Antioxidants extend lubricant life by interrupting the auto-oxidation chain — phenolic antioxidants donate hydrogen to peroxy radicals; aminic antioxidants trap nitroxide intermediates; ZDDP and sulphurised olefins decompose hydroperoxides before propagation. Together (with ZDDP as secondary AO) they give engine oil 15,000-30,000 km drain interval and turbine R&O oil 5-15 years of service. This guide covers AO chemistry families, the phenolic + aminic synergy, treat rates by service, RPVOT and the Indian supplier landscape.
| AO Type | Mechanism | Best Temp Range | Colour | Volatility | Lubechem Code |
|---|---|---|---|---|---|
| BHT (food-grade) | Phenolic H-donor | 100-140 °C | Colourless | High | LC-AO-PHEN-BHT |
| 2,6-DTBP | Phenolic H-donor | 120-180 °C | Pale yellow | Moderate | LC-AO-PHEN-DTBP |
| Methylene-bis-DTBP | Phenolic H-donor | 150-220 °C | Pale yellow | Low | LC-AO-PHEN-MBP |
| Alkylated DPA | Aminic H-donor + radical trap | 150-220 °C | Amber to brown | Low | LC-AO-AMIN-DPA |
| PAN | Aminic H-donor + radical trap | 160-240 °C | Brown | Very low | LC-AO-AMIN-PAN |
| Mixed Phen+Aminic 1:1 | Synergistic | 100-220 °C | Yellow-amber | Moderate-low | LC-AO-SYN-PA |
| Service | Phenolic % | Aminic % | ZDDP / Other % | RPVOT Target (min) |
|---|---|---|---|---|
| PCMO API SP / GF-6 | 0.3-0.5% | 0.3-0.5% | 0.9-1.1% ZDDP | 300-600 |
| HDD API CK-4 | 0.6-1.0% | 0.6-1.0% | 1.0-1.4% ZDDP | 500-1,000 |
| Turbine R&O ISO 32/46/68 | 0.5-0.9% | 0.3-0.5% | 0 (no ZDDP) | 1,000-3,000 |
| Hydraulic HVI / HV | 0.3-0.6% | 0.2-0.4% | 0.3-0.6% ZDDP | 700-1,500 |
| Gear EP GL-5 | 0.3-0.6% | 0.3-0.5% | Sulphurised olefin EP | N/A (FZG drives) |
| Industrial Circulating | 0.5-1.0% | 0.3-0.6% | 0.5-1.0% sulphurised | 1,500-3,000 |
| Compressor (rotary screw) | 0.4-0.8% | 0.3-0.5% | 0.3-0.5% phosphite | 800-2,000 |
Both are hindered phenol antioxidants. BHT (butylated hydroxytoluene, 2,6-di-tert-butyl-4-methylphenol) is a small-molecule food-grade phenol — cheap, volatile, used as commodity antioxidant in food, plastics and low-temperature lubricants. 2,6-DTBP (2,6-di-tert-butylphenol) is the lubricant-grade hindered phenol — slightly larger, less volatile, better thermal stability.
Both work by donating H to peroxy radicals (ROO• + ArOH → ROOH + ArO•). Methylene-bis-DTBP and other dimerised hindered phenols give still lower volatility for high-temperature turbine oil and industrial circulating.
RPVOT = Rotating Pressure Vessel Oxidation Test (ASTM D2272). 50 g oil + 5 g water + Cu wire coil in a sealed pressure vessel, pressurised with O2 to 90 psi, rotated at 100 rpm in a 150 °C bath. Time to oxygen pressure drop of 175 kPa is the RPVOT life.
Used for turbine oil, hydraulic, transformer oil and gear oil oxidation life specification. ISO 32/46/68 turbine R&O target ≥ 1,000 minutes; premium turbine 2,000-3,000 min. Phenolic + aminic AO synergy is the main lever to extend RPVOT.
Hindered phenols are colourless when fresh but darken on oxidation — the oxidised quinone form is yellow / amber. High phenolic treat rate gives darker finished oil colour over time, which OEMs (especially aerospace, turbine, ATF) sometimes specify against.
Premium turbine and ATF formulations use lower phenolic and rely more on aminic + ZDDP for AO. PCMO and HDD don't have colour specs, so phenolic can be used freely up to 1-2% treat.
DPA (diphenylamine) and PAN (phenyl-α-naphthylamine) aminic antioxidants are extremely effective but can sludge under certain conditions — oxidised aminic forms dark sludge that deposits on bearing surfaces. Turbine oil specs (GE GEK 32568, Siemens TLV) limit aminic content and require RPVOT plus sludge tests (ASTM D4310, D7873 varnish potential).
Modern turbine formulations use alkylated DPA (Naugalube 438L, Vanlube 81) which has the antioxidant function but much lower sludge tendency.
ZDDP is primarily anti-wear, but also has secondary antioxidant function via two mechanisms. (1) Peroxide decomposition: ZDDP reduces ROOH peroxides to ROH alcohols before the peroxide can propagate radical chain. (2) Mo-DTP variants (when present) trap nitroxide radicals.
Net effect: a PCMO with ZDDP at 0.8-1.2% plus modest phenolic (0.3-0.5%) plus aminic (0.3-0.5%) gives ~1,000-1,500 minutes RPVOT — sufficient for 15,000-30,000 km PCMO drain. Without ZDDP, the same oil would need 2-3x more primary AO.
Long-drain HDD CK-4 for fleet operation (50,000 km drain) needs heavier AO than standard PCMO. Typical treat: phenolic 0.6-1.0% + aminic 0.6-1.0% + ZDDP 1.0-1.4% (primary-dominant).
The phenolic + aminic 1:1 synergy gives the AO depth needed for long drain. Mack T-13 oxidation test (200 hr at 130 °C with NOx exposure) is the qualifier — finished oil viscosity rise <100% at end of test.
Industrial circulating oil (paper machine, large gearbox, hydraulic system) target life is 5-15 years (40,000-130,000 hr operation). AO budget: phenolic 0.5-1.0% (methylene-bis-DTBP or similar low-volatility), aminic 0.3-0.6% (alkylated DPA), peroxide decomposer (sulfurised olefin) 0.5-1.0%.
Plus careful base oil selection (Group II or III preferred — natural oxidation life better than Group I). RPVOT specification 1,000-3,000 min depending on OEM.
BASF Vanlube / Irganox L-series (L57, L74, L101 — phenolic and aminic), Albemarle Ethanox 4710 / 4733 (phenolic, methylene-bis-DTBP), LANXESS Naugalube 438L (alkylated DPA), SI Group Wingstay (phenolic), Songwon SongNox 1076/1024 (phenolic), King Industries (multifunctional AO/CI).
India landed ₹350-650 / kg depending on chemistry and grade. Lubechem custom equivalents ₹250-450 / kg (28-35% saving).
Share your target service (PCMO, HDD, turbine, hydraulic, industrial), RPVOT target and volume. We respond within one business day with a phenolic + aminic + peroxide-decomposer blend plan and indicative cost.