Meaning
Primary antioxidant formulations incorporating bulky alkyl substituents around a phenolic hydroxyl group terminate free radical propagation during polymer thermal processing. Incorporating hindered phenols into thermoplastic formulations protects polyolefins and styrenic resins from thermo-oxidative chain scission during extrusion and molding. The chemical structure donates steric hydrogen atoms to peroxyl radicals, forming stable phenoxyl radicals that cannot abstract hydrogen from polymer chains.
This protection extends to melt processing and long-term heat aging, though secondary phosphite antioxidants are required to decompose hydroperoxides created during oxidation.
Radical Scavenging
Steric hindrance prevents reactive phenoxyl radicals from initiating new polymer chain cleavage reactions. Adding hindered phenols stops auto-oxidation cascades by intercepting carbon-centered and oxygen-centered radicals early in the degradation cycle. Sterically hindered structures like tetrakis(methylene(3,5-di-tert-butyl-4-hydroxyhydrocinnamate))methane provide long-term protection against oxidative degradation.
Low radical scavenging efficiency occurs when thermal loss or leaching depletes additive concentrations in thin polymer films.
Discoloration Risk
Oxidized phenolic structures can transform into quinoid chromophores that alter final part appearance. Formulations containing hindered phenols may undergo gas fading when exposed to nitrogen oxides in warehouse storage environments. Yellowing defects appear on light-colored molded parts without reducing mechanical tensile strength.
Blending secondary phosphites reduces quinoid formation and maintains original color consistency.
Processing Performance
Melt flow rate stability depends on effective radical neutralization during high-temperature molding cycles. Utilizing hindered phenols maintains melt viscosity across multiple regrind reprocessing passes. Extrusion compounding without primary antioxidants leads to rapid crosslinking in polyethylene or scission in polypropylene.
Resin datasheets specify additive concentration limits to prevent bloom onto molded part surfaces.
