Pigments

 













Inorganic or organic substances that are insoluble or substantially insoluble in water or organic medium. Impart colour, opacity ,mechanical rigidity and reinforcement to the continuous phase in which they are dispersed.[1] Impart certain special properties to the coatings such as reflection of infrared radiation.[2] Pigments can differ in their weatherability, chemical resistance, and other durability criteria. Pigments are classified on the basis of their performance as decorative and protective . Alter the appearance of the coating by selective absorption or by scattering of light.

 









The sun’s energy that reaches the earth’s surface is divided in to three main parts.(Fig 1) UV - 5% Visible - 43% Infra red - 52% The heat-producing region of the infrared radiations ranges from 700 1100 nm. These radiations on absorption result in heating up of the surface[1] Coatings colored with conventional inorganic pigments tend to absorb the invisible near-infrared (NIR) radiation.

Spectral Solar PowerDistribution

 







Researchers are developing new complex inorganic pigments that exhibit dark color in the visible spectrum and high reflectance in the near infrared. This class of pigments increases the near infrared reflectance of exterior finishes and paints, there by dropping the surface temperatures of roofs and walls. The inorganic NIR reflective pigments are mainly metal oxides and are primarily useful in two major applications: (i) visual camouflage and (ii) reducing heat build up.

 











Infrared reflective inorganic pigments are complex inorganic color pigments, which reflect the wavelengths in infrared region in addition to reflecting some visible light selectively selectively.. The reflectivity and absorptivity of the pigment are independent of each other. Thus an IR reflective pigment pi gment may have any color. color. They do not absorb in near infrared region. They either reflect it or transmit it. Their 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 . In such a case, any reflection in the near infrared region would be due to 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 blacks 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. In this case, a beam of light with a wavelength of 1500 nm is too low in energy to cause any electronic transitions in the material. Thus it will not be absorbed. Instead the 1500 nm light beam is refracted,r eflected and scattered leading to diffuse reflection of NIR light.

 







Pigments are synthesized by subjecting mixtures of metal hydroxides, nitrates, acetates or even oxides, to very high temperatures by calcination. Metal oxides or salts are blended together and a nd strongly heated, generally at temperatures of over 1000 °C. At the calcining temperature the solids themselves become reactive. Metal and oxygen ions in the solids rearrange to form new,, more stable crystal structures [4]. new

 





Individual Pigment selection: selection: Pigments having highest reflectivity in the near infrared region must be selected for making infrared reflective coatings. Dispersion:: These pigments are Dispersion compatible with all types of solvent and water-based coating systems such as acrylics, polyesters and fluoropolymer systems.

Representation of CIE lab system

 











Blending pigments: pigments: A combination of two infrared reflective pigments can increase the total reflection of the coating. But in some cases where different pigments absorb in different regions, the total reflectance becomes less than the reflectance of the individual pigments. Opacity :The :The infrared reflective pigments have high visible opacity.. These pigments only scatter or transmit the infrared opacity radiations. higher coating thickness may be required for the coating to be opaque to infrared. Pigment volume concentration also plays an important role in deciding the opacity of a coating to infrared radiations.

 









Contamination :Contamination occurs when two infrared reflecting pigments absorbing at different regions are mixed together.. It drastically affect the total solar reflectance on the together coatings [3] Particle Size: Size: For highest reflectivity reflectivity,, the particle size should be more than half the wave length of the light to be reflected. Thus for reflecting infra red light of 700-1100 700-1100 nanometers wave length, particle size should be at least 0.35 to 0.55 microns [2]. Excessive. grinding and dispersion may therefore be counter productive

 

General benefits  Longer life-cycle due to less polymer degradation and thermal expansion due to lower temperature.  Aesthetically pleasing colors.  Cooler to touch for better handling.  Improved system durability and less thermal degradation. Roofing benefits  Less heat to transfer into buildings.  Reduced ‘Urban heat island effect’.    Low energy demand for air conditioning, particularly in equatorial regions.

 







Reduction in air pollution due to low energy usage, power plant emissions, and reduction in urban air temperatures. Installation crews can work longer during the day d ay before the roof gets too hot to work on. Very high durability coatings. Some coatings have been in use for as long as 25 years[1] 

 













The amount of total energy absorbed and emitted by a topcoat determines the heat build-up of a coated c oated surface and results in a final surface temperature after reaching an equilibrium state. Coating pigmentation has the main influence on heat build-up performance. Pigment reflects infrared radiation (heat) from the sun and keeps the coated item cooler, independent of the color of the coating. This can help to minimize air conditioning unit work load and operational cost. Increasing the cooling load of the air conditioning unit may lead to the release of Chloro fluoro flu oro carbons(CFC) Automobile engines are being downsized to reduce the weight and improve fuel economy.

 









They are less able to handle the power drain of the larger air conditioners . Thus, there is a need for new technologies to reduce the solar heat loads and to allow reduction of air conditioner size. IR reflecting pigments would result in reduced CFC emission and increased vehicle fuel efficiency efficiency.. Over all reduction of carbondioxide emanating from fossil fuels can be possible by this way.

Smart’s New Electric Ve Vehicle:Heat hicle:Heat shield made of reflective pigments

 













The ‘reflector’ ,the dog coat that takes the heat out of the heat To achieve cool surfaces, the pigments need to reflect as much energy as possible. This reflection ability can be expressed as the Total Solar Reflectance value (100%=total reflection; 0 % = total absorption). Pigments with high TSR values show a high reflection combined with low heat build-up, and vice versa. TiO2 , a white pigment with a high NIR solar reflectance of about 87.0% [22] is currently regarded as the best pigment for coating materials. One prefers a dark colour instead of white, while pigments with the lowest reflection are based on carbon black.

 









There is a choice of using dark inorganic complex pigments like Chromium Iron Oxide with a medium IR reflectivity (50-57%) [22]and showing higher TSR vaues than carbon black. A maximum NIR reflectance of 82.8% is reported for pigments prepared from a composition of Cr 2O3, TiO2, Al2O3 and V2O5 of 80, 4, 14, and 2wt% respectively respectively.[23] .[23] 6g of this pigment powder in 10g ceramic glaze gives a NIR reflectance of 74.5% and is useful for reflective ceramic roof coating. The near-IR reflective property of the Ga doped ZnO film (72%) is much better than that of the most reported IR-reflective ZnO:Al films(50%[14]

 

 



Y6MoO12 exhibits 92% NIR reflectance at 1 1100 100 nm region. Doping of Praseodymium gives brick red colour with 88% NIR reflectance. Doping of Silicon gives yellow pigments with NIR reflectance 98%. The pigments Y2Ce1.5Mo0.5O7 exhibited yellow and reddish brown respectively with high NIR reflectivity(>90%) and Y 2Ce1.7Pr0.3O7  showed colours from brick red to dark brown with good reflectivity.[24]

Blue Pigments  Intense blue colour is rep reported orted with YIn0.9Mn0.1O3 and the colour darkens with increasing Mn concentration.

 

 

Pigment

Colour

Remarks

Y2BaCuO5, Sm2BaCuO5  and Yb2BaCuO5 

Green

Thermally/chemically stable

(Bi2O3) 0.7(Ln2O3) 0.3  (Ln=Eu,Gd,Tm,Yb,Lu)

Yellow-orange

Reflectance(74-86%) Heat resistant

Sm2Ce2-xMoxO7+δ (x = 0 to

Cream to yellow

0.4) Ce1-xMxW2O8 (M = Zr or Ti, 0 < x < 0.6)

Intense yellow

Ca-doped Nd2S3

Green

ZrCe1-xYxO4-δ 

Yellow

ZrCe1-xPrxO4-δ 

Orange red

TiCe1-x Pr x O 4-δ

Brick red

Sm2Ce2-xPrxO7+δ (x = 0 to 0.4)

Dark brown

CePS4, Ce6Si4S17, and Ce4Si3S12 

Yellow

Reflectnce-90%

chromatic properties similar to CdS, PbCrO4 and PbMoO4,thermally/chemically stable

 









Industry has realized the potential of pigments for various applications and the research is focused on developing hygienically and environmentally friendly rare earth based pigments. There is no known method to predict the infrared reflectivity of any substance. The only technique to find out if a pigment reflects infrared radiations is to test its reflectivity. Any specific physical property of the pigment cannot be correlated wi with th its infrared reflectivity. reflectivity. It should be possible to screen a large number of inorganic pigments manufactured so far to look for IR reflecting pigments.