Progress in Organic Coatings
Volume 72, Issues 1–2,
September–October 2011
, Pages 65-72
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Abstract
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.
Introduction
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:
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scattering granular pigments—in the case of which it is difficult to achieve a higher reflectivity than 0.3 [20];
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metal flake pigments with a particle size of 10–50μm—reflectivity is 0.7–0.75 and after modification 0.8–0.85 [26];
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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;
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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]:
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
Samples
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.
Summary
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
Acknowledgment
The authors wish to express their gratitude to the Ministry of Science and Higher Education for sponsoring this research project.
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This paper presents a series of passive intelligent cooling materials with different colors, which can be used to alleviate the heat island effect applying in buildings. The cooling composites with intelligent discoloration and reflectance adjustment were prepared based on low density polyethylene (LDPE). Inorganic particles with different colors and thermosensitive black were introduced into LDPE to satisfy the requirements of color and cooling properties. The chromatic aberration values measured at 25°C and 35°C indicate that the variation of color appears on LDPE composites with intelligent color. Taking advantage of the 31°C discoloration of thermosensitive black, the color of the samples tends to be dimmed at low temperature due to the thermosensitive black appears black, which weakens the reflectance ability of sunlight reducing cooling property and achieves partial heat retention. Above 31°C, the color of thermosensitive black fades causing the samples to develop color. The bright colors bring higher solar reflectance reducing heat to achieve intelligent cooling. In the outdoor cooling property test, the final temperature of the glass is 65.2°C while all samples reduce the temperature to 45.6°C–50.6°C, performing excellent cooling property. In addition, after thermal aging at 80°C for 48h as well as cold and thermal cycle tests (0°C–80°C) for 10 times, the composites still have good reversible thermochromic behavior showing excellent thermal aging, durability and reproducibility.
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Citation Excerpt :
Besides, the environmental protection cooling roof is also widely concerned to alleviate this problem. The energy of sunlight consists of three parts, including ultraviolet (wavelength range 280–400 nm), visible (400–700 nm), and near-infrared (700–2500 nm) bands, accounting for 5%, 43%, and 52% respectively [11]. Infrared radiation is the main source of heat and most of the energy is transferred to the building surface [12].
Building energy consumption is an important part of energy consumption. Cooling materials can be applied to reduce building energy consumption. In this study, chromium trioxide (Cr2O3) and titanium dioxide (TiO2) were introduced into low-density polyethylene (LDPE) matrix to prepare the solar-infrared reflective layers, then styrene–butadiene-styrene triblock copolymer (SBS) and surface functionalized nano-silicon dioxide (SiO2) superhydrophobic coating was constructed on the surface forming bilayer composites applying for building roof. It was found that SBS/SiO2 coating had higher roughness, which provoked a soaring contact angle of about 160° realizing superhydrophobic surfaces. Withal, the contact angle was invariant nearly 160° after dropping the solution of calcium chloride (pH=6), acid (pH=2), and alkali (pH=13) on the surface, which engendered chemical stability and anti-pollution property. Subsequently, the emissivity of LDPE and 10wt% Cr2O3 composites coated with SBS/SiO2 coating (LDPE/Cr-10/S) as well LDPE and 10wt% TiO2 composites coated with SBS/SiO2 coating (LDPE/Ti-10/S) were increased to 83% and 83.9% along with high near-infrared reflectance was 47% and 63.1% separately. Simultaneously, when the initial temperature was 24°C for 1h under strong light irradiation in indoors, the final temperature of glass was 69°C while the finished temperature of LDPE/Cr-10/S and LDPE/Ti-10/S were reduced to 30.5°C and 25.8°C respectively, showing the excellent cooling property.
Synthesis and characterization of CoFe<inf>2</inf>O<inf>4</inf> thin films for solar absorber application
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Spinel oxides synthetized by the electrochemical route can be used in different technological areas. In the present work, cobalt ferrite (CoFe2O4) thin films were deposited from sulfate bath on FTO-coated glass substrate by linear sweep voltammetry technique (LSV). We have investigated the effect of applied magnetic field B (AMF B) up to 0.5T on the electrochemical response, the crystallographic texture, the morphology, the semiconductor type and the optical properties of the elaborated films. The structural and compositional analysis confirmed the synthesis of a well-crystallized CoFe2O4 with a good stoichiometry after an annealing treatment at 500°C. Moreover, the CoFe2O4 films exhibited a p-type semiconductor as confirmed by the photo-electrochemical and Mott-Schottky tests. In addition, an increasing in the absorbance in the visible-UV region is observed under AMF B. Consequently, the magneto-electrodeposited CoFe2O4 thin films have a direct band gap energy of 1.5eV.
New environmental-friendly yellow pigments Y<inf>4–x</inf>A<inf>x</inf>MoO<inf>9+δ</inf> (A =Ta, Tb)
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Citation Excerpt :
Thus, it is optional to choose inorganic pigments with high NIR reflectance as coolers to lower solar energy absorption, which can save the cooling energy consumed by air conditioning in hot seasons.6–8 A wide area of applications of NIR reflective pigments is to be found in the thermal protection of surfaces especially in chemical containers to cool the liquid inside, and in coatings of roof tops, vehicles and storage compartments to decrease the utilization of air conditioning.9,10 Therefore, it is necessary to develop the cool inorganic pigments with high near infrared solar reflectance.
A series of inorganic yellow pigments with general formula Y4–xAxMoO9+δ (A=Ta, Tb), where x=0, 0.05, 0.1, 0.2, 0.4 for Ta and x=0, 0.005, 0.01, 0.03, 0.05 for Tb, were synthesized by a conventional ceramic method at 1400°C for 6h in air. The samples were characterized by XRD, EDS, XPS, SEM, TG-DSC, UV-vis-NIR reflectance spectroscopy and CIE L*a*b* color scales. It is found that the substitution of A (A=Ta, Tb) for Y3+ in Y4MoO9 decreases the NIR reflectance of the pigment samples, but the developed pigments Y4–xAxMoO9+δ (A=Ta, Tb) still exhibit impressive NIR solar reflectance. The brighter yellow color of inorganic pigments Y4–xAxMoO9+δ (A=Ta, Tb) is available when x is about 0.1 for Ta and 0.01 for Tb. The results make them a series of potential candidates as ecological yellow pigments because of their high reflectance, lightness, intense coloration and excellent thermal and chemical stability.
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Citation Excerpt :
In the past decades, solar energy conversion has been one of the most interesting research topics due to energy crisis and environmental issues. Solar-thermal process encompass a wide variety of technologies, most of them start by converting the incident sunlight into heat [2]. Devices which can collect solar-thermal energy are called solar collectors.
In this study, cupper chromite Spinel (CuCr2O4) pigment was synthesized via two-step combustion–calcination procedure. Mixtures of citric acid and glycine were used as fuel and nitrate ions were used as oxidizers. The samples were synthesized at different pHs. The synthesized sample was characterized by x-ray diffraction, scanning electron microscopy, Fourier-transform infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC), thermogravimetric analysis (TG) and energy-dispersive X-ray spectroscopy (EDX). The X-ray diffraction (XRD) patterns of the samples revealed that the single-phase CuCr2O4 spinel was formed. The SEM results showed that the spinel particles were synthesized in octahedral morphology. Furthermore, pH and amount of fuel have remarkable effect on size of particles. Increasing the pH leads to increasing the size of particles. Also, the size of particles was significantly increased by doubling the amount of fuel. In the next step, the synthesized pigments were used as spectrally selective absorber in solvent-based acrylic coatings. The functional groups of the coatings were identified by the attenuated total reflectance (ATR) analysis. The solar absorptance (αs) and the thermal emittance (εT) of the coatings were determined from the corresponding diffuse reflectance spectra. The optimum selective sample were calculated αs = 0.9 and εt = 0.3.
Preparation and thermal reflectivity of nickel antimony titanium yellow rutile coated hollow glass microspheres composite pigment
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Citation Excerpt :
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