15 Mooney viscosity changes of commercial NR samples during storage (Yunyongwattanakorn and Sakdapipanich, 2006).
The mechanisms of storage hardening have yet to be conclusively explained. However, the hardening process has been postulated as linked with the reactions between the polyisoprene chains and some non-isoprene components such as proteins and phospholipids (Yunyongwattanakorn, 2003) and/or abnormal groups such as aldehyde, epoxide, carbonyl, and lactone. Moreover, other factors possibly involved in storage hardening of natural rubber were reported, such as the effect of sugar (Sekhar, 1963),