Secondary Antioxidants
- Mechanism
- Organophosphorus Compounds
- Thiosynergists
Mechanism
Secondary antioxidants, frequently referred to as hydroperoxide decomposers, decompose
hydroperoxides into non-radical, non-reactive, and thermally stable products.
They are often used in combination with primary antioxidants to yield synergistic
stabilization effects.
Hydroperoxide decomposers prevent the split of hydroperoxides into extremely reactive
alkoxy and hydroxy radicals.
Organophosphorus compounds and
Thiosynergists antioxidants widely used hydroperoxide decomposers.
Organophosphorus Compounds
Organophosphorus compounds are secondary antioxidants that decompose peroxides and
hydroperoxides into stable, non-radical products. They are extremely effective stabilizers
during processing and are normally used in combination with a
primary antioxidant.
Trivalent phosphorus compounds are excellent hydroperoxide decomposers.
Generally, phosphites (or phosphonites) are used and react according to the following
general reaction, generating phosphates.
Some of these compounds are sensitive to water and can hydrolyze, leading to formation
of acidic species. While the addition of an acid scavenger can minimize the effect,
the industry has generally converted to hydrolysis-resistant compounds.
Commercially available phosphites differ by the nature of the aryl groups. Some
typical structures are shown below.
Thiosynergists
Among sulfur-based hydroperoxide decomposers, esters of 3,3-thiodipropionic acid
play an important role. Thiosynergists react according to the following general
reaction, generating sulfoxides and sulfones.
Although thiosynergists do not improve the melt stability of polymers during polymer
processing, they are very efficient for long-term thermal aging applications. Sulfur
based hydroperoxide decomposers are mainly used in combination with
hindered phenol antioxidants.
The most common commercially available thiosynergists are based on either lauric
or stearic acid.
