Heat reflective coatings

Excerpt: This review article focusses on various aspects of the heat/IR reflective coatings, particularly decorative applications. The types of pigments, processes, mechanisms, advantages etc are highlighted

Research & Technology Centre, Asian Paints Limited,

Plot No. C3-B/1, TTC MIDC, Pawane, Thane – Belapur Road, Navi Mumbai,

Maharashtra 400 703, India

ABSTRACT

Paints and coatings offer various advantages along with the aesthetic value to the substrates to which it is applied. Binders and pigments majorly contribute to various advantages of the coatings. Particularly, pigments contribution is obviously for color and opacity to the paint. Apart from color and opacity the properties such as particle size, shape, and chemistry determines the performance of the coatings. Pigments with exceptionally high light fastness, heat/IR reflectance properties are essential for exterior coatings. Preferably the roofs which are exposed most to the solar radiation impact on the high heat build-up, and add to the load on cooling. To overcome such issues mixed metal oxide/complex inorganic pigments are generally used in heat/IR reflective coating. These pigments with high total solar reflectance (TSR), results in reflecting the IR radiation and keep the surface or interior cooler. Thus they contribute to energy savings, sustainability, provide long life to coating as well as the buildings. This review article focusses on various aspects of the heat/IR reflective coatings, particularly decorative applications. The types of pigments, processes, mechanisms, advantages, and contribution to the sustainability are highlighted.

Introduction

Paints/coatings are one of the most consumed materials in the market. These are very essentials for automotive, decorative, industrial applications etc. for aesthetic and protective purpose[1]. Paint is the mixture of pigments/pigment dispersion, binder, solvents, extenders, and a small amount of additives. These ingredients determine the final properties of the paint/coatings. Every ingredient will play its respective role to achieve the desired performance. Pigments impart color to the surface to which paint was applied. Pigments are largely classified as inorganic or organic in nature, and these are insoluble in water or the organic continuous phase in which they are dispersed[2]. Appropriately dispersed pigment dispersion imparts acceptable coloristic values, better hiding, and mechanical, properties to the coating[3,4]. The characteristic of the color is mainly due to the selective absorption or reflectance of the visible region of solar light. Conventional color paints results large heat build-up at the roof/wall surface due to the absorption of the IR/NIR radiations of the solar energy and which percolate into the building leading to increased inside temperature. Hence maintaining the building cooler is expensive due to high energy requirements. Certain pigments apart from selective absorption or reflectance of the visible light to impart color, also impart special properties to the coatings such as reflection of infrared (IR), and/or near IR (NIR) radiations[5-7]. Since these pigments reflect the IR/NIR radiations the heat build-up at the surface and surface's temperature will be significantly low. Development of such pigments was a need to fulfil the demand of the customers from a sustainability point, to enhance the durability of the coatings, energy savings etc.

Most of the total solar energy is absorbed by our atmosphere and never reaches the Earth's surface. The solar spectrum spans a wide range of wavelengths, the electromagnetic radiations that reaches the earth surface ranges from 295-2500 nm in wavelength. While the human eye is sensitive to only a small part of the electromagnetic spectrum, whereas interactions of pigment particles with wavelengths outside the visible range can result in interesting effects on coating properties. Apart from the visible range, the key range of spectrum is the IR, specifically the NIR region. This spectral region is the major contributor for the heat built at the surface[6]. Since the roof is generally horizontal or slightly tilted, most of the solar radiation falls on it. Roof is the contributor/transmitter of the heat to the building and the indoor climate[8]. Due to the heat build-up, in air conditioned buildings the energy consumption is also a concern[9]. There are various methods to overcome heat build-up in buildings as shown in Figure 1[10]. Architectural methods involve maintaining the proper configuration and the appropriate geometry of the roof which can provide better heat passivation. These methods have to be incorporated during the construction of the buildings. The other method, non-architectural approach, involves use of materials which can prevent heat build-up or reflect the solar radiation. These can be installed/applied to the existing buildings. One of the non-architectural methods to control heat built-up is through reflective roof coatings containing pigments which can reflect solar spectrum.

One of the ways to increase IR reflectivity in paints/coatings is by the use of white pigments like titanium dioxide (TiO2)[11], hollow microsphere [12], glass beads [13] etc. TiO2 reflects the visible and infrared radiations of the solar spectrum[14,15]. However, the key challenge is to have paints with IR/heat reflective coatings with the color. The colored paint should have pigments characteristics such as selective reflectance in the visible to produce color and complete reflectance in the IR for coolness. In order to fulfil these demands, various manufacturers have developed highly engineered pigments which are generally known as IR-reflective pigments [Appendix]. The challenge for the IR reflective color pigments is that, visual color has to be at par with the conventional counterpart [16]. This will enable the formulator to match the tone/shade of the paint as well. Figure 2 shows the selected shade for the comparison of IR reflective and conventional pigments. No significant difference was found in the visual appearance of the shades.

Table 1 (see the Appendix) show some of the major manufacturers with their products for the IR reflective coatings [17-21], and the list is not limited w.r.t vendors and their products. These pigments with various colors allow the formulation of paints/coatings tomeet the need for IR reflectivity, long-term durability along with deep and rich colors. Apart from the pigments, there are various suppliers such as BASF [17], Clariant [22], Chromaflo [23] etc. offering IR reflective pigment preparations/dispersions for ready use.

Solar spectrum and heat build up

The concern is due to the exposure of the coatings to the solar spectrum particularly to the Ultra Violet (UV), and IR region. Due to the IR radiation and the local heat build-up, the quality of the paint deteriorates. Not just the deterioration of the quality and durability of the paint, the heat built at surface increases the inside temperature as well [8]. Hence, it is required to keep the outer surface cool. The surfaces with white color tend to reflect most of the radiation from the solar spectrum (UV, visible and IR), whereas colored coatings reflect only a part of the radiation. In general, surfaces coated with dark colors absorb large amount of radiation in the visible region. Paints or coatings that look opaque in the visible area might still have transparency in the IR region, there are few ways to maximizing the IR reflection by a) increasing the thickness of the coating, b) increasing the loading of the pigment in the coating, or c) use of IR-reflective pigments.

As shown in Figure 3, the solar spectrum is mainly divided into 3 spectral regions UV, VISIBLE, and IR[16]. The ultraviolet region covers the spectral range of 295 – 400 nm. UV accounts for about 5% of the sun's energy that reaches the earth's surface. The UV radiations are of high energy and are sufficient enough to break several organic linkages such as polymers [1]. It is responsible for degradation of binder of the paints/coatings and hence deteriorates the performance and durability.

The VISIBLE region of the solar spectrum covers 400 – 700 nm, and accounts for ~ 42% of the Sun's energy. White materials reflect all the VISIBLE light, and that of black materials absorb all the VISIBLE light. Whereas, colored materials/pigments selectively absorb the VISIBLE light and reflect the remaining e.g. a green pigment absorbs all wavelengths of the VISIBLE range except green and hence it appears as green.

The IR spectrum ranges from 700 - 2,500 nm, and accounts for ~ 50% of the total solar energy. Figure 3 evidences that the majority of the energy is in the IR range of 700 - 1,200 nm. The absorption of these IR radiation/energy leads to heat build-up at coated surfaces [7,24]. Use of appropriate IR reflective coatings can minimise the absorption or reflect most of the IR radiation and thus keep the surface cool. In case of white and light colored surfaces much of IR radiation is reflected, for darker colors this is not always true. Hence the IR reflective pigments/coatings have been in use to enhance the reflectance of the IR radiation, and reflect selected wavelength in VISIBLE region to impart characteristic color.

Total Solar Reflectance (TSR)

Total Solar Reflectance (TSR) describes how much of the sun's energy an object reflects. Higher the TSR of the pigments betters will be the reflectance of the IR radiation. The TSR of a pigment/pigment dispersion/coating can be quantified as below:

% TSR = (ʃ (% R * Idλ) / ʃ Idλ) * 100

Where: R = reflectance percentage; I = solar irradiance; dλ = wavelength interval of integration (300 to 2500nm).

Typical white coating has TSR ≥ 75% and absorbs 25% of incident solar energy. Black coating based on carbon black pigmentation has TSR ~ 4% and absorbs ~ 96% of incident solar energy[6].

Types of IR reflective pigments

Inorganic Pigments

An exhaustive survey of the IR reflective inorganic pigments is available elsewhere[7]. Varieties of inorganic IR reflective pigments are tabulated in Table 1 (Appendix). Rutile TiO2 is the widely used white pigment in the manufacture of paints and plastics. It is chemically inert, insoluble, and exhibit good heat resistance[25]. Due to its chemical characteristics the surface reflectance through refraction and diffraction of the light is much higher. When a ray of light passes through a TiO2 particle, the physical phenomenon like the bending, or refraction occur because light travels more slowly through the pigments than it does through the binder. The film containing the pigment with higher refractive index bends the light more than the film containing the lower refractive index pigment as shown in Figure 4[26]. Since the reflectance of the surface increased the surface opacity also increases through refraction induced by the TiO2 particles [2,26].

There are pigments suitable for IR reflective applications for various coatings. Generally, the conventional black pigments like carbon black, black iron oxide and copper chromite black absorb most of the solar energy with a total solar reflectance (TSR) of ~5-6 %[27,28]. IR reflective black pigment results in higher reflectance of the IR region without compromising with the reflectance of the visible light. Figure 5 shows comparison of reflectance curves for several types of black pigments at varied TSR values [28]. With increase in the TSR value, the per cent of reflectance increased and hence a cooler surface can be expected along with less degradation reactions and therefore a long lasting coating.

A correlation between TSR value of some black pigments and the surface temperature can be established as shown in Figure 6. Higher the TSR of a pigment the lower will be the roof surface temperature and hence the cooler the interior of the building. This figure indicates that for 10% increase in TSR value there will be a drop in surface temperature by ~ 5 °C [29].

There are pigments which are being commonly used have good TSR values e.g. phtalocyanine blue; however, the IR reflectance is poor. Figure 7 shows comparison of reflectance curves of inorganic cobalt blue and the organic phtalocyanine blue. It is evident that the cobalt blue reflects more energy in NIR region (700 – 1100 nm) compared to phtalocyanine blue [30]. Though, the phtalocyanine blue has larger reflectance range of the spectrum, cobalt blue reflects more energy in the IR region, and hence less heat build-up in coatings with cobalt blue when compared to coatings with phtalocyanine blue. Lanthanum-strontium copper silicate blue pigments are another

class of inorganic blue pigments which shown

high reflection in NIR region [31].

Complex Inorganic Color Pigments (CICPs)

CICPs are often produced by blend of simple metal oxides which are heated above 600°C for few hours. At such high temperature the metal ions transfer to and fro. Thus, the mixture of oxides results in matrix/complex of multiple metals ions and oxygen[32,33]. These CICPs provide better hiding, UV blocking, increased heat and thermal stability and IR reflective performances[34,35]. Very efficient, aesthetically pleasing dark roofing can be formulated with CICPs[36]. The CICPs which exhibit dark color in VISIBLE region and high reflectance in IR region are used in paints for military equipment and personnel to camouflage by matching the reflectance of background foliage in the VISIBLE and IR spectrum[37]. The chlorophyll of the leaves boosts the reflectance of the plant leaf from 0.1 to about 0.9 at 750 nm (Figure 8). This itself explains why a dark green leaf remains cool on a summer day. The tailor made CICPs for high IR reflectance similar to that of chlorophyll, are very suitable for roof applications to keep the roof surface cool. Comparison of solar spectrum, solar reflectance of white pigment, green leaf, and a CICP is shown in Figure 8 [26]. A CICP consisting of a mixture of black IR reflective pigments, chromic oxide (Cr2O3) and ferric oxide (Fe2O3) boosts the total hemispherical reflectance of carbon black from 0.05 to 0.26 (e.g. CICP:Fe2o3:Cr2O3). In IR spectral region the CICP enhances the reflectance to ~ 0.7.

Mechanism of cool effect

Any objects reflect, and/or absorb, and/or transmit the solar energy. Figure 9 shows a schematic representation of the principles of light when fall on a surface/object. As mentioned earlier the solar light consists of mainly UV, VISIBLE and IR radiations. The reflection/absorption/transmittance of these radiations depends on the nature of the surface. Total Solar Reflectance (TSR) describes how much of the sun's energy an object reflects. Surfaces with white color reflects > 85 % of the solar light whereas, surfaces with black color reflects ~ 5 % [5].

Reflectivity can be manipulated by the careful selection of high-IR-reflective pigments. The key is to reflect infrared, and absorb and reflect in the VISIBLE region to produce the needed color. The infrared reflective pigments possess the properties like, they do not absorb in near infrared region. They either reflect it or transmit it.

IR reflective pigment's refractive index is different from that of the binder in the infrared region. This causes diffused reflection in IR region. If the refractive index of the pigments in the IR region is similar to that of the binder's refractive index in the IR region, the pigment would be transparent to near infrared light (NIR). If any reflection observed in the near IR region, that would be from the undercoat.

Absorption of light occurs when light energy promotes electrons from one bonding state to another. If light of a different wavelength is used to cause this energy transition, it will not be absorbed e.g. iron chrome black absorb light through the visible region. This means there are electronic transitions responsible for absorbing light with wavelengths of energy from 400 - 700 nm. Light of lower energy (>700nm) is not absorbed. The light of wavelength 1500 nm being low energy is unlikely to affect any electronic transitions in the materials. Thus it will not be absorbed. Instead the 1500 nm light beam is refracted, reflected and scattered (depending on the refractive index) leading to diffuse reflection of NIR light.

In principle, the IR reflective pigments/coatings with high TSR values significantly reflect the IR radiations of the solar spectrum, and selective reflect in the VISIBLE range to impart respective color. Thus the surface remains cool and hence the inside the building remains cool.

Applications and advantages

IR/heat reflective coatings have found applications on various surfaces such as roof, military camouflage, automotive, cement concrete and pavers etc.

Roof/Roof tile

A building's cooling load can significantly be reduced by IR-reflective coatings for the roof, and this will contribute to the energy savings. Thermal expansion and contraction of the roofs, and metal cladding can be minimized by incorporating IR-reflecting pigments and this enhances the life time of the coatings. A significant enhancement in life time or the durability of the coating consisting of IR reflective pigment is shown in Figure 10. Cool roof color materials (CRCMs) prepared using CICPs as IR reflective pigments applied on PVDF metal roof sheets in Florida, USA showed that, there was tremendous improvement in the durability of CRCMs against the conventional coatings [38].

Apart from the surface of roof/roof tiles, the IR reflective coatings are very useful in wood coatings where the wood surface is exposed to the high temperature region.

Otherwise, the durability of binder system used for the wood coating may deteriorate and damage the wood [39].

Military

To hideout the military equipment and personnel, synthetic green pigments are incorporated into materials, mimicking the surrounding green leaves[37]. The chlorophyll present in the natural leaves reflect IR radiation. Therefore the coatings used for military applications demands coatings with IR reflective pigments since, conventional green pigments do not resemble chlorophyll in the infrared, and they absorb IR radiations. As a result, an improperly formulated camouflage color appears black against a bright background when viewed through IR-imaging equipment. IR- reflecting pigments make possible the formulation of materials that look like foliage to the human eye and also the IR camera.

Automotive

The dark colored vehicles absorb solar energy and the interior of the vehicle warms up to a greater level. Hot instrument panels, consoles and dashboards become brittle over the time, and exude plasticizer and other organic compounds. In such situations the IR/heat reflective coatings found to be very useful in controlling the heat absorption and obviously the durability.

IR/heat reflective pigments or coatings are good substitute for the coating applications where unbearable heat build-up, due to which the load on cooling become expensive. Here the main advantage includes significant energy savings, enhancement of the durability of the coatings, lowering the thermal expansion of the construction materials. It was estimated that the energy saving for a commercial building was ~ 5% by using cool roof coatings[40]. The cooling load decreases with increase of solar reflectance of the roof[41]. It was reported that the IR/heat reflective coating for exterior walls/roofs contribute in summer time in terms of reducing the load on cooling the interior, however it will be expensive in winter time since as the IR radiations are not absorbed, the interior temperature drops significantly and need to be maintained which costs the energy and hence combination of reflective coating for interior and exterior of the building suits best for milder environment [42]. IR reflective pigments are not just limited to the coating applications, they have been a good candidates for composite matrix/materials [6].

Summary

The paints/coatings with conventional pigments for exterior applications are not sufficient enough to reduce the heat build-up due to their low TSR values. Such pigmented systems increase the energy consumption to cool the interior and hence the cost. Apart from energy consumption, due to the heat build-up the quality of the coatings deteriorates and hence the less durability. The heat built at the surface increases the thermal expansion of the construction materials, drastically reduces the life of the buildings/materials. The alternate route to overcome such issues is to reflect the IR/NIR radiations of the solar energy which is the culprit of the above all causes. Inorganic metal oxide/complex color pigments which reflect IR radiation, and selected reflection in VISIBLE region to impart color are of great help for IR reflective paint/coatings. Since these pigments show significantly high TSR value over their conventional pigments counter parts, the visual appearance and the IR/heat reflectiveness of these pigments made them as strong candidates for exterior applications. This review article brought some insight on IR reflective pigments/coatings, their cooling mechanism, applications and advantages.

Acknowledgments

The authors would like to thank Asian Paints Limited for support and encouragement to publish.

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