Sulfur Vulcanisation System

It should be recognised that appreciable shifts in properties are sometimes made possible by special compounding variations. For instance, the heat resistance of natural rubber vulcanisates may be improved considerably by variation of the vulcanising recipe. The normal sulfur vulcanisation system is capable of many variants which will govern the chemical nature of sulfur crosslinks, i.e., whether it is essentially a mono, di or polysulfide linkage. The nature of sulfur crosslinks can have considerable influence on the heat and chemical resistance of vulcanisates. 

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. 

It has been shown that during sulfur vulcanisation of EPDM the C=C peak of the residual ENB unsaturation at 1685 cm 1 seems to decrease in intensity in agreement with the observations by Fujimoto and co-workers [73,74] (see Section 6.2.2.1). However, in Section 6.2.2.2 it was shown that sulfur vulcanisation of the low-molecular-weight ENBH results in a shift of the Raman C=C peak from 1688 to 1678 cm 1. Taking this into account a closer inspection of the FT-Raman spectra reveals that the original C=C peak at 1690 cm"1 decreases in intensity, and a new peak is observed at 1681 cm"1. Actually, the C=C peak broadens towards lower wave numbers, but in a first approximation the total area remains constant. So, the sulfur substitution reaction of the allylic hydrogens is confirmed for the polymer system. This corresponds to the observation by Koenig and co-workers, namely that upon sulfur vulcanisation of cz s-BR, the C=C peak at 1650 cm 1 decreases in intensity and that of a new peak at 1633 cm-1 increases its intensity [19, 58]. 

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 NMR crosslink density results were compared with the crosslink density obtained by the mechanical measurements. In the determination of the crosslink density by mechanical methods, the contributions of the topological constraints on the results were neglected and the density was expressed as G/2RT. The 13C and mechanical-crosslink densities were obtained for both sulfur and dicumyl peroxide (DCP)-cured samples to ensure the effect of wasted crosslinks (pendent or intramolecular type sulfurisations), which are expected in the typical sulfur-vulcanisation of NR. In the major range of crosslink densities, the crosslink densities for those two systems are described by the same linear function with a slope of 1.0. Based on these observations, it is shown that the crosslink density of the sulfur-vulcanised NR as determined by 13C is identical with the true crosslink density, and the influence of the wasted or ineffective crosslinks (pendent and cyclic crosslinks) and chain ends is negligible. However, this conclusion seems to be only valid if the effect of topological constraints or entrapped entanglements on the mechanical modulus is negligible which is rarely the case in real systems. 

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... 

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. 

Solid-state 13C NMR spectra of carbon black filled, uncured and sulfur-vulcanised HR were recorded at 22.6 MHz. The line broadening of the filled polymer relative to the unfilled polymer is attributed to incomplete motional narrowing of the NMR lines [53, 54] Incorporation of filler also results in a decrease in the signal-to-noise ratios in the spectra, but fundamentally it does not obscure the qualitative and quantitative nature of the spectra for the moderately cured elastomer systems. 

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. 

Another study used 13C NMR to examine the thermal ageing of NR with both peroxide and conventional and EV sulfur vulcanised samples [59]. The samples were heat aged at 70 °C in air with 13C NMR recorded after 3 and 21 days. The vulcanisate structures vulcanised by peroxide and the EV sulfur system showed low sensitivity to ageing effects while peaks in the conventional sulfur system almost disappeared after 21 days of ageing. In addition, the main chain isoprene carbon peaks showed considerable broadening due to increasing stiffness of the network. 

Balasubramanian [16] has described a devulcanisation process that uses a counter-rotating twin-screw extruder to devulcanise GTR. The DR was then blended with virgin NR in various proportions and the blends revulcanised using a sulfur cure system. The Mooney viscosity, cure characteristics and mechanical properties of the resulting vulcanisates were characterised and a four-parameter rheometric equation, based on the standard logistical model for the curing behaviour of extrusion processed blends, was derived and validated for the different levels of virgin NR. 

Guzman and co-workers [27] investigated whether it is possible to use waste tyre crumb as a replacement for zinc oxide as an activator in the sulfur vulcanisation of natural rubber (NR). They used the unsaturated organic compound squalene as a model compound for NR in their work, and followed the course of the vulcanisation reaction using the analytical technique high-performance liquid chromatography. The results confirmed that waste rubber crumb was an alternative to zinc oxide as an activator in the curing of NR compounds by sulfur-based cure systems. 

Since these early days, the process and the resulting vulcanised articles have been greatly improved. In addition to natural rubber, many synthetic rubbers have been introduced over the years. Furthermore, many substances other than sulfur have been introduced as components of curing (vulcanisation) systems. 

Most accelerators used in the sulfur vulcanisation of other high diene rubbers are not applicable to the metal oxide vulcanisation of CR. An exception is the use of a so-called mixed curing system for CR, in which metal oxide and accelerated sulfur vulcanisation are combined. Along with the metal oxides, TMTD, DOTG and sulfur are used. This is a good method to obtain high resilience and dimensional stability. 

Sulfur crosslinks have limited stability at elevated temperatures and can rearrange to form new crosslinks. This results in poor permanent set and creep for vulcanisates when exposed for long periods of time at high temperatures. Resin cure systems provide C-C crosslinks and heat stability. Alkyl phenol formaldehyde derivatives are usually employed for tyre bladder applications. A typical vulcanisation system is shown in Table 26 (mix 03). Perkalink 900 has been added at 0.5 phr (mix 04)... 

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. 

The effect of the accelerator to sulfur ratio on the vulcanisation chemistry was also investigated by comparing the vulcanisation products from conventional, SEV and EV formulation systems. With increase in accelerator to sulfur ratio (from conventional to EV) there is a lowering in the sulfur rank. Also, the cis-to-trans isomerisation increases with the amount of accelerator. 

The second phenomenon due to carbon black on a proposed scheme based on the vulcanisation process shown in Figure 9.12. During the vulcanisation process, the reactions (1) and (2), and (3) and (4) form polysulfidic Ale and Bit, respectively. Considering the fact that Ale polysulfide reduces its sulfur rank to monosulfide and reaction (3) and (4) is the predominant process in the later stage of cure, the system may favour reaction (3) and (4) rather than (1) and (2) in the whole reaction scheme. BtSH is formed from the... 

Vulcanisation of EPDM with sulfur systems was studied by H-NMR using ethylidene norbornane (ENBH) as a model of ENB [62]. The use of ENBH was also effective to elucidate the curing reaction of EPDM with phenol-formaldehyde resin [63]. Similarly, halogenation reaction of HR was studied by H-NMR using 2,2,4,8,8-pentamethyl-4-nonene as a model [64],... 

When a rubber is subject to vulcanisation, with sulfur or any other curing agent, it is converted into a thermoset system that is lightly crosslinked, with a molecular weight (Mw) between each crosslink of approximately 10,000 Da. This leaves the majority of the rubber chain free and, as a consequence, the glass transition temperature of the material is below ambient temperature. This gives rubber its... 

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