and constant viscosity grade rubbers (CV rubber) are now available on the market. Although Sekhar (1961) reported that CV rubber has an average increase in Mooney viscosity of only 4-8 units after 4-5 years of storage at ambient temperature, but Yunyongwattanakorn and Sakdapipanich (2006) reported that STR CV60 showed increasing in Mooney viscosity, gel content, P