What is ageing, and what does an antioxidant prevent?
Vulcanised rubber hardens, cracks or softens over time. Three mechanisms drive this: chain scission and crosslinking by oxygen (thermo-oxidative ageing), cracks opened by ozone on a strained surface (ozone cracking) and fatigue accelerated by repeated deformation. Antioxidants target the first, antiozonants the second and third; in practice 'antidegradant' covers all of them.
Antioxidant classes
Amine types (PPDs and TMQ) are the strongest protectors but stain; phenolics (BHT, BKF) do not stain but are weaker; mercaptobenzimidazoles (MMBI, ZMMBI) are moderate alone and strong synergists in combination. Choose by three questions: is the part black or coloured, is there dynamic load, what is the service temperature?
Black, dynamic parts: 6PPD + TMQ
For black, dynamically loaded parts such as tyres, bellows, belts and mounts the standard system is 1–2 phr 6PPD (Vulkanox 4020) with 1–2 phr TMQ (Vulkanox HS) plus 1–3 phr antiozonant wax (Antilux). 6PPD handles ozone and fatigue, TMQ heat ageing, wax static ozone. For faster static protection add 77PD (Vulkanox 4030).
Light and coloured parts
PPDs cannot be used. BHT or BKF (0.5–1.5 phr) give the base oxidation protection; for ozone add Vulkazon AFS (1–3 phr) and wax. At high temperature BKF is preferred over BHT.
High temperature and cable
In EPDM and NBR cable and high-temperature parts, TMQ + ZMMBI (Vulkanox ZMB2) or phenolic + MMBI combinations give long life; in peroxide cure MMBI derivatives are particularly effective.
Common mistakes
Raising a single antioxidant brings bloom and cost, not protection; the right move is a synergistic pair. Wax above 3 phr causes tack and appearance problems. Remember that TMQ gives no ozone protection and BHT is volatile.