Ebonite is a brand name for a material generically known ashard rubber orvulcanite, obtained viavulcanizingnatural rubber for prolonged periods. Ebonite may contain from 25% to 80%sulfur andlinseed oil.[1][2] Its name comes from its intended use as an artificial substitute forebony wood. The material has also been calledvulcanite,[3] although that name formally refers to the mineralvulcanite.
Charles Goodyear's brother, Nelson Goodyear, experimented with the chemistry of ebonite composites. In 1851, he used zinc oxide as a filler.[4]Hugh Silver was responsible for giving it its name.[5]
The sulfur percentage and the applied temperatures and duration of vulcanizing are the main variables that determine the technical properties of the hard rubberpolysulfideelastomer. The occurring reaction is basically addition of sulfur at the double bonds, forming intramolecular ring structures, so a large portion of the sulfur is highlycross-linked in the form of intramolecular addition. As a result of having a maximum sulfur content up to 40%, it may be used to resist swelling and minimize dielectric loss. The strongest mechanical properties and greatest heat resistance is obtained with sulfur contents around 35% while the highest impact strength can be obtained with a lower sulfur content of 30%. The rigidity of hard rubber atroom temperature is attributed to thevan der Waals forces between the intramolecular sulfur atoms. Raising the temperature gradually increases the molecular vibrations that overcome the van der Waals forces making it elastic. Hard rubber has a content mixture dependent density around 1.1 to 1.2. When reheated hard rubber exhibits shape-memory effect and can be fairly easily reshaped within certain limits. Depending on the sulfur percentage hard rubber has a thermoplastic transition or softening temperature of 70 to 80 °C (158 to 176 °F).
The material is brittle, which produces problems in its use in battery cases for example, where the integrity of the case is vital to prevent leakage ofsulfuric acid. It has now been generally replaced bycarbon black-filledpolypropylene.
Under the influence of the ultraviolet portion of daylight, hard rubber oxidizes. Subsequent exposure to moisture bonds water with free sulfur on the surface, creating sulfates andsulfuric acid at the surface that are veryhygroscopic. The sulfates condense water from the air, forming a hydrophilic film with favorablewettability characteristics on the surface.[6] These aging processes will gradually discolor the surface grayish green to brown and cause rapid deterioration of electric surface resistivity.
Contaminated ebonite was problematic when it was used for electronics. During manufacturing the ebonite was rolled between metal foil sheets, which were peeled off, leaving traces of metal behind. For electronic use the surface was ground to remove these metal particles.[7]
Hard rubber was used in early 20th centurybowling balls; however, it was phased out in favor of other materials (the Ebonite name remains as a trade name for one of the major manufacturers of polymer balls).[8] It has been used in electric plugs,tobacco pipe mouthpieces (in competition withLucite), fishing reels,hockey pucks,fountain pen[9][10] bodies and nib feeds,saxophone andclarinetmouthpieces, as well as complete humidity-stable clarinets.[11][better source needed] Hard rubber is often seen as the wheel material incasters. It is also commonly used inphysics classrooms to demonstratestatic electricity, because it is at or near the negative end of thetriboelectric series.[citation needed]
Hard rubber was used in the cases of automobilebatteries for years, thus establishing black as their traditional colour even long after stronger modern plastics likepolypropylene were substituted. It was used for decades in hair combs made by Ace, now part ofNewell Rubbermaid, although the current models are known to be produced solely with plastics.[12]
Ebonite is used as an anticorrosive lining for various (mainly storage) vessels that contain dilutedhydrochloric acid. It forms bubbles when storinghydrofluoric acid at temperatures above room temperature, or for prolonged durations.[13]
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