by J. Picken, P.J. Martin, Tun Abdul Razak Research Center, UK
The degree of epoxidation of natural rubber (NR) has a linear relationship with its density, glass transition temperature (Tg) and polarity of the rubber. Previous work proposed a relationship between the Tg and the epoxidation level for epoxidized natural rubber (ENR) compounds of 1°C increase for each mole percent oxidation. Previously, master curves for the dynamic properties of natural rubber and epoxidized natural rubber (ENR) with 25 and 50 mol% epoxidation levels, denoted ENR-25 and ENR-50, respectively, were generated based on the time-temperature superposition principle. These master curves and the Cole-Cole plot of their dynamic properties showed the epoxidation level did not affect the relationship between the storage modulus (G’) and loss modulus (G”). The master curves were merely shifted along the frequency axis by a value related to the Tg.
This work was performed on NR (SMR-L grade) and samples of dry ENR obtained from either the Rubber Technology Center, Malaysia, or from Felda Rubber Industries, Malaysia. Nominally, the ENR grades were ENR-15, ENR-25, ENR-35 and ENR-50, where the numbers represent the molar epoxidation levels. The SMR-L, ENR-25 (Ekoprena- 25) and ENR-50 (Ekoprena-50) grades are commercially available as 33.3 kg bales. Testing was performed either on samples from the raw rubber bales, or on vulcanizates compounded according to the formulations. These vulcanizates were cured at 150°C to tmax (the time at which the peak value of torque in the rheometer trace is observed).