In considering of the storage of halon-like agents, the long-term effects of the agent on elastomeric sealing gaskets and lubricants in the storage system must be considered. In this connection, a specific evaluation of various fluorocarbons (FCs), hydrofluorocarbons (HFCs), and hydrochlorofluorocarbons (HCFCs), acting on relevant elastomeric gasket materials and lubricants, has been carried out by McKenna et al.3 at NIST. The elastomers investigated included silicones, fluorosilicones, fluorocarbons, neoprene, and nitriles. The lubricants included a fluorinated grease, a perfluoropolyether grease, and a commercial aircraft grease.
Two methods of evaluation were employed. The first method involved measurement of the degree of swelling of the elastomer or lubricant on exposure to agent vapor. If the amount of solvent absorbed was small, the agent was reckoned to have good compatibility; i.e., it did not damage the elastomer or the lubricant. Bad compatibility was implied by excessive swelling, and fair compatibility was intermediate between the two. The measurements were carried out at 35°C. This method is indirect in the sense that it does not measure the effect on the mechanical properties of the elastomer or lubricant. Good compatibility implies equilibrium sorption of a weight fraction less than 0.22, while bad compatibility implies sorption greater than 0.38.
The second method involved measurement of mechanical properties of the elastomers and lubricants after extended exposure (weeks) to agent vapor at elevated temperature and pressure (150°C, 5.86 Mpa). The authors concluded that testing at 150°C was too severe, but the data are indicative if not interpreted absolutely. For elastomers the mechanical properties are compression set resistance, i.e., the tendency to spring back after compression, and elongation reduction in ultimate tensile testing, i.e., the loss of stretchability. These mechanical factors are directly relevant to gasket performance. Bad compatibility is indicated when the ultimate elongation decreases by 65% after a 2-week exposure to agent. Fair compatibility, i.e., marginally acceptable, is indicated by a 65% loss in 4 weeks.
Lubricants responded to exposure to an agent in a different manner. The mobile fraction of the lubricant is extracted by the agent over time, leaving a powdery residue that is unsuitable as a lubricant. Bad compatibility signifies an agent that leaves the lubricant powdery after 4 weeks of exposure. Good compatibility implies that the lubricant does not become powdery after 6 weeks of exposure.
Results are given in Tables C.4 and C.5. The first letter (B, F, or G) is the poorer of the two mechanical ratings, i.e., compression set resistance or reduction of elongation. The second letter is based on swelling measurements. In drawing conclusions from the ratings one must be aware of the authors' admonition that the exposure conditions prior to mechanical testing (150°C) were too severe. Even so, until further test results are available, the information may be used for tentative screening. Examination of Table C.4 suggests that neither of the nitriles is promising as a gasket material with halon-like agents. Fluorocarbon elastomers are similarly unsuitable. Silicone, fluorosilicone, and neoprene elastomers emerge as superior candidates, but there are sufficient negative entries to make further testing necessary before selecting a material. In assessing the compatibility of lubricants, the mechanical property measure is probably most relevant. That is, the tendency to become powdery appears to be the failure mechanism, and swelling may not measure this tendency. Even so, the mechanism may be more complex, and further testing under milder conditions of exposure is indicated. The data in Table C.5 do not offer unambiguous clues for distinguishing among the three lubricants.
The study of McKenna et al. is an excellent beginning but, as the authors point out, further testing is required. Their study contains a wealth of interpretation not included herein. The results do lead to optimism that effective, long-lived gasket and lubricating materials can be found for a given halon-like fire fighting agent. It should be noted that other classes of elastomers could be considered (e.g., polyacrylates and polyphosphazenes). It is recommended that specific tests be carried out before designing a system.