Basic properties of elastomeric seal compounds

Although elastomeric compounds used in aerospace seals are derived from relatively few base polymers (such as nitrile, fluoroelastomer, and ethylene propylene), each seal manufacturer usually develops special compounds of these base polymers to enhance or suppress different chemical or physical properties to fit specific requirements of an application.

Proprietary formulations of these compounds are kept secret. Even the analysis of a finished elastomer seal presents an incomplete picture of the original elastomer compound because some ingredients are consumed in processing.

Of all compound properties, the most critical are the changes that occur. Every property of every compound changes with age, temperature, fluid, pressure, squeeze, and other factors. Standardized tests have been developed to provide comparability in changes among compounds. Compounds with the least tendency to change properties are the easiest to work with; they produce a seal that is adaptable to more applications.

The number of properties evaluated for an application depends on the severity of conditions. Factors are highly interdependent, but typically include resilience and memory, abrasion resistance, coefficient of friction, and fluid compatibility. Let's take a closer look at each of these.

Resilience and memory are defined as a compound's ability to return to original shape and dimensions after a deforming force is removed. Resilience implies a rapid return, while memory implies a slow return. In seals, resilience is important because it permits a dynamic seal to follow variations in the sealing surface. Although elastomer resilience is frequently measured on a Bashore resiliometer, field experience is required to relate ratings to seal performance. Additional attention is required for low-temperature applications. When temperature is too low, a compound loses its memory.

Abrasion resistance - resistance to wear when in contact with a moving surface - is the product of other properties, including resilience, hardness, thermal stability, fluid compatibility, and tear/cut resistance. It also is influenced by the compound's ability to hold a film of protective lubricant on its surface.

Harder compounds are usually more resistant to wear, so dynamic seals of 85-durometer compounds are common. However, if the seals encounter high temperatures, it may be good practice to specify an even harder material to compensate for the softening effect of heat. In low-temperature applications, a softer material might be preferred because elastomers tend to harden a temperatures drop.

Coefficient of friction (usually only important in dynamic seals) is compound-specific and different for running and break-out. Usually break-out friction is higher. Break-out friction increases with time between cycles.

Coefficient of friction is affected by temperature, lubrication, and surface finish. Aging and the influence of service fluids on the compounds may also affect hardness and, in return, both breakout and running friction.

As far as fluid compatibility is concerned, a fluid is considered incompatible with a compound if the fluid causes enough property changes to reduce sealing function and/or shorten the working life of the compound. Dissimilar chemical structure is the key to fluid compatibility. For non-polar liquids - such as hydrocarbon fuels and oils - nitriles, fluorocarbons, or fluorosilicone polymers are normally used. For polar liquids, such as phosphate ester hydraulic fluids, ethylene propylene compounds are most satisfactory.

Sealing pipe-thread fittings

A review of the important performance properties of compounds of tetrafluoroethylene (TFE) resin and filler materials shows that the resin performs well in many applications without fillers. In fact, fillers can lessen TFE's outstanding electrical and chemical properties. In mechanical applications, however, compounds of TFE and inorganic fillers offer improved wear-resistance, reduced initial deformation and creep, and increased stiffness and thermal conductivity. Hardness is increased, and the coefficient of thermal expansion is decreased. These compounds can make it possible to gain the advantages of TFE in applications where the unfilled resin cannot be used.

Many different fillers can be blended with TFE, but most application requirements have been met with five filler materials: glass fiber, carbon, graphite, bronze, and molybdenum disulfide. The properties of any compound depend on filler type and concentration, and processing conditions. Compounds - such as plain TFE - are made into finished parts by molding, extrusion, or machining.

One example of the application of TFE resin and fillers is O-rings made of TFE. They are used where resistance to solvents and other chemicals, or extremely high- or low-temperature resistance is required. These are applications where elastomeric materials are not suitable. An additional benefit of TFE O-rings, in certain applications, is the material's low coefficient of friction and anti-stick properties. Typical applications are rotary, piston, and valve seals, and gaskets.

Four methods can seal pipe threads:

Trapped dope. The use of drying or non-drying dopes is the oldest and least costly thread-sealing method. Made from ingredients ranging from crushed walnut shells in shellac to other fillers and oils, usually with some thinning volatiles, they are inherently weak, and will shrink when the volatiles evaporate.

Yielding metal. The sealing interface is limited in area and unlimited in force so that yielding takes place. Metal flow fills misalignment and leak paths. These dryseal joints can be effective, but they usually cannot be disassembled and reused without leaking.

Trapped elastomer. Confined O-rings can seal effectively, but also can suffer from sloppy assembly. Damaged threads or pinched rings also can contribute to leakage. O-rings typically are used in high-pressure fluid power systems where the extra cost is more easily justified and freedom from contamination is especially desirable.

Curing resins. Sometimes called machinery adhesives, these anaerobic acrylic materials develop strength by curing. They are very forgiving of tolerances, tool marks, and slight misalignment. They make tapered fittings as effective as O-rings at a fraction of the cost. They lock free-standing fittings - such as gages. They can also improve the 98% effectiveness of yielding-metal joints to 100%. The correct grade must be selected because of their wide range of strengths so that disassembly will not be hampered.

Curing materials are so effective in sealing threads that they are often used on straight threads which enter or plug pressure vessels. In addition, the curing materials are effective even when tapered threads are lightly torqued. Lightly torqued threads (straight or tapered) do not leave high residual stresses in housings or valves that can distort valve bodies to the point of inoperation or long-term fatigue failure.

Design engineers must choose between these options to assure that equipment will function as planned. Specifications should not be left to assembly workers - as is often done.

There probably is little argument that the most significant event in sealing fittings during the past 25 years was the appearance of anaerobic pipe sealant with TFE materials. Since the first appearance of these materials, many other companies have added anaerobic thread sealants to their lines. The new sealant technology offers a variety of benefits:

Convenient curing. Being anaerobic, it cures in the absence of the air, remaining uncured until the parts are assembled. There is no evaporation, hardening beforehand, or other work-life problems.

Lubricity. Containing TFE filler, the material eliminates galling or other component-assembly problems. These products prevent over-torquing to affect a seal.

Fills threads. Due to high wetting ability, the material fills threads so well that leakage past nicks, scratches, and dents does not occur.

Fitting movement. Systems being assembled with anaerobic sealant can be initially readjusted without breaking the seal in the threads.

Vibration resistance. Anaerobic sealant does not permit a fitting to be loosened by vibration. Reusability. Fittings sealed with acrylic and latex-based materials can be disassembled and reused with sealant in the field without danger of leakage.

Freedom from contamination. Unlike the tape most often replaced by the anaerobic material, sealant does not break up to contaminate lines and valves.