Industry | Porous polymer coating can dynamically control light and heat

This porous polymer coating changes from white to transparent when it is wetted, and can be placed in a plastic shell to make a panel that controls the light and temperature of the building.
The Columbia Engineering Building uses more than 30% of its energy for heating, cooling and lighting systems.
Passive designs such as cool roof coatings have come a long way in reducing this usage and its impact on the environment and climate, but they have a key limitation-they are usually static and therefore impaired by daily use.
Or there is no response to seasonal changes.
Image source: Jyotirmoy Mandal Columbia University engineering researchers have developed porous polymer coatings ( PPCs ) that can control light and heat in buildings in an inexpensive and scalable way.
They used the optical switchability of PPCs at solar wavelengths to adjust the solar heating and daylighting, and extended this concept to thermal infrared wavelengths to modulate the heat radiated by objects.
The results of this research were published in the journal "Joule" on October 21, 2019.
PPCs lead author of the study, Yuan Yang (Yuan Yang) before laboratory PhD, assistant professor of materials science and engineering Jyotirmoy Mandal, said: "Our research shows that wetting PPCs by common liquids such as water or alcohol, we can reversible Change their transmittance in the sunlight and thermal light wavelengths.
" "By placing such PPCs in hollow plastic or glass panels, we can create building envelopes that can regulate indoor temperature and light.
" The team’s The design is similar to that of smart windows, but with higher optical switchability, and the use of simpler and cheaper materials to construct, it can be realized on a large scale.
It builds on the foundation of earlier work and demonstrates a paint-like fluoropolymer coating with nanometer to micrometer pores that can cool buildings.
However, the coating is static.
"In a place like New York where the summer is warm and the winter is cold, a design that can switch between heating and cooling modes is more useful," Yuan Yang said.
When Mandal noticed a few drops of alcohol splashed on the white fluoropolymer PPC to make it transparent, the team accidentally started work on the PPCs optical switch.
"The mechanism we see is the same as the one that makes the paper translucent when wet, but close to the optimal level," Mandal said.
"The physical principles of this phenomenon have been explored before, but the drastic transformations we saw led us to explore this particular situation and how to use it.
"Porous materials such as paper look white because the refractive index of the air in the pores is different (?1) from the refractive index of the porous material (?1.
5), causing them to scatter and reflect light.
When wetted by water, the refractive index (approximately 1.
33) is closer to the substance, the scattering is reduced, and more light passes through, making it translucent.
When the refractive index approaches, the transmittance increases.
The researchers found that the refractive index of their fluoropolymer (?1.
4) and typical alcohol (?1.
38) are very close.
"Therefore, when the porous polymer is wetted, its optical properties become uniform," Yang said.
"Light is no longer scattered, but like passing through solid glass, the porous polymer becomes transparent.
" Image source: Jyotirmoy Mandal Researchers proved that ibuychem.
com/new/sjk?search_name=ppc&search_type=1" target="_blank">ppc ppc- based roofs can be switched between reflection and transmission states, and can be used To control the indoor temperature of the building.
Since the refractive indices of alcohol and fluoropolymer are almost exactly matched, the team can change the solar transmittance of its PPC by approximately 74%.
For the visible part, the change is 80%.
Although the switching speed is slower than typical smart windows, the change in transmittance is much higher, which makes PPCs attractive for controlling sunlight in buildings.
Researchers also studied how to use optical switches for temperature regulation.
"We envision that the roofs are white in summer to keep the buildings cool, and in winter they are darkened to heat them, which can greatly reduce the building's air conditioning and heating costs," Yang said.
In order to test their ideas, the researchers placed a panel containing PPC on a dollhouse with a black roof.
One panel is dry and reflective, while the other panel is damp and translucent, with a black roof underneath.
In the summer sun at noon, the temperature of the white roof is about 3 degrees Celsius/5 degrees Fahrenheit cooler than the surrounding air, while the temperature of the black roof is about 21 degrees Celsius/38 degrees Fahrenheit warmer than the surrounding air.
The team also studied the switching of thermal infrared wavelengths and observed a new switch between the "ice room" and "greenhouse" states by wetting infrared transparent polyethylene PPC.
When dry, the porous polyethylene PPC reflects sunlight, but emits radiant heat, like an "igloo".
Wetting PPCs causes them to transmit sunlight, and because typical liquids absorb heat wavelengths, they block radiant heat like a greenhouse.
Since they are simultaneously adjusted solar radiation and heat radiation, and therefore may be white day regulate the heat at night.
Tianhe “Although this transition is simple, it is very unusual compared to transitions in other optical systems, and it may be the first report,” Mandal said.
Yang's team also tested other potential applications, such as thermal camouflage and coatings that respond to rain.
The latter can be used to cool or heat buildings in the Mediterranean climate zone and coastal areas of California, which have dry summers and rainy winters.
Researchers are now looking for ways to scale up designs and explore opportunities to deploy and test them on a large scale.
Yang said: "In view of the scalability and performance of PPC-based designs, we hope that their applications can be widely used, especially their potential applications on building facades make us excited.
" Mandal is currently the University of California, Los Angeles.
A Schmidt scientific researcher who is doing post-doctoral research, he added: “We deliberately chose general-purpose polymers and simple designs for our work.
The goal is to make them locally manufacturable and implementable in developing countries, because In developing countries, they will have the greatest impact.
" The title of this study is "Porous polymers with adjustable light transmittance for optical and thermal regulation.
"