Stabilizers and antioxidants

Definition (what it is)

Stabilizers and antioxidants are additive classes used in polymers, elastomers, lubricants, fuels, coatings, and adhesives to slow or prevent degradation caused by heat, oxygen, light, moisture, ozone, metal ions, and mechanical stress. Antioxidants are a subset of stabilizers that primarily interrupt oxidative radical chain reactions; the broader term stabilizers also includes light (UV) stabilizers, thermal/processing stabilizers, hydrolysis stabilizers, metal deactivators, and antiozonants.

What they do and how they work (key mechanisms)

  • Oxidation control
    • Primary (chain-breaking) antioxidants donate hydrogen to radical species (e.g., ROO•, R•), terminating the autoxidation chain. Typical chemistries: hindered phenols (e.g., BHT; octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate; tetrakis[methylene(3,5-di-tert-butyl-4-hydroxyhydrocinnamate)]methane). Aminic antioxidants (alkylated diphenylamines) are widely used in oils and rubbers but can stain plastics.
    • Secondary (peroxide-decomposing) antioxidants convert hydroperoxides (ROOH) into non-radical products, limiting new radical formation. Typical chemistries: phosphites/phosphonites (e.g., tris(2,4-di-tert-butylphenyl) phosphite), organosulfur thioethers/thioesters (e.g., distearyl or dilauryl thiodipropionate).
    • Synergy: primary + secondary systems provide both processing color control and long-term heat stability.
  • Light/UV stabilization (photo-oxidation control)
    • UV absorbers (benzotriazoles, benzophenones, hydroxyphenyl-triazines) absorb harmful UV and dissipate the energy as heat.
    • Hindered amine light stabilizers (HALS) scavenge radicals generated by light exposure through a regenerative nitroxyl cycle, providing long-term outdoor durability.
    • Often combined (HALS + UV absorber) and influenced by pigments/fillers (e.g., carbon black screens UV; certain TiO2 grades are surface-treated to reduce photocatalysis).
  • Thermal/processing stabilization
    • Neutralize acids and catalytic species, inhibit chain scission/crosslinking during melt processing and service at elevated temperatures.
    • Typical chemistries: organophosphites, calcium/zinc stearates, hydrotalcites. In PVC, Ca/Zn or organotin systems scavenge HCl and suppress dehydrochlorination.
  • Hydrolysis stabilization
    • Reduce water-driven chain scission in susceptible polymers. Typical approaches: carbodiimides (for PET, PBT, TPU), chain extenders/epoxies (rebuild molecular weight), end-group capping agents (reduce acid end groups).
  • Metal deactivation
    • Chelate or passivate catalytic metal ions (Cu, Fe, Mn) that accelerate oxidation. Used in polyolefins contacting copper (wires, foils) and in lubricants. Typical chemistries: benzotriazole derivatives, salicylaldoximes, Schiff bases.
  • Antiozonants (elastomers)
    • Sacrificially react with ozone and slow fatigue cracking in unsaturated rubbers (NR, SBR, BR). Typical chemistries: p-phenylenediamine (PPD) derivatives; paraffin wax “bloom” can add a physical ozone barrier.
  • Resulting benefits
    • Preserve mechanical properties (toughness, elongation, impact), color and gloss, dimensional stability, dielectric strength, and surface appearance.
    • Enable higher processing temperatures, longer residence times, and multiple reprocessing/recycling cycles with reduced melt degradation, gel formation, and odor.
    • In fluids, limit acid formation, varnish/sludge, and viscosity increase.

Where they are used (materials and sectors)

  • Polymers: polyolefins (PE, PP), styrenics (PS, ABS), engineering thermoplastics (PA, PBT, PC, PPS, POM), PVC.
  • Elastomers: EPDM, NR, SBR, NBR, CR; tires, seals, mounts.
  • Coatings, inks, adhesives, sealants, composites (GFRP/CFRP).
  • Lubricants and fuels: phenolic and aminic antioxidants; multifunctional additives such as ZDDP in engine and gear oils; oxidation inhibitors in e-drive fluids and coolants.
  • Electrical and electronics: wire/cable insulation and jacketing, encapsulants, films; metal deactivators for copper contact.
  • EV-relevant examples: battery module housings and potting compounds; high-voltage cabling and connectors; exterior plastics and composites exposed to UV; power electronics encapsulants; e-axle oils and thermal management fluids; tires subjected to high torque.

Common classes and typical examples

  • Primary antioxidants: hindered phenols (BHT; octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate; tetrakis[methylene(3,5-di-tert-butyl-4-hydroxyhydrocinnamate)]methane); aminic antioxidants (alkylated diphenylamines, mainly in lubricants/rubbers).
  • Secondary antioxidants: phosphites/phosphonites (e.g., tris(2,4-di-tert-butylphenyl) phosphite); organosulfur thioesters (DLTDP, DSTDP) and thioethers.
  • Light/UV stabilizers: HALS (e.g., piperidyl-based), UV absorbers (benzotriazoles, benzophenones, triazines).
  • Thermal/processing/acid scavengers: organophosphites; Ca/Zn stearates; hydrotalcites. PVC uses Ca/Zn or organotin (methyltin/butyltin carboxylates); lead-based systems are restricted in many regions.
  • Metal deactivators: benzotriazole derivatives (e.g., MD-1024 type), salicylaldoximes, chelating Schiff bases.
  • Hydrolysis stabilizers: carbodiimides (for polyesters/TPU); epoxy/oxazoline chain extenders; end-group cappers.
  • Antiozonants: PPD derivatives (rubber), typically combined with paraffin waxes for surface protection.

Incorporation and typical dosage

  • Added during polymerization (reactive or grafted stabilizers) or, more commonly, during melt compounding as powders, pastilles, granules, liquids, or masterbatches; in rubbers during internal mixing or milling; in lubricants/fuels as additive concentrates during blending.
  • Typical ranges: antioxidants 0.05–1.0 wt%; UV systems (absorber and/or HALS) 0.1–2.0 wt% depending on thickness, pigment, and exposure; PVC heat stabilizers ~1–5 phr; antiozonants in rubber ~1–3 phr.
  • Oligomeric/encapsulated/reactive stabilizers reduce migration, extraction, fogging, and odor.

Selection, synergy, and compatibility

  • Choose based on base polymer, part thickness, service temperature and life, UV/ozone/humidity/chemical exposure, electrical needs, regulatory limits, and interaction with fillers, pigments, plasticizers, and flame retardants.
  • Common synergies: hindered phenol + phosphite/phosphonite; HALS + UV absorber; antioxidants + metal deactivator in copper-contact applications.
  • Interactions and constraints: acidic FRs or fillers can consume phosphites; HALS may interact with acidic pigments or halogenated FRs; transition metals catalyze oxidation unless deactivated. Aminic/PPD antioxidants can stain; some stabilizers can migrate or bloom, affecting paint/adhesion and electrical creepage.

Testing and qualification

  • Weathering: ISO 4892, ASTM G154/G155; measure color/gloss change and mechanical retention.
  • Thermal/electrical aging: oven aging per UL/IEC/ISO; dielectric strength and insulation resistance.
  • Oxidation metrics: DSC oxidation induction time/temperature (OIT/OITP/OOT); FTIR carbonyl index; gel count/melt flow changes; in oils, acid number, sludge/varnish tendency and viscosity rise.
  • Chemical resistance: immersion tests in relevant fluids (oils, coolants, fuels, electrolytes).

Regulatory, safety, and sustainability

  • Compliance with REACH, RoHS, and regional limits on heavy metals, certain amines, and alkylphenol-derived phosphites is common. Low-VOC, non-staining, and food-contact-compliant grades exist.
  • Halogen- and heavy-metal-free stabilizer systems are increasingly used.
  • Recycling and circularity: “rejuvenation” packages for recycled plastics compensate for prior thermal/oxidative history and contaminants; reactive/oligomeric stabilizers help maintain performance and reduce migration in second-life applications.

Related terms and synonyms

Polymer stabilizers, plastic stabilizers, oxidation inhibitors, ageing inhibitors, thermo-oxidative stabilizers, antidegradants (rubber), radical scavengers, peroxide decomposers, light stabilizers, heat stabilizers, hydrolysis stabilizers, metal deactivators, antiozonants.