Curing characteristics of nitrile and natural rubber blends

The dependence of Mooney scorch time and cure index on blend ratio of nitrile rubber/natural rubber (NBR/SMR L) and nitrile rubber/epoxidized natural rubber (NBR/ENR 25) blends were determined in the temperature range from 120 degreesC to 160 degreesC using a Monsanto automatic Mooney Viscometer. Semi-efficient vulcanization system was used throughout the study. Results indicate that scorch time decreases with increasing SMR L and ENR 25 contents, an observation which is attributed to the increasing solubility of sulfur, as SMR L and ENR 25 composition is increased, However, for temperature greater than 140 degreesC, the dependence of scorch time on blend ratio diminishes, as enough thermal energy is available to overcome the activation energy of vulcanization. The cure index of NBR/SMR L blend passes through a maximum at 25% SMR L composition whereas in the case of NBR/ENR 25, generally it shows a gradual drop with increasing ENR 25 content. The differing observation between the two blends in terms of cure index is explained by the compatibility factor of the respective blends. For the apparent activation energy of vulcanization of the two blends, similar dependence behavior on blend ratio as that of cure index is obtained.