Vulcanisation accelerators

Rubber chemicals are materials that are added ia minor amounts to mbber formulations in order to improve their properties and make them commercially useful. Raw mbber polymer has very limited practical appHcations because of tackiness, flow, and other undesirable features. Rubber chemicals are added to assist processing, promote cross-linking, and provide longevity to the part in service. Vulcanising adjacent polymer chains together by cross-links prevents flow, increases strength, and provides recovery from deformation. The most widely used method of cross-linking polymer chains is by heating with elemental sulfur (vulcanisation). Accelerators speed up the reaction of sulfur with polymer to improve the economics of manufacture and prevent degradation that would otherwise occur upon prolonged heating. Peptizers and process aids assist flow during the mixing and shaping operations. Antidegradants protect the part in service from heat, oxygen, ozone, and repeated flexing. Other mbber chemicals function as blowing agents, adhesion promoters, and activators or retarders which modify the onset of cross-linking.  [c.219]

It is now common practice to use sulphur in conjunction with several other additives. First amongst them are vulcanisation accelerators, of which there are many types. In the absence of an accelerator about 10 parts of sulphur is required, the vulcanisation time may be a matter of hours and much of the sulphur is  [c.282]

A. B. Sullivan, C. J. Harm, and G. H. Kuhls, "Vulcanisation Chemistry— Fate of Elemental Sulfur and Accelerator during Scorch Delay as Studied by Modem HPLC", Paper No. 9, presented at the MGS Tubber Division Meeting Toronto, Canada, May 21 —24, 1991, American Chemical Society, Washington, D.C., 1991.  [c.229]

This regulation is only concerned with volatile nitrosamines in the workplace air. A principal problem in enforcement is in the detection method. Only certain analytical laboratories are certified and reproducibiUty is difficult to achieve. Epidemiological studies have shown volatile nitrosamines to be carcinogenic in animals (45). Volatile nitrosamines are formed when secondary amine compounds break down and are nitrosated. In mbber this occurs primarily during the vulcanisation stage, where accelerators, which are predominandy secondary amine compounds, decompose, forming lower molecular weight compounds, and are nitrosated either from oxides of nitrogen in the air or from nitrate—nitrite salts in the vulcanisa tion process. Other sources of these secondary amines are as contaminants in compounding ingredients and as trace amounts in emulsion SBR, from the residue of certain shortstopping chemicals used in its manufacture.  [c.501]

Figure 11.15. Typical chemical groupings in a sulphur-vulcanised natural rubber network, (a) Monosulphide cross-link (b) disulphide cross-link (c) polysulphide cross-link (j = 3-6) (d) parallel vicinal cross-link (n = 1-6) attached to adjacent main-chain atoms and which have the same influence as a single cross-link (e) cross-links attached to common or adjacent carbon atom (f) intra-chain cyclic monosulphide (g) intra-chain cyclic disulphide (h) pendent sulphide group terminated by moiety X derived from accelerator (i) conjugated diene (j) conjugated triene (k) extra-network material (1) carbon-carbon cross-links (probably absent) Figure 11.15. Typical chemical groupings in a sulphur-vulcanised natural rubber network, (a) Monosulphide cross-link (b) disulphide cross-link (c) polysulphide cross-link (j = 3-6) (d) parallel vicinal cross-link (n = 1-6) attached to adjacent main-chain atoms and which have the same influence as a single cross-link (e) cross-links attached to common or adjacent carbon atom (f) intra-chain cyclic monosulphide (g) intra-chain cyclic disulphide (h) pendent sulphide group terminated by moiety X derived from accelerator (i) conjugated diene (j) conjugated triene (k) extra-network material (1) carbon-carbon cross-links (probably absent)
Accelerated sulphur systems also require the use of an activator comprising a metal oxide, usually zinc oxide, and a fatty acid, commonly stearic acid. For some purposes, for example where a high degree of transparency is required, the activator may be a fatty acid salt such as zinc stearate. Thus a basic curing system has four components sulphur vulcanising agent, accelerator (sometimes combinations of accelerators), metal oxide and fatty acid. In addition, in order to improve the resistance to scorching, a prevulcanisation inhibitor such as A -cyclohexylthiophthalimide may be incorporated without adverse effects on either cure rate or physical properties.  [c.283]

Natural rubber is generally vulcanised using accelerated sulphur systems although several alternatives have been used. At the present time there is some limited use of the cold cure process using sulphur chloride in the manufacture of rubber proofings. This process was first discovered by Alexander Parkes in 1846, which was some years before his discovery of Parkesine (see Chapter 1) and this is sometimes known as the Parkes Process. (Another Parkes Process is that of separating silver from lead ) Peroxides are also very occasionally used, particularly where freedom from staining by metals such as copper is important. Nitroso compounds and their derivatives, including the so-called urethane cross-linking systems, may also be employed. The latter in particular give a uniform state of cure to thick sections as well as an improved level of heat resistance compared to conventional sulphur-cured systems.  [c.288]

The rubbers may be vulcanised by conventional accelerated sulphur systems and also by peroxides. The vulcanisates are widely used in petrol hose and seal applications. Two limiting factors of the materials as rubbers are the tendency to harden in the presence of sulphur-bearing oils, particularly at elevated temperatures (presumably due to a form of vulcanisation), and the rather limited heat resistance. The latter may be improved somewhat by Judicious compounding to give vulcanisates that may be used up to 150°C. When for the above reasons nitrile rubbers are unsatisfactory it may be necessary to consider acrylic rubbers (Chapter 15), epichlorohydrin rubbers (Chapter 19) and in more extreme conditions fluororubbers (Chapter 13).  [c.294]

The close structural similarities between polychloroprene and the natural rubber molecule will be noted. However, whilst the methyl group activates the double bond in the polyisoprene molecule the chlorine atom has the opposite effect in polychloroprene. Thus the polymer is less liable to oxygen and ozone attack. At the same time the a-methylene groups are also deactivated so that accelerated sulphur vulcanisation is not a feasible proposition and alternative curing systems, often involving the pendant vinyl groups arising from 1,2-polymerisation modes, are necessary.  [c.295]

Because of their saturated structure the raw polymers could not be vulcanised using accelerated sulphur systems and the less convenient peroxide curing systems were required. Binary copolymers of this type are designated as EPM rubbers. Such ethylene-propylene rubbers were accepted reluctantly by the rubber industry. Such reluctance was largely due to the real risks of mixing EPM stocks with those of diene rubbers and thus causing considerable problems because of the different curing systems. In addition peroxide curing systems are much more liable to premature vulcanisation (scorch) than accelerated sulphur systems and this can lead to high scrap rates.  [c.299]

In consequence ethylene-propylene rubbers were introduced with a small amount (3-8%) of a third, diene, monomer which provided a cross-link site for accelerated sulphur vulcanisation. Such ethylene-propylene-diene monomer ternary copolymers are designated as EPDM rubbers.  [c.300]

Whilst polyisobutene is a non-rubbery polymer exhibiting high cold flow (see Section 11.3), the copolymer containing about 2% isoprene can be vulcanised with a powerful accelerated sulphur system to give moderately rubbery polymers. The copolymers were first developed in 1940 by Esso and are known as butyl rubbers and designated as HR. As they are almost saturated they have many properties broadly similar to the EPDM terpolymers. They do, however, have two properties that should be particularly noted  [c.302]

Such low Tg values are indicative of a very rubbery polymer. Furthermore, careful control of the polymerisation enables highly stereoregular polymers to be produced, i.e. polymers that are essentially cis or essentially trans. This gives the possibility that the rubber may crystallise and thus show that great virtue of natural rubber-self-reinforcement, brought about by crystallisation on extension. In practice the of the c -polymer at 1°C is too low to be of any consequence at room temperature but the trans-polymer with a of +15°C is closer to that of natural rubber and the polymer exhibits high green strength, good building tack, and generally good processability. The polymer may be vulcanised using conventional accelerated sulphur systems to give vulcanisates with good strength and abrasion resistance.  [c.305]

Whilst the blend has a good green strength it is usual to vulcanise the rubber by an accelerated sulphur system using a higher than usual accelerator sulphur ratio.  [c.306]

The ebonite compound before cure is a rather soft plastic mass which may be extruded, calendered and moulded on the simple equipment of the type that has been in use in the rubber industry for the last century. In the case of extruded and calendered products vulcanisation is carried out in an air or steam pan. There has been a progressive reduction in the cure times for ebonite mixes over the years from 4-5 hours down to 7-8 minutes. This has been brought about by considerable dilution of the reactive rubber and sulphur by inert fillers, by use of accelerators and an increase in cure temperatures up to 170-180°C. The valuable effect of ebonite dust in reducing the exotherm is shown graphically in Figure 30.3.  [c.861]

In general, the reaction mechanism of elastomeric polymers with vulcanisation reagents is slow. Therefore, it is natural to add special accelerators to rubber compounds to speed the reaction. Accelerators are usually organic compounds such as amines, aldehyde-amines, thiazoles, thiurams or dithio-carbamates, either on their own or in various combinations.  [c.939]

In more recent years, lining compounds have been developed that vulcanise at ambient temperatures. Most polymers can be used for such compounds, although most materials are based on natural rubber, acrylonitrile-butadiene rubber and polychloroprene. These compounds contain accelerators which usually give rise to a material which has a delay in the onset of vulcanisation with a subsequent rapid rise in cross-link formation to give full vulcanisation in 6 to 8 weeks. Such materials, unless to be used within a few days of manufacture, are refrigerated to arrest the sel f-vulcanisation.  [c.940]

Large, non-pressure vessels, which are usually lined on site in their final location, are normally vulcanised at 100°C using steam at ambient pressure or boiling water. Using such techniques, vulcanisation times will be extended and it is often necessary to shield the outside of the unit to minimise heat loss. Self vulcanising or chemical cure linings usually take up to eight weeks to fully vulcanise, but this process may be accelerated by injecting steam or hot air into the unit.  [c.947]

Uses. Methylglutaronitnle is readily hydrogenated to give 2-methyl-1,5-pentanediamine (DYTEK A, MPMD), used as a comonomer in polyamide fibers and resins, as a curing agent for epoxy coatings, and as its isocyanates in specialty urethanes. A co-product of the DYTEK A process is 3-methylpiperidine which can be used to produce vulcanisation accelerators for mbber curing or can be converted to 3-picoline, an intermediate used to make niacin and niacinamide. 3-Picoline may also be made by direct conversion of MGN or alternatively from DYTEK A.  [c.226]

The production of hexamethylenetetramine consumes about 6% of the U.S. formaldehyde supply (115). Its principal use is as a thermosetting catalyst for phenoHc resins. Other significant uses are for the manufacture of RDX (cyclonite) high explosives, in mol ding compounds, and for mbber vulcanisation accelerators. Some hexamethylenetetramine is made as an unisolated intermediate in the manufacture of nitfilotriacetic acid.  [c.497]

Peroxides decompose when heated to produce active free radicals which ia turn react with the mbber to produce cross-links. The rate of peroxide cure is coatroUed by temperature and selection of the specific peroxide, based on half-hfe considerations (see Initiators, free-RADICAL Peroxy compounds, organic). Although some chemicals, such as bismaleimides, triaHyl isocyanurate, and diaHyl phthalate, act as coagents ia peroxide cures, they are aot vulcanisation accelerators. lastead they act to improve cross-link efftcieacy (cross-linking vs scissioa), but aot rate of cross-link formatioa.  [c.236]

As in dry rubber compounding, neoprenes require sulfur and an accelerator for vulcanisation, but sine oxide can also be used to accelerate the cure and to function as an acid acceptor. Unlike dry neoprene compounding, no magnesium salt is used because of its destabilising effect on the latex. Organic accelerators are used to improve the physical properties of neoprene latex films but their effect is not generally as great as when used with other polymers. The most effective accelerators in neoprene latex are thiocarbanilide (yy -diphenylthiourea) used either alone or in combination with DPG, or combinations ofTETD and SBUD.  [c.256]

The selection of the type of mbber to be used for a specific product depends on the technical requirements of the product, the properties attainable by compounding, and economic factors. By far the largest volume of mbber is used in the manufacture of pneumatic tires for passenger cars, tmcks, aircraft, and farm machinery. The polymers used for these appHcations are natural mbber, SBR, polybutadiene, polyisoprene, halogenated butyl, and some EPDM mbber. Butyl, EPDM and natural mbber are also used for innertubes. The essential requirement for a typical passenger tire include resistance to abrasion, cutting, and chipping reduced rolling resistance resistance to cracking and crack growth adequate flexibiUty at the lowest temperature encountered in service a sufficientiy high coefficient of friction between the tread and road to minimise slipping and skidding adequate adhesion of the tread to the carcass sufficient stabiUty of the material with time so that excessive deterioration does not occur in the normal life of the tire and a moderate hysteresis so that excessive temperatures do not develop in service. When compounded with 70 parts of N339 (HAE-HS) or N234 (ISAE-HS) carbon black, vulcanised with a suitable sulfenamide accelerator system, and adequately stabilized with antiozonants, the various grades of SBR, polybutadiene, and natural mbber provide these requirements. Generally, SBR and 25—35 wt % polybutadiene are used in passenger tires.  [c.257]

In producing latex products, the chemicals required for vulcanisation, stiffening, coloring, antioxidant protection, or other purposes are added as solutions, emulsions, or fine dispersions to the latex before forming the product. Because no heat is generated during this mixing, it is possible to use ultrafast accelerators that would cause scorch problems in dry mbber compounds. Indeed, it is desirable to use ultra-accelerators in latex products because it allows lower vulcanization temperatures to be used, which reduces the potential of oxidative degradation in these thin-waHed products. The most widely used accelerators for latex products are the dithiocarbamates, particularly zinc diethyldithiocarbamate (ZDEC) and zinc dibutyl dithiocarbamate (ZDBC or ZBUD). Thiazoles such as zinc mercaptobenzothiazole (ZMBT) are often used as secondary accelerators with dithiocarbamates, and thiurams such as TM l L) are sometimes used in place of dithiocarbamates, especially where low sulfur systems are required. Another effect of wet mixing of chemicals that distinguishes latex products from dry mbber products is that mastication does not occur and hence the original high molecular weight of the polymer is retained.  [c.274]

Accelerators are chemical compounds that iacrease the rate of cure and improve the physical properties of the compound. As a class, they are as important as the vulcanising agent itself. Without the accelerator, curing requires hours or even days to achieve acceptable levels. Aldehyde amines, thiocarbamates, thiuram sulfides, guanidines, and thiasoles are aU. classified as accelerators. By far, the most widely used are the thiasoles, represented by mercaptobensothiasole (MBT) and bensothiasyl disulfide (MBTS).  [c.499]

The earliest study describing vulcanised polymers of esters of acryUc acid was carried out in Germany by Rohm (2) before World War I. The first commercial acryUc elastomers were produced in the United States in the 1940s (3—5). They were homopolymers and copolymers of ethyl acrylate and other alkyl acrylates, with a preference for poly(ethyl acrylate) [9003-32-17, due to its superior balance of properties. The main drawback of these products was the vulcanisation. The fully saturated chemical stmcture of the polymeric backbone in fact is inactive toward the classical accelerators and curing systems. As a consequence they requited the use of aggressive and not versatile compounds such as strong bases, eg, sodium metasiUcate pentahydrate. To overcome this limitation, monomers containing a reactive moiety were incorporated in the polymer backbone by copolymerisation with the usual alkyl acrylates.  [c.474]

Except for standard ebonites and the speciality ambient vulcanising or chemical cure linings, most compounds have the capability of vulcanising both in an autoclave at 150° C or more and in ambient pressure steam or hot water at 100°C. Ebonites are of two types, the autoclave version for factory applications and a highly accelerated version for vulcanising on site at 100°C. Site-lining ebonites with their high acceleration levels are limited with respect to operational temperatures. At temperatures above 70°C, they tend to post vulcanise leading to increased hardness and subsequent brittleness. Leaching of excess, unreacted accelerator can also cause problems, especially in electroplating operations, where contamination may occur with the deposited elemental metal.  [c.939]

In the past chemical cure linings have been employed on a wide scale. These linings, usually based on natural rubber or acrylonitrile-butadiene rubber consist of a standard lining compound with a chemical activator such as dibenzylamine incorporated in the formulation. Prior to the application of the lining to the substrate, the individual sheets of rubber are dipped or brush coated with carbon disulphide or a solution of a xanthogen disulphide in a solvent. The carbon disulphide or xanthogen disulphide permeates the rubber and combines with the dibenzylamine to form an ultra-fast dithiocar-bamate accelerator in situ, and thus the rubber rapidly vulcanises at ambient temperature.  [c.940]

See pages that mention the term Vulcanisation accelerators : [c.243]    [c.283]    [c.306]    [c.861]   
Plastics materials (1999) -- [ c.282 ]