Recycled Automotive and Construction Wastes in Three-Layer Particleboards: Thermophysical Properties, Sound Absorption, and Radiant-Heat Mass-Loss Behavior

Recycled polymer-rich residues can alter several functions of wood-based panels, but claims of multifunctionality require property-specific evidence and transparent treatment of replication. Three-layer spruce particleboards were therefore prepared with 10 wt.% painted or unpainted polypropylene bumper granules, high-density polyethylene fuel-tank granules, tire rubber, seal-and-carpet residues, or electrical-cable fractions in the core layer. Two hybrid formulations contained 10 wt.% recycled rubber-rich filler plus 10 wt.% expandable graphite. Transient plane source measurements were evaluated at the specimen-pair level (two pairs per formulation); normal-incidence sound absorption and radiant-heat mass loss were complementary descriptive screens because the archived datasets contained one spectrum or one exposed specimen per formulation. Mean thermal conductivity varied only from 0.1908 to 0.2075 W m−1 K−1 (−5.3% to +3.0% relative to the reference). The graphite hybrids showed the clearest change in transient response: thermal diffusivity increased by 19.6–20.1%, whereas volumetric heat capacity decreased by 14.5–16.7% and thermal effusivity by 6.5–8.7%. SC10G10 had the highest mean absorption coefficient over 126–6400 Hz (0.210; +46.2%) and the lowest mass loss after 600 s at 30 kW m−2 (37.48%; −17.72%). Macroscopic and polarizing optical images showed formulation-dependent filler distribution and visible interfacial spaces, but they did not establish bonding mechanisms or quantify porosity. The results identify promising formulation-dependent responses while also defining the replication and structural measurements needed before application-level claims can be made.