Antioxidant 168 is a phosphite-based secondary antioxidant used primarily to protect polymers during high-temperature processing. Its main job is to react with hydroperoxides formed during polymer oxidation and convert them into non-radical, less reactive products before they can accelerate degradation. For manufacturers, this can mean better melt stability, less discoloration, more consistent molecular weight, and improved processing reliability. Antioxidant 168 is especially effective when combined with a primary hindered phenolic antioxidant such as Antioxidant 1010, because the two additives protect the polymer at different stages of the oxidation cycle.
The material is widely used in polyolefins, engineering plastics, elastomers, adhesives, and other organic substrates. It is valued for its low volatility, strong processing stabilization, and good resistance to hydrolysis compared with many conventional phosphite stabilizers. However, it should not be treated as a universal stand-alone solution: the correct antioxidant package depends on the polymer, processing temperature, residence time, recycled content, end-use conditions, and regulatory requirements.
Antioxidant 168 at a Glance
| Property | Practical Meaning |
| Chemical type | Organo-phosphite / secondary antioxidant |
| Chemical name | Tris(2,4-di-tert-butylphenyl) phosphite |
| CAS number | 31570-04-4 |
| Molecular weight | 646.9 g/mol |
| Typical appearance | White, free-flowing powder; granular forms are also available |
| Typical melting range | 183-186°C |
| Primary function | Processing stabilization by hydroperoxide decomposition |
| Typical use strategy | Often combined with a primary phenolic antioxidant |
| Common polymer families | PE, PP, EVA, polybutene, PC, PA, polyesters, styrenics, elastomers |
Polymer degradation does not begin only after a finished plastic product enters service. A significant portion of oxidative damage can start much earlier, during compounding, extrusion, pelletizing, molding, recycling, or other high-temperature processing steps. Heat and shear can generate reactive species inside the polymer. In the presence of oxygen, this can lead to hydroperoxide formation and a chain of reactions that changes the polymer structure.
Depending on the resin and processing conditions, oxidation may appear as yellowing, loss of melt strength, changes in viscosity or melt flow, chain scission, unwanted crosslinking, odor generation, or a decline in mechanical performance. Processing stabilizers are used to interrupt this chemistry before the damage becomes difficult to control.
What type of antioxidant is Antioxidant 168?
Antioxidant 168 belongs to the phosphite class of secondary antioxidants. The word “secondary” does not mean less important. It describes the part of the oxidation mechanism that the additive targets. Primary antioxidants, especially hindered phenols, are designed mainly to neutralize free radicals. Secondary antioxidants such as Antioxidant 168 act mainly on hydroperoxides, which are important intermediates in thermo-oxidative degradation.
How Antioxidant 168 works during processing
During polymer autoxidation, hydroperoxides can decompose into highly reactive radical species that continue the degradation cycle. Antioxidant 168 reacts with these hydroperoxides and converts them into non-radical products. By reducing the concentration of hydroperoxides before they decompose destructively, the stabilizer helps slow process-induced oxidation and also reduces the burden on the primary antioxidant in the formulation.
This explains why Antioxidant 168 is often used together with a hindered phenolic antioxidant. The phosphite provides strong short-term protection during melt processing, while the primary antioxidant contributes radical-scavenging protection and can support longer-term thermal stability after fabrication. The combination is generally more effective than relying on either mechanism alone when both processing and service-life protection are required.
Read more: Antioxidant 1010 vs Antioxidant 1076 | Which One Should You Choose?
Technical Properties
The technical identity of Antioxidant 168 is tris(2,4-di-tert-butylphenyl) phosphite, CAS 31570-04-4. BASF technical information for Irgafos 168 lists a molecular weight of 646.9 g/mol, a typical melting range of 183-186°C, a specific gravity of about 1.03 at 20°C, and white free-flowing powder or granule product forms. These values are useful for material identification and formulation work, but purchasing specifications should always be taken from the actual manufacturer’s current TDS and COA for the supplied grade.
Chemical nature and stabilization mechanism
The phosphite functionality is the key to Antioxidant 168’s role as a processing stabilizer. It is consumed as it reacts with oxidation intermediates, so its performance should be considered as part of the total antioxidant package rather than as a permanent, unlimited protective reservoir. This is one reason formulation design must account for processing severity, repeated heat history, and whether the material contains recycled resin.
Low volatility and high-temperature processing
Low volatility is an important practical advantage for a polymer additive used at elevated temperature. A stabilizer that is lost too easily through volatilization may not remain in the polymer long enough to provide the intended protection. Antioxidant 168 is widely selected for demanding polymer-processing conditions because it can remain effective through common compounding and fabrication operations while helping control oxidation-related changes in the melt.
Hydrolytic stability and moisture control
Commercial Antioxidant 168 is known for relatively strong hydrolytic stability among phosphite processing stabilizers. That does not mean moisture can be ignored. Phosphites can be sensitive to prolonged exposure to humidity or unsuitable storage conditions, and hydrolysis can reduce active phosphite content and alter product quality. In industrial handling, the material should therefore be stored in accordance with the supplier’s SDS and TDS, protected from contamination, and kept in properly closed packaging.
Solubility and compatibility
Antioxidant 168 has very low solubility in water and greater solubility in several organic media. In polymer applications, however, the more important issue is effective dispersion and compatibility within the resin or additive package. Poor dispersion can produce local under-stabilization even when the overall dosage appears correct. Masterbatching, pre-blending, and suitable feeding equipment can help improve distribution in large-scale compounding operations.
Industrial Applications
Antioxidant 168 is used across a broad range of polymers because process-induced oxidation is not limited to one resin family. Its strongest fit is generally in formulations that need protection during melting, compounding, extrusion, molding, fabrication, or recycling. The exact additive package should be selected according to resin chemistry and the final application.
Polyethylene and polypropylene
Polyethylene and polypropylene are among the most common application areas. During extrusion and repeated thermal history, polyolefins can undergo molecular-weight changes that affect melt flow and product consistency. Antioxidant 168 helps limit hydroperoxide-driven degradation during processing. It is frequently paired with a hindered phenolic antioxidant to give the formulation both processing stability and broader thermo-oxidative protection.
Typical use cases include resin compounding, film and sheet extrusion, injection molding, pipe and profile production, fiber processing, and recycled-polymer upgrading. The stabilizer package can be particularly important when a material passes through several heat histories because each processing cycle can consume part of the antioxidant reserve.
EVA, polybutene, and olefin copolymers
BASF lists Antioxidant 168 applications in olefin copolymers such as ethylene-vinyl acetate as well as polybutene. These materials may be used in adhesive, film, encapsulation, or specialty compounding systems where heat exposure during manufacturing can affect color and physical properties. A phosphite processing stabilizer can help preserve processing consistency without relying solely on the primary antioxidant.
Read more: Why Antioxidants Are Essential in Plastic Manufacturing
Engineering plastics: polycarbonate and polyamide
Polycarbonate and polyamide processing can involve demanding melt temperatures and tight requirements for color and property retention. Antioxidant 168 is used in selected engineering-plastic formulations to reduce oxidation during processing. In these systems, additive compatibility, thermal history, and interaction with other stabilizers or pigments should be evaluated carefully because formulation requirements can differ substantially from those of polyolefins.
Polyesters and styrenic polymers
Polyesters and styrene homo- and copolymers are also listed within the broader application range of Antioxidant 168. In styrenic systems, maintaining color and limiting heat-related degradation may be important for both appearance and processing consistency. In polyester systems, the selected stabilizer package must be compatible with the polymer chemistry, processing conditions, and end-use requirements.
Elastomers, adhesives, and tackifier resins
Antioxidant 168 can also be used in elastomers such as BR, SBS, and SEBS, as well as adhesives and tackifier resins. These formulations may experience elevated temperatures during mixing, compounding, hot-melt processing, or application. Oxidative degradation can affect viscosity, color, odor, and performance, so secondary antioxidants can play a useful role in maintaining processing stability.
Application Summary
| Material Family | Typical Context | Role of Antioxidant 168 |
| PE / PP | Extrusion, molding, compounding, recycling | Helps control process oxidation and melt-property changes |
| EVA / olefin copolymers | Films, adhesives, compounds | Supports heat and color stability during processing |
| PC / PA | Engineering-plastic fabrication | Helps reduce high-temperature oxidative damage |
| Styrenics / polyesters | Compounding and fabrication | Supports color and processing consistency |
| Elastomers | Rubber and TPE compounding | Helps protect during thermal mixing and processing |
| Adhesives / resins | Hot-melt and specialty formulations | Helps control viscosity and discoloration |
Advantages
1. Strong processing stabilization
The main advantage of Antioxidant 168 is its ability to protect polymers at the stage where heat and shear can rapidly generate oxidation intermediates. By decomposing hydroperoxides, it helps reduce process-induced degradation before those intermediates create additional radical reactions.
2. Better color retention
Thermo-oxidative degradation often contributes to yellowing or other unwanted color changes. Antioxidant 168 can help reduce discoloration associated with processing damage. This is especially valuable in natural, white, pastel, transparent, and lightly pigmented polymer products where small changes in color are easy to notice.
3. Helps preserve molecular weight and rheology
Oxidation can cause chain scission or, in some polymers, unwanted crosslinking. Both mechanisms can change molecular weight distribution and therefore affect melt flow, viscosity, processability, and final properties. BASF specifically identifies prevention of process-related molecular-weight changes as a key performance benefit of Irgafos 168.
4. Synergy with primary antioxidants
One of the most important practical benefits is synergy with hindered phenolic antioxidants. Antioxidant 168 deals with hydroperoxides, while the primary antioxidant intercepts radicals. This complementary mechanism can increase the efficiency of the stabilization package and extend the useful performance of the primary antioxidant during processing.
5. Low volatility
Low volatility supports retention of the additive during high-temperature processing. This is important in extrusion, compounding, molding, and other operations where an additive may otherwise be lost from the melt before completing its stabilizing role.
6. Broad application range
The stabilizer can be used across multiple polymer and organic-substrate families, allowing formulators to work with a familiar secondary-antioxidant chemistry in many applications. This does not eliminate the need for validation, but it makes Antioxidant 168 a widely recognized reference point in polymer stabilization.
Read more: Primary vs Secondary Antioxidants: What’s the Difference?
Practical Selection and Use Considerations
A good antioxidant is only effective when it is correctly selected, dosed, dispersed, and stored. The following points are especially important when evaluating Antioxidant 168 for an industrial formulation.
- Do not select dosage from a generic number alone. Required loading depends on polymer type, processing temperature, residence time, oxygen exposure, recycled content, target service life, and the rest of the additive package.
- Evaluate the primary/secondary antioxidant ratio as a formulation variable. A system optimized for processing may not be optimized for long-term heat aging, and vice versa.
- Review the supplier TDS, SDS, and batch COA. Confirm identity, appearance, assay or active-content criteria where specified, melting behavior, and any quality parameters relevant to your process.
- Protect the material from unsuitable storage conditions. Hydrolytic stability is an advantage, but prolonged moisture exposure should still be avoided.
- Run process trials when changing supplier, grade, carrier system, or antioxidant ratio. Melt flow, color, odor, OIT, mechanical properties, and accelerated aging can be useful validation tools depending on the application.
- Check regulatory suitability separately for food contact, medical, automotive, electrical, or other regulated applications. A chemical name alone does not establish compliance for every use.
Antioxidant 168 vs. a primary antioxidant
| Comparison Point | Antioxidant 168 | Primary Phenolic Antioxidant |
| Main target | Hydroperoxides | Free radicals |
| Typical role | Processing stabilization | Radical scavenging and longer-term thermal protection |
| Typical chemistry | Phosphite | Hindered phenol |
| Common strategy | Use with a primary antioxidant | Use with a secondary antioxidant when processing protection is needed |
Frequently Asked Questions
What is Antioxidant 168 used for?
Antioxidant 168 is mainly used as a secondary antioxidant and processing stabilizer in polymers. It decomposes hydroperoxides generated during thermo-oxidative processing and helps reduce discoloration, molecular-weight changes, and loss of processing stability.
Is Antioxidant 168 a primary or secondary antioxidant?
It is a secondary antioxidant. Chemically, it is an organo-phosphite. Its main function is hydroperoxide decomposition, while primary hindered phenolic antioxidants mainly act as radical scavengers.
Why are Antioxidant 168 and Antioxidant 1010 used together?
They protect the polymer through complementary mechanisms. Antioxidant 168 provides strong processing stabilization by reacting with hydroperoxides, while Antioxidant 1010 is a hindered phenolic primary antioxidant that intercepts radicals and supports longer-term thermo-oxidative stability.
Which polymers can use Antioxidant 168?
Common application families include polyethylene, polypropylene, polybutene, EVA and other olefin copolymers, polycarbonate, polyamide, polyesters, styrenic polymers, elastomers, adhesives, and tackifier resins. Actual suitability should be confirmed for the specific grade and application.
Is Antioxidant 168 resistant to hydrolysis?
It is recognized as a hydrolytically stable phosphite compared with many conventional phosphites, but moisture control still matters. Storage and handling should follow the supplier’s current TDS and SDS.
Can Antioxidant 168 be used alone?
It can provide processing stabilization, but many polymer systems use it together with a primary antioxidant because the two mechanisms are complementary. Whether a stand-alone or combined system is appropriate depends on processing conditions and the required service-life protection.
How should the dosage of Antioxidant 168 be selected?
Dosage should be determined from the resin, process temperature, residence time, recycled content, target performance, and the other stabilizers in the formulation. Supplier guidance and laboratory or production trials are more reliable than applying one fixed percentage to every polymer.
Conclusion
Antioxidant 168 is one of the most widely recognized phosphite processing stabilizers for polymers. Its value comes from a clear technical role: decomposing hydroperoxides before they can accelerate thermo-oxidative degradation during processing. This mechanism helps protect color, molecular weight, melt behavior, and overall processing consistency across polyolefins, engineering plastics, elastomers, adhesives, and other polymer systems.
Its performance is particularly strong when it is incorporated into a balanced antioxidant package with a primary hindered phenol. For industrial users, the best results come from treating Antioxidant 168 not as a universal additive with one fixed dosage, but as a formulation component that must be matched to the resin, process severity, service-life target, regulatory needs, and quality of the supplied grade.
For a broader view of antioxidant selection across polymers, connect this article to your main category or pillar content on stabilization additives.
