Initial tack and viscoelastic properties of MF/PVAc hybrid resins used as adhesives for composite flooring materials

The initial tack and viscoelastic properties of blends of melamine-formaldehyde (MF) resin and poly(vinyl acetate) (PVAc) emulsion on decorative veneer and plywood surfaces were investigated using a texture analyzer and dynamic mechanical thermal analysis (DMTA) at room temperature with respect to the 'open time', the time delay between applying an adhesive to a surface and contacting that surface with the other material. We determined and compared the effects of PVAc content, at 0, 30, 50, 70 and 100 wt%, in blends with MF resin. As PVAc is curable at room temperature, PVAc markedly affected the initial tack, as well as DMTA results. The degree of 'tack' was determined quantitatively based on the probe tack data. As the adhesion in an engineered flooring should be achieved in 2-3 min, the acceptable tack range is between 100 and 200 g. From the DMTA results, i.e., the tan(E″) values, the maximum loss modulus (Δ″) and the rigidity (ΔE) of MF/PVAc blends at room temperature, it was found that tan δ and loss modulus (E′) increased with increasing MF resin content. Moreover, the rigidities of the 70:30 and 50:50 MF/PVAc blends were higher than those of the other blends, especially in comparison with the pure 100% PVAc and 100% MF. We conclude that the initial tack correlates with the MF/PVAc blend ratio during the adhesion process at room temperature.