SAE AIR 1412D-2018 Designing for Long Life with Elastomers.pdf

1、 _ SAE Technical Standards Board Rules provide that: “This report is published by SAE to advance the state of technical and engineering sciences. The use of this report is entirely voluntary, and its applicability and suitability for any particular use, including any patent infringement arising ther

2、efrom, is the sole responsibility of the user.” SAE reviews each technical report at least every five years at which time it may be revised, reaffirmed, stabilized, or cancelled. SAE invites your written comments and suggestions. Copyright 2018 SAE International All rights reserved. No part of this

3、publication may be reproduced, stored in a retrieval system or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of SAE. TO PLACE A DOCUMENT ORDER: Tel: 877-606-7323 (inside USA and Canada) Tel: +1 724-776-49

4、70 (outside USA) Fax: 724-776-0790 Email: CustomerServicesae.org SAE WEB ADDRESS: http:/www.sae.org SAE values your input. To provide feedback on this Technical Report, please visit http:/standards.sae.org/AIR1412D AEROSPACE INFORMATION REPORT AIR1412 REV. D Issued 1976-11 Revised 2018-01 Supersedin

5、g AIR1412C Designing for Long Life with Elastomers RATIONALE This revision incorporates several updates and latest information on evolving technology in the field of seal life prediction. FOREWORD The properties of elastomers change with time and temperature; in some cases, these changes are substan

6、tial. As a result of long-term storage stability problems with some early elastomeric materials, the aerospace industry to a large extent has become accustomed to the application of age controls on O-rings, hoses, and certain other rubber products. This has proven to be very costly, time-consuming,

7、and unwieldy. Additionally, elastomeric materials qualified for service based on the results of short-term simulation tests conducted only at service temperature extremes have not always performed adequately in the field. Accelerated tests, when required, should be performed significantly above the

8、continuous service temperature to provide a meaningful estimate of life at reduced temperatures. However, the aging temperature must not be so high as to change the degradation mechanism and thereby give misleading results (see references 8 to 18 below). Replacement and reassembly of parts have been

9、 found to lower reliability. Maintenance on very complex aerospace products is difficult to carry out because of compactness of these products and disassembly required to gain access to seals or other rubber goods. The reliability and cost requirements of aerospace components are very high, hence sh

10、ort life, unreliable elastomeric parts cannot be tolerated. Long life elastomers are available for use in aerospace designs. It, therefore, follows that designing for long life is a much more viable approach. The designer must convey a specific requirement to all concerned that he is building critic

11、al aerospace equipment intended for long life and high reliability. This can be as straightforward as a detailed drawing note citing the life requirement in years and the expected environments. This overall requirement has to be backed up by specific elastomer material performance and mechanical pro

12、perty specification requirements, as well as due consideration to application requirements. Moreover, the designer cannot assume that published specifications or proprietary callouts will automatically provide elastomeric performance to meet his specific needs. SAE INTERNATIONAL AIR1412D Page 2 of 8

13、 1. SCOPE This document lists those guidelines recognized as being essential for consideration by the designer who is preparing to select an elastomer as part of an aerospace design. 1.1 Purpose To provide guidelines to the aerospace designer in the testing and selection of elastomers so that long l

14、ife service will be realized in critical components. 2. APPLICABLE DOCUMENTS The following publications form a part of this document to the extent specified herein. The latest issue of SAE publications shall apply. The applicable issue of other publications shall be the issue in effect on the date o

15、f the purchase order. In the event of conflict between the text of this document and references cited herein, the text of this document takes precedence. Nothing in this document, however, supersedes applicable laws and regulations unless a specific exemption has been obtained. 2.1 Related Publicati

16、ons The following publications are provided for information purposes only and are not a required part of this SAE Aerospace Technical Report. 1. F. R. Eirich, ed., “Science and Technology of Rubber“, Academic Press, New York, 1978. 2. A. V. Tobolsky and H. F. Mark, ed., “Polymer Science and Material

17、s“, John Wiley and Sons, New York, 1980. 3. H. Liebowitz, ed., “Fracture - An Advanced Treatise“, Vol. VII, Academic Press, New York, 1972. 4. H. F. Mark and N. G. Garylor, N. J. Bikales, ed., “Encyclopedia of Polymer Science and Technology“, Vol. 8, p. 419, Interscience Publishers, New York, 1968.

18、5. U. Meier, J. Kuster, and J. F. Mandel, “Rubber Chemistry and Technology“, Vol. 54, 254 (1984). 6. L. P. Smith, “The Language of Rubber”, Butterworth-Heinemann Ltd., Oxford, UK, 1993. 7. A.N. Gent, ed., “Engineering with Rubber How to Design Rubber Components”, 2nd Edition, HanserGardner Publicati

19、ons Inc., Cincinnati, Ohio, 2001. 8. K. T. Gillen, J. Wise and R. L. Ciough, “Novel Techniques Applied to Polymer Lifetime Predictions”, http:/www.osti.gov/scitech/servlets/purl/10113498/. 9. K. T. Gillen, “Modulus Profiling of Polymers”, Polymer Degradation and Stability, Vol. 17, pp. 31-47, 1987.

20、10. J. Wise, “An Ultrasensitive Technique for Testing the Arrhenius Extrapolation Assumption for Thermally Aged Elastomers”, Polymer Degradation and Stability, Vol. 49, pp. 403-418, 1995. 11. K. T. Gillen, “Extrapolation of Accelerated Aging Data Arrhenius or Erroneous?”, Trends in Polymer Science,

21、Vol. 5 (8) pp. 250-257, August 1997. 12. M. Celina, “Oxidation Profiles of Thermally Aged Nitrile Rubber”, Polymer Degradation and Stability, Vol. 60, pp. 493-504, 1998. 13. K. T. Gillen, “The Wear-Out Approach for Predicting the Remaining Lifetime of Materials”, Polymer Degradation and Stability, V

22、ol. 71, pp.15-30, 2001. 14. K. T. Gillen, M. Celina and R. Bernstein, “Validation of Improved Methods for Predicting Long-term Elastomeric Seal Lifetimes from Compression Stress-Relaxation and Oxygen Consumption Techniques”, Polymer Degradation and Stability, Vol. 1, pp. 25-35, 2003; http:/ SAE INTE

23、RNATIONAL AIR1412D Page 3 of 8 15. K. T. Gillen, “Methods for Predicting More Confident Lifetimes of Seals in Air Environments”, Rubber Chemistry http:/rubberchemtechnol.org/doi/abs/10.5254/1.3547590. 16. K. T. Gillen, “New Method for Predicting Lifetime of Seals from Compression-Stress Relaxation E

24、xperiments”, Die Angewandte Makromol. Chemie, 261/262, 4619, pp. 83-92, 1998. 17. K. T. Gillen, “Predicting and Confirming the Lifetime of O-rings”, Polymer Degradation and Stability, 87, pp.257-270, 2005. 18. M. Celina, “Accelerated Aging and Lifetime Prediction: Review of Non-Arrhenius Behavior Du

25、e to Two Competing Processes”, Polymer Degradation and Stability, Vol. 90, pp. 395-404, 2005. 19. MIL-HDBK-695, Rubber Products: Recommended Shelf Life, Dept. of Defense. 2.2 SAE Publications Available from SAE International, 400 Commonwealth Drive, Warrendale, PA 15096-0001, Tel: 877-606-7323 (insi

26、de USA and Canada) or +1 724-776-4970 (outside USA), www.sae.org. AMS2810 Identification and Packaging Elastomeric Products AMS2817 Packaging and Identification of Molded Elastomeric Seals and Sealing Components ARP5316 Storage of Elastomer Seals and Seal Assemblies Which Include an Elastomer Elemen

27、t Prior to Hardware Assembly 2.3 ASTM Publications Available from ASTM International, 100 Barr Harbor Drive, P.O. Box C700, West Conshohocken, PA 19428-2959, Tel: 610-832-9585, www.astm.org. ASTM D2990 Tensile, Compressive, and Flexural Creep and Creep Rupture of Plastics ASTM D3045 Heat Aging of Pl

28、astics Without Load ASTM D6147 Vulcanized Rubber and Thermoplastic Elastomer Determination of Force Decay (Stress Relaxation) in Compression ASTM E1641 Standard Test Method for Decomposition Kinetics by Thermogravimetry Using the Ozawa/Flynn/Wall Method 2.4 ISO Publications Copies of these documents

29、 are available online at http:/webstore.ansi.org/. ISO 3384 Rubber, vulcanized Determination of stress relaxation in compression at ambient and at elevated temperatures ISO 6056 Rubber, vulcanized or thermoplastic Determination of compression stress relaxation (rings) ISO 27996 Aerospace Fluid Syste

30、ms Elastomer Seals Storage and Shelf Life 2.5 UL Publications Available from UL, 333 Pfingsten Road, Northbrook, IL 60062-2096, Tel: 847-272-8800, . UL 746B Polymeric Materials: Long Term Property Evaluations SAE INTERNATIONAL AIR1412D Page 4 of 8 3. GUIDELINES The following minimal guidelines are p

31、resented to aid the designer in the selection of long life elastomeric parts: a. The elastomer compound should be resistant to oxidative attack. b. The elastomer compound should be resistant to ozone cracking. c. The elastomer compound should be resistant to the service fluid media involved. d. The

32、elastomer compound should suitably resist permanent set, creep, and/or stress relaxation. e. The elastomer compound should possess a sufficient safety factor in mechanical properties to allow for known degradation. f. The elastomer compound should possess sufficient resistance to cut, tear, and abra

33、sion to give the required life. g. The elastomer compound should resist special environments such as temperature, water, humidity, radiation, fungus, hard vacuum, corrosion, cleaning and processing media, and the like when required. h. Functional hardware should be artificially aged as part of norma

34、l qualification testing. i. The quality and product control systems should be explicit and be enforced to make certain the specified elastomer is received and is properly packaged and stored. It must be recognized that it is not necessary that an elastomer meet all of the above guidelines in all cas

35、es. If, for example, ozone resistance is not a major item of consideration, some polymers such as nitrile-butadiene rubber (NBR) with poor ozone resistance could perform satisfactorily. 4. DISCUSSION Discussions covering each of the guidelines are as follows: 4.1 Oxidative Attack Most elastomers are

36、 subject to oxidative attack, but antioxidants have been developed to mitigate against this mode of degradation. Certain elastomers, such as fluorocarbons, silicones or ethylene propylenes, have a greater resistance to oxidation and may require only a lower level of antioxidants, or possibly, even n

37、one at all. The control of oxidation is a complex empirical problem. Hence, specifications do not specify the chemical type(s) of antioxidants or the amount to be used. Rather, they define the effect using accelerated heat oven tests to set limits to the extent of degradation. The designer must insu

38、re that a suitable test of this type is included in the specification invoked. The time and temperature of an accelerated test such as defined in ASTM D3045 can provide an indication as to how long this material may be used safely in a design at a rated service temperature. However, the accelerated

39、heat aging temperature must not be so high as to change the degradation mechanism and thereby give misleading results (see references 8 to 8 above). Antioxidant technology has been developing for almost 75 years and extensive tests have shown that most quality antioxidant protected elastomer compoun

40、ds will resist gross degradation due to oxidation for periods greater than ten years at ambient temperature. 4.2 Ozone Cracking Chemically unsaturated elastomers are subject to ozone cracking. This is familiar to most persons who have seen ozone cracking on the sidewalls of their automotive tires. I

41、n former times, this was incorrectly attributed to sunlight. Ozone is an extremely active form of oxygen. Its attack is not to be confused with oxidative attack as discussed above. Chemically saturated elastomers can be used with little or no concern for ozone cracking. These include among others: p

42、olyacrylates, ethylene propylene, fluorocarbons, silicones, butyl rubber, and polysulfides. SAE INTERNATIONAL AIR1412D Page 5 of 8 4.2.1 The more common chemically unsaturated elastomers which may be attacked by ozone include: neoprene, nitrile-butadiene rubber (NBR), polybutadiene, natural, isopren

43、e and styrene-butadiene rubber (SBR). These should be used only where protected from atmospheric or other source of ozone, or provided by addition of antiozonants or by physical separation from exposure to ozone. As with antioxidants, the requirement is implemented by tests for resistance to ozone c

44、racking, rather than by tests for antiozonant content. 4.3 Fluid Media Elastomers can be dissolved, swollen, softened, or degraded by fluids that have chemical structures or solubilities similar to their own. There is no such thing as an elastomer that resists all fluid media. The rubber must be com

45、patible with any operational fluid media (or nonoperational fluid such as lubricants or cleaning fluids) with which it comes in contact at any time. True compatibility will depend on the concentration of the fluid, temperature, duration of exposure, and the state of cure of the elastomer. 4.3.1 Elas

46、tomer specifications often have fluid immersion tests in various standard fluids under heat-accelerated conditions. Volume change is one measurement. These tests may be in fluids never seen in application since the intent is to verify appropriate state of cure of the rubber specimen. Negative volume

47、 change (shrinkage) is almost always considered unacceptable, but positive changes up to 25% or even higher with some designs may be acceptable depending upon the application. Unless carefully structured it has been found that heat-accelerated volume change tests may not always predict what will hap

48、pen at room temperature, i.e., positive volume changes were found in the heat-accelerated tests, but shrinkage was experienced in long-term usage at room temperature. Pneumatic sealing conditions may vary from those typically experienced with fluids contained in hydraulic systems and engine componen

49、ts. In some applications, more squeeze may be required, perhaps double the squeeze required in hydraulic systems. The use of nonstandard O-rings and/or modification of groove dimensions may be advisable. In others, less squeeze may be required, for example, in dynamic pneumatic cylinders where high sealing forces tend to produce binding. 4.3.2 Changes i