Significance and Use
Sulfur is one of the principal rubber vulcanizing agents. It is a critical additive. When chemically combined with rubber, sulfur develops basic performance properties in the vulcanized compound such as: tensile strength, elongation, modulus, and hardness. In soft or elastic rubber compounds, sulfur is an essential but minor additive. In semi-hard rubber and ebonite, sulfur becomes a major compounding material while retaining its role as a vulcanizing agent.
The most stable molecular form of sulfur at ambient conditions is a ring structure containing eight sulfur atoms. Depending on conditions these molecules orient into one of two crystalline structures. At room temperature the crystals are rhombic and above 95°C they rearrange to monoclinic. Less than 1.5 % of either crystalline structure of sulfur is soluble in any rubber at room temperature.
The second common molecular form of sulfur is polymeric sulfur, made up of unbranched chains of sulfur atoms. It is commonly referred to in the rubber industry as insoluble sulfur. When this material is created by rapid heating to above 160°C and quenching to room temperature, the sulfur is amorphous. If formed under other conditions, the polymer chains may develop regions of pseudo crystallinity.
Insoluble sulfur is an important form of sulfur used only in the rubber industry. It is not soluble in any type of rubber hydrocarbon. When it is mixed in rubber, it disperses but remains undissolved in the rubber. The use of insoluble sulfur prevents the development of a supersaturated solution of sulfur in rubber that occurs when rhombic sulfur is used. No sulfur bloom will develop on the surface of uncured rubber pieces when the rubber cools after mixing or processing; therefore, building tack is preserved. At curing temperatures, insoluble sulfur rapidly transforms to a soluble species, dissolves in the rubber, and enters into the vulcanization process.
1.1 This classification covers the variety of sulfur grades used in the rubber industry. Typical chemical and physical properties for sulfur are shown. Sulfur is principally used in unsaturated rubbers as a vulcanizing agent.
2. Referenced Documents (purchase separately) The documents listed below are referenced within the subject standard but are not provided as part of the standard.
D4569 Test Method for Rubber Compounding Materials--Determination of Acidity in Sulfur
D4571 Test Methods for Rubber Compounding Materials--Determination of Volatile Material
D4572 Test Method for Rubber Chemicals--Wet Sieve Analysis of Sulfur
D4573 Test Method for Rubber Chemicals--Determination of Oil Content in Oil-Treated Sulfur
D4574 Test Methods for Rubber Compounding Materials--Determination of Ash Content
D4578 Test Methods for Rubber Chemicals--Determination of Percent Sulfur by Extraction
insoluble sulfur; rhombic sulfur; sulfur ; Chemical analysis--rubber chemicals/materials; Classification (standards)--rubber; Insoluble matter content--rubber; Rhombic sulfur; Rubber compounding materials; Sulfur--rubber
ICS Number Code 83.040.20 (Rubber compounding ingredients)
ASTM International is a member of CrossRef.
Citing ASTM Standards
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