A GUIDE TO UNDERSTANDING O-RING MATERIAL COMPOUND TEST REPORT DATASHEETS
A test report on a rubber compound which is being considered for use in O-rings, or for any other type of elastomer seal, is of value only if the user understands how to interpret and utilize the data it contains. This brief guide is intended to explain the various terms used in common elastomer test reports, and to assist the reader in relating material test results to potential seal performance.
Test equipment and specific methods of testing are not included here. They are however, fully described in ASTM D1414, Standard Methods of Testing Rubber
O-rings. RMA Publication OR-9, A Guide for Preparing An O-ring Specification , should also be helpful. These documents may be obtained from the American Society for Testing and Materials, and the Rubber Manufacturers Association, respectively. Their addresses are listed at the end of this guide.
TEST REPORT USES
Test reports are used by both the seal manufacturer and the customer for various purposes. A seal manufacturer, for example, uses them for:
- Compound development and evaluation
- Quality control
- Process control
- Conformance testing to specifications
THE CUSTOMER USES TEST REPORTS FOR:
- Quality assurance
- Compound evaluation and selection
- Specification development
- Performance prediction
- Supplier material evaluation
WHAT A TEST REPORT CANNOT DO
A compound test report is not intended, and under no circumstances should be used, as a substitute for in-service testing and evaluation of seal materials . In the first place, as with any test, there is the problem of experimental error, which, in the case of seal material testing, can be quite large due to the basic nature of the elastomeric materials and subtle differences in laboratory procedures and test equipment calibration. The main value of the test report lies in the use of the data for intelligent reasoning. Material test reports are not unlike a road map which can guide you in the general direction of your destination. When the road map says there is a cross-road at a specific point however, and you arrive to find no cross-road -- believe
your eyes, not the map. Likewise, with compound test reports -- physical evidence from actual in-service experience should supersede test report results.
The most commonly used seal elastomer tests are:
- Original physical properties (with 5 subsets)
- Compression set (High Temperature)
- Fluid immersion aging
- Air aging
- Low temperature Flexibility (TR-10)
These are the tests which are described in this guide, though it is only rarely that all of these will be included in a single material test report.
There are a number of highly specialized material tests which are used to determine specific attributes of elastomers. These tests are not provided in the average material test report but may be called for by the customer, a material specification such as ASTM D2000 or a military specification such as MIL-G-83528. All the standard tests described, plus many of the more specialized tests, are completely explained in available ASTM documents. Some of these specialized elastomer tests are:
- Low temperature compression set (ASTM D1229)
- Tear resistance (ASTM D624, Die B & C)
- Compression/Deflection (ASTM D575)
- Brittleness (ASTM D2137)
- Friction & abrasion (ASTM D3389)
- Crush test
- Fatigue test (ASTM D430)
- Corrosion & adhesion (ASTM D429)
- Curing Characteristics (Mooney Rheometer) (ASTM D1646)
- Humidity aging
- Ozone Deterioration (ASTM D1149)
- Radiation Resistance
- Stress Relaxation (ASTM 1390)
- Permeation (water & gas)
- Vacuum (outgassing & weight/volume loss)
- Electrical properties:
- Volume resistivity
- Shielding effectiveness (EMI shielding) Electrical stability
Detailed information on these special tests, and how they relate to seal performance is available from the referenced ASTM documents or you may contact Parker Seal Group Technical Services.
DESCRIPTION OF GENERAL TESTS
Following is a brief description of the five general material tests usually available from a seal manufacturer without additional cost.
ORIGINAL PHYSICAL PROPERTIES
The original physical properties most commonly reported are:
- Tensile strength
- Ultimate elongation
- Specific gravity
Hardness (ASTM D2240)
The hardness of an O-ring or other seal material is measured in accordance with ASTM test method D2240. This method employs an instrument called a "durometer," first manufactured by the Shore Instrument Company. There are two different scales for measuring rubber hardness, Type A, and Type D.
Type A readings are used for most common O-ring and seal materials. A 65 to 75 Type A range is considered mid-range for O-ring elastomers, and is the range in which most O-ring and other seal materials exhibit the best combination of properties. The Type D scale is used for harder materials such as filled thermoplastics. By way of comparison, a common red eraser has a shore hardness of about 50 Type A, while a bowling ball has a hardness of approximately 80 Type D.
It should be noted that hardness values are not consistently described in precisely these terms. For instance, a specimen having a durometer hardness of 75 Type A may be referred to as a "75 SHORE A" material or a "75 durometer" material.
Tensile Strength (ASTM D412 (Dumbbell)\D1414 O-Ring)
Tensile Strength is the stress in pounds per square inch (PSI) or pascals (Pa) that develops in an O-ring just before it breaks when being pulled apart. (Imagine pulling a rubber band between two fingers until it breaks.) The test
method most often employed is ASTM D1414. High tensile strength is seldom required in normal O-ring service, and by itself does not assure the acceptable level of any other physical property.
The test is valuable, however, because it is easily reproducible and its consistency is a good indicator of consistent quality of the elastomer. Tensile strength at rupture will vary with the cross-section and therefore, size 2-214 O-rings having a cross-section of .139" are generally used for tensile tests unless otherwise noted or requested for special requirements .
Ultimate Elongation (ASTM D412 (Dumbbell)\D1414 O-Ring)
Ultimate elongation, generally referred to simply as "elongation", is the amount an O-ring will stretch before breaking, expressed as a percent of original length. The test method employed is ASTM D1414. A higher elongation value is important in some designs (for example where O-rings in piston seal grooves must be stretched during installation.)
Small diameter O-rings must sometimes be stretched to nearly twice their original circumference when they are assembled over the end of a piston. Harder rubber materials generally have lower elongation values than softer compounds. If a hard material is needed for extrusion resistance, for example, a compromise will be required . See "COMPROMISE" for more information.
Modulus (ASTM D412 (Dumbbell)\D1414 O-Ring)
The modulus of an O-ring material is the stress in pounds per square inch (PSI) or pascals (Pa) required to produce a given percentage of elongation (usually 100%). This test is usually conducted to ASTM D1414 standards. Modulus is critical to O-ring sealing performance. It correlates fairly well with extrusion resistance and, in dynamic applications, with wear resistance as well. Higher modulus values generally provide better wear and extrusion resistance.
Specific Gravity (ASTM D792)
The specific gravity of any material is a comparison (ratio) of its weight with the weight of an equal volume of water. A specific gravity of 2 indicates that the material weighs twice as much as the same volume of water.
Specific gravity does not, in and of itself, correlate with seal performance, but it is valuable as a quality control tool because it can be measured accurately, and it is the most consistent physical property of an elastomer compound. It can also be used in some cases to determine the probable base polymer of an unknown compound. For example, the fluorocarbon compounds generally have a specific gravity between 1.80 and 2.20, the highest of the common O-ring polymers, while the nitriles generally fall in the 1.10 to 1.40 range . Within a given polymer group, compounds with higher carbon black content tend to have higher specific gravity values .
COMPRESSION SET (ASTM D395)
Without a doubt, the most important of all the measured properties of an O-ring or other elastomeric seal material is compression set. This is a measure of the ability of the material to recover some portion of its original cross-section thickness after long term compression. (Compression set should not be confused with resilience or bounce, which is very short term.)
As a general rule, lower compression set values indicate better sealing ability because lower compression set numbers mean that the material recovers more nearly its original thickness after being compressed for a long period of time. It is this tendency to recover while being compressed that produces pressure of the seal material against the mating surfaces, thus maintaining the sealing line that is essential whenever a fluid must be contained within a joint.
Elastomers do not recover completely (to 0% set) after being compressed unless the test is conducted with the specimen immersed in a fluid that swells the rubber significantly. The standard ASTM D395 compression set test provides a means of comparing the ability of different materials to recover under a given set of conditions.
The ASTM D395 test is quite simple. An O-ring or test disk is compressed between two flat metal plates until its thickness is reduced to 75% of its original value (25% initial compression). The confined specimen is then placed in an oven which is maintained at the desired temperature for a specified time, generally 22 to 70 hours. The test specimen is then removed from the confining fixture and after a period of time (usually 30 minutes) its cross section is measured and compared to the original cross section thickness. the compression set figure is usually expressed as the percent of the original compression which is not recovered.
Original cross-section = .139 inches
Cross section when compressed 25% = .104 inches
Recovered cross-section = .122 inches
Amount of original compression: (25% of .139) = .035 inches
Unrecovered compression (.139 - .122) = .017 inches
Compression set = .017 divided by .035 = 49%
Therefore this material has 49% compression set at the temperature and time specified for the compression set test. Compression set tests in fluids have not been commonplace in the past, but they are conducted more frequently now to provide valuable data for evaluating anticipated seal performance in actual service conditions.
FLUID AGING (ASTM D471)
The properties covered thus far are useful for quality control and comparison purposes but the vital question remains -- How will the material function in the sealed fluid?
This question can be answered in part by the standard fluid aging tests outlined in ASTM test method D471. These procedures measure the change in hardness and the percent change in tensile strength, elongation, modulus, and volume of a material when immersed in a specific test fluid for a specific time at a specific temperature. The difference between the original properties and the properties obtained after fluid aging are usually expressed as a percent change although both the percent change and the actual values are often included in the test report.
The intent of fluid aging is to accelerate the aging process of the elastomer in a particular fluid in order to obtain an early indication of the effects the fluid will have on the seal material in service. The aging process is "speeded up" by testing at a higher temperature than the seal will be exposed to in the application. Unfortunately, this extra high temperature will sometimes cause changes in the seal material that are different from the changes which occur in actual use.
It is not practical, however, to wait for a five year test to be completed before selecting a seal material for a new application, so a 70 hour test at elevated temperature must usually suffice. This is another reason why the ultimate test is in-service experience . Though fluid aging cannot give a precise prediction of seal performance in the tested medium, it does provide a means of comparing the effects of the fluid on different seal materials. Thus, it provides a basis for choosing the most likely candidate materials from a number of compounds. To a person experienced in sealing technology, fluid aging can often provide a rough indication of how well a seal fabricated of the rubber material tested, or a similar compound, will actually perform in service when in contact with the tested fluid, or a similar fluid.
A large change (%) in any physical property either up or down, usually indicates that the test fluid has caused an undesirable change in the rubber. Volume change in the test fluid is a particularly significant piece of information . In fact, volume change is sometimes the only test conducted. The seal material may absorb some of the sealed fluid, resulting in volume swell. This can sometimes be beneficial to the seal application if the increase is small, as it increases the sealing force and may, at least
partially, compensate for compression set. Excessive volume swell, however, can force the seal to extrude and has been known to develop enough pressure to damage mating metal components. In dynamic applications, excessive swell will also increase friction and wear, and can lock up moving parts.
Volume shrinkage, caused by migration of oils or other components of the seal material out into the fluid, can pull the seal element completely away from the mating surfaces causing total seal failure. This is more likely to occur when combined with excessive compression set.
Volume change, similar to original physical property changes, is reported as a percent of the original volume, a plus percentage indicating swell and a minus change indicating shrinkage.
AIR AGING (ASTM D865 or D573)
Air aging, like fluid aging, is simply another means of evaluating a given elastomeric material for possible deterioration of physical properties in an intended environment, (in this case, air at an elevated temperature). Again, the usual physical properties; hardness, tensile strength, elongation, and modulus, plus, for air aging, weight are measured before and after air aging and all except hardness change are expressed as percent change from the original physical properties. The test procedure is conducted according to ASTM D865 or D573.
Weight change after air aging will be either zero (no change) or a minus quantity (weight loss). Like volume shrinkage, a loss of weight indicates that something has been physically removed from the seal. In the case of air aging, weight loss is caused by the evaporation of plasticizing or other volatile components. Significant weight loss is often accompanied by an increase in hardness, loss of elasticity and major changes in other physical properties. (See "COMPROMISE")
LOW TEMPERATURE TEST (TR-10) (ASTM D1329)
The TR-10 test is conducted according to ASTM D1329. The procedure requires that a cut O-ring specimen be stretched until it is 50% longer than its original length, frozen in an extremely cold alcohol bath and released. Heat is then applied to the solution at a specific rate. As the temperature rises, the frozen specimen begins to retract. The temperature at which it recovers 10% of the original stretch is the TR-10 temperature for that material. The term, "TR-10" stands for "temperature retraction, 10% ."
This test is a good indicator of the ability of an elastomer to remain sufficiently elastic that it will maintain the sealing line during pressure cycling and thermal movement of the mating parts in low temperature service. As a rule of thumb, a seal will generally function at a temperature as low as 15°F below its TR-10 temperature although this can be in error depending upon the application or if the seal shrinks or swells in the service medium. It may also be modified by the way the seal is used, i.e., a rod type seal assembly will usually hold fluid at a lower temperature than a piston type.
Seal designers seek the "perfect seal material" to cover all possible operating conditions. In reality, compound development involves a series of compromises to achieve the most benefits with the fewest disadvantages. Most of the physical properties of elastomeric sealing compounds are interrelated, and improving one property through formulation and/or processing adjustment will usually change one or all of the others in some measure.
The relationship between hardness and elongation has been mentioned as one example. High temperature and low temperature service limits are another. In attempting to increase the high temperature capability of a given material, there is usually a corresponding decrease in the low temperature performance and vice versa. There are a myriad of similar interrelationships in any elastomer. A rubber compound is a complex mixture of chemical ingredients and no single material can ever have all the physical and fluid resistance desired by either the manufacturer or the user.
Consider the analogy of an imaginary window of fixed dimensions which can be moved as desired. The window can be placed so that sunlight falls on any area of the room.
Moving the window to provide more illumination in one area however, will reduce the amount of light in another. Elastomeric seal materials behave in much the same way. Moving the "window" to improve any one property may well degrade one or more of the others.
Used properly, test reports are a valuable tool for determining the best seal material compromise for a given application -- the material that will provide the best possible sealing performance, for the longest time, at the most reasonable cost.
For more information on RMA and ASTM test documents, contact:
Rubber Manufacturers Association, inc.
400 K Street, N . W . Washington, DC 20005
American Society for Testing and Materials
1916 Race Street
Philadelphia, PA 19103