Analysis of damage in commercial natural rubber through NMR spectroscopy

Damage to natural rubber (NR) during processing was investigated through liquid state NMR spectroscopy. Various commercial NR were used in the present study such as Pale Crepe (PC), Rubber Smoked Sheet (RSS3), Technically Specified Sheet (TSS8®), Standard Thai Rubber (STR5L), and Standard Malaysian Rubber (SMR20). As-cast films of the rubbers were prepared by solution casting method and followed by purification by acetone extraction. Chemical structures of the rubbers were elucidated through 1H, 13C-NMR, and DEPT spectroscopy. The 13C-NMR signals at 39.8 and 40.0 ppm were assigned to the trans-1,4-isoprene unit in the trans-trans and cis-trans sequences, respectively, while the signals at 60.8 and 64.5 ppm were assigned to C2 and C3 of an cis epoxy group. The cis epoxy groups were identified for PC, STR5L, RSS3, TSS8®, and SMR20, whereas the trans-1,4-isoprene units from cis-trans isomerization were detected for TSS8® and SMR20. The amount of these groups in TSS8® and SMR20 were higher than those in PC, STR5L, and RSS3. Mechanical properties of TSS8® and SMR20 were found to be inferior to those of PC, STR5L and RSS3. The mechanical properties depended mainly upon the damage of the samples during processing.