Report on selected lignin-based binders (polymerization techniques, properties and processability)

Deliverable 3.2 focuses on the development and optimization of enzymatically polymerized lignosulfonates (LS) as sustainable binders for fully bio-based wallboards with enhanced thermal performance and energy storage capabilities. This work supports the BIOBUILD project’s goal of advancing circular, low-carbon construction materials that promote energy savings and environmental sustainability in buildings. One of the three bio-binders investigated for wallboard production in the project is lignin. Lignin is an underutilized by-product of the pulp and paper industry and a promising alternative to conventional fossil-based binders due to its polyaromatic structure. The enzymatic polymerization process reported in this deliverable utilizes laccase enzymes to modify LS under mild conditions. The resulting LS polymers were applied as binders for test panels, which serve as prototypes for the final wallboards, using two production approaches. Modifying lignosulfonate with vinyl oleate via esterification reduces its hydroxyl group content, as shown by decreased O–H stretching (~3300 cm⁻¹) by Fourier-transform infrared spectroscopy (FTIR). This lowers hydrophilicity by replacing polar OH groups with non-polar aliphatic chains from oleate, evidenced by new C–H signals (2923, 2855, 710 cm⁻¹). Consequently, the modified lignosulfonate exhibits improved moisture resistance, with reduced water uptake and better dimensional stability in humid environments compared to unmodified lignosulfonate. The modified material can be polymerized enzymatically applying the same process used for unmodified lignosulfonate. Cold-pressed composites demonstrated promising mechanical stability and avoided leaching of phase change material (PCM) during production due to the low processing temperature. However, the drying process remains energy-intensive, and the brittleness of dried LS polymers necessitates the use of plasticizers to improve the elasticity and durability of the composites. Among the tested plasticizers, sorbitol emerged as the most promising option due to its biobased origin, cost-effectiveness, and ability to enhance mechanical and hydrophobic properties. Increasing the dry matter content of the binder further improved composite mechanical performance while reducing drying time and energy consumption. Hot-pressing was identified as a fast and effective production method with the potential to achieve higher mechanical strength and durability. This approach eliminates the need for plasticizers and drying, potentially reducing production costs and aligning with established industrial processes, making it a promising alternative for large-scale applications. However, challenges such as PCM leaching and mat stability need to be addressed to ensure the viability of this process. Binder stability studies showed that heat inactivation by autoclaving caused severe viscosity loss, led to crumbling boards, and is therefore not recommended. Sodium azide can delay microbial growth in wet binders for up to 3 months. However, due to its toxicity it is unsuitable for scale-up and should be replaced by safer anti-fungal alternatives.