Conventional vulcanization cure system

Conventional sulfur vulcanization cure systems, 21 801 Convention du Mtre, 24 434 Convergent dendrimer synthesis method, 26 787-788... 

Figure 5.16 Tearing energy versus crack propagation rate of vulcanized (conventional sulfur cure system) HAF (N330) black-filled (50 pphr) NR at 23 °C. (Reproduced from ref 8.).

Polyacrylic elastomers contain chemically saturated backbones. Since these rubbers are not unsaturated, they cannot be vulcanized with conventional sulfur cure systems. Monomers with cure-sites must therefore be introduced into the polymer chain. 

Sulfur cures do not always use elemental sulfur. Sometimes with special EV (efficient vulcanization) cure systems no elemental sulfur is used at all. Instead a sulfur donor chemical is used. The most common sulfur donor is dithiodimorpholine (DTDM), which donates two sulfur atoms from the center of its molecule to participate in the sulfur vulcanization process. These EV cures are more expensive than conventional sulfur cures based on elemental sulfur. This is because sulfur donors such as DTDM are more expensive per pound than sulfur itself. Elowever, the EV cure will usually impart better air aging resistance than that of a conventional sulfur cure system using a significant concentration of elemental sulfur. 

Double-Bond Cure Sites. The effectiveness of this kind of reactive site is obvious. It allows vulcanization with conventional organic accelerators and sulfur-based curing systems, besides vulcanization by peroxides. Fast and controllable vulcanizations are expected so double-bond cure sites represent a chance to avoid post-curing. Furthermore, blending with other diene elastomers, such as nitrile mbber [9003-18-3] is gready faciUtated. 

Material containing conventional vulcanization system, once formed (e.g., profile) is normally cured immediately. If the product is off-size, or undercured, it is not possible to run again. In the case of EB-cross-Iinked equivalent, the product, in the case of being undercured, can be treated with additional dose to make up to the required level. Hence, EB-processing of polymers is expected to generate less scrap [47]. 

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. 

Because of the diene component, nitrile rubbers can be vulcanized with sulfur. A conventional curing system consists of 2.5 parts sulfur, 5.0 parts zinc oxide, 2.0 parts stearic acid, and 0.6 parts N-t-butylbenzothiazole-2-sulfenamide (TBBS) per 100 parts polymer. 

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. 

In fact, the physical properties obtained depend on the types of cross-hnk formed and the extent of main-chain modification by side reactions. This is usually being largely determined by the vulcanization system, although cure time and temperature also have an important effect. Generally, there are three types of typical sulfur vulcanization systems, namely, conventional vulcanization, efficient vulcanization and semi-efficient vulcanization. 

More recently, this has led to the development of liquid injection molding (LIM), in which the reactive sdicone oligomer system is injection molded at 200 to 250°C and cures in a few seconds, a great advance over conventional vulcanization systems. 

Cure System Design. There are three generally recognized classifications for sulfur vulcanization conventional, efficient (EV) cures, and semiefficient (semi-EV) cures. These differ primarily in the type of sulfur cross-links that form as shown in Figure 12, which in turn significantly influences the vulcanizate properties (33). The term efficient refers to the number of sulfur atoms per cross-link (32). 

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

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... 

Tread. Tread is the wear resistance component of the tyre and is in direct contact with the road. It must provide traction, wet skid and good cornering characteristics with minimum noise generation and also low heat build-up. Tread components can consist of blends of NR, polybutadiene (BR) and SBR, compounded with carbon black, silica, oils and vulcanizing chemicals." Among recently reported formulations for tyre tread with economic and environmental merits is the work of Rattanasom, in which a blend of NR and tyre tread reclaimed rubber (RR) was prepared and mechanically characterized. Their results showed that the blends prepared with different curing systems, i.e. conventional vulcanization (CV) and efficient vulcanization (EV), exhibit an increase in their hardness and modulus with increasing RR content, while other mechanical properties were adversely affected. ... 

The low degree of unsaturation of butyl rubber, whilst having certain advantages, has the consequences of a low cure rate when using the conventional vulcanization systems used in diene rubbers. In turn this... 

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). 

Halogenated Butyl. Butyl elastomers have a low density of unsaturation, which results in a low cure rate when conventional vulcanization systems are used. Butyl copolymers containing a small amount of combined chlorine or bromine are vulcanized more quickly than normal butyl rubber because the halogen atoms provide additional sites for the cross-linking process. Butyl rubber has poor adhesive properties to metals and other rubbers because of the lack of polar groups. This is the reason for using halogenation. 

Films of carboxylated latexes vary considerably in properties but generally are of lower elongation and higher modulus than their non-carboxylated counterparts. Cure systems are not normally necessary but may be used in some special applications for the development of specific properties. Conventional sulfur vulcanization can be used in some cases (lower styrene, lower gel) but more often materials that react with the functional groups are used (e.g., zinc oxide or melamine formaldehyde resins). 

Uncured ethylene-propylene copolymers are soluble in hydrocarbons and have rather poor physical properties useful technological properties are developed only on vulcanization. As mentioned above, the saturated copolymers are vulcanized by heating with peroxides whilst the terpolymers are vulcanized by conventional sulphur systems. The peroxide-cured rubbers have somewhat better heat aging characteristics and resistance to compression set but sulphur-cured rubbers are more convenient to process and allow greater compounding freedom. 

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