Ambient curable latex films and adhesives based on natural rubber bearing Acetoacetoxy functionality

The study described in this paper first demonstrates that a newly modified form of natural rubber, namely graft copolymers of natural rubber with poly (acetoacetoxyethyl methacrylate), NR-g-PAAEM, is able to undergo a cross-linking reaction at room temperature by reaction with a water dispersible polyisocyanate based on hexamethylene diisocyanate (poly-HDI). Attenuated total reflectance Fourier transform infrared (ATR-FTIR) analysis indicated that amide groups were formed by the reaction of the acetoacetyl groups (AcAc) present in the grafted poly (acetoacetoxyethyl methacrylate) (PAAEM) chains with the poly-HDI. This observation was accompanied by a noticeable increase in the tensile strength of the NR-g-PAAEM latex films when adding poly-HDI to the latex prior to film formation. DMTA analyses also revealed a shift in the tan delta peaks, corresponding to the transitions of both NR-g-PAAEM and free PAAEM phases, to higher temperatures. These results provide firm evidence of cross-linking between NR-g-PAAEM chains by reaction with poly-HDI during film formation under ambient conditions. Adhesives for bonding wood to wood based on the NR-g-PAAEM latex were then prepared, using poly-HDI as the cross-linker. The lap shear strength of the resulting adhesives exhibited a maximum value of 2657 KPa when a poly-HDI:AAEM molar ratio of 3:1 was employed. It was also observed that the adhesive attained about approximately 89% of the highest lap shear strength after it was allowed to set at 30 degrees C for 24 hours. Hence, the use of poly-HDI in cross-linking NR particles bearing grafted PAAEM offers great potential for developing latex adhesives and coatings capable of curing under ambient conditions.