Yellowing Index

Color Measurement

Spectrophotometers measure spectral reflectance or transmittance across visible wavelengths to calculate standard tristimulus values according to ASTM E313 protocols. Calculating the yellowing index involves comparing color coordinates against standardized white or clear calibration references. Increases in this optical index reflect polymer chain degradation, additive depletion or antioxidant consumption inside processed polymer matrices.

Automated colorimeters evaluate optical clarity on clear extruded sheet and injection molded plaques.

Thermal Degradation

Excessive barrel temperatures and extended melt residence times induce thermal oxidation, generating chromophores that discolor molten polymer. Tracking changes in yellowing index reveals degradation during compounding or re-pelletizing operations involving post-industrial regrind streams. Adding secondary phosphite antioxidants suppresses chromophore formation, preserving initial resin color purity.

Resin specifications establish maximum allowable index increases across consecutive heat histories.

Quality Standard

Optical criteria specify upper index limits for commercial resin lots. Off-color resin lots are rejected prior to final component moulding operations.