Peroxide, Radiation, and High Pressure Vulcanisation

Dicumyl peroxide vulcanisation of high-cis BR and NR samples at different peroxide levels were investigated by solid-state 13C NMR [42], As the peroxide level increased, decreased signal intensity and peak broadening was observed in the main chain peaks in both BR and NR due to decreased segmental motion. In addition, cis-trans isomerisation was observed during the vulcanisation process in both rubbers. New peaks at 35 and 44 ppm were observed in the BR spectra while peaks at 14.9, 21.4, 30.6, 37.5, and 45.0 

Network structure and reaction mechanisms in high pressure vulcanisation (HPV) and peroxide vulcanisation of BR was studied by 13C solid-state NMR [43]. Different samples of polybutadiene (51% trans, 38% cis, and 11 % vinyl) were peroxide cured with dicumyl peroxide on a silica carrier and by the HPV conditions of 250 °C and 293 MPa. The 13C NMR spectra from peroxide and HPV cures were compared to a control samples heated to 250 °C for 6 minutes under atmospheric pressure. Although no new isolated strong peaks were detected in either the peroxide or HPV vulcanisations, small increases in both spectra were observed at 29.5, 36.0, 46.5, and 48.0 ppm. These peaks compare favourably with calculated shifts from structures that arise from main chain radical addition to the pendent vinyl groups. These assignments are further reinforced by the observation that the vinyl carbon concentration is substantially reduced during vulcanisation in both peroxide and HPV curing. Two peaks at 39.5 and 42.5 ppm appear only in the peroxide spectrum. Cis-trans isomerisation was absent in both cures. 

Solid-state 13C NMR was used to characterise the structure produced in gamma irradiated NR [45]. Cis-trans isomerisation increases from 5.1% at 40 Mrad to 10.3% with 161 Mrad. Peaks in the NMR spectra were observed during irradiation due to the formation of a quaternary carbon (44 ppm) and its attached methyl group (21 ppm), a methine carbon (38 ppm), and a g-carbon of a cis isomeric unit adjacent to a trans isomeric unit (30.1 ppm). 

The mechanism of radiation vulcanisation of NR with 2-ethyl hexyl acrylate (EHA) was examined by 13C NMR [46]. All peaks in spectrum due to the acrylate decrease in intensities and broaden with increasing radiation dose. Using the solid-state CP technique, all peaks corresponding to EHA and NR were observed. The olefin peak at 129 ppm in the acrylate was absent, however, indicating that all the EHA had polymerised. 

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