Natural fibrous sepiolite-reinforced emulsion polymer isocyanate for timber structural adhesive

The natural fibrous sepiolite is introduced into emulsion polymer isocyanate (EPI) consisting of aqueous vinyl acetate copolymer-based main component (MC) and polymeric methylene diisocyanate (pMDI) for the first time to prepare a high-performance, environmentally friendly, and cost-effective timber adhesive. We have studied the effect of the sepiolite content on the tensile stress-strain curves, dynamical mechanical properties, essential work of fracture of the cured glue layer, and the EPI bonding properties. In addition, we have also investigated the chemical group, fractional free volume, and morphology of EPI polymers with various sepiolite contents. The results indicate that adding sepiolite can significantly increase the tensile strength, elastic modulus, dynamic stiffness, and fracture toughness of cured EPI films. Remarkably, EPI-bonded structures exhibit superior resistance to compressive shear damage upon ageing in boiling water. For MC with 6.0-9.0 wt% sepiolite, only the addition of 10.0 wt% general pMDI crosslinker is required to achieve the JIS K6806 structural adhesive. The underlying enhancement mechanism is the significant hydrogen-bonding coupled chemical reaction capability of sepiolite for the available groups in the EPI system.