By Industrias Hernol · Technical Engineering Series · Bogotá, Colombia
The energy sector is undergoing a structural shift. Vegetable dielectric oils — such as Cargill’s FR3® fluid — are increasingly replacing mineral oils in power and distribution transformers. The reasons are well documented: higher flash point, biodegradability, improved thermal performance, and greater overload capacity.
But this transition does not stop at the fluid. Every component inside the transformer must be thermally and chemically aligned with the new operating conditions. And one component that is frequently underestimated — until it fails — is the rubber gasket.
A transformer is designed for a 20 to 25-year service life. The gaskets must never fail during that window. If they do, the dielectric fluid oxidizes, insulation resistance drops, and the entire asset is compromised.
Why the Gasket Is the Most Demanding Component in a Transformer
Engineers who design transformer enclosures typically focus on three structural elements:
- The welded steel housing
- The anti-corrosion coating system
- The rubber gaskets (flat seals and o-rings)
Of these three, the rubber gasket operates under the most demanding conditions. Steel and paint are passive materials under static loads. A rubber gasket is not.
The gasket works under permanent compression — sometimes for decades. Its function is to generate a continuous elastic recovery force that maintains hermeticity, prevents oil leaks, and blocks air infiltration.
The rubber gasket is a dynamic material in a static condition. That is its technical complexity — and precisely why material selection is a strategic decision, not a procurement formality.
Root Causes of Transformer Failures: Where Gaskets Fit In
Industry surveys consistently identify two primary causes of transformer failure:
- Thermal overloads
- Dielectric oil leaks
Oil leaks are directly related to gasket integrity. A gasket that loses elasticity — through creep, chemical degradation, or thermal breakdown — can no longer maintain the seal. The consequences include oil oxidation, reduction in dielectric strength, accelerated aging of paper insulation, and potential arc discharge or transformer failure.
The shift to vegetable ester fluids adds another dimension: these fluids are hygroscopic — they absorb moisture from the air. A failed seal in a transformer with natural ester oil does not just mean a leak — it means accelerated moisture ingress, which degrades both the oil and the cellulose insulation.
Vegetable Dielectric Oil Operates at Higher Temperatures — So Must the Gasket
FR3® and similar vegetable ester fluids are selected in part because they allow transformers to operate at higher thermal loads — a significant advantage for grid infrastructure and renewable energy integration.
But higher operational loads translate directly to higher gasket temperatures. A transformer using vegetable oil at elevated capacity will expose gaskets to thermal conditions significantly above standard mineral oil service ratings.
It is not sufficient that the oil handles higher temperatures. All components — including the rubber seals — must be thermally aligned with the new operating conditions. A system is only as reliable as its weakest component.
Material Selection: HNBR, FKM, or Engineered High-Performance NBR?
For high-temperature transformer applications, engineers typically consider:
- HNBR (Hydrogenated Nitrile): rated up to 150°C, excellent oil resistance — cost 3x–5x standard NBR
- FKM / Viton®: rated up to 200°C, outstanding chemical resistance — cost 6x–10x standard NBR
- Fluorosilicone (FVMQ): rated -60°C to 175°C — cost at premium tier
These materials perform exceptionally. But their cost creates a real engineering challenge for large-volume transformer gasket procurement. The core question becomes: can an NBR compound be engineered to reach significantly higher thermal performance while remaining cost-competitive?
At Industrias Hernol, we answered that question through material engineering.
Compound 130: Hernol’s High-Performance NBR for Transformer Applications
Working in direct collaboration with Cargill — the developer of FR3® dielectric fluid — Industrias Hernol developed Compound 130 (TH-130 NBR), a proprietary formulation specifically engineered for transformer gasket applications in both mineral and vegetable dielectric oil environments.
The development objective was clear: achieve compression set performance, thermal stability, and elastic recovery under elevated temperatures — validated under ASTM protocols — without moving to premium elastomers such as HNBR or FKM.
A note on test temperature
Standard ASTM test protocols for NBR materials specify aging and compression set tests at 100°C. Hernol’s qualification program for Compound 130 was intentionally elevated to 125°C — conditions that exceed the standard NBR test requirement and more accurately represent real service conditions in transformers using vegetable ester fluids under high load.
Testing at 125°C on a material normally tested at 100°C is not standard practice — it is a deliberate engineering decision to validate performance under the most demanding conditions Compound 130 will encounter in service.
Performance Data: Compound 130 vs. Standard NBR
The following table presents laboratory results for Compound 130 alongside industry-typical reference values for standard commercial NBR compounds. All Compound 130 testing was conducted at Industrias Hernol’s laboratory facility in Bogotá under ASTM protocols. Ozone resistance was independently verified at the Rubber and Plastics Institute (ICIP), Medellín, Colombia.
| Property / Test | ASTM | Condition | Compound 130 (Hernol) | Std NBR (generic ref.) | Unit | ✓ |
| Hardness | D2240 | As received, 23°C | 70 | 60–75 | Shore A | ✓ |
| Tensile Strength | D412 | As received, 23°C | 10.80 MPa (1,566 psi) | ≥ 10 MPa | MPa | ✓ |
| Elongation at Break | D412 | As received, 23°C | 457% | ≥ 300% | % | ✓ |
| Specific Gravity | D792 | As received | 1.22 | 1.14–1.30 | g/cm³ | ✓ |
| Hardness Change (air aging) | D573 | 70h @ 125°C¹ | +5 | ≤ +12 max (~+8 typical) | Δ Shore A | ✓ |
| Tensile Change (air aging) | D573 | 70h @ 125°C¹ | -15% | -15% max (~-20 typical) | Δ% | ✓ |
| Compression Set (air) | D395-B | 70h @ 125°C¹ 25% compression | 28% | 30% max (~45–55 typical) | % | ✓ |
| Compression Set (air) | D395-B | 28 days @ 100°C 25% compression | 24% | 40% max (~50–60 typical) | % | ✓ |
| Volume Change (oil immersion) | D471 | 70h @ 125°C veg. ester oil | +2.3% | 0 to +20% (~+8–12 typical) | Δ Vol% | ✓ |
| Tensile Change (in oil) | D412 | 70h @ 125°C veg. ester oil | -9.5% | -35% max (~-25 typical) | Δ% | ✓ |
| Compression Set (in oil) | D395-B | 70h @ 125°C veg. ester oil | 34% | 50% max (~55–70 typical) | % | ✓ |
| Compression Set (in oil) | D395-B | 28 days @ 100°C veg. ester oil | 38% | 50% max (~60–75 typical) | % | ✓ |
| Oil Dielectric Change | D877 | 70h @ 125°C | +5.1 kV | -5 kV max | ΔkV | ✓ |
| Oil Power Factor Change | D924 | 70h @ 125°C @ 60 Hz, 100°C | 0.049 | 0.20 max | — | ✓ |
| Oil Acidity Change | D974 | 70h @ 125°C | 0.0037 | 0.015 max | mg KOH/g | ✓ |
| Ozone Resistance | D1149 D518 | 50 ppm, 70h @ 40°C (independent lab²) | No cracks | Cracks typical at 125°C aging | — | ✓ |
| UV / Weathering (QUV) | G154 Cycle 1 | 500h exposure 0.89 W/m²/nm | No cracks (slight dulling only) | Surface crazing common >200h | — | ✓ |
| Cold Flexibility | — | -30°C (3 methods) | No cracks | NBR: No cracks (down to -40°C) | — | ✓ |
| Flame test (halogen-free) | Copper wire | Chloride absence | No green flame | Varies by formulation | — | ✓ |
¹ Tested at 125°C — above the standard 100°C protocol for NBR, by design, to simulate elevated service conditions with vegetable ester dielectric fluids.
² Ozone test independently conducted at Rubber and Plastics Institute (ICIP), Medellín, Colombia, August 29, 2025. Standard NBR reference values are industry-typical ranges from published ASTM D2000 compound data.
A Critical Note on Compression Ratio: Protecting Seal Integrity in the Field
During the testing program, Compound 130 was also evaluated at 50% compression in oil — a condition far beyond recommended installation practice.
Important: No rubber gasket — regardless of compound — should be installed at 50% compression. The correct installation range for transformer flat gaskets is 20–30% compression. Beyond 30%, permanent deformation accelerates and sealing force degrades significantly over time.
The 50% compression test is a destructive qualification test, not a service condition. Its purpose is to evaluate how a compound behaves under the worst-case field misassembly scenario — over-tightened bolting or incorrect groove design.
Under this extreme condition, a standard commercial NBR compound typically shows complete or near-complete loss of elastic recovery — the material remains permanently deformed and cannot re-establish a seal.
Compound 130 achieved compression set values of 34% (70 hours at 125°C in vegetable oil) and 38% (28 days at 100°C in vegetable oil) — both within the 50% maximum acceptance limit — with no cracking. This means that even under severe over-compression, the compound retains structural integrity.
This is not a performance claim for operation at 50% compression. It is evidence of material robustness under abuse conditions — the kind of robustness that matters in real field installations where perfect assembly is not always guaranteed.
Ozone, UV, and Cold Temperature Performance
Standard NBR compounds have a known weakness: sensitivity to ozone and UV degradation. Surface cracking caused by ozone exposure is a serious failure mode — micro-cracks propagate into the gasket body, destroying hermeticity.
For transformers in outdoor installations — pad-mount units, substation transformers, pole-mounted distribution units — ozone resistance is a baseline requirement. Compound 130 incorporates anti-ozonant systems and UV stabilizers validated under ASTM D1149 and ASTM G154.
- Ozone resistance (50 ppm, 70h @ 40°C): No cracks across all 8 specimens and both elongation levels — independently verified
- QUV weathering (500h, 0.89 W/m²/nm, ASTM G154 Cycle 1): No cracks after full exposure. Slight surface dulling only
- Cold flexibility (-30°C, three methods): No cracks in any test configuration
In natural ester transformer applications, hermeticity is particularly critical. Vegetable ester fluids are hygroscopic — they absorb atmospheric moisture. A failed seal allows moisture ingress that degrades both oil quality and paper insulation, accelerating aging throughout the transformer’s service life.
Applicable Standards for Transformer Gasket Specification
The following standards govern the specification and testing of elastomeric seals for power and distribution transformers:
- ASTM D395 — Compression Set (Method B)
- ASTM D412 — Tensile Strength and Elongation at Break
- ASTM D471 — Fluid Immersion (oils and natural esters)
- ASTM D573 — Heat Aging in Air Oven
- ASTM D877 — Dielectric Breakdown Voltage of Insulating Liquids
- ASTM D924 — Dissipation Factor and Permittivity of Insulating Liquids
- ASTM D974 — Acid Number of Petroleum Products
- ASTM D1149 — Ozone Resistance
- ASTM D2240 — Shore A Hardness
- ASTM D2000 — Classification of Rubber Materials for Automotive Applications
- ASTM D6871 — Natural (Vegetable Oil) Ester Fluids for Electrical Apparatus
- ASTM G154 — UV Weathering (Cycle 1)
- IEEE C57.147 — Guide for Acceptance and Maintenance of Natural Ester Insulating Liquids
- ISO 815 — Compression Set Determination
- ISO 1817 — Resistance to Liquids
- NTC 1759 — Elastomeric Oil-Resistant Gaskets for Electrical Transformers
Hernol’s Capabilities for Transformer OEMs and Utilities
Industrias Hernol has been manufacturing technical rubber components for the electrical sector for over 30 years, supplying transformer manufacturers and utilities across many countries including the United States, Costa Rica, Ecuador, Peru, Colombia, Bolivia and Panama.
- Custom flat gaskets from engineering drawings (DXF, PDF, or physical samples)
- O-rings and cord sets in Compound 130 (TH-130), TH-115AU, HNBR, and FKM
- Compression molding, transfer molding, extrusion, and die-cutting processes
- Full ASTM test capability: D395, D412, D471, D573, D877, D924, D974
- Independent laboratory verification: ICIPC (Medellín, Colombia)
- Ozone testing: Rubber and Plastics Institute, Medellín
- CIDET certification (Colombian Electrotechnical Testing Institute)
- ISO 9001:2015 and ISO 14001:2015 certified
The Energy Transition Requires Component-Level Technical Alignment
Switching to vegetable dielectric oil is a sound engineering decision for many transformer applications. But it is not a fluid change in isolation — it is a system upgrade that touches every component inside the enclosure.
The gasket is not a commodity. It is the component that maintains hermeticity for the full service life of the asset. Selecting a gasket material designed for mineral oil service and continuing to use it in a vegetable oil application is a reliability risk that compounds over time — quite literally.
Compound 130 was developed specifically to close this gap: NBR-based, cost-competitive, thermally validated at 125°C, and proven compatible with both mineral and vegetable ester dielectric oils. Performance data available upon request.
Request technical data sheets, compound samples, or a custom gasket quote: comercial@hernol.com.co
| +57 320 398 5260 | hernol.com.co
