Phosphite and phosphonite antioxidants are secondary antioxidants that protect plastics during melt processing by reducing polymer hydroperoxides to alcohols, and they are added at about 0.05-0.2 wt% next to a phenolic antioxidant. Because each phosphorus atom is used up as it turns into a phosphate, the choice of grade decides how long the melt stays protected and how the additive survives humid storage, so which phosphite suits which polymer? Phosphites are the processing half of almost every polyolefin stabilizer package, which makes them one of the highest-volume groups of plastic additives. US EPA Chemical Data Reporting for 2023 lists Irgafos 168 at 40 to under 55 million pounds, the same production band as Irganox 1010.
This reference covers how phosphites reduce hydroperoxides, why they are combined with hindered phenols, the 5 structural types and their named grades (Irgafos 168, Antioxidant 626, Doverphos S-9228, P-EPQ, TNPP and their relatives), a grade comparison, why some phosphites hydrolyse and how that is prevented, how much is used in polyolefins, recyclates, engineering plastics and PVC, how performance is tested, and which grades are listed for food contact in the EU and the US.
- 0.05-0.2 wt% Irgafos 168 in polyolefins
- 1:1 to 4:1 phosphite:phenol ratio in standard blends
- 0.6 mg/kg EU migration limit for Antioxidant 626, the lowest phosphite SML in this reference
- 6.18 % CAGR, the fastest-growing antioxidant type 2026-2031 (Mordor Intelligence)
What Are Phosphite and Phosphonite Antioxidants?#
Phosphite and phosphonite antioxidants are esters of trivalent phosphorus that act as secondary (preventive) antioxidants: they convert the hydroperoxides formed during polymer oxidation into stable alcohols before those hydroperoxides can split into new radicals. They sit in the secondary antioxidant class alongside thioesters and disulfides, next to the primary antioxidants (hindered phenols, aminics) and the carbon-radical scavengers (benzofuranone lactones, hydroxylamines, acrylated phenols). Phosphites and phosphonites are also called processing stabilizers or melt stabilizers, because they work mainly during melt processing and contribute little to a part's long-term heat ageing once it is in service.
Phosphites are one of the classes compared on the hub page for antioxidants for plastics, next to hindered phenols, aminics and thioesters.
How do phosphite antioxidants work?#
Phosphite antioxidants work by reducing a polymer hydroperoxide (ROOH) to an alcohol (ROH) while the phosphite itself is oxidised to a phosphate, which removes the molecule that would otherwise split into two new radicals. The reaction is written P(OR)3 + ROOH -> O=P(OR)3 + ROH. Left alone, that hydroperoxide would undergo chain branching in the Bolland-Gee autoxidation scheme, splitting into an alkoxy radical and a hydroxyl radical (ROOH -> RO• + *OH) that start new oxidation chains. Gerald Scott and K.J. Humphris at Aston University described the mechanism in "Mechanisms of antioxidant action. Phosphite esters", Pure and Applied Chemistry, 1973, and K. Schwetlick at Dresden extended it in "Mechanisms of Antioxidant Action of Phosphite and Phosphonite Esters".
The phosphite's action has 3 consequences in the melt.
- Hydroperoxides are removed before they can reach chain branching.
- The phosphite is consumed stoichiometrically: one P(III) atom neutralises one hydroperoxide.
- Phenols released when a phosphite hydrolyses or transesterifies add their own primary-antioxidant activity, so phosphites are sometimes considered multifunctional.
The full autoxidation cycle, from initiation to chain branching, is set out under polymer oxidation and antioxidant mechanisms.
What is the difference between a phosphite and a phosphonite?#
A phosphite carries three P-O-R ester bonds on its phosphorus atom, while a phosphonite such as P-EPQ has one direct phosphorus-carbon bond, and that difference makes phosphonites the most reactive class in hydroperoxide reduction. Schwetlick's reactivity order for reduction of ROOH runs: phosphonites, then alkyl phosphites, then aryl phosphites, then hindered aryl phosphites. Hydrolytic stability runs roughly the opposite way: the most reactive grades tend to be the least resistant to moisture, which is why hindered aryl phosphites such as Irgafos 168 are both the least reactive and the most hydrolytically stable of the group.
Are phosphite and phosphate the same?#
No: a phosphite contains trivalent phosphorus and is the active antioxidant, while a phosphate contains pentavalent phosphorus and is the spent form the phosphite turns into after it has reduced a hydroperoxide. Irgafos 168, for example, is oxidised to tris(2,4-di-tert-butylphenyl) phosphate (known as AO168=O) as it works. This is a different substance class from triphenyl phosphate or the phosphate-ester flame retardants, and triphenyl phosphite is abbreviated TPPi on this site, never "TPP", to avoid the same confusion.
Why Are Phosphite Antioxidants Combined with Phenolic Antioxidants?#
Phosphites are combined with phenolic antioxidants because the phenol stops peroxy radicals by turning them into hydroperoxides, and the phosphite then destroys those hydroperoxides, so each additive removes the other's weak point. The phenol traps a peroxy radical (ROO•) and forms a hydroperoxide (ROOH); the phosphite then reduces that hydroperoxide to an alcohol before it can chain-branch. Because the phosphite is consumed sacrificially during processing, it spares the phenol for its own long-term role, and it also reduces the coloured quinoid species that can otherwise build up in the melt.
Grades and dosages of the partner class are on phenolic antioxidants (hindered phenols).
Formulators should keep the phosphite share at or above the phenol share (1:1 to 4:1) when melt-flow retention and colour matter more than long-term heat ageing; patent literature shows the phenol-to-phosphite ratio can range far wider, from about 20:1 to 1:10, depending on the target property. BASF's standard commercial blends fix the ratio for each product:
| Blend | Phosphite | Phenol | Ratio (phosphite:phenol) |
|---|---|---|---|
| Irganox B 215 | Irgafos 168 | Irganox 1010 | 2:1 (67/33) |
| Irganox B 225 | Irgafos 168 | Irganox 1010 | 1:1 |
| Irganox B 900 | Irgafos 168 | Irganox 1076 | 4:1 (80/20) |
Blend names are BASF's; equivalents exist from other makers, see the blends page.
Ratios and equivalents of every commercial one-pack are compared under antioxidant blends and synergy. Irgafos 168 has one processing advantage the phenolics do not share: it can be used in direct gas-fired ovens, because phenolic antioxidants discolour under the NOx present in those ovens while the phosphite does not. Commercial PE, PP and PVC packaging typically contains Irganox 1010 or 1076 together with Irgafos 168 and traces of its phosphate.
What Are the 5 Types of Phosphite and Phosphonite Antioxidants?#
The 5 types of phosphite and phosphonite antioxidants are hindered aryl phosphites, pentaerythritol diphosphites, phosphonites, liquid alkylaryl phosphites and the PVC co-stabilizer phosphites, and the hindered aryl phosphite Irgafos 168 is the volume leader.
1. Hindered aryl phosphites: Irgafos 168 and Irgafos 38#
Hindered aryl phosphites carry bulky tert-butyl groups next to each P-O bond, and Irgafos 168 (tris(2,4-di-tert-butylphenyl) phosphite, CAS 31570-04-4) is the reference grade: a hydrolytically stable white powder that melts at 183-186 °C. It is made from 2,4-di-tert-butylphenol and phosphorus trichloride, carries about 4.8 % phosphorus (theoretical), and is used in polyolefins at 0.05-0.2 wt%.
| Grade | CAS | MW (g/mol) | mp (°C) | P content |
|---|---|---|---|---|
| Irgafos 168 | 31570-04-4 | 646.9 | 183-186 | ~4.8 % (theoretical) |
| Irgafos 38 | 145650-60-8 | 514.7 | not established | not established |
Identity, grades and the full regulatory matrix are on Irgafos 168 (Antioxidant 168). A related grade, Irgafos 38 (bis(2,4-di-tert-butyl-6-methylphenyl) ethyl phosphite, CAS 145650-60-8, EC 416-140-4), is an ethyl phosphite from the same hindered aryl family, and it is not the fluorophosphonite Ethanox 398, a different substance despite the similar naming.
2. Pentaerythritol diphosphites: Antioxidant 626, PEP-36, S-9228 and Weston 618#
Pentaerythritol diphosphites carry two phosphorus atoms in one spiro ring system, which gives Antioxidant 626 (Ultranox 626 / Irgafos 126 / ADK STAB PEP-24) more activity per gram than Irgafos 168 but also lower resistance to hydrolysis. Antioxidant 626 (CAS 26741-53-7, EC 247-952-5, 604.7 g/mol, mp 170-180 °C) is also sold as Songnox 6260 and Doverphos S-9432, among other trade names.
A second generation of pentaerythritol diphosphites addresses that hydrolysis weakness. Doverphos S-9228 (also Antioxidant 9228, Antioxidant 852, Songnox 9228), bis(2,4-dicumylphenyl) pentaerythritol diphosphite, CAS 154862-43-8, EC 421-920-2, 853.0 g/mol, melting above 228 °C, is described by its makers as a second-generation diphosphite with better hydrolysis resistance than Antioxidant 626. ADK STAB PEP-36 (CAS 80693-00-1, EC 410-290-4, 632.7 g/mol) is a further spiro diphosphite in this family, and Weston 618 (distearyl pentaerythritol diphosphite, CAS 3806-34-6, EC 223-276-6, 733.0 g/mol) is an alkyl spiro-diphosphite variant used in olefin polymers, styrenics and PVC.
| Grade | CAS | MW (g/mol) | mp (°C) |
|---|---|---|---|
| Antioxidant 626 | 26741-53-7 | 604.7 | 170-180 |
| Doverphos S-9228 | 154862-43-8 | 853.0 | >228 |
| ADK STAB PEP-36 | 80693-00-1 | 632.7 | not established |
| Weston 618 | 3806-34-6 | 733.0 | not established |
Storage and TIPA-stabilised grades are covered on the Antioxidant 626 substance page. Doverphos S-9228 is the hydrolysis-resistant second-generation diphosphite most commonly chosen when both high temperature and moisture resistance are needed.
3. Phosphonites and fluorophosphonites: P-EPQ and Ethanox 398#
Phosphonites such as P-EPQ are the most reactive phosphorus antioxidants, and P-EPQ itself is a reaction product rather than a single molecule: about 70 % active, with 36-46 % of the 4,4'-biphenylene isomer and 9-18 % Irgafos 168 in the EU specification. Its reaction product carries CAS 119345-01-6 (EC 432-130-2), while its main component, tetrakis(2,4-di-tert-butylphenyl) 4,4'-biphenylenediphosphonite, carries CAS 38613-77-3 (EC 254-037-4) and a molecular weight of 1,035.4 g/mol. P-EPQ is sold as Irgafos P-EPQ, Hostanox P-EPQ and Songnox PQ, and it is used in recycled LLDPE at 0.05-0.1 wt% for gel reduction.
Both CAS numbers are explained on the P-EPQ phosphonite antioxidant substance page. A second reactive class, the fluorophosphonites, is represented by Ethanox 398 (2,2'-ethylidenebis(4,6-di-tert-butylphenyl) fluorophosphonite, CAS 118337-09-0, 486.6 g/mol), made by SI Group after its 2014 acquisition of Albemarle's antioxidants business. The fluorophosphonite Ethanox 398 adds low volatility and hydrolysis resistance to the phosphonite class.
4. Liquid alkylaryl phosphites: TNPP and Weston 705#
Liquid alkylaryl phosphites are pumpable secondary antioxidants for PE, PVC and styrenic rubbers, and the legacy grade TNPP (tris(nonylphenyl) phosphite, CAS 26523-78-4) is being replaced by nonylphenol-free liquids such as Weston 705. TNPP (689.0 g/mol, a viscous liquid with a pour point of 6 °C) stabilizes SBR/BR, PE, PVC and polymer latexes, and carries about 4.4 % phosphorus; it is also sold as Weston TNPP, Irgafos TNPP, Naugard TNPP, Doverphos 4 and Sumilizer TNP.
The SVHC history and alternatives are on the TNPP (tris(nonylphenyl) phosphite) substance page. Weston 705 (a mixed amylphenyl phosphite triester, CAS 939402-02-5, about 5.2 % phosphorus, specific gravity 1.004 at 25 °C, acid number below 0.5 mg KOH/g) is marketed as a drop-in TNPP replacement for HDPE, LLDPE, SBR, NBR, SBC, SBS, ABS and PVC. SI Group states that Weston 705 needs about 20 % lower loading than TNPP and produces far less plate-out and fewer gels than AO-168-type phosphites; these are supplier performance claims, not independently verified figures, and Weston 705's REACH and FDA listing status is not established in this reference. Weston 705 is sold as a nonylphenol-free TNPP replacement, with its own substance page covering identity and status in detail.
5. PVC co-stabilizer phosphites: triphenyl, diphenyl alkyl and trialkyl phosphites#
PVC co-stabilizer phosphites such as triphenyl phosphite (TPPi) and diphenyl isodecyl phosphite (DPDP) are liquid phosphites added to calcium-zinc and barium-zinc stabilizers, where they bind zinc chloride and keep early colour and clarity. In Ca/Zn and Ba/Zn systems, zinc chloride formed during PVC degradation is Lewis-acidic and drives "zinc burning"; PVC phosphites chelate that zinc chloride and reduce hydroperoxides, working alongside polyols, beta-diketones and hydrotalcite to control the effect.
Phosphites work next to polyols, beta-diketones and ESBO among the PVC co-stabilizers.
| Substance | CAS | Class | P content | ECHA mapping, typical level |
|---|---|---|---|---|
| Triphenyl phosphite (TPPi) | 101-02-0 | Triaryl phosphite | 10 % | 3 wt% (upper bound) |
| Diphenyl isodecyl phosphite (DPDP) | 26544-23-0 | Alkylaryl phosphite (liquid) | 8.3 % | 3 wt% |
| Diisodecyl phenyl phosphite (PDDP) | 25550-98-5 | Alkylaryl phosphite (liquid) | 7.0 % | 3 wt% |
| 2-Ethylhexyl diphenyl phosphite (EHDP) | 15647-08-2 | Alkylaryl phosphite (liquid) | 9.0 % | 2 wt% |
| Triisodecyl phosphite (TDP) | 25448-25-3 | Trialkyl phosphite | not established | 2 wt% |
| Triisotridecyl phosphite (TTDP) | 77745-66-5 | Trialkyl phosphite | not established | 2 wt% |
ECHA mapping values are typical upper-bound concentrations from the ECHA plastic additives initiative mapping exercise, not recommended dosages.
Triphenyl phosphite has the highest phosphorus content of the PVC liquids in this table, at 10 %; its substance page covers its full regulatory status.
Irgafos 168 vs Antioxidant 626 vs S-9228 vs P-EPQ: Which Phosphite Antioxidant to Choose?#
Irgafos 168 suits most polyolefin processing because it is hydrolytically stable and listed without an EU SML, while Antioxidant 626 gives more activity per gram, S-9228 adds high-temperature and hydrolysis resistance, and P-EPQ gives the highest reactivity and gel control in LLDPE. Buyers should compare phosphites by CAS number and phosphorus content, not by trade name, since one CAS number can carry six or more trade names across suppliers.
| Grade (trade names) | CAS | MW (g/mol) | mp (°C) | P content | Hydrolysis resistance | EU 10/2011 FCM / SML | FDA 178.2010 cap in olefin polymers |
|---|---|---|---|---|---|---|---|
| Irgafos 168 (AO-168, Songnox 1680, Alkanox 240, Doverphos S-480, Everfos 168, Hostanox PAR 24) | 31570-04-4 | 646.9 | 183-186 | ~4.8 % (theoretical) | Hydrolytically stable (BASF) | FCM 671, no SML (OML applies) | 0.25 % propylene polymers; 0.2 % ethylene polymers |
| Antioxidant 626 (Ultranox 626, Irgafos 126, PEP-24, Songnox 6260) | 26741-53-7 | 604.7 | 170-180 | not established | Lower than 168; TIPA-stabilised grades exist | FCM 652, SML 0.6 mg/kg | 0.1 % (grade with ≤1 % TIPA) |
| Doverphos S-9228 (Songnox 9228) | 154862-43-8 | 853.0 | >228 | not established | Better than 626 ("second generation") | FCM 773, SML 5 mg/kg (sum with phosphate and 2,4-dicumylphenol) | 0.15 % all polymers |
| ADK STAB PEP-36 | 80693-00-1 | 632.7 | not established | not established | not established | FCM 746, SML 5 mg/kg (sum phosphite + phosphate) | 0.25 % PP; 0.05 % olefin films ≤100 µm |
| P-EPQ (Irgafos P-EPQ, Hostanox P-EPQ, Songnox PQ) | 119345-01-6 (main component 38613-77-3) | 1,035.4 (main component) | 85-110 (FDA spec) | min 5.4 % (FDA spec) | not established | FCM 760, SML 18 mg/kg | 0.1 % (PE ≥0.94); 0.15 % (PE <0.94) |
| Ethanox 398 | 118337-09-0 | 486.6 | not established | not established | Hydrolysis resistant | FCM 759, SML 6 mg/kg | 0.25 % (listed food types); 0.1-0.25 % PP/HDPE for other food types |
| Irgafos 38 | 145650-60-8 | 514.7 | not established | not established | not established | FCM 769, SML 5 mg/kg (sum phosphite + phosphate) | 0.3 % olefin polymers (listed food types); 0.1 % propylene polymers |
"Not established" means the value is not confirmed in our verified source library. FDA limits are legal maxima, not recommended dosages.
Phenolic and phosphite grades side by side, including Irganox 1010 and BHT, are in the antioxidant grade comparison.
Request quotes for Irgafos 168, Antioxidant 626, S-9228 or P-EPQ: grade or CAS number, TIPA or hydrotalcite grade, volume, polymer and country, through the plastic additive supplier finder.
Why Do Phosphite Antioxidants Hydrolyse?#
Phosphite antioxidants hydrolyse because water attacks the P-O ester bonds on the phosphorus atom, and the most reactive grades, such as the spiro-diphosphite Antioxidant 626, are also the least resistant to moisture. Hydrolytic stability runs roughly opposite to reactivity in hydroperoxide reduction: Irgafos 168 is described by BASF as hydrolytically stable, while Antioxidant 626 has lower hydrolytic stability and is often supplied with a stabilising trace amine.
What happens when a phosphite antioxidant hydrolyses?#
A hydrolysed phosphite splits into a phenol, such as 2,4-di-tert-butylphenol, and an acidic hydrogen phosphite, which lowers the remaining antioxidant activity and can cause feeding problems, black specks and acidity in the compound. Schwetlick's work shows that aryl phosphites hydrolyse at 150-180 °C into phenols and hydrogen phosphites, some of which act catalytically on further hydrolysis; the released phenols do add some synergistic antioxidant activity, but the net effect on the processor is negative. Papanastasiou and Allen, in Polymer Degradation and Stability 91 (2006) 2675, describe the hydrolysis mechanism of the Antioxidant 626 diphosphite (marketed as Alkanox P-24) in detail.
The 4 effects of phosphite hydrolysis on processing are listed below.
- Caking and feeding problems in the extruder hopper
- Black specks in the finished part
- Acidity, which carries a corrosion risk for processing equipment
- Loss of active phosphite available to protect the melt
How is phosphite hydrolysis prevented?#
Phosphite hydrolysis is prevented by choosing a hindered or second-generation grade, buffering it with a trace amine or hydrotalcite, and keeping the powder dry from warehouse to feeder.
- Choose a hydrolysis-resistant grade, such as Irgafos 168, Doverphos S-9228 or Ethanox 398.
- Use amine-stabilised grades that contain up to about 1 % triisopropanolamine (TIPA).
- Use hydrotalcite-buffered blends, such as Songnox 6280 (Songnox 6260 combined with Mg/Al hydrotalcite in a 93:7 ratio).
- Store the additive in moisture-barrier packaging, such as aluminium-coated bags; Songwon ships its phosphites 6260, 6280, 9228, 9228T and PQ this way in 20 kg bags.
- Add an acid scavenger, such as hydrotalcite or calcium stearate, to the finished compound.
Hydrotalcite and calcium stearate act as acid scavengers in the compound itself, buffering any acidity from a hydrolysed phosphite, and are compared under acid scavengers.
How Much Phosphite Antioxidant Is Used?#
Phosphite antioxidants are used at 0.05-0.2 wt% in polyolefins, normally 1 to 4 times the weight of the phenolic antioxidant, while PVC liquid phosphites appear at up to about 2-3 wt% in ECHA's mapping of typical concentrations. FDA percentages listed elsewhere on this page are legal maxima, not dosages, and should not be read as supplier-recommended levels.
| Polymer / use | Phosphite | Level | Basis |
|---|---|---|---|
| Polyolefins (PP, PE) | Irgafos 168 | 0.05-0.2 wt% | Supplier guidance |
| Recycled PP / HDPE | Irganox 1010 + Irgafos 168 binary blend | 0.1-0.3 wt% of blend | Songwon |
| PP recyclate (study) | Irgafos 168 with Irganox 1010 | 1,000 ppm 168 + 500 ppm 1010 | Study dose |
| Recycled LLDPE | P-EPQ | 0.05-0.1 wt% | Songwon multipass and gel-count data |
| Polyamide | Phenolic + phosphite | 0.2-0.8 % (total) | SpecialChem summary (supplier source) |
| Flexible and rigid PVC | TPPi, DDPP | 3 wt% typical | ECHA mapping, upper bound |
| Flexible and rigid PVC | EHDP, TDP, TTDP | 2 wt% typical | ECHA mapping |
Supplier or mapping ranges; trials decide the final level. FDA 21 CFR 178.2010 percentages are legal maxima and are not listed here as dosages.
PVC recipes quote phosphites in phr; convert with the PHR to weight percent calculator.
Phosphite dosage in polypropylene and polyethylene#
Polypropylene and polyethylene take 0.05-0.2 wt% Irgafos 168, usually paired 1:1 or 2:1 with Irganox 1010, because PP loses melt viscosity by chain scission and LLDPE forms gels when hydroperoxides survive the extruder. Commodity PP grades typically carry less than 400 ppm phenolic antioxidant as base stabilisation, so the phosphite share of the package does most of the melt-protection work. One supplier reports a 20-50 % rise in melt flow rate after a single extrusion pass at 230 °C for unstabilised PP, illustrating the scale of the unprotected chain scission the phosphite package addresses.
Full PP packages are on antioxidants for polypropylene. Gel control in LLDPE film is covered on antioxidants for polyethylene.
Phosphite dosage in recycled polyolefins#
Recycled PP and HDPE are restabilised with 0.1-0.3 wt% of an Irganox 1010 / Irgafos 168 blend, because the phosphite left in post-consumer resin is partly spent and the recyclate carries oxidised groups from its first life. Songwon's multipass data covers restabilisation at 250 °C for PP and 220 °C for HDPE. Rudolf Pfaendner defines restabilisation of recycled plastics in Polymer Degradation and Stability 203 (2022) 110082, and P-EPQ is used in recycled LLDPE at 0.05-0.1 wt% specifically for its gel-reduction effect.
Recyclate packages are set out under restabilization of recycled plastics.
Phosphites in polycarbonate, polyamide and PET#
Polycarbonate, PET and other high-temperature polymers use hydrolysis-resistant diphosphites such as Doverphos S-9228 mainly to protect colour during processing. FDA caps for these polymers are legal maxima, not dosages: S-9228 is capped at 0.2 % in polycarbonate and 0.3 % in polyetherimide, Antioxidant 626 at 0.25 % in polycarbonate, P-EPQ at 0.1 % in polycarbonate, and Irgafos 168 at 0.3 % in polycarbonate and 1 % in nylon (conditions E-G). In a study of glass-filled PA56T aged at 150 °C, antioxidants including Irganox 1098 and Doverphos S-9228 slowed the ageing rate but did not change the underlying ageing pathway.
Phosphites in PVC#
PVC uses liquid phosphites as co-stabilizers inside calcium-zinc and barium-zinc systems, at up to about 2-3 wt% in ECHA's mapping, to chelate zinc chloride and hold early colour. Flexible PVC flooring commonly uses phosphite boosters together with liquid Ca/Zn, Ba/Zn or K/Zn stabilizers and ESBO. Antioxidant 626 carries an FDA cap of 0.86 % in PVC (not for foods above 15 % alcohol). TNPP remains a legacy phosphite for this use, but its SVHC status (below) means it is not a recommended choice for new formulations.
Zinc burning and its co-stabilizers are explained on calcium-zinc stabilizers.
How Is the Performance of Phosphite Antioxidants Tested?#
The performance of a phosphite antioxidant is tested by extruding the compound several times and measuring melt flow rate and yellowness index after each pass, because phosphites protect melt viscosity and colour rather than long-term heat resistance. Melt flow rate is measured per ISO 1133 or ASTM D1238-26 (PP at 230 °C / 2.16 kg), and multipass extrusion trials commonly run at 250 °C for PP and 220 °C for HDPE. Yellowness index follows ASTM E313-20 (reapproved 2025); the withdrawn standard ASTM D1925 is never used on current testing pages.
- Melt flow rate after repeated extrusion passes, tracking the phosphite's effect on chain scission and gel formation
- Yellowness index (ASTM E313), tracking colour retention through processing
- Gel count in LLDPE film, tracking the phosphonite P-EPQ's specific benefit
- Oxidative induction time (OIT), which mainly detects residual phenolic antioxidant rather than the phosphite itself
A stable melt flow rate over several extrusion passes is the main phosphite target in these trials. OIT is measured by differential scanning calorimetry per ISO 11357-6:2018 and ASTM D3895, typically at 190-220 °C; because phosphites and thioesters contribute little to the OIT signal at those temperatures, the test mostly reflects the phenolic antioxidant and underrates the phosphite's contribution to processing stability. Why oxidative induction time (OIT) underrates phosphites is explained on the OIT test page.
Which Phosphite Antioxidants Are Allowed in Food Contact?#
Food-contact plastics may contain only phosphite antioxidants listed in Regulation (EU) No 10/2011 (within each substance's specific migration limit) or in US 21 CFR 178.2010 (within each polymer's weight limit), and Weston 618 is one grade that the EU list does not include. Neither system authorises a use outside its listed conditions, and this reference never describes a listing as "FDA approved"; the correct phrasing is "listed" or "authorised".
Phenolic and thioester limits are on the food contact antioxidants page.
EU 10/2011 SMLs and FDA 21 CFR 178.2010 limits for phosphites#
Antioxidant 626 shows how far the two systems differ: the EU limits its migration to 0.6 mg/kg of food (FCM 652), while the US caps it at 0.1 wt% of olefin polymers and 0.86 wt% of PVC under 21 CFR 178.2010.
| Substance | CAS | EU 10/2011 FCM, SML | FDA 21 CFR 178.2010 (abridged) |
|---|---|---|---|
| Irgafos 168 | 31570-04-4 | FCM 671, no SML (OML 10 mg/dm2 applies) | 0.25 % propylene polymers; 0.2 % ethylene polymers; 0.2 % EVA; 0.5 % elastomers; 1 % nylon (E-G); 0.3 % PC; 0.2 % PS/HIPS |
| Antioxidant 626 | 26741-53-7 | FCM 652, SML 0.6 mg/kg | 0.1 % olefin polymers (≤1 % TIPA grade); 0.86 % PVC (not >15 % alcohol); 0.25 % PC |
| Doverphos S-9228 | 154862-43-8 | FCM 773, SML 5 mg/kg (substance + phosphate + 2,4-dicumylphenol) | 0.15 % all polymers; 0.2 % PC; 0.3 % PEI |
| ADK STAB PEP-36 | 80693-00-1 | FCM 746, SML 5 mg/kg (phosphite + phosphate) | 0.25 % PP; 0.05 % olefin films ≤100 µm |
| Irgafos 38 | 145650-60-8 | FCM 769, SML 5 mg/kg (phosphite + phosphate) | 0.3 % olefin polymers (listed food types); 0.1 % propylene polymers |
| P-EPQ | 119345-01-6 (main component 38613-77-3) | FCM 760, SML 18 mg/kg (main component FCM 688, SML 18 mg/kg) | 0.1 % olefin polymers (PE ≥0.94); 0.15 % PE <0.94; 0.1 % PC; 0.2 % PS; 0.3 % HIPS |
| Ethanox 398 | 118337-09-0 | FCM 759, SML 6 mg/kg | 0.25 % all polymers (listed food types, conditions B-H); 0.1-0.25 % PP, propylene copolymers, HDPE (other food types, ≤375 µm) |
| TNPP | 26523-78-4 | FCM 69, SML 30 mg/kg | Listed as tri(mixed mono- and dinonylphenyl) phosphite |
| Weston 618 | 3806-34-6 | Not listed in Annex I | 0.25 % olefin polymers (items 1.1, 2.1, 3.1); 0.20 % PS/HIPS (E-G); 0.1 % EVA (E-G) |
| Triphenyl phosphite | 101-02-0 | Not listed in Annex I | Not established |
FDA limits are maximum use levels, not recommended dosages. EU values are checked against the consolidated text of 16 March 2025.
Food types and conditions of use are decoded on the 21 CFR 178.2010 regulation page. SML and OML rules are explained on EU 10/2011.
NIAS from phosphites: Irgafos 168 phosphate and 2,4-DTBP#
The main non-intentionally added substances from phosphites are their own breakdown products: Irgafos 168 turns into its phosphate (AO168=O) as it works and releases 2,4-di-tert-butylphenol (2,4-DTBP) when it hydrolyses. 2,4-DTBP is a NIAS, not itself on the Union list, and has been measured at up to 45.568 ± 31.513 mg/kg in BOPP and LDPE food-contact products; AO168=O has also been detected in urban PM2.5 in China at up to 851 ng/m3 (median 153 ng/m3). Several EU SMLs in the table above, including those for PEP-36, S-9228 and Irgafos 38, are expressed as sums that include the phosphate or phenol breakdown product, tying the SML directly to NIAS assessment under Article 19 of the regulation. Markley et al. published an FDA safety assessment of Irgafos 168 in 2023, setting an acceptable daily intake of 1 mg/kg body weight per day and a cumulative estimated daily intake for Irgafos 168 plus its phosphate of 0.09 mg/kg body weight per day.
Risk assessment of breakdown products is covered under NIAS (non-intentionally added substances).
TNPP: SVHC status and nonylphenol-free replacements#
TNPP has been on the REACH Candidate List since 16 July 2019 as an endocrine disruptor for the environment, and since the update of 21 January 2025 the listing covers TNPP itself, not only its 4-nonylphenol content. The original listing applied to TNPP containing 0.1 % w/w or more of 4-nonylphenol under Article 57(f); the 21 January 2025 update (the 32nd Candidate List update) extended the entry to cover TNPP's own intrinsic endocrine-disrupting properties. TNPP is not in Annex XIV as of September 2026. An SVHC present in an article above 0.1 % w/w triggers a REACH Article 33 communication duty and SCIP database notification. TNPP also carries the harmonised classifications Skin Sens. 1 (H317), Aquatic Acute 1 (H400) and Aquatic Chronic 1 (H410), and it remains listed in EU 10/2011 (FCM 69, SML 30 mg/kg) despite its SVHC status. Weston 705, a nonylphenol-free liquid phosphite, is marketed as a drop-in replacement for TNPP in these applications.
Every additive on the SVHC Candidate List is tracked with its date.
Who Makes Phosphite Antioxidants?#
Phosphite antioxidants are made by BASF (Irgafos), SI Group (Weston, Alkanox, Ultranox, Ethanox), Songwon (Songnox), Adeka (ADK STAB), Dover Chemical (Doverphos) and Clariant (Hostanox), and Irgafos 168 alone is sold under at least 6 trade names. SI Group's antioxidant range includes brands acquired from Albemarle in 2014. Other producers named in this reference include Everspring (Everfos), Galata Chemicals (Markphos, for PVC liquids) and Rianlon (Rianox). Songwon describes itself as the world's second-largest polymer stabilizer producer, a company claim rather than an independently verified ranking. US EPA Chemical Data Reporting for 2023 puts Irgafos 168 production at 40 to under 55 million pounds and TNPP at 10 to under 50 million pounds, both far above Antioxidant 626 and Doverphos S-9228, each at 1 to under 5 million pounds.
Buyers should specify the CAS number, whether the grade is TIPA- or hydrotalcite-stabilised, and the packaging, not only the trade name, since one CAS number is sold under several brand names with potentially different moisture protection.
| Same CAS, different trade names (not a performance-equivalence claim) |
|---|
| Irgafos 168 = Songnox 1680 = Alkanox 240 = Doverphos S-480 = Everfos 168 = Hostanox PAR 24 (31570-04-4) |
| Antioxidant 626 = Ultranox 626 = Irgafos 126 = ADK STAB PEP-24 = Alkanox P-24 = Doverphos S-9432 = Songnox 6260 = Weston 626 (26741-53-7) |
| Doverphos S-9228 = Songnox 9228 (154862-43-8) |
| P-EPQ = Irgafos P-EPQ = Hostanox P-EPQ = Songnox PQ (119345-01-6) |
| TNPP = Weston TNPP = Irgafos TNPP = Naugard TNPP = Doverphos 4 = Sumilizer TNP (26523-78-4) |
| Triphenyl phosphite = Weston TPP = ADK STAB TPP = Mark CH 66 = Rostabil TPP (101-02-0) |
| Triisotridecyl phosphite = Rianox 571 (77745-66-5); triisodecyl phosphite = Lankromark LE164 (25448-25-3) |
Plants and grades by company are in the directory of polymer antioxidant manufacturers.
Request quotes for phosphite and phosphonite antioxidants from multiple makers at once with the plastic additive supplier finder.
What Other Antioxidants Work Alongside Phosphites?#
Phosphites cover melt processing, so a complete antioxidant package adds a hindered phenol for radical scavenging and, where needed, a thioester for long-term heat or a hydroxylamine for phenol-free systems. Antioxidant classes for plastics divide into primary antioxidants (hindered phenols, aminics), secondary antioxidants (phosphites, phosphonites, thioesters, disulfides) and carbon-radical scavengers (benzofuranone lactones, hydroxylamines, acrylated phenols). The usual phenolic partner is Irganox 1010.
Thioester antioxidants for long-term heat ageing#
Thioester antioxidants such as DLTDP and DSTDP complement phosphites: phosphites protect the melt, thioesters protect the part during long-term heat ageing at about 100-150 °C. Thioesters are antagonistic toward HALS light stabilizers, because their acidic sulfur oxidation products deactivate the HALS. DLTDP (FCM 294), DSTDP (FCM 368) and ditetradecyl thiodipropionate (FCM 894) share EU group restriction 14, with a combined SML(T) of 5 mg/kg expressed as the sum of the three substances and their oxidation products.
DLTDP and DSTDP are covered under thioester antioxidants.
Phenol-free stabilization with hydroxylamines and phosphites#
Phenol-free stabilization pairs a phosphite with a hydroxylamine instead of a hindered phenol, which prevents the gas fading that NOx causes in phenol-stabilized PP fibres. NOx converts phenolic antioxidants into coloured quinone methides; N,N-dibenzylhydroxylamine, covered in US patent 4,590,231, prevents this gas fading when paired with a phenolic antioxidant in PP. Irgastab FS 042 (CAS 143925-92-2, EU FCM 768) is used at up to 0.1 % in polyolefins in this type of system.
Hydroxylamine and lactone systems are set out under phenol-free stabilization. Causes of pinking and gas fading are compared on the discoloration page.
Is phosphite safe for humans?#
The phosphite antioxidants used in plastics are assessed substance by substance: FDA set an acceptable daily intake of 1 mg/kg body weight per day for Irgafos 168 in 2023, while TNPP carries a harmonised skin-sensitiser classification and is an SVHC. PEP-36 and Doverphos S-9228 carry the harmonised classification Aquatic Chronic 4 (H413), while Irgafos 168 is reported as not classified in aggregated self-classification notifications, which are not a harmonised CLP entry. This answer covers only the phosphite esters used as plastics antioxidants, not agricultural phosphite salts.
Is "Irganox 168" the same as Irgafos 168?#
Yes: "Irganox 168" is a common misspelling of Irgafos 168, because BASF uses the Irganox name for phenolics and blends and the Irgafos name for phosphites.
Is phosphite a fungicide?#
Agricultural "phosphite" fungicides and fertilisers are separate products used on crops, and this reference covers only phosphite esters used as antioxidants in plastics.