Meaning
Highly reactive organic intermediates are generated during the thermal degradation or oxidation of hindered phenolic antioxidants in polymer compounds. The formation of a quinone methide occurs when these stabilizers consume free radicals during high temperature melt processing or long term storage. These intermediates are intensely colored, often causing a distinct yellow or pink discoloration in the finished plastic part.
This aesthetic defect can ruin the value of clear or light colored moulded components even when the mechanical properties remain intact.
Antioxidant Degradation
Phenolic stabilizers protect the polymer by donating hydrogen atoms to neutralize destructive alkyl and peroxy radicals. This protective action alters the chemical structure of the stabilizer itself, converting it into a quinone methide under oxidizing conditions. This conversion occurs more rapidly when the melt is exposed to high shear forces or excessive processing temperatures.
Polymer Discoloration
Conjugated double bond systems of these intermediates absorb light in the visible spectrum. This absorption creates a yellowing effect that is particularly noticeable in large, thick walled parts or transparent packaging. This phenomenon, often called gas fading, can occur during storage if the polymer is exposed to nitrogen oxides from gas fired warehouse heaters.
Processing Prevention
Co stabilizers like phosphites are added to the resin blend to prevent the formation of these colored species. These secondary antioxidants reduce the oxidized phenolic compounds back to their inactive, colorless forms during melt processing.
