AbstractSamples of polyester plastics, upon exposure to solar or carbon‐arc radiant energy, were found to form oxidation products which, when reacted withN,N‐dimethyl‐p‐phenylenediamine (DMPDA), produced yellow to brown reaction products. The extent of the reaction was determined directly by the amount of diamine reacted or indirectly by differential colorimetry. Research with this color reaction was extended into the development of tests for measuring the photooxidative degradation of polyesters, rigid poly(vinyl chloride), acrylics, and cellulose acetate–butyrate plastics. The basic colorimetric method was modified as needed for each of these plastics types, by directly measuring the absorbance of the DMPDA solution instead of measuring the amount of DMPDA reacted, by the use ofo‐,m‐, andp‐phenylenediamines as replacements for DMPDA in the color reaction, or by a change in the solvent. The reliability of “yellowness” as an indicator of the photochemical stability of pl
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