Polymer Oxidation and Antioxidant Mechanisms: The Autoxidation (Bolland-Gee) Cycle and 4 Ways Antioxidants Break It

Polymer oxidation is a free-radical chain reaction with atmospheric oxygen that runs in 4 stages (initiation, propagation, chain branching and termination), and antioxidants slow it by breaking that chain at 4 different points. The scheme was worked out by J. L. Bolland and G. Gee in the 1940s from the oxidation of rubbers and lipids and is still the basis of every stabilizer package; which step does each antioxidant block, and why are they almost always used in pairs?

Antioxidants are the stabilizers found in almost every polyolefin, part of a USD 5.41 billion global market in 2025 according to Mordor Intelligence, within plastic additives.

This page traces that chemistry from the 5 sources that start polymer oxidation, through the sigmoidal oxygen-uptake curve and its 2018 revision by Michelle Coote's group at the Australian National University, to what oxidation does to polypropylene and polyethylene. It then maps the 4 antioxidant mechanisms onto named grades, including Irganox 1010, Irgafos 168, Irganox HP-136 and Irganox MD 1024, explains why phenols, phosphites and thioesters are blended, compares processing with service-life protection, catalogues the side reactions that antioxidant chemistry causes, and closes with the 4 methods used to measure oxidation and antioxidant activity.

Key figures

• 4 stages make up the Bolland-Gee autoxidation cycle: initiation, propagation, chain branching, termination
• 10⁷-10⁹ L mol⁻¹ s⁻¹: the rate constant of R• + O₂, close to diffusion-controlled
• About 2 peroxyl radicals are trapped per hindered-phenol molecule before it is spent
• 0.05-0.4 wt% is the typical Irganox 1010 dosage in polyolefins (BASF)

What Is Polymer Oxidation?#

Polymer oxidation, also called thermo-oxidative degradation, is the reaction of a polymer with oxygen through a free-radical chain mechanism that cuts or links its chains and changes its molecular weight, colour and mechanical properties. The reaction runs on the same alkyl, peroxyl and alkoxyl radicals at every stage of a plastic's life, from the hot melt inside an extruder to a finished part sitting on a warehouse shelf, and it is the single most common reason polyolefins need a stabilizer package at all.

Polymer oxidation runs in 2 time windows: melt processing at 150-320 °C, where shear and residual oxygen drive the reaction within minutes, and storage and service at roughly 60-150 °C, where the same chemistry proceeds far more slowly over months or years. Heat without oxygen, light and water break plastics down through other routes, compared together as polymer degradation elsewhere on this site.

What starts polymer oxidation?#

Polymer oxidation starts from 5 sources of free radicals: heat and shear during processing, catalyst residues, transition-metal ions such as copper and iron, UV light, and hydroperoxides already present in the resin. The 5 initiation sources are listed below.

• Heat and shear during extrusion or injection moulding, which break polymer C-C and C-H bonds homolytically and generate the first alkyl radicals (R•)
• Catalyst residues left over from polymerization, such as titanium and aluminium compounds from Ziegler-Natta catalyst systems
• Transition-metal ions, including copper, iron, manganese and cobalt, which cycle between oxidation states and split hydroperoxides into new radicals
• Light, mainly UV radiation above 290 nm absorbed by chromophores already present in the polymer
• Hydroperoxides already present in the resin, which break down homolytically (ROOH → RO• + •OH) once heat is applied

Any one of these 5 sources is enough to generate the first alkyl radical that the cycle then multiplies. Catalyst residues are usually countered with acid scavengers and catalyst neutralizers rather than antioxidants, since the metal itself is not a radical, only a trigger.

Where does oxygen attack polypropylene, polyethylene and rubber-modified plastics?#

Oxygen attacks the weakest carbon-hydrogen bond first: the tertiary C-H of polypropylene, the allylic C-H next to the double bonds of rubbers and the polybutadiene phase of ABS, and the less reactive secondary C-H of polyethylene. The polymer's backbone structure, not any additive, decides which bond breaks first, and the outcome differs by polymer even though the underlying radical chemistry is identical.

Polypropylene's tertiary carbon gives up a hydrogen atom more easily than any other site in a polyolefin, so the resulting tertiary alkoxyl radical undergoes β-scission and cuts the chain in two. Grade choices that follow from this chemistry are on antioxidants for polypropylene.

Polyethylene behaves differently: its secondary C-H bonds are less reactive, so alkyl radical recombination competes with scission, and chromium-catalyzed LLDPE and HDPE grades most prone to gel formation combine both routes at once. Gel control in film and pipe grades is covered under antioxidants for polyethylene.

How Does the Autoxidation (Bolland-Gee) Cycle Work?#

The autoxidation (Bolland-Gee) cycle turns the first radicals into a self-accelerating chain in 4 stages: initiation creates alkyl radicals, propagation converts them into hydroperoxides, chain branching splits the hydroperoxides into new radicals, and termination removes radicals in pairs. J. L. Bolland and G. Gee developed this basic autoxidation scheme (BAS) in the 1940s from the oxidation of rubbers and lipids, and it was later generalized to all polymers.

1. Initiation: how the first radicals form#

Initiation is the step in which a polymer C-C or C-H bond breaks homolytically under heat and shear, or with help from catalyst residues and metal ions, to form alkyl radicals (R•). Once a small population of alkyl radicals exists, the reaction no longer needs an external trigger to continue, because oxygen is almost always present in the melt or the surrounding air. In the presence of oxygen, an alkyl radical becomes a peroxyl radical (ROO•) almost instantly, which is why initiation is usually the shortest and least visible stage of the whole cycle.

2. Propagation: peroxyl radicals and hydrogen abstraction#

Propagation is a two-step loop: an alkyl radical reacts with oxygen to form a peroxyl radical (R• + O₂ → ROO•), and the peroxyl radical abstracts hydrogen from a neighbouring chain to form a hydroperoxide and a new alkyl radical (ROO• + RH → ROOH + R•). The first step is close to diffusion-controlled, with a rate constant of 10⁷-10⁹ L mol⁻¹ s⁻¹, which explains why alkyl radicals are so hard to trap directly and why most antioxidant classes are built to intercept the peroxyl radical instead.

The second step, hydrogen abstraction, is usually rate-determining and is favoured at tertiary and allylic C-H sites, the same positions identified in the polymer-by-polymer comparison above. If every turn of this loop leaves one hydroperoxide behind, what happens to those hydroperoxides next?

3. Chain branching: why hydroperoxides make oxidation autocatalytic#

Hydroperoxides make polymer oxidation autocatalytic because each ROOH that splits yields 2 new radicals (ROOH → RO• + •OH), so the number of growing chains multiplies instead of staying constant. A single hydroperoxide can decompose through 3 routes: unimolecular homolysis (ROOH → RO• + •OH), reaction with a neighbouring C-H bond (ROOH + RH → RO• + R• + H₂O), or bimolecular reaction with a second hydroperoxide (2 ROOH → RO• + ROO• + H₂O). The term hydroperoxide is defined in the glossary with its role in stabilization.

Transition-metal ions such as copper, iron, manganese and cobalt accelerate all 3 routes by cycling between oxidation states, which is why metal contamination from pigments, fillers or wire conductors is treated as a separate stabilization problem. In polypropylene, the alkoxyl radicals formed here are mostly tertiary, and tertiary alkoxyl radicals undergo β-scission, the chain-scission event responsible for the rising melt flow rate described earlier.

4. Termination: how radical chains end#

Termination ends radical chains when 2 radicals meet and combine, for example 2 peroxyl radicals forming non-radical products or 2 alkyl radicals forming a new C-C bond (R• + R• → R-R). Termination is a bimolecular event, so its rate depends on how many radicals are present at once, which is one reason the reaction accelerates once chain branching has built up a large radical population.

In polyethylene, alkyl-alkyl recombination (R• + R• → R-R) is one route to the crosslinks and gels that lower melt flow rate, alongside the chain-scission route that dominates in PP, though it is not the only crosslinking pathway in PE.

Why does polymer oxidation start slowly and then accelerate?#

Polymer oxidation starts slowly and then accelerates because hydroperoxides must first build up before chain branching takes over, which gives the oxygen-uptake curve its S shape with an induction period. During the induction period, antioxidants are consumed as fast as radicals are generated, holding the hydroperoxide concentration low; once the antioxidant is used up, chain branching multiplies radicals faster than termination can remove them, and oxygen uptake accelerates sharply. Oxidation induction time (OIT) measurement by differential scanning calorimetry is built directly on this behaviour, timing exactly how long that delay lasts under controlled conditions.

Is the Bolland-Gee autoxidation scheme still correct?#

The Bolland-Gee scheme still describes the kinetics of polymer oxidation well, but calculations by Michelle Coote's group at the Australian National University, published in Accounts of Chemical Research in 2018, show that its key propagation step, ROO• + RH, is strongly disfavoured in saturated polymers such as PE and PP. L. M. Smith, H. M. Aitken and Michelle Coote calculated that hydrogen transfer from a peroxyl radical to a saturated C-H bond is thermodynamically unfavourable and becomes favourable only at allylic or otherwise unsaturated defect sites; in place of that step, the group proposes that peroxyl-peroxyl termination forming alkoxyl radicals carries much of the chain in saturated polymers, building on earlier work from the same research line on revising the polymer autooxidation mechanism.

In practice this means that defects, branch points and residual unsaturation, not the idealized repeat unit, control how fast a resin oxidizes, which is why catalyst residues and processing history matter as much as the base polymer's nominal structure.

What Does Oxidation Do to Plastics?#

Oxidation changes plastics in 6 measurable ways: chain scission, crosslinking and gel formation, embrittlement, discoloration, surface cracking and the build-up of carbonyl groups. These 6 outcomes trace directly back to the cycle described above, and which ones dominate depends on the polymer.

• Chain scission, dominant in polypropylene, where β-scission of tertiary alkoxyl radicals cuts chains and raises melt flow rate
• Crosslinking and gel formation, dominant in polyethylene, where alkyl radical recombination competes with scission and lowers melt flow rate
• Embrittlement, the mechanical consequence of both chain scission and heavy crosslinking, as molecular weight distribution shifts away from the original design
• Discoloration, including the yellowing and pinking covered later in this page, driven by coloured reaction products
• Cracking and loss of elongation, most visible at the surface, where oxygen concentration is highest
• Carbonyl group build-up, the ketones and other oxidation products formed by β-scission and measurable directly by FTIR

Rubber-modified plastics such as HIPS and ABS show a combined pattern: their polybutadiene phase hardens and yellows through allylic oxidation while the matrix resin scissions or crosslinks separately.

How Do Antioxidants Stop Polymer Oxidation? The 4 Mechanisms#

Antioxidants stop polymer oxidation in 4 ways: chain-breaking donors trap peroxyl radicals, hydroperoxide decomposers destroy hydroperoxides before they branch, carbon-radical scavengers catch alkyl radicals, and metal deactivators bind the metal ions that speed up hydroperoxide decomposition. Each mechanism corresponds to one step of the cycle described above, and a stabilizer package almost always combines more than one.

The first 2 mechanisms correspond to the classic split into primary and secondary antioxidants. Classes, dosage tables and grade selection for all antioxidants for plastics sit on the family hub; this page focuses on the reaction chemistry behind each class rather than on grade-by-grade dosing.

1. Chain-breaking donors: hindered phenols and aromatic amines#

Chain-breaking donors are antioxidants that give a hydrogen atom to a peroxyl radical faster than the polymer can (ROO• + ArOH → ROOH + ArO•), leaving behind a radical too stable to continue the chain. The phenoxyl radical (ArO•) that results is stabilized by resonance and by ortho tert-butyl groups, and it ends its life as a non-radical product, typically a quinone methide or a peroxycyclohexadienone, rather than restarting the chain.

Hindered phenols such as Irganox 1010, Irganox 1076 and BHT dominate this class. All commercial phenolic antioxidants (hindered phenols) are compared by molecular weight and use on the class page, since molecular weight, not the radical-trapping chemistry itself, is what separates a low-volatility long-term thermal stabilizer from a fast-migrating processing aid.

Commodity PP resin typically carries less than 400 ppm of phenolic antioxidant as base stabilization straight from the reactor, enough for initial handling but not for demanding long-term service. Aromatic amines work by the same donor chemistry (Ar₂NH + ROO• → Ar₂N• + ROOH) and are more effective radical scavengers than phenols, but they discolour strongly on oxidation. Diphenylamines and TMQ are covered under aminic antioxidants; in practice, aminic antioxidants such as Irganox 5057 (0.1-0.4 % in polyols) and Naugard 445 are reserved for carbon-black rubber, polyurethane foam and nylon or TPU/TPE compounds, never for light-coloured articles or food packaging.

Why are hindered phenols "hindered"?#

Hindered phenols are called hindered because 2 bulky tert-butyl groups sit next to the O-H group, shielding it and stabilizing the phenoxyl radical so that it traps a second radical instead of starting a new chain. Modern phenolic antioxidants build this hindered structure onto a common propionate "head", 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, attached to anchors of very different molecular weight: BHT at 220 g/mol, Irganox 1076 at 531 g/mol, Irganox 1330 at 775 g/mol, Irganox 3114 at 784 g/mol and Irganox 1010 at 1,178 g/mol. The tert-butyl groups do the chemical work; the anchor decides how long the molecule stays in the plastic.

What happens to a phenolic antioxidant after it reacts?#

A phenolic antioxidant is used up as it works: its phenoxyl radical traps a second peroxyl radical and ends as a quinone methide or peroxycyclohexadienone, so each phenol group removes about 2 radicals before it is spent. This stoichiometric factor of about 2 has been measured for alpha-tocopherol, PMHC, BHA and BHT alike, and it is the reason antioxidants are described as sacrificial rather than catalytic: they are consumed in proportion to the oxidation they prevent, not regenerated indefinitely.

BHT itself has an inhibition rate constant of (1-2) × 10³ M⁻¹ s⁻¹ against growing PMMA radicals, measured by DSC at 70 °C, with a stoichiometric factor of 1 to 2; this value was measured against PMMA radicals specifically, not a generic peroxyl radical. Propionate-type quinone methides can rearrange once, back to a hydroxycinnamate that partially regenerates a phenol, though this recovery is limited. Quinone methides are also coloured, which links this reaction to the yellowing and gas-fading chemistry covered later on this page.

2. Hydroperoxide decomposers: phosphites, phosphonites and thioesters#

Hydroperoxide decomposers are secondary antioxidants that turn hydroperoxides into stable alcohols before they can split into new radicals, removing the fuel for chain branching. Phosphites and phosphonites dominate melt processing, while thioesters carry the same job during long-term heat ageing, and the two groups rarely overlap in their effective temperature range.

How phosphite and phosphonite antioxidants reduce hydroperoxides#

Phosphite and phosphonite antioxidants reduce a hydroperoxide to an alcohol and are themselves oxidised to a phosphate in the process: P(OR)₃ + ROOH → O=P(OR)₃ + ROH. Reactivity toward hydroperoxides falls in this order.

1. Phosphonites, the most reactive class, such as P-EPQ
2. Alkyl phosphites, more reactive than aryl types but less hydrolytically stable
3. Aryl phosphites, including Irgafos 168, balanced for reactivity and stability
4. Hindered aryl phosphites, the most hydrolytically stable but slowest to react, such as Ultranox 626 and its spiro-diphosphite relatives

This reactivity order, and the roughly reverse order for hydrolytic stability, was established by Humphris and Scott at Aston University in Pure and Applied Chemistry in 1973 and still guides grade selection today. Phosphites are consumed mainly during processing and contribute little to long-term protection once the part has cooled; Irgafos 168 (CAS 31570-04-4, MW 646.9 g/mol) is used at 0.05-0.2 wt% in polyolefins, most often paired with a phenolic at a phosphite-to-phenol ratio of 1:1 to 4:1. Grades and hydrolysis data for all phosphite and phosphonite antioxidants are on the class page.

How thioester antioxidants work in long-term heat ageing#

Thioester antioxidants such as DSTDP and DLTDP decompose hydroperoxides mainly during long-term heat ageing at about 100-150 °C, when they are oxidised to sulfoxides and sulfones that keep destroying hydroperoxides. Because thioesters need time and moderate heat to reach full activity, they contribute little during the fast, high-temperature pass through an extruder and instead protect the part over months or years in service.

Allen et al., writing in the Journal of Vinyl and Additive Technology in 2021, aged polypropylene at 150 °C and found that a 20:80 ratio of phenolic antioxidant to DSTDP gave the best long-term thermal stability, while an 80:20 ratio gave the best processing stability, a reversal that shows why formulators tune the phenol-to-thioester ratio to the failure mode they are protecting against. DLTDP, DSTDP and their group standard migration limit are covered under thioester antioxidants.

3. Carbon-radical scavengers: lactones, hydroxylamines and acrylated phenols#

Carbon-radical scavengers trap alkyl radicals (R•) directly, which ordinary phenols cannot do because R• reacts with oxygen at 10⁷-10⁹ L mol⁻¹ s⁻¹; the 3 commercial types are benzofuranone lactones, hydroxylamines and acrylated phenols. Because the alkyl radical is consumed by oxygen almost as soon as it forms, only a reaction that is itself close to diffusion-controlled can intercept it before it becomes a peroxyl radical, which is why this class exists as a separate mechanism from the peroxyl-radical chemistry of phenols.

Irganox HP-136 reacts at the weak C-H bond of its lactone ring and gives only medium chain-breaking performance in polypropylene at 180-200 °C on its own, but it is reported as an excellent UV co-stabilizer alongside a hindered amine light stabilizer. The Irganox HP-136 benzofuranone lactone substance page carries its full regulatory status. Irgastab FS 042 traps alkyl and peroxyl radicals to form nitrones and is used together with a phosphite in phenol-free systems, which are covered in more depth under phenol-free stabilization with hydroxylamines.

4. Metal deactivators: blocking metal-catalysed hydroperoxide decomposition#

Metal deactivators bind copper, iron and other transition-metal ions in stable complexes so they can no longer cycle between oxidation states and split hydroperoxides into radicals. Catalytic metals reach a polyolefin through catalyst residues (titanium, aluminium) or through direct contact with a metal substrate, and wire and cable insulation touching a copper conductor is the case where metal-catalysed oxidation is most severe.

Irganox MD 1024 (CAS 32687-78-8) combines a hindered-phenol structure with a metal-chelating hydrazide group, giving it both antioxidant and metal-deactivating function in a single molecule; Naugard XL-1 is a comparable hybrid built on an oxamide chelator. Hydrazide and oxamide types are covered on metal deactivators, the class page for this function across all polymer families, not only antioxidant-adjacent grades.

Which Antioxidant Class Acts at Which Step of the Autoxidation Cycle?#

Each antioxidant class blocks one specific step of the autoxidation cycle, which is why the table below reads like a map of where stabilizer packages intervene.

Class pages marked with a later publication day are unlinked until they are live; the phenolic, phosphite and metal-deactivator pages are already linked in the sections above, so their table cells stay unlinked here to keep each target linked once per page.

Why Are Antioxidant Blends Used? Synergy and Antagonism#

Antioxidant blends are used because the partners protect each other: a hindered phenol turns peroxyl radicals into hydroperoxides, and a phosphite then reduces those hydroperoxides to alcohols before they can branch. This relay is the single most common combination in commercial stabilizer packages, and BASF's Irganox B-series blends codify it directly: B 215 combines Irgafos 168 and Irganox 1010 at 2:1, B 225 at 1:1, and B 900 combines Irgafos 168 and Irganox 1076 at 4:1.

Patent literature records phenol-to-phosphite weight ratios from about 20:1 to 1:10 across different applications, far wider than the 1:1 to 4:1 range used in the standard commercial blends above, which shows how much a specific polymer and process can shift the optimal ratio. Formulators should not combine thioesters with HALS in outdoor parts, because acidic sulfur oxidation products deactivate the HALS. Ratios and cross-supplier equivalents for these combinations are listed under antioxidant blends and synergy, and interactions with HALS, pigments and fillers beyond antioxidants alone are mapped under additive interactions: synergy and antagonism.

Download the AO + HALS Pairing Chart (PDF): which antioxidant and light-stabilizer combinations work, and which cancel each other out. (email, role and company required)

Request quotes for phenol-phosphite blends, thioesters or phenol-free packages by grade or CAS number, volume and polymer, using the plastic additive supplier finder.

How Do Antioxidants Protect During Processing and During Service Life?#

Antioxidants protect a plastic in 2 separate windows: during melt processing at 150-320 °C, where phosphites and phenols carry the load, and during service at about 60-150 °C, where high-molecular-weight phenols and thioesters take over. The classes that dominate one window are often nearly inactive in the other, which is the practical reason no single antioxidant class is sold as a complete stabilizer package on its own.

Hindered amine light stabilizers are consistently less effective than phenolic antioxidants during melt processing, a finding reported by Gensler et al. in 2000 and confirmed by Gijsman and Fiorio in a 2023 study in Polymer Degradation and Stability, which is why HALS packages are always paired with a phenolic or phosphite for the processing step. Commodity PP resin carries under 400 ppm of phenolic base stabilization as it leaves the reactor.

Why do Irganox 1010 and Irganox 1076 outlast BHT?#

Irganox 1010 and Irganox 1076 outlast BHT because they are 2.4 to 5.3 times heavier (1,177.6 and 530.9 g/mol against 220.35 g/mol), so they evaporate and migrate far less during processing and service. Vapour pressure confirms the same ranking directly: Irganox 1076 has a vapour pressure of 2.5 × 10⁻⁷ Pa at 20 °C and Irganox 1010 only 7 × 10⁻¹⁰ Pa, more than 2 orders of magnitude lower, while higher molecular weight also slows the rate at which a molecule can diffuse to the part's surface and be lost.

Irganox 1010 (CAS 6683-19-8) was invented by M. Dexter and co-workers at Geigy, with a priority date of 5 January 1962, and remains the volume leader among plastic antioxidants today according to Pfaendner's 2025 history of plastic additives. Primary antioxidants in general sit in a molecular weight range of roughly 300-1,000 g/mol; below that range, volatility and extraction losses cut into the sacrificial protection an antioxidant can provide before it ever reacts.

BHT as an antioxidant for plastics remains in use where its low cost outweighs its volatility, typically in lower-temperature applications, while Irganox 1076 sits between BHT and Irganox 1010 on both molecular weight and volatility, a common choice in polyolefin film and fibre.

What Side Reactions Do Antioxidant Mechanisms Cause?#

The chemistry that makes antioxidants work also causes 3 side effects: coloured quinone methides from phenols, hydrolysis of phosphites, and transformation products such as 2,4-di-tert-butylphenol that migrate from the plastic. None of these side reactions negates the protection an antioxidant provides; each one is simply the visible trace of the sacrificial chemistry described earlier in this page.

Yellowing, pinking and gas fading from quinone methides#

Phenolic antioxidants turn plastics yellow or pink when nitrogen oxides from gas-fired heaters or forklift exhaust convert them into coloured quinone methides, an effect called gas fading. BHT converts specifically into stilbenequinone under this pathway, and the effect is most visible in white PP fibre, film and TPO stored in warehouses that use gas-fired forklifts or heaters.

Phenolics are not suitable for use in direct gas-fired ovens, while Irgafos 168 is; the practical remedy for gas fading is a phenol-free system built on a hydroxylamine and a phosphite instead of a hindered phenol, since N,N-dibenzylhydroxylamine has been shown to prevent gas fading of PP that contains a phenolic antioxidant. Pinking of white PE and PP is aggravated by low-treated titanium dioxide, nitrogen oxides, high pH, moisture and storage in darkness, and zinc stearate is reported to form colourless zinc-quinone complexes that counteract it. Troubleshooting steps for discoloration in general are collected under why plastics turn yellow or pink.

Phosphite hydrolysis#

Phosphites react with water as well as with hydroperoxides, and the hydrolysis releases phenols and acidic phosphorus species that cause black specks, feeding problems and corrosion. Aryl phosphites and phosphonites hydrolyse at 150-180 °C, releasing phenols such as 2,4-di-tert-butylphenol along with acidic hydrogen phosphites.

Spiro-diphosphites such as Ultranox 626 combine high hydroperoxide-decomposing activity with comparatively poor hydrolysis resistance, which is why suppliers add a trace amine such as triisopropanolamine to extend shelf life, or co-formulate with hydrotalcite, as in Songnox 6280, a 93:7 blend of Ultranox 626 with hydrotalcite. Second-generation spiro diphosphites, such as Doverphos S-9228, were developed specifically for better hydrolysis resistance than the first-generation grades.

Irgafos 168 transformation products and 2,4-DTBP#

Irgafos 168 is oxidised to its phosphate (AO168=O) as it destroys hydroperoxides, and its hydrolysis releases 2,4-di-tert-butylphenol (2,4-DTBP), a non-intentionally added substance (NIAS) found in food-contact films. Irgafos 168 itself carries no SML under EU Regulation (EU) No 10/2011 (FCM 671, overall migration limit applies), but its transformation products are tracked separately as occurrence data rather than as a regulated substance in their own right.

2,4-DTBP has been measured up to 45.568 ± 31.513 mg/kg in BOPP and LDPE food-contact products in one published survey, and AO168=O has been measured in urban PM2.5 in China at up to 851 ng/m³, with a median of 153 ng/m³. These figures describe measured occurrence, not a risk assessment, and they carry no health claim on this page; regulatory limits for Irgafos 168 and its phosphate are on the substance page.

How Are Polymer Oxidation and Antioxidant Activity Measured?#

Polymer oxidation and antioxidant activity are measured with 4 methods: oxidation induction time by DSC, melt flow rate after multiple extrusion passes, oven ageing to embrittlement, and the growth of carbonyl groups and yellowness. Each method targets a different stage of the oxidation and stabilization chemistry described above.

Oxidation induction time (OIT) by DSC#

Oxidation induction time (OIT) is the time a sample survives in pure oxygen at a fixed temperature, typically 190-220 °C, before the DSC records the exotherm of oxidation, and it measures how much active antioxidant is left. The test switches the DSC atmosphere from nitrogen to oxygen once the sample reaches temperature, and the clock starts at that switch; ISO 11357-6:2018 calls the result "oxidation induction time" and ASTM D3895 calls the equivalent measurement "oxidative-induction time".

OIT is mostly sensitive to residual phenolic antioxidant; phosphites and thioesters contribute little to the signal at 200 °C, and the test does not predict service life at the much lower temperatures a part actually experiences, roughly 60-110 °C. Knoben et al., in a 2025 study, used oxidation induction temperature rather than time to track antioxidant depletion through repeated recycling loops. Test conditions and limits of oxidative induction time (OIT) are covered on the test page.

Multiple-pass extrusion and melt flow rate#

Multiple-pass extrusion tests processing stability directly: the compound is extruded several times and the melt flow rate is measured after each pass, rising in PP as chains break and falling in PE as they crosslink. This is the standard processing-stability test for both polymers, with melt flow rate measured per ISO 1133, typically at 230 °C and 2.16 kg for PP.

Songwon's recyclate studies run multipass tests at 250 °C for PP and 220 °C for HDPE, and a 3-pass comparison of vitamin E against Irganox 1010 in HDPE, reported by Al-Malaika at Aston University, is a commonly cited example of the method in use. A rising melt flow rate in PP signals chain scission, the same outcome the Bolland-Gee cycle predicts for that polymer.

Long-term heat ageing in the oven#

Long-term heat ageing measures how long a stabilized plastic lasts in a hot-air oven before it embrittles, and it is the test that shows the value of thioesters and high-molecular-weight phenols. Samples are held at an elevated temperature, commonly in the range used for PP long-term thermal stability studies, and checked periodically until mechanical failure or visible embrittlement occurs; UL 746B uses a related protocol to assign a Relative Thermal Index for electrical and electronic parts.

Allen et al.'s 2021 study of PP aged at 150 °C, discussed above under thioester chemistry, is a direct example of this method distinguishing formulations that look identical by OIT. Oven methods and RTI ratings are explained in more depth under long-term heat aging.

Carbonyl index and yellowness index#

The carbonyl index tracks oxidation chemically, as the growth of the FTIR carbonyl band relative to a reference band, while the yellowness index to ASTM E313 tracks the colour that oxidation and antioxidant by-products create. The carbonyl index rises directly with the ketones and other carbonyl-containing products formed by β-scission, making it a chemical marker rather than a mechanical one.

Yellowness index is always measured to the current ASTM E313 edition, never to the withdrawn ASTM D1925 standard. How to calculate the carbonyl index from FTIR spectra is explained on the testing page.

How Does Thermo-Oxidation Differ from Other Types of Polymer Degradation?#

Thermo-oxidation differs from other types of polymer degradation because it needs both oxygen and heat and runs through the radical autoxidation cycle, whereas light, water and heat alone break polymers by other routes. All types of polymer degradation share overlapping radical chemistry, but each one starts from a different trigger, and 3 of the routes below do not need oxygen at all.

Thermal degradation without oxygen proceeds by depolymerization, or unzipping, as in PMMA, by side-group elimination, as in the hydrogen chloride loss from PVC, or by random chain scission; PET degrades mainly by hydrolysis, which cuts chains without any radical step. A short definition of thermo-oxidation sits in the glossary.

Photodegradation of plastics and HALS#

Photodegradation of plastics runs through the same peroxyl and hydroperoxide chemistry, but sunlight rather than heat supplies the initiating energy, which is why outdoor parts add light stabilizers to the antioxidant package. Chromophores already present in the polymer, including hydroperoxides, carbonyls and metal salts, absorb UV radiation above 290 nm and feed radicals into the same cycle, often through Norrish type II scission of ketone groups. Outdoor formulations pair antioxidants with hindered amine light stabilizers (HALS), which work through a separate, regenerative cycle.

HALS act through the Denisov cycle, named for E. T. Denisov's 1991 paper in Polymer Degradation and Stability, in which aminoxyl radicals trap alkyl radicals and are regenerated rather than consumed, a mechanism confirmed computationally by Hodgson and Coote at the Australian National University in Macromolecules in 2010. This light-driven chemistry is covered in full under photodegradation of plastics.

PVC thermal degradation and dehydrochlorination#

PVC degrades mainly by losing hydrogen chloride from its chain rather than by autoxidation, so PVC heat stabilizers are HCl scavengers, a different class from the antioxidants on this page. As dehydrochlorination proceeds, the chain builds up conjugated double bonds, and once a polyene sequence reaches 8 or more conjugated carbon-carbon double bonds it becomes visibly coloured. This naming overlap between PVC "heat stabilizers" and the antioxidants on this page is explained fully under PVC thermal degradation and dehydrochlorination.

Oxo-degradable plastics: oxidation on purpose#

Oxo-degradable plastics use the autoxidation cycle in reverse: iron, manganese or cobalt salts are added to speed up hydroperoxide decomposition so that the plastic fragments faster. These pro-oxidant salts accelerate exactly the chain-branching step that antioxidants exist to slow, turning a stabilization mechanism into a degradation one on purpose. The evidence and the rules on oxo-degradable plastics are reviewed separately from the mechanism described here.

Restabilization of recycled plastics#

Recycled plastics carry partly consumed antioxidants and oxidized groups from their first life, so restabilization adds a fresh phenol-phosphite package, typically 0.1-0.3 wt% of an Irganox 1010 / Irgafos 168 blend. Pfaendner's 2022 review in Polymer Degradation and Stability describes restabilization as replacing depleted primary and secondary antioxidant while also correcting for oxidized groups already present in the backbone, not simply topping up whichever additive ran out first.

PP regranulate reprocessed once without added antioxidant has been measured retaining over 150 ppm of intact Irgafos 168, enough for 1 to 2 further processing steps; in closed-loop recycling, where fresh antioxidant is added every cycle, Knoben et al.'s 2025 study found antioxidant can instead build up over successive loops. Dosage studies for PP and HDPE recyclate are on restabilization of recycled plastics.

Can plastics oxidize at room temperature?#

Yes: plastics oxidize at room temperature too, only much more slowly, and unstabilized polypropylene degrades over time even without heat, which is why resin producers add antioxidants before the pellets leave the plant. The same induction period described earlier still applies at room temperature; it simply stretches from minutes or hours at processing temperature to months or years in ambient storage.

Are antioxidants in plastics the same as antioxidants in food?#

Antioxidants in plastics and in food use the same radical-trapping chemistry, and 2 molecules in particular, vitamin E and BHT, serve both, but plastics antioxidants are chosen for heat resistance, low volatility and migration limits. Vitamin E, or alpha-tocopherol (CAS 10191-41-0), protects PE and PP at 100-300 ppm according to work by Al-Malaika at Aston University, and BHT carries a prior-sanctioned status under 21 CFR 181.24 for food packaging in the US, but this page covers only its plastics-stabilization use. Vitamin E and other natural and bio-based antioxidants for plastics are compared in full on a separate page.

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Sources#

• L. M. Smith, H. M. Aitken and Michelle Coote, "The Fate of the Peroxyl Radical in Autoxidation," Accounts of Chemical Research 51 (2018) 2006, https://pubs.acs.org/doi/abs/10.1021/acs.accounts.8b00250
• "Autoxidation," Wikipedia, https://en.wikipedia.org/wiki/Autoxidation
• E. T. Denisov, Polymer Degradation and Stability 34 (1991) 325-332, https://doi.org/10.1016/0141-3910(91)90126-C
• Allen et al., "Effect of antioxidant blend ratio on polypropylene thermal stability," Journal of Vinyl and Additive Technology (2021), https://4spepublications.onlinelibrary.wiley.com/doi/10.1002/vnl.21813
• BHT inhibition rate constant study, Chemistry and Physics of Lipids (2004), https://doi.org/10.1016/j.chemphyslip.2004.03.005
• PP regranulate residual Irgafos 168 study, Materials (2025), https://pmc.ncbi.nlm.nih.gov/articles/PMC11990476/
• Knoben et al. (2025), closed-loop recycling antioxidant depletion, https://pubmed.ncbi.nlm.nih.gov/40271840/
• 2,4-DTBP occurrence in food-contact films, https://pubmed.ncbi.nlm.nih.gov/30517180/
• AO168=O occurrence in urban PM2.5, https://pubmed.ncbi.nlm.nih.gov/32786564/
• Mordor Intelligence, "Plastic Antioxidant Market," 2025, https://www.mordorintelligence.com/industry-reports/plastic-antioxidant-market