Meaning
Spectrophotometric metrics measuring the increase in the yellow hue of a plastic material after exposure to heat quantify the degree of polymer degradation. The thermal yellowing index represents the change in color that occurs when a resin is processed at high temperatures or stored in hot environments. This value is calculated from colorimetric measurements of the coordinates along the yellow-blue color axis.
Degradation Mechanism
High melt temperatures and presence of oxygen trigger the formation of chromophoric compounds in polymer chains. A rising thermal yellowing index is often caused by the oxidation of residual catalysts or polymer stabilizers during extrusion. When the stabilizer package is exhausted, the polymer begins to break down, forming conjugated structures that absorb blue light.
This degradation results in a progressive color shift from clear to yellow.
Moulding Variable
Cycle time and melt temperature in the barrel are primary process variables that affect final part color. If the melt resides too long in the injection moulding barrel, the thermal yellowing index of the finished part will increase. Operators must minimize dwell times and use lower melt temperatures to prevent color defects.
This is especially essential for optical-grade parts where maximum light transmission is required. High shear rates from narrow gates or runners also contribute to this heat generation and accelerate color shifts.
Resin Quality
Incoming quality control laboratories measure color stability to ensure consistent appearance across production runs. A low thermal yellowing index indicates that the resin has a stable stabilizer formulation with high thermal endurance. Moulders use this index to compare the performance of recycled grades against virgin resins.
This comparison helps determine the maximum percentage of regrind that can be added to the melt.
