Efficient vulcanization

New efficient vulcanization systems have been introduced in the market based on quaternary ammonium salts initially developed in Italy (29—33) and later adopted in Japan (34) to vulcanize epoxy/carboxyl cure sites. They have been found effective in chlorine containing ACM dual cure site with carboxyl monomer (43). This accelerator system together with a retarder (or scorch inhibitor) based on stearic acid (43) and/or guanidine (29—33) can eliminate post-curing. More recently (47,48), in the United States a proprietary vulcanization package based on zinc diethyldithiocarbamate [14324-55-1]... 

Polychloroprene rubbers are not efficiently vulcanized by sulfur. The chlorine atoms deactivate the double bonds toward reaction with sulfur. Vulcanization is achieved by heating with zinc and magnesium oxides. Crosslinking involves the loss of... 

Certain thio-substituted 1,2,4-thiadiazoles (422), (423), (424) have been patented as efficient vulcanization accelerators (76MI42501,75USP3899502, 75USP3904619). 

Here is 1 or 2 in efficient vulcanization systems but may be as high as 8 under other conditions where cyclic and other structures are also formed in the reaction. The rubber article is essentially fixed in shape once it is vulcanized, and it is... 

Figure 1.4 Vulcanization of natural rubber with sulfur, (a) Linear polyisoprene (natural rubber), (b) An idealized structure produced by vulcanization with sulfur. The number (x) of sulfur atoms in sulfide cross-linkages is 1 or 2 in efficient vulcanization systems but may be as high as 8 under conditions where cyclic and other structures are also formed in the reaction, (c) The effect of cross-linking is to introduce points of linkage or anchor points between chain molecules, restricting their slippage.

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 later work, Lattimer et al. identified eight oligomeric series in a commercial version of poly-TMDQ by using the LC/FD-MS combination. Components of f-octylphenol/formaldehyde resins were also characterized using this method. In a series of studies, ozonation products of several p-phenylenediamine compoimds (rubber antiozonants) were separated by LC and identified by using FD-MS. In another study, model "efficient vulcanization" products were separated by colunm chromatography and characterized by FD-MS. "... 

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. 

For semi-efficient vulcanization systems, intermediate sulfur level of 1-2 phr and 2.5-1 phr of accelerator are often used. The vulcanizates have physical properties intermediate between those of conventional vulcanization and efficient vul-canizationvulcanizates. hi fact, they give some improvements in reversion, ageing resistance and compression set compared with conventional vulcanizationvulca-nizates, but resistance to fatigue and low temperature crystallization is impaired. However, they have higher scorch safely, particularly when sulfenamide accelerators are used in the system. 

Peroxides are highly efficient vulcanizing agents for elastomers and plastics. In addition they are used as polymerization catalysts in the coatings industry. Tliese materials are commercially available in several physical forms and container types to suit the needs of users. Sulfur vulcanization of rubber was discovered in the early 1800s. The cross-linking of natural rubber with a peroxide was discovered in 1914 by a Russian, who used benzoyl peroxide. However, benzoyl peroxide vulcanizates have lower strength and poorer heat resistance than do sulfur vulcanizates. In 1950... 

Mukhopadhyay, R De, SK. Effect of Vulcanization Temperature and Different Fillers on the Properties of Efficiently Vulcanized Natural Rubber. Rubber Chem. Technol, 1979, vol.52, JV22, 263 - 111. 

Plot of permanent set against recombination efficiency n2/ i of conventional vulcanization system (CV) and efficient vulcanization system (EV) black-filled (50 pphr HAF) NR vulcanizates. 

Sulfur. Low sulfur compounds and efficient vulcanized (EV) sulfur-accelerated systems have better aging resistance. Normally, the oxidation rate increases with the amount of sulfur used in the cure. The increased rate may be due to activation of adjacent C—H groups by high levels of combined sulfur. Saturated sulfides are more inert to oxidation than allylic sulfides. Polysulfidic cross-links impart excessive hardening of SBR as compared to more stable mono-sulfidic cross-links. 

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. ... 

As a result of these reactions, the vulcanizate will exhibit reduced resistance to compression set and creep. It may be noted that although vulcanizates with predominantly monosulphide cross-links show better heat resistance, they may not have the excellent overall physical properties of predominantly polysulphide cross-linked networks. For this reason, a compromise is usually aimed for in practice and so-called semi-efficient vulcanizing systems are used. 

It is also desirable that the salts obtained during the in situ reaction between the zinc oxide and fatty acid should be soluble since their separation out can markedly affect creep and low compression set characteristics. The zinc salts of branched fatty acids, such as zinc 2-ethyl hexanoate, can be particularly useful especially in conjunction with a soluble efficient vulcanization (EV) system. 

Ethylidene norbornene (2-ethylidene-bicyclo(2,2,l)-5-heptane)— which leads to rapid and efficient vulcanization. 

ENR possesses fewer double bonds than NR and therefore would be expected to be more resistant to oxidation. However, the oven air ageing of a conventionally cured ENR vulcanizate (S, 2-5 sul-phenamide accelerator, 0 5 phr) was found to be poor. A rapid hardening occurred on air ageing. This oxidative hardening is not an intrinsic property of ENR since when peroxide and efficient vulcanizing systems (S, 0 3 sulphenamide, 2 4 TMTD, 1 6 phr) were used their ageing characteristics were observed to be similar to those of the corresponding NR vulcanizates (Fig. 16). 

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