Most elastomeric seals are designed to operate within ambient pressure to about 1,500 psi. At very high pressures, the seal must have sufficient strength to resist extrusion into the clearance gap. The chart at right illustrates the recommended limits of the combination of clearance gap (diametral), seal hardness, and pressure differential.
Techniques to avoid extrusion in high-pressure applications include decreasing the clearance gap, increasing the elastomer modulus (Mod 100) and the use of backup rings. Backup rings can be made of many rigid polymeric materials and are used on the low-pressure side within the gland to help prevent extrusion. Standard size backup rings are available in many materials.
Changes in Pressure/Vacuum
Cycling pressure can cause the seal to move back and forth within the gland. This can be especially damaging to seals with poor dynamic properties or in applications with low compression, which will allow for more motion.
The low-temperature limit is generally 15ÉF below TR-10 for static seals. For dynamic seals the TR-10 is more relevant. The TR-10 is the temperature at which an elastomer is able to retract 10%.
Low-temperature performance is generally a reversible process.
For design purposes compression is generally increased. The chemical media may cause swelling which may act as a plasticizer and lower the service temperature.
Glass Transition Temperature:
TR Tests: ISO 2921, ASTM D1414, D1329
Stiffness: ISO 1432, ASTM D1053, D2240
Brittleness: ISO R812, ASTM C509
Crystallization: ISO 3387, 6471
The high-temperature limit is generally considered a 30Ò50% loss of physical properties and typically represents a maximum temperature for 1,000 hours continuous service. It represents an irreversible change in the backbone or cross-link network.
The effect of high temperature can be compounded by the interaction with the chemical media. Chemical reactions typically double with a 10ÉC increase in temperature.
The following chemical guide is intended to assist the user in determining the suitability of various elastomers in many different chemical environments. The ratings are based on a combination of published literature, laboratory tests, actual field experience and informed judgments.
NOTE: Volume swell is only one indicator of elastomer fluid compatibility and may be based on the solubility parameter alone. Fluid attack on the backbone of the polymer may show up as a change in physical properties such as tensile strength, elongation at break, and hardness.
Elevated temperatures and extended exposure times may create more aggressive conditions than cited in this guide.
In some cases, specific elastomer compounds within a material family may provide improved compatibility. These cases are noted by an asterisk. Please contact Applications Engineering for assistance.
The information provided in this guide is believed to be reliable, but no representation, guarantees or warranties of any kind are made to its accuracy or suitability for any purpose. The information presented is based on laboratory testing and does not necessarily indicate end-product performance. It is recommended that users of ISC products conduct their own evaluations to determine suitability for the intended application.
The most common measure of chemical compatibility is volume swell. The following formula is used in reporting volume swell measurements. This takes into account dimensional changes in all three dimensions, and is more relevant than specific dimensional change readings for most sealing applications.
VS (%) = ((Weight in Air Ò Wt. in Water)final Ò (Wt. in Air Ò Wt. in Water)initial / (Weight in Air Ò Weight in Water)initial) x 100