Transformer Oil Base Stock (TOBS)
Highly refined base stock for transformer oil manufacturers and blenders — covering both naphthenic grades (very low pour point, DBDS-free) and paraffinic grade (high VI, ultra-low dielectric loss). Naphthenic V9 & V9-TBU plus Paraffinic 60N.
Transformer oil base stock, engineered for blenders
Sinolook supplies transformer oil base stock (TOBS) — the highly refined mineral oil that transformer oil manufacturers and blenders use to produce finished electrical insulating oil. Our TOBS line covers both naphthenic grades (inhibited V9 and uninhibited V9-TBU) and a paraffinic grade (60N), meeting the current IEC 60296:2020 standard, with SGS verification on every shipment.
Refined from naphthenic crude · lowest pour point · highest solvency
The classic transformer oil base stock. Naphthenic chemistry delivers exceptional low-temperature fluidity, high dielectric strength and additive solvency — ideal for distribution and power transformers.
High viscosity index · very low dielectric loss · high flash point
Paraffinic-based transformer oil base stock with high viscosity index (112) and exceptionally low dielectric dissipation. Ideal for applications favoring oxidation stability and high flash point.
Two grades: inhibited V9 and uninhibited V9-TBU
Both grades are high-purity naphthenic transformer oil base stocks engineered for electrical insulation and cooling duty. The key difference is oxidation-inhibitor content — which determines the IEC 60296 antioxidant classification and suits different customer preferences.
A high-purity naphthenic transformer oil with zero aromatic carbon (Ca = 0%) and high naphthenic carbon (Cn = 66%), delivering an exceptionally clean composition with excellent low-temperature fluidity and outstanding dielectric properties.
- Zero aromatic carbon (Ca = 0%) — exceptionally clean, superior oxidation resistance
- Breakdown voltage 43.3 kV — strong dielectric strength
- Dissipation factor 0.00123 — very low dielectric loss
- Pour point −54°C — reliable cold-climate operation
- Antioxidant 0.038% — long service life, stable performance
An uninhibited (Type U) naphthenic transformer oil free of antioxidant additives, metal passivators and DBDS. Delivers a clean, additive-free insulating fluid with outstanding low-temperature performance and a strong health, safety and environmental profile.
- Uninhibited (Type U) — antioxidant not detected, additive-free fluid
- DBDS not detected — protects copper windings
- Non-corrosive sulphur — long-term transformer reliability
- Pour point −60°C — outstanding cold-climate performance
- PCB not detected, PCA <1% — strong HSE profile
Naphthenic TOBS V9
InhibitedTypical properties — values represent typical production results, not guaranteed limits. Batch-specific COA provided with each shipment.
| Item | Unit | Specification | Typical Result | Method |
|---|---|---|---|---|
| Appearance | — | Clear & Bright | Clear & Bright | Visual |
| Density (20°C) | kg/m³ | ≤895 | 891.5 | SH/T 0604 |
| Kinematic Viscosity @ 40°C | mm²/s | ≤12 | 8.605 | GB/T 265 |
| Water | mg/kg | ≤40 | 21 | GB/T 6283 |
| Breakdown Voltage | kV | ≥30 | 43.3 | GB/T 507 |
| Flash Point, COC | °C | ≥135 | 138.0 | GB/T 261 |
| Pour Point | °C | ≤−40 | −54 | GB/T 3535 |
| Sulphur | mg/kg | Report | 0.5 | SH/T 0689 |
| Antioxidant Additive | % | ≤0.08 | 0.038 | NB/SH/T 0802 |
| Acid Number | mg KOH/g | ≤0.01 | 0.008 | NB/SH/T 0836 |
| Dielectric Dissipation Factor (90°C) | — | ≤0.005 | 0.00123 | GB/T 5654 |
| Aromatic Carbon Content (Ca) | % | Report | 0 | ASTM D2140-24 |
| Naphthenic Carbon Content (Cn) | % | Report | 66 | ASTM D2140-24 |
| Paraffinic Carbon Content (Cp) | % | Report | 34 | ASTM D2140-24 |
Naphthenic TOBS V9-TBU
Uninhibited · Type BTypical properties — values represent typical production results, not guaranteed limits. Batch-specific COA provided with each shipment.
| Item | Unit | Specification | Typical Value | Method |
|---|---|---|---|---|
| Functional Properties | ||||
| Appearance | — | Clear & Bright | Clear & Bright | Visual |
| Pour Point | °C | ≤−20 | −60 | ASTM D97 |
| Kinematic Viscosity @ 40°C | mm²/s | ≤12 | 7.964 | ASTM D445 |
| Kinematic Viscosity @ −10°C | mm²/s | ≤1800 | 121.2 | ASTM D445 |
| Water Content | mg/kg | ≤30 | 25 | ASTM D1533 |
| Breakdown Voltage (untreated) | kV | ≥30 | 39 | ASTM D877 |
| Density (20°C) | kg/m³ | ≤895 | 880.8 | ASTM D1298 |
| Dielectric Dissipation Factor (90°C) | — | ≤0.005 | 0.00048 | ASTM D924 |
| Refining & Stability Properties | ||||
| Color | — | ≤1.5 | <0.5 | ASTM D1500 |
| Acid Number | mg KOH/g | ≤0.01 | 0.005 | ASTM D974 |
| Water-Soluble Acid or Alkali | — | None | None | ASTM D1093 |
| Interfacial Tension | mN/m | ≥40 | 48 | ASTM D971 |
| Total Sulphur | mg/kg | Report | 1.4 | ASTM D4294 |
| Corrosive Sulphur | — | Non-corrosive | Non-corrosive | ASTM D1275 |
| Potentially Corrosive Sulphur | — | Non-corrosive | Non-corrosive | ASTM D1275 B |
| DBDS Content | mg/kg | Not detected | Not detected | ASTM D7649 |
| Antioxidant Additive Content | wt% | Not detected | Not detected | ASTM D2668 |
| Metal Passivator Content | mg/kg | Not detected | Not detected | ASTM D8010 |
| 2-Furfural and Related Components | mg/kg | Not detected | Not detected | ASTM D5837 |
| Gassing Tendency | mm³/min | No general requirement | 27.4 | ASTM D2300 |
| Carbon-Type Composition | ||||
| Aromatic Carbon Content (Ca) | % | ≤10 | 3.18 | ASTM D2140 |
| Naphthenic Carbon Content (Cn) | % | ≥40 | 54.19 | ASTM D2140 |
| Paraffinic Carbon Content (Cp) | % | Report | 42.63 | ASTM D2140 |
| Health, Safety & Environmental Properties | ||||
| Flash Point (closed cup) | °C | ≥135 | 138.0 | ASTM D93 |
| PCA Content | wt% | <3 | <1 | IP 346 |
| PCB Content | mg/kg | Not detected | Not detected | ASTM D4059 |
Paraffinic TOBS 60N — high VI, ultra-low dielectric loss
A paraffinic-based transformer oil base stock that complements our naphthenic line. Where naphthenic TOBS leads on low-temperature performance, Paraffinic TOBS 60N offers a high viscosity index (112), exceptionally low dielectric dissipation (tan δ 0.0002) and a high flash point (180°C open cup) — suited to applications favoring oxidation stability and thermal robustness.
A highly refined, colorless and transparent paraffinic transformer oil base stock with high viscosity index and very low dielectric loss. Non-corrosive sulphur, low moisture and excellent interfacial tension indicate a well-refined product with strong insulating performance.
- Viscosity index 112 — high VI for stable viscosity across temperature
- tan δ 0.0002 (90°C) — exceptionally low dielectric dissipation
- Breakdown voltage 43.0 kV — strong dielectric strength
- Flash point 180°C (open cup) — high thermal safety margin
- Interfacial tension 50 mN/m — well-refined, strong aging resistance
- Non-corrosive sulphur (0.077 mg/kg) — protects copper windings
Paraffinic TOBS 60N
ParaffinicTypical properties — values represent typical production results, not guaranteed limits. Batch-specific COA provided with each shipment. Production reference: 2026/03.
| Item | Unit | Specification | Typical Value | Method |
|---|---|---|---|---|
| Appearance | — | Colorless & transparent | Colorless & transparent | Visual |
| Density (20°C) | g/cm³ | Report | 0.8212 | ASTM D4052 |
| Viscosity Index | — | ≥105 | 112 | ASTM D2270 |
| Kinematic Viscosity @ 100°C | mm²/s | 1.9–2.8 | 2.497 | ASTM D445 |
| Kinematic Viscosity @ 40°C | mm²/s | Report | 8.762 | ASTM D445 |
| Kinematic Viscosity @ −20°C | mm²/s | Report | 147.8 | ASTM D445 |
| Pour Point | °C | ≤−30 | <−30 | ASTM D97 |
| Cloud Point | °C | Report | −23 | ASTM D2500 |
| Flash Point, Open Cup | °C | ≥150 | 180 | ASTM D92 |
| Flash Point, Closed Cup | °C | Report | 161.5 | ASTM D93 |
| Sulphur | mg/kg | ≤5 | 0.077 | ASTM D5453 |
| Corrosive Sulphur | — | Report | Non-corrosive | ASTM D130 |
| Acid Number | mg KOH/g | ≤0.01 | Not detected | ASTM D664 |
| Moisture (trace) | mg/kg | Report | 24.75 | ASTM D95 |
| Breakdown Voltage | kV | Report | 43.0 | ASTM D1816 |
| Dielectric Dissipation Factor (tan δ, 90°C) | — | Report | 0.0002 | ASTM D924 |
| Interfacial Tension | mN/m | Report | 50.0 | ASTM D971 |
| Water-Soluble Acids & Alkalis | — | None | None | GB/T 259 |
| Saybolt Color | — | ≥30 | >30 | ASTM D156 |
Naphthenic vs Paraffinic TOBS — at a glance
Both are premium transformer oil base stocks — the right choice depends on your climate and formulation priorities.
| Property | Naphthenic TOBS (V9 / V9-TBU) | Paraffinic TOBS 60N |
|---|---|---|
| Base chemistry | Naphthenic (cyclic) | Paraffinic (chain) |
| Viscosity Index | Lower (naphthenic nature) | 112 (high) |
| Pour point | −54 / −60°C (very low) | <−30°C |
| Low-temperature performance | Best | Good |
| Dielectric dissipation (tan δ) | 0.00048–0.00123 | 0.0002 (lowest) |
| Flash point | 138°C | 180°C (open cup) |
| Best suited for | Cold climates, low-temp reliability | Oxidation stability, thermal robustness |
Understanding IEC 60296:2020 designations
The current fifth edition of IEC 60296:2020 classifies mineral insulating oils using a four-letter ordering code. Knowing where each grade sits helps you specify exactly what your transformer application requires.
Which grade should you choose?
Three grades across two base chemistries. Choose based on your climate, oxidation-stability preference and target market requirements.
Choose V9 (Inhibited) if you need…
- Maximum oxidation stability and long oil service life
- Higher dielectric strength (43.3 kV breakdown voltage)
- The cleanest carbon composition (zero aromatic carbon, Ca = 0%)
- To blend an inhibited (Type I) finished transformer oil
- Very low pour point (−54°C) for cold climates
Choose V9-TBU (Uninhibited) if you need…
- An additive-free (Type U) insulating fluid, IEC designation TVBU
- DBDS-free, non-corrosive sulphur for copper-winding protection
- The strongest HSE profile (PCB not detected, PCA <1%)
- The lowest pour point (−60°C) for extreme cold climates
- The lowest dielectric dissipation among naphthenic grades
Choose Paraffinic 60N if you need…
- A high viscosity index (112) for viscosity stability
- Exceptionally low dielectric dissipation (tan δ 0.0002)
- A high flash point (180°C open cup) for thermal safety
- High interfacial tension (50 mN/m) & strong aging resistance
- Paraffinic chemistry for oxidation-stability-focused blends
Where naphthenic TOBS is used
Distribution transformers
The primary application. Low pour point and high dielectric strength support reliable insulation and cooling in pole-mount and pad-mount distribution transformers.
Power transformers
High-voltage power transformers in substations and generation plants, where oxidation stability and long service life are critical to grid reliability.
Switchgear & capacitors
Oil-filled switchgear, tap-changers and capacitors that require stable dielectric performance and low-temperature fluidity.
A reliable TOBS supply partner for blenders
IEC 60296:2020 compliant
Grades engineered to meet the current fifth-edition standard, with clear antioxidant classification for inhibited and uninhibited fluids.
SGS verified shipments
Independent third-party inspection with full batch traceability and Certificate of Analysis on every shipment.
DBDS-free & HSE-focused
The V9-TBU grade delivers non-corrosive sulphur, DBDS not detected, PCB not detected and PCA below 1% — meeting stringent modern requirements.
Global supply & bonded storage
Serving transformer oil manufacturers and traders across 60+ countries, with a global bonded storage network for reliable delivery.
Frequently asked questions
What is transformer oil base stock (TOBS)?
Transformer oil base stock is highly refined mineral oil supplied to transformer oil manufacturers and blenders as the base for finished electrical insulating oil. Naphthenic TOBS is refined from naphthenic crude to achieve very low pour point, high dielectric strength and excellent oxidation resistance. Sinolook supplies naphthenic TOBS in inhibited (V9) and uninhibited Type B (V9-TBU) grades meeting IEC 60296:2020.
What is the difference between inhibited and uninhibited transformer oil?
Inhibited transformer oil contains a synthetic antioxidant additive to boost oxidation stability. Uninhibited transformer oil relies only on the natural antioxidants in the highly refined base oil. Sinolook TOBS V9 is inhibited (antioxidant 0.038%), giving high oxidation stability. TOBS V9-TBU is uninhibited Type B (antioxidant not detected), offering a clean additive-free fluid preferred where an uninhibited oil is required — IEC designation TVBU.
Does Sinolook TOBS meet IEC 60296?
Yes. Sinolook naphthenic TOBS grades are engineered to meet the general specifications of IEC 60296:2020, the current fifth-edition standard for mineral insulating oils. The V9-TBU grade corresponds to IEC designation TVBU (Transformer, Virgin, Type B, Uninhibited), and additionally features DBDS not detected, non-corrosive sulphur, PCB not detected and PCA below 1%.
Why is DBDS-free transformer oil important?
DBDS (dibenzyl disulphide) is a corrosive sulphur compound that can react with copper windings inside a transformer, forming copper sulphide deposits that degrade insulation and can cause transformer failure. DBDS-free transformer oil — such as Sinolook TOBS V9-TBU where DBDS is not detected — protects copper windings and supports long-term reliability. This is increasingly required by transformer manufacturers and utilities worldwide.
What is the difference between naphthenic and paraffinic transformer oil base stock?
Naphthenic TOBS offers very low pour point and high solvency — ideal for cold-climate transformers. Paraffinic TOBS offers a high viscosity index and high flash point, favoring oxidation stability and thermal robustness. Sinolook supplies both: naphthenic V9 / V9-TBU (pour point down to −60°C) and paraffinic 60N (VI 112, flash point 180°C, tan δ 0.0002).
Can I get samples before placing a bulk order?
Yes. Sinolook provides laboratory samples of TOBS V9 and V9-TBU for your qualification testing and formulation evaluation. Contact our technical team to request samples and the full technical data sheet (TDS) for each grade.
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Request TOBS specifications & pricing
Get the full technical data sheets, samples and a quote for naphthenic TOBS V9 and V9-TBU. Our technical team supports transformer oil manufacturers and traders worldwide.