Severe Shrinkage of Heat-sensitive Substrates (Such as Certain Degradable Plastics PLA) During Coating Curing

The product shows overall or local yellowing and color darkening; in severe cases, burnt 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 conditions. PVC degradation releases HCl that corrodes equipment; PE/PP degradation produces oxidized chromophoric groups that cause product yellowing. Degradation not only affects appearance but also substantially reduces mechanical properties—impact strength can drop to below 50% of normal values.

1. Comprehensive factors: PLA's low Tg and crystallization behavior cause significant shrinkage stress. 2. Substrate surface condition factors: The surface tension, cleanliness, and microscopic roughness of the substrate are the basic conditions for coating adhesion and leveling. When the substrate surface tension is lower than the coating fluid surface tension, wetting is insufficient and cratering occurs; surface contamination causes local adhesion failure; this is the primary direction for troubleshooting coating defects. 3. Curing process factors: Curing temperature and time determine the completion degree of the coating crosslinking reaction—insufficient temperature or insufficient time leads to incomplete curing (tacky, low hardness, poor solvent resistance), excessive temperature or excessive time leads to over-curing (embrittlement, yellowing, cracking); the curing window of the coating fluid system must be precisely matched.

1. Process parameter optimization and adjustment: Switch to UV or EB curing instead of thermal curing. 2. Mold/tooling improvement and maintenance: Or use more heat-resistant modified PLA (such as adding stereocomplex crystallization, etc.). 3. Systematic troubleshooting method: Troubleshooting of coating problems follows the order of "substrate → coating fluid → coating head → curing → environment". First check the substrate surface tension (corona treatment effect) and cleanliness, then verify the coating fluid viscosity, solid content, and degassing status, then check the coating head gap and cleanliness, further verify the curing temperature and time, and finally evaluate the influence of ambient temperature, humidity, and cleanliness. 4. 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 that the 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 material's recommended range, control the residence time to avoid excessively long retention, and control the amount of heat stabilizers and antioxidants in the formulation to fully protect the molecular chains.