This International Standard applies to a variety of substituted p-phenylenediamine (PPD) antidegradants used in the rubber industry. The three general classes of PPD are dialkyl, alkyl-aryl and diaryl, which are used to impart ozone resistance to rubber. The test methods of greatest significance in assessing the purity of production PPDs, and hence their suitability for use in rubber, are specified in this International Standard.