Enhancing the performance of natural rubber latex with polymeric isocyanate as cold-pressing and formaldehyde free adhesive for plywood

A cold-pressing and formaldehyde free adhesive for plywood has been prepared by incorporating natural rubber latex (NRL) and poly(vinyl alcohol) (PVA) at a ratio of 1:1 (w/w) with different contents of polymeric 4,4-diphenyl methane diisocyanate (pMDI) (i.e., 0, 1, 3, and 5% based on NRL-PVA solids content). Plywood panel has been manufactured with a glue spread of 200 g/cm2 under a cold-pressing of 5.0 MPa at temperature of 27 ± 2°C for 24 hr. The optimum level of pMDI in NRL-based adhesive is then used to prepare plywood for different pressing times (2, 6, and 12 hr). The result shows that addition of pMDI-created urethane linkages from the reaction between pMDI and NRL-PVA. Increasing pMDI content shortens the gel time of NRL-based adhesive, lowers its curing temperature and activation energy, and increased the solids content and viscosity. Adhesion performance of plywood bonded with NRL-PVA/pMDI adhesive was enhanced and formaldehyde emission was reduced with increasing in pMDI content. Statistical analysis suggested that NRL-PVA/3%pMDI adhesive can be used to manufacture plywood under cold-pressing of 5.0 MPa at temperature of 27 ± 2°C for 12 hr with a dry tensile shear strength of .87 MPa and .09 mg/L of formaldehyde emission.