Standard

SAE AIR4275B

Published

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Abstract

This information report covers two distinct projects to formulate Jet Refrence Fluids (JRF) for testing of material compatibility. The first effort began in 1978 and focused on producing a formulation (JRF-2) that simulated JP-4 and included composition with metallic ions that reproduced chalking of fuel tank sealants. This effort resulted in the preparation of AMS2629 that defined the formulation of JRF-2 (Type 1) and the same formulation with metallic ions (Type 2). The second effort began in 2002 and focused on preparing a JRF that simulated Jet A, JP-5 and JP-8. This effort went through multiple iterations, but eventually resulted in a JRF-3 formulation composed of Jet A plus military additives spiked to 25% aromatic content and high levels of sulfur experienced in the global fuel supply. Since the metallic ions added to JRF-2 demonstrated their ability to simulate a chalking reaction, chalking was not tested with the ions added to JRF-3. AMS2629 was changed multiple times to reflect the onging changes of this project and included both Type 1 and Type 2 formulations. Background Standard reference fluids, or test fluids, have long been used to evaluate the effects of hydrocarbon fuels on various materials, such as integral fuel tank sealants. Standard fluids are required because hydrocarbon fuels, such as JP-4, vary widely in composition depending on crude source, refining techniques, and other factors. To ensure reliable and reproducible results when determining the fuel resistance of materials, reference fluids of known composition, using worst-case fuel compositions, are used. The original Jet Reference Fluid (JRF) called out in military sealant specifications was developed in the mid-1950s specifically as a JP-4-type test fluid formulation to be used for the accelerated laboratory testing of integral fuel tank sealants. In August, 1978, chalking of the polysulfide sealant in integral fuel tanks of some new aircraft at Edwards Air Force Base in California was discovered after only 1 year of service. Chalking is a phenomenon that occurs in polymeric materials used in the fuel tanks of military and civil aircraft. It manifests as a white, chalky deposit on the surface of the material (sealant, seal, slosh coating, etc.) and is a consequence of chemical attack by certain constituents of aviation fuel (metallic ions, sulfur-containing compounds, naphthenic compounds, etc.). Reversion of the organic matrix renders it soluble in the aviation fuel, depositing the inorganic fraction (fillers, extenders, rheology agents) on the surface as it dissolves. The degree of chalking can be classified as “Slight,” “Moderate,” or “Heavy” depending upon the amount of inorganic material deposited. Although chalking of polysulfide sealants had been observed occasionally in the past, the rate of chalking was unprecedented. The results of an investigation showed that the rapid chalking of the polysulfide sealant was caused by a chemical reaction involving metal ions (copper, cadmium, lead, and iron) and mercaptan sulfur in the fuel. It was also noted that qualification testing of the sealant used had not predicted the chalking that occurred in service. Further investigation disclosed that the sealant had passed the chalking test in the military specification because the JRF used in the specifications chalking test did not contain trace metal ions as did the fuel removed from the tanks of the affected aircraft. The special Air Force investigating team included in its final report a recommendation that the JRF specification be reviewed and revised. The above chalking incident coupled with concerns resulting from deficiencies observed with the original JRF, and from changing sources of JP-4, indicated that an update of the JRF formulation in the sealant specifications was needed. A proposal was made to the SAE Aerospace Sealing Committee (G-9) which then formed a subcommittee for the development of the original Jet Reference Fluid (JRF) for the evaluation of integral fuel tank sealants. In the 1990s the Air Force converted from JP-4, a kerosene/gasoline blend, to JP-8 which is a kerosene based fuel. The Navy had been using JP-5, a higher flash point kerosene-based fuel, for aircraft on carriers since the 1950s. The change for the Air Force stimulated the need to develop JRF-3 as a simulant to JP-5 and JP-8. A subcommittee was formed in AMS G-9 in the Fall of 2002 with active members from government and industry. The first JRF-3 formulation (AMS2629 Revision C) was a synthetic mixture incorporating discrete fuel and additive components but resulted in a surrogate that was costly to produce. Recognizing that Jet A-1 (and subsequently Jet A) was the basis for JP-8, the final approach (AMS2629 Revision E) was to spike Jet A to the maximum allowed aromatic and sulfur concentrations. Other changes included adjustments to accommodate high global fuel supply sulfur and mercaptan levels and reductions in icing inhibitor concentrations to follow reductions in fuel specification limits. AMS2629 was revised four times during this effort and ultimately delivered a reference fuel composition of reasonable cost, ease of production, maximized consistency with in-serice fuels, and an option open to many fuel system materials. Program Organization JRF-2 The organization of the first committee and its functions were discussed at a meeting in Long Beach, CA, on 23 May 1979. The prime purpose of this meeting was to define objectives, establish the scope of effort, and to develop a program plan so that it could get started on the technical effort. Simply stated, the objective of this committee was to develop a new JRF for sealant evaluation which would reasonably reflect the worst to be expected from fuels derived from existing and expected sources, provide reliable sealant differentiation, and have none of the known deficiencies of the original JRF. JRF-3 The SAE G-9 JRF subcommittee was activated at the 46th SAE G-9 meeting in Oklahoma City, OK. The goal of the subcommittee was to develop a new JRF composition to more closely simulate JP-5 and JP-8 fuels used in service and still maintain the objectives of the JRF-2 effort. The committee worked with AFRL/RQ fuels branch to determine the best approach. Approach JRF-2 The original JRF called out in MIL-S-8802 and MIL-S-83430 sealant specifications was selected as a suitable starting point for discussion and planning. Knowledge of the rationale used for the development of the original JRF and the selection of its components coupled with the problems encountered with its use was considered essential to the development of a new replacement fluid. Establishing the scope of effort proved to be difficult. However, there was general agreement that a broad, two-level program to define the requirements for, develop the composition of, and fully evaluate a new JRF was needed. It consisted of a short-term effort addressing current, urgent problems and a long-term effort to address the full spectrum of fuels, seal and sealant materials, and potential environments. As a minimum, the new JRF should address the following: a. Both military and commercial requirements, with an emphasis on the military b. Composition of JRF to simulate existing fuels (i.e., JP-4, JP-5, JP-8, and Jet A) and future alternate fuels as developed c. The effects of JRF on the following classes of materials: polysulfides, fluorosilicones, fluorocarbons, and nitriles d. Problems experienced with the original JRF e. Analytical techniques and handling/storage requirements to ensure adequate quality control of the JRF f. The potential requirement for more than one JRF (i.e., high and low aromatics content) for different applications g. An appropriate method for governing the new JRF with provisions for future review and revisions h. Appropriate and adequate testing Although a detailed program plan was not accomplished at that time, the following considerations for the Short-Term Program and the Long-Term Program were initially identified: a. Short-Term Program 1. Consider polysulfide sealants only 2. Concentrate on reliable sealant differentiation 3. Establish a common source 4. Establish analytical techniques for quality control 5. Establish problem contaminant content (i.e., metal ions and mercaptan sulfur) 6. Revise original chalking test 7. Formulate to better represent the worst to be expected from fuels derived from existing and expected sources b. Long-Term Program 1. Evaluate mercaptan content (a) Kind (b) Level 2. Formulate to represent future fuel compositions 3. Evaluate potential problem fuel components (a) Sulfur compounds (b) Additives (c) Aromatics composition and content (d) Nitrogen compounds 4. Consider other sealant materials (i.e., fluorosilicones, fluorocarbons, and nitriles) It was later decided that the mercaptan study, as well as aromatics, nitrogen compounds, and other sulfur compounds and additives, needed to be addressed in the short-term program in order to properly formulate a new JRF. JRF-3 The second Jet Reference Fluid (JRF-2), in use since 1989, was formulated to closely represent JP-4, a kerosene/gasoline blend fuel widely used by the Air Force. Since that time, JP-8, a kerosene-based fuel, formulated with Jet A-1 (ultimately Jet A), replaced JP-4; JP-5, used by the Navy since the 1950s, was also a kerosene-based fuel. To update the reference fluid to more closely represent fuel compositions in use by both the Air Force and Navy, an SAE G-9 subcommittee was formed to investigate and develop a new formulation. The goal for this new reference fluid, similar to JRF-2 goals, was to exhibit “worst case” conditions within the limits of the current fuel specifications. The resultant test fluid, designated JRF-3, was created using commercially available paraffin and aromatic blends and maximized sulfur, mercaptan, and additive packages to be consistent with fuel specifications. Screening tests for JRF-3 were performed on eleven sealants and testing was ultimately expanded to include compatibility with nine groups of non-metallic materials currently used in military aircraft fuel systems. This was done not only to validate JRF-3 as a fuel surrogate but to expand the potential for use of JRF-3, via AMS2629, in additional material specifications. Based on compatibility testing, this JRF-3 composition was incorporated into AMS2629 Revision C. Before there was any significant transfer to JRF-3, it became clear that the synthetic formula was too costly for labs to produce and/or procure. The G-9 JRF subcommittee changed the approach and investigated basing JRF on Jet A that is spiked to the highest allowed specification limits for aromatics and sulfur content. With help of AFRL/RQ, a large batch of JRF-3 was produced and tested. This formulation change was incorporated into AMS2629 Revision E in 2017. In recent years, a number of lighter-weight fuel tank sealants have been developed for qualification to AMS3281 and used in military aircraft applications. However, there is some concern that these newer sealants will not hold up after long-term fuel aging. The purpose of this effort was to compare the tensile strength and elongation, Shore A hardness, and peel properties of these sealants to the older, existing sealants that have been in use in the military for years, after continuous aging in JRF-3-1 at 200 °F.

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