Kunming, China – Scientists at the Southwest Forestry University in Kunming have developed a novel non-isocyanate polyurethane urea adhesive based on tannin. It has strong adhesive properties, and good water resistance. They believe the urea in the formulation acts as a cross-linking agent, connecting with the tannin-based component
The team had previously investigated the potential non-isocyanate PU adhesives using tannin, with dimethyl carbonate also included in the system to form an intermediate at the relatively low temperature of 50–70°C, before reacting with an excess of hexamethylenediamine (HMDA) at 90–95°C to form a carbamate.
These NIPU products have been used as adhesives to bond plywood and particleboard, giving good adhesion strength under wet conditions. However, they have their drawbacks, including the volatilisation of residual HMDA.
They thought that using urea as an additional low-cost crosslinking agent could have potential. The urea would also be able to capture free amino groups from HMDA or oligomers formed from it. It could also enhance bonding performance.
The team evaluated the bonding performance of the new tannin-based non-isocyanate PU-urea adhesive by creating three-layer plywood from poplar veneers, making particleboard, and bonding bamboo.
The adhesive strength was improved by about 10% on the original, non-urea-containing adhesive in the plywood, and its performance could be improved further by hot water treatment. Further advantages include the lack of formaldehyde emissions, and it gave better performance than current biomass-based adhesives. Both other substrate tests proved successful, too.
Overall, they suggested that the new NIPU adhesive that includes urea in the formulation might overcome the drawbacks found with tannin-based non-isocyanate adhesives.
The work has been published in the journal Industrial Crops and Products.
