Purdue Ultrawhite Radiative Cooling Paint
Overview
Purdue University researchers developed BaSO4 (barium sulfate) nanoparticle acrylic paint achieving 98.1% solar reflectance — the whitest paint ever created — designed for passive radiative cooling of buildings without electricity.
Cooling Mechanism
The paint cools through two complementary processes:
- Reflecting incoming solar radiation (0.3-2.5 μm wavelengths)
- Emitting infrared heat through Earth's atmospheric transparency window (8-13 μm) directly to outer space
Unlike typical white paints that absorb and retain heat, this formulation maintains surfaces below ambient temperature even during daytime.
Material Composition
- BaSO4 at 60% volume concentration in acrylic base
- BaSO4 selected from over 100 candidate materials for:
- High electron bandgap → low solar absorptance
- Phonon resonance at 9 μm → high sky window emissivity
- ~50 formulations per material tested over 6 years of research
- Builds on radiative cooling paint concepts from the 1970s
Particle Geometry — The Key Innovation
The paint contains BaSO4 nanoparticles with deliberately varied sizes. This heterogeneous distribution is critical:
- Light scattering depends on particle diameter
- A wider size range scatters more wavelengths across the solar spectrum
- "A high concentration of particles that are also different sizes gives the paint the broadest spectral scattering"
- This maximizes reflectance across visible and near-infrared regions
Performance Data
| Metric | Value |
|---|---|
| Solar reflectance | 98.1% (vs. 80-90% commercial, 95.5% prior CaCO3 formulation) |
| Sky window emissivity | 0.95 |
| Average cooling power | 117 W/m² |
| Daytime cooling (noon sun) | 8°F (4.4°C) below ambient |
| Nighttime cooling | 19°F (10.6°C) below ambient |
| Winter cooling (43°F ambient) | 18°F reduction |
| Roof cooling power (1,000 sq ft) | ~10 kW — equivalent to or exceeding typical residential AC |
Practical Considerations
- Compatible with commercial paint manufacturing processes
- Particle concentration cannot be increased indefinitely — too high makes paint brittle and prone to peeling
- Minimal room for further whiteness improvement beyond 98.1%
- Represents practical path from lab to building-scale deployment
Broader Context — Radiative Cooling Materials Landscape
Four dominant technology clusters for passive radiative cooling (per 2026 patent landscape):
- Metamaterial nanostructured emitters: Metal-plated tapered nanopores via anodic aluminum oxide (PARC); near-unity emissivity
- Polymer/particle composite films: Fluoropolymers, PDMS, or polyethylene loaded with TiO2, SiO2, CaCO3, SiC, ZnO, Al2O3
- Photonic multilayer stacks: Alternating dielectric layers (HfO2/SiO2) engineering spectrally selective properties
- Vacuum-insulated far-infrared radiators: ≥0.80 emissivity in 8-13 μm band
Emerging frontier: radiative cooling engines (modified Stirling) converting surface-to-sky temperature difference into electricity.
Bio-Inspired Radiative Cooling Surfaces
- Saharan silver ant (Cataglyphis bombycina): Triangular-cross-section hairs enhance reflectivity in visible/NIR while increasing emissivity in mid-infrared. Survives ground temperatures exceeding 70°C.
- Morpho butterfly: Wing nanostructures control reflection spectrum via geometric design at nanoscale
- LDO microflower coating: ~97.58% solar reflectivity, ~98% thermal emissivity, ~7°C cooling under 1400 W/m² solar radiation
- Hierarchical PMMA film: >0.95 solar reflectance, ~0.98 LWIR emittance, 6-9°C sub-ambient cooling