Tannin-Based Insulation Foams: The Effect of the Substitution of Formalin with Glutaraldehyde

This study investigates the potential of tannin-based foams as sustainable thermal insulations, analysing how the type and ratio of crosslinker (formalin and glutaraldehyde) and the addition of natural fibres can affect their mechanical, moisture-related, hygrothermal and fire-related performance, in comparison to commercial polystyrene. Formulations prepared with a single crosslinking agent (formalin or glutaraldehyde) exhibited lower densities (50.81 kg/m3 and 53.37 kg/m3, respectively), whereas the combined use of both crosslinkers resulted in higher apparent densities, with the maximum values observed for the intermediate substitution levels. The 100% glutaraldehyde foams showed higher compressive and flexural strengths than foams with 100% formalin (79 vs. 77 kPa and 160 vs. 120 kPa, respectively). A non-linear relationship between crosslinker composition and mechanical performance was observed, as the hybrid formulations did not consistently display intermediate properties. The 50/50 formulation showed about 40–46% lower compressive strength than the other foams; the loss appeared to be recovered by the addition of fibres (with Cs passing from 43 to 76 kPa). These results indicate that fibre addition is beneficial when higher stiffness is required, such as for sandwich panels or impact-resistant components. The substitution of F with G at 75% introduced an increase in the flexural strength of approx. 43% and the highest recovery after deformation (after 24 h, 75% of the original thickness). Tannin foams show high water vapour permeability (1.04–1.17 × 10−10 kg/(m·s·Pa)), potentially favouring moisture transport in permeable walls, although their sensitivity to liquid water highlights the need for protective layers in moisture-exposed applications. Thermal conductivity remains within the expected range for bio-based insulations (0.043–0.054 W/(m·K)). Tannin foams also show no flaming droplets or released smoke when ignited and high char formation in preliminary tests, reinforcing their potential as safer alternatives to petroleum-based insulations. The substitution of formalin with glutaraldehyde did not show any difference in heavy-metal-related toxicity. Overall, tannin-based foams represent a promising class of bio-insulations for protected sandwich panels, suitable for the implementation of low-carbon and circular construction strategies.