Ebonites vulcanization

Ebonite media are manufacmred from partially vulcanized rubber, which is crushed, pressed and vulcanized. These media are resistant to acids, salt solutions and alkalies. They may be used for filtration at temperamres ranging from -10 to + 110 C. 

Initially, vulcanization was accomplished by heating elemental sulfur at a concentration of 8 parts per 100 parts of rubber (phr) for 5 h at 140°C. The addition of zinc oxide reduced the time to 3 h. Accelerator in concentrations as low as 0.5 phr have since reduced time to 1-3 min. As a result, elastomer vulcanization by sulfur without accelerator is no longer of commercial significance. An exception is the use of about 30 or more phr of sulfur, with httle or no accelerator, to produce molded products of hard mbber called ebonite. 

Which has the higher cross-linked density (a) ebonite or (b) soft vulcanized rubber ... 

Accelerators are second in importance only to sulphur. Their function is to accelerate the normally slow rubber-sulphur reaction, increase the rate of vulcanization, and increase productivity. Accelerators are classified into two main classes by types, namely organic and inorganic. The inorganic accelerators such as lime, litharge and other lead compounds and magnesia were employed extensively before the introduction of organic accelerators. They are still used mainly to produce hard rubber or ebonite products. Litharge is used in rubberized fabrics, insulated wires and cables and shoe compounds as well as chemical resistant rubber products... 

However, for construction purposes, solid ebonites were chosen. As is known from rubber chemistry, solid ebonite, commonly known as hard rubber, is a polymer material with sulfur content used for vulcanization. Ebonite, like elastomeric or flexible rubber, is made from a combination of sulfur with polydienes (unsaturated rubbers containing double bonds). The sulfur and polydienes are combined with some auxiliary additives and heated to produce vulcanization. Typical mass ratios of sulfur to rubber are 2 100 for elastomeric rubber and 40 100 for hard rubber. Due to the large degree of sulfide cross linking formed in the vulcanization process, solid ebonite is a hard, non-flexible, plastic-like material possessed of... 

The elastic modulus of ebonite in its high elastic stage (i.e., at high temperature) is some ten times that of a pure gum soft vulcanized... 

The coefficient of vulcanization is usually defined as the number of units of weight of sulphur combined with 100 units by weight of unsaturated hydrocarbon. Ebonites prepared from natural rubber are... 

Type of rubber Theoretical coefficient of vulcanization at ebonite stage Coefficient after correcting for rubber constituents... 

Basic operations such as mastication, mixing, calendering and extrusion are similar for ebonite and soft rubber. The problems, especially in the compounding and vulcanization process which arise in the manufacture of ebonites, are quite different from those with soft rubbers and different tests are used for control of manufacture and for the assessment of the quality of the product. The outstanding differences between ebonites and soft rubbers are detailed below. 

Because of the dark colour of ebonites when vulcanized the possibilities of colouring is very limited and this is so with natural rubber ebonites. 

The vulcanization times for ebonites are very long when compared with those of soft rubber. 

Since all ebonite compounds contain high proportions of sulphur and usually large quantities of ebonite dust, the consistency of the compound during mixing and calendering prior to vulcanization is never that of a pure gum stock. 

If we take a rubber-sulphur ratio of 68 32 in an ebonite compound and then cure it at 155°C, the vulcanization coefficient increases practically to a constant value of 47 after about five hours and the uncombined sulphur decreases during the first four hours. This state may be called a full cure in the chemical sense. There is also a reduction in volume of about 6%. It is known that after the combination of the first few percent of sulphur, the material passes through a leathery stage with low strength and poor resistance to oxidation and with time passing a true ebonite is formed with increased impact strength. 

The effects of reducing the rubber-sulphur ratio from 68 32 to 72 28 are very significant. The uncombined sulphur can be reduced, the vulcanization coefficient being about 40. Tensile and cross braking strength are a little lower. Impact strength is much higher. An ebonite... 

Vulcanized rubber can be carved, or, more importantly, molded into almost any shape. The uses for this versatile, water-proof, flexible material increased with time. These included household and decorative objects, apparel, personal items, insulation, and equipment and materials for use in medicine, the military, and industry. Vulcanized rubber also goes under the commercial names vulcanite and ebonite. 

Both linear and branched polymers are thermoplastics. However, cross-linked three-dimensional, or network-polymers are thermoset polymers. The cross-linked density may vary from the low cross-linked density in vulcanized rubber to high cross-linked density observed in ebonite (hard rubber highly cross-linked natural rubber). 

If the sulfur atoms are not part of the polymeric backbone, the polymers are not included in this class. For example, in the vulcanization process of the polymers with unsaturated carbon chain backbone, -S-S- bonds are introduced in the polymer, but the resulting product is not classified as polymer with C-S bonds in the backbone, although in hard rubber (ebonite), for example, the content of sulfur can be as high as 32%. 

Hard rubber, vulcanite, or ebonite, is a hard, tough variety of vulcanized rubber, susceptible of a good polish, and a non-conductor of electricity. It contains 20 to 35 per cent, of 8 (the ordinary vulcanized rubber contains 7 to 10 per cent.). 

The properties of the vulcanized product vary with the amount of sulphur used. In the preparation of rubber for most purposes from 5 to 7 per cent of the weight of the rubber is used. In making ebonite, or hard rubber, up to 30 per cent is employed. It is probable that in vulcanization a part of the sulphur unites chemically with the rubber and a part is held by physical adsorption. The exact nature of the process is not yet understood. 

Poisson s ratio is a measure of the reduction in the cross section accompanying stretching and is the ratio of the transverse strain (a contraction for tensile stress) to longitudinal strain (elongation). Poisson s ratio for many of the more brittle plastics such as polystyrene, the acrylics, and the thermoset materials is about 0.3 for the more flexible plasticized materials, such as cellulose acetate, the value is somewhat higher, about 0.45. Poisson s ratio for rubber is 0.5 (characteristic of a liquid) it decrease to 0.4 for vulcanized rubber and to about 0.3 for ebonite. Poisson s ratio varies not only with the nature of the material but also with the magnitude of the strain for a given material. All values cited here are for zero strain. 

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