Isocyanate Micro/Nano Fillers as Functional Cross-linkers in Wood Adhesives with Cold-pressing Bonding Strength

Isocyanate is a highly reactive compound that quickly reacts with active hydrogen, posing challenges in its use in emulsion adhesives. Micro/nanoencapsulation technology involves enclosing a core material with a protective shell under specific conditions. Encapsulating isocyanate via micro/nanoencapsulation extends its lifespan in emulsion adhesives and enables controlled release, improving the mechanical strength of the adhesive. In this study, polyurea was used as the shell material and isophorone diisocyanate as the core to prepare isocyanate micro/nanofillers via micro/nanoencapsulation. Isocyanate micro/nanofillers were prepared through interfacial polymerization in an oil-in-water system with process optimization. The obtained isocyanate micro/nanofillers had an active group content of 22.1   wt %. The shielding effect of the shell effectively prolonged the lifespan of isocyanate groups in the emulsion. After six weeks of storage, the isocyanate micro/nanofillers retained most of their activity, showing satisfactory stability. When used as functional cross-linking agents in emulsion adhesives, pressure-induced rupture of the isocyanate micro/nanofillers triggered reactions with compounds containing active hydrogen, forming strong chemical bonds and significantly improving lap shear strength (4.01   MPa), which is 4.22 times that of the original latex. Transforming isocyanate from liquid to solid via micro/nanoencapsulation not only extended its lifespan but also improved bonding performance. The prepared adhesive was solvent-free, energy-efficient, environmentally friendly, clean, low-cost, and offered excellent bonding strength. These findings will enable broad practical applications in wood structural joints, engineering assemblies, and related fields.