Introduction to Antioxidant 168 (Irgafos 168)
Antioxidant 168 (CAS 31570-04-4), widely known by the trade name Irgafos 168, is tris(2,4-di-tert-butylphenyl) phosphite. It is one of the most important secondary antioxidants in the plastics industry, used to protect polymers — above all polyolefins — from the damage caused by heat and oxygen during processing and service.
Where a phenolic “primary” antioxidant traps the radicals that start oxidation, a phosphite “secondary” antioxidant destroys the hydroperoxides that carry the reaction forward. The two are complementary, and the classic combination of a hindered phenolic antioxidant (such as Irganox 1010) with a phosphite (such as Irgafos 168) is one of the most widely used stabiliser systems in the world.
This article covers the identity, properties, chemistry, production, specification, applications and sourcing of antioxidant 168, with particular attention to the practical issue that dominates its handling: hydrolysis. PolyBlueChem can supply antioxidant 168 (CAS 31570-04-4); see the product page for the current specification.
Chemical Identity and Structure
Antioxidant 168 is a triaryl phosphite: a central phosphorus(III) atom bonded to three 2,4-di-tert-butylphenoxy groups. The bulky tert-butyl groups are not decoration — they protect the phosphorus–oxygen bonds from hydrolysis and make the molecule an effective, relatively hydrolytically stable phosphite.
| Property | Value |
|---|---|
| Common names | Antioxidant 168; Irgafos 168; tris(2,4-di-tert-butylphenyl) phosphite |
| Abbreviation | AO-168 / P-168 |
| CAS number | 31570-04-4 |
| Molecular formula | C42H63O3P |
| Molecular weight | Approx. 646.9 g/mol |
| Class | Organophosphorus secondary antioxidant (phosphite) |
| SMILES | CC(C)(C)c1ccc(OP(Oc2ccc(C(C)(C)C)cc2C(C)(C)C)Oc3ccc(C(C)(C)C)cc3C(C)(C)C)c(c1)C(C)(C)C |
Physical and Chemical Properties
| Property | Typical value / behaviour |
|---|---|
| Appearance | White crystalline powder |
| Melting point | Approx. 183–187 °C |
| Water solubility | Practically insoluble |
| Solubility in organics | Soluble in aromatic and other common organic solvents (clear solution) |
| Phosphorus content | Approx. 4.8% (theoretical) |
| Hydrolytic stability | Good for a phosphite, but hydrolysis must still be controlled |
| Main hydrolysis product | 2,4-Di-tert-butylphenol (DTBP) |
The white crystalline form, high melting point and very low water solubility are all commercially relevant: they make the product easy to handle as a powder, but they also mean that protecting it from moisture is essential, since a hydrolysed phosphite is no longer an effective antioxidant.
How Phosphite Antioxidants Work
Polymer oxidation proceeds by a well-understood chain reaction. Heat, light and shear generate radicals; those radicals react with oxygen to give peroxy radicals; the peroxy radicals abstract hydrogen to form hydroperoxides (ROOH); and the hydroperoxides then decompose to give more radicals, which propagate the cycle and ultimately break the polymer chains. Antioxidants interrupt this cycle at different points:
- Primary (radical-trapping) antioxidants: hindered phenols and amines donate hydrogen to peroxy radicals, stopping chain propagation. They are the main defence against long-term thermal-oxidative ageing.
- Secondary (hydroperoxide-decomposing) antioxidants: phosphites and thioethers convert hydroperoxides into non-radical products (alcohols), removing the reservoir of reactive species before they can decompose into new radicals.
Antioxidant 168 is a secondary antioxidant. Its value is greatest during melt processing, when the polymer is hot, sheared and exposed to oxygen — the conditions that generate hydroperoxides fastest. By mopping up hydroperoxides as they form, the phosphite protects both the polymer and the phenolic antioxidant (which otherwise has to deal with the hydroperoxides itself), giving a combination that is far more effective than either component alone. This is why processing stabiliser packages almost always pair a phosphite with a phenol.
Industrial Production
Antioxidant 168 is made by reacting phosphorus trichloride with 2,4-di-tert-butylphenol. The reaction is a straightforward nucleophilic substitution that releases hydrogen chloride, and the key engineering tasks are achieving complete conversion, removing HCl cleanly and delivering a product with low free DTBP and high transmittance.
| Stage | Purpose | Notes |
|---|---|---|
| 1. Raw materials | Provide P and ArO groups | Phosphorus trichloride (PCl3) and 2,4-di-tert-butylphenol (DTBP) |
| 2. Reaction | Form the triaryl phosphite | 3 ArOH + PCl3 → (ArO)3P + 3 HCl; controlled temperature; HCl scavenging |
| 3. HCl removal | Purify the product | Removal of hydrogen chloride and by-product control |
| 4. Isolation and purification | Obtain a clean solid | Crystallisation/filtration; washing; drying |
| 5. Finishing and QC | Meet the specification | Milling; assay, DTBP content, transmittance, acid value |
The 2,4-di-tert-butylphenol used as the feedstock is also the molecule’s hydrolysis product, which is why “free DTBP” appears as a specification parameter: a well-made, well-stored product contains very little of it.
Specifications and Quality Control
Antioxidant 168 is controlled by assay, melting point, colour/transmittance and the free-DTBP and acid-value limits that indicate the material is intact and not hydrolysed. A representative specification is shown below.
| Parameter | Typical specification |
|---|---|
| Appearance | White powder |
| Main content | ≥ 99% |
| Melting point range | 186–187 °C |
| Heating decrement (volatiles) | ≤ 0.3% |
| Solubility | Clear solution |
| Transmittance | ≥ 98% at 425 nm and ≥ 98% at 500 nm |
| Acid value | ≤ 0.3 mg KOH/g |
| Hydrolysis resistance | Qualified |
| 2,4-Di-tert-butylphenol (DTBP) content | ≤ 0.2% |
Transmittance is a colour/purity indicator for a white powder destined for clear or light-coloured polymers. The acid value rises as the phosphite hydrolyses, so together with the DTBP content it is the most direct check that the material has not been degraded by moisture.
Hydrolysis: The Key Practical Issue
Phosphites are susceptible to hydrolysis, and antioxidant 168 is no exception. Water attacks the phosphorus–oxygen bond and converts the phosphite into 2,4-di-tert-butylphenol (and phosphorus acid species), destroying its antioxidant activity in the process. Because hydrolysis is the main way the product degrades, it drives the way the material is specified, packed and stored.
- What hydrolysis does. It consumes the active phosphite, releasing DTBP; the product loses antioxidant power and can develop acidity and odour.
- How it is controlled. The bulky tert-butyl groups provide steric protection, and the specification limits both the free DTBP and the acid value.
- How it is managed in practice. Store cool and dry in sealed packaging, keep stock rotating, and avoid exposing the powder to humid air. In some formulations the phosphite is supplied pre-blended or in a form that reduces moisture contact.
- How it is detected. A rising acid value or DTBP content, or a fall in transmittance, signals that the material has taken up moisture.
Grades and Forms
- Standard powder: the white crystalline powder used in most plastics processing.
- Blended / one-pack stabilisers: antioxidant 168 is frequently supplied blended with a phenolic antioxidant (for example Irganox 1010) as a ready-to-use stabiliser system.
- Low-dust and pre-dispersed forms: for easier, cleaner dosing in continuous lines.
- Food-contact and regulated grades: where the polymer is intended for regulated applications, the grade and its documentation must match.
Applications: Polyolefins and Plastics
The dominant use of antioxidant 168 is in polyolefins, where it acts as a processing stabiliser. The table below summarises the main applications.
| Application | Role of antioxidant 168 |
|---|---|
| Polypropylene (PP) | Processing stabiliser; protects the melt and helps retain colour and melt flow |
| Polyethylene (PE, HDPE, LDPE, LLDPE) | Processing and colour stabilisation |
| Elastomers and rubbers | Protection during compounding and service |
| Adhesives and sealants | Colour and heat stability |
| Engineering plastics and compounds | Stabilisation in demanding melt-processing conditions |
| Masterbatches and one-pack systems | Delivered together with a phenolic antioxidant |
In polypropylene especially, where processing temperatures are high and the polymer is prone to chain scission, a phosphite is close to indispensable: it is the additive that keeps the polymer from yellowing and losing melt-flow stability when it is extruded or moulded.
The Irganox 1010 + Irgafos 168 Combination
The most common way antioxidant 168 is used is together with a hindered phenolic primary antioxidant, above all Irganox 1010 (pentaerythritol tetrakis(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate)). The table compares their roles.
| Property | Antioxidant 168 (phosphite) | Irganox 1010 (phenol) |
|---|---|---|
| Type | Secondary (hydroperoxide decomposer) | Primary (radical trapper) |
| Main benefit | Melt-processing and colour stability | Long-term thermal-oxidative stability |
| Typical stage | During processing (hot, sheared melt) | During processing and long-term service |
| Combination | Used together for a complete stabiliser system | Used together for a complete stabiliser system |
Used alone, a phosphite protects the polymer well during processing but leaves long-term ageing less well covered; used alone, a phenol is effective long-term but works harder during processing because it has to cope with hydroperoxides as well. Together, they divide the work and give both good melt-processing stability and good long-term performance. See the related reading for a fuller treatment of how the two work together.
Other Applications
- Rubber and elastomers: processing and colour protection during compounding.
- Adhesives, sealants and coatings: heat and colour stability in hot-melt and solvent systems.
- Lubricants and functional fluids: as a secondary antioxidant in appropriate formulations.
- Polymer recycling: reprocessing exposes polymers to repeated heat history, so phosphite antioxidants are relevant to the quality of recycled polyolefins.
Formulation and Dosage Guidance
- Dosage. Phosphite antioxidants are typically used at a low level (commonly in the range of a few hundred to a few thousand ppm, agreed with the stabiliser supplier); the exact level balances processing protection against cost and any regulatory limits.
- Balance with the phenol. The phosphite-to-phenol ratio is tuned to the polymer and the processing conditions; the supplier’s guidance and the customer’s trials set the final figure.
- Feeding. The powder is usually fed as part of a masterbatch or a one-pack blend, which improves dispersion and reduces dust.
- Moisture control. Keep the powder dry; moisture uptake hydrolyses the phosphite and raises the acid value and DTBP content.
- Storage. Store cool and dry in sealed packaging, and rotate stock.
Safety, Health and Environment
Antioxidant 168 is a solid industrial additive and is handled as a normal chemical powder. The main practical points are:
- Dust. Avoid generating and inhaling dust; use appropriate ventilation and personal protective equipment, following the SDS.
- Skin and eye contact. Avoid contact; wear gloves and eye protection where dust may be generated.
- Environment. Prevent release to drains and watercourses; dispose of waste in line with local regulations.
- Documentation. The SDS and, for food-contact uses, the relevant food-contact statements should accompany the product.
Regulatory Status
Antioxidant 168 is an established industrial polymer additive. It is registered under REACH in the EU and appears on the TSCA inventory in the US, and it is widely used in food-contact-approved grades where the relevant regulations allow. Buyers should confirm the grade and its documentation against the intended application:
- SDS and per-batch COA.
- Food-contact or other regulatory statements where required.
- REACH registration and TSCA status.
Market and Supply Overview
- Demand driver. Driven by global polyolefin production, so demand tracks the plastics cycle.
- Production base. Made by a number of specialty additive producers, mainly in Asia, Europe and North America.
- Price drivers. The cost of 2,4-di-tert-butylphenol and phosphorus trichloride, energy, and the grade and documentation required.
- Quality differentiators. Assay, DTBP content, transmittance and hydrolytic stability.
- Packaging. Typically 25 kg bags or drums with moisture-tight liners.
Procurement and Sourcing Guide
- Assay and DTBP. Confirm the main content and the free-DTBP limit, since these define both purity and hydrolysis level.
- Transmittance. Specify transmittance for clear or light-coloured polymers.
- Hydrolysis resistance and acid value. Check these to confirm the material is intact.
- Grade and documentation. Match the grade to your application and request the SDS, COA and any food-contact statements.
- Form. Consider one-pack or pre-dispersed forms to simplify dosing.
- Storage discipline. Confirm packaging is moisture-tight and plan for dry storage.
PolyBlueChem can supply antioxidant 168 (CAS 31570-04-4) as a bulk raw material. To request current specifications, a COA or a quotation, contact the PolyBlueChem team.
Why Polypropylene Needs a Phosphite
Polypropylene is the polymer where phosphite antioxidants earn their place. PP is especially sensitive to thermal-oxidative degradation during melt processing: at extrusion and moulding temperatures it undergoes chain scission, which shows up as a rise in melt-flow rate, loss of impact strength and yellowing. The chemistry is dominated by the hydroperoxide route, which is exactly the reaction a phosphite controls.
The practical consequences of leaving a phosphite out of a PP formulation are familiar to compounders:
- Melt-flow drift. Without hydroperoxide control, the polymer chains break and the melt-flow rate changes, upsetting downstream processing.
- Yellowing. Oxidised species give the polymer a yellow tint, unacceptable in natural, white or light-coloured articles.
- Reduced long-term stability. Hydroperoxides left in the polymer after processing act as a reservoir that shortens service life, even if a phenolic antioxidant is present.
Because the damage happens fastest when the polymer is hot and sheared, the phosphite is at its most valuable during processing — which is why it is often called a processing stabiliser. The table below contrasts the two stages at which a stabiliser acts.
| Stage | Dominant threat | Most useful antioxidant |
|---|---|---|
| Melt processing (extrusion, moulding) | Hydroperoxide formation under heat and shear | Phosphite (secondary), e.g. antioxidant 168 |
| Long-term service (years) | Slow radical chain oxidation | Hindered phenol (primary), e.g. Irganox 1010 |
| Both together | Full life cycle | Phosphite + phenol in one package |
This division of labour is why the two are almost always combined, and why a formulator who wants the best of both worlds uses antioxidant 168 alongside a phenolic antioxidant rather than either on its own.
Phosphites and Other Secondary Antioxidants
Phosphites are not the only secondary antioxidants; thioether (thio-synergist) antioxidants such as DLTDP and DSTDP are also used to decompose hydroperoxides. Each has a different performance and odour profile, and the choice depends on the polymer, the processing temperature and the end use.
| Secondary antioxidant | Mechanism | Notes |
|---|---|---|
| Phosphite (e.g. antioxidant 168) | Reduces hydroperoxides to alcohols | Excellent processing/colour stability; near-colourless; widely used in polyolefins |
| Thioether (e.g. DLTDP/DSTDP) | Decomposes hydroperoxides | Very effective long-term; can give odour/taste issues in some uses |
| Hindered amine (HALS) | Radical scavenging (light stabilisation) | Primarily a light stabiliser, complementary to antioxidants |
Antioxidant 168’s advantages are its very low colour contribution, its effectiveness at low use levels and its good (for a phosphite) hydrolytic stability. Its main vulnerability is hydrolysis, which is why thioethers may be preferred where moisture exposure cannot be controlled, and why the specification of 168 places such emphasis on DTBP content and acid value. In practice, phosphites and thioethers are sometimes combined with a phenol to build a stabiliser package tuned to a specific service life.
Antioxidant 168 in Recycling and Sustainability
As polymer recycling grows, processing stability has become a sustainability issue as well as a technical one. Recycled polyolefins have already experienced one or more heat histories, so their hydroperoxide burden is high and their remaining stability is low. Reprocessing them without adequate stabilisation accelerates degradation, giving recycled resin with poor colour and mechanical properties.
Phosphite antioxidants are relevant here for two reasons:
- They restore processing stability to resins that have already been degraded once, helping recyclers reach acceptable colour and melt-flow values.
- They protect the phenol in the stabiliser package, improving the cost-effectiveness of the whole system.
The practical implication for buyers is that demand for antioxidants such as 168 is increasingly tied not only to virgin polymer production but to the reprocessing and upgrading of recycled streams — an area where the grade, dosing and documentation must be matched to the recycler’s process and end market.
Handling, Packaging and Storage
- Packaging. Supplied in 25 kg bags or drums with moisture-tight inner liners; keep the liner sealed in use.
- Moisture. The single most important storage factor; moisture hydrolyses the phosphite and raises the acid value and DTBP content.
- Temperature. Store at ambient temperature, away from heat and direct sunlight.
- Stock rotation. Use older stock first so that material does not sit for long periods where it may take up moisture.
- Dust. Avoid dust generation and use appropriate ventilation and protective equipment during handling and dosing.
- Shelf life. The product is stable for a defined shelf life when stored correctly; confirm the shelf life and retest policy with the supplier.
Frequently Asked Questions
What is antioxidant 168 and what is its CAS number?
Antioxidant 168 (Irgafos 168) is tris(2,4-di-tert-butylphenyl) phosphite, a secondary antioxidant for plastics; its CAS number is 31570-04-4. See the product page for specifications.
Is antioxidant 168 the same as Irgafos 168?
Yes. Irgafos 168 is a trade name for tris(2,4-di-tert-butylphenyl) phosphite, the same substance as antioxidant 168.
What is the molecular formula of antioxidant 168?
C42H63O3P, with a molecular weight of about 646.9 g/mol.
How is antioxidant 168 made?
By reacting phosphorus trichloride (PCl3) with 2,4-di-tert-butylphenol to form the triaryl phosphite, with removal of hydrogen chloride and purification to a white crystalline powder.
Why is hydrolysis important for antioxidant 168?
Phosphites hydrolyse on contact with water, converting antioxidant 168 into 2,4-di-tert-butylphenol and destroying its activity. The specification’s DTBP and acid-value limits, and dry storage, exist to keep the material intact.
Is antioxidant 168 a primary or secondary antioxidant?
Secondary. It decomposes hydroperoxides, whereas phenolic antioxidants such as Irganox 1010 are primary radical-trapping antioxidants. They are used together.
Which polymers use antioxidant 168?
Mainly polyolefins (polypropylene and polyethylene), where it stabilises the melt during processing and protects colour and melt-flow; also elastomers, adhesives and other plastics.
How should antioxidant 168 be stored?
Cool, dry and in sealed, moisture-tight packaging, with stock rotation, to prevent hydrolysis.
Is antioxidant 168 used in food-contact plastics?
Yes, food-contact-approved grades are available where the applicable regulations permit; confirm the grade and its documentation for the intended food-contact application.
References and Disclaimer
References. Substance identification data for tris(2,4-di-tert-butylphenyl) phosphite (CAS 31570-04-4); published chemistry of phosphite secondary antioxidants and of polymer antioxidant mechanisms; manufacturer technical data for the powder grade. Specifications quoted are representative and should be confirmed against the current certificate of analysis.
Disclaimer. This article is provided for general technical and commercial information only and does not constitute a specification or a warranty of fitness for any particular use. Users are responsible for confirming regulatory status, safe handling and suitability of the material for their own application.