Ebonite

Ebonite applications from the 19th century

Ebonite was a brand name for very hard rubber first obtained by Charles Goodyear by vulcanizing rubber for prolonged periods. For vulcanizing rubber he received patent number 3633 from the United States Patent Office on June 15, 1844.^[1] Besides natural rubber Ebonite contains about 25% to 80% sulfur and linseed oil.^[2][3] Its name comes from its intended use as an artificial substitute for ebony wood.

The material is known generically as hard rubber and has formerly been called "vulcanite",^[4] although that name now refers to the mineral vulcanite.

History

Charles Goodyear's brother Nelson Goodyear experimented with the chemistry of ebonite composites. In 1851 he used zinc oxide as a filler.^[5]

Properties

Schematic presentation of two strands (blue and green) of natural rubber after vulcanization with elemental sulfur

The sulfur percentage and the applied temperatures and duration during 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 intramoleculer ring structures, so a large portion of the sulfur is highly cross-linked in the form of intramoleculer addition. High sulfur content up to 40% may be used for greatest resistance to swelling and minimal dielectric loss. The stongest mechanical properties and greatest heat resistance is obtained with sulfur contents around 35%. Best impact strength is obtained with somewhat lower sulfur contents around 30%. The rigidity of hard rubber at room temperature is attributed to the van der Waals forces between the intramoleculer 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 can be fairly easy 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.

Under the influence of the ultraviolet portion in daylight sulfuric acid is produced at the surface that will gradually discolor the surface gray green to white and cause rapid deterioration of electric surface resistivity.

Contamination

Ebonite contamination was problematic when it was used for electronics. 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 metal particles.

Applications

Green/black rippled ebonite fountain pen made in 2014

It is often used in bowling balls, electric plugs, smoking pipe mouthpieces (in competition with lucite), fountain pen bodies and nib feeds, saxophone and clarinet mouthpieces as well as complete clarinets (barrels/upper & lower joints/bells). Hard rubber is often seen as the wheel material in casters. It is also commonly used in physics classrooms to demonstrate static electricity.

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 famously for many decades in hair combs made by Ace, now part of Newell Rubbermaid, although the current models are known to be produced solely with plastics. (An easy way to identify a hard rubber comb is to rub part of its surface vigorously, then immediately smell the comb. Hard rubber's scent, resulting from the sulfur in the ebonite, can usually be detected temporarily. The same effect can often be produced by running the comb under hot tap water.) Ebonite is used as an anticorrosive lining for various (mainly storage) vessels that contain hydrochloric acid. It forms bubbles when storing hydrofluoric acid at temperatures above room temperature, or for prolonged durations.

References

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