Bonding of natural rubber and epoxidized natural rubber to poly(ethylene terephthalate)

Hydrolysis of PET in aqueous NaOH was used to increase surface energy. Contact angle measurements were employed to characterize wettability of PET. Peel tests for natural rubber (NR) and epoxidized natural rubber (ENR), with a molar ratio of 25% or 50% of epoxide groups (ENR-25 and ENR-50), bonded to PET film were carried out at different peel rates and temperatures, and mastercurves were constructed. For uncrosslinked materials, at low peel rates, samples failed "cohesively" within the rubber. As peel rate increased, peel strength increased to a maximum. and fracture locus moved closer to the rubber-PET interface, followed by a sharp decrease, as failure became "interfacial" between rubber and PET. Near the transition, samples failed in a mixed mode manner for ENR, while NR showed stick-slip behavior. Hydrolyzed PET resulted in a higher percent cohesive failure in mixed mode failure for ENR, but, for NR, the transition peel rate was shifted to a higher value. When samples failed interfacially, hydrolyzed PET resulted in higher peel strength than untreated PET. Relative adhesion (hydrolyzed: untreated) for NR > ENR-25 > ENR-50. For crosslinked rubbers (0.95, 1.2 and 2.5 phr of crosslinking agent for NR, ENR-25 and ENR-50, respectively), failure was only interfacial. Peel strength decreased as temperature increased. At lower crosslinking agent concentration (1.25 phr for ENR-50), peel strength was less dependent on test temperature. At even lower concentration (0.475 phr for NR and 0.6 phr for both of the ENRs), as temperature increased, peel strength increased (up to 65 or 85°C) and then decreased at higher temperatures. It is thought that strain-induced crystallization caused the unusual behavior. Highly crosslinked NR (1.9 phr), in which crystallization was suppressed, showed a normal response.;The cohesive strength of uncrosslinked ENR-25 showed usual behavior. However, 0.6 and 1.2 phr peroxide crosslinked rubber had unusual behavior, similar to adhesion results for the 0.6 phr peroxide cured ENR. Dynamic mechanical analysis showed that 0.6 and 1.2 phr peroxide cured materials obeyed the WLF relation. Amorphous SBR with a wide range of crosslink densities did not show abnormal response of peel energies with changing temperature.