Meaning
Quantitative metrics used to track the loss of whiteness in polymers and coatings provide a standard for assessing environmental degradation. A high yellowing index indicates that the material has undergone chemical changes due to heat, light, moisture or atmospheric pollutants. This value is calculated using spectrophotometric data according to international standards like ASTM E313.
The measurement is restricted to the visible spectrum and does not account for changes in structural strength or electrical insulation properties.
Optical Measurement
Instruments measure the light reflected from the surface at specific wavelengths.
Quality Threshold
Manufacturers set a maximum allowable value for the yellowing index to ensure that consumer goods maintain their aesthetic appeal throughout their expected service life. If the material exceeds this limit during a stress test, the formulation is rejected or modified with higher concentrations of stabilizers. This limit is often specified in the supply contract as a grounds for batch rejection.
Atmospheric Impact
Outdoor exposure accelerates the rate of color change. Sunlight and oxygen break the molecular bonds in the plastic, leading to the rise in the yellowing index over months of use. Protective coatings are applied to slow this process and extend the time before the change becomes visible to the naked eye.
