This term is used to indicate the wear resistance of a compound. It is concerned with scraping or rubbing of the surface and must therefore always be considered for dynamic seals. Compounds of medium hardness (about 70 durometer) are usually more resistant to abrasion than harder or softer compounds.
The term "aging" is applied to the physical and chemical changes that take place in a given elastomer over a period of time, including both storage time and service time. The changes generally contribute to deterioration, which affects the useful properties of the elastomer.
The aging process and its effects can be minimized by both careful selection of the compound used, and the addition of age inhibitors. Well-planned storage helps to prevent exposure to common harmful elements such as heat, sunlight, ozone, oxygen, moisture and radiation.
While natural aging may cover months or years, careful formulation for maximum life under normal storage and operating conditions through the development of many tests that accelerate the natural aging process give the data necessary for thisw formulation.
ESI maintains the most stringent controls over their inventory. When O-Rings become over age, they are destroyed. Where O-Rings are manufactured to Military Specifications, maximum accumlated age allowances for O-Rings based on current government policies and regulations are strictly adhered to.
"Compression Set" is the failure of the elastomer to return to its original shape after compression load is released. If an O-Ring takes an appreciable compression set it may no longer exert adequate sealing force. In practice, however, this may not be as serious as one might expect. Under constant temperatures and pressures, an O-Ring will continue to seal after taking 100% compression set because it is still in contact with the mating serfaces and is activated by the fluid pressure. Furthermore, swelling due to the media can compensate for compression set. For critical applications, ESI has special compounds with outstanding resistance to compression set.
"Corrosion" is the chemical action of the media and/or the compound upon the metal surface of the gland. Media corrosion of the gland metal can cause a change of finish that can affect the seal. In general, compound ingredients are formulated so they will not act adversely on the gland surface.
The chemical effect on a compound by the media almost always results in a permanent reduction of physical properties. However, a reduction of physical properties by swelling is not always considered "deterioration." Compounds can be enlarged by a high swell media and while in appearance have impaired physical properties, once the media is removed will test almost as originally.
The terms "media" or "fluid" denote whatever substance is to be retained by the seal. This includes gases, liquids, or mixtures of both.
An important part in the selection of a compound is to make sure the compound selected will not be affected through a chemical reaction of the media on the compound material. If the media is such that it causes excess deterioration of a seal such as excessive swell or shrinkage, a marked increase or decrease in hardness or a definite or extreme change in elongation or tensile strength, then the operational characteristics and the life expectancy of the seal will be completely altered.
The Compound Selection Guide on this site identifies the compound most serviceable in specific fluids. This extensive listing has been prepared for the greatest possible convenience in selecting a compound for specific applications.
An understanding of both break-out and running friction is often helpful in an analysis of a specific seal problem. "Break-Out Friction" is usually defined as initial friction developed when the O-Ring is employed as a moving or dynamic seal. The amount of break-out friction which an O-Ring will develop is dependent upon the length of time the O-Ring has been in contact with the mating surfaces, the O-Ring material and the type of mating surface.
"Running Friction" is defined as friction developed after the seal is in motion during the use of the O-Ring as a moving seal. The problem involved with high running friction may cause difficulty by wearing soft metal parts, as well as the seal itself -- especially with high pressures. Harder O-Rings generally produce less running friction. In addition to special compounds containing dry lubricants.(?)
The "hardness" of a compound is usually determined through the use of an instrument known as a "durometer." This instrument registers high or low resistance to pressure or the "hardness" of a compound. This is important, for example, in O-Rings where a higher hardness will reduce the tendency of the O-Ring to be forced or extruded into the narrow gap beyond the groove. Therefore, extrusion can usually be prevented by a harder compound. In low pressure seals, a lower hardness rating means the material will flow more easily into the microfine grooves of the mating parts.
"Hardness" has an appreciable effect on frictional forces. Hardness must also be considered when the compression force necessary for assembly is limited. The lower the hardness rating, the less force is required. Seventy durometer is the optimum for most O-Ring seals, but 80 durometer is preferable for rotary seals. The upper limit for O-Rings is normally 90 for effective sealing.
Hysteresis is sometimes used to describe the amount of drag caused by the elastomer as measured in lbs., oz., etc. Hysteresis is also used to describe a delay or lag between two events. The rate of motion of the seal face axially must be the same in both directions or the seal faces will separate in not returning as fast as it was thrust away from the stationary ring. This minute separation caused by motion, drag and hysteresis depends then on not only the amount of drag, but the size of the seal and the speed. Hysteresis is the underlying reason for face separation, leakage, premature life and abrasive face damage that can occur in seals. 1
The tendency of gaseous media to pass or diffuse through the elastomer is defined as "permeability." There is sometimes confusion between this term and "leakage," which is the tendency of media to go around the seal. In vacuum service and some pneumatic applications, permeability is a critical factor. In most other cases it is not. Permeability increases as temperature rises, and different gases have different permeability rates. The more a seal is compressed, the geater its resistance to permeability. Butyl rubber is one of the best elastomers for low permeability. Silicone rubber is one of the poorest.
Good resilience is an important attribute in a moving seal. Resilience in most cases is dependent on proper compounding techniques, which have been developed through production and field test experience. Of the base polymers, however, natural silicone and rubber have the best resilience. Original resilience of a compound is not the whole story, as resistance to compression set is very important in prolonged service.
"Stretch," particularly in O-Rings, can be extremely harmful to seals. Normally, O-Rings should not be permanently stretched more than 5% because any stretch in excess of this amount will result in rapid deterioration of many elastomers. Only in very small O-Rings is there variation in this rule, since small sizes usually perform satisfactorily with a permanent stretch factor somewhat greater than 5%.
Stretch, in addition to being harmful to the seal, also causes a reduction in cross-section. This condition, as well as loss in plasticizer, may result in leakage.
There are a few compounds that are almost unaffected by higher stretch for prolonged periods. ESI's staff can provide complete information upon request.
A very important point to consider in seal design is "squeeze." Too much squeeze tends to overstress the compound and cause it to wear or rupture. Maximum squeeze varies with the compound being used. Rubber may be considered an incompressible fluid, and it is always necessary to provide space for the displaced material.
Minimum squeeze for all seals, regardless of cross-section, is usually set at about 0.004 or 0.005 inches. This rule has been established due to the fact that exceedingly light squeeze causes almost all elastomers to quickly take 100% compression set at moderate temperature (200° to 225° F).
Most compounds show relatively good tear strength; however, if the tear strength is very low (less than 100 pounds/inch) there exists the danger of cutting or nicking the seal during assembly if it passes over any burrs or sharp edges. Compounds with poor tear resistance also usually have much less resistance to flexing or stress, especially once a crack is started. In dynamic seals, low tear strength usually also indicates poor abrasion resistance.
"Tensile Strength" is a production control measurement used to ensure uniformity of a compound and also is used as a method of showing deterioration of the compound after contact with the media for long periods. It is measure in psi (pounds per square inch) required to rupture a specimen of a particular material. This measurment is not a proper indication of resistance to extrusion and is not generally used in design calculations. In dynamic applications a minimum of 1000 psi is usually specified to assure good stength characteristics.
"Elongation" is defined as an increase in length presented numerically as a percentage of the initial length. In most caseds it is expressed as ultimate elongation or the increase over the original dimension at break. Pertaining to seals, it is usually used to indicate the stretch which can be tolerated during the installation of the seal.
"Modulus" refers to stress at a predetermined elongation, generally 100%. Modulus is one of the best indicators of the toughness of a compound. It is also used as a quality control value because it is more consistent than ultimate elongation or tensile strength.
One of the most important differences between low- and high-temperature effects on elastomers is that through the warming process, most elastomers will regain their original properties after low-temperature exposure, while high-temperature effects (compression set, loss of strength, etc.) are usually permanent. In determining the effects of temperature the swelling or shrinkage effect of the media being sealed must also be taken into account. If the seal swells in the media, it absorbs fluid which may act in much the same way as a low temperature plasticizer. In other words, it may allow the seal to remain more flexible at low temperature than was possible before the absorption of the fluid.
If the seal shrinks, something is being extracted from the compound. In this case, the seal may lose some of its flexibility at low temperature.
One side-effect of low temperature, especially in dynamic applications, is crystallization. When a compound crystallizes, it becomes rigid and loses its resilience. The end result is seal leakage. Tendency to crystallize, therefore, is an important consideration in selecting compounds for low temperature service.
The sealing ability of a compound deteriorates with total accumulated time at elevated temperatures. Therefore, continuous service at temperatures beyond the specified range is not recommended. Short-time exposure is often entirely satisfactory.
Temperature ranges listed for ESI compounds in this manual are for normal applications and in many cases are based on tests conducted to give continuous seal reliability for 1000 hours or more.
Temperature ranges for both storage and operation are important factors in design but in many cases, this area is over-specified.
Since elastomers have a coefficient of expansion about ten times that of steel this can be a critical factor at high temperatures if squeeze is marginal.
In order to help the designer overcome the difficulties inherent in finding a compound for a specific job without over-specifying temperature ranges, a great deal of time and work has been invested to develop compounds that have realistic temperature ranges with a safety margin for specific seal purposes. Maximum high operating temperature ranges have been established, based on long term functional service and proven by extensive laboratory tests.
When a seal is completely confined and the gland completely filled, the coefficient of the thermal expansion of the compound becomes the dominant force and in many cases a seal has ruptured its gland due to expansion when heated. It is desireable in designing a seal gland to fill the groove no more than 95% after adverse tolerances have been considered.
"Toughness" is a general term used to indicate the combination of resistance to physical forces rather than a chemical action. It is most often used as a relative term in general practice and is not a measured property. Tensile properties are often indications of the toughness of a compound.
This is a measure of the increase or decrease of the volume of an elastomer after contact with a fluid. Linear change varies as the cube root of volumetric change and the effect on a seal might not be as great as one might expect. A 20% swell (volume increase) would result in an O-Ring cross-section diameter increase of only 6%, as an example.
Swell is almost always accompanied by a decrease in hardness. This softening of the material naturally leads to reduced abrasion and tear resistance and extrusion of the seal under high pressure.
In the case of shrinkage (decrease in volume), there is usually an increase in hardness. Shrinkage may cause the O-Ring to pull away from the sealing surface, permitting leakage.