Ebonite

Ebonite is a brand name for a material generically known as hard rubber or vulcanite, obtained via vulcanizing natural rubber for prolonged periods. Ebonite may contain from 25% to 80% sulfur and linseed oil. Its name comes from its intended use as an artificial substitute for ebony wood. The material has also been called vulcanite, although that name formally refers to the mineral vulcanite.

Charles Goodyear's brother, Nelson Goodyear, experimented with the chemistry of ebonite composites. In 1851, he used zinc oxide as a filler. Hugh Silver was responsible for giving it its name.

Composition

Ebonite, like all rubber mixtures, is composed of virgin rubber, sulfur, accelerators, antioxidants, plasticizers, ebonite powder, inert fillers and, sometimes, graphite. Chemically it can only be obtained with the 1/3 coefficient of neutral (sublimated) sulfur.

Logically, the quantities and proportions are therefore variable, depending on the use and characteristics of the specification. However, its molecular structure, due to sulfur, gives it a crystalline state, which is its great advantage over other elastomers for a wide range of applications.

Properties

The sulfur percentage and the applied temperatures and duration of vulcanizing are the main variables that determine the technical properties of the hard rubber polysulfide elastomer. The occurring reaction is basically addition of sulfur at the double bonds, forming intramolecular ring structures, so a large portion of the sulfur is highly cross-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 at room temperature is attributed to the van 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 of sulfuric acid. It has now been generally replaced by carbon black-filled polypropylene.

Compared to phenolic resins (phenolic resins, Bakelite), ebonite is somewhat softer and offers very high impact strength. It is considered to be very long-term stable and extremely resistant to many chemicals. Only when exposed to ultraviolet radiation (e.g., sunlight) does it discolor over time.

Production

Only highly unsaturated rubbers (with many C=C double bonds) can be used to produce ebonite in order to achieve the high degree of crosslinking required for strength.

A natural rubber and sulfur mixture is melted in a furnace. The molten rubber is then poured into a metal mold and heated to 100 to 200 °C for 15 minutes to vulcanize. The mold is then removed, and the contents are heated again in the furnace for about one and a half days. The long reaction time leads to a complete reaction of the mixture and thus to the greatest possible crosslinking of the available double bonds in the rubber chains with sulfur bridges. This allows the natural rubber and sulfur to fully react with each other. This process gives ebonite its high density and durability.

The amounts of sulphur used to produce ebonite are between 25 and 80%, usually 30 to 40 parts by weight of sulphur per 100 parts of rubber.

Ultraviolet and daylight exposure

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 and sulfuric acid at the surface that are very hygroscopic. The sulfates condense water from the air, forming a hydrophilic film with favorable wettability characteristics on the surface. These aging processes will gradually discolor the surface grayish green to brown and cause rapid deterioration of electric surface resistivity. 

Contamination

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.

Applications

Hard rubber was used in early 20th century bowling 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). It has been used in electric plugs, tobacco pipe mouthpieces (in competition with Lucite), fishing reels, hockey pucks, fountain pen bodies and nib feeds, saxophone and clarinet mouthpieces, as well as complete humidity-stable clarinets. Hard rubber is often seen as the wheel material in casters. It is also commonly used in physics classrooms to demonstrate static electricity, because it is at or near the negative end of the triboelectric series.

Hard rubber was used in the cases of automobile batteries for years, thus establishing black as their traditional colour even long after stronger modern plastics like polypropylene were substituted. It was used for decades in hair combs made by Ace, now part of Newell Rubbermaid, although the current models are known to be produced solely with plastics.

Ebonite is used as an anticorrosive lining for various (mainly storage) vessels that contain diluted hydrochloric acid. It forms bubbles when storing hydrofluoric acid at temperatures above room temperature, or for prolonged durations.

Ebonite, likeglass, is characterized by its excellent insulation; this allows for experimental studies ofstatic electricityor electrostatic electricity. The resulting body is black, very hard, and can be polished to a high shine. It is used for contact pin handles, tabletops, and supports for telegraph equipment that require good insulation.

In fiction

Ebonite is a fictional volcanic ore in the Elder Scrolls universe, particularly heavy, valuable, and durable. It is used in particular for forging weapons and armor.

Professor Calculus 's ear trumpet, in Destination Moon, is made of ebonite, but the horn is designed as gold. Captain Haddock inadvertently mistakes it for his pipe, fills it with tobacco and lights it, triggering a copious release of pestilential smoke, immediately provoking the intervention of the firefighters (the gag takes place in the ultra-secure Sbrodj space center in Syldavia).

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