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Friday, May 14, 2021

Explained: The ‘whitest ever’ paint that can reflect 99 per cent of sunlight

Buildings coated with this paint may be able to cool them off enough to reduce the need for air conditioning, the researchers have said.

Written by Explained Desk | New Delhi |
Updated: April 22, 2021 9:21:51 am
Purdue researchers Xiulin Ruan (left) and Joseph Peoples use an infrared camera to compare the cooling performance of white paint samples on a rooftop. (Purdue University photo/Jared Pike)

Engineers from Purdue University in the US have created what they are calling the whitest paint yet. Buildings coated with this paint may be able to cool them off enough to reduce the need for air conditioning, the researchers have said.

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What is the whitest paint?

The team of researchers at the university created an ultra-white paint in October pushing the limits of how white paint can be. This older formulation was made of calcium carbonate, while the new one is made up of barium sulphate, which makes it more white.

The newer paint is whiter and keeps the surface areas it is painted on cooler than the formulation before this could. If this new paint was used to cover a roof area of 1,000 square feet, it may be able to get a cooling power of 10 kilowatts, the team has said.

Most ovens use up about 2.3 kilowatts to run for an hour and a 3 ton 12 Seasonal Energy Efficiency Ratio (SEER) air conditioner uses up about 3 kilowatts to run for an hour.

The team has also claimed that this paint may be the closest equivalent to the blackest black paint called “Vantablack” that is able to absorb up to 99.9 per cent of visible light.

A press release issued by Purdue University notes that typical commercial white paint gets warmer rather than cooler and that paints currently available in the market that are designed to reject heat reflect only 80-90 per cent of the sunlight and therefore, they can’t make surfaces cooler than their surroundings.

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But first, how do we see colours and what determines if a colour absorbs or reflects light?

To understand how this works one needs to note that whenever an object is seen by the eye, it is either because of sunlight or the artificial light in the room. This light is made up of seven different colours (Violet, Indigo, Blue, Green, Yellow, Orange and Red or VIBGYOR). Specifically, light is made up of wavelengths of different colours.

If an individual is looking at a sofa that is green, this is because the fabric or material it is made up of is able to absorb all the colours except green. This means that the molecules of the fabric reflect the green coloured wavelengths, which is what the eye sees. Therefore, the colour of any object or thing is determined by the wavelength the molecules are not able to absorb. Which is to say that whichever wavelength of colour is not absorbed by an object, will be the colour that the eye sees.

For instance, if a person is looking at a bowl of vanilla ice cream or a plain white wall, it is because the molecules that make up these two are absorbing none of the wavelengths of light. This means, the entire spectrum of these coloured wavelengths if reflected from the surface, results in the white colour that the eye notices.

What determines which wavelength of light will be reflected and absorbed?

This is dependent on how electrons are arranged in an atom (the building block of life, an atom is made up of electrons, protons and neutrons. These three particles make up everything in the known universe from mountains, planets, humans to pizza and cake). In contrast, if an object is black, it is because it has absorbed all the wavelengths and therefore no light is reflected from them. This is the reason that darker objects, as a result absorbing all wavelengths tend to heat up faster (during absorption the light energy is converted into heat energy).

So, what makes the paint so white?

There are two features, one is the paint’s high concentration of a chemical compound called barium sulfate, which is also used to make photo paper and cosmetics white. The second feature is that the team has used different sized particles of this chemical compound, which means different sizes scatter different amounts of light. In this way, a varying size of particles of the compound make sure that the paint can scatter more of the light spectrum from the sun.

“There is a little bit of room to make the paint whiter, but not much without compromising the paint,” the press release said. This is because while a higher particle concentration can be used to make something whiter, tweaking this concentration or making it higher too much makes it easier for the paint to break or peel off.

Significantly, because the paint is so white, the researchers demonstrated outdoors that the paint can keep surfaces 19 degrees Fahrenheit cooler than their ambient surroundings at night. It can also cool surfaces 8 degrees Fahrenheit below their surroundings under strong sunlight during noon hours.

The research is a result of six years of research building on attempts going back to the 1970s to develop radiative cooling paint as a feasible alternative to traditional air conditioners. The team tested hundreds of different materials, narrowed them down to ten and tested about 50 different formulations for each material.

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