Non-polar solvent; fuel, oil, gasoline & lubricant will not dissolve in polar rubber.
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 long term 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.
HIGH TEMPERATURE EFFECTS
All rubber is subject to deterioration at high temperature. Volume change and compression set are both greatly
influenced by heat. Hardness is influenced in a rather complex way. The first effect of increased temperature is to
soften the compound. This is a physical change, and will reverse when the temperature drops. However, it must
be considered in high pressure applications because a compound that is sufficiently hard to resist extrusion at
room temperature may begin to flow and extrude through the clearance gap as the temperature rises, due to this
softening effect.
With increasing time at high temperature, chemical changes slowly occur. These generally cause an increase in
hardness, along with volume and compression set changes as mentioned above. Changes in tensile strength and
elongation are also involved. Being chemical in nature, these changes are not reversible.
Parker has applied a realistic temperature range with a margin of safety when setting the general operating
temperature range for seal compounds. The maximum temperature recommendation for a compound is based on
long term functional service. If it is subjected to this temperature continuosly, it should perform reliably for 1,000
hours. Time at less than maximum temperature will extend life. Similarly, higher temperature will reduce it.
The high temperature limits assigned to compounds in the figure below are conservative estimates of the
maximum temperature for 1,000 hours of continuous service in the media the compounds are most often used to
seal. Since the top limit for any compound varies with the medium, the high temperature limit for many
compounds is shown as a range rather than a single figure. This range may be reduced or extended in unusual
fluids.
Since some fluids decompose at a temperature lower then the maximum temperature limit of the elastomer, the
temperature limits of both the seal and the fluid must be considered in determining limits for a system.
