Accelerators of vulcanization

Ostromyslensky [90] observed that rubber could be vulcanized in the absence of sulphur or its compounds if small quantities of aromatic nitro compounds, e.g. nitrobenzene, dinitro- or sym- trinitro-benzene, tetranitronaphthalene plus certain oxidizing compounds, such as benzoyl peroxide, were added to it. The resultant vulcanized rubber had mechanical properties not inferior to those of a product vulcanized by sulphur alone, and when free from accelerators it exhibited a greater resistance to ageing. Some metal oxides, such as PbO, CaO, BaO, promoted vulcanization by nitro compounds. Urea played the role of an accelerator of vulcanization. These observations have been confirmed in more recent studies by other workers. 

Furfural is an insecticide, fungicide, and germicide. It has multiple industrial uses, including production of durite, solvent refining of petroleum oils, acceleration of vulcanization, and a component of rubber... 

Mercaptobenzothiazole is a rubber chemical, an accelerant of vulcanization. It is contained in the mercapto mix . The most frequent occupational categories are metal industry, homemakers, health services and laboratories, building industries, and shoemakers. It is also used as a corrosion inhibitor in cutting fluids or in releasing fluids used in the pottery industry. 

In comparison with the developments in new rubbers that have occurred this century, developments concerned with the chemistry of the reactions of the already-formed rubbery polymers have been less immediately spectacular. It has already been pointed out that for about ISO years sulphur has been the dominant vulcanizing agent, almost exclusively used with diene rubbers. It must however be stressed that the efficiency with which the sulphur is used and the quality of the vulcanizates is today vastly superior. In part this is due to systematic semi-empirical studies which led to the development of a wide range of accelerators of vulcanization. It is also in part due to the excellent scientific studies undertaken by many chemists throughout the world but particularly by the Malaysian Rubber Producers Research Association (and its forerunners the Natural Rubber Producers Research Association and the British Rubber Producers Research Association). As a consequence of this work the mechanism of vulcanization and its control, at least in the major diene rubbers, is reasonably well understood. 

Copolymerization can be carried out with styrene, acetonitrile, vinyl chloride, methyl acrylate, vinylpyridines, 2-vinylfurans, and so forth. The addition of 2-substituted thiazoles to different dienes or mixtures of dienes with other vinyl compounds often increases the rate of polymeriza tion and improves the tensile strength and the rate of cure of the final polymers. This allows vulcanization at lower temperature, or with reduced amounts of accelerators and vulcanizing agents. 

Piperidines. A significant use of piperidine (18) has been ia the manufacture of vulcanization accelerators, eg, thiuram disulfide [120-54-7] (115) (see Rubber chemicals). Mepiquat dichloride [24307-26-4] the dimethyl quaternary salt of (18), is used as a plant growth regulator for cotton (qv). Piperidine is used to make vasodilators such as dipyridamole [58-32-2] (116) and minoxidil [38304-91-5] (117), and diuretics such as etozoline [73-09-6] (118). 

Other Accelerators. Amine isophthalate and thiazolidine thione, which are used as alternatives to thioureas for cross-linking polychloroprene (Neoprene) and other chlorine-containing polymers, are also used as accelerators. A few free amines are used as accelerators of sulfur vulcanization these have high molecular weight to minimize volatility and workplace exposure. Several amines and amine salts are used to speed up the dimercapto thiadiazole cure of chlorinated polyethylene and polyacrylates. Phosphonium salts are used as accelerators for the bisphenol cure of fluorocarbon mbbers. 

Accelerators. During sulfur vulcanization of rubber, accelerators serve to control time to onset of vulcanization, rate of vulcanization, and number and type of sulfur cross-links that form. These factors in turn play a significant role in determining the performance properties of the vulcanizate. 

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). 

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. The mbber industry consumes finely ground metallic selenium and Selenac (selenium diethyl dithiocarbamate, R. T. Vanderbilt). Both are used with natural mbber and styrene—butadiene mbber (SBR) to increase the rate of vulcanization and improve the aging and mechanical properties of sulfudess and low sulfur stocks. Selenac is also used as an accelerator in butyl mbber and as an activator for other types of accelerators, eg, thiazoles (see Rubber chemicals). Selenium compounds are useflil as antioxidants (qv), uv stabilizers, (qv), bonding agents, carbon black activators, and polymerization additives. Selenac improves the adhesion of polyester fibers to mbber. 

Rubber Chemicals. Sodium nitrite is an important raw material in the manufacture of mbber processing chemicals. Accelerators, retarders, antioxidants (qv), and antiozonants (qv) are the types of compounds made using sodium nitrite. Accelerators, eg, thiuram [137-26-8J, greatly increase the rate of vulcaniza tion and lead to marked improvement in mbber quaUty. Retarders, on the other hand (eg, /V-nitrosodiphenylamine [156-10-5]) delay the onset of vulcanization but do not inhibit the subsequent process rate. Antioxidants and antiozonants, sometimes referred to as antidegradants, serve to slow the rate of oxidation by acting as chain stoppers, transfer agents, and peroxide decomposers. A commonly used antioxidant is A/,AT-disubstituted Nphenylenediamine which can employ sodium nitrite in its manufacture (see Rubber chemicals). 

A new process to develop interface vulcanization is grafting of selective accelerators onto a polymer chain, which in the subsequent process of vulcanization acts as an effective cure accelerator for the second polymer component in the blend. Beniska et al. [6] prepared SERFS blends where the polystyrene phase was grafted with the accelerator for curing SBR. Improved hardness, tensile strength, and abrasion resistance were obtained. Blends containing modified polystyrene and rw-1,4-polybutadiene showed similar characteristics as SBS triblock copolymers. 

The accelerated sulfur vulcanization of general-purpose diene rubbers (e.g., NR, styrene-butadiene rubber [SBR], and butadiene rubber [BR]) by sulfur in the presence of organic accelerators and other rubbers, which are vulcanized by closely related technology (e.g., ethylene-propylene-diene monomer [EPDM] mbber, butyl rubber [HR], halobutyl mbber [XIIR], nitrile rubber [NBR]) comprises more than 90% of all vulcanizations. 

Table 14.2 provides comparisons of the different classes of accelerators based on their rates of vulcanization. The secondary accelerators are seldom used alone, but are generally found in combination with primary accelerators to gain faster cures. 

FIGURE 14.3 Development of accelerated sulfur vulcanization of natural rubber (NR). (From A.Y. Coran, Chem. Tech., 23, 106, 1983.)... 

Most accelerators used in the accelerated sulfur vulcanization of other high diene rubbers are not applicable to the metal oxide vulcanization of CR. An exception is the use of so-called mixed-curing system for CR, in which metal oxide and accelerated sulfur vulcanization are combined. Along with the metal oxides, TMTD, DOTG, and sulfur are used. This is a good method to obtain high resilience and dimensional stability. 

The silver white, shiny, metal-like semiconductor is considered a semimetal. The atomic weight is greater than that of the following neighbor (iodine), because tellurium isotopes are neutron-rich (compare Ar/K). Its main use is in alloys, as the addition of small amounts considerably improves properties such as hardness and corrosion resistance. New applications of tellurium include optoelectronics (lasers), electrical resistors, thermoelectric elements (a current gives rise to a temperature gradient), photocopier drums, infrared cameras, and solar cells. Tellurium accelerates the vulcanization of rubber. 

Standard method of test for accelerated ozone cracking of vulcanized rubber [Test method D-1149-64 (reapproved 1970)], pp. 554-560. In 1972 Annual Book of ASTM Standards. Part 28. Rubber Carbon Black Gaskets. Philadelphia American Society for Testing and Materials, 1972. 

Uses. Rubber accelerator and vulcanizer, as an activator of thiazole accelerators, and as a plasticizer in neoprene pharmaceutical grade used in treatment of alcoholism... 

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