New test procedure for measuring the ageability of elastomers and plastics.

The demand for improved reliability and longer service life in automotive uses for rubber has created a need for being able to forecast and measure the expected service life of elastomer and thermoplastic components. The warranties and consumer demands for a "better" product require the elastomer parts to last longer and maintain the rubber-like properties over a longer period of time in ever increasing severe service environments.

A materials engineer from General Motors, John Beckett, was interested in being able to use short term, high temperature agings to forecast rubber part life. He observed that most of the basic automotive specifications in SAE J200 are based on 70 hours of aging. Many assumptions about longer life cycles are based on these short term agings since there is no other provision.

Beckett was interested in knowing what the upper temperature limits were for continuous service for elastomers. His research showed that 1,000 hours had been used in radial lip seal life cycle testing. The data showed that the property changes based on this aging at simulated service temperatures correlated well with end product performance. If one property was key to this it was elongation retention.

Beckett then requested the SAE Committee for Automotive Rubber Specifications (CARS) establish a committee to set up a procedure for measuring continuous upper temperature limits (CUTL). Beckett was selected. chairman and has completed establishing the SAE J2236 CUTL test procedure.

The procedure calls for measuring at what temperature a commercial practical rubber compound will retain 50% of its tensile strength and elongation for 1,008 hours. The eight hours was added to the basic 1,000 hours to ensure that the sample would be aged 42 days and come out of the oven at convenient times. The specification is specifically targeted at rubber and thermoplastic elastomers with Shore A durometer less than 90. The specification reads as follows:

Scope (1)

This method is intended to define the continuous upper temperature resistance (CUTR) of thermoplastic elastomers and thermoset rubber of 90 Shore A or less, to oxidation or other degradation when exposed solely to hot air for an extended period of time.

This method establishes the upper thermal aging limits of commercially available compounds as measured at 23 [degrees] C by retention of at least 50% original elongation and tensile at break after 1,008 hours of heat aging. This method does not take into account nor measure the effects of stress, environment or temperature variations on other thermal aging characteristics of the materials tested.

This method may involve hazardous materials, operations and equipment. This standard does not address the safety problems associated with its use. It is the responsibility of the user of this standard to consult and establish appropriate safety and health practices and determine the applicability of regulatory limitation prior to use.

This test method is based on SI units.

Referenced documents (2)

The latest revision of all documents should be used.

ASTM standards:

* D 412 - Standard test methods for rubber properties in tension. Method A.

* D 573 - Standard test method for rubber - deterioration in an air oven.

* D 1418 - Standard practice for rubber and rubber latices - nomenclature.

* D 2240 - Standard test method for rubber property - durometer hardness.

* D 5046 - Standard practice for plastics - nomenclature.

SAE standards:

* J200 - Classification system for rubber materials.

* J1344 - Marking of plastic parts.

* J3000 - Standard classification system for thermoplastic elastomers.

Terms specific to this standard (3)

Continuous upper temperature limit (CUTL) - the temperature at which the material retains 50% minimum of both the original elongation and tensile at break after 1,008 hours in an air circulating oven per ASTM D 573, Type IIA or IIB. Note per ASTM D 573, Type IIB ovens are not suitable for test temperatures above 70 [degrees] C.

Test method - a definitive procedure for the identification, measurement and evaluation of one or more qualities, characteristics or properties of a material, product, system or service that produces a test result.

Production compound - an identifiable, homogeneous quantity of material from a standard production period, with consistency and properties demonstrated through testing and use.

Commercial material - a finished compound developed primarily, but not exclusively for heat resistance, from readily available ingredients and processed on conventional industry equipment.

Significance and use (4)

Data obtained by this method are applicable to the material under conditions of this test and are not necessarily the same as those obtained in end use applications. The information can be used for comparison, selection or qualification of commercially available compounds where a level of proficiency is desired beyond short, quality control tests.

Ultimate elongation was selected over other physical property measurements because of its greater sensitivity to the various effects of air oven aging on elastomers. Tensile strength is also used since some compounds maintain more than 50% of their original elongation while losing considerable tensile strength after heat aging.

A material classification CUTL represents the highest temperature value the heat compound(s) can achieve per guidelines currently under development. The materials are classified according to ASTM D 1418, D 5046 or SAE J1344. Not all compounds within a material category can reach the listed maximum temperature.

This document requires aging at elevated temperatures in a hot air circulating oven per ASTM D 573, regardless of what is used in SAE J200 recommended practice. Care must be taken to test only similar generic compounds (i.e. silicone to silicone), to avoid cross contamination from volatile products and subsequent variation in test data.

General test conditions (5)

Unless otherwise specified, the material shall be tested to ASTM D 412, Method A requirements at 23 [degrees] C.

Aging temperatures - variation around a specified temperature shall be within [+ or -] 2 C.

Aging temperature increments - unless otherwise specified, test new materials (per SAE J200 table 1, plus 135 [degrees] C and 165 [degrees] C) to establish the upper continuous temperature limit to the definition in 3.1. Material tables shall be established using this criterion to minimize testing burden.

Aging time - 1,008h. [+ or -] 2h.

Uniform aging conditions - tests of samples shall ensure that all surfaces be exposed and the temperature uniform.

Sample requirements (6)

Thermoplastic elastomer and thermoset rubber elongation and tensile at break data used to establish tables or qualify new materials, shall be the median for a minimum of five samples per batch from five batches.

Sample dimensions for thermoset rubber - follow ASTM D 412 requirements.

Thermoplastic elastomers (for SAE J3000) - unless otherwise noted, samples are to be die-cut from injection molded rectangular plaques 3.0 [+ or -] 0.4mm thick. Specimens of other thickness will not necessarily give comparable results. Plaque dimensions must be sufficient to permit this. Five samples are to be tested in the direction of highest tensile strength and the median value is reported for both elongation and tensile at break.

Certain elastomers may require post curing or conditioning to achieve optimum heat resistance properties.

Table requirements (7)

The material heat resistance table shall be developed using temperatures from SAE J200 table 1, and additional temperatures 135 [degrees] C and 165 [degrees] C) appropriate to industry needs. If a material meets a temperature requirement, it is presumed that it will meet all lower temperatures.

A technician tests commercially available compounds to section five conditions for 1,008 hours, then determines the percent elongation and tensile at break prior to and immediately after the test period per ASTM D 412 for thermoset and thermoplastic materials of 90 Shore A hardness or less.

Report the results at 23 [degrees] C for the number of samples as stated in items 6.1 and 6.3 per ASTM D 412.

A material will qualify for a temperature classification table when test data satisfies the requirements of item 3.1.

Precision and bias (8)

No precision statement exists for 1,008 hour oven aging per ASTM D 573. See ASTM D 573 for the precision of oven aging after 48 and 96 hours and ASTM D 412 for precision of tensile testing.

Conclusion

Comments from automotive engineers demonstrate the practical utility of the specifications. Randy Dickerman of Chrysler Corp. said "SAE J2236-92 will serve as guidelines for elastomers based on long term heat resistance. This will be an excellent tool when used with J200 to determine the best material for an application. The design engineer will be better able to weigh fluid resistance vs. heat resistance vs. cost when choosing a material for an application."

He said that being able to make these comparisons will result in less over-engineered/high cost parts and reduce the amount of low cost/poor performing parts. "J2236 will also assist engineers in evaluating long term heat resistance of current under-the-hood elastomers, since the temperatures they are exposed to will increase as we move to lighter weight, lower exhaust emission automobiles," he added.

Al Edmonson, laboratory specialist in the polymers section of Ford agreed stating "there are continuing efforts to improve longevity and usefulness of rubber and polymer parts in automotive applications. The service environments are getting more and more severe. This new test procedure should allow us to better monitor the performance under long term exposure and predict performance for 100,000-125,000 miles on the vehicle."

GM's Beckett is interested in finding rubber companies that are interested and willing to participate in wound-robin and correlated testing to establish tables for classifying the existing automotive materials. Interested parties should contact John T. Beckett, Materials Engineer, Materials and Fastener Engineering Room 209-37, General Motors Corp., 30001 Van Dyke Ave., Warren, MI 48090-9020. His phone is [phone].