Sustainable non-isocyanate polyurethanes bio-adhesives for engineered wood panels are revealed as promising candidates to move from formaldehyde-based alternatives

The main driving forces on the development of eco-friendly wood adhesives are based on environmental sustainability, costs savings, recyclability, reusability and health benefits, in comparison with synthetic resins. Lignin, tannin, proteins and carbohydrates are the main renewable raw materials being studied. Taking as a premise the technical performance of different bio-based alternatives, in comparison with formaldehyde-based resins, it is necessary to evaluate the environmental profile of such products in order to assess the pros and cons. In this regard, this manuscript addresses the industrial-scale design and environmental evaluation, through the Life Cycle Assessment methodology, of four formaldehyde-free bio-adhesives. For this purpose, the use of renewable resources such as Organosolv (OSL) and kraft (KL) lignins, soy (SPI) and tannins (MT)), crosslinked and hardened with NIPU (non-isocyanate polyurethanes) were considered. The impact results obtained showed that OSL-NIPU bio-adhesive, with a single environmental score of 35.27 mPa, has the best environmental profile, followed by SPI-NIPU, with a value of 63.36 mPa. Therefore, both could be considered as potential substitutes for synthetic resins. On the other hand, it has been identified that hexamethylenediamine (HDMA), used as cross-linking agent for the formulation of the bio-adhesives, is one of the main hotspots of the environmental profiles of OSL, KL and MT NIPU bio-adhesives. In the case of SPI-NIPU adhesives, it is the soy protein isolation process that leads to a higher environmental contribution. Thus, future research should focus on trying to reduce the dose of HMDA and on improving the soy protein isolation process.