The Whitest Paint: Purdue University's Ultrawhite BaSO4 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:

1. Reflecting incoming solar radiation (0.3-2.5 μm wavelengths)
2. 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):

1. Metamaterial nanostructured emitters: Metal-plated tapered nanopores via anodic aluminum oxide (PARC); near-unity emissivity
2. Polymer/particle composite films: Fluoropolymers, PDMS, or polyethylene loaded with TiO2, SiO2, CaCO3, SiC, ZnO, Al2O3
3. Photonic multilayer stacks: Alternating dielectric layers (HfO2/SiO2) engineering spectrally selective properties
4. 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