Efficient vulcanization systems

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]... 

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.

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. 

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. 

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

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. 

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). 

Morpholine reacts with sulfur monochloride to make DTDM (a commonly used sulfur donor In efficient vulcanization systems). 

The balance between these two reactions was found to determine, in large measure, the type of vulcanizate network formed. If the concentration in the rubber of zinc accelerator-thiolate complexes such as (1) and (2) is high, the polysulfide crosslinks are desulfurated rapidly to stable monosulfides and a heat-resistant network with a high degree of crosslinking is formed — the so-called efficient vulcanization system. If the zinc complexes are present in low concentration or are insufficiently soluble, desulfuration is slow and, unless the vulcanization temperature is low, the polysulfide crosslinks suffer thermal decomposition with consequent modulus reversion and extensive modification of the main chains (conventional or inefficient vulcanization). 

It has now become possible to enlarge on the outline in Scheme 1 and the reactions (1) and (2). The similarity in structure of the rubber-bound intermediate RS X and the polysulfide crosslink RS R suggests that the intermediate, like the crosslinks, should be susceptible to desulfuration and to thermal decomposition. Some evidence for the first of these processes was originally obtained by CampbelF and both reactions were later invoked by Newell, Porter and Tidd" to explain the reduction in crosslinking efficiency displayed by efficient vulcanization systems at high temperatures (see also ref. 44). The recent synthesis of model pendent groups containing benzothiazolyl functions" has enabled their thermal behavior to be studied directly. 

The heat-resistant nature of NR vulcanizates containing very high proportions of monosulfide crosslinks (as obtained from an adequately cured efficient vulcanization system) is now well known. However, the thermal stability of these monosulfide-crosslinked networks is still about an order of magnitude lower than the stability of carbon-carbon crosslinked NR, which shows only very slow modulus reversion even at 200 °C (Figure 2). An extensive investigation of the chemical reactivity... 

The comparative estimation of ZnCFO efficiency depending on type of vulcanization system is given. 

Taking into account above told, in the work the efficiency of zinccontaining carbamide-formaldehyde composite (ZnCFO) as vulcanization active component of various vulcanization systems for rubbers of general and special assignment is investigated for the first time. 

An estimation of ZnCFO efficiency as vulcanization active component was carried out in modelling unfilled elastomeric compositions on the basis of isoprene, butadiene-nitrile, chloroprene and butyl rubbers of sulphur, thiuram, peroxide, metaloxide and resin vulcanization systems. 

The comparative estimation of efficiency of zinc oxide and ZnCFO similar concentrations (3,0 5,0 7,0 phr) as the agents of metaloxide vulcanization system was carried out on example of modelling unfilled elastomeric compositions from chloroprene rubber of recipe, phr chloroprene rubber - 100,0 magnesium oxide - 7,0. Kinetic curves of rubber mixes curing process at 155°C are shown on fig. 8. The analysis of the submitted data testifies, that at increase of zinc oxide contents vulcanization kinetics is changed as follows the scorch time and optimum cure time are decreased, the cure rate is increase. Vulcanization... 

Thus, from the analysis of results of experimental researches on estimation of ZnCFO vulcanization activity in comparison with zinc oxide in structure of various vulcanization systems (VS) follows, that its efficiency decreases in line (fig. 10) ... 

It is possible to explain the decrease of ZnCFO efficiency as component of various vulcanization systems for rubbers of general and special assignment in the earlier submitted line (fig. 10) also by character of formed morphology of compositions. So, at use of ZnCFO as the activator of sulfur vulcanization the structure of rubbers with the minimal value of parameter r is formed, and at transition from sulfur to peroxide vulcanization of elastomeric compositions the particles size of heterophase is increased (fig. 11 b). 

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