Scorch delay

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. 

Another commercially available retarder for sulfur vulcanization is based on an aromatic sulfenamide. Like CTP, this product is most effective ki sulfenamide cure systems, but it also works well ki thiazole systems. Performance properties are generally not affected except for a slight modulus kicrease. In some cases this feature allows for the use of lower levels of accelerator to achieve the desked modulus with the added potential benefits of further scorch delay and lower cost cure system (23). 

Functionally, accelerators are classified as primary or secondary. Primary accelerators provide considerable scorch delay, medium fast cure, and good modulus development. Secondary accelerators, on the other hand, are usually scorchy and provide very fast cure. There are a wide variety of accelerators available to the compounder including accelerator blends this number well over 100. In order to rationalize the extensive range of materials it is useful to classify them in terms of their generic chemical structure listed below and shown in Figure 14.4. 

The cure reaction is obviously highly complex, and expressing such a system of reactions in a simple first-order reaction with an activation energy results from a strong assumption. As the problem is vast, only two studies are considered the one concerned with the sulfur vulcanization process, and the other more specifically based on the explanation of the scorch delay kinetics. 

In order to explain the rather long delay period frequently enconntered in accelerated sulfur vulcanization, especially wherein thiazole sulfonamides or other scorch delay accelerators are nsed, the following scheme has been proposed ... 

If the reaction through k is much faster than through k, very little crosslink formation can occur until A is essentially depleted. Both the reaction through k and that through fe4are assumed to be much faster that the reaction through k2l hence, after the delay required for the depletion of A, crosslink formation will proceed in a first-order fashion. As said by Coran [17], the proposed scheme for scorch delay vulcanization was largely substantiated by experimental observations. 

Coran A.Y. 1964. Vulcanization. Part VI. A model and treatment for scorch delay kinetics. Rubber Chem. Technol. 37 689-97. 

This figure also illustrates the three main regions of rubber vulcanization. The first regime is the induction period, or scorch delay, during which accelerator complex formation occurs. The second time period is the cure period, in which the network or sulfurization structures are formed. The network structures can include crosslinks, cyclics, main chain modification, isomerization, etc. The third regime is the overcure, or reversion regime. 

Although acceptable cure characteristics are obtained with a conventional system based on 2 5 phr sulphur and 0 5 phr sulphenamide, this formulation is not recommended for ENR as the vulcanizates have poor ageing characteristics compared with unmodified NR. The cause of this is discussed in Section 6.9. Semi-EV or EV-type cure systems have been found to be the most satisfactory for ENR. Examples of such systems are recorded in Table 1. The need to compound ENR with a base affects the vulcanization characteristics, and one of the factors to be considered in the choice of base is the processing safety of the mix. " Stronger bases can markedly reduce scorch delay by base catalysis of the vulcanizing system and should only be employed at low levels, whereas the less basic compounds can be employed at higher concentrations (Fig. 9). 

MBT is used as both a primary (when used alone) and secondary accelerator (in combination with other accelerators) for curing compounds based on general-purpose elastomers. It is a very potent vulcanization accelerator, giving a very fast rate of crosslinking. Unfortunately, its scorch delay is quite poor. For this reason the sulfenamide accelerators, derived from MBT, were developed. 

Determining the scorch safety of the samples (devulcanised rubber alone and in blends with virgin rubber) using rheometry and Mooney scorch delays. 

A study was made of the mechanisms by which tetraisobutylthiuram monosulphide (TiBTM) acts both as a retarder and a secondary accelerator in curing systems containing benzothiazole sulphenamides as primary accelerators. Model reactions between TiBTM andTBBS, CBS or MBTS were followed by HPLC to identify intermediates formed between TiBTM and its various analogues and the primary accelerators. Some of these intermediates were independently synthesised and evaluated as retarders and secondary accelerators in SBR/ polybutadiene compounds. The isobutyl groups were responsible for imparting scorch delay characteristics to one of these intermediates, while another intermediate was most likely responsible for the secondary accelerator effect. 14 refs. 

The disodium salt of hexamethylene bisthiosulphate (HTS) was evaluated as an accelerator in the zinc oxide mediated vulcanisation of tyre sidewall compounds containing a brominated isobutylene-p-methylstyrene copolymer. Rheometer profiles of blends with polybutadiene cured with sulphenamide, sulphur, zinc oxide and HTS showed a characteristic delayed action fast cure kinetic profile, while the same blends cured without HTS exhibited undesirable cure characteristics, with long early cure and fast late cure. Many vulcanisation characteristics and vulcanisate properties were largely dominated by a single cure component. State of cure was dominated by sulphur level, scorch delay and early cure time by HTS loading, and the time to reach t90 by the sulphenamide level. Time of flight secondary ion mass spectroscopy imaging showed that HTS favourably affected the distribution of curatives in blends with SBR. 11 refs. 

An examination is made of the characteristics obtained by vulcanisation with tetrabenzylthiuram disulphide (TBzTD)/sulphenamide accelerator systems, including reversion resistance, efficient vulcanisation with sufficient scorch delay, and increased nitrosamine safety. It is shown that the combination of properties imparted by these accelerator blends results from low sulphur rank crosslinks provided by the thiuram and the scorch safety provided by the sulphenamide. The advantages of these systems over sulphur donor/sulphenamide and lower molecular weight thiuram/sulphenamide systems are illustrated by the evaluation of blends of TBzTD andN-t-butyl-2-benzothiazole-2-sulphenamide in NR tyre compounds. 13 refs. 

A polyacetylene coating applied on sulfur does not negatively influence its activity and speed as curing agent, but it can increase the scorch time. This effect is probably due to a delayed release of the sulfur out of the polymeric shell. In the SBR/EPDM blend, on the other hand, the plasma-treated sulfur results in higher torque values, an indication that the distribution of the plasma-treated sulfur over the different rubber phases is more homogenous, which is the main effect aimed for in the context of this study. 

Premature cure at processing tenqperature ("scorch at 121 0 is conveniently delayed or prevented by snail amounts of various phenolic and amine free-radical inhibitors and antioxidants. When the inhibitor concentration is properly chosen, the retarding effect disappears on the curing temperature (177 0. 

The effect of the addition of small concentrations of the premature vulcanization inhibitor (PVI), N-(cyclohexylthio)phthalimide, is also given by Figure 7.11. This retarder (Coran and Kerwood, 1970) is frequently used to independently control scorch resistance with little effect on the rate of crosslinking (Coran, 1978). Before the development of N-(cyclohexylthio)phthalimide as a PVI, acidic retarders like salicylic acid, acetylsaUcylic acid, phthaUc anhydride, and benzoic acid were used. These additives improved scorch resistance but also gave greatly reduced rates of crosslink formation after the delay. Another retarder of the past was N-nitrosodiphenylamine, which is less active and not now used because of toxicological concerns. 

Organic chemical accelerators were not used until 1906 (65 years after the Goodyear-Hancock development of unaccelerated vulcanization [Fig. 8]), when the effect of aniline on sulfur vulcanization was discovered by Oenslager [11]. This could have been, at least partially, in response to the development of pneumatic tires and automobiles near the turn of the century. Aniline, however, is too toxic for use in rubber products. Its less toxic reaction product with carbon disulfide, thiocarbanilide, was introduced as an accelerator in 1907. Further developments lead to guanidine accelerators [12]. Reaction products formed between carbon disulfide and aliphatic amines (dithiocarba-mates) were first used as accelerators in 1919 [13]. These were and are still the most active accelerators with respect to both crosslinking rate and extent of crosslink formation. However, most of the dithiocarbamate accelerators give little or no scorch resistance and their use is impossible in many factoryprocessing situations. The first delayed-action accelerators were introduced in 1925 with the development of 2-mercaptobenzothiazole (MET) and 2-... 

Another study by Sui et al. shows the vulcanization curves of the neat NR and NR composites. In Figure 21.11 it can be seen that the scorch time (measurement of premature vulcanization) decreased after adding CNTs or carbon black into the NR. This may be because when CNTs are added, they would absorb the basic accelerator species and delay the onset of vulcanisation. The reason for this is that the addition of the CNTs could improve thermal... 

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