Nano-scale blocking mechanism of MMT and its effects on the properties of polyisocyanate-modified soybean protein adhesive

• A water-resistant soybean protein adhesive with good technological applicability is formulated.
• MMT is found to retard the properties of the soybean protein adhesive.
• This retarding effect is further confirmed by XRD, FTIR and chemical determination.
• A nano-blocking mechanism is proposed to explain the MMT retarding effect.

A novel soybean protein-based wood adhesive with improved water resistance was developed using thermal-caustic degraded soybean proteins with modifications by polyisocyanate and nano-scale montmorillonite (MMT). A gel permeation chromatography (GPC) analysis, X-ray diffraction (XRD) measurement, Fourier transform infrared spectroscopy (FTIR) analysis, traditional chemical analysis and plywood evaluation indicated that the thermal-caustic degradation process effectively reduced the viscosity of the high-concentration soybean protein solution due to the sharp decrease in the molecular weight of the soybean proteins. Crosslinking by polyisocyanate increased the molecular weight and produced soybean protein with a crosslinked structure, which improved the water resistance but clearly shortened the work life of the polyisocyanate-modified soybean protein adhesive. The addition of 3% MMT effectively prolonged the work life of the modified soybean protein adhesive and reduced the bond strength to some extent. A nano-scale blocking mechanism of MMT that retards the effects on some properties of soybean protein adhesive was presented because the exfoliated nano-scale MMT platelets not only reduced the number but also increased the steric hindrance of some active groups of soybean proteins via hydrogen and electrostatic bonds.