Vulcanizing systems

Although filled systems are not as sensitive to variations in this type of cure behavior, certain correlations can be made with practical vulcanizing systems (29). 

Because of the different vulcanization chemistry involved in each commercial ACM, a vulcanization system specific to the cure site present has to be adopted. Many cure systems for labile chlorine containing ACM have been proposed (45). Among these the alkali metal carboxylate—sulfur cure system, or soap—sulfur as it is called in the United States, became the mainstay of acryflc elastomer technology in the early 1960s (46), and continues to be widely used. 

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]... 

Poor resistance to organic solvents and oils. This can be partially reduced in vulcanized systems. 

Like the 1-RTV systems, the two-part room temperature vulcanization systems (2-RTV) cure to produce flexible elastomers that resist humidity and other harsh environments. Interestingly, they display primerless adhesion property to many substrates, and are used in silicone adhesives, sealants, seals, and gaskets, to name a few. 

Since these early days, the process and the resulting vulcanized articles have been greatly improved. In addition to NR, many synthetic rubbers have been introduced over the years. Furthermore, many substances other than sulfur have been introduced as components of curing (vulcanization) systems. 

Sulfur cross-links have limited stability at elevated temperatures and can rearrange to form new cross-links. These results in poor permanent set and creep for vulcanizates when exposed for long periods of time at high temperatures. Resin cure systems provide C-C cross-links and heat stability. Alkyl phenol-formaldehyde derivatives are usually employed for tire bladder application. Typical vulcanization system is shown in Table 14.24. The properties are summarized in Tables 14.25 and 14.26. 

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]. 

The ZnCFO action as vulcanization active component of elastomeric compositions on the basis of rubbers of general and special assignment with various vulcanization systems is investigated. 

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

Despite of 150-year s history of vulcanization process, it is impossible to consider that fundamental and applied researches in direction of vulcanization systems perfection are completed. For today one of the ways of rubbers properties improvement is the synthesis and application of the new chemicals-additives, including, vulcanization active, that is connected, first of all, with reduction of global stocks of zinc ores as basic raw material for reception of traditional activator - zinc oxide. Besides, modem increase of industrial potential and the accumulation of big quantity wastes derivate the problems of ecological character, which require the emergency decision. Therefore creation of resourcesaving technologies of the new compounds reception from products of secondary raw material processing has paramount importance. 

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. 

Figure 7. Cure curves of vulcanization process of modeling unfilled elastomeric compositions on the basis of nitrile-butadiene rubber at 155°C with various vulcanization systems.

Influence of the ZnCFO contents (3,0 5,0 7,0 phr) on crosslink kinetics of the modelling unfilled rubber mixes from NBR-26 of sulfur, thiuram and peroxide vulcanization of recipe, phr NBR-26 - 100,0 sulfur - 1,5 2-mercaptobenzthiazole - 0,8 stearic acid - 1,5 tetramethylthiuramdisulfide - 3,0 peroximon F-40 - 3,0, is possible to estimate on the data of fig. 7. As it is shown, the increase of ZnCFO concentration results in increase of the maximum torque and, accordingly, crosslink degree of elastomeric compositions, decrease of optimum cure time, that, in turn, causes increase of cure rate, confirmed by counted constants of speed in the main period (k2). The analysis of vulcanizates physical-mechanical properties testifies, that with the increase of ZnCFO contents increase the tensile strength, hardness, resilience elongation at break and residual deformation at compression on 20 %. That is, ZnCFO is effective component of given vulcanization systems, as at equal-mass replacement of known zinc oxide (5,0 phr) the cure rate, the concentration of crosslink bonds are increased and general properties complex of rubber mixes and their vulcanizates is improved. 

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) ... 

Figure 10. Change of elastomeric compositions properties (%) with various vulcanization systems at ZnCFO presence (in comparison with ZnO) ...

Figure 11. Dependence of particles size of heterophase of modeling unfilled compositions on the basis of various rubbers with different vulcanization systems from the ZnO (a) or ZnCFO (b) contents...

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). 

Generalizing results of researches of the ZnCFO efficiency as the component of various vulcanization systems for rubbers of general and special assignment, it is possible to make the following conclusions ... 

ZnCFO is the effective vulcanization active component of the sulfur, thiuram, peroxide and metaloxide vulcanization systems for isoprene, nitrile-butadiene and chloroprene rubbers at the same time it is not effective in resin vulcanization system for butyl rubber. On a degree of positive influence on the properties of elastomeric compositions vulcanization systems with ZnCFO are arranged in a line ... 

The phenyl modified polymers possess the optimum combination of high temperature and elastomeric properties and were used in the study of formulation parameters These variables can have an important effect on the thermal stability and property profile of vulcanized systems For example, the use of reinforcing silicas, peroxide content, and oxidative stabilizers have been shown to be important ( 3, 10, in However, polymer-silica interactions had the most pronounced effect on retaining properties during high temperature aging studies ... 

Modification of the polymer backbone by the incorporation of trifluoropropyl groups leads to substantial decreases in swelling. In vulcanized systems reinforced with hydrophilic silica (30 phr) the swelling decreased with increasing CH2CH2CF3 content as shown in Table III. 

Sulfur vulcanization leads to a variety of cross-link structures as shown in Figure 1. All the sulfur does not result in cross-links some of it remains as pendent accelerator polysulfide groups and internal cyclic polysulfides. These alternative structures do not contribute to load bearing or strength properties and are more prevalent in unaccelerated or weakly accelerated vulcanization systems. Additional heating can also reduce the polysulfide rank of the cross-links. In some elastomers, this leads to a larger number of cross-links. However, in natural mbber or its synthetic polyisoprene equivalent, the overall result is a loss of cross-links, especially at temperatures over 160°C. 

In summaiy, theie aie a lange of vulcanizing systems which can be used for natural rubber, and the choice is dependent on the combination of properties required. No single one offers ideal, all-around properties combined with good heat resistance. The end user has to be selective, according to the properties required foi the final application. Certain properties such as oil resistance and gas permeability have been omitted from Table 3, because in legaid to these properties natural mbbei is substantially inferior to synthetic mbbers such as acrylonitrile rubber and halobutyl rubber (see Elastomers,... 

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