When rubber sits in a fluid, the fluid can swell it, extract ingredients, or chemically attack the polymer. Immersion testing in a standard method gives data you can compare between compounds. The standard routes are ASTM D471 and ISO 1817.
How the test is run
Specimens of known size and weight are immersed in the test fluid at a set temperature for a set time. After removal, mass, volume, hardness and tensile properties are measured and compared with the originals. Changes are usually reported as percent changes (volume, mass, tensile, elongation) and point changes (hardness).
Reference fluids
For comparison between labs, the standards define reference fluids.
- IRM 901, 902 and 903 oils. Reference oils that replaced the older ASTM Nos 1, 2 and 3. They differ in aniline point and give low, medium and high swelling. IRM 903 is the high-swelling reference oil and is the usual choice for immersion tests on seal compounds.
- Reference fuels A, B and C. Fuel A is iso-octane. Fuel B is a mix of iso-octane and toluene at 70:30, and Fuel C is 50:50. Higher aromatic content gives more swell.
- Distilled water and other media for specific tests.
Reading the results
- Volume swell. A common guide is that seals should swell by less than about 15% to 20% in the service fluid for static use, and less for dynamic use. A small shrinkage is common when extractable plasticiser is removed. Limits depend on the design.
- Hardness change. A large drop suggests softening by swelling. An increase after immersion can mean further cure or extraction.
- Tensile and elongation change. Large changes of more than about 25% to 30% suggest chemical attack, not only swell.
- Visual condition. Cracks, blisters or a sticky surface are red flags regardless of the numbers.
- After drying. Properties after the fluid is removed show whether the effect is reversible.
Traps
- A single temperature is not enough. Chemical attack rises quickly with temperature. Test at or above service temperature.
- Real fluids are mixtures. Corrosion inhibitors, amines and additives in oilfield and coolant fluids can behave differently from the base oil. Test in the real fluid, or in a close synthetic version.
- Specimen size matters. The cut edge and thickness change how fast fluid penetrates.
- Immersion is not service. Pressure, flow, cycling and gas content change the result. For gas service, test decompression too.
- Fluid age. Fluid degrades during a long test. Replace it according to the method.
Sources and further reading
- ASTM D471 and ISO 1817 for immersion tests. ASTM D1418 for polymer designations.
About this guide. It is an original summary written from public sources and standards. It is not a copy of any standard or book, and it is not engineering advice. Confirm values against the current edition of the standard and test in your own application.