Ambient curable natural rubber latex adhesive cross-linked with polymeric isocyanate for bonding wood

As a way to develop an alternative wood adhesive to the conventional formaldehyde-based adhesives, this study investigated a formaldehyde-free ambient curable adhesive based on the natural rubber latex (NRL). NRL-based adhesive was prepared by mixing NRL (50% w/v) and poly(vinyl alcohol) (PVOH, 15% w/v), which acted as adhesion promoter. Calcium carbonate (CaCO3, 30% w/v) was added as filler. Approximately 15% of polymeric 4,4-methylene diphenyl diisocyanate (pMDI) was used as cross-linker based on the adhesive’s solids content. Physical, chemical, thermal, and morphological features of NRL-based adhesive were characterized using several techniques. The adhesion strength was examined in laminated wood using a universal testing machine. Incorporation of PVOH reduced the solids content and increased viscosity of adhesive. Addition of pMDI remarkably increased the viscosity of adhesive to around 1610 mPa.s which alter the phase of NRL-based adhesives form viscous liquid to gel. Fourier Transform Infrared spectroscopy revealed the formation of urethane linkages from the reaction of –NCO groups of pMDI and the –OH groups of NRL-PVOH. Differential scanning calorimetery analysis showed that the NRL-based adhesives cured at ambient temperature of around 23.1 °C with addition of 15% pMDI. Adhesion strength of laminated wood bonded with neat NRL-PVOH/0% pMDI was five times lower than that of samples bonded with NRL-PVOH/15% pMDI. The highest shear strength of around 5.0 MPa was obtained at laminated wood made of Mersawa bonded with NRL-PVOH/15% pMDI. This study showed that NRL-PVOH/15% could be used as alternative adhesive to manufacture laminated wood at ambient temperature of around 23 ± 2 °C.