Hydraulic seal material compatibility with synthetic and biodegradable fluids

As hydraulic systems shift away from mineral oils that are conventional towards biodegradable and synthetic fluids, sealing compatibility is now an extremely important and least understood aspects of reliability of the system. Seals that work flawlessly using mineral-based hydraulic oils will fail in a matter of weeks if the system is switched to an ester made of synthetic material or biodegradable fluid, not due to the seal being damaged, but due to the chemical composition of the fluid reacting differently with the structure of the elastomer's polymer. In excess, swelling, shrinkage, hardening, or chemical attack could all be a result of a fluid-seal incompatibility, and the results can range from a small leakage to a catastrophic extrusion of the seal under pressure. Knowing how various seal materials react to biodegradable and synthetic fluids is crucial to anyone who specifies, converts, or maintains hydraulic equipment.

Understanding the concept of synthetic hydraulic fluids

Synthetic fluids for hydraulics are base stocks engineered specifically to outperform mineral oils under certain conditions, like extremely high temperature, fire resistance, or a longer service life. The main synthetic categories are polyalphaolefins (PAOs) as well as synthetic esters and polyalkylene glycols (PAGs) as well as phosphate esters. Each one has distinctive chemical characteristics that interact with elastomers in a different way in comparison to oils derived from petroleum. PAOs generally behave similarly to mineral oils with respect to sealing compatibility, which makes conversion fairly simple in most cases. Synthetic esters are polar compounds that may result in significant swelling of certain elastic materials while shrinking other elastomers, based on the specific ester's chemical composition and the polymer composition of the seal. Phosphate esters, which are commonly used in applications involving fire-resistant fluids, are especially abrasive to the majority of elastomers used in everyday applications, such as the standard nitrile rubber. They generally require special seals like ethylene propylene monomer rubber. PAGs, however, are highly polar and could cause problems with sealing that is designed specifically for mineral oils with nonpolar properties and, in many cases, cause severe shrinkage that can cause leakage.

Biodegradable hydraulic fluids HETG, HEES, HEPG, and HEPR

Biodegradable hydraulic fluids are classified according to various standard classifications according to their chemical base. The HETG fluids originate from triglycerides. They are typically vegetable oils like sunflower or rapeseed oil. These are synthetic esters that have been designed specifically to be biodegradable and have improved stability in oxidation when compared with natural triglycerides. These fluids are based on polyglycols. They also utilize other synthetic bases that are not covered by the earlier categories. Each category is a seal material that interacts in a different manner. The HETG liquids, as natural esters, have some similar features with synthetic esters but are more susceptible to oxidation that can create acidic byproducts over time, which further strain seal materials. HEES fluids generally have superior thermal and oxidative durability to HETG fluids; however, they have the same fundamental polarity issues, which affect the elastomer's swelling. The biodegradability offered by these fluids is not related to a mild chemical reaction towards seals. However, in certain instances biodegradable liquids are more aggressive towards certain elastomers compared to the mineral oils they substitute.

The behavior and properties of the seal material include shrinkage, swelling and hardening

In trying to determine compatibility, three major failure mechanisms must be identified. Swelling occurs when liquid molecules are absorbed by the polymer network in the elastomer, which causes the material to expand. A moderate amount of swelling, usually within the range of just a few percentage points in volume, is usually beneficial since it maintains the pressure of seals and helps compensate for compression changes over time. In excess, however, it could cause the seal to expand beyond its groove for retaining or lose its sealing geometry or soften and be prone to damage from nibbling in dynamic interfaces. Shrinkage is a different issue that is caused by certain fluid chemistries that extract plasticizers or other substances from the elastomer and cause the material to expand. Shrinkage is more dangerous than swelling, as it reduces the pressure at the seal's interface and can cause leakage paths, which get worse over time. The process of hardening happens when a fluid causes the elastomer's flexibility to decrease, usually by chemical crosslinking or loss of plasticizer. This results in a seal that will not be molded to surface imperfections or allow dynamic movement without cracking.

NBR (Nitrile) Compatibility

Nitrile rubber is the most widely used seal material used in traditional hydraulic systems and is a material that has varying compatibility with biodegradable and synthetic fluids. It generally works well with HETG and a variety of HEES fluids, but the byproducts of oxidation from biodegradable liquids that are degraded may attack Nitrile over time. NBR performs poorly when used with phosphate esters that cause massive expansion and rapid degrading and is typically not suitable for PAG-based fluids due to substantial shrinkage. In systems that are converting to biodegradable esters, NBR is often left in service; however, the expected service life is to be reduced and inspection intervals cut.

FKM (Fluoroelastomer/Viton) Compatibility

Fluoroelastomers provide greater resistance to chemical attack than nitrile. They are typically the most preferred option in the process of converting systems to biodegradable or synthetic fluids. FKM is able to handle phosphate esters with ease and is the preferred choice for applications that require fire protection. It is also compatible with synthetic esters as well as biodegradable ester-based fluids. It shows little swelling and long-term stability. The main drawback of FKM is its cost, and in some formulations, decreased low-temperature flexibility as compared to the nitrile that is essential for equipment that is used outdoors or in cold climates.

EPDM compatible

EPDM is the preferred material that is used in phosphate ester systems as well as certain PAG-based fluids. It provides outstanding resistance. But EPDM has a critical drawback: it is incompatible with mineral oils derived from petroleum and with the majority of ester-based synthetic and biodegradable fluids, such as the HEEG as well as HEES formulas. This is what makes EPDM an unsuitable general-purpose option unless the system is devoted exclusively to phosphate esters or compatible glycol-based fluids. Cross-contamination is a major danger; even tiny amounts of mineral oil injected into an EPDM-sealed system for phosphate isomers can cause a rapid seal failure.

Polyurethane and PTFE-related considerations

Polyurethane seals are renowned for their resistance to abrasion and load-bearing capacity that is high in cylinder applications and generally have excellent compatibility with mineral oils and PAO-based synthetics, but they can be prone to hydrolysis when in conditions of fluids that contain water or ester chemistries, especially when temperatures are elevated. This makes it a less suitable choice for biodegradable esters in high-humidity or high-temperature environments. The PTFE alternative is chemically inert and resistant to nearly all hydraulic fluids. This includes PAGs and phosphate esters with a high degree of aggressiveness and is therefore a good option for seals that are static as well as backup rings in situations of fluid conversion. The main drawback of PTFE is its absence of elastic memory. This means it requires energy-generating elements like O-rings or springs to keep its sealing force. Additionally, it is not able to make up for wear in the same way that rubber seals do.

The right seal material for the type of fluid

The selection of seals should always begin with the particular fluid classification and the manufacturer's compatibility document rather than making general assumptions about the fluid's categories. As a guideline generally, phosphate ester fluids work best when combined when used with FKM or EPDM but not with nitrile. Ester-based biodegradable liquids, such as Hetgas and HEES, generally are compatible with FKM and are often compatible with nitrile. However, FKM provides longer-lasting endurance. The PAG-based liquids need careful matching, usually choosing EPDM or specially formulated nitrile compounds since conventional nitrile shrinks too much. PAO-based synthetics are the most forgiving category and often allow continued use of existing mineral-oil-compatible seals with minimal risk.

Verification prior to conversion of fluids

Before changing the hydraulic systems to biodegradable or synthetic liquid, compatibility must be checked using the manufacturer's elastomer compatibility chart and immersion tests when the documentation is not available as well as a study of the original specifications of the manufacturer of the seal. A lot of fluid manufacturers have published volume swell and variations across tests and temperatures, providing an accurate basis to compare them. When you are converting the existing equipment, all components made of elastomer must be taken into consideration, not only the pump and cylinder seals. This is true for O-rings and hoses in valve bodies and accumulator bladders. Any fluid that is compatible with the main seals could not be able to treat smaller elastomeric components or other parts of the circuit.

The transition to biodegradable and synthetic hydraulic fluids can bring real performance as well as environmental benefits, but it adds an additional layer of materials science that must not be overlooked. Seal material selection isn't an all-inclusive decision. It is dependent on the particular fluid's chemistry operating temperature, as well as the duration of use. It is important to ensure that you match the seal elastomer with the type of fluid prior to conversion, rather than finding out that the seal is incompatible due to malfunctions in the field, which is still the most cost-effective way to ensure reliability in the hydraulic system.