A novel method to crosslink natural rubber latex adhesive at ambient temperature

Proof-of-concept laboratory experiments demonstrated grafting diacetone acrylamide (DAAM) onto natural rubber (NR) latex particles and crosslinking these sites via the keto-hydrazide reaction. Graft copolymers of NR and PDAAM (NR-g-PDAAM) were prepared by seeded emulsion polymerization at 50 °C. The morphology of the obtained graft copolymers was characterized by transmission electron microscopy. The degree of grafting of poly(diacetone acrylamide), PDAAM, phase in NR-g-PDAAM was evaluated by ¹H NMR technique. The keto-hydrazide crosslinking reaction of the ketone carbonyl of grafted PDAAM with adipic acid dihydrazide (ADH) was initially investigated using attenuated total reflection Fourier transform infrared (ATR-FTIR). The FTIR spectra of the NR-g-PDAAM film with added ADH lacked the absorption peak of ketone carbonyl in PDAAM. This result is consistent with the observations that the tensile strength of the cast films with ADH was higher than those without ADH, corroborating that the crosslinking reaction was taking place in the cast NR-g-PDAAM latex film. Then, the adhesive performance of NR-g-PDAAM latexes with various DAAM contents was determined, using ADH as the crosslinking agent. The lap shear strength increased noticeably with the amount of DAAM in the latex adhesives. This is attributed to the crosslinking in the adhesive film increasing with DAAM content and giving correspondingly improved cohesive strength. The effect of tackifier (i.e., gum rosin) content on the shear strength of NR-g-PDAAM latex adhesive was subsequently investigated. Adhesion tests indicated that the optimal gum rosin loading was 50 phr for attaining the maximum shear strength.