Sulphur vulcanisation

A term that was used as a generic name for a new family of elastomers related to EPT. They are terpolymers and sulphur-vulcanisable. Sulphur atoms separate each of the ethylene and propylene repeating units in the backbone of the polymer chain, hence the name thiamer. Thickening Agent... 

Vulcanisation of rubber Natural rubber becomes soft at high temperature (>335 K) and brittle at low temperatures (<283 K) and shows high water absorption capacity, it Is soluble in non-polar solvents and Is non-resistant to attack by oxidising agents. To improve upon these physical properties, a process of vulcanisation is carried out. This process consists of heating a mixture of raw rubber with sulphur and an appropriate additive at a temperature range between 373 K to 415 K. On vulcanisation, sulphur forms cross links at the reactive sites of double bonds and thus the rubber gets stiffened. 

Sulphur is used in the manufacture of matches and fireworks, as a dust insecticide and for vulcanising rubber. Most of the world supply of sulphur, however, is used for the manufacture of sulphuric acid (p. 296). 

Like sulphur, selenium has been used in the vulcanisation of rubber. It is also used in photoelectric cells. 

Diselenium dichloride acts as a solvent for selenium. Similarly disulphur dichloride is a solvent for sulphur and also many other covalent compounds, such as iodine. S Clj attacks rubber in such a way that sulphur atoms are introduced into the polymer chains of the rubber, so hardening it. This product is known as vulcanised rubber. The structure of these dichlorides is given below ... 

In extensions of this work on vulcanisation, which normally involved only a few per cent of sulphur, both Goodyear and Hancock found that if rubber was heated with larger quantities of sulphur (about 50 parts per 100 parts of rubber) a hard product was obtained. This subsequently became known variously as ebonite, vulcanite and hard rubber. A patent for producing hard rubber was taken out by Nelson Goodyear in 1851. 

The vulcanisation of natural rubber, a long chain polyisoprene, with sulphur involves a similar type of cross-linking. 

The first type includes vulcanising agents, such as sulphur, selenium and sulphur monochloride, for diene rubbers formaldehyde for phenolics diisocyanates for reaction with hydrogen atoms in polyesters and polyethers and polyamines in fluoroelastomers and epoxide resins. Perhaps the most well-known cross-linking initiators are peroxides, which initiate a double-bond... 

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)...

In the case of polychloroprene the chlorine atom so deactivates both the double bond and the a-methylenic group that a sulphur-based vulcanisation system is ineffective and special techniques have to be employed. 

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... 

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. 

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). 

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. 

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. 

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 ... 

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. 

Vulcanisation can be effected by diamines, polyamines and lead compounds such as lead oxides and basic lead phosphite. The homopolymer vulcanisate is similar to butyl rubber in such characteristics as low air permeability, low resilience, excellent ozone resistance, good heat resistance and good weathering resistance. In addition the polyepichlorohydrins have good flame resistance. The copolymers have more resilience and lower brittle points but air impermeability and oil resistance are not so good. The inclusion of allyl glycidyl ether in the polymerisation recipe produces a sulphur-curable elastomer primarily of interest because of its better resistance to sour gas than conventional epichlorhydrin rubbers. 

Another approach has been adopted by the Du Pont Company with Adiprene C. This is a urethane-type polymer with unsaturated groups in the polymer. Because of the unsaturation the polymer may be vulcanised with sulphur, the standard vulcanising agent of the rubber industry. This is a clear-cut example of a product being modified to suit the processor rather than that of a processor adapting himself to meet new products. Whereas Adiprene C has poor tensile strength when unfilled, the use of carbon black leads to appreciable reinforcement (as is the case with SBR and to some extent natural rubber. 

Rubbers containing traces of vinyl groups can be cross-linked by weaker peroxide catalysts, the reaction involving a vinyl group. It is, however, unlikely that vinyl-to-vinyl linking occurs. Where there is a high vinyl content (4-5% molar) it is possible to vulcanise with sulphur. 

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. 

Hard products may also be made by vulcanising rubber (natural or synthetic) using only about two parts of sulphur per 100 parts of rubber. In these cases either the so-called high-styrene resins or phenolie rubber compounding resins are ineorporated into the formulation. These compounds are processed using the methods of rubber technology but, like those of ebonite, the produets are more akin to plastics than to rubbers. Examples of the usage of these materials are to be found in battery boxes, shoe heels and ear washer brushes. 

As with c -polyisoprene, the gutta molecule may be hydrogenated, hydro-chlorinated and vulcanised with sulphur. Ozone will cause rapid degradation. It is also seriously affected by both air (oxygen) and light and is therefore stored under water. Antioxidants such as those used in natural rubber retard oxidative deterioration. If the material is subjected to heat and mechanical working when dry, there is additional deterioration so that it is important to maintain a minimum moisture content of 1%. (It is not usual to vulcanise the polymer.)... 

Copper and silver tarnish readily in sulphide atmospheres, and copper in contact with sulphur-vulcanised rubber will sometimes react with the sulphur, devulcanising it in the process. The growth of conducting sulphide whiskers on silver is noteworthy as these whiskers may give rise to short circuits across silver-plated contacts. Ammonia has little effect on most metals, but traces will tarnish many copper alloys and cause stress-corrosion cracking of certain stressed brasses. 

Rubber vulcanised-determination of adhesion to, and corrosion of, metals Corrosion of metals and alloys determination of dezincification resistance of brass Sulphur dioxide test with general condensation of moisture... 

A vulcanising agent particularly for silicone rubber and fluoroelastomers it has been used as a non-sulphur vulcanising agent for natural rubber. It is also a catalyst in emulsion polymerisation. Beta Rays... 

Chlorinated butyl rubber. Chlorination or bromination of butyl rubber overcomes the difficulty of vulcanising butyl rubber in mixtures with more highly unsaturated substances due to the preferential absorption of the sulphur by the more highly unsaturated component. Chlorohydrin Rubbers... 

A non-sulphur vulcanising agent for natural rubber and some types of synthetic rubber. One trade name is Dicup. /V,A/-Dicyclohexyl-2-benzothiazole sulphenamide... 

Also known as vulcanite and (mainly in the USA) hard rubber . The hard, horn-like product obtained when natural rubber and some synthetic rubbers such as nitrile (NBR) are vulcanised with a high proportion of sulphur or organic nonsulphur vulcanising agent. Butyl rubber and polysulphide rubber do not form ebonites. Ebullioscopy... 

A term apphed to vulcanisation systems in which sulphur or a sulphur donor is used very efficiently for crosslinking the rubber. EV systems produce vulcanisates with crosslinks that are mainly monosulphidic, which are thermally and mechanically stable. 

The amount of sulphur which has not combined with the rubber during the vulcanisation reaction. The determination of the free sulphur content is widely used in assessing the state of cure of a rubber product. The addition of the free sulphur curve to the modulus and tensile/time curves of the particular compound enables the state of cure of the vulcanised product to be estimated. 

Charles Goodyear (1800-1860) is acknowledged by some as the discoverer of vulcanisation by the heating of a rubber-sulphur mixture. Many others including Ludersdorf and Hancock were also researching means to impart long term stability to rubber at around the same time. Goodyear did not patent his discovery until 1844, whereas Hancock obtained a British Patent for a process of vulcanisation in 1843. 

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