ASTM-D4528 Standard Classification for Rubber Compounding Materials—Sulfur

Show Complete Document History

Document Center Inc. is an authorized dealer of ASTM standards.
The following bibliographic material is provided to assist you with your purchasing decision:

Standard Classification for Rubber Compounding Materials—Sulfur



Want this as a site license?


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.

Significance and Use

3.1 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.

3.2 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.

3.3 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.

3.4 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.


insoluble sulfur; rhombic sulfur; sulfur ;; ICS Number Code 83.040.20 (Rubber compounding ingredients)

To find similar documents by ASTM Volume:

09.01 (Rubber, Natural and Synthetic -- General Test Methods; Carbon Black)

To find similar documents by classification:

83.040.20 (Rubber compounding ingredients Including carbon black, kaolin clay, etc.)

This document comes with our free Notification Service, good for the life of the document.

This document is available in either Paper or PDF format.

Document Number


Revision Level

1988 R17 EDITION



Modification Type


Publication Date

Feb. 15, 2017

Document Type


Page Count

2 pages

Committee Number