Competitive reaction effect of vinyl acetate on preparation and characterization of thermosetting starch-based wood adhesives grafted by N-methylol acrylamide

N-methylol acrylamide (NMA) was introduced into a formaldehyde-free starch-based adhesive to improve certain characteristics and achieve a range of important thermosetting properties. The content of NMA in the adhesive was raised to 4% with the addition of 6% vinyl acetate (VAc), in which the wet strength reached 1.4 MPa and the starch accounted for 86.6% of the total solids content. It was observed from thermogravimetric analysis (TGA), Fourier-transform infrared (FTIR) spectroscopy and a residual rate test that VAc could reduce the graft density to avoid gelation, which could be caused by local overgrafting of NMA. In addition, Raman microscopy was applied to prove that uniformity of NMA had been improved by VAc and the preparation of starch-based adhesive with fluidity and better stability was achieved. Chemical shifts in the ¹H NMR spectrum and the appearance of a significant shoulder in ¹³C-CPMAS spectra proved that VAc and NMA were inclined to react via hydroxyl groups at the C ₆ sites of starch to form a competitive reaction effect in order to inhibit overreaction between NMA and starch. Statistical analysis of the Raman spectra showed that the ingredients of starch reacted with both VAc and NMA, which accounted for 40.3% of the total particles, which played an important role in the water resistance of the resulting adhesives.