Multiscale-engineered polyolefin bilayer composites for long-term energy-saving cool roofs

Passive cool roofs can substantially reduce building cooling energy consumption, yet commercial roofing materials remain constrained by limited initial solar reflectance (R_solar) and its severe long-term decay. We developed a multiscale-engineered polyolefin bilayer composite that integrates a titanium dioxide (TiO₂)-loaded thermoplastic polyolefin (TPO) substrate with a thin antifouling propylene-based elastomer (PBE)/boron nitride (BN) surface layer. Through precise multiscale structural regulation, the composite achieves high mid-infrared emissivity (ε_mir = 90.7%) and broadband R_solar, delivering a high initial R_solar of 94.1%. In parallel, the microphase-separation-free surface layer effectively suppresses dust adhesion, imparting strong antifouling capability. As a result, the composite shows exceptional optical durability under coupled aging-contamination conditions: after 5600 h of xenon-arc accelerated weathering, corresponding to a ten-year-equivalent exposure under the adopted GB/T 18244-based protocol, its R_solar remains as high as 83% after re-contamination, corresponding to 88% retention of the initial value. This aged-and-fouled R_solar outperforms representative three-year benchmarks of commercial products listed by the Cool Roof Rating Council (CRRC), including the highest-rated single-ply membranes that typically decay from R_solar = 91% to 73% after three years, positioning this polyolefin composite as an industrially scalable and durable cool-roof membrane platform.