A seal is the typical Decode job: a small part, an unknown rubber and a demanding duty. The checklist below keeps the analysis proportional.
Start with the duty
Ask what the seal sees: fluid, temperature range, pressure, static or dynamic, any rapid pressure drops. This tells you which families to consider before you test.
Non-destructive first
- Weigh it. Measure its dimensions and cross-section. Photograph.
- Measure hardness (small parts need micro-hardness or IRHD).
- Measure density with a small-sample method. Density gives a polymer family hint.
Sample budget
A 3.5 mm cross-section O-ring has a few grams of rubber. Plan the split:
| Use | Typical amount |
|---|---|
| Extraction and TGA | 1 to 2 g |
| IR, pyrolysis, DSC | 0.1 to 0.5 g |
| Ash and elements | 1 to 2 g |
| Swelling and crosslink | 0.5 g |
| Physical tests | limited, often on a similar-lot part |
If more parts are available from the same lot, use them. Never assume parts from different batches or years are identical.
Polymer by elimination
Combine density, IR, DSC and elemental tests. Fluoroelastomer, nitrile or HNBR, EPDM, silicone and chloroprene all separate quickly.
Seals have special traps
- Absorbed service fluid: a used seal may be swollen and plasticised by oil. Dry to constant weight.
- Surface coatings: PTFE, molybdenum disulfide or silicone coatings on seals show up as silicon or fluorine in surface scans. Remove before bulk analysis.
- Post-cure: many seal compounds are post-cured. It matters for compression set.
Match for the seal's job
Compression set at temperature, stress relaxation, hardness, fluid swell and low-temperature tests. For gas service add rapid gas decompression. A seal that matches on tensile alone is not a match.
