Hydraulic Fluid and Seal Material Compatibility

Polymer seals must resist degradation, chemical degradation and maintain their engineering properties over the service life. Fluid power seal materials will have interactions with their environment and fluids, which they have contacts with (or with their vapours, gases).  That is why it is important to select seal materials what are compatible with the medias/environment in which they are used. It is critical, as it is going to determine the system performance.

When we select the seal material, we must consider that beside the hydraulic fluid in the systems seal materials can be exposed by

• other fluids from machinery (such as lubricants (oils and) greases, fuels, coatings)
• environmental factors (such as humidity or radiation)
• contaminants, additives in the system and degradation/reaction of the fluids can create additional chemicals

Interaction(s) between seal material and fluids:

• Physical reaction
• Chemical reaction
• Combined physical/chemical reaction

Such interactions between the seal material and hydraulic fluid will impact the seal performance and expected seal service life. Results of physical and chemical reactions:

• Discoloration of the seal material  
• Swelling (change in seal material volume – increase)
• Shrinking (change in seal material volume – decrease)
• Softening of the seal material (loss of physical strength)
• Hardening of the seal material (loss of flexibility, elasticity)
• Degradation of the material (degradation of the polymer chain or crosslinking)
• Material aging (through local fracturing of bonds, oxidation and other chemical reaction)

Some of the consequences:

• Swelling:  excessive swelling can lead to softening and abrasion and loss of resilience. Excessive extrusion is expected as the seal flows out of the seal cavity.
• Shrinking: excessive shrinkage will reduce sealing force and seal can shrinks away from the mating surface
• Softening of the seal material: loss of the physical strength or dissolving the seal material
• Hardening of the seal material: loss of elasticity and flexibility
• Degradation of the seal material: lose resilience, changes in tensile strength, elongation etc.  

The physical or chemical reaction between the fluid and seal material can be accelerated at elevated temperature or pressure, resulting in faster changes in physical properties and degradation of the seal material. In addition, the elevate temperature can cause the degradation of the fluid or its additives, leading to the deterioration of the seal material.