified pigments into six categories on the basis of reflectivity measurements at the wavelength of λ=800nm, separating them into highly reflective, highly absorbent, transparent and in between. In accordance with the K–M theory, scattering or absorption of incident radiation on the coating affects pigment opacity which reduces or eliminates the penetration of radiation to the substrate. In the case of formulating “cool coatings”, the aim is to achieve maximum IR reflection, thus the desired opacity may only be controlled by scattering. Maximum scattering takes place when there is a large difference between the refractive indexes of the pigment and the medium, and the pigment particle sizes are approximately equal in size to half a wavelength of incident radiation. Larger particles therefore improve IR scattering, but at the same level of pigment mass, surface areas and the number of pigment particles are reduced, causing a reduction in opacity. Brady and Wake [16] suggest that determination of radiation scattering through coatings in infrared is an effective method for evaluating pigment dispersion. When IR-reflecting and IR-absorbing pigments are mixed, absorption is preferred and the coating TSR does not achieve an average value. Consistent with the K–M theory, the role of optical absorption increases with scattering reduction. The K–M theory, which functions in colour matching at wavelengths from 400 to 700nm, can be useful in predicting IR curve reflection.

Levinson et al. [9], [10] presented a model for computing the scattering S and absorption K coefficients based on spectrophotometric measurements in a function of solar spectrum wavelengths (300–2500nm). To illustrate this method, many widely used pigments have been used. It has been confirmed that S values measured for a typical white titanium dioxide pigment are approximately consistent with values calculated on the basis of Mie theory, supplemented with a simple model of multiple scattering. Pigments of broad applications, among those which reflect a part of infrared (NIR) solar radiation, e.g. CICPs, were tested. On the basis of test results a database of pigments of varying colour, including browns, blues, purples, greens, and reds, was developed. They reflect between 15% and 40% more solar radiation than conventional pigments, in spite of being nearly identical in colour.

Bendiganovale and Malshe [13] confirm, as a result of implementing a broad programme of tests with the aim of synthesis and developing IR reflective pigments, that the theory of predicting properties such as IR reflection is inconclusive—this property is unpredictable. The one useful technique is direct testing of IR reflectance, because none of the physical properties such as density, transparency or opacity, refractivity, chromaticity, thermal or electric conductivity, is firmly correlated with pigments’ capacity for IR reflection.

The heat build-up of 29 coated steel plates exposed to summer heat in Australia was tested by Reck and Moerk [27]. The authors state that the results of their measurements are firmly correlated with the predicted heat build-up of plates calculated based on laboratory measurements in accordance with ASTM D4803, the spectral reflectivity measurement at wavelength λ=2400nm and lightness L* in the CIE LAB system.

The purpose of our research was to trace the dependence between solar radiation reflection and certain spectral properties, morphology as well as the physical and chemical structure of pigments which could be used for predicting the heat build-up on coated surfaces.

Section snippets


27 Complex inorganic colour pigments (CICPs) of rutile, spinel and hematite structure were used in the tests (Table 1). The pigments tested were divided into five colour groups: yellows, greens, blues, browns and reds. Black pigments of high total solar reflectance (TSR) were also included in the tests, allowing paint formulation for dark colours, which do not cause excessive heat build-up on coated surfaces.

Mono-pigmented coatings of thickness of 80–100μm, demonstrating optimal opacity and

Results and discussion

As a result of the tests conducted, a sizeable number of quantitative variables was obtained (Table 3). In order to find a relationship between them, a correlation matrix was created, containing all possible factors of linear correlation between particular variables (Fig. 1). Correlation analysis gives the same, or even more complete information about the positions of individual variables than discriminatory analysis, because we are working with quantitative rather than qualitative variables.


From the tests carried out we conclude that within certain limits the influence of pigments on heat build-up of coated surfaces can be predicted based on measuring selected parameters, such as lightness (L*), reflectance in the 700–1200nm range (ρs700–1200) or at wavelength 1000nm (ρs1000).

Heat build-up of surfaces, both as calculated and as measured in external conditions, is correlated strongly with TSR, and therefore results of laboratory tests in the heat box permit optimal selection of


The authors wish to express their gratitude to the Ministry of Science and Higher Education for sponsoring this research project.

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