Accelerated Sulfur-vulcanisation of NR and IR

Accelerated sulfur formulations are the most common vulcanisation systems used in commercial and industrial applications. Therefore, research on both the fundamental and applied aspects of accelerated sulfur vulcanisation is ongoing. Several reviews of the chemistry and/or physics of accelerated sulfur-vulcanisation of elastomers have been published [13, 14, 22, 23]. 

Model compounds based on 2-methyl-2-pentene were studied to supplement the 13C chemical shift assignments of the products from accelerated sulfur vulcanisation of NR. It is observed in the model compound data that it may not be possible to distinguish between a 13C NMR resonance which is due to disulfidic crosslinks and a peak due to pendent accelerator groups, while a large chemical shift difference ( 3 ppm) is observed for the monosulfidic bonds. The MBS-accelerated sample shows similar new resonances as seen in the TMTD accelerated systems. In this comparison however, the quantitative aspects of the data might be obscured due to the differences in the state of cure among the different formulations. 

The 13C chemical shifts were assigned in more detail for monosulfidic and polysulfidic crosslinks occurring in the accelerated sulfur vulcanisation of NR [18]. The NR was cured with a pure thiuram formulation (TMTD alone) in order to predominantly prepare monosulfidic bridges in the network. The distortionless enhancement by polarisation transfer (DEPT) experiments, in which the carbons with different level of protonation can be distinguished [22-24], were performed for the NR cured with extended levels of sulfur. Based on the DEPT results and previously reported model compound results [20], the chemical shifts of the resonances occurring in the spectra were assigned. 

In the DEPT experiments, both peaks around 50 and 58 ppm are divided into three components and the levels of the protonation for these six individual resonances are evaluated. The peaks at 57.4, 58.0 and 58.6 ppm are assigned to the polysulfidic crosslinks in the Al, B1 and B2-type structures, respectively. The peaks at 37.2 and 50.7 ppm are due to Al-type polysulfidic crosslinks. There was no apparent structural match for the quaternary peak at 50.2 ppm. 

Solid-state 13C MAS NMR has been applied for quantitative determination of crosslink density in accelerated sulfur-vulcanised NR [27]. The concepts used to calculate the crosslink density by 13C NMR are the same as the one mentioned above, but different resonances were used for the quantitative treatment based on the different assignments 

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The amount of the sulfurisation occurring in the carbon black (N347) filled, TBBS accelerated sulfur-vulcanisation of NR and IR have been determined by quantitative... 

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