Sulfur vulcanisation spectroscopy

Over the last decade the development of advanced analytical techniques, such as Fourier transform (FT) Raman and solid-state NMR spectroscopy, have been impressive, resulting in a great deal of progress in the field of the sulfur vulcanisation of unsaturated elastomers [22-25]. 

The results of the optical spectroscopy studies into sulfur vulcanisation of polydiene rubbers correspond well with the results obtained via low molecular weight model olefin studies and solid state 13C NMR studies. From all these studies the mechanism for accelerated sulfur vulcanisation as shown in Figure 6.2 has emerged [14-18], which is... 

The mechanism of the accelerated sulfur vulcanisation of EPDM is probably similar to that of the highly unsaturated polydiene rubbers. The vulcanisation of EPDM has been studied with emphasis on the cure behaviour and mechanical and elastic properties of the crosslinked EPDM. Hardly any spectroscopic studies on the crosslinking chemistry of EPDM have been published, not only because of the problems discussed in Section 6.1.3 but also because of the low amount of unsaturation of EPDM relative to the sensitivity of the analytical techniques. For instance, high-temperature magic-angle spinning solid-state 13C NMR spectroscopy of crosslinked EPDM just allows the identification of the rubber type, but spectroscopic evidence for the presence of crosslinks is not found [72]. 

Only two spectroscopic studies on sulfur vulcanisation of EPDM by Fujimoto and coworkers are available [73-74], Using attenuated total reflectance (ATR) IR spectroscopy they showed that during sulfur/TMTD/MBT/ZnO/stearic acid vulcanization, the C=C bands at 3035, 966 and 870 cm 1 of the residual unsaturations of the EPDM third monomers, DCPD, 1,4-hexadiene (HD) and 5-methylidene-2-norbornene (MNB), respectively, decreased in intensity as a function of time at 140 and 150 °C. The relative decrease in intensity was shown to correlate with the increase in crosslink density. In Sections 6.2.2.2 and 6.2.2.3 it will be shown that this decrease of intensity should not be interpreted as a loss of unsaturation during sulfur vulcanisation of EPDM. 

The FT-Raman spectra of the sulfur vulcanisates of the various model olefins do not contain the characteristic disulfide signal at 510 cm"1, but do contain the typical higher sulfide bands at 490, 460 and 440 cm"1 (Table 6.2). In addition, a new band at about 475 cm 1 is observed for the vulcanisates of 2-methyl-2-pentene and 3-hexene, which has not yet been assigned (hexasulfide ). Results of HPLC analysis have shown that the vulcanisate of 2,3-dimethyl-2-butene consists mainly of a mixture of disulfide to pentasulfide with about 15 mole% of disulfide [79]. This illustrates that FT-Raman spectroscopy is not very sensitive for the identification of disulfides. Because of an overlap of signals, FT-Raman does not provide detailed, quantitative information on the presence of the individual higher sulfides (S>2). 

NMR spectroscopy and solid-state and 13C NMR relaxation-time experiments. However, the sensitivity of solid-state 13C NMR is not as high as that of Raman and IR spectroscopy. For instance, solid-state 13C NMR of sulfur-vulcanised EPDM could only be performed when the ENB unsaturation of EPDM was fully isotopically enriched with 13C NMR [124]. 

Solid-state 13C NMR spectroscopy was used to study accelerated [33] and unaccelerated [34] sulfur-vulcanisation and sulfur-donor (TMTD) [35] vulcanisation of czs-polybutadiene (BR). Olefinic and methylene carbons of the czs-BR repeating unit typically resonate at 129.5 and 27.5 ppm, respectively. The dominant products occurring in the vulcanisation... 

Microstructural changes of an accelerated sulfur vulcanisation of HR with TMTD/ZnO/ sulfur has been studied by solid-state 13C NMR spectroscopy [47]. The HR containing 2% isoprene and 98% isobutylene were formulated using EV and cured at 160 °C for several cure times. The resonances at 20.3 and 24.4 ppm, which are due to trans isoprene units in the HR, decrease with cure, while the resonances at 26.9 and 25.2 ppm which arise from cis isoprene units increase with cure times. The cis trans ratio increases up to a maximum ratio of approximately 4 1 at a cure time of 60 minutes. New resonances are observed at 15, 21, 23.6 and 49 ppm. The peak at 49 ppm is assigned to the mixture of the isoprene units in czs-IIR, polysulfidic Alt and polysulfidic Ale structures. The resonance peaks at 15, 21 and 23.6 ppm are assigned to the isoprene units in mono- and polysulfidic Bit, mono- and polysulfidic Blc and polysulfidic Alt structures, respectively. No reaction occurs in the isobutylene units. No migration of the double bond saturation, internal cyclisation or sulfurisation resulting in Clt and Clc structures is observed. 

As a result of the high polarisability of C—S and S—S bonds, Raman spectroscopy is especially suitable for studying sulfur vulcanisation of elastomers. However, conventional Raman studies of elastomers are limited on account of sample fluorescence (often due to impurities). 

It is of interest to examine the development of the analytical toolbox for rubber deformulation over the last two decades and the role of emerging technologies (Table 2.9). Bayer technology (1981) for the qualitative and quantitative analysis of rubbers and elastomers consisted of a multitechnique approach comprising extraction (Soxhlet, DIN 53 553), wet chemistry (colour reactions, photometry), electrochemistry (polarography, conductometry), various forms of chromatography (PC, GC, off-line PyGC, TLC), spectroscopy (UV, IR, off-line PylR), and microscopy (OM, SEM, TEM, fluorescence) [10]. Reported applications concerned the identification of plasticisers, fatty acids, stabilisers, antioxidants, vulcanisation accelerators, free/total/bound sulfur, minerals and CB. Monsanto (1983) used direct-probe MS for in situ quantitative analysis of additives and rubber and made use of 31P NMR [69]. 

The chemical microstructures of cis-polyisoprene (HR) vulcanised with sulfur and N-t-butyl-2-benzothiazole sulfenamide (TBBS) accelerator were studied as a function of extent of cure and accelerator to sulfur ratio in the formulations by solid-state 13C NMR spectroscopy at 75.5 MHz [29]. Conventional (TBBS/Sulfur=0.75/2.38), semi-efficient (SEV=1.50/1.50) and efficient (EV=3.00/1.08) vulcanisation formulations were prepared, which were cured to different cure states according to the magnitude of increase in rheometer torque. The order and types of the sulfurisation products formed are constant in all the formulation systems with different accelerator to sulfur ratios. However, the amount of sulfurisation has been found to vary directly with the concentration of elemental sulfur. 

High resolution MAS techniques of 13C, DEPT, correlated spectroscopy (COSY), total correlation spectroscopy (TOCSY), heteronuclear chemical shift correlation (HETCOR) were used to examine conventional CBS and efficient TMTD vulcanisation of polybutadiene [37]. In conventional CBS vulcanisation, the major vulcanisate 13C NMR peak occurred at 44.9 ppm and was assigned to a trans allylic structure (-C=C-C-Sx with X=3 or 4). The efficient TMTD vulcanisation yielded as main product a 13C NMR peak at 54.0 ppm and was assigned to a cis allylic vulcanisate (-C=C-C-Sx x=l). While cyclic sulfur by-products were observed in both vulcanisation systems, the CBS formulations gave rise to a higher percentage postulated to be formed via a episulfide intermediate. 

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