HIGH-PRESSURE PERFORMANCE

 
High Pressure/Extrusion 

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.

 
 

THERMAL STABILITY

 
Low Temperature

The low-temperature limit is generally 15F 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. 

Definition of:  

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

  Thermal Conductivity (g-cal/sec/cm2/cm/C) Specific Heat (cal/g/C)
VMQ 0.330-0.515 x 10-3 0.28-0.35
SC 1030/SC1020 0.6/0.8 x 10-3 0.2

High Temperature 

The high-temperature limit is generally considered a 3050% 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 10C increase in temperature. 

Definition of:  
Thermal Conductivity: ASTM C177, C518  
Thermal Expansion: ASTM D864  
Heat Aging: ISO 188, ASTM D454, D572, D573, D865

Thermal Expansion
Material Units Therm. Stab. x10-5 / C x10-6 / F
FKM    200C 16 90
NBR    120C 23 62
VMQ    230C 59-79 103
FFKM    300C 23   
EPDM    150C 16   
Stainless    - 1.04   
Aluminum    - 1.3   
TEFLON    230C 5-8   
KEL-F    280C 4-7   
Polyimide    275C 5   

 

 
 

CHEMICAL COMPATIBILITY

 

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.

 

Chemical Compatibility Guide Rating System
Rating Description Volume change Comments
1 Little or no effect <10% Elastomer may exhibit slight swelling and/or loss of physical properties under severe conditions.
2 Possible loss of physical properties 10-20% Elastomer may exhibit swelling in addition to a change in physical properties. May be suitable for static applications.
3 Noticeable change 20-40% Elastomer exhibits a noticeable change in swelling and physical properties. Questionable performance in most applications.
4 Excessive change >40% Elastomer not suitable for service.
- Insufficient information     Insufficient information available for rating.
 

Test Methods: 

ISO 1817 (Liquids) 

ASTM D471, D1460, D3137 (Liquids)

 

Volume Swell  

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 Wt. in Water)final (Wt. in Air Wt. in Water)initial / (Weight in Air Weight in Water)initial) x 100 

 
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