Epoxidized Natural Rubber

When natural rubber is epoxidized under carefully controlled conditions, it can be converted to a totally new polymer with some properties more akin to speciality rubbers and some properties that appear to be advantageous for tire treads. Three levels of epoxidation have been extensively evaluated. They were 50, 25 and 10 mole% epoxidized NR referred to as ENR-50, ENR-25 and ENR-10 respectively. ENR-50 has been found to undergo strain crystallization like NR, but with oil resistance similar to a medium acrylonitrile NBR and gas permeability similar to butyl rubber. It is a highly damping rubber with a very low room-temperature resilience. ENR-50 and ENR-25 both exhibit good wet grip characteristics and have been examined as tire tread materials. In particular, ENR-25 compounds containing silica or silica/black have been found to give lower rolling resistance than NR and better wet traction than OESBR, so providing an ideal combination of these two properties for tire treads. Unfortunately, wear data is as yet incomplete, but it is anticipated, from truck tire experience, that the black/silica compounds will present no problems. ENR-25 and ENR-50 exhibit this unique feature with silica of reinforcement equivalent to black without the use of a coupling reagent. Thus, these rubbers have potential of providing white or colored vulcanizates with properties previously associated only with black-filled compounds, so extending applicational areas, or even resulting in colored tires or other automotive parts. ENR-10 provides a damping grade of NR when lower resiliences are required. Alternatively, blends of ENR-25 or -50 with NR may be used. Many engineering applications are calling for reduced resilience, and this new form of NR can give precisely this.