Volume change is the increase or decrease of the volume of an elastomer after it has been in contact with a fluid, measured in percent (%).
Swell or increase in volume is almost always accompanied by a decrease in hardness. As might be surmised, excessive swell will result in marked softening of the rubber. This condition will lead to reduced abrasion and tear resistance, and may permit extrusion of the seal under high pressure.
For static O-ring applications volume swell up to 30% can usually be tolerated. For dynamic applications, 10 or 15% swell is a reasonable maximum unless special provisions are made in the gland design itself. This is a rule-of-thumb and there will be occasional exceptions to the rule.
Swell may actually augment seal effectiveness under some circumstances. For instance, (1) swell may compensate for compression set. If a seal relaxes 15% and swells 20%, the relaxation (compression set) tends to be canceled by the swell (see table below), (2) absorbed fluid may have somewhat the same effect on a compound as the addition of plasticizers, softening and thus providing more seal flexibility at the low temperature end of its operating range. These “potential” good effects however, should not be relied upon when choosing compound for an application. Awareness of these facts is of interest as they can and frequently do contribute to enhanced seal performance. The amount of volume swell after longterm immersion - stabilized volume - is seldom reported because it takes several readings to identify. The usual 70-hour ASTM immersion test will indicate a swelling effect, whereas a long-term test shows shrinkage. Thus swell indicated by short-term testing may only be an interim condition.
Shrinkage or decrease in volume is usually accompanied by an increase in hardness. Also, just as swell compensates for compression set, shrinkage will intensify the compression set effect causing the seal to pull away from sealing surfaces, thus providing a leak path. It is apparent then, that shrinkage is far more critical than swell. More than 3 or 4% shrinkage can be serious for dynamic seals. In some instances, fluids may extract plasticizers, causing the seal to shrink when the fluid is temporarily removed and the seal is allowed to dry out. Such shrinkage may or may not be serious; depending on its magnitude, gland design, and the degree of leakage tolerable before the seal re-swells and regains its sealing line of contact. However, even if the seal does re-swell there is the danger that it may not properly reseat itself. If any shrinkage is a possibility in an application, it must be considered thoroughly and carefully.
|Compression Set vs. Volume Change|
|Parker Compound: Butyl||Temperature: 74°C (165°F)|
|Time: 168 hrs||Deflection used: 25%|
|Volume Change %||0||+19.5||-0.4|
|Set % of Original Deflection||25.4||0||20.7|