Antioxidants for Polypropylene: 5 Stabilizer Classes, Dosage and Selection

Antioxidants for polypropylene are stabilizers, mainly a hindered phenol such as Irganox 1010 (0.05-0.4 wt%) combined with a phosphite such as Irgafos 168 (0.05-0.2 wt%), that stop oxygen from breaking PP chains during extrusion and in service. PP needs them more than any other commodity plastic because a tertiary carbon on every repeat unit makes it the most oxidation-sensitive; so which antioxidant, at which ratio, suits which PP product?

Polypropylene takes 34.68 % of the revenue of the plastic antioxidant market (Mordor Intelligence, 2025), the largest share of any polymer, which makes it the main user of this group of plastic additives.

This guide explains the oxidation chemistry behind that sensitivity, the 5 antioxidant classes that counter it and the classes to avoid, why phenol-phosphite blends dominate, the package for each PP application, dosage, additive interactions, oxidative-stability testing, food-contact limits and suppliers.

Key figures

• 0.05-0.4 wt% Irganox 1010 in polyolefins (BASF technical data sheet)
• 1:1 to 4:1 phosphite:phenol in standard PP blends
• Less than 400 ppm phenolic antioxidant in commodity PP grades leaving the reactor
• 0.25 wt% FDA legal maximum for Irgafos 168 in propylene polymers (21 CFR 178.2010)

Why Does Polypropylene Need Antioxidants?#

Polypropylene needs antioxidants because every repeat unit carries a tertiary C-H bond, the weakest point for oxidation, so heat and oxygen cut PP chains (β-scission) during extrusion, moulding and long-term use. This tertiary carbon, absent from polyethylene's chain, is the main reason PP oxidizes faster than any other commodity polyolefin, and every commercial PP resin already carries a base antioxidant package before it reaches a converter.

The hub on antioxidants for plastics compares the same classes across all polymers, from the phenol-phosphite package used in PP to the different balance point needed by polyethylene, PVC or engineering plastics. Unlike PE, which crosslinks and forms gels as it oxidizes, PP undergoes chain scission, so its melt flow rate rises rather than falls as oxidation proceeds, a contrast covered in full below.

How does polypropylene oxidize during processing and use?#

Polypropylene oxidizes through autoxidation: heat and shear form alkyl radicals, oxygen turns them into peroxyl radicals, these create hydroperoxides that split into new radicals, and the tertiary alkoxyl radicals cut the PP chain.

The autoxidation cycle in PP runs in 5 steps.

1. Initiation: heat, shear and residual Ziegler-Natta or metallocene catalyst metals (titanium, aluminium), together with trace copper, iron, manganese and cobalt ions picked up during compounding, generate alkyl radicals (R•) on the polymer backbone.
2. Propagation to peroxyl radicals: each alkyl radical reacts with dissolved oxygen at a diffusion-controlled rate of 10^7 to 10^9 L mol-1 s-1, forming a peroxyl radical (ROO•).
3. Hydrogen abstraction: the peroxyl radical abstracts a hydrogen atom, most readily from a tertiary C-H bond, producing a hydroperoxide (ROOH) and a new alkyl radical that restarts the cycle.
4. Chain branching: the hydroperoxide splits into an alkoxyl radical and a hydroxyl radical (RO• and •OH), each able to start a fresh oxidation chain; this branching makes the process autocatalytic, with a measurable induction period before oxygen uptake rises sharply (sigmoidal uptake curve).
5. Chain scission: the tertiary alkoxyl radical undergoes β-scission, breaking the PP backbone into two shorter chains and raising the melt flow rate.

J. L. Bolland and G. Gee first described this basic autoxidation scheme at the British Rubber Producers' Research Association in the 1940s. Smith, Aitken and Coote (Australian National University, Acc. Chem. Res. 51 (2018) 2006) later refined step 3, showing that direct hydrogen abstraction by ROO• from a saturated polymer such as PP is thermodynamically disfavoured, so chain defects and alkoxyl radicals carry a larger share of the propagation than the textbook cycle alone suggests. The metal traces named in step 1 accelerate branching by cycling between oxidation states (for example Fe2+/Fe3+ or Cu+/Cu2+); the full kinetics are covered under polymer oxidation and antioxidant mechanisms.

What are the signs of oxidative degradation in polypropylene?#

Oxidatively degraded polypropylene shows 4 typical signs: a rising melt flow rate, falling elongation at break, embrittlement, and yellowing or pinking.

• Rising melt flow rate, because β-scission shortens the average chain length and lowers the melt viscosity.
• Falling elongation at break, as the narrowing molecular weight distribution reduces the polymer's ability to draw before it fractures.
• Embrittlement, the practical end point used in oven-ageing tests once the part can no longer flex without cracking.
• Yellowing or pinking, caused by the coloured quinone methides formed when the phenolic antioxidant itself is consumed.

Can polypropylene be processed without antioxidants?#

No: unstabilized polypropylene cannot be processed commercially, because it loses molecular weight in the extruder and degrades slowly even at room temperature. Even the base commodity grades that leave the polymerization reactor already carry less than 400 ppm of a phenolic antioxidant, because extrusion and injection moulding melt temperatures, typically above 200 °C, would otherwise degrade the resin within a single pass.

What Are the 5 Types of Antioxidants for Polypropylene?#

The 5 types of antioxidants used in polypropylene are hindered phenols, phosphites and phosphonites, thioesters, hydroxylamines and benzofuranone lactones, and almost every PP grade combines the first two. Hindered phenols and phosphites (or phosphonites) form the standard package used across commodity, fibre, automotive and food-contact PP. Table 1 lists the role, radical target, main PP job and example grades for each class, in the order used throughout this guide.

Table 1. The 5 antioxidant classes used in PP

1. Hindered phenols (primary antioxidants)#

Hindered phenols are primary antioxidants that give a hydrogen atom to peroxyl radicals in PP, stopping the oxidation chain; two ortho tert-butyl groups stabilize the phenoxyl radical left behind, so each phenol group traps about 2 radicals.

The reaction ROO• + ArOH → ROOH + ArO• converts a reactive peroxyl radical into a stable hydroperoxide and a resonance-stabilized phenoxyl radical. The bulky ortho tert-butyl substituents shield the phenolic oxygen and keep that phenoxyl radical from starting a new chain. The stoichiometric factor of about 2 radicals trapped per phenol group means the antioxidant is progressively consumed, and its main end products are coloured quinone methides.

Most commercial grades are tetrafunctional or monofunctional esters with a molecular weight of 300-1000 g/mol, balancing volatility loss against melt dispersion. Every grade and its dosage are listed under phenolic antioxidants (hindered phenols). Irganox 1010, patented by M. Dexter and co-workers at Geigy on 5 January 1962, remains the industry's volume leader (Pfaendner, 2025).

Irganox 1010 and Irganox 1076: the reference phenols for PP#

Irganox 1010 (1,177.6 g/mol) and Irganox 1076 (530.9 g/mol) are the two reference phenols for PP: the tetrafunctional 1010 is the less volatile of the two and carries long-term heat stability, while 1076 is a monofunctional octadecyl ester used in film and fibre applications.

Irganox 1010 is the volume leader among polymer antioxidants, used at 0.05-0.4 wt% in polyolefins per BASF's technical data sheet, with a melting point of 110-125 °C and a vapour pressure of 7 x 10^-10 Pa at 20 °C that keeps loss during compounding low.

Table 1a. Reference phenols for PP

Irganox 1076 carries an EU specific migration limit of 6 mg/kg (FCM 433) against Irganox 1010's unrestricted overall migration limit, and its lower melting point and higher vapour pressure make it the more volatile of the two. Supplier use tables list 1076 for PE and PP film; a direct performance verdict between the grades is answered in the supplementary comparison section below.

Low gas-fade phenols for white PP: Irganox 3114, Irganox 1425 and Cyanox 1790#

Irganox 3114, Irganox 1425 and Cyanox 1790 are phenols chosen for white PP fibre and film because they discolour less under NOx exposure (gas fading) than standard propionate phenols.

• Irganox 3114 (CAS 27676-62-6), an isocyanurate-type phenol with a molecular weight of 784.1 g/mol and a melting point of 218-223 °C, listed under EU FCM 661 with an SML of 5 mg/kg and an FDA maximum of 0.25 % in PP.
• Irganox 1425 (CAS 65140-91-2), a calcium phosphonate phenol used for gas-fade-resistant fibre stabilization, listed under EU FCM 715 with an SML of 6 mg/kg and an FDA maximum of 0.25 % in PP.
• Cyanox 1790 (CAS 40601-76-1), a partially hindered isocyanurate phenol, listed under EU FCM 689 with an SML of 6 mg/kg and an FDA maximum of 0.1 % in olefin polymers.

A typical PP dosage for these three grades is not established in current supplier technical literature; formulators size them against the FDA maxima above and confirm the final level in trial.

2. Phosphites and phosphonites (Irgafos 168 type)#

Phosphites and phosphonites are secondary antioxidants that reduce hydroperoxides to harmless alcohols while being oxidized to phosphates, which is why they protect melt flow and colour during PP extrusion but contribute little once the part is in service.

The reaction P(OR)3 + ROOH → O=P(OR)3 + ROH destroys the hydroperoxide before it can split into new radicals, breaking the autoxidation cycle at the branching step rather than the propagation step phenols intercept. Reactivity follows the order phosphonites > alkyl phosphites > aryl phosphites > hindered aryl phosphites, and because phosphites are consumed stoichiometrically during melt processing, they add little to long-term heat ageing once compounding is complete. The hydroperoxide decomposer role of phosphite and phosphonite antioxidants makes them essential wherever melt flow retention and initial colour matter, and hydrolysis data for the full class sit on that page.

Irgafos 168 (CAS 31570-04-4, MW 646.9 g/mol, mp 183-186 °C) is the reference phosphite for PP at 0.05-0.2 wt% in polyolefins. It is hydrolytically more stable than several alternatives, which lets it survive humid storage before compounding, and it degrades in service to tris(2,4-di-tert-butylphenyl) phosphate (AO168=O) and 2,4-di-tert-butylphenol. Irgafos 168 also tolerates the direct gas-fired ovens where standard phenolics discolour, because a phosphite alone does not form the coloured quinone methides gas fading produces.

Hydrolysis-resistant phosphites: antioxidant 626, S-9228 and PEP-36#

Antioxidant 626 is a more active phosphite than Irgafos 168 but hydrolyses more easily, so grades such as Doverphos S-9228 and hydrotalcite-buffered 626 blends exist for humid storage and high-temperature PP processing.

Hydrolysis of a phosphite at 150-180 °C releases free phenols and acidic hydrogen phosphites, causing feeding problems and black specks in the extruder, so higher-activity phosphites need protection or a more hydrolysis-stable backbone. Antioxidant 626, also sold as Irgafos 126 and ADK STAB PEP-24 (CAS 26741-53-7, MW 604.7 g/mol, mp 170-180 °C), is often supplied with a trace amine stabilizer and, as SONGNOX 6280, pre-blended with magnesium-aluminium hydrotalcite at 93:7 for extra hydrolysis protection. Doverphos S-9228 (CAS 154862-43-8, MW 853, mp above 228 °C) is a second-generation phosphite built for better hydrolysis resistance, and ADK STAB PEP-36 (CAS 80693-00-1) fills the same role.

Table 1b. Hydrolysis-resistant phosphites

3. Thioesters (DSTDP and DLTDP)#

Thioesters such as DSTDP and DLTDP are thiosynergists: they decompose hydroperoxides through sulfoxide and sulfone intermediates, which makes them effective in long-term heat ageing of PP at about 100-150 °C but of little use in the melt.

Distearyl thiodipropionate (DSTDP, CAS 693-36-7, mp 63.5-68.5 °C, also sold as Irganox PS 802) and dilauryl thiodipropionate (DLTDP, CAS 123-28-4, mp 38-41 °C, also sold as Irganox PS 800) oxidize stepwise to sulfoxides and then sulfones, and each intermediate keeps decomposing hydroperoxides as the thioester is consumed. Both share EU 10/2011 group restriction 14, with a combined SML(T) of 5 mg/kg. A blend ratio of Irganox 1010 to DSTDP of 20:80 gives the optimal long-term thermal stability of PP at 150 °C, while 80:20 favours processing stabilization instead.

Thioesters are kept out of outdoor PP that relies on HALS, because their acidic sulfur oxidation products deactivate it. The chemistry of thioester antioxidants is covered on the class page, including their odour and colour trade-offs. A fixed phenol-to-thioester dosage ratio for PP is not established in sourced literature, so formulators size DSTDP and DLTDP against the 150 °C study ratio above rather than a rule of thumb.

4. Hydroxylamines for phenol-free stabilization#

Hydroxylamines such as Irgastab FS 042 replace the phenol in PP fibre and white TPO: combined with a phosphite at 0.05-0.15 %, they trap alkyl and peroxyl radicals without forming the quinone methides that cause gas fading.

Irgastab FS 042 (oxidized bis(hydrogenated tallow alkyl)amine, CAS 143925-92-2, melting point 90-98 °C) is typically used with a phosphite such as Irgafos 168 in PP fibre and automotive TPO. Because it produces no phenoxyl radical, the white or light-coloured parts it stabilizes resist the yellow and pink discolouration that phenolic systems can develop under NOx exposure.

Irgastab FS 042 is listed under EU FCM 768 at a maximum of 0.1 % in polyolefins, not for fatty foods under simulants D1 and D2, and the FDA sets the same 0.1 % maximum in PP. Phenol-free stabilization compares hydroxylamine and lactone systems, including N,N-dibenzylhydroxylamine (US patent 4,590,231, 1986), used against gas fading in phenolic PP.

5. Benzofuranone lactones#

Benzofuranone lactones such as Irganox HP-136 are carbon-radical scavengers: they intercept alkyl radicals before oxygen reaches them, so they are dosed at very low levels into phenol-phosphite blends for demanding PP processing.

Irganox HP-136 (5,7-di-tert-butyl-3-(3,4-dimethylphenyl)-3H-benzofuran-2-one, CAS 181314-48-7, MW 350.5 g/mol) traps both carbon-centred alkyl radicals and oxygen-centred radicals. Alkyl radicals react with oxygen at a diffusion-controlled rate, so only lactones, hydroxylamines and acrylated phenols can intercept them before oxidation proceeds, which is why HP-136 is used at very low levels inside phenol-phosphite blends rather than alone. It shows medium chain-breaking activity in PP at 180-200 °C, listed under EU FCM 26 with an SML of 5 mg/kg and an FDA maximum of 0.1 % in olefin polymers for listed food types or 0.02 % in PP for other food types. A typical PP dosage is not established beyond these maxima, and HP-136 has been marketed as a processing additive for demanding PP compounds, though its current availability in every region is not verified.

Which Antioxidants Do Not Suit Polypropylene?#

5 antioxidant types are poor choices for polypropylene: aminic antioxidants (they discolour), BHT (it evaporates), TNPP and AO-2246 (both on the EU SVHC Candidate List), and thioesters in HALS-stabilized outdoor parts.

• Aminic antioxidants, more effective radical scavengers than phenols in some polymers but prone to strong discolouration, which keeps them out of food packaging and light-coloured PP articles; Naugard 445 is FDA-listed only up to 0.3 % in PP for non-fatty foods.
• BHT (butylated hydroxytoluene, CAS 128-37-0, MW 220.35 g/mol), too small and volatile for modern high-temperature PP processing, and displaced in commodity PP by the higher-molecular-weight phenols such as Irganox 1010 and 1076.
• TNPP (tris(nonylphenyl) phosphite, CAS 26523-78-4), on the EU SVHC Candidate List since 16 July 2019 for endocrine-disrupting properties to the environment under REACH Article 57(f).
• AO-2246 (2,2'-methylenebis(4-methyl-6-tert-butylphenol), CAS 119-47-1), on the SVHC Candidate List since 17 January 2022 as a reproductive toxicant, harmonised Repr. 1B H360F.
• Thioesters in HALS-stabilized outdoor PP, because the acidic sulfur oxidation products of DSTDP and DLTDP deactivate the hindered amine light stabilizers that protect the part against UV.

BHT as an antioxidant for plastics explains its remaining niche uses and its regulatory status in more detail; it is not on the SVHC Candidate List, and it remains listed under EU FCM 315 with an SML of 3 mg/kg for the applications where its volatility is not a drawback.

Why Are Phenol and Phosphite Antioxidant Blends Used in Polypropylene?#

Phenol and phosphite antioxidant blends are used in polypropylene because the two work in sequence: the phenol stops peroxyl radicals and forms hydroperoxides, and the phosphite destroys those hydroperoxides before they split into new radicals.

This synergy also reduces the coloured quinoid species that a phenol alone can generate, because the phosphite consumes the hydroperoxide intermediate before it degrades into a chromophore. Standard PP blends set the phosphite-to-phenol ratio between 1:1 and 4:1, well inside the wider 20:1 to 1:10 range seen across patent literature for other polymer systems.

Table 2. Standard phenol-phosphite blends

Blend compositions per supplier technical data; equivalents are listed in the suppliers section below.

Irganox B 215 combines Irgafos 168 and Irganox 1010 at a 2:1 phosphite-to-phenol ratio, that is 67 % phosphite and 33 % phenol by weight, a composition some distributor sources outside the primary literature state in reverse. Irganox B 225 uses the same two components at 1:1, and Irganox B 900 pairs Irgafos 168 with Irganox 1076 at 4:1. Every B-blend and its equivalents are listed under antioxidant blends and synergy, including the SONGNOX 11B and 21B binary blends used to restabilize recycled PP.

Which Antioxidant Package Is Best for Each Polypropylene Application?#

The best antioxidant package for polypropylene depends on processing severity, service temperature, colour demands and food contact: a phenol-phosphite blend for most grades, a hydroxylamine system for white fibre and TPO, and an added thioester for parts that age at high temperature.

Table 3. Master antioxidant package table for PP applications

Supplier TDS and study levels; trials decide the final level. FDA percentages are legal maxima, not recommended dosages.

Request quotes for antioxidants and antioxidant blends for polypropylene: grade or CAS, volume, form (powder, pastille, masterbatch), application, country. Get supplier quotes

Download the AO + HALS Pairing Chart for PP (PDF). Enter your email, role and company to receive the pairing chart, shared with the UV stabilizer sibling guide.

Commodity PP resin and injection-moulded parts#

Commodity PP resin leaves the reactor with a base package of less than 400 ppm phenolic antioxidant plus a phosphite and an acid scavenger, and converters add more only when the part faces repeated processing or heat.

Mayer and co-workers at Fraunhofer LBF (Polymers, 2023) measured this base level across commercial PP grades and confirmed it stays below 400 ppm phenolic antioxidant, consistent with Dopico-García and co-workers (2007), who found commercial PP packaging typically contains Irganox 1010 and 1076 alongside Irgafos 168 and its oxidized phosphate. Calcium stearate covers the acid-scavenger role in the same base package, and moulders raise the phenol or phosphite level above this floor only for parts facing multiple regrind passes or elevated service temperature.

PP fibres, nonwovens and film#

PP fibres, nonwovens and white film need gas-fade-resistant antioxidants, because NOx from gas-fired forklifts and heaters turns standard phenols into yellow or pink quinone methides during storage.

A hydroxylamine such as Irgastab FS 042 combined with a phosphite, or a low gas-fade phenol such as Irganox 3114, Irganox 1425 or Cyanox 1790, avoids the gas-fading reaction that standard propionate phenols undergo. Irganox 1076 is the phenol most often named for PE and PP film in supplier use tables, where its lower melting point aids dispersion in thin sections. The full fibre package is on additives for synthetic fibres, textiles and nonwovens.

Automotive PP and TPO#

Automotive PP and TPO combine a high-molecular-weight phenol and a phosphite with a thioester for under-the-bonnet heat, or with a hydroxylamine where white or light-coloured parts must not gas-fade.

Under-hood parts add long-term heat resistance through a thioester such as DSTDP alongside the phenol-phosphite base, while interior trim and light-coloured TPO more often use Irgastab FS 042 to avoid gas fading. Fogging resistance is a selection criterion for interior compounds, though sourced fogging values are not established for PP antioxidants specifically, and thioester and HALS antagonism must be checked before combining a thioester package with an outdoor UV system. Additives for automotive plastics covers the interior and under-the-bonnet packages in full.

PP pipes and parts in copper contact#

PP pipes and parts in contact with copper need two additions to the base package: a thioester for long-term heat ageing and a metal deactivator such as Irganox MD 1024, because copper ions catalyse the breakdown of hydroperoxides.

PP-R (random copolymer PP) is used for potable plumbing and hydronic heating pipe, where decades of service at elevated temperature demand long-term thermal stabilization beyond the standard phenol-phosphite package. Irganox MD 1024 (CAS 32687-78-8) combines a phenol with a copper deactivator, listed under EU FCM 675 with an SML of 15 mg/kg for PE and PP wire, cable and pipe in contact with copper, since copper ions accelerate hydroperoxide decomposition through redox cycling between Cu+ and Cu2+. EN 13476-3 Annex D lists oxidation induction time among the agreed specification items for PP recyclate in multilayer pipe systems. Additives for plastic pipes covers full pressure-pipe requirements; a metal deactivator complements the base package rather than replacing it.

Talc- and mineral-filled PP#

Talc- and mineral-filled PP needs a stronger antioxidant package than unfilled PP, because talc adsorbs part of the antioxidant and fillers bring metal impurities that speed up oxidation.

Talc grades from T10 to T40 dominate automotive interior and under-hood PP compounds, and calcium carbonate fillers at 20-40 % loading are common elsewhere. Some clays adsorb or deactivate part of the antioxidant charge, so formulators either raise the antioxidant level or add an epoxy-based talc deactivator; a numeric uplift factor is not established in current supplier literature, so the increase is set by trial. Fillers for polypropylene details talc and calcium carbonate loadings.

Food-contact PP packaging#

Food-contact PP packaging uses the same phenol-phosphite chemistry as technical PP, but only grades listed in EU Regulation (EU) No 10/2011 and 21 CFR 178.2010, such as Irganox 1010, Irganox 1076 and Irgafos 168, within their migration or weight limits.

The food-contact status of every grade named on this page is set out in the additives for food packaging guide and in the dedicated food-contact section below, which lists the EU specific migration limits and FDA maxima side by side.

Recycled PP: restabilization#

Recycled PP is restabilized with 0.1-0.3 wt% of a phenol-phosphite blend, but the dose should follow a measurement of the residual antioxidant, because closed-loop studies show that re-dosing every cycle builds up unused phosphite.

R. Pfaendner, formerly of Ciba and now at Fraunhofer LBF, laid out the restabilization concept in a 2022 review (Polym. Degrad. Stab. 203 (2022) 110082), and Songwon's SONGNOX 1010/168 binary blends are tested at 0.1-0.3 wt% in multipass extrusion at 250 °C. Knoben, Vanhouttem, Wypkema and Subramanian (Materials 18 (2025) 1640) measured un-restabilized PP regranulate still carrying more than 150 ppm of intact Irgafos 168; in a closed-loop trial re-dosing 500 ppm primary plus 1,000 ppm secondary antioxidant every cycle, residual Irgafos 168 built from about 650 ppm at cycle 1 to more than 1,200 ppm by cycle 5, while oxidation induction temperature rose from 198 °C unstabilized to 257 °C restabilized. The Packaging and Packaging Waste Regulation (EU) 2025/40 sets recycled-content targets from 2030, pushing compounders toward measured restabilization over a fixed top-up dose. Dosing strategies are covered under restabilization of recycled plastics.

How Much Antioxidant Does Polypropylene Need?#

Polypropylene typically needs 0.05-0.4 wt% of a phenolic antioxidant and 0.05-0.2 wt% of a phosphite, with the exact level set by processing severity, service temperature, colour demands and fillers.

4 factors set the exact antioxidant level within these ranges.

• Processing severity, meaning the number of extrusion or moulding passes and the peak melt temperature the resin sees.
• Service temperature and expected lifetime of the finished part.
• Colour demands and NOx exposure, such as proximity to gas-fired ovens or forklifts.
• Fillers, pigments and metal contact, each of which can consume or catalyse the antioxidant package.

A masterbatch containing 10 wt% of a 1:1 Irganox 1010 to Irgafos 168 blend, let down at 2 % into the base resin, gives 0.2 wt% total antioxidant in the finished part (0.10 x 2 % = 0.2 %, or 0.1 wt% of each component). This is an arithmetic worked example rather than a specific supplier product, and the same calculation can be checked against the site's let-down ratio calculator. Vitamin E is used at a lower level, 100-300 ppm, when a bio-based melt stabilizer is chosen instead of a synthetic phenol. FDA maxima quoted elsewhere in this guide are legal ceilings for the amount reasonably required to achieve a technical effect, never a recommended dosage.

How Do Antioxidants Interact with Other Additives in Polypropylene?#

Antioxidants in PP work inside a larger package: acid scavengers protect them from catalyst acids, fillers and some pigments consume or discolour them, and thioesters, if added, weaken the HALS of outdoor grades.

Table 4. Co-additive effects on the PP antioxidant package

Ziegler-Natta and metallocene catalyst residues leave trace acids in PP that attack antioxidants and equipment alike, hydrolyse phosphites and antagonize both phenols and hindered amine light stabilizers, which is why an acid scavenger belongs in every PP formulation rather than only in filled or pigmented grades. Additive interactions maps synergy and antagonism across every additive family on the site, including the hydrotalcite-phosphite protection described below.

Acid scavengers: calcium stearate and hydrotalcite#

Calcium stearate at 0.05-0.20 % or hydrotalcite neutralises the chloride left by Ziegler-Natta catalysts in PP, which protects phosphites from hydrolysis and leaves more Irgafos 168 intact after compounding.

Calcium stearate (CAS 1592-23-0) is used at 0.05-0.20 % as an acid scavenger in polyolefins, up to about 1,000 ppm per supplier technical literature (Peter Greven); Espelage and co-workers (Polymers, 2025, with Borealis-supplied material) found more Irgafos 168 survives compounding when it is present. Acid scavengers for polyolefins compares it against hydrotalcite and zinc oxide in full.

Hydrotalcite (CAS 12304-65-3, EU FCM 604, or CAS 11097-59-9, FCM 592) is the alternative acid scavenger in PP and PE, chosen for catalyst-residue neutralization in BOPP and raffia; its FDA food-contact notification status is not listed in 21 CFR 178.2010. Calcium stearate also serves as a lubricant in the same formulations.

Why does antioxidant-stabilized PP turn yellow or pink?#

Antioxidant-stabilized PP turns yellow or pink when its phenolic antioxidant is over-oxidized into coloured quinone methides, most often by NOx from gas-fired equipment (gas fading) or by interaction with low-treated titanium dioxide.

4 measures prevent yellowing and pinking in PP.

1. Switch to a phenol-free system that pairs a hydroxylamine with a phosphite.
2. Raise the phosphite share of the phenol-phosphite blend, since phosphites do not form quinone methides.
3. Use a low gas-fade phenol such as Irganox 3114, Irganox 1425 or Cyanox 1790 in white fibre and film.
4. Add zinc stearate to TiO2-pigmented grades, which forms colourless zinc-quinone complexes instead of pink ones.

Pinking specifically follows from low-treated rutile titanium dioxide, high pH, moisture and storage in darkness acting together with the phenolic antioxidant. More detail on other discolouration causes across the additive system is under why plastics turn yellow or pink.

How Is the Oxidative Stability of Polypropylene Tested?#

The oxidative stability of polypropylene is tested by multiple-pass extrusion with melt flow and colour checks, by oxidation induction time in a DSC, and by oven ageing to embrittlement.

Multiple-pass extrusion: melt flow rate and colour#

Multiple-pass extrusion is the standard processing-stability test for PP: the compound is re-extruded several times, and melt flow rate (230 °C / 2.16 kg) and yellowness index are measured after each pass.

Recycled PP is commonly tested through 5 passes at 250 °C as supplier practice, while the resin's melt flow rate is measured to ISO 1133-1 or ASTM D1238-26 at 230 °C and 2.16 kg. Yellowness index follows ASTM E313-20 (reapproved 2025), never the withdrawn ASTM D1925 (1995). No numeric pass or fail criterion is established in current standards for this test; the end point is set by the individual product specification.

Oxidation induction time (OIT)#

Oxidation induction time measures how long a PP sample resists oxidation in pure oxygen at a fixed temperature, typically 190-220 °C, in a DSC to ISO 11357-6 or ASTM D3895, and it mainly reflects the phenolic antioxidant still active.

The DSC heats the sample under nitrogen to the test temperature, then switches to oxygen at that constant temperature and records the time to the onset of an exothermic oxidation peak; ISO 11357-6:2018 also allows a dynamic oxidation onset temperature (OOT) measurement in the same series, and ISO 11357-6 and ASTM D3895-19 are not technically equivalent. OIT mainly reflects the residual phenolic antioxidant present at the test temperature and can overrate or underrate volatile grades (ASTM D3895 Note 2); phosphites and thioesters contribute little at 200 °C, and OIT does not predict service life at a part's actual, lower operating temperature. Knoben and co-workers (2025) used oxidation induction temperature instead to screen recycled PP. Test conditions are detailed on oxidative induction time (OIT).

Oven ageing to embrittlement (ASTM D3012)#

Oven ageing to ASTM D3012 rates the long-term heat stability of moulded PP as the number of days a rotating specimen survives at a set temperature, such as 150 °C, before it embrittles.

The standard specifies a forced-draft oven with a specimen rotator, and the result is the number of days to embrittlement rather than a pass or fail value. In a supplier trial on recycled PP at 150 °C, unstabilized material embrittled after about 25 days, while material carrying 0.2-0.4 % of a Songwon test stabilizer lasted about 37 to 42 days, driven by the same phenolic and thioester chemistry described earlier. ISO 4577 is a related method; its detailed parameters are not established in our source library. Long-term heat aging compares oven methods across the site.

Measuring antioxidant content in PP (ASTM D6042)#

The antioxidant content of PP is measured to ASTM D6042 by extracting the additives with a cyclohexane and methylene chloride mixture and quantifying Irganox 1010, 1076, 3114 and Irgafos 168 by liquid chromatography down to about 2 ppm.

The method extracts PP additives under reflux or ultrasonic conditions with a cyclohexane:methylene chloride mixture, then separates and quantifies them by liquid chromatography with UV detection at 200 nm against an internal standard. Besides the antioxidants named above, the same run also covers erucamide and vitamin E. Additive analysis covers the other extraction and spectroscopic methods used across the family, including HPLC, GC-MS and FTIR.

Which Antioxidants Are Allowed in Food-Contact Polypropylene?#

Food-contact PP may contain only antioxidants listed in Regulation (EU) No 10/2011 (within each substance's specific migration limit, or the overall limit of 10 mg/dm2 where none is set) or in 21 CFR 178.2010 (within each polymer's weight limit), which rules out unlisted grades such as Irganox 1135 in the EU.

Food contact antioxidants covers the same EU and FDA framework across every polymer, not PP alone. The two systems differ structurally: the EU sets a specific migration limit in mg per kg of food, or the overall migration limit where none is set, while the FDA caps the weight percentage usable in the polymer under 178.2010(a), the amount reasonably required to achieve the intended technical effect.

EU 10/2011 and FDA 21 CFR 178.2010 limits for PP antioxidants#

Irgafos 168 shows how the two systems differ: the EU sets no specific migration limit for it (FCM 671, overall limit applies), while the US caps it at 0.25 wt% of propylene polymers under 21 CFR 178.2010.

Values in EU 10/2011 below are checked against the consolidated text of 16 March 2025.

Table 5. Food-contact matrix for PP antioxidants

EU values checked against the consolidated text of 16 March 2025. FDA limits are maximum use levels, not recommended dosages. Check food type and condition of use in 21 CFR 178.2010.

21 CFR 178.2010 sets these maxima by polymer type and, in several cases, by food-contact condition of use, so the percentage that applies to PP is not automatically the same one that applies to another polymer using the same substance. FDA maxima are legal ceilings, never recommended dosages.

SVHC-listed antioxidants: TNPP, AO-2246 and 2,4,6-TTBP#

3 antioxidant-related substances are on the EU SVHC Candidate List: TNPP (since 16 July 2019), AO-2246 (since 17 January 2022) and 2,4,6-tri-tert-butylphenol (since 23 January 2024), while Irganox 1010, Irganox 1076 and Irgafos 168 are not listed as of September 2026.

TNPP (CAS 26523-78-4) was added for endocrine-disrupting properties to the environment under REACH Article 57(f), where the substance contains 0.1 % or more of 4-nonylphenol; its entry was updated on 21 January 2025 to also cover its own intrinsic endocrine-disrupting properties, and it carries harmonised hazard statements H317, H400 and H410, triggering Article 33 communication and SCIP notification above 0.1 % w/w in an article. AO-2246 (CAS 119-47-1) was added for reproductive toxicity, harmonised Repr. 1B H360F, yet it remains authorised in EU 10/2011 under FCM 285, group restriction 13. 2,4,6-tri-tert-butylphenol (CAS 732-26-3), an antioxidant intermediate and impurity rather than a standalone product antioxidant, was added on 23 January 2024 for reproductive toxicity and PBT properties. The SVHC Candidate List tracks every listed plastic additive as new entries appear.

NIAS from PP antioxidants: 2,4-DTBP and oxidized Irgafos 168#

The main NIAS from PP antioxidants are the breakdown products of Irgafos 168: its phosphate (AO168=O) and 2,4-di-tert-butylphenol, which Qian and co-workers (PLoS One, 2018) found at up to about 45.6 mg/kg in BOPP and LDPE food-contact products.

2,4-di-tert-butylphenol is not itself on the EU 10/2011 Union list and is treated as a non-intentionally added substance rather than an authorised additive. Several food-contact SMLs in the matrix above are expressed as a sum with oxidation or hydrolysis products for this reason, including those for PEP-36, S-9228 and ADK STAB AO-80. Markley and co-workers at the FDA's Center for Food Safety and Applied Nutrition (Food Chem. Toxicol., 2023) assessed Irgafos 168 with an acceptable daily intake of 1 mg/kg body weight per day, against an estimated dietary intake for the phosphite and its phosphate of 0.09 mg/kg body weight per day. NIAS: non-intentionally added substances covers the assessment rules for breakdown products like these.

Who Supplies Antioxidants for Polypropylene?#

Antioxidants for polypropylene come from BASF (Irganox, Irgafos), Songwon (SONGNOX), SI Group (Anox, Alkanox, Ultranox), Adeka (ADK STAB), Clariant (Hostanox), Dover Chemical (Doverphos), Syensqo (Cyanox) and Sumitomo Chemical (Sumilizer), and the key molecules are sold by several of them under different names.

BASF, formerly Ciba, remains the reference producer for the Irganox and Irgafos ranges used throughout this guide, and Irgastab is its hydroxylamine line. Songwon is the world's number two producer of polymer stabilizers by volume, having doubled its OPS antioxidant capacity to 14,000 t/a in 2012, and SI Group expanded its own antioxidant portfolio by acquiring Albemarle's antioxidants business in 2014. Adeka, Clariant, Dover Chemical, Syensqo and Sumitomo Chemical complete the group of producers whose PP grades appear throughout this guide. Buyers should compare antioxidants by CAS number, form (powder, pastille) and food-contact status, not by trade name.

Table 6. Same molecule, different trade names (not a performance-equivalence claim)

Plants and grades by company are in the directory of polymer antioxidant manufacturers and suppliers. Send one request to several antioxidant suppliers with the plastic additive supplier finder.

─── What else stabilizes polypropylene ───

What Other Stabilizers Does Polypropylene Need?#

Polypropylene needs a complete stabilizer package in which antioxidants are the first layer, followed by an acid scavenger and, for outdoor parts, UV stabilizers; nucleating agents, fillers and impact modifiers then set its mechanical profile.

The full PP package, covering acid scavengers, UV stabilizers and HALS, nucleating and clarifying agents, antistatic and slip additives, fillers and impact modifiers, is on additives for polypropylene.

UV stabilizers for polypropylene#

Outdoor PP adds UV stabilizers, mainly oligomeric HALS such as Chimassorb 944 at 0.1-1.4 wt% in fibres, on top of its phenol-phosphite package, because antioxidants alone do not stop photo-oxidation.

HALS dosage in PP ranges from 0.05-1.0 wt% in thick sections and films up to 0.1-1.4 wt% in fibres. Unlike phenolic antioxidants, HALS regenerate through the Denisov cycle rather than being consumed in a single event, though a thioester in the same formulation weakens their protection. UV stabilizers for polypropylene covers grades and dosage in full.

Antioxidants for polyethylene compared#

Polyethylene uses the same phenol-phosphite chemistry as PP, but it crosslinks and forms gels when it oxidizes, so PE packages target gel control, while PP packages target chain scission and melt flow.

Typical PE packages pair Irganox 1076 or 1010 with Irgafos 168; gel-prone grades such as chromium-catalysed HDPE or LLDPE add P-EPQ at 0.05-0.1 wt% in recycled LLDPE for gel reduction, and vitamin E serves the same melt-stabilizing role in food-contact and medical PE. Antioxidants for polyethylene covers film and pipe packages for that polymer.

Irganox 1010 vs Irganox 1076: which is better for PP?#

Irganox 1010 is usually the better choice for PP parts that need long-term heat stability, because its tetrafunctional structure (1,177.6 g/mol) makes it far less volatile than the monofunctional Irganox 1076 (530.9 g/mol).

The vapour pressure gap (7 x 10^-10 Pa for 1010 against 2.5 x 10^-7 Pa for 1076) explains most of the practical difference, alongside their FDA maxima (0.5 % against 0.25 %) and EU status (no SML against a 6 mg/kg SML). The antioxidant grade comparison adds Irgafos 168 and BHT against both phenols.

Is vitamin E a natural antioxidant for polypropylene?#

Yes: vitamin E (α-tocopherol) melt-stabilizes PE and PP at only 100-300 ppm, although it yellows the polymer and needs a phosphite where colour matters.

S. Al-Malaika at Aston University has published on tocopherol's melt-stabilizing effect in polyolefins, and Mayer and co-workers (Fraunhofer LBF, 2023) found that sinapic acid stearyl ester performs comparably to, or better than, Irganox 1076 in PP processing trials. Vitamin E is listed under EU FCM 110 with no SML and is GRAS in the US under 21 CFR 182.3890.

What are the best antioxidants for plastic?#

The most widely used antioxidants for plastics are Irganox 1010 and Irgafos 168, each produced at 40 to under 55 million lb a year in the US (EPA CDR 2023), usually combined as a phenol-phosphite blend.

Irganox 1076 follows at 25 to under 40 million lb a year in the same EPA Chemical Data Reporting cycle. Class-by-class selection for all polymers, not PP alone, is on the hub for plastic antioxidants.

Are antioxidants in polypropylene safe?#

The standard PP antioxidants are regulated for food contact and assessed by the FDA: Irgafos 168 has an acceptable daily intake of 1 mg/kg body weight, against an estimated dietary intake of 0.09 mg/kg for the phosphite and its phosphate.

Neal-Kluever, Bailey and Hatwell at the FDA (2015) re-evaluated Irganox 1076 with a no-observed-adverse-effect level of 64 mg/kg body weight per day and a cumulative estimated daily intake of 4.5 mg per person per day, a margin of exposure of about 850. The SVHC exceptions named earlier, TNPP and AO-2246, show that safety assessment is substance-specific, not a single verdict for the whole class.

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