Conventional cure systems

Conventional cure systems use relatively high levels (2.5 + phr) of sulfur combkied with lower levels of accelerator(s). These typically provide high initial physical properties, tensile and tear strengths, and good initial fatigue life, but with a greater tendency to lose these properties after heat aging. 

The sulphur vulcanisation of NR generally requires higher added amounts of sulphur, and lower levels of accelerators than the synthetic rubbers. Sulphur contents of 2-3 phr, and accelerator levels of 0.2-1.0 phr are considered to be conventional cure systems. 

This analysis of the vulcanization problems that arise when VF2 /mVE/TFB polymers are subjected to conventional curing systems has led to development of specifically peroxide-curable polymers. Thus, the entire prlem of undesirable response to basic curatives has been tqnpassed and excellent vulcanization b tavior and vulcanizate properties, especiidly low temperature service fluid resistance, have been obtained. 

Reinforcement of SBR with carbon black leads to vulcani2ates which resemble those of natural rubber, and the two products are interchangeable in most applications. As with natural rubber, accelerated sulfur systems consisting of sulfur and an activator comprising a metal oxide (usually zinc oxide) and a fatty acid (commonly stearic acid) are used. A conventional curing system for SBR consists of 2.0 parts sulfur, 5.0 parts zinc oxide, 2.0 parts stearic acid, and 1.0 part N-r-butylbenzothiazole-2-sulfenide (TBBS) per 100 parts polymers. 

Thermo-oxidative stability is primarily a function of the vulcanization system. Peroxide vulcanization or cure systems tend to perform best for reversion resistance as a result of the absence of sulfur and use of carbon-carbon crosslinks. Efficient vulcanization (EV) systems that feature a low sulfur level (0.0-0.3 phr), a high acceleration level, and a sulfur donor similarly show good heat stability and oxidation resistance. Such systems do, however, have poor resistance to fatigue because of the presence of predominantly monosulfidic crosslinks. Conventional cure systems that feature a high sulfur level and low accelerator concentration show poor heat and oxidation resistance because the polysulfidic crosslinks are thermally unstable and readily oxidized. Such vulcanization systems do, however, have better fatigue resistance. Semi-EV cure systems, which are intermediate between EV and conventional systems, are a compromise between resistance to oxidation and required product fatigue performance. 

Material Conventional cure system, phr Semi-EV (semiefficient vulcanization system, phr EV (efficient vulcanization system), phr... 

Fig. 13. Aging properties of cured natural rubber. A conventional cure system, B semi-EV, C EV system, tensile strength loss (%), fatigue life, andhardness change.

Vulcanization System Components. Tire compounds are almost exclusively cured (cross-linked) with sulfur. Sulfenamides, thiazoles, thiurams, guanidine, and carbamates are the most popular choices to accelerate curing. Efficient vulcanization (EV), semiefficient (semi-EV), and the conventional curing systems... 

Polybutadienes with vinyl contents of 10 7 and 72.4% have been selected for further fatigue-to-failure analysis. Fatigue life measurements have been performed on a large number of specimens by means of the Fatigue-to-Failure Tester (Monsanto). Carbon-black-filled compounds with 50 phr N330 carbon black were cured with conventional curing system at 145°C. Curative levels have been chosen to obtain vulcaniza-tes at several levels of 300% modulus. Fatigue tests were performed on un-notched specimens at room temperature and at several deformation amplitudes. Data discussed here were obtained at 136% strain amplitude. 

Sulphur systems. Conventional cure systems for most current, practical, tyre-related and mechanical goods formulations consist of zinc oxide, plus sulphur or sulphur-donors, accelerated with sulphenamides or benzothiazoles. These types of accelerator function initially as retarders but ultimately produce very high states of cure. MBT (mercaptobenzothiazole) is an exception in that it will scorch bromobutyl stocks, but it can be used... 

Purified natural rubber (PNR) is of interest because of its potentially lower toxicological effects than whole natural rubber (WNR) due particularly to the reduction in protein content. Improved dynamic mechanical properties have also been reported. Comparison of both gum and filled compounds, vulcanised using conventional cure systems (CV) and efficient vulcanisation systems (EV) prepared from PNR and WNR indicate that generally properties of the PNR are poorer than WNR. The exception is in the flex cracking resistance and the heat build up in the filled samples where PNR shows an improvement. Using the EV cure system on filled PNR gives properties almost comparable to WNR. A study of the distribution of the types of sulphur crosslink in both pNR and WNR vulcanisates indicates a more uniform distribution of monosulphidic (S), disulphidic (S2) and polysulphidic (Sx) crosslinks in the PNR samples (38.7/25/36.4 in PNR compared to 64.1/29.7/6.6 in WNR respectively). This is believed to be the reason for the better dynamic properties of PNR vulcanisates. 7 refs THAILAND... 

The optimising of a tyre compound to improve reversion resistance is discussed with reference to the use of two new rubber chemicals developed by Flexsys. They are 1,3 bis(citraconimidomethyl benzene) (BMI-MX) and hexamethylene-1,6-bisthiosulphate disodium salt dihydrate (HTS), which are claimed to provide two different ways to improve the thermal ageing resistance of sulphur-based vulcanisates as compared to existing technology. Each functions by uniquely different mechanisms of action and each provides different levels of performance improvements. A comparison is made of HTS and BCI-MXto conventional cure systems and semiefficient cure systems to illustrate the potential benefits of these approaches to address reversion in tyre compounds. 5 refs. 

The curing acceleration effect of bis(dialkyl) thiophosphoryl sulphide (DaPDS) are investigated. The order of acceleration effect is ethyl, butyl, 2-ethylhexyl. Ethyl and butyl derivatives indicate the function of a sulphur donor. It is assumed that the active species is the zinc salt of each DaPDS. In contrast with the conventional curing system, curing of the DaPDS/S curing system is activated by silica. Physical properties of silica-filled vulcanisates are superior to those of conventional curing systems. 4 refs. 

A rubber curative based on adducts of quinone dioxime and toluene diisocyanate has been developed that offers improved reversion resistance and heat ageing properties compared to conventional cure systems. This comprehensive article supplies a detailed examination of the use of these adducts as curatives for natural rubber and their effect on bondability. 

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