Irgafos 168 (Antioxidant 168): Properties, Uses in Plastics and Regulatory Status

Irgafos 168 (Antioxidant 168, tris(2,4-di-tert-butylphenyl) phosphite; CAS 31570-04-4) is an aryl phosphite used as a secondary antioxidant and processing stabilizer in polyolefins, where it decomposes the hydroperoxides that form in the melt between 150 and 320 °C (302 and 608 °F). A phosphite is consumed while the polymer is being processed rather than during its service life, so the first question a formulator asks is not how much to use but what it is paired with.

The regulatory position is settled in four instruments. Irgafos 168 is registered under REACH in ECHA dossier 15253, is absent from the REACH Candidate List as of 23 September 2026, carries no harmonised classification under the CLP Regulation, and is listed for EU food contact as FCM substance 671 with no specific migration limit. US production and import stood at 40 to under 55 million lb in 2023 (EPA Chemical Data Reporting, via PubChem). Irgafos 168 is one of 43 antioxidant substance pages in our directory of plastic additives, each carrying the same identity, dosage and regulatory fields.

This page holds the data sheet and the compliance file in one document: the identity and the eight trade names that share CAS 31570-04-4, the hydroperoxide-decomposition mechanism, the physical constants published by BASF and Songwon, dosage in polypropylene, polyethylene, recyclate and the engineering plastics, the performance indicators and their standards, what the molecule turns into inside the polymer, the dated EU and US regulatory matrix, the four phosphite alternatives and the five producers.

Table T1. Irgafos 168 identity card.

Footnote: identity and physical data from PubChem CID 91601, the BASF Irgafos 168 (ED) technical data sheet and the Songwon antioxidants technical sheet V5; EU entries from the consolidated text of Regulation (EU) No 10/2011 of 16 March 2025. Status as of 23 September 2026.

What Is Irgafos 168 (Antioxidant 168)?#

Irgafos 168 is the phosphorous acid triester of 2,4-di-tert-butylphenol, a hindered aryl phosphite that acts as a secondary antioxidant: it does not trap radicals, it destroys the hydroperoxides that create them. Three 2,4-di-tert-butylphenyl groups are bound through oxygen to one trivalent phosphorus atom, giving the formula C42H63O3P, a molecular weight of 646.9 g/mol and a phosphorus content of about 4.8 % by theory. The molecule is made by reacting 2,4-di-tert-butylphenol with phosphorus trichloride. What separates a secondary antioxidant from the hindered phenols it is sold with?

The two classes attack different species in one degradation chain. Primary antioxidants, the hindered phenols Irganox 1010, Irganox 1076 and BHT, sit between 300 and 1,000 g/mol and donate a hydrogen atom to peroxyl radicals; secondary antioxidants such as Irgafos 168 reduce the hydroperoxide itself and never touch the radical. Within the phosphorus class the reactivity order runs phosphonites, then alkyl phosphites, then aryl phosphites, then hindered aryl phosphites, which places Irgafos 168 at the least reactive and most hydrolysis-resistant end of the scale. As a secondary antioxidant it is one half of the standard stabilizer package; the hub on antioxidants for plastics compares all of the classes side by side.

What is the chemical name of Antioxidant 168?#

The chemical name of Antioxidant 168 is tris(2,4-di-tert-butylphenyl) phosphite, registered as phosphorous acid, tris(2,4-di-tert-butylphenyl) ester under CAS 31570-04-4. Both forms describe the same substance, formula C42H63O3P, and PubChem files it under CID 91601 with the systematic name as the record title, which is why database entries and supplier catalogues can look like separate products while carrying one CAS number.

Is "Irganox 168" the same as Irgafos 168?#

There is no product called Irganox 168: Irganox is BASF's line of phenolic antioxidants and blends, Irgafos its line of phosphites, so buyers searching for "Irganox 168" are looking for Irgafos 168. The company applies the same logic to a third line, Irgastab, which covers its special stabilizers. The confusion is recorded as a frequent misspelling in the trade literature, and it matters commercially: an order written as Irganox 168 gives a distributor two plausible readings, the phosphite Irgafos 168 or one of the Irganox phenolics, which are different products with different functions.

Which trade names are the same substance as Irgafos 168?#

Eight trade names on the market are the same molecule, CAS 31570-04-4: Irgafos 168 (BASF), Songnox 1680 (Songwon), Alkanox 240, Doverphos S-480 (Dover Chemical), Everfos 168 (Everspring), Hostanox PAR 24 (Clariant), Lowinox 242 and Naugard 524. The Asian generic convention drops the brand and keeps the grade number, which is why "antioxidant 168" is used as a product name in its own right across Chinese, Korean and Indian catalogues, and why a UK specification may call the same powder a processing stabiliser without naming any brand at all. Every equivalent under one CAS is searchable in the plastic additive trade name lookup.

Table T2. Trade-name equivalence for CAS 31570-04-4.

How Does Irgafos 168 Protect Polymers?#

Irgafos 168 protects a polymer by reducing hydroperoxides to alcohols before they split into radicals: the trivalent phosphorus takes an oxygen atom and is itself oxidised to the corresponding phosphate. The reaction is stoichiometric, one phosphite for one hydroperoxide.

ROOH + P(OR')3 → ROH + O=P(OR')3

Why does removing a hydroperoxide matter more than removing a radical during extrusion? Because the hydroperoxide is the branching point of the autoxidation cycle. An alkyl radical formed by shear or heat reacts with oxygen at 10^7 to 10^9 L mol-1 s-1 to give a peroxyl radical, the peroxyl radical abstracts hydrogen from the polymer to give a hydroperoxide and a new alkyl radical, and the hydroperoxide then decomposes into an alkoxy radical and a hydroxyl radical, turning one chain into two. A hydroperoxide decomposer removes the branching step, and a radical scavenger does not.

That job is done where the hydroperoxides are made, which is the extruder rather than the finished part. Plastics melt processing runs between 150 and 320 °C (302 and 608 °F), and the phosphite is largely consumed inside that window, so Irgafos 168 is a processing stabilizer, not a long-term thermal stabilizer. Its structural class sets how far it goes: among the phosphorus stabilizers the reactivity order towards hydroperoxides runs phosphonites first, then alkyl phosphites, then aryl phosphites, then hindered aryl phosphites such as Irgafos 168, while hydrolytic stability runs roughly in reverse. The full autoxidation cycle that produces these hydroperoxides is set out in polymer oxidation and antioxidant mechanisms.

Why is Irgafos 168 always used with a hindered phenol?#

A phosphite and a hindered phenol do different jobs: Irgafos 168 removes hydroperoxides in the melt, while a phenolic antioxidant traps peroxyl radicals over the product's service life. Neither covers the other's ground, which is why the two are bought, dosed and, in the ready-made blends, shipped together. The partner class, phenolic antioxidants (hindered phenols), carries the long-term thermal load that a phosphite cannot.

The division of labour inside a phenol and phosphite package has three parts.

• The phenol handles radicals. A hindered phenol donates hydrogen to peroxyl radicals and gives long-term thermal stability during service, where the phosphite contributes little.
• The phosphite handles hydroperoxides. Irgafos 168 reduces them to alcohols during melt processing, protecting melt flow rate and colour through each extrusion pass.
• Hydrolysis adds a bonus phenol. Above 150 to 180 °C (302 to 356 °F) aryl phosphites release phenols and hydrogen phosphites, and those phenols act synergistically with the primary antioxidant.

In polypropylene this pairing is the base of the standard package, phenol plus phosphite plus an acid scavenger. Irganox 1010 (Antioxidant 1010) is the phenol it is paired with most often.

What phosphite-to-phenol ratio is used?#

Formulators dose Irgafos 168 and a hindered phenol at a phosphite-to-phenol ratio between 1:1 and 4:1, which is exactly the range BASF sells ready-mixed. In polypropylene the AO-1010 and AO-168 combination is normally run at 1:1 or 1:2 phenol to phosphite. Irganox B 225 is the 1:1 mixture with Irganox 1010, and Irganox B 215 is 67 % Irgafos 168 with 33 % Irganox 1010, which is the composition BASF publishes and the reverse of the ratio supplier blogs print. The ready-mixed ratios are compared on antioxidant blends and synergy.

What Are the Physical and Chemical Properties of Irgafos 168?#

The physical constants that matter to a compounder are the melting range, the two solubilities and the bulk density, and they are set out below with the supplier that publishes each value.

Table T3. Physical and chemical properties of Irgafos 168.

Irgafos 168 is a white free-flowing powder that melts at 183-186 °C (362-367 °F), has a density of 1.03 g/cm3 at 20 °C (68 °F) and dissolves in xylene to about 20 % while staying below 0.1 % in water. The water figure is the one that decides service behaviour: a stabilizer that does not dissolve in water is not washed out of a film surface, a pipe wall or a washed recyclate flake, so the additive that survives compounding stays in the article. Suppliers publish slightly different melting ranges, BASF 183-186 °C (362-367 °F) and Songwon 181-187 °C (358-369 °F), so quote the range from the grade's own data sheet rather than from a generic database entry.

Is Irgafos 168 hydrolytically stable?#

Yes: Irgafos 168 is the hydrolytically stable member of the phosphite family, because the two tert-butyl groups on each phenyl ring shield the phosphorus atom from attack by water. That shielding is why it stores and feeds without the additives the pentaerythritol diphosphites need: Ultranox 626 buys higher activity at the price of lower hydrolytic stability and carries a trace amine, triisopropanolamine at up to 1 % under its FDA listing, to extend storage life, while Songnox 6280 is the same 626 chemistry buffered with magnesium and aluminium hydrotalcite at 93:7.

The stability has a ceiling. Above 150 to 180 °C (302 to 356 °F) aryl phosphites hydrolyse as well, releasing phenols and hydrogen phosphites, and that reaction is the origin of the degradation products covered further down this page. How each structure class resists hydrolysis is compared on phosphite and phosphonite antioxidants.

Which Polymers Use Irgafos 168, and at What Dosage?#

Irgafos 168 is dosed at 0.05 to 0.2 wt% (500 to 2,000 ppm) in polyolefins, almost always together with a hindered phenol at a phosphite-to-phenol ratio of 1:1 to 4:1. That band is supplier guidance rather than a legal figure, and it covers polypropylene, polyethylene and the polyolefin copolymers that carry the bulk of the volume.

How do wt% and ppm line up on an antioxidant data sheet? One is the other multiplied by 10,000: 0.1 wt% is 1,000 ppm is 1 g per kg of compound, so a 0.05 to 0.2 wt% range is a 500 to 2,000 ppm range and a 0.3 wt% recyclate dose is 3,000 ppm. Typical ranges per family, in wt% and ppm, sit on additive dosage levels in plastics.

Table T4. Irgafos 168 levels by polymer or compound.

The percentages printed in 21 CFR 178.2010 further down this page are legal maxima for food-contact use and never recommended dosages, and the two must not be read from the same column.

Irgafos 168 in polypropylene#

Polypropylene is the largest use for Irgafos 168, because oxidation attacks PP's tertiary carbon and splits the chain, so the melt flow rate climbs with every extrusion pass. The counter to beta-scission is the standard PP package: a phenol and a phosphite, usually AO-1010 and AO-168 at 1:1 or 1:2, with an acid scavenger, either calcium stearate at up to 1,000 ppm or hydrotalcite. Commodity PP grades leave the reactor carrying less than 400 ppm of phenolic antioxidant as base stabilization, so the compounder supplies most of the protection.

Filled grades need more. Talc and calcium carbonate adsorb antioxidant onto the filler surface and carry metal impurities, so a filled PP compound takes a stronger package than an unfilled one. Polypropylene carries the largest share of antioxidant demand, 34.68 % of plastic antioxidant revenue in 2025 according to Mordor Intelligence. The full PP package, phenol, phosphite, acid scavenger and thioester, is on antioxidants for polypropylene.

Irgafos 168 in polyethylene#

In polyethylene the failure mode is the opposite of PP: degraded PE crosslinks and forms gels rather than losing molecular weight, so the film-grade package pairs Irgafos 168 with Irganox 1076 or Irganox 1010. Gels and fisheyes are counted per square metre on a film line, which makes gel formation the practical criterion rather than melt flow rate alone. Plate-out, the deposition of additive on chill rolls and dies, is the second film-line concern that drives grade selection among the phosphites.

Pipe compounds are judged on a different number. A PE pressure-pipe compound has to reach an oxidative induction time of at least 20 minutes at 210 °C (410 °F) under EN 12201-1 and ISO 4427-1, a long-term criterion the phenolic carries rather than the phosphite. Film, pipe and XLPE packages are separated on antioxidants for polyethylene.

Irgafos 168 in recycled polyolefins#

Recyclate is where Irgafos 168 is measured rather than assumed: a polypropylene regranulate that received no new antioxidant still carried more than 150 ppm of intact Irgafos 168 after extrusion, enough to survive one or two further thermal processing steps. That residual is the starting point for restabilization rather than a reason to skip it, because the consumed fraction has already become phosphate.

Two dose figures anchor the practice. Songwon's multipass data, run at 250 °C (482 °F) for PP and 220 °C (428 °F) for HDPE, put the restabilization dose of an Irganox 1010 and Irgafos 168 binary blend at 0.1 to 0.3 wt%, and a 2025 PP recyclate study used 500 ppm of AO-1010 with 1,000 ppm of AO-168. Knoben and colleagues (Materials, 2025) recorded an oxidation induction temperature of 198 °C (388 °F) for a closed-loop recycled polypropylene without added antioxidant against 257 °C (495 °F) with it. Doses, OIT targets and multipass data are collected on restabilization of recycled plastics.

Irgafos 168 in polycarbonate, polyamide, PET and ABS#

Outside the polyolefins, Irgafos 168 is used mainly to hold colour: in polycarbonate, polyamide, PET and ABS compounds it limits the yellowing that comes from processing at 240 to 320 °C (464 to 608 °F). Engineering plastics run hotter than the commodity polyolefins and spend longer in the melt, so suppressing hydroperoxide branching during that residence time protects the optical properties as much as the mechanical ones. PET takes very little antioxidant, typically a phosphite for colour alone, and ABS compounding pairs Irgafos 168 with phenolics such as Irganox 1076 and Irganox 245.

No supplier dosage range for these polymers sits in our source library, and the only published numbers are the US food-contact ceilings: 0.3 % in polycarbonate, 1 % in nylon under conditions of use E to G, and 0.2 % in polystyrene and HIPS. Those are maxima under 21 CFR 178.2010, not formulation targets. The complete PC package is on additives for polycarbonate.

What Is Irgafos 168 Used For? 3 Application Areas in Plastics#

Irgafos 168 is used in 3 plastics application areas: food-contact film and packaging, colour-critical parts and fibres, and the restabilization of recycled polyolefin compounds. The three are listed below in order of volume.

• Food-contact film, sheet and packaging, where the substance is cleared in both the EU and the US and is found analytically in commercial packages.
• Colour protection in fibres, moulded parts and high-temperature processing, including the direct gas-fired ovens where phenolic antioxidants fail.
• Restabilization of recycled polyolefin compounds, where the incoming material arrives with its stabilizer partly consumed.

Food-contact film, sheet and packaging#

Packaging is the single largest end use: when Sara Dopico-García and colleagues analysed commercial PE, PP and PVC packages (Journal of Agricultural and Food Chemistry, 2007) they found Irganox 1010 and Irganox 1076 alongside Irgafos 168 and its phosphate in most of them. That survey is why the phosphite and the phosphate are treated as one analytical pair in food-contact work rather than as parent and impurity.

The clearance behind that use is specific. Irgafos 168 is FCM substance 671 on the Union list of Regulation (EU) No 10/2011 with no specific migration limit, so only the overall migration limit applies. Packaging accounted for 39.22 % of plastic antioxidant revenue in 2025 according to Mordor Intelligence. Which additives are cleared for each packaging polymer is tabulated on additives for food packaging.

Colour protection in fibres, moulded parts and high-temperature processing#

Irgafos 168 carries the colour load where phenolic antioxidants cannot: phenolics react with the NOx in a direct gas-fired oven and yellow the part, which is the defect the industry calls gas fading. The chemistry is specific to the phenol: NOx converts hindered phenols to quinone methides, with BHT giving stilbenequinone, and the coloured product sits in the article rather than washing out.

Two remedies follow, and both involve the phosphite. BASF states in its Irgafos 168 data sheet that the substance is usable in direct gas-fired ovens and prevents thermally induced discolouration; where colour requirements are absolute, the industry moves to a phenol-free system built on a hydroxylamine plus a phosphite. The mechanisms behind yellowing, pinking and gas fading are worked through with their additive fixes.

Restabilization of recycled polyolefin compounds#

Recycled polyolefins reach the compounder with their antioxidant partly consumed, so Rudolf Pfaendner's restabilization approach (Polymer Degradation and Stability, 2022) tops the package back up with a phenol and a phosphite such as Irgafos 168. Restabilization means adding phenol and phosphite packages, with HALS or UV absorbers where needed, to compensate for antioxidant depletion and the oxidised groups the previous life cycle left behind, at 0.1 to 0.3 wt% of an Irganox 1010 and Irgafos 168 binary blend. Demand for it is written into law: Article 7 of the Packaging and Packaging Waste Regulation (EU) 2025/40 sets minimum recycled content from 2030 at 30 % for contact-sensitive PET packaging, 10 % for other contact-sensitive packaging, 30 % for single-use plastic beverage bottles and 35 % for other plastic packaging.

How Does Irgafos 168 Perform? Melt Stability, Colour and OIT#

Irgafos 168 is judged on three numbers: how far the melt flow rate moves over repeated extrusion passes, how far the yellowness index moves, and how long the oxidative induction time lasts. Processing stability is read as a shift in melt flow rate (MFR / MFI / MVR) between extrusion passes, measured under ISO 1133-1 or ASTM D1238-26 at 230 °C with a 2.16 kg load for polypropylene and at 190 °C with the same load for polyethylene. Colour is read as yellowness index under ASTM E313-20 (R2025), which replaced the withdrawn ASTM D1925 and is the only current method for the measurement.

Does a phosphite improve long-term heat ageing? No: that is the work of the hindered phenol and, where service temperatures are high, of a thioester. Irgafos 168 is consumed during melt processing, so an oxidative induction time measured on a finished article reports mainly what the primary antioxidant is still doing, not what the phosphite did in the extruder. Our source library holds no Irgafos 168-specific MFR retention, yellowness index or OIT value, and none is stated here; the values below are the method conditions and the one published recyclate measurement.

Table T5. Performance indicators and their test methods.

ISO 11357-6 and ASTM D3895 are explained on oxidative induction time (OIT).

How is the performance of Irgafos 168 measured?#

The standard laboratory test is a multipass extrusion: the same compound is extruded five times at 250 °C for polypropylene or 220 °C for HDPE, and the melt flow rate and colour are read after each pass. The test reproduces in one afternoon the thermal history a recyclate accumulates over repeated life cycles, and it is how Songwon derives the 0.1 to 0.3 wt% restabilization figure quoted earlier.

Oxidative induction time is the calorimetric complement. A specimen is heated in a differential scanning calorimeter under nitrogen to a fixed temperature between 190 and 220 °C (374 and 428 °F), the gas is switched to oxygen, and the time to the oxidation onset is recorded. ASTM D3895 warns in Note 2 that volatile antioxidants can give a low OIT and still perform in service, which is why residual antioxidant content is also measured directly by chromatography on recyclate.

What Happens to Irgafos 168 in the Polymer? Oxidation and Degradation Products#

Irgafos 168 is meant to be consumed: the oxidation it prevents converts it into tris(2,4-di-tert-butylphenyl) phosphate, written AO168=O, and hydrolysis converts it into 2,4-di-tert-butylphenol. Both routes are built into the mechanism rather than signs of a defective additive: the phosphite reduces a hydroperoxide by giving up its own oxidation state, and aryl phosphites hydrolyse above 150 to 180 °C (302 to 356 °F), releasing phenols and hydrogen phosphites. The practical consequence is that any analysis of a finished article finds a mixture of the parent and its transformation products, which is why both appear in migration studies and in food-contact assessments.

Table T6. Degradation and transformation products of Irgafos 168.

Irgafos 168 phosphate (AO168=O), the oxidised form#

The oxidised form, tris(2,4-di-tert-butylphenyl) phosphate, is the reason regulators assess Irgafos 168 as a pair: the US FDA's 2023 exposure estimate covers the phosphite and its phosphate together. Markley and colleagues at the FDA's Center for Food Safety and Applied Nutrition (Food and Chemical Toxicology 178:113877, 2023) set the cumulative estimated daily intake for the two substances combined at 0.09 mg per kg of body weight per day and identified no neurotoxicity concern for the phosphate.

The phosphate also travels beyond the polymer. Shi, Cai and colleagues (Environmental Science and Technology 54:10570, 2020) measured tris(2,4-di-tert-butylphenyl) phosphate in urban fine particulate matter in China at up to 851 ng/m3 with a median of 153 ng/m3, and Zhou and colleagues (Food Chemistry 448:139144, 2024) reported AO168 contamination in dairy products. These are analytical findings in environmental and food matrices, and they establish that the oxidised form is detectable outside the article, not that any exposure limit is exceeded. Its onward transformation product, bis(2,4-di-tert-butylphenyl) phosphate, carries no EU or US limit in our source library.

2,4-di-tert-butylphenol (2,4-DTBP) and NIAS in food-contact plastics#

2,4-di-tert-butylphenol is the hydrolysis product that turns up in migration studies: Jing Qian and colleagues (PLoS One, 2018) measured up to 45.568 ± 31.513 mg/kg of it in BOPP and LDPE food-contact products, and it is not on the EU Union list. The phenol is the same building block the molecule is made from, released when water attacks the phosphorus centre at processing temperature, so it is present in articles that never contained it as an ingredient.

A substance in that position has a name in EU law. Article 3(9) of Regulation (EU) No 10/2011 defines non-intentionally added substances, and Article 19 requires the business operator to assess them for safety even though they carry no Union list entry and no specific migration limit. 2,4-DTBP is not a Substance of Very High Concern in the sources checked, while its analogue 2,4,6-tri-tert-butylphenol (CAS 732-26-3) has been on the REACH Candidate List since 23 January 2024, so the two must not be treated as interchangeable. Article 19 is what turns a degradation product into a compliance task; see NIAS: non-intentionally added substances.

How Does Irgafos 168 Interact with Other Additives?#

Irgafos 168 is a team player with one known conflict on the team: it works with hindered phenols and acid scavengers, while the thioesters used for long-term heat ageing antagonise HALS. The three interactions that decide a polyolefin package are listed below.

• Synergy with hindered phenols. The phenol removes peroxyl radicals and the phosphite removes hydroperoxides, and the phenols released when the aryl phosphite hydrolyses above 150 to 180 °C (302 to 356 °F) add to the primary antioxidant present.
• Co-dosing with acid scavengers. Calcium stearate at up to 1,000 ppm or hydrotalcite travels with the phenol and phosphite package in polypropylene, neutralising catalyst residues that attack both the polymer and the stabilizer.
• Antagonism between thioesters and HALS. Thioesters such as DSTDP give long-term heat ageing, but their acidic sulfur oxidation products deactivate the basic hindered amine light stabilizer, so one package cannot maximise both.

Fillers change the arithmetic rather than the chemistry. Talc and calcium carbonate adsorb antioxidant onto their surfaces and carry metal impurities, so a filled polypropylene compound needs a stronger package than an unfilled one to reach the same melt stability. The one interaction a formulator cannot design around is the thioester and HALS conflict, because no dosage adjustment restores a hindered amine that acidic decomposition products have deactivated.

What Is the Regulatory Status of Irgafos 168?#

Irgafos 168 is REACH-registered, is not a Substance of Very High Concern, is listed for EU food contact as FCM substance 671 without a specific migration limit, and is cleared in the United States under 21 CFR 178.2010 with polymer-by-polymer maxima (status 23 September 2026). The full matrix is below, with the instrument, the entry and the date for each line.

Table T7. Irgafos 168 regulatory matrix, status 23 September 2026.

Registration, evaluation and the Candidate List process are explained on REACH and plastic additives.

Is Irgafos 168 REACH registered, and is it an SVHC?#

Yes, Irgafos 168 is registered under REACH (Regulation (EC) No 1907/2006) in dossier 15253, and no, it is not a Substance of Very High Concern: it is absent from the Candidate List as of 23 September 2026. The same absence applies to Annex XIV (authorisation) and Annex XVII (restrictions), so no EU authorisation or restriction condition attaches to its use in plastics.

The contrast inside the phosphite family is instructive. TNPP has been on the SVHC Candidate List since 16 July 2019, identified under Article 57(f) as endocrine disrupting for the environment where it contains 0.1 % or more 4-nonylphenol, with the entry updated on 21 January 2025; Irgafos 168 has never been added. Candidate List statements are dated rather than permanent, and the ECHA update due in January 2027 is the next point at which this line is re-verified.

Is Irgafos 168 allowed in food-contact plastics in the EU?#

Yes: Irgafos 168 is on the Union list of Regulation (EU) No 10/2011 as FCM substance 671 (reference 74240) with no specific migration limit, so the overall migration limit of 10 mg/dm2 is the figure that applies. That limit becomes 60 mg/kg for articles intended for infants and young children, and it covers everything that migrates from the article rather than the antioxidant alone. The values here are verified against the consolidated text of 16 March 2025.

Within the antioxidant family the absence of an SML is not unusual but it is not universal either. Irganox 1010 (FCM 496) and Irganox 1330 (FCM 428) also carry no specific migration limit, while Ultranox 626 (FCM 652) is capped at 0.6 mg/kg and Doverphos S-9228 (FCM 773) at 5 mg/kg expressed as a sum with its phosphate and 2,4-dicumylphenol. The Union list, the overall migration limit and the fat reduction factor are explained on EU 10/2011, and every antioxidant SML in one table sits on food contact antioxidants.

Is Irgafos 168 FDA approved? The 21 CFR 178.2010 limits#

"FDA approved" is the wrong frame: Irgafos 168 is listed in 21 CFR 178.2010 as an antioxidant and stabilizer for polymers, with a maximum of 0.25 % in propylene polymers, 0.2 % in ethylene polymers, 0.3 % in polycarbonate and 1 % in nylon under conditions of use E to G. The FDA does not approve indirect additives one by one; it lists them with conditions, and a compliant use stays inside those conditions.

Every one of these figures is a legal ceiling rather than a recommendation, and paragraph (a) of 21 CFR 178.2010 adds a second constraint on top of the numbers: the quantity used must not exceed the amount reasonably required to accomplish the intended technical effect. A formulation at 0.1 wt% therefore complies, and a formulation at 0.25 wt% in propylene polymers complies only if the technical case for that level can be made. How 21 CFR 178.2010 relates to food contact notifications and prior sanctions is set out on FDA food contact rules for plastic additives.

Is Irgafos 168 listed under California Proposition 65?#

Proposition 65: we are verifying this entry against the current OEHHA list and will publish the result with its date. Until that check is complete this page states no listing status for CAS 31570-04-4 in either direction, because a Proposition 65 answer without a verification date is worth nothing to a compliance file. Listing dates for every plastic additive are on California Proposition 65.

Is Irgafos 168 Safe? Health, Safety and Environmental Profile#

Irgafos 168 has no harmonised hazard classification under the EU CLP Regulation, and the US FDA's 2023 assessment set an acceptable daily intake of 1 mg per kg of body weight per day against an estimated dietary intake of 0.09 mg/kg bw/day for the substance and its phosphate together. The estimated intake is therefore about eleven times below the acceptable daily intake, an arithmetic comparison of the two published figures rather than a safety conclusion of its own. Our source library holds no LD50 and no NOAEL for this substance, and none is stated here.

The profile rests on three separate kinds of evidence.

• Hazard classification. No harmonised classification exists under Regulation (EC) No 1272/2008. The classification and labelling notifications aggregated by PubChem are self-classifications by registrants rather than agreed EU entries, and they report the substance as not classified.
• Exposure assessment. Markley and colleagues (Food and Chemical Toxicology 178:113877, 2023) published the FDA's assessment with an acceptable daily intake of 1 mg/kg bw/day, a cumulative estimated daily intake of 0.09 mg/kg bw/day covering the phosphite and its phosphate, and no neurotoxicity concern for the phosphate. That assessment is completed work, not an open review.
• Environmental occurrence. Shi, Cai and colleagues (Environmental Science and Technology 54:10570, 2020) found the oxidised form, tris(2,4-di-tert-butylphenyl) phosphate, in urban fine particulate matter in China at up to 851 ng/m3, against a US production and import volume of 40 to under 55 million lb in 2023.

What migration studies actually measure is summarised on chemicals migrating from plastic food packaging.

What Are the Alternatives to Irgafos 168?#

The 4 direct alternatives to Irgafos 168 are Antioxidant 626, Doverphos S-9228, ADK STAB PEP-36 and P-EPQ, and they trade hydrolytic stability for reactivity in roughly that order. A fifth, TNPP, is the liquid phosphite that Irgafos 168 and the other solids have been replacing since its Candidate List entry.

Table T8. Six phosphorus stabilizers compared.

Footnote: EU values follow the consolidated text of Regulation (EU) No 10/2011 of 16 March 2025. FDA percentages are legal maxima, not recommended dosages.

Phenolics and phosphites are set side by side on the antioxidant grade comparison.

Irgafos 168 vs Antioxidant 626 (Ultranox 626, Irgafos 126)#

Antioxidant 626 is the more active phosphite and Irgafos 168 the more robust one: the spiro diphosphite carries two phosphorus atoms per molecule but hydrolyses more readily, which is why it is sold with a trace amine or buffered with hydrotalcite. Six trade names cover the single CAS 26741-53-7, namely Ultranox 626, Irgafos 126, ADK STAB PEP-24, Alkanox P-24, Doverphos S-9432 and Songnox 6260, at 604.7 g/mol and a melting point of 170-180 °C (338-356 °F).

The difference shows up in storage and in the compliance file alike. Triisopropanolamine at up to 1 % under the FDA listing extends the storage life of 626, and Songnox 6280 is the same chemistry buffered with magnesium and aluminium hydrotalcite at 93:7. Antioxidant 626 (Ultranox 626 / Irgafos 126) carries a specific migration limit of 0.6 mg/kg under FCM 652; Irgafos 168 has none.

Irgafos 168 vs Doverphos S-9228 and ADK STAB PEP-36#

Doverphos S-9228 and ADK STAB PEP-36 are the high-temperature diphosphites: both carry EU specific migration limits of 5 mg/kg that are expressed as the sum of the substance and its oxidation products, which Irgafos 168 does not need because FCM 671 has no SML at all. An SML expressed as a sum changes the analytical task: a laboratory quantifies the parent, its phosphate and, for S-9228, 2,4-dicumylphenol, then adds them before comparing with the limit.

Doverphos S-9228 was designed as the second-generation, hydrolysis-resistant answer to the 626 diphosphite, at a molecular weight of 852 g/mol and US maxima of 0.15 % in all polymers, 0.2 % in polycarbonate and 0.3 % in polyetherimide. ADK STAB PEP-36, at 632.7 g/mol, is capped at 0.25 % in polypropylene and 0.05 % in olefin films up to 100 µm under 21 CFR 178.2010.

Irgafos 168 vs P-EPQ#

P-EPQ is a phosphonite, not a phosphite, and phosphonites sit one step above aryl phosphites in hydroperoxide reactivity, which is why recyclers reach for it at 0.05 to 0.1 wt% in recycled LLDPE when gels are the problem. The commercial product is about 70 % active, carries CAS 119345-01-6 with 38613-77-3 as the main component, and has a molecular weight of about 1,035 g/mol.

The trade-off is clearance as much as chemistry. P-EPQ phosphonite antioxidant is listed as FCM 760 with a specific migration limit of 18 mg/kg and a US maximum of 0.1 % in the listed olefin polymers and in polycarbonate, so it works under a numeric EU limit where Irgafos 168 works under the overall migration limit only. For routine polyolefin processing the aryl phosphite is the default and the phosphonite is the escalation.

Irgafos 168 vs TNPP and Weston 705#

The liquid phosphites are a different conversation: TNPP has been on the REACH Candidate List since 16 July 2019 when it contains 0.1 % or more 4-nonylphenol, and Irgafos 168 is one of the solid grades that replaced it. The identification was made under Article 57(f) as endocrine disrupting for the environment, the entry was updated on 21 January 2025, and TNPP remains listed for food contact as FCM 69 with a specific migration limit of 30 mg/kg. TNPP (tris(nonylphenyl) phosphite) is the phosphite the Candidate List caught.

Liquid phosphites did not disappear with it. Weston 705 (nonylphenol-free phosphite), CAS 939402-02-5, is SI Group's nonylphenol-free liquid replacement for TNPP; SI Group states about 20 % lower loading than TNPP and ten times less plate-out with fewer gels than AO-168, and those performance figures are supplier claims our source library records as unverified.

Who Manufactures Irgafos 168? Grades and Suppliers#

Irgafos 168 is made by BASF, which owns the Irgafos name, and by Songwon (Songnox 1680), Dover Chemical (Doverphos S-480), Everspring (Everfos 168) and Clariant (Hostanox PAR 24). Three further trade names circulate without a current owner recorded in our source library: Alkanox 240, Lowinox 242 and Naugard 524. Songwon describes itself as the world's second-largest polymer stabilizer producer, a company claim that CHEManager supported in 2013 by ranking it second behind BASF.

Table T9. Producers and their Irgafos 168 trade names.

Buyers should ask each supplier for the grade's own TDS and SDS, because the melting range and the product form differ between producers, as the 183-186 °C and 181-187 °C ranges published by BASF and Songwon show. Volume supports dual sourcing: US production and import stood at 40 to under 55 million lb in 2023 (EPA Chemical Data Reporting, via PubChem), and Mordor Intelligence puts phosphites and phosphonites as the fastest-growing antioxidant class at a 6.18 % compound annual growth rate. More producers, their plants and their certifications are in the directory of polymer antioxidant manufacturers, and price drivers by additive family are tracked under plastic additive prices.

How Does Irgafos 168 Fit into the Antioxidant Family?#

Irgafos 168 is the reference secondary antioxidant of the plastics industry, the phosphite that the whole class is compared against, and it sits beside the hindered phenols, the thioesters and the carbon-radical scavengers in the antioxidant family. That family has four branches: primary antioxidants (hindered phenols and aminics), secondary antioxidants (phosphites, phosphonites, thioesters and disulfides), carbon-radical scavengers (benzofuranones, hydroxylamines and acrylated phenols) and metal deactivators. Mordor Intelligence sizes the plastic antioxidant market at USD 5.41 billion in 2025 and USD 5.69 billion in 2026, with phosphites and phosphonites the fastest-growing class; other analysts put the 2025 figure at USD 5.86 to 5.9 billion, so the number quoted always needs its source. Where each class sits is mapped under the types of antioxidants for plastics.

Irganox B 215, B 225 and B 900: the ready-made phenol and phosphite blends#

Three BASF blends do the phosphite-plus-phenol dosing for the compounder: Irganox B 215 (67 % Irgafos 168, 33 % Irganox 1010), Irganox B 225 (1:1 with Irganox 1010) and Irganox B 900 (80 % Irgafos 168, 20 % Irganox 1076). Supplier blogs print the B 215 ratio reversed; the composition below is the one BASF publishes.

A blend removes two weighing errors and one segregation risk from the compounding line, which is why recyclers quote restabilization doses as a percentage of the binary blend. Songwon sells equivalent binary blends as Songnox 11B and Songnox 21B. Irganox B 900 pairs the phosphite with Irganox 1076 (Antioxidant 1076) at 80:20.

Irgafos 168 outside plastics: coatings, adhesives and elastomers#

Outside plastics, Irgafos 168 is used in powder and coil coatings at 0.5 to 1 wt% on solids, in adhesives, and in elastomers, where the US food-contact maximum is 0.5 % under 21 CFR 177.2600. The coatings use turns on the same gas-fading property as the plastics one, since BASF's coatings data sheet records the substance as usable in direct gas-fired ovens where phenolic antioxidants discolour. Coatings and adhesives sit outside this site's scope, and these figures are given for completeness only.

Is Irgafos 168 banned anywhere?#

No: Irgafos 168 is not restricted under REACH Annex XVII, not on the Candidate List, not on the EU POPs list and not withdrawn from 21 CFR 178.2010 (status 23 September 2026). Among the phosphites it is TNPP that carries the SVHC listing, since 16 July 2019 and only where it contains 0.1 % or more 4-nonylphenol, not Irgafos 168.

Does Irgafos 168 need an SDS?#

Yes: every supplier issues a safety data sheet for Irgafos 168 as for any traded chemical, even though the substance carries no harmonised CLP classification. Request the sheet from the producer of the grade being bought, because the document is grade-specific and version-dated.

Is Antioxidant 168 the same as Antioxidant 626?#

No: Antioxidant 168 is tris(2,4-di-tert-butylphenyl) phosphite (CAS 31570-04-4) and Antioxidant 626 is bis(2,4-di-tert-butylphenyl) pentaerythritol diphosphite (CAS 26741-53-7). The numbers are grade codes inherited from the Asian generic naming convention, which drops the brand and keeps the digits, so they look like positions on one scale while denoting two different molecules with two different EU food-contact entries.