Degradation yellowing

The product shows overall or local yellowing and darkened color; in severe cases scorched black spots are visible, 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 which corrodes equipment; PE/PP degradation produces oxidative chromophoric groups that yellow the product. Degradation not only affects appearance but also substantially reduces mechanical properties — impact strength can drop to below 50% of normal values.

1. Comprehensive factors: Melt temperature too high or residence time too long causes thermo-oxidative degradation. 2. Material/formulation factors: Insufficient thermal stability of Ca/Zn stabilizer in the PVC formulation. 3. Material/formulation factors: Phenolic antioxidants discolor at high temperature. 4. Equipment factors: Local degradation at screw dead corners. 5. Die design factors: The geometry of the die flow channel, the land length, and the outlet dimensions directly affect melt flow stability and product dimensional accuracy. Unreasonable flow channel design (presence of dead corners, excessively large convergence angle) is an important root cause of melt fracture, material buildup, and flow marks, and must be addressed starting from optimization of the die structure.

1. Process parameter optimization and adjustment: Reduce processing temperature or use low-temperature processing grade resin. 2. Mold/tooling improvement and maintenance: Optimize the heat stabilizer system. 3. Equipment inspection and system maintenance: Select high-temperature resistant antioxidants. 4. Material/formulation optimization: Check and eliminate dead corners in the screw and die flow channel. 6. 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 5Why analysis to trace the root cause, and solidify 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 chain scission and chemical changes of polymer molecular chains under high temperature and high shear conditions. The core of the solution is "three controls": control processing temperature within the material recommended range, control residence time to avoid excessive retention, and control the dosage of heat stabilizers and antioxidants in the formulation to fully protect molecular chains.