Physicochemical and Thermo-Mechanical Characterization of Sheep Wool/Phenolic Novolac Panels for Sustainable Thermal...

This study reports the physicochemical characterization and structure-property relationships of rigid sheep wool/phenolic novolac panels developed as bio-based thermal insulation for building envelopes. Mixed Polish sheep wool was washed, mechanically opened, and formed into nonwoven mats, then impregnated with either neat or flame-retardant novolac resin to obtain lightweight boards with a fiber content of about 50 wt%. Elemental analysis, ICP-OES, FTIR spectroscopy, and laser and electron microscopy were used to evaluate the fiber composition, keratin structure, morphology, and fiber-matrix interfaces. Mechanical performance under three-point bending and shear, differential scanning calorimetry, thermogravimetric analysis, and transient hot-probe thermal-conductivity measurements were applied to link microstructure with functional behavior. Novolac impregnation transformed the compliant wool mat into self-supporting panels, increasing the flexural modulus to the 0.8-1.4 GPa range and flexural strength to approximately 48-52 MPa, while the shear modulus and work to failure rose by more than an order of magnitude relative to the loose wool reference. Thermal conductivity remained in a typical range for natural-fiber insulations (λ = 0.061 W·m^-1·K^-1 for the wool mat and 0.071-0.074 W·m^-1·K^-1 for the composites), although higher than that of expanded polystyrene. DSC and TGA confirmed that wool fibers remain thermally stable up to about 200-220 °C, that the novolac resin cures around 140 °C, with typical phenolic reaction enthalpies, and that both formulations generate high char residues of roughly 60-80 wt% at 600 °C under nitrogen, evidencing a strong charring propensity rather than directly quantifying fire resistance. Overall, the results position sheep wool/novolac panels between conventional bio-based insulation and structural composites and highlight their potential as sustainable, circular insulation materials for energy-efficient building envelopes.