Zinc-accelerator-sulfur complex

The hydrophilic nature of silica also affects the cure characteristics of rubber compounds, the properties of vulcanized rubber and also the compatibility with non-polar rubber such as natural rubber (NR). Silica retards the vulcanization as it reacts with zinc-accelerator-sulfur complex. These drawbacks can be overcome through the use of silane coupling agents. The most common silane coupling agent used is bis(3-triethoxysilylpropyl) tetrasulfide (TESPT). A silane... 

The aim of the present chapter is to review the course of sulfur vulcanization in the light of recently obtained information on the reactivity of some of the intermediates in the process and on the stability of monosulfide crosslinks. All the recent results relate to NR, but reference to previous work with BR allows some major differences in behavior to be discerned. In the case of NR, the key role of zinc accelerator-thiolate complexes in promoting various reactions has been emphasized by the new findings. In addition, the importance of the position of substitution of sulfur on the rubber backbone in determining the subsequent fate of the system has been further underlined. It turns out that neither of these features can play any significant part in the sulfur vulcanizations of BR. 

The concentration of zinc accelerator-thiolate complexes in the rubber is not the only factor determining the balance of the two reactions in NR. Both the rate of desulfuration of polysulfide crosslinks and the rate of their thermal decomposition depend upon the positions of attachment of the sulfur chains to the backbone rubber chains and the detailed structure of the hydrocarbon at the ends of the crosslinks. In the course of normal accelerated vulcanization there are three different positions of attack on the polyisoprene backbone two of these are methylene groups in the main chain (labelled d and a in 3), and the third is the side chain methyl group (labelled b in 3). Direct analysis of the distribution of the sites of attack cannot yet be made on actual rubber vulcanizates, and information has had to be obtained solely by sulfuration of the model alkene 2-methyl-2-pentene and, more recently, 2,6-dimethyl-2,6-octadiene. The former (4) models the a-methylic site but only one of the two a-methylenic sites of polyisoprene the latter (5) models all three sites, but at the present time these are not all supported by the synthesis of relevant sulfides. Because allylic rearrangements are common in subsequent reactions of the sulfurated rubber, sulfur substituents appear not only on allylic carbon atoms but on isoallylic carbon atoms. Thus, from 2-methyl-2-pentene, the groups shown in Scheme 2 are formed. 

Zinc accelerator-thiolate complexes play a central role in controlling the balance between the various reactions because they promote the primary sulfuration of the rubber to form polysulfidic pendent groups and the conversion of these to crosslinks they are the agents which desulfurate both pendent groups and crosslinks they catalyze polysulfide exchange reactions and in some cases they promote the decomposition of crosslinks. Other factors which affect the balance between these reactions are the temperature and the structure of the main rubber chain in the immediate vicinity of the crosslink. The latter is, in turn, at least partly controlled by the structure and concentration of zinc accelerator-thiolate complexes. 

Although sulfur vulcanisation was discovered over one hundred and fifty years ago, the exact mechanism of vulcanisation is still being examined. This arises not only from the complexity of the reactions and products formed but also to the fact that the mechanism of accelerated sulfur vulcanisation changes is dependent on the class of accelerators/ activators used. Typically, benzothiazole or sulfenamide are used as accelerators, zinc... 

It is known that zinc cations of the ZnO and/or zinc compounds react with the organic accelerators thus giving zinc complexes accelerators. This is one of the most important stages of the vulcanization scheme [50, 51]. It has been suggested that the better zine dispersing in the system ends with zinc complex generating and the zine ions are free to form active accelerator complexes. By reason of that, zine ion is supposed to be the central atom in the accelerated sulfur vulcanization [46]. This idea has remained indivertible for the last 25 years. 

The chemistry of the sulfur system is extremely complex, but the generally accepted mechanism involves the formation of zinc-accelerator complexes, which interact with sulfur to form a zinc perthio salt [7]. 

The irreversible binding of accelerator moieties by desulfuration or by some other reaction limits the formation of zinc accelerator complexes and subsequently hinders desulfuration of both pendent groups and crosslinks. Although direct evidence for this pathway being responsible is lacking, it does appear that with NR at high temperatures, or with butadiene-based rubbers at all temperatures, removal of the accelerator by combination with the polymer gives the system many of the characteristics of an unaccelerated sulfur vulcanization" (see Section 4.7). 

Activators. Activators are chemicals that increase the rate of vulcanization by reacting first with the accelerators to form mbber soluble complexes. These complexes then react with the sulfur to achieve vulcanization. The most common activators are combinations of zinc oxide and stearic acid. Other metal oxides have been used for specific purposes, ie, lead, cadmium, etc, and other fatty acids used include lauric, oleic, and propionic acids. Soluble zinc salts of fatty acid such as zinc 2-ethyIhexanoate are also used, and these mbber-soluble activators are effective in natural mbber to produce low set, low creep compounds used in load-bearing appHcations. Weak amines and amino alcohols have also been used as activators in combination with the metal oxides. 

The role of activators in the mechanism of vulcanization is as follows. The soluble zinc salt forms a complex with the accelerator and sulfur. This complex then reacts with a diene elastomer to form a mbber—sulfur—accelerator cross-link cursor while also Hberating the zinc ion. The final step involves completion of the sulfur cross-link to another mbber diene segment (18). 

Halobutyl Cures. Halogenated butyls cure faster in sulfur-accelerator systems than butyl bromobutyl is generally faster than chlorobutyl. Zinc oxide-based cure systems result in C—C bonds formed by alkylation through dehydrohalogenation of the halobutyl to form a zinc chloride catalyst (94,95). Cure rate is increased by stearic acid, but there is a competitive reaction of substitution at the halogen site. Because of this, stearic acid can reduce the overall state of cure (number of cross-links). Water is a strong retarder because it forms complexes with the reactive intermediates. Amine cure may be represented as follows ... 

Zinc dithiocarbamates have been used for many years as antioxidants/antiabrasives in motor oils and as vulcanization accelerators in rubber. The crystal structure of bis[A, A-di- -propyldithio-carbamato]zinc shows identical coordination of the two zinc atoms by five sulfur donors in a trigonal-bipyramidal environment with a zinc-zinc distance of 3.786 A.5 5 The electrochemistry of a range of dialkylthiocarbamate zinc complexes was studied at platinum and mercury electrodes. An exchange reaction was observed with mercury of the electrode.556 Different structural types have been identified by variation of the nitrogen donor in the pyridine and N,N,N, N -tetra-methylenediamine adducts of bis[7V,7V-di- .vo-propyldithiocarbamato]zinc. The pyridine shows a 1 1 complex and the TMEDA gives an unusual bridging coordination mode.557 The anionic complexes of zinc tris( V, V-dialkyldithiocarbamates) can be synthesized and have been spectroscopically characterized.558... 

Complexes with SCN throw light on the relative affinities of the two metals for N-and 5-donors. In [Zn(NCS)4] the ligand is A-bonded whereas in [Cd(SCN)4] it is 5-bonded. SCN can also act as a bridging group, as in [Cd S=C(NHCH2)2 2(SCN)2] when linear chains of octahedrally coordinated Cd are formed (Fig. 29.3c). A number of zinc-sulfur compounds are used as accelerators in the vulcanization of rubber. Among these are the dithio-carbamates, of which [Zn(S2CNEt2)2]2, and the isostructural Cd and Hg compounds achieve... 

Zinc complexes of dithiocarbamates and of other sulfur compounds are important accelerators in the vulcanization of rubber by sulfur. The iso-structural Zn and Cd compounds, [M(S2CNEt2)2]2, achieve 5-coordination... 

Typically a recipe for the vulcanization system for one of these elastomers contains 2-10 phr of zinc oxide, l phr of fatty acid (e.g., stearic), 0.5-4 phr of sulfur, and 0.5-2 phr of accelerator. Zinc oxide and the fatty acid are vulcanization-system activators. The fatty acid with zinc oxide forms a salt, which can form complexes with accelerators and reaction products, formed between accelerators and sulfur. Accelerators are classified and illustrated in Table 7.1. 

To improve the efficiency of the vulcanization reaction, various accelerators were developed. Among them are zinc oxide combined with fatty acids, and/or amines. Zinc oxide forms zinc mercaptides like (XS)2ZnL2, where X is an electron-withdrawing substituent and L is a ligand from a carboxyl or an amine group. The function of the ligand is to render the complex soluble. The mercaptide complexes are believed to react with additional sulfur to form zinc perthiomeraptides. 

Manufacture of rubber products requires the incorporation of fillers such as carbon black, silica, and clay, pigments, sulfur, accelerators, retarders, resins, antioxidants, antiozonants, extending oils, zinc oxide, and a variety of other elastomers. The complexity and the variety of compounding ingredients normally present in articles containing natural rubber necessitate the use of a multitude of analytical techniques depending on the information required. 

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