Surface Cracking of Rubber Calendered Products Caused by Environmental Ozone

Fine cracks appear on the coating surface, with crack directions randomly or parallelly arranged, and in severe cases forming a network of cracks. Coating cracking is the result of internal stress exceeding the fracture toughness of the coating—the difference in shrinkage rate between the coating and the substrate generates interfacial stress during curing, an overly thick coating increases the absolute value of stress, and an overly fast curing speed leaves no time for stress relaxation. A cracked coating not only loses its protective and decorative functions, but the cracks also become channels for corrosive media to invade the substrate.

1. Comprehensive factors: increased ozone concentration in the ambient air produces ozone cracking under stress. 2. Nip adjustment factors: The crown and cross-angle compensation of calender rolls is the core means of controlling the transverse thickness distribution of the film. Crown design is based on roll deflection calculation; if it does not match the actual process load, it will lead to abnormal transverse distribution of film thickness—thicker in the middle and thinner at both ends, or reverse bow-shaped distribution. 3. Feed uniformity factors: The temperature uniformity of the calender feed and the stability of the material bank shape directly affect the uniformity of nip feeding. A bank that is too narrow, skewed, or unevenly rolled will all cause transverse thickness abnormalities in the product; it is necessary to ensure the temperature and flow stability of the internal-mixer or extruder feed.

1. Process parameter optimization and adjustment: Use antiozonants (p-phenylenediamine type) in the formulation or microcrystalline wax to bloom out and form a protective film. 2. Systematic troubleshooting method: Troubleshooting of calendering problems follows the order of "nip → roll temperature → feed → tension → cooling". First check and precisely adjust the nip crown and cross angle, then confirm the uniformity of the temperature distribution of each roll, then verify the feed bank shape and rolling behavior, further adjust the winding tension control parameters, and finally optimize the cooling roll temperature and contact method. Change only one variable at a time to confirm the effect. 3. 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 coating cracking is that the internal stress exceeds the fracture toughness of the coating. The core of the solution is "stress reduction + toughening": control the coating thickness to avoid excessive stress accumulation, adopt mild curing conditions to allow stress relaxation, add toughening agents to improve the coating's fracture toughness, and select a coating system with a shrinkage rate close to that of the substrate.