Mechanism
Ozone breaks the double bonds of unsaturated rubber (NR, SBR, BR, NBR, CR) at the surface. If the part is under strain (critical strain ~5–25%) micro-cracks open, fresh surface is exposed and the crack grows. Saturated EPDM is ozone resistant for this reason.
Static protection: wax
Waxes such as Antilux 111 and 654 migrate to the surface and build a physical barrier. The film is excellent on a part at rest; it breaks on a flexing part. Choose wax by service temperature: 111 for variable/low temperature, 654 for high temperature, a blend for a wide window.
Dynamic protection: PPD antiozonants
6PPD (Vulkanox 4020) reacts with ozone faster than the rubber, acting as a sacrificial scavenger and forming a protective layer; it works on flexing parts too. 77PD (Vulkanox 4030) gives faster static protection. Together they are the standard tyre-sidewall system.
In coloured parts
Because PPDs stain, Vulkazon AFS (enol ether) + wax is used. Since protection is lower than with PPDs, reducing strain in the design (thicker section, radiused corners) matters.
Testing and verification
ISO 1431-1 (static, 20% strain, 50 pphm, 40 °C, 72 h) and its dynamic variant. Condition specimens 24–48 h before testing so the wax film can form; otherwise the wax effect is invisible.