When a tote weeps at the outlet, the instinct is to blame the valve. The valve is almost never the problem. Buttress threads seal on a flat gasket against a seat face — the thread provides clamping force and nothing else — so a leak is a gasket problem, a seat-face problem, or a compound-selection problem.
The six compounds you will actually meet
| Compound | Best for | Fails against | Temp limit | Relative cost |
|---|---|---|---|---|
| EPDM | Aqueous, caustics, surfactants, glycols | Oils, fuels, aromatic solvents | 250 °F | Lowest |
| Viton (FKM) | Solvents, fuels, oils, acids | Hot caustics, ketones, amines | 400 °F | High |
| Silicone | Food contact, high temperature | Solvents, fuels, steam over time | 350 °F | Middle |
| PTFE envelope | Almost everything chemically | Mechanical abuse, low clamping force | 450 °F | Highest |
| Buna-N (NBR) | Petroleum products, mineral oils | Oxidisers, ozone, aromatics | 250 °F | Low |
| Neoprene (CR) | Mild chemistry, weathering | Strong acids, aromatics | 250 °F | Low |
EPDM is the default fitted on most reconditioned units because it covers the majority of industrial IBC traffic. It is also precisely wrong for oil service, which is the single most common mis-specification we see.
A four-question selection path
- 01Is the product petroleum-based, an oil, or a hydrocarbon? If yes, you need Viton or Buna-N. EPDM will swell and fail. This question alone resolves most mis-specifications.
- 02Is it a solvent — aromatic, chlorinated or ketone? Viton for aromatics and chlorinated; PTFE if ketones are involved, because ketones attack Viton.
- 03Is it food contact or above 250 °F? Silicone for food and heat; PTFE if chemical resistance is also needed.
- 04Otherwise — aqueous, caustic, surfactant, glycol, fertiliser, brine? EPDM, and you are done.
The four failure modes
Swelling
The compound absorbs the product and increases in volume. Early on this can actually improve the seal, which is why swelling failures often appear suddenly after months of apparently fine service. Eventually the gasket distorts, extrudes out of its seat, or loses elasticity entirely. A gasket that has visibly grown is a swelling failure.
Hardening and embrittlement
Plasticiser is leached out, or the polymer is oxidised, and the gasket loses compliance. It can no longer deform to fill the microscopic irregularities in a seat face, so it weeps. A gasket that has gone hard and shows fine surface cracking is an embrittlement failure.
Compression set
Not a chemical failure at all. The gasket has been clamped for long enough that it has permanently taken the shape of the compression, and it no longer springs back when loosened and retightened. This is why reusing a gasket is a false economy — the second installation starts with a part that has already used up its compliance.
Mechanical damage
Over-tightening, grit on the seat face, or a nicked seat. PTFE-envelope gaskets are the most vulnerable here because they are chemically superb and mechanically unforgiving. Always inspect the seat face before fitting a new seal.
Diagnosing from the old gasket
| Observation | Diagnosis | Action |
|---|---|---|
| Visibly larger, soft, spongy | Swelling — wrong compound | Change compound |
| Hard, cracked, brittle | Embrittlement — wrong compound or overheated | Change compound; check temperature |
| Flattened, permanent impression, still supple | Compression set — normal wear | Fit new, same compound |
| Nicked, torn, extruded on one side | Mechanical — over-tightened or dirty seat | Inspect seat; hand-tighten |
| Discoloured but otherwise sound | Cosmetic staining | Fit new, same compound |
| Partly dissolved or tacky | Chemical attack — badly wrong compound | Change compound urgently |
Practical rules
- Never reuse a gasket. It is the cheapest part on the container and the most common cause of a leak.
- Match the compound to your product, not to what the unit arrived with. The previous owner may have had different chemistry.
- Inspect the seat face for scoring, moulding flash and debris before fitting.
- Hand-tighten buttress closures. A wrench cracks the collar and does not improve the seal.
- Keep a spare kit on the shelf. Gaskets are the only genuine consumable in tote handling.
- If you run more than one chemistry, label your gasket stock by compound. Mixed unlabelled stock guarantees the wrong one gets fitted eventually.
What to tell your supplier
Four facts let anyone specify a gasket properly, and they are the same four facts that govern container material selection generally.
- 01The product, by chemical class rather than trade name where possible
- 02The concentration
- 03The service temperature, including the maximum rather than the typical
- 04The contact time — a two-week transport is a different problem from six months of storage
With those four, a compound recommendation is straightforward. Without them, anybody specifying a gasket is guessing, and the guess will default to EPDM — which is right most of the time and expensively wrong in oil service.