A Comprehensive Guide to the Mechanism and Selection of Auxiliary Antioxidants in High-Temperature Materials
High-temperature engineering plastics (PPS, PEEK, high-temperature nylon, polysulfone) are widely used in aerospace, automotive, electronics and other fields due to their excellent comprehensive properties. However, they are prone to thermo-oxidative degradation during melt processing above 300°C and long-term high-temperature service. This causes molecular chain scission or cross-linking, manifesting as yellowing, embrittlement, deteriorated mechanical properties and unstable melt fluidity. To address this issue, a combined system of primary antioxidants (hindered phenols for radical scavenging) and secondary antioxidants (phosphites/thioesters for hydroperoxide decomposition) is commonly adopted. It forms a dual protective barrier via the synergistic mechanism of "eliminating generated radicals and decomposing precursor substances", which cost-effectively inhibits degradation and maintains the material performance.
Ⅰ、Phosphite Antioxidants
1.Basic type: Antioxidant 168
Mechanism：A classic general-purpose phosphite antioxidant with high cost performance and good hydrolysis resistance.
Performance：It delivers excellent results in polyolefins. However, it features poor thermal stability (decomposition temperature: approx. 240°C), so it is not suitable for high-temperature engineering plastics such as PPS and PEEK with processing temperatures above 300°C.
2.High-performance type
Antioxidant 626 / 627A
Mechanism：It features a pentaerythritol spiro structure with high molecular rigidity.
Performance：It has better thermal stability than Antioxidant 168 (decomposition temperature: approx. 280°C), suitable for PC, PET and some high-temperature nylons. It can also be applied to PEEK processed at relatively low temperatures.
Antioxidant 619F
Mechanism：A high-molecular-weight phosphite antioxidant containing long-chain aliphatic groups.
Performance：It features extremely low volatility and good extraction resistance. Suitable for polyolefins, polyesters and applications requiring long-term thermal stability.
Antioxidant PEP-36
Mechanism：It has a similar structure to Antioxidant 626, yet features larger steric hindrance on benzene ring substituents.
Performance：Boasts excellent thermal stability, effectively inhibiting yellowing and melt index fluctuation during high-temperature processing. It is applicable to high-temperature nylon and polysulfone.
3.Premium performance grade
Antioxidant PEPQ
Mechanism：As a phosphite antioxidant, it contains P-C bonds and delivers superior chemical stability.
Performance：It possesses ultrahigh thermal stability (above 350°C), effectively maintaining processing stability of PPS, PEEK and polysulfone under extreme high temperatures, and provides outstanding color protection. Its main drawback is poor hydrolysis resistance.
Antioxidant S9228 / Revonox® 608
Mechanism：It is a spirocyclic phosphite with large sterically hindered cumyl groups and a high molecular weight.
Performance：It features the best overall properties. With ultrahigh thermal stability (>350°C) and excellent hydrolysis resistance, it is an ideal choice for PPS, PEEK, high-temperature nylon (highly recommended) and polysulfone, ensuring both processing stability and hydrolysis resistance simultaneously.
Ⅱ、Thioester Antioxidants
Core mechanism: The thioether groups (-S-) in the molecule decompose hydroperoxides. Its decomposition products also exhibit antioxidant activity, delivering long-term thermal stability.
Antioxidant DSTP
Mechanism：A conventional long-chain aliphatic thioester.
Performance：It is a cost-effective long-term thermal stabilizer, yet with relatively low thermal stability. Not recommended for high-temperature engineering plastics such as PPS and PEEK.
Antioxidant 412S
Mechanism：A high-molecular-weight multifunctional thioester with four thioether linkages.
Performance：It features high thermal stability (>300°C) and is an upgraded version of DSTP. Specially designed for long-term thermal aging resistance, it is often compounded with high-performance phosphites (e.g. S9228) to provide full-cycle protection for PPS, PEEK and other materials during processing and service.
Ⅲ、Summary: Phosphite Antioxidants (Processing Stabilizers)
Mechanism of action: Trivalent phosphorus atoms (P(III)) reduce hydroperoxides (ROOH) to alcohols, while being oxidized into pentavalent phosphate esters without generating new free radicals.
Ⅳ、Formulation Design Recommendations
The optimal antioxidant solution is a composite system:
- Protection during processing: High-performance phosphites (PEPQ / S9228 / Revonox® 608) – decompose hydroperoxides to prevent processing degradation and yellowing
- Long-term service protection: High-performance thioester (412S) – continuously decompose slowly generated hydroperoxides and retard the deterioration of mechanical properties
- Free radical scavenging: High-molecular-weight hindered phenolic primary antioxidants (e.g. 1010, 1076, 1098) – eliminate generated free radicals
Ⅴ、Quick Reference Guidelines for Antioxidant Selection
- Priority to thermal stability: The 10% weight loss temperature from TGA test of the antioxidant shall be at least 30–50 °C higher than the plastic processing temperature to guarantee sufficient safety margin.
- Hydrolysis resistance: Hydrolysis-resistant grades are mandatory for moisture-sensitive materials such as high-temperature nylon (S9228 / Revonox® 608 are top choices; spiro-structured products like 626 shall be avoided).
- Low volatility: High molecular weight and high melting point are the core factors for low volatility.
- Color protection: PEPQ delivers outstanding performance in polysulfone; S9228 and Revonox® 608 feature the widest compatibility.
- Long-term stability: As a thioester antioxidant, 412S exerts synergistic effects with phosphites to provide full-lifecycle protection.
- Cost consideration: Antioxidant 168 and DSTP are cost-effective yet inferior in performance; S9228, Revonox® 608 and PEPQ offer premium performance with relatively higher costs.