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Energy efficient paints ... using stealth technology.

The recent dramatic rise in oil and natural gas prices has once again raised awareness among consumers of the need to increase energy efficiency. To date, emphasis has been primarily placed on reducing energy consumption in automobiles and commercial/industrial facilities and operations. At some point that focus will expand to include residential buildings. When this happens, QinetiQ, a company spun out of the UK's Ministry of Defense research group, will be ready to supply an insulating paint that will reduce the demand for heating and cooling, and thus improve the energy efficiency of homes.

Insulating materials currently used in today's buildings largely rely on conduction or convection to move heat to or away from desired areas. Very few materials, though, control the transfer of energy that occurs by means of thermal radiation, according to Dr. Eoin S. O'Keefe, QinetiQ Fellow and senior technical consultant for stealth materials with the company.

There are paints that possess some radiative energy control ability, but each has its own limitations. Paints containing reflective granular pigments can only achieve minimal reflectivity in the thermal infrared ([IR.sub.T]) waveband. Coatings formulated with large quantities of good-sized metal flakes (typically aluminum) that act like reflectors can achieve much higher [IR.sub.T] reflectivities but are considered aesthetically unappealing due to their silver-metallic appearance. Paints that possess a metallic flake layer beneath a thin binder/pigment layer initially overcome the appearance issue. However, alignment of the metallic flakes is disrupted due to the presence of the pigment particles, so reflectivity is decreased. In addition, the binder/pigment layer must be very thin in order not to reduce [IR.sub.T] reflectivity further, and thus, quickly wears away through scuffing and abrasive cleaning.

O'Keefe's research in the stealth area relates to the control of interactions between energy and matter (absorption, radiation, and directionality of detectable energy). One aspect of that research involves investigating how [IR.sub.T] energy reacts with matter via radiation. His experience in this area led to the idea for a new flake structure that would overcome these difficulties.

QinetiQ's material is comprised of a metal core sandwiched between two layers of organic polymer that is transparent in the [IR.sub.T] waveband and has embedded in it stabilized dyes that also do not absorb thermal infrared energy. "The advantage of this design is that the reflective material is incorporated directly into the binder along with the colorant," notes O'Keefe. Eliminating the two-part system increases reflectivity due to more effective alignment of the flakes both with themselves and with the surface. Abrasion is also no longer an issue because the color exists throughout the paint and not just in a thin top layer.

The polymer is typically an acrylic resin but can also be a polyolefin or polyalkane. O'Keefe selected stabilized dyes over traditional pigments because they exhibit very intense visual colors and therefore very little material is required. "The dye center must be chosen carefully, though, with respect to [IR.sub.T] absorption. Surprisingly, a number of compounds such as azo dyes, phthalocyanines, and anthroquinones have high specific absorption in the visible waveband due to electronic transitions, but weak specific absorption at [IR.sub.T] wavelengths from molecular vibrations," O'Keefe explains.

The flakes are produced in a series of steps. First, a release layer is formed on a carrier film, typically a polyester. The first color layer--acrylic resin and stabilized dye--is then deposited. An aluminum layer (or other appropriate metal) is next evaporated onto the organic polymer layer. Finally, the second color layer is deposited. QinetiQ uses either a water- or heat-soluble release agent to remove the finished material. Typical grinding techniques are employed to form the flakes.

Altering the choice of metal for the core of the flake makes it possible to control the optical behavior of the flake itself. Using indium tin oxide, for example, which is optically transparent but still reflective, an insulating clear lacquer can be produced. The lacquer can be colorless as well if no dyes are incorporated into the transparent organic polymer.

Paint formulations incorporating the flakes have been developed in a variety of colors and with different levels of gloss, etc. They are low-VOC coatings with no heavy metals that meet current regulatory requirements. Because of the thin layer of metal in the core of the flakes, these paints have excellent opacity. In general, a two-flake thickness layer (~ 2 microns) of these coatings will provide 100% optical coverage. "The paint obviously will not be robust at this thickness, but it may be possible to apply thinner layers than what is typically used for traditional architectural paints," O'Keefe comments.

QinetiQ has tested these new coatings, and although no specific cost or energy savings have been enumerated, a real advantage has been observed, according to O'Keefe. Some modeling studies have also been conducted, but because the results are dependent on the individual factors in each case, no general statement about the level of benefit can be made.

Currently, the company is discussing this new technology with various paint manufacturers, both large and small. So far, O'Keefe has found a higher level of interest from specialty, niche producers and QinetiQ is working with several of them to develop commercial grade formulations. "We expect, though, that as pressure for improving energy efficiency in residential buildings becomes stronger, the major paint companies will begin to take a more active interest. At this point consumers remain focused on style and aesthetics, so that is what large paint manufacturers will continue to focus on."

QinetiQ has been awarded a patent for its reflective coating technology (WO 2005/007754). For more information, please contact Dr. Eoin O'Keefe at +44.2392. 582521 or esokeefe@QinetiQ.com, or visit www.QinetiQ.com.

RELATED ARTICLE

Although not used for residential energy efficiency, one product has already been generated based on QinetiQ's technology. Mirage[TM] vehicle marking film is used by some UK police forces to clearly indicate the identity of police vehicles for observers conducting airborne surveillance. Prior to the introduction of this marking system, at night and in poor weather conditions, airborne observers could not distinguish one police car from another, nor police cars from those belonging to suspected criminals or citizens. Black lettering in the Mirage material can easily be viewed in the visible and thermal infrared range, allowing for clear identification of police vehicles. "We have also demonstrated that this technology can work well for military, emergency, and other vehicles transporting high value cargos."
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Title Annotation:Coatings Xperience
Publication:JCT CoatingsTech
Date:Mar 1, 2007
Words:1080
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