A Cocoon-Inspired Integrated Phase Change Envelope with Inner Regulation and Outer Insulation for Building Thermal Management

Phase change materials (PCMs) can regulate heat flow in building envelopes through reversible heat storage and release. However, under high-heat-flux conditions, building envelopes still face continuous environmental heat input, insufficient buffering of indoor-side temperature fluctuations, and limited regulation duration in single-layer PCM systems. Inspired by the hierarchical thermal protection mechanism of silkworm cocoons, this study proposes a biomimetic integrated phase change envelope with an inner-regulation/outer-insulation architecture. The outer layer provides thermal insulation and thermal buffering, whereas the inner layer promotes rapid heat spreading and temperature regulation. An octadecane-lauric acid (OC-LA) eutectic was selected as the phase-change core to prepare melamine-formaldehyde-based microcapsules, and polyethylene polyamine (PEPA) was introduced for source-level formaldehyde scavenging. An inner high-thermal-conductivity phase change regulation panel and an outer low-thermal-conductivity phase change aerogel thermal-protection layer were fabricated and integrated into a bilayer structure. After PEPA treatment, the free formaldehyde content decreased from 16 to 1.5 μg/100 g. Phase change microcapsules/carbon black/flake graphite (77%MPCM/CB/FG) exhibited a latent heat of 146.5 J/g, a thermal conductivity of 1.108 W/(m·K), and good photothermal responsiveness. Phase change microcapsules/poly(vinyl alcohol)-borax/fumed silica (70%MPCM/PVA-B/SiO2) showed a latent heat of 133.8 J/g, a low thermal conductivity of 0.065 W/(m·K), and good mechanical stability. Building model tests demonstrated that the bilayer wall significantly delayed indoor temperature rise and can serve as a year-round basic thermal-management unit. Under winter conditions or in regions with large diurnal temperature differences, MPCM/CB/FG can further function as a climate-responsive roof enhancement module, thereby prolonging the indoor thermal comfort period. Overall, this work establishes a modular phase change envelope design framework that integrates temperature-window matching, indoor-use safety, and adaptability to high-heat-flux conditions, offering a new strategy for energy-efficient building materials and structures.