Melt Degradation/Yellowing

The product shows overall or local yellowing and darkening of color, and in severe cases burnt black spots can be seen, accompanied by a pungent odor. Degradation is the process in which polymer molecular chains undergo breakage and chemical changes under high temperature, high shear or long residence time conditions. PVC degradation releases HCl that corrodes equipment, and PE/PP degradation produces oxidative chromophoric groups that cause yellowing of the product. Degradation not only affects appearance, but also substantially reduces mechanical properties — impact strength can drop to below 50% of the normal value.

1. Comprehensive factor: the spinning temperature is too high or the residence time is too long. 2. Material/formulation factor: oxygen or moisture mixed in the chips causes thermal oxidative and hydrolytic degradation. 3. Equipment factor: dead corners in the equipment produce degraded charred material that is intermittently discharged. 4. Cooling air field factor: the temperature, wind speed and uniformity of the side-blowing air or ring-blowing air determine the solidification and forming quality of the fiber — too fast cooling leads to rough surface and increased internal stress, uneven cooling leads to inconsistent thickness of individual filaments in the same bundle, and low wind speed leads to filament sticking and filament breakage; the air field parameters need to be precisely controlled.

1. Process parameter optimization and adjustment: lower the spinning temperature and shorten the residence time in the melt pipeline. 2. Mold/tooling improvement and maintenance: strengthen chip drying and nitrogen protection to prevent oxidation. 3. Equipment inspection and system maintenance: eliminate dead corners in pipelines and equipment. 4. Troubleshooting of spinning problems follows the sequence of "chips → temperature → metering → spinneret → cooling air". First check the chip dryness and IV value, then verify the temperatures of each section of the spinning box and pipeline, next verify the metering pump accuracy and pressure stability, further check the spinneret cleanliness and hole shape condition, and finally adjust the cooling air field parameters to make the wind speed and temperature evenly distributed. 5. Preventive management: establish standardized process parameter cards and quality control limits for each mold/product, and regularly conduct process capability index (CPK) evaluations. For recurring problems, use fishbone diagrams or the 5Why analysis method to trace the root cause, and solidify the countermeasures as PFMEA control points. Establish a regular equipment maintenance system, and implement life management for key tooling such as dies/spinnerets/calender rolls/coating heads.

The essence of degradation yellowing is that polymer molecular chains undergo chain scission and chemical changes under high-temperature and high-shear conditions. The core of the solution is "three controls": control the processing temperature within the recommended range of the material, control the residence time to avoid excessive retention, and control the dosage of heat stabilizers and antioxidants in the formulation to fully protect the molecular chains.