Infrared Reflecting Pigment

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    68 Recent Patents on Chemical Engineering, 2008, Vol. 1, No. 1 Malshe et al

    are independent of each other. Thus an IR reflective pigmentmay have any color. These pigments are synthesized bysubjecting mixtures of metal hydroxides, nitrates, acetates oreven oxides, to very high temperatures in a process calledcalcination. Metal oxides or salts are blended together andstrongly heated, generally at temperatures of over 1000C.At the calcining temperature the solids themselves becomereactive. Metal and oxygen ions in the solids rearrange to

    form new, more stable crystal structures such as spinel orrutile structures [4].

    IR reflective pigments are increasingly used for roof andbuilding coatings because of their excellent weatherability.They have an ability to maximize reflectivity in the nearinfrared region. These IR-reflective pigments find increaseduse as the formulators make an attempt to produce darkcoatings and minimize heat buildup in the underlyingstructure. Nickel manganese ferrite blacks (Pigment Black30) and iron chromite brown-blacks (CI Pigment Green 17,CI Pigment Browns 29 and 35) are some of the infraredreflective pigments that are used to provide dark colors withreduced heat buildup. Other commercially available infraredreflective pigments are Pigment Blue 28 Pigment Blue 36,Pigment Green 26, Pigment Green 50, Pigment Brown 33,Pigment Brown 24, Pigment Black 12 and Pigment Yellow53 [5].

    In urban areas, the design of roofs has a major influenceon the heat absorption of sunlight. The hot buildings alsoknown as Concrete Jungle radiate heat and warm the air inthe surrounding. If there are several such buildings in thevicinity, the combined effect leads to a phenomenon knownas Urban Heat Island Effect. The amount of heat radiated inthe surroundings varies depending on the roof construction,type, elevation and also the color of the coating used.Significant amount of heat is also absorbed into the buildingby means of conduction. With such increasing heat energy in

    the building, there is a need for variable energy in the formof air-conditioning to keep the interiors of the building cooland tolerable for people to work and live in them. To reducethe increasing demand for energy consumption for airconditioning, there is a need for cooler roofs. Reflectingmost of the suns heating energy minimizes the amount ofenergy absorbed by the building.

    These pigments are highly stable and chemically inert.They can withstand the chemically aggressive environmentsand still retain their color. They do not fade in the presenceof ozone, acid rain, SOx, NOxor other air pollutants commonin industrial areas. They even remain colorfast in thepresence of strong acids, bases, oxidizing or reducing agents.They are non-migratory, and do not dissolve or bleed whenin contact with solvents. Because of these properties, thesepigments last as long as 30 years in outdoors. Formulatingpaints with them is a major challenge since the bindersdegrade much faster. The most expensive component of theformulation is the IR reflective pigment.

    In addition to excellent chemical stability, these pigmentsare also stable to high temperatures. Due to high heatstability, they can be used for high-heat coatings, such asmuffler and stove coatings, fireplace paint, and high-heatpowder coatings. Porcelain enamel and decorative ceramiccoatings also use these pigments.

    REFLECTION MECHANISM OF INFRARED

    RADIATIONS

    The infrared reflective pigments have the followingproperties[6].

    They do not absorb in near infrared region. They eithereflect it or transmit it.

    Their refractive index is different from that of the binde

    in the infrared region. This causes diffused reflection in IRregion. If the refractive index of the pigments in the IRregion is similar to that of the binders refractive index in theIR region, the pigment would be transparent to near infraredlight (NIR). In such a case, any reflection in the near infraredregion would be due to the undercoat.

    Absorption of light occurs when light energy promoteselectrons from one bonding state to another. If light of adifferent wavelength is used to cause this energy transition, iwill not be absorbed e.g. iron chrome blacks absorb lighthrough the visible region. This means there are electronictransitions responsible for absorbing light with wavelengthsof energy from 400 - 700 nm. Light of lower energy (>700

    nm) is not absorbed. In this case, a beam of light with awavelength of 1500 nm is too low in energy to cause anyelectronic transitions in the material. Thus it will not beabsorbed. Instead the 1500 nm light beam is refractedreflected and scattered (depending on the refractive indexleading to diffuse reflection of NIR light. There is no methodto predict the IR reflectivity of an inorganic or organic compound. This property appears to be an inherent characteristicproperty just like density, thermal conductivity, colorrefractive index etc.

    BENEFITS OF INFRARED REFLECTIVE COATINGS

    General benefits:

    Longer life-cycle due to less polymer degradation andthermal expansion due to lower temperature.

    Aesthetically pleasing colors.

    Cooler to touch for better handling

    Improved system durability and less thermadegradation.

    In addition to the above mentioned benefits, the IRreflective coatings also have certain 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 usagepower plant emissions, and reduction in urban aitemperatures.

    Installation crews can work longer during the daybefore the roof gets too hot to work on.

    Very high durability coatings. Some coatings havebeen in use for as long as 25 years.

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    FACTORS AFFECTING INFRARED REFLECTIVITY

    Some factors that can affect the coatings infraredreflectivity are individual pigment selection, milling anddispersion, particle size, mixing infrared reflective pigments,opacity and contamination.

    Individual pigment selection: Pigments having highestreflectivity in the near infrared region must be selected

    for making infrared reflective coatings. Pigments shouldbe selected on the basis of required shade depending onthe L, a and b value of the pigment.

    Dispersion: These pigments are compatible with alltypes of solvent and water-based coating systems suchas acrylics, polyesters and fluoropolymer systems. Forcomplete dispersion and optimum properties, pigmentsshould be dispersed in a small media mill to obtain therequired fineness of grind. The pigments should not beover ground, as additional grinding will break theparticles, affecting the color and also the infraredreflectivity of the pigments.

    Blending pigments: Care must be taken while formu-

    lating a coating with more than one pigment. Acombination of two infrared reflective pigments canincrease the total reflection of the coating. But in somecases where different pigments absorb in differentregions, the total reflectance becomes less than thereflectance of the individual pigments. In such a case,the absorption overpowers scattering. Thus, care mustbe taken while selecting a combination of pigments tomake infrared reflective coatings.

    Opacity: The infrared reflective pigments have high

    visible opacity. These pigments only scatter or transmitthe infrared radiations. Thin films may not scatter andreflect all the infrared radiations from the coating andmay allow the radiations to pass through to the substrate.Thus, such coatings have visual opacity but are notcompletely opaque to infrared light. Thus higher coatingthickness may be required for the coating to be opaqueto infrared. Apart from film thickness, pigment volumeconcentration also plays an important role in decidingthe opacity of a coating to infrared radiations.

    Contamination: Contamination occurs when two infra-red reflecting pigments absorbing at different regionsare mixed together. It is worsened when an infraredreflective pigment is mixed with an infrared absorbingpigment. Such contaminations drastically affect the totalsolar reflectance on the coatings [3].

    Particle Size: Particle size of the pigment is a very

    important parameter. For highest reflectivity, the particlesize should be more than half the wave length of thelight to be reflected. Thus for reflecting infra red light of700-1100 nanometers wave length, particle size shouldbe at least 0.35 to 0.55 microns [1].Excessive grindingand dispersion may therefore be counter productive.

    Various attempts have been made to synthesize IRreflective pigments and coatings. The synthesis strategiescan be roughly divided in three parts.

    1.

    Coating a thin film of glass or mica with an infra redreflecting substance and holding it there by physicaforces.

    2.

    Coating a thin film of glass or mica with a metallicsurface. Metals are known to reflect almost entire visibleand near IR radiations. (For visible region, the welknown exceptions are gold and copper which arerespectively yellow and red)

    3. Synthesize inorganic crystalline substances, which havegood pigmentary properties and find out if these are alsoIR reflective

    Few attempts are listed as follows:

    Infrared reflective metallic pigments are manufactured byThe Shepherd Color Co. using Micro Mirror TechnologyThese pigments have high brilliance and sparkle and alsogood infrared reflectivity. Borosilicate glass, having highstrength and excellent chemical resistance, is used as thesubstrate in these pigments. A sub-micron layer of metallicsilver is deposited on the glass surface. The uniformity of thesilver coating is important to cover the glass surface

    completely and also to minimize the surface roughness thawould diffuse the incident radiation. These pigments areresistant to stress and shear. There is no need to soak themfor pre-dispersion. They can be incorporated in coatingsusing low shear mixing equipments. They are capable ofsurviving a degree of vigorous mixing without changing theappearance or brilliant effect. These pigments are producedin two different crystal shapes - lamellar and spherical. Thelamellar grades are based on glass plate substrate havingthickness of 5 microns. They have a very narrow particle sizedistribution for maximum contrast. There are two types ospherical grades. One of them has a solid glass substrate andthe other one has silver coated hollow microspheres. Theorientation of these particles results in excellent reflection

    Most of the applications of these pigments are based on theiaesthetic properties. They are used in coatings for cannedbeverages, packaging of consumer goods, candles, decorative ornaments, musical instruments and fishing lures. Theeffect of these pigments is prominent in sunlight. Theyproduce dazzling effects on custom automobiles, motorbikesbicycles, boats, skis, spas and swimming pools. Thesepigments also find applications in cosmetics, apparelssunglasses and cell phones. Thus, silver coated glass flakepigments are used to achieve brilliant effects withoulimitations of conventional metallic pigments. They can beincorporated in any coat of the paint system, in combinationwith transparent or opaque colorants [7].

    US Patent 20060159922A1 describes synthesis oinfrared reflective pigments, in form of flakes, for use inpaints, composite gelcoats , varnishes and other coatingformulations [8]. The flakes comprise an infrared reflectivecore having thickness of less than 0.2m and an infraredtransparent material coated on the surface of the core. Thetransparent layer comprises a binder with optionally acolored material. The transparent layer provides color andmechanical strength along with chemical resistance to thecore material. The core comprises metallic or conductiveoxide material. Different combination of colorants can be

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    used on either side of the core surface to provide differentshades.

    In another invention, Anthony David Skelhorn et al. [9]have developed a coating system with high infraredreflectivity and low thermal conductivity. These coatingscomprise water-based, solvent based, single component ormulti component, cement or gypsum based binder systems.The coatings also comprise infrared reflective extenders,

    pigments and hollow micro-spheres made up of glass,ceramic or organic polymers. Extenders like calciumcarbonate, crystalline and amorphous silica, silicate mineralssuch as Talc, kaolin, calcined clay, wollastonite, nephelinesyenite, feldspar, mica, attapulgite clay, bentonite andorganically modified bentonite, alumina trihydrate, alumi-num oxides, barytes and lithopone are known to haveconsiderable infrared reflectivity. Hollow micro-spheres likeglass micro-spheres of different glass compositions, havingdifferent diameter to wall thickness values and differentparticle diameters, Ceramic micro-spheres such as 3M's Z-light Spheres, Cenospheres, fly ash; or micro-spheres basedon organic polymer composites like polymers or copolymersof acrylic materials in form of dry powder or dispersionssuch as Rhopaque by Rohm and Haas, or copolymers ofvinylidene chloride and acrylonitrile such as Expancel byExpancel, Inc. were used in the composition. Commerciallyavailable infrared reflective pigments were also used in thecoating composition. The hollow micro-spheres reduced thethermal conductivity of the coatings, whereas the IRreflective pigments and extenders increased the infraredreflectivity of the coatings. The thermal primers and coatingsprepared by using the above mentioned raw materials werefound to have excellent infrared reflectivity and low thermalconductivity.

    U.S. patent 6454848 describes development of solidsolutions having corundum-hematite crystalline structure

    which are useful in inorganic pigments [10]. The solidsolutions contain a host component having a corundum-hematite structure and a guest component of one or moreelements such as aluminum, antimony, bismuth, boron,chrome, cobalt, gallium, indium, iron, lanthanum, lithium,magnesium, manganese, molybdenum, neodymium, nickel,niobium, silicon, tin, titanium, vanadium, and zinc.Thoroughly mixing the compounds, containing the host andthe guest components and calcining at high temperatures toform solid solutions having corundum-hematite crystallinestructure formed the solid solutions. These compoundsexhibit dark colors and excellent reflectivity in the near-infrared region.

    Another invention describes preparation of IR reflectivepigments by coating a white pigment with IR reflectingcolorants [11]. Yanagimoto Hiromitsu et al. used colorantslike azo, anthraquinone, phthalocyanine, perinone/perylene,indigo/thioindigo, dioxazine, quinacridone, isoindolinone,isoindoline, diketopyrrolopyrrole, azomethine, and azome-thine-azo for coating on the white pigments. White pigmentslike titanium dioxide or zinc white and extenders likecalcium carbonate, barium sulfate, alumina, silica, clay,activated clay, aluminum powder, stainless steel powder, andorganic plastic pigments were used in different combinationsas the white pigment base. The colorants and white pigment

    base were used in the form of dispersions. The dispersantused were high molecular weight components obtained bycopolymerizing hydrophilic monomers such as acrylic acidmethacrylic acid, dimethylaminoethyl methacrylate withstyrene or methacrylate ester, having hydrophilic end groupsThe IR reflective colorant dispersions and white pigmendispersions were mixed in a conventional mixer such asdissolver, where the white pigment was coated with the IR

    reflective colorant. The IR reflective pigment was obtainedby drying the resulting mixture. Even though the coloringcomponent is organic, the temptation of thinking of these asIR reflecting organic pigments must be completely avoidedThe IR reflective component is the inorganic part.

    Several existing inorganic pigments in the range omanufacturers servicing the ceramic industry were found tohave IR reflecting character. These have found additionaapplications in the cool roof coatings. Figure 1 shows arange of different commercially available infrared reflectivepigment shades.

    APPLICATIONS OF INFRARED REFLECTIVEPIGMENTS

    Building products

    Exterior building products are designed such that the heabuild-up can lead to part failure and increased energy costsIR-reflecting pigments are used for different buildingapplications.

    Coatings: Infrared reflective pigments are widely used inroof coatings. They are suitable for all types of architecturafinishes; including highly-alkaline masonry coatings (e.gsilicate paints).

    Apart from roof coatings, window coatings have alsobeen synthesized. These coatings transmit the visible lighand reflect the infrared light from the suns radiations

    Modern architectural designs employ the use of large glasareas. These contribute to the aesthetic appearance of thebuilding and reduced maintenance, but also increase theenergy consumption. Large glass surfaces in buildingaccount for large heat losses during the winter season andexcessive heating by direct solar radiation in summer. Thelatter effect is a major contribution to energy cost since icosts three to six times more to cool a building than to heat i[12].

    The infrared reflective properties of thin metallic films ometals such as gold can be utilized for making windowcoatings. These films, when sufficiently thin, transmit visiblelight and reflect the infrared portion of the incoming

    sunlight. Heat reflective gold coatings on architecturaglazing have two main disadvantages. The thin gold filmseven when transmitting visible light, are highly reflective noonly to infrared radiation, but also to visible sunlight. Thisproperty results in a metallic glare, which is objectionable tothe observers. In addition, gold is expensive even though it isused as only a thin film. The cost disadvantage of a goldcoating can be overcome by replacing it with non-noblemetal films. However, such films still exhibit metallic glareand also have stability problems due to corrosion, thusrequiring the deposition of additional layers for protectionagainst oxidation. Considering these factors, Haacke

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    Gottfried has used certain semiconductor metal layers as analternative to heat reflective metallic films [13]. He hasstated that if the energy gap of these materials is large(approximately 3 eV), they are transparent to visible light.Also, if the free electron concentration in these semi-conductors exceed to a large extent, these films also give

    high infrared reflectivity.Cadmium stannate (Cd2 SnO4) is one of the most

    transparent, heat reflecting semiconductors, which HaackeGottfried has used for making infrared reflective coatings.CdO, SnO2 and CuCl are mixed thoroughly to make ahomogenous mixture and heated in an alumina crucible forsix hours at 1050C. The cadmium stannate crystals haveshown orthorhombic structure. Cadmium stannate films on asilica plate reflect infrared light at approximately 1.5 micronthickness. These films give 80% reflectance at 2 micronsfilm thickness and 90% reflectance at 6 microns thickness.These properties make cadmium stannate films highlysuitable for greenhouse window applications. It has alsobeen found that doping of cadmium stannate with copperincreases the infrared reflectivity and films made out of thismaterial can also be utilized for architectural windowcoatings.

    Vinyl window and siding: Rigid PVC is a temperaturesensitive product and it distorts, as it gets hotter. Vinyl sidingand window and door profiles tend to wrap and twist out ofshape if not correctly manufactured. Regular pigments usedto achieve color often make the warping tendency muchworse. Infrared reflective pigments can be used in suchcases. They enable colors other than white to be made andalso reduce the warping and twisting.

    Cement, concrete and pavers: Infrared reflective pigments do not fade and counter heat build-up when used incements. Infrared reflectance allows the color to stay true foa longer period of time. Cement pavements remain coolerduring hot climates.

    Automotive application: Dark colored cars get hotter

    during summer. Dark seats may be easier to maintain, buthey can get uncomfortably warm. Hot instrument panelsconsoles and dashboards become brittle with age, and exudeplasticizer and other organic compounds. Moreoverincreasing the cooling load of the air conditioning unit toreduce heat discomfort may lead to certain other problemsAutomobile engines are being downsized to reduce theweight and improve fuel economy. They are less able tohandle the power drain of the larger air conditioners. Airconditioners are a major source of chlorofluorocarbons(CFC) released into the atmosphere. Increased cooling loadleads to larger air conditioning units, which increases thisproblem. Thus, there is a need for new technologies toreduce the solar heat loads and to allow reduction of airconditioner size. These cooling alternatives would result inreduced CFC emission and increased vehicle fuel efficiencyThus there is a possibility of over all reduction of carbondioxide emanating from fossil fuels and making roadtransport more economic.

    Infrared reflective pigments can be used in the coatingformulation to reduce the heat build-up of the car. Thesepigments give best performance with respect to colorfastness, IR-reflection and other properties for several years.

    Fig. (1).Commercially available IR reflective pigment shades

    V-9415Yellow

    Solar Ref = 66.5%

    V-9416Yellow

    Solar Ref = 67.4%

    10415Golden Yellow

    Solar Ref =

    58.0%

    10411Golden Yellow

    Solar Ref =

    61.8%

    10364Brown

    Solar Ref =

    39.2%

    10201Eclipse Black

    Solar Ref = 22.6%

    V-780

    IR BRN BlackSolar Ref = 30.0%

    10241Forest Green

    Solar Ref = 40.0%

    V-9248Blue

    Solar Ref = 30.0%

    V-9250

    Bright BlueSolar Ref = 29.2%

    F-5686Turquoise

    Solar Ref = 31.0%

    10202Eclipse Black

    Solar Ref = 32.5%

    V-13810

    RedSolar Ref = 39.0%

    V-12600

    IR Cobalt GreenSolar Ref = 28.2%

    V-12650

    Hi IR GreenSolar Ref = 40.4% V-778

    IR Brn BlackSolar Ref = 27.6%

    V-799

    Brn BlackSolar Ref = 25.1%

    10203

    Eclipse Blue BlackSolar Ref = 27.4%

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    The above-mentioned coatings only reduce the infraredreflection of the coatings from the metal surface. Theautomobile glazing still contributes to the heat problem byallowing infrared radiations to pass through. US Patent5405680 describes the use of a coating comprising asemimetal and a selectively emissive metal [14]. The semi-metal film reflects the incident solar radiation and transmitsvisible light. The selective emissive material provides a

    means for radiating the infrared radiation, thereby coolingthe enclosure interior via radiation. There are several knownsemimetals that reflect in the range of 650-800 nm. Some ofthem are rare earth and other metal borides and chalco-genides, such as LaB6, LaTe, and SbS3. It has been seen thatthe semimetal film made by using LaB6 exhibits strongreflection through the entire infrared region. The selectiveemissive materials used are metal oxides, particularly heavymetal oxides, such as zirconium oxide and thorium oxide. Insome cases, aluminum oxide has also been used. These havebeen studied by applying separately on the glass surface asthin films, combining together and applying as a single thinfilm, converting them into a paint wherein they are retainedas suspended particles and embedding them into the glass

    structure itself. In all the four cases, these materials haveshown good infrared reflectivity.

    Apart from automobiles, these coatings can also be usedin domestic household applications.

    Military application: Chlorophyll is the pigment thatmakes plants green. To camouflage military equipment andpersonnel, synthetic green pigments are incorporated in to thecoatings. However, conventional green pigments do notresemble chlorophyll in the infrared region. They absorbinfrared light, whereas chlorophyll reflects it [10]. (Chloro-phyll thus appears to be the only known Organic IRreflecting pigment and this also explains why the shade oftrees is cooler than the surroundings in hot summer). As a

    result, an improperly formulated camouflage color appearsblack against a bright background when viewed through IR-imaging equipment. The use of infrared reflecting pigmentsmakes possible the formulation of materials that look likefoliage to the human eye, and also to the infrared camera.These pigments also reflect solar energy from the decks ofships. Thus, solar-induced heat build-up is minimized, andso is the energy consumption to cool the vessels interior.

    US Patent US6468647 describes the use of coloredmetallic pigments such as aluminum and mica flakes forgood infrared reflectivity [15]. According to the patent, colorhas been incorporated on the metallic surfaces in such a waythat it does not interfere with the ability of metallic pigmentsto control infrared reflectivity. Many commercially availablepigments have been burnished into the metallic surface toyield a modified surface, which retains the color of thepigment, and gives very high infrared reflectivity. Thisburnished surface technique holds the pigment particles sostrongly that the pigment cannot be removed by normalwashing or by solvents. The amount of pigment burnishedinto the surface may be varied to achieve various shades ofcolor. Pigments with good hardness and dispersed particlesize of less than 1 micron have been used for the burnishingprocess. The burnishing process can be carried out indifferent ways. The metal particles and the pigment particles

    can be subjected to thorough mixing in a vibratory tumblerThey can also be subjected to ball-milling for several hoursto get a proper coating on the metal surface. By this processthe pigment particles are mechanically bound to the surfaceof the metal and are not easily removable by washing ohandling. The pigment particles remain on the metallicsurface when the colored particles are utilized in coatingformulations. These coatings can be applied to metal, plastic

    composite, or fabric substrates to yield products, which havethe desired infrared reflectivity and the desired visual colorThese materials can find wide applications in militaryapplications.

    Infrared reflection from fire: Fire-resistant paints havebeen formulated for two main reasons. One is to reflect heaand the other is to insulate from heat. These coatings thuskeep the temperature of the coated combustible substratebelow the ignition temperature.

    There are two types of f ire retardant coatings:

    1.

    Coatings that do not burn when exposed to firegenerally referred to as "fire resistant"

    2.

    Coatings that insulate the flammable substrate, keepingits temperature lower than the combustion pointgenerally referred to as "intumescent".

    In addition to these coatings, some pigments chemicallyinhibit fire. E.g. antimony trioxide when combined withhalogenated organic compounds, gives the productantimony oxy halide, which smothers the fire by excludingoxygen. An important quality for the effectiveness of fireretardancy is the ability of the coating to reflect heat. Theradiation of heat from a fire to an unaffected area of thestructure is a decisive factor in the spread of the fire [16]Use of white or pastel paints has been made in most of thecases for heat reflection. A combustible structure is coatedwith a white or pastel paint to reflect infrared radiations and

    delay the spread of fire. Titanium dioxide is a common whitepigment that is being used for this application as it has goodinfrared reflectivity.

    The US Patent US5811180 describes the use of certaininfrared reflective pigments to the coating composition toenable the reflection of the fire radiation [17]. Thesepigments have particle size between 1-2 microns. Theinfrared reflective pigments include metallic flakes such asaluminum flakes, and mica flakes coated with a material ohigh refractive index. The infrared reflective property of themetal or coated-mica flakes is not dependent on pigmensize. Some materials having high refractive index describedin the patent for coating on mica are Fe2O3 - a red pigment

    anatase and rutile TiO2, Cr2O3 - a green pigment, ZnSSb2O3, ZrO2, and ZnO. Thicker coatings than that are neededfor colored coatings increase the infrared reflectivity of thepigments. Coated synthetic f luorophlogopite micas in whichIR absorbing OH groups have been largely eliminated by theuse of fluorine substitution also act as an infrared reflectivepigment substrate. To achieve high reflectance over a widespectral range, more than one type of coated mica can beused.

    Another type of infrared reflective pigment for use incoatings, which is not IR transparent, is aluminum metal inthe form of thin flakes. Aluminum is highly reflective in the

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    IR due to the high concentration of mobile electrons."Leafing" aluminum paints are particularly more reflective.These coatings contain overlapping pigment flakes, whichare parallel to and concentrated near the surface of thebinder.

    Coatings are made from these materials and used forcoating the surfaces of combustible materials such as wood,polymers, fabrics, paper, etc. These coatings reduce the fire

    hazards associated with these materials. These coatings canprevent substrate ignition despite the proximity of a heatsource and, if ignition occurs, can cause a decrease in therate of fire spread. These coatings are not limited only for thesubstrates that are directly combustible. Flammable liquidsare often stored in non-combustible, painted, metal con-tainers. These containers can be coated with such an infraredreflective coating to keep the container surface cooler, andthus its flammable liquid contents cooler, when a fire isnearby.

    EXPERIMENTATION

    We undertook a very ambitious research program forsynthesis and evaluation of infra red reflecting pigments.Little did we know that theories were simply not available topredict physical properties such as IR reflectivity. A series ofdiscussions involving some of our colleagues from physicsand dyes departments met with a dead end. Thinking thatTitanium was an important ingredient of infra red reflectingmaterials, we tried some synthetic procedures for preparationof titanium bearing pigments including elements fromtransition group known to impart colour. Several pigmentswere synthesized using Cobalt, Iron, Nickel and Manganesein different atomic proportions and evaluated for their IRreflectivity. None of the products so synthesized displayedany significant IR reflectivity, though most displayedexceptional pigmentary properties and a range of colours.

    The parameters responsible for IR reflectivity of apigment seem to be unknown. Just like other physicalproperties of matter such as density, transparency or opacity,refractive index, color, thermal conductivity, electricalconduction or resistance, there seems to be no method topredict if a substance would be able to reflect infraredradiations or not. We were reminded of the extensiveresearch work carried out all over the world in thousands oflaboratories for inventing super conductors and semiconductors, which were inorganic materials. Our search tocorrelate structural parameters to any of the physicalproperties using available theories of physics (known to us)met with a dead end. Why an element or a compoundbehaves in a particular way appears to be as yet unpredic-

    table. An IR reflecting pigment may contain a given numberof elements in a specific proportion. Altering the proportionor the temperature of calcination results in completelydifferent product. In order to correlate the IR reflectivity of apigment with different parameters, the only available methodwas to screen and search. This was adopted to look for anyunreported infrared reflective compounds. As our studieshave indicated, this approach appears to have the biggestpotential to find new products.

    Ceramic pigments are inorganic compounds with highpigmentary properties. Several commercial ceramic

    pigments were screened by a simple laboratory test. Certaincommercially available ceramic pigments were used for thetest as they cover the entire transition elements series (excepnoble metals) based on the information provided by thesuppliers. These have good pigmentary properties and arealso manufactured by calcination at high temperatures as thaof the commercially available infrared reflective pigmentsThese pigments were obtained from Ceramitec Industries

    Thane (Maharashtra, India). Some commercially availableinfrared reflective pigments were also received fromShepherd Color Company, USA, for the test.

    Experimental

    1.Chemical Analysis

    No efforts were made to chemically analyze thepigments.

    2.Preparation of Paints

    The pigments were used as received and only dispersed(not ground) for preparation of the paints. The pigmentswere converted into coatings using Styrene-acrylic emulsionwith 50% solids as the binder with pigment to binder ratio o1: 1.

    3. Evaluation of performance

    The samples were evaluated using the Lab test devisedby us to evaluate the pigment samples used in the presenstudy. The coatings were applied on cement roof sheepanels with 6 mm thickness. The panels were exposed toPhilips 250 W, 220-230 V Infrared lamp. The distancebetween the panel and the lamp was maintained at 30 cm. Athermocouple at the other side of the panel recorded thetemperature. The panels were allowed to equilibrate for anhour before the temperature was recorded. The sameprocedure was followed for an uncoated panel. Thedifference in the temperatures at the reverse side of both thepanels showed whether the pigments used were infraredreflective or not. Figure 2shows the experimental setup fomeasuring the infrared reflectivity of pigments and Table 1shows a list of all the pigments and coatings tested for IRreflectivity.

    Fig. (2).Experimental setup for measuring the infrared reflectivity

    of pigments.

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    (Table 1) Contd

    Sample Manufacturer Temperature of

    Uncoated Panel (C)

    Temperature of Coated

    Panel (C)

    Temperature

    Difference (C)

    Blue 217 (VI)

    Cement roof Panel Shepherd Color 77 60 17

    Green 179

    Cement roof Panel Shepherd Color 77 62 15

    Black 411

    Cement roof Panel Shepherd Color 77 59 18

    Green

    Cement roof Panel Shepherd Color

    77

    64 13

    White

    Insulating Coating

    Cement roof Panel

    Local Manufacturer 60 52 08

    Co, Al, Zn Ceramic

    Pigment

    Cement roof Panel

    Ceramitec Industries 60 53 07

    Co, Al, Fe, Zn,

    Sn

    Ceramic pigment

    Cement roof Panel

    Ceramitec Industries 60 55 05

    Co, Al, Si, Zn

    Ceramic Pigment

    Cement roof Panel

    Ceramitec Industries

    60 61 - 01

    Co, Cr, Fe, Al, Zn

    Ceramic Pigment

    Cement roof Panel

    Ceramitec Industries 60 55 05

    Fe, Cr, Mn, Co,

    Ni, Ceramic pigment

    Cement roof Panel

    Ceramitec Industries

    60 61 - 01

    1342

    Brown

    Cement roof Panel

    Ceramitec Industries

    60 57 03

    1432

    Brown

    Cement roof Panel

    Ceramitec Industries

    60 57 03

    2133

    Brown

    Cement roof Panel

    Ceramitec Industries 60 58 02

    2143

    Brown

    Cement roof Panel

    Ceramitec Industries 60 58 02

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    76 Recent Patents on Chemical Engineering, 2008, Vol. 1, No. 1 Malshe et al

    (Table 1) Contd

    Sample Manufacturer Temperature of

    Uncoated Panel (C)

    Temperature of Coated

    Panel (C)

    Temperature

    Difference (C)

    4231

    Brown

    Cement roof Panel

    Ceramitec Industries 60 54 06

    2261

    Black

    Cement roof Panel

    Ceramitec Industries 60 59 01

    431515C

    Black

    Cement roof Panel

    Ceramitec Industries 60 61 - 01

    43205

    Black

    Cement roof Panel

    Ceramitec Industries 60 56 04

    1GY95

    Black

    Cement roof Panel

    Ceramitec Industries 60 57 03

    161515

    Black

    Cement roof Panel

    Ceramitec Industries 60 60 00

    6012

    Green

    Cement roof Panel

    Ceramitec Industries 60 55 05

    23112

    Green

    Cement roof Panel

    Ceramitec Industries 60 55 05

    (Undisclosed composition)D

    Cement roof Panel Ceramitec Industries 60 51 09

    (Undisclosed composition) K

    Cement roof Panel Ceramitec Industries 60 51 09

    Violet Fine

    (2% Loading)

    (Mica coated pigments)

    Cement roof Panel

    Sudarshan Chemicals

    Pune, India 60 55 05

    Violet Fine

    (25% Loading)

    (Mica coated pigments)

    Cement roof Panel

    Sudarshan Chemicals

    Pune, India 60 49 11

    Silver Fine

    (2% Loading)

    (Mica coated pigments)

    Cement roof Panel

    Sudarshan Chemicals

    Pune, India 60 58 02

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    Infrared Reflective Inorganic Pigments Recent Patents on Chemical Engineering, 2008,Vol. 1, No. 1 77

    (Table 1) Contd

    Sample Manufacturer Temperature of

    Uncoated Panel (C)

    Temperature of Coated

    Panel (C)

    Temperature

    Difference (C)

    Silver Fine

    (25% Loading)

    (Mica coated pigments)

    Cement roof Panel

    Sudarshan Chemicals

    Pune, India 60 49 11

    Silver Fine

    (50% Loading)

    (Mica coated pigments)

    Cement roof Panel

    Sudarshan Chemicals

    Pune, India 60 41 19

    Aluminum Paste

    Cement roof Panel

    Sudarshan Chemicals

    Pune, India 60 46 14

    Hi Tech Red Coating

    Cement roof Panel Hi Tech 60 54 06

    Hi Tech White Coating

    Cement roof Panel Hi Tech 60 46 14

    Hi Tech Green Coating

    Cement roof Panel Hi Tech 60 54 06

    All the pigments listed above are inorganic pigmentsobtained from different manufacturers. They contain diffe-rent components and are of different particle sizes. Most ofthese pigments have shown good IR reflectivity. Some of thepigments having the similar composition have shown diffe-

    rent reflectivity.

    Another method to determine the infrared reflectivity ofpigments is Drift Infrared Spectroscopy. The samples wereanalyzed by Perkin Elmer Spectrum One NTS system. Thistechnique is able to accurately measure the infraredreflectivity with little or no sample preparation. It is alsoversatile and fast and can be used for wide range of samples.Figs. 3 and 4 show the drift infrared spectra of SyntheticRutile (enriched ilmenite by reduction and acid treatment toremove iron) obtained from CMRL. These pigments havecrystal structure same as that of rutile titanium dioxidecontaining traces of iron. These pigments have shown goodreflectivity in the wavelength of 700 to 2400 nm.

    CURRENT & FUTURE DEVELOPMENTS

    Industry has realized the potential of these pigments andproducts for refrigerated transport containers, roofs of thebuildings are already in use. Further use of these coatings inpetroleum refinery to reduce evaporation losses of hydro-carbons is being explored. Published information onpreparation of these pigments is not available. It is expectedthat a good deal of research may be published in comingyears. We have found from our tests that there is no knownmethod 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 physicaproperty of the pigment cannot be correlated with its infraredreflectivity. Like other physical properties of matter such asdensity, transparency or opacity, refractive index, colorthermal conductivity, electrical conduction or resistance

    semi conductivity there is no method to predict the infraredreflectivity of a substance. It should be possible to screen alarge number of inorganic pigments manufactured by severamanufacturers world over to look for IR reflecting pigmentsOrganic pigments , which are relatively simple to synthesizeand are far more explored could be screened for their IRreflectivity. Only one compound of about 100 synthesizedqualifies for a commercial pigment. There are thus thousandof products that need to be studied for their IR reflectivityThe starting point is chlorophyll. Collaborative researchprograms between physicists, organic chemists and paintechnologists are more likely to yield results.

    ACKNOWLEDGEMENTS

    1.

    CMRL (Cochine Minerals and Rutiles Ltd) foproviding scholarship to carry out the research work(AKB).

    2.

    Mr. S.L. Garg, Chairman, CMRL for valuablediscussions during the research work

    3.

    Mr. Mohan Tamhankar, Ceramatic Industries, W-88T.T.C Ind. Area, MIDC Rabale, Navi Mumbai - 400701India for providing the ceramic colours

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    78 Recent Patents on Chemical Engineering, 2008, Vol. 1, No. 1 Malshe et al

    4. Shepherd Color Company, 4539 Dues Dr., Cincinnati,OH 45246-1006, USA for a liberal supply of the entirerange of their IR reflecting products

    5.

    Sudarshan Chemicals, 162 Wellesley Road, Pune,Maharashtra, 411 001, India for their developmentalproducts

    6. Perkin Elmer, 102 Gateway Plaza, Hiranandani Gar-dens, Powai, Mumbai, 400 076, India, for the instru-mental analysis.

    7.

    My (VCM) colleagues from Physics department, ProfAparna V.Deshpande, Prof. N.C.Debnath, and fromDyes department Dr.Ganapati Shankarling and Dr. N

    Fig. (3).Drift IR Scan of Synthetic Rutile (Enriched Ilmenite) Sample.

    Fig. (4).Drift IR Scan of Synthetic Rutile (Hydrated Titanium Dioxide) Sample.

    14000.0 13000 12000 11000 10000 9000 8000 7000 6000 5000 4000.0

    61.77

    62.0

    62.2

    62.4

    62.6

    62.8

    63.0

    63.2

    63.4

    63.6

    63.8

    64.0

    64.2

    64.4

    64.6

    64.8

    65.0

    65.2

    65.4

    65.6

    65.8

    65.93

    cm-1

    %T

    4511.28,62.12

    13257.26,65.47

    12482.23,65.66

    10275.56,64.43

    7089.34,65.04

    5119.48,61.79

    4161.46,65.46 4043.05,65.56

    14000.0 13000 12000 11000 10000 9000 8000 7000 6000 5000 4000.0

    64.14

    64.5

    65.0

    65.5

    66.0

    66.5

    67.0

    67.5

    68.0

    68.5

    69.0

    69.5

    69.95

    cm-1

    %T

    10360.85,65.37

    4509.50,65.67

    7089.34,68.82

    5173.30,64.13

    4150.69,69.034064.58,69.07

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