Antioxidants protect rubber from hardening, cracking, loss of strength, and embrittlement. They are additives used to slow down oxidative degradation of rubber caused by:

• Heat

• Oxygen

• Mechanical stress

• Time

Why do we need antioxidants?

Rubber polymers contain unsaturated double bonds, which are vulnerable to oxidation. Oxidation leads to:

• Chain scission → loss of strength and elongation

• Additional crosslinking → hardening and brittleness

• Cracking under stress (especially dynamic)

This happens even at moderate temperatures, just more slowly.

How do they work?

Most rubber antioxidants act by:

• Scavenging free radicals

• Interrupting oxidation chain reactions

• Decomposing peroxides

They don’t stop aging completely—they slow it down.

Main classes of antioxidants

1. Amine antioxidants (very effective)

Examples: IPPD, 6PPD, DPPD
Traits:

• Excellent protection against heat + oxygen

• Also protect against fatigue and flex cracking

• Can migrate to the surface (bloom)

• Cause staining and discoloration

Used in: Tires, belts, hoses (non-light-colored goods)

6PPD is the most widely used tire antioxidant but currently under research for safe toxicity levels. 6PPD breaks down into 6PPD quinone, which is proven to be toxic for aquatic life.

2. Phenolic antioxidants (non-staining)

Examples: BHT, AO-2246, Irganox types
Traits:

• Good heat-aging resistance

• Non-staining

• Less effective under severe dynamic conditions

Used in: Light-colored, medical, consumer goods

3. Phosphite / phosphonite antioxidants (secondary)

Examples: TNPP
Traits:

• Decompose hydroperoxides

• Often used in combination with phenolics

• Improve long-term thermal stability

Antiozonants vs antioxidants (important distinction)

• Antioxidants → protect against oxidation

• Antiozonants → protect against ozone cracking

Many amine antioxidants (like 6PPD) also act as antiozonants, but not all antioxidants do.

Why migration matters

Some antioxidants are designed to migrate to the surface:

• They form a protective layer

• Especially useful for ozone protection

Trade-off:

• Blooming

• Surface discoloration

• Environmental concerns (e.g., 6PPD transformation products)

How to choose an antioxidant?

• Polymer type (NR, SBR, BR, NBR, EPDM)

• Service temperature

• Dynamic vs static use

• Color requirements

• Regulatory/environmental constraints

• Expected service life

Typical dosage (very general)

• Amine antioxidants: ~1–2 phr

• Phenolic antioxidants: ~0.2–1.0 phr

More is not always better—overuse can:

• Affect cure

• Cause bloom

• Reduce mechanical properties

Practical examples

• Tire tread: 6PPD (fatigue + ozone protection)

• Black industrial rubber: IPPD or 6PPD

• White/light goods: phenolic antioxidant

• High-heat parts: phenolic + phosphite system