Volume change is the increase (swell) or decrease (shrinkage) in the volume of a specimen that has been in contact with a fluid. This contact may range from occasional “splashing” to constant immersion. Any resulting volume change can range from minor (indicating there is a relative compatibility between the fluid and the specimen) to major (indicative of incompatibility). Volume change is typically noted as a percentage of the original volume. For example, a specimen that swells to twice its original volume is said to have undergone a 100% increase. Figure 31 shows the swelling of a shaft seal’s elastomeric lip.

An elastomeric material typically becomes softer as a result of swell, whereas shrinkage generally hardens the material. A slightly swollen seal is, in most cases, still functional. A limited amount of swell may compensate for other variables, such as compression set. Shrinkage, on the other hand, can exacerbate an already-existing compression set problem. With some of its soluble components (such as plasticizer) having been extracted by system fluid, a rubber O.D. seal that has undergone shrinkage is more prone to leaks between the seal O.D. and the housing I.D.

As described in ASTM test method D 471, volume change testing typically employs ASTM and Industry Reference Material (IRM) oils, as well as ASTM Reference Fuels, Service Liquids, and Type IV Reagent Water. Regardless of the liquid in use, testing involves immersing a material sample (of known properties) in the liquid for a specific period of time (e.g. 70 hours) at a specific temperature (e.g. 100° C ± 2°), both variables based on the conditions expected in service. Material deterioration (if any) is then determined based on changes in physical properties, including volume.

CHEMICAL PROPERTIES MAIN PAGE