Curb global warming with ultra-white paint

Following "Whitest Paint," Purdue researchers discuss ultrawhiteness and how barium sulfate can be used to create coatings and films that achieve significant daytime subambient radiative cooling

By Xiangyu Li, Joesph Peoples, Peiyan Yao, School of Mechanical Engineering, Purdue University, USA

To curb global warming, engineers at Purdue University have developed the whitest paint yet

Previously, the team created an ultra-white paint that pushed the limits of "white" paint

"If you cover a 1,000-square-foot roof with this paint, we estimate you're getting the equivalent of 10 kilowatts of cooling power

The researchers believe this white may be most similar to the blackest black, "Vantablack," which absorbs 99.

Typical commercial white paint will warm rather than cool

The team's research paper, showing how the coating works, was originally published in the journal ACS Applied Materials & Interfaces and is now reproduced here

Radiative cooling is a passive cooling technology that is of great significance in reducing space cooling costs, combating the urban island effect, and mitigating global warming

Attempts have been made to achieve daytime passive radiative cooling with a single coat of paint, but usually require a thick layer of paint or show only localized daytime cooling
.

In this study, we experimentally demonstrate that nano-BaSO4 particle films and BaSO4 nanocomposite coatings have remarkable whole-day sub-ambient cooling performance
.
BaSO4 has a high electronic bandgap at low solar absorption and a phonon resonance at 9 μm for high skylight emissivity
.
With suitable particle size and wide particle size distribution, the solar reflectance of the prepared nano BaSO4 particle film reaches 97.
6%, and the skylight emissivity reaches 0.
96
.
In field tests, the BaSO4 film was more than 4.
5°C lower than the ambient temperature, or reached an average cooling power of 117W/m2
.
The BaSO4-acrylic paint has a volume concentration of 60% for increased reliability in outdoor applications, achieving 98.
1% solar reflectance and 0.
95 skylight emissivity
.
Field tests show that its cooling performance is similar to that of BaSO4 films
.

Overall, BaSO4-acrylic paint has a standard figure of merit of 0.
77 and is one of the highest radiative cooling solutions available, while offering extreme reliability, proper paint form, ease of use, and compatibility with commercial paint manufacturing processes
.

Radiant cooling has shown great potential to reduce space cooling costs in a variety of cooling applications
.
In contrast to active cooling, which requires electricity to drive the refrigeration cycle, radiative cooling utilizes atmospheric transparent windows (“skylights”) to emit thermal radiation directly into the deep sky without consuming any energy
.
Its passive nature has the potential to mitigate the urban island effect by lowering building temperatures and reduce global warming by reducing carbon dioxide emissions for cooling applications
.
Since the high emissivity of the skylight exceeds incoming solar absorption, it is possible for the surface to remain below ambient temperature even in direct sunlight
.

Early research into radiative cooling coatings continued for decades, but these coatings have not yet achieved radiative cooling throughout the day
.
Among them, a study showed that sub-ambient cooling of 2°C can be achieved after coating a layer of TiO2 coating on an aluminum substrate, but the high solar reflectance should mainly come from the metal substrate, not the coating itself
.
Many coatings are based on titanium dioxide and have low particle concentrations, which limit radiative cooling performance due to insufficient solar reflection due to solar absorptivity in the ultraviolet band
.
To this end, wide-bandgap materials have been investigated as fillers to eliminate UV absorption, while their smaller refractive indices result in weaker photon scattering
.

Heat-reflective coatings have also been developed, but they are still limited to less than 91% solar reflectance and do not exhibit sub-ambient cooling throughout the day
.
Additionally, photonic structures and multilayer films have recently demonstrated whole-day sub-ambient cooling capabilities, which has stimulated a strong interest in radiative cooling
.
Other studies have explored scalable non-coating methods such as double layers including a metal layer, polyethylene aerogels, and delignified wood
.
However, these methods are limited in one or more aspects, such as complex structures, intervening metal layers, and large thicknesses, limiting their various applications
.
Therefore, the development of high-performance radiative cooling coatings is still an important task
.

Recently, non-metallic bilayer designs have been proposed, consisting of a TiO2 top layer for solar reflectance and a bottom layer for thermal emission, which enables localized daytime cooling without metallic components
.
A dense film made of SiO2 nanoparticles with partial daytime cooling capacity as a monolayer coating
.
Porous, paint-like polymers have been developed that provide cooling throughout the day
.
A strategy is proposed to further improve the solar reflectance of particle-binder coatings by employing a broad particle size distribution rather than a single particle size
.
Combined with high filler concentrations, CaCO3-acrylic coatings with layer thicknesses of 200−400 μm were prepared and demonstrated to have sub-room-day cooling throughout the day
.

Another study also proposed wide-bandgap nanoparticle coatings and high filler concentrations to achieve high radiative cooling performance, but the reported coating thickness of 1 mm still poses challenges for potential applications of radiative cooling coatings
.
Considering the existing research, developing thin-layer, low-cost, easy-to-apply, and scalable high-performance single-layer coatings remains a challenging and urgent task to take full advantage of radiative cooling in a wide range of applications
.

In this study, we experimentally demonstrate the use of nano-BaSO4 particle films and BaSO4-acrylic coatings for whole-day sub-ambient cooling
.
We chose BaSO4 because of its high electronic bandgap, low solar absorption, and phonon resonance at 9 μm with high skylight emissivity
.
With suitable particle size and wide particle size distribution, the solar reflectance of the prepared nano BaSO4 particle film reaches 97.
6%, and the skylight emissivity reaches 0.
96
.

Field tests show that the surface temperature is more than 4.
5°C lower than the ambient temperature or the average cooling power is 117W/m2, the highest cooling capacity reported
.
To improve the reliability of the coating, a BaSO4-acrylic paint with a concentration of 60% by volume was developed
.
The higher filler concentration and wider particle size distribution overcome the low refractive index of BaSO4, making it solar reflectance of 98.
1% and skylight emissivity of 0.
95
.

In field tests, the BaSO4 coating also had the same high cooling performance
.
Displaying a 0.
77 standard quality factor, our BaSO4-acrylic paint is one of the highest radiative cooling solutions available, while offering extreme reliability, proper paint form, ease of use, and compatibility with commercial paint manufacturing processes
.

The results of this paper are contained in a provisional patent filed on October 3, 2018, and a non-provisional international patent filed on October 3, 2019 and published on April 9, 2020 (PCT/US2019 / 054566) pending
.

Results and discussion

Commercial white paints, such as TiO2-acrylic paints, fail to achieve passive whole-day sub-ambient cooling, due in large part to their sensitivity to the UV band (due to the 3.
2 eV electronic bandgap of TiO2) and the near-infrared band (due to acrylic acid) absorption) high absorption
.
In this experiment, we fabricated BaSO4 particle films with a thickness of 150 μm on silicon wafers (Fig.
1a)
.
A commercial white paint (DutchBoy Maxbond Ultra White Acrylic Exterior Paint) was selected as a control sample
.
The scanning electron microscope (SEM) image of the BaSO4 film in Fig.
1b shows the presence of pores in the film
.
The interface between the nano-BaSO4 particles and air voids enhances the scattering of photons in the film, thereby increasing the overall solar reflectance
.
To improve the reliability of coatings under long-term outdoor exposure, commercial coating forms are often preferred as filler-binder composites
.
A key challenge in using BaSO4 as a filler material in a polymer matrix is ​​its low refractive index compared to other common filler materials such as TiO2
.
To achieve strong scattering in the composite, we used a high filler volume concentration of 60%, whereas most commercial coatings have much lower concentrations
.
In addition, the broad particle size distribution contributes to solar reflectivity
.
The BaSO4-acrylic nanocomposite coating is shown in Fig.
1a, and Fig.
1c is its SEM image
.
The addition of acrylic helps to bond the filler and leads to better reliability
.
Some air voids are present in the BaSO4 paint, which also increases solar reflectance
.
Based on our previous theoretical work, we chose an average particle size of 400 nm, which is beneficial to the overall solar reflectance
.
Furthermore, in previous experiments, we found that a wider particle size distribution (>100 nm) can significantly improve the overall solar reflectance compared to a uniform particle size distribution
.
The purchased BaSO4 particles were measured by scanning electron microscopy, and their particle size distribution was 398±130nm, which met our needs and our design guidelines
.

Figure 1: Radiative cooling coating and SEM images
.
(a) BaSO4 film sample, BaSO4-acrylic paint sample and white commercial paint
.
The thickness of the BaSO4 film on the silicon wafer is 150 μm
.
The BaSO4 coating and the commercial coating were freestanding unsubstrate samples of 400 μm thickness
.
All samples are 5 square meters
.
(b) SEM image of BaSO4 thin film
.
(c) SEM image of BaSO4-acrylic composite coating, 60% filler concentration
.
(b, c) The particle size distribution was estimated to be (398±130nm) from SEM images
.
Pores were introduced in both BaSO4 films and BaSO4 coatings
.

To achieve high radiative cooling performance and daytime sub-ambient cooling, coatings require high reflectivity in the solar spectrum (contributed by particles), and high emissivity in skylights (contributed by particles and/or binders) such as shown in Figure 2a
.
Here, we employ BaSO4 with a high electronic bandgap ~6 eV to reduce the absorption in the ultraviolet band
.
Due to the 9 μm phonon resonance in the skylight, the particle size design can allow the monolayer BaSO4 particle film to have both skylight emission and sunlight reflection functions
.
Therefore, no base material is required to achieve daytime sub-ambient cooling
.
The lack of an acrylic matrix also reduces NIR absorption
.

The average particle size of BaSO4 is selected to be 400 nm, which can reflect the visible and near-infrared wavelengths of solar radiation
.
The use of a broad particle size distribution can further improve the solar reflectance
.
Detailed theoretical and experimental studies can be found in previous studies
.
Overall, the solar reflectance of the BaSO4 film reaches 97.
6% and the skylight emissivity is 0.
96 (Fig.
2b)
.
Its solar reflectance is significantly higher than commercial white paint (400 μm thickness), especially in the UV and NIR range
.

Although heat-reflective commercial coatings have been introduced to the market, their solar reflectance (about 80-91%) is still much lower than BaSO4 films
.
The solar reflectance of the BaSO4 film is also higher than that of the recently reported CaCO3-acrylic paint
.
The silicon base plate is only used as a supporting substrate, which can neither increase the solar reflectivity nor the emissivity of the skylight
.

Figure 2: Radiant energy transfer for cooling paint, emissivity test results and Monte Carlo simulation of solar reflectance
.
(a) To obtain high cooling power using passive radiative cooling, high reflectivity in the solar spectrum and high emissivity in the skylight are required
.
Fillers can enhance solar reflectivity, while the infrared emissivity of skylights can be contributed by fillers and/or binders
.
For particle films, the particles both reflect sunlight and emit infrared heat in the skylight
.
(b) The emissivity of BaSO4 thin films and BaSO4 coatings were tested in the range of 0.
25–20 μm, compared with 400 μm thick commercial white coatings, and the solar reflectance of particle films and nanocomposite coatings while maintaining high skylight emissivity significantly enhanced
.
(c) The solar reflectance of BaSO4 thin films with different thicknesses and different substrates was measured, indicating that the solar reflectance of BaSO4 thin films of 150 μm is independent of the substrate
.
(d) Monte Carlo simulation of a BaSO4 coating with a thickness of 400 μm showing that both the high filler concentration and the broad particle size distribution increase the total solar reflectance
.
(e) Comparison of solar reflectance of BaSO4-acrylic coatings with 60% filler particle concentration and different film thicknesses with Monte Carlo simulation method results
.
A thin coat of paint is applied to the PET film
.

To avoid the influence of the substrate on the cooling performance, we tested the solar reflectance of different substrates in relation to the coating film thickness, as shown in Fig.
2c
.
For the BaSO4 coating, with higher filler concentration (60%) and larger particle size distribution, the 400 μm film thickness of the individual coating (without baseplate) sample achieves similar optical performance to the baseplate coating (98.
1% solar reflectance) rate, 0.
95 sunroof emissivity)
.

Monte Carlo simulations were performed using the modified Lorentz-Mie theory, shining the light on the physics to make the paint have a higher solar reflectivity, the results are shown in Figure 2d shown
.
The refractive index of BaSO4 was obtained by first-principles simulation
.
At the same filler concentration, the simulation results show that the particle size distribution is wider, which can further improve the solar reflectance
.
This simulation slightly underestimates solar reflectance because it does not capture the effect of voids
.
A 400 μm thick BaSO4 coating was applied to ensure that the measured emissivity was independent of the substrate
.
Using an applicator to control the wet film thickness, thin films were prepared on transparent polyester films
.
Due to the low transmittance of the coating film and the similar refractive index between the coating and the PET film, the effect of the PET substrate on the solar reflectance is negligible
.
When the coating thickness is 200, 224 and 280 μm, the solar reflectance reaches 95.
8, 96.
2 and 96.
8%, respectively (Fig.
2e)
.

The Monte Carlo simulation results show similar trends to the experimental data, both following the diffusive properties of the propagation
.
The Monte Carlo simulation slightly underestimated the solar reflectance, probably because the voids introduced in the paint were not captured by the Monte Carlo simulation
.
Future studies could include such effects to more accurately predict solar reflectance
.

Field tests demonstrate the full daytime sub-ambient cooling function of the BaSO4 film
.
Shown in Figure 3a is 64% relative humidity on March 14-16, 2018 at 12:00 a.
m.
, and 50% relative humidity at 12:00 a.
m.
on March 15, 2018, in West Lafayette, IN, BaSO4 film throughout the day Sub-ambient cooling reached 907W/m2
.
The sample temperature was 10.
5°C lower than ambient temperature at night and kept at 4.
5-10°C lower than ambient temperature during the day
.
The commercial coating is 6.
8°C higher than the ambient temperature at 2-3 pm
.
A direct test in Reno, Nevada showed 29% relative humidity at 2:00 AM on July 28, 2018 and 15% relative humidity at 12:00 PM on July 28, 2018 , the cooling power reached an average of 117W/m2 within 24 hours, see Fig.
3b
.
In the absence of sun exposure, we observed similar cooling power during daytime and nighttime, both above 110W/m2
.
The higher the surface temperature during the day, the greater the thermal emission power, which offsets the effect of higher solar absorption
.

Figure 3: Field test results of nano-BaSO4 thin films and BaSO4-acrylic nanocomposite coatings
.
(a) The temperature of the nano-BaSO4 film and commercial white paint was compared to ambient temperature over 24 hours
.
(b) The feedback heater directly characterizes the cooling ability of nano-BaSO4 thin films
.
(c) The temperature of the BaSO4 coating was compared with the ambient temperature
.
(d) The cooling capacity of the BaSO4 coating was measured during the day and at night
.
Sun exposure is indicated by the orange area
.

Therefore, reporting only cooling power without considering surface temperature may be a misleading measure of cooling performance
.
In this case, the thermal emission power of BaSO4 film reaches 106W/m2 at 15℃
.
Overall, our BaSO4 films maintain a constant high cooling power regardless of the sun exposure
.
Through field tests, we further demonstrate the cooling performance of BaSO4 coatings, as shown in Fig.
3c,d
.
Under sun exposure up to 993W/m2 (65% relative humidity at 12:00 am on April 2, 2019, 46% relative humidity at 12:00 pm), the BaSO4 coating remained cooler than ambient temperature for more than 24 hours
.
The cooling power measurement shows that when the surface temperature is as low as −10°C, the average cooling power exceeds 80W/m2, which is equivalent to 15°C (77% relative humidity at 12 am on February 9, 2019, 57% relative humidity at 2 pm on February 9, 2019).
) when the cooling power is 113W/m2
.

Here the figure of merit RC is used to fairly evaluate radiative cooling performance independent of weather conditions, i.
e.

RC = ε sky – r(1 – Rsolar)

where εsky is the skylight emissivity, Rsolar is the solar reflectance, and r is the ratio of solar irradiance to blackbody emission through the skylight
.
When the surface temperature is 300K and r is 10, our BaSO4 film and BaSO4 coating achieve RCs of 0.
72 and 0.
77, which are 0.
32, 0.
53, 0.
35, 0.
49 and 0.
57 higher than the state-of-the-art radiative cooling solutions, respectively.
The quality factor is to address the effect of weather on the field test results
.
It enables a fair assessment of the potential of any radiative cooling scheme under different weather conditions
.
Accurately predicting the effects of weather conditions on radiative cooling capacity is still being investigated along with other studies
.
We believe that the insights from these field studies, combined with our figure of merit, will be critical in predicting actual radiative cooling power and greatly shortening the development cycle of any future radiative cooling solution
.

To demonstrate reliability, we performed abrasion tests, outdoor weatherability and viscosity tests on BaSO4 coatings
.
Abrasion tests were performed with a Taber abraser according to ASTM D4060 and the results are shown in Figure 4a
.
Place two grinding wheels with a load of 250g on the surface to be tested
.
The wheel surface needs to be re-sharpened after every 500 cycles of rubbing.
The mass loss is monitored after 3000 rubbing cycles.
The wear index is described as the weight loss (μg) per cycle
.
The BaSO4 coating has an abrasion index of 150, which is comparable to the abrasion index of 104 for commercial exterior paints
.
The BaSO4 coating was exposed to outdoor for 3 weeks for weather resistance test (Fig.
4b)
.
The solar reflectance remains constant within experimental uncertainty
.
At the beginning and end of the test period, the emissivity of the sunroof was measured at 0.
95
.
In Figure 4c, the viscosity of the BaSO4 coating is similar to that of the commercial coating
.

Figure 4: BaSO4 coating reliability test
.
(a) Abrasion tests were performed on BaSO4 coatings and commercial exterior paints according to ASTM D4060
.
BaSO4 coatings exhibit abrasion resistance comparable to commercial exterior paints
.
(b) The BaSO4-painted samples were exposed outdoors for three weeks
.
(c) The viscosity of the BaSO4 paint was tested and was comparable to that of solvent-borne and water-borne commercial paints
.

Experimental part and conclusion 

In this study, we experimentally demonstrated the radiative cooling throughout the day for nano-BaSO4 thin films and BaSO4-acrylic nanocomposite coatings
.
With suitable particle size and wide particle size distribution, the nano-BaSO4 film can obtain a high solar reflectance of 97.
6% and a high skylight emissivity of 0.
96
.
Field tests show that the surface temperature is more than 4.
5°C lower than the ambient temperature, or the average cooling power is 117W/m2, one of the highest cooling powers reported
.
To improve the reliability of the coating, the volume concentration of BaSO4-acrylic paint is 60%
.
The high filler concentration and broad particle size distribution help achieve a solar reflectance of 98.
1% and a skylight emissivity of 0.
95
.
In field testing, BaSO4 coatings have the same high cooling capacity while providing high reliability, proper coating form, ease of use and compatibility with commercial coatings
.

Fabrication of Nano BaSO4 Particle Film and BaSO4-Acrylic Coating

BaSO4 particle films with a thickness of 150 μm were prepared on silicon wafers
.
Mix 400 nm BaSO4 particles, deionized water, and ethanol in a mass ratio of 2:1:1 and coat the substrate until completely dry
.
The average particle size was chosen to be 400 nm to reflect visible and near-infrared light from solar radiation
.
To prepare the BaSO4-acrylic nanocomposite coating, dimethylformamide and nano BaSO4 particles (400 nm in diameter, Research Nanomaterials, USA) were mixed and dispersed ultrasonically for 15 minutes using a Fisherbrand Model 505 sonic disperser
.
The mixture was degassed to remove air bubbles introduced during ultrasonic dispersion
.
Then slowly add the acrylic base (Elvacite 2028 from Lucite International) and mix until completely dissolved
.
The mixture was poured into molds to dry well overnight to obtain individual (without bottom plate) samples of 400 μm thickness to eliminate the effect of the bottom plate on the overall coating performance
.
In addition, a film applicator (BYK film preparation machine) was used to prepare a series of thinner BaSO4 particle films and paint films to study the effect of film thickness
.
Dry film thickness was measured with a coordinate measuring machine (Brown & Sharp MicroXcel PFX)
.

Measurement of spectral emissivity

Perkin Elmer Lambda 950 UV - Vis - NIR250nm2.
5μm
。Spectralon
。AM 1.
5
。0.
0050.
5%
。2.
520μmNicolet iS50 FTIR，PIKE
。（、Gemini、1.
51.
0mm）
。PIKE Technologies，0.
02
。

，，5
。
。，，，
。Apogee SP-510
。（5a）T
。
。

（LDPE）
。（5b），，
。，
。，，
。

Figure 5: Test setup for field testing of temperature and cooling capacity (a) The sample is suspended in a polystyrene foam cavity to minimize heat conduction and forced convection
.
During the temperature measurement, the sample and ambient temperature are monitored
.
(b) To directly measure the cooling power of the sample, a feedback heater is used to synchronize the sample temperature with the ambient temperature
.
The cooling power of the sample is obtained by monitoring the power consumption of the heater
.

Therefore, in our work, we choose to evaluate cooling performance mainly from the measurement of direct cooling power, which minimizes parasitic heat conduction and convective heat conduction
.

Monte Carlo simulation

Monte Carlo simulations were performed according to a modified Lorentz-Mie theory algorithm with dependent scattering correction due to high concentrations
.
Photonic packets start on nanocomposites
.
The initial weight of the photonic package is uniform and flies vertically to the top composite interface
.
When a photon packet propagates through the bottom-air composite interface, we consider it to be transported
.
If it goes back above the medium, it's considered a reflection
.
A total of 500,000 photons were used in each simulation, covering 226 wavelengths from 0.
25 to 20 μm
.