Sulfur vulcanisation accelerated

A. B. Sullivan, C. J. Harm, and G. H. Kuhls, "Vulcanisation Chemistry— Fate of Elemental Sulfur and Accelerator during Scorch Delay as Studied by Modem HPLC", Paper No. 9, presented at the MGS Tubber Division Meeting Toronto, Canada, May 21 —24, 1991, American Chemical Society, Washington, D.C., 1991. 

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]. 

W. H. Helt and D. Sikora, Accelerated Sulfur Vulcanisation, at the ACS Meeting of the 147th Rubber Division, Philadelphia, Pa., May 2—5, 1995, American Chemical Society, Washington, D.C. 

Most EPDM applications require crosslinking except when used as an impact modifier for PP, polystyrene (PS) and polyamides or as an oil additive, e.g., as viscosity index improver or dispersant. Most commonly, accelerated sulfur vulcanisation is used for the crosslinking of EPDM. As a result of the low amount of unsaturation in EPDM (< 1 mole/ kg versus NR -15 mole/kg), sulfur vulcanisation of EPDM is rather slow and a relatively large amount of accelerators is needed. Because of the low polarity of EPDM the solubility of polar accelerators is limited, often resulting in low effectivity and/or blooming. Typically, up to 5 different accelerators are used in EPDM formulations. As for other rubbers environmental issues, such as nitrosamine formation and may be in the future the presence of zinc, are prompting the development of new accelerator systems. 

Chen and co-workers tentatively assigned new signals in the FT-IR spectra of accelerated sulfur-vulcanised NR to the formation of C-S and S-S bonds corresponding to monosulfides, disulfides and polysulfides [68]. The vulcanisation of NR was retarded when clay was added to the NR compound. 

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]. 

Although sulfur vulcanisation was discovered over one hundred and fifty years ago, the exact mechanism of vulcanisation is still being examined. This arises not only from the complexity of the reactions and products formed but also to the fact that the mechanism of accelerated sulfur vulcanisation changes is dependent on the class of accelerators/ activators used. Typically, benzothiazole or sulfenamide are used as accelerators, zinc... 

There is wide variety of vulcanisation agents and methods available for crosslinking rubber materials including peroxide, radiation, urethane, amine-boranes, and sulfur compounds [20]. Because of its superior mechanical and elastic properties, ease in use, and low cost, sulfur vulcanisation is the most widely used. Although vulcanisation with sulfur alone is not practical compared to the accelerated sulfur vulcanisation in terms of the slower cure rate and inferior physical properties of the end products, many fundamental aspects can be learned from such a simply formulated vulcanisation system. The use of sulfur alone to cure NR is typically inefficient, i.e., requiring 45-55 sulfur atoms per crosslink [21], and tends to produce a large portion of intramolecular (cyclic) crosslinks. However, such ineffective crosslink structures are of interest in the understanding of complex nature of vulcanisation reactions. 

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. 

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... 

Similar vulcanisation chemistry is observed with the N- -butyl-2-benzothiazole sulfenimide (TBSI) accelerated sulfur-vulcanisation of HR [26] compared to the TBBS accelerated systems... 

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... 

Similar vulcanisation products with reduced structural modifications are obtained in the sulfur-donor (TMTD) vulcanised BR [35]. By comparing the GHPD and DEPT results of sulfur-vulcanised BR with the results of sulfur-donor vulcanisation, detailed chemical shift assignments are possible where the peaks from inter- and intra-molecular sulfurisations and those from accelerator fragments can be distinguished. 

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. 

Similar trends have been observed in the carbon black (N347) filled, TBBS accelerated sulfur-vulcanisation of high-c/s-IIR. In contrast to the NR/CB system, the reversion reactions, i.e., the cis-to-trans isomerisation and the chain scission at 3,4-isoprene units, increase with black content during the overcuring. 

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... 

A change in T2 relaxation during accelerated sulfur vulcanisation of EPDM rubber is shown as an example in Figure 10.17 [179]. 

Figure 10.17 Dependency of T2 relaxation time as a function of the actual vulcanisation time during accelerated sulfur vulcanisation of EPDM [179]. The line has been included to guide the eye. The vulcanisation temperature was 413 K (140 °C)...

Accelerators, e.g. zinc oxide and fatty acids, increase the rate of vulcanisation of rubber by sulfur and they reduce the amount of sulfur required from 10% to <3%. Certain sulfur-donating accelerators, like thiuram disulfides (1) and mercaptobenzothiazole (2), will effect vulcanisation without added sulfur to yield products with greatly enhanced ageing properties.1... 

The accelerated sulfur vulcanisation of general purpose diene rubbers (e.g., natural rubber (NR), Styrene butadiene rubber (SBR), and butadiene rubber (BR)) in the presence of organic accelerators and other rubbers, which are vulcanised by closely related technology (e.g., ethylene-propylene-diene terpolymer (EPDM) rubber, butyl rubber (HR), halobutyl rubber (XIIR), nitrile rubber (NBR)) comprises more than 90% of all vulcanisations. 

Initially, vulcanisation was accomplished by heating elemental sulfur at a concentration of 8 parts per hundred parts of rubber (phr) for 5 h at 140 °C. The addition of zinc oxide reduced the time to 3 h. Accelerators in concentrations as low as 0.5 phr have since reduced times to 1-3 min. As a result, elastomer vulcanisation by sulfur without accelerator is no longer of commercial significance. An exception is the use of about 30 or more phr of sulfur, with little or no accelerator, to produce moulded products of a hard rubber called ebonite. 

Organic chemical accelerators were not used until 1906, 65 years after the Goodyear-Hancock development of unaccelerated vulcanisation, when the effect of aniline on sulfur vulcanisation was discovered by Oenslayer (405, a.2). 

Accelerated sulfur vulcanisation is the most widely used method. This method is useful to vulcanise NR, SBR, BR, HR, NBR, chloroprene rubber (CR), XIIR and EPDM rubber. The reactive moiety present in all these rabbers is ... 

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