2-Mercaptobenzothiazole vulcanization accelerator

A larger amount of sulfur added to natural rubber (20-30%) generates a different product, vulcanite. Besides sulfur, during the vulcanization process other chemical compounds are commonly added to rubber. One group of such compounds consists of vulcanization accelerators (A in the previous scheme). Substances such as diphenylguanidine, mercaptobenzothiazole, tetramethythiuram disulfide, N-oxydiethylene-2-benzothiazolylsulfenamide, and cyclohexylbenzothiazolylsulfenamide are utilized as accelerators. 

The vulcanization accelerators include the thiazole 2-mercaptobenzothiazole (MBT) (169), and its derivative benzothiazole disulfide (dibenzoythiazyl disulfide, 2,2 -dithiobis (benzothiazole), MBTS) (170). Organic accelerators enable reduction in time of vulcanization, more effective use of sulfur in formation of cross-links and use of low processing temperatures. MBTS delays vulcanization, when compared with MBT alone. They are used in production of conveyor belts, footware, etc. The 2-mercaptobenzothiazole zinc salt (MBTZ) (171) is also important, and is used in latex products. Other sulfur donors include 2-morpholinodithiobenzothiazole, 2-(4-morpholinyldithio)benzothiazole (MBSS, MORFAX) (172). 

Kawaoka, Y. Studies of rubber vulcanization accelerators. VII. Dimethyl-amine salt of mercaptobenzothiazol. J. Soc. Chem. Ind. Japan 43, Suppl. 223 (1940) Chem. Abstracts 35, 2368 (1941). 

At the present time, an accelerated sulfur vulcanization system is used for RubCon curing. This system consists of sulfurs as the structuring agent of vulcanization, tetramethylthiuram disulfide and 2-mercaptobenzothiazole as accelerators, and zinc oxide as the activator of this process. 

Mercaptobenzothiazole (2-) A nitrogen- and sulfur-containing polyheterocyclic organic thiol used as vulcanization accelerator for rubber. Requires zinc oxide as an activator. Its vulcanizates have a good aging resistance. A yellowish powder with distinctive odor. Combustible. Also called MET. 

Fig. 45 Structual formulae of vulcanization accelerators (a) ethylenethiourea, NPV/C (b) 2-mercaptobenzothiazole,M(c) N-cydohexyl-2-benzothiazylsulfenamide, CZ (d) tetra-methylthiuram monosulfide, TS (e) zinc dimeftyldithiocarbamate, PZ...

One of the main applications for cyclohexylamine is in the manufacture of the vulcanization accelerator, cyclohexylbenzothiazolesulfenamide (CBS) from cyclohexylamine and 2-mercaptobenzothiazole (MBT). The dicyclohexylamine derivative is also used as a vulcanization accelerator (DCBS). 

Other important vulcanization accelerators based on 2-mercaptobenzothiazole are dibenzothiazyl disulfide (MBTS), obtained by the oxidation from MBT (e.g. with chlorine), and 2-morpholinobenzothiazylsulfenamide (MBS), (which more accurately should be referred to as N-(oxydiethylene)-2-benzothiazylsulfenamide). 

The thiophthalimide (CTP) and sulfenamide classes of retarders differ from the organic acid types by thek abiUty to retard scorch (onset of vulcanization) without significantly affecting cure rate or performance properties. Much has been pubUshed on the mechanism of CTP retardation. It functions particularly well with sulfenamide-accelerated diene polymers, typically those used in the the industry. During the initial stages of vulcanization, sulfenamides decompose to form mercaptobenzothiazole (MBT) and an amine. The MBT formed reacts with additional sulfenamide to complete the vulcanization process. If the MBT initially formed is removed as soon as it forms, vulcanization does not occur. It is the role of CTP to remove MBT as it forms. The retardation effect is linear with CTP concentration and allows for excellent control of scorch behavior. 

Rubber base adhesives can be used without cross-linking. When necessary, essentially all the cross-linking agents normally used in the vulcanization of natural rubber can be used to cross-link elastomers with internal double carbon-carbon bonds. A common system, which requires heat to work, is the combination of sulphur with accelerators (zinc stearate, mercaptobenzothiazole). The use of a sulphur-based cross-linking system with zinc dibutyldithiocarbamate and/or zinc mercaptobenzothiazole allows curing at room temperature. If the formulation is very active, a two-part adhesive is used (sulphur and accelerator are placed in two separate components of the adhesive and mixed just before application). 

Vulcanizing latex adhesives are used in the manufacture of textiles, rugs and carpets. The vulcanizing ingredients are sulphur, zinc oxide and accelerators (for example, zinc dibutyldithiocarbamate and zinc mercaptobenzothiazole to produce vulcanization at room temperature). 

Type IV reactions are due to chemicals added during manufacture of NRL, which include accelerators, antioxidants, antiozo-nants, emulsifiers, stabilizers, extenders, colorants, retarders, stiffeners, and biocides. Accelerators primarily control the rate, uniformity, and completeness of vulcanization. The most common accelerators include thiurams, carbamates, and mercaptobenzothiazoles. These chemicals are covered in detail in their specific monographs in this volume. 

Captax (Structure 15.21) is used to the extent of 1% with hevea rubber and accounts for the major part of the over 30,000 t of accelerators used annually in the United States. Other accelerators widely used include 2-mercaptobenzothiazole sulfenamide (Santocure Structure 15.22), used for the vulcanization of SBR dithiocarbamates and thiuram disulfides. Thiuram disulfide (Structure 15.23) is a member of a group called ultra-accelerators, which allow the curing of rubber at moderate temperatures and may be used in the absence of sulfur. 

The mechanism of vulcanization by XVI involves the initial formation of 2,2 -dithio-bisbenzothiazole (XVII) via cleavage of XVI to 2-mercaptobenzothiazole followed by oxidative coupling. 2,2 -Dithiobisbenzothiazole reacts with sulfur to form the accelerator... 

The second important class of accelerators are zinc dialkyldithiocarbamates (XXI). These are more active with faster vulcanization rates than the 2-mercaptobenzothiazole sulfena-mides. Also, activators are not usually needed since the zinc is incorporated into the accelerator molecule. Tetralkylthiuram disulfides (XXII) in combination with activators S... 

The chemistry of the vulcanization of rubber is complex. The reaction of rubber with sulfur is markedly expedited by substances called accelerators, of which those commonly known as mercaptobenzothiazole and tetramethyl-thiuram disulfide are examples ... 

Accelerators are chemical compounds that increase the rate of cure and improve the physical properties of the compound. As a class, they are as important as the vulcanizing agent itself. Without the accelerator, curing requires hours or even days to achieve acceptable levels. Aldehyde amines, thiocarbamates, thiuram sulfides, guanidines, and thiazoles are all classified as accelerators. By far, the most widely used are the thiazoles, represented by mercaptobenzothiazole (MBT) and benzothiazyl disulfide (MBTS). 

High-tensile-strength butyl compounds generally use FEF- or GPF-grade carbon blacks. Vulcanization systems tend to be based on thiazole accelerators such as mercaptobenzothiazole disulfide (MBTS) and thiuram accelerators such as tetramethylthiuram disulfide (TMTD). Low-tensile-sfrengfh compounds will use a clay or silica reinforcing filler in place of carbon black. 

Organic chemical accelerators were not used until 1906 (65 years after the Goodyear-Hancock development of unaccelerated vulcanization [Fig. 8]), when the effect of aniline on sulfur vulcanization was discovered by Oenslager [11]. This could have been, at least partially, in response to the development of pneumatic tires and automobiles near the turn of the century. Aniline, however, is too toxic for use in rubber products. Its less toxic reaction product with carbon disulfide, thiocarbanilide, was introduced as an accelerator in 1907. Further developments lead to guanidine accelerators [12]. Reaction products formed between carbon disulfide and aliphatic amines (dithiocarba-mates) were first used as accelerators in 1919 [13]. These were and are still the most active accelerators with respect to both crosslinking rate and extent of crosslink formation. However, most of the dithiocarbamate accelerators give little or no scorch resistance and their use is impossible in many factoryprocessing situations. The first delayed-action accelerators were introduced in 1925 with the development of 2-mercaptobenzothiazole (MET) and 2-... 

The accelerators that are most commonly used are derivatives of 2-mercaptobenzothiazole. They are very effective when used in combinations of metal oxides with fatty acids (referred to as activators). The favorite activators are zinc oxide combined with stearic acid. The combinations permit rapid vulcanizations that take minutes compared to hours when sulfur is used alone. In the process of vulcanization, 2,2 -dithiobisbenzthiazole forms initially and then reacts with sulfur to form polysulfides [273] ... 

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