ASTM_D_4528_-_88_2021.pdf

Designation:D452888(Reapproved 2021)Standard Classification forRubber Compounding MaterialsSulfur1This standard is issued under the fixed designation D4528;the number immediately following the designation indicates the year oforiginal adoption or,in the case of revision,the year of last revision.A number in parentheses indicates the year of last reapproval.Asuperscript epsilon()indicates an editorial change since the last revision or reapproval.1.Scope1.1 This classification covers the variety of sulfur gradesused in the rubber industry.Typical chemical and physicalproperties for sulfur are shown.Sulfur is principally used inunsaturated rubbers as a vulcanizing agent.1.2 This international standard was developed in accor-dance with internationally recognized principles on standard-ization established in the Decision on Principles for theDevelopment of International Standards,Guides and Recom-mendations issued by the World Trade Organization TechnicalBarriers to Trade(TBT)Committee.2.Referenced Documents2.1 ASTM Standards:2D4569 Test Method for Rubber Compounding MaterialsDetermination of Acidity in SulfurD4571 Test Methods for Rubber Compounding MaterialsDetermination of Volatile MaterialD4572 Test Method for Rubber ChemicalsWet SieveAnalysis of SulfurD4573 Test Method for Rubber ChemicalsDeterminationof Oil Content in Oil-Treated SulfurD4574 Test Methods for Rubber Compounding MaterialsDetermination of Ash ContentD4578 Test Methods for Rubber ChemicalsDeterminationof Percent Sulfur by Extraction3.Significance and Use3.1 Sulfur is one of the principal rubber vulcanizing agents.It is a critical additive.When chemically combined withrubber,sulfur develops basic performance properties in thevulcanized 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 rubberand ebonite,sulfur becomes a major compounding materialwhile retaining its role as a vulcanizing agent.3.2 The most stable molecular form of sulfur at ambientconditions is a ring structure containing eight sulfur atoms.Depending on conditions these molecules orient into one oftwo crystalline structures.At room temperature the crystals arerhombic and above 95C they rearrange to monoclinic.Lessthan 1.5%of either crystalline structure of sulfur is soluble inany rubber at room temperature.3.3 The second common molecular form of sulfur is poly-meric sulfur,made up of unbranched chains of sulfur atoms.Itis commonly referred to in the rubber industry as insolublesulfur.When this material is created by rapid heating to above160C and quenching to room temperature,the sulfur isamorphous.If formed under other conditions,the polymerchains may develop regions of pseudo crystallinity.3.4 Insoluble sulfur is an important form of sulfur used onlyin the rubber industry.It is not soluble in any type of rubberhydrocarbon.When it is mixed in rubber,it disperses butremains undissolved in the rubber.The use of insoluble sulfurprevents the development of a supersaturated solution of sulfurin rubber that occurs when rhombic sulfur is used.No sulfurbloom will develop on the surface of uncured rubber pieceswhen the rubber cools after mixing or processing;therefore,building tack is preserved.At curing temperatures,insolublesulfur rapidly transforms to a soluble species,dissolves in therubber,and enters into the vulcanization process.4.Basis of Classification of Sulfurs4.1 Rhombic Sulfur(ordinary ground sulfur)Rhombicsulfur,which is the ordinary form of sulfur under normalconditions,is ground and classified to meet specific particlesize requirements.The various grades of this type of sulfurcontain less than 1%polymeric sulfur.The ground types ofsulfur may also contain additives to enhance performance.Oilis added to sulfur to help control sulfur dust and improvedispersion in rubber.Finely ground solid minerals are alsoadded to improve dispersion in rubber.Ordinarily the totaladditive level is 5%or less of the sulfur formulation.Many ofthese types of sulfur may be added to rubber compounds as100%sulfur.1This classification is under the jurisdiction of ASTM Committee D11 onRubber and Rubber-like Materials and is the direct responsibility of SubcommitteeD11.20 on Compounding Materials and Procedures.Current edition approved Nov.1,2021.Published December 2021.Originallyapproved in 1988.Last previous edition approved in 2017 as D4528 88(2017).DOI:10.1520/D4528-88R21.2For referenced ASTM standards,visit the ASTM website,www.astm.org,orcontact ASTM Customer Service at serviceastm.org.For Annual Book of ASTMStandards volume information,refer to the standards Document Summary page onthe ASTM website.Copyright ASTM International,100 Barr Harbor Drive,PO Box C700,West Conshohocken,PA 19428-2959.United StatesThis international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for theDevelopment of Intern