Determining Antioxidant Depletion Thresholds in Recycled Polypropylene Extrusion Processing
Oxidation induction time testing at 200 °C establishes true additive depletion in recycled polypropylene before melt processing causes chain scission.
Yellow Index
Meaning
Numerical values derived from colorimetric data indicate the degree of departure from a clear or white state toward a yellow hue. The yellow index is a standard metric used in the plastics industry to track the degradation of resins during processing or aging. It is calculated from spectrophotometric measurements using specific formulas such as those in astm e313.
A rising value typically signals that the material has been exposed to excessive heat or UV light.
Colorimetric Shift
Virgin polymers are usually colorless or a crisp white. A shift in the yellow index happens when chemical changes in the polymer chains create chromophores. These structures absorb blue light and cause the material to appear yellowish.
Thermal Degradation
Exposure to high temperatures in the extruder barrel can break down the molecular structure of the resin. The yellow index serves as an indicator of how much thermal damage the material has sustained. Processing the polymer at the lower end of its melt range can help keep this value low.
Monitoring the index ensures that the final parts meet the aesthetic requirements of the customer.
Purity Metric
Raw material suppliers provide these values to guarantee the quality of the incoming resin. A high yellow index in a fresh shipment might indicate contamination or poor stabilization. It is a documented part of the quality control process for clear packaging and medical devices.
Maintaining a consistent index is essential for brand consistency across different production lots and ensures consumer trust in the product.