Primary Antioxidants
- Mechanism
- Hindered Phenols
- Secondary Aromatic Amines
Mechanism
- Primary or free radical scavenging antioxidants inhibit oxidation via chain terminating
reactions.
- They have reactive OH or NH groups. (Hindered
phenols and Secondary aromatic amines)
- Inhibition occurs via a transfer of a proton to the free radical species.
- The resulting radical is stable and does not abstract a proton from the polymer
chain.
Hindered Phenols
Phenolic stabilizers are primary antioxidants that act as hydrogen donors. They
react with peroxy radicals to form hydroperoxides and prevent the abstraction of
hydrogen from the polymer backbone. Often used in combination with
secondary antioxidants, phenolic stabilizers are offered in an extensive
range of molecular weights, product forms, and functionalities.
Sterically hindered phenols, are the most widely used stabilizers of this type.
They are effective during both processing and long term thermal aging and many have
FDA approvals.
ROO* radicals are deactivated by hindered phenol via the following reaction.
The phenoxy radical generated are very stable due to their ability to build numerous
mesomeric forms.
Secondary Aromatic Amines
Secondary aromatic amines act as primary antioxidants and are excellent hydrogen
donors.
Secondary Aromatic Amines - Mechanism
Also available in an extensive range of molecular weights and product forms, aromatic
amines are often more active than hindered phenols, because of less steric hinderance.
Aromatic amines, however, are more discoloring than hindered phenols, especially
on exposure to light or combustion gases (gas fade) and have limited FDA approvals.
