The greatest strides in seal materials have been made with thermoplastic polyurethanes (TPUs). Initial limitations of the early TPUs have been overcome. Current TPUs can tolerate system operating temperature up to 250°F without suffering serious loss of lip preload and generally do not require O-ring energization. Hydrolysis resistance in some formulations is now so good that TPU seals are used in underground mining cylinders that operate on high-water-based, fire-resistant fluids.

Pneumatic cylinder designers also have benefitted from the advances in TPUmaterials. Calls for very low friction and ultra-long service life have been accommodated by TPU seals which offer 50% of nitrile's breakout friction and have lasted for 12 × 106 cycles in 2-in. bore, 10-in. stroke cylinders with non-lubricated air.

Thermoplastic polyester elastomers (TPE) have also improved. It is possible to chemically engineer TPEs to produce such desirable properties as outstanding wear and fluid resistance. These characteristics have made them a first choice in many sealing applications - particularly as piston seals where, with suitable energization, extremely efficient performance can be produced. Many of these TPE seals compete with PTFE elements where the elastomeric nature of TPE makes them more easy to install and also prevents piston drift. An example is in truck-mounted crane outriggers, where the elastomer can bond into the adjacent surface finish. TPEs with their superior wear resistance and tensile strength are ideal for this use.

In Europe, TPEs have a growing importance in specialty sealing applications such as the mining and steel industries. TPE's heat and fluid resistance perform well in rolling mills, for instance. For port-passing applications, such as phasing cylinders, by exploiting the wear resistance and hardness of TPE, seals can be designed specifically to overcome problems often associated with this type of cylinder design.

The key to success in today's industry for the seal maker lies in combining the latest material technology with innovative profiles to provide the customer with solutions which work.

Future trends

As environmental issues continue to influence almost all industries, the hydraulics sector will be no exception. In Europe and the U.S., so-called environmentally friendly fluids are being developed. Vegetable oils, such as rape and sunflower seed, have been tried, but they can cause problems for the system (forming resin above 180°F) and for the seals and other components (forming acid in any water present that can attack elastomers). Other fluid contenders include polyglycols and synthetic esters, but these also present problems - not the least of which is a cost up to ten times that of mineral oil. New materials and blends will be required to combat the effects of these fluids while still providing the sealing integrity users expect. Preliminary work indicates that there is a long road ahead if this issue becomes a reality.

Table 3
Material Applications Positive factors Precautions
Nitrile Fluid power cylinders Inexpensive; good resistance to set Not tough enough to withstand very smooth surface finishes (<0.4 µin. CLA)
Carboxylated nitrile Better wear resistance than nitrile Limited low-temperature flexibility, compared with standard nitrile
EPDM Exposure to fire-resistant fluids Resistant to HFD fluids and Skydrol Not resistant to mineral oils, greases, other hydrocarbons
Fluoroelastomer High temperatures (to 400°F) Resistant to most hydraulic fluids Relatively expensive and difficult to process
PTFE General sealing Low friction Not elastomeric, requires energization
Polyurethane General sealing elements Good wear resistance and resistance to set; energization not required First generation subject to hydrolysis effects of water above 120°F
Polyester Rubbing faces of seals; Anti-extrusion elements Elastomeric; good resistance to wear and fluids Poor resistance to set; requires energization

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.

Dealing with pipe thread fittings

There is is no question that pipe threads should not be specified in new equipment deisgns. Pipe threads are prone to leakage, especially after being disassembled and reassembled. Furthermore, many more-modern thread forms are widely available that offer long-term, leak-free performance, even after being assembled and reassembled several times. Still, despite their poor perfomrance, pipe threads continue to be used throughout a variety of industries. So accepting that pipe threads will still be encountered, this discussion reviews methods for reducing shortcomings of pipe fittings.

Four methods are commonly used to seal pipe threads:

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 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.

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 gauges. 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 that 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.

Probably the most significant event in sealing fittings has been the development of anaerobic pipe sealant with TFE materials. Since the first appearance of these materials, many  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.

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.