Boeing Research and Technology has assembled a cross-functional team with University of California-Berkeley, Royal Adhesives and Sealants, Air Force Research Laboratory and Naval Air Systems Command, for this project to incorporate novel metal organic framework (MOF) corrosion inhibitors into polysulfide sealant formulations. Hexavalent chromium (Cr6+) inhibited sealants have been the standard in protecting aircraft from corrosion in areas such as structural and non-structural bonding applications or as gap fillers. These account for an estimated 9,200 pounds over a 12-month period in the Department of Defense (DoD) process category. The objectives of this study are to develop and characterize MOF corrosion inhibitors as Cr6+ replacement, and to incorporate the MOF inhibitors in polysulfide polymer formulations. The hypotheses are that the polysulfide sealants will provide a compatible polymer matrix for the MOF inhibitors and that the polysulfide sealants are more cost-effective than polythioether-based polymers. In order to ensure ease of development and implementation, a lifecycle framework and sustainment analysis of the MOF inhibited sealants will be projected and compared to the incumbent Cr6+ sealants at the Fleet Readiness Center, Air Logistics Centers and base operations.
Boeing has developed a MOF inhibitor technology wherein the corrosion inhibiting metal ions or metal clusters from the rare earth metal group react with one or more corrosion inhibiting organic ligands (such as exocyclic sulfur groups) to form a three-dimensional network. Boeing has demonstrated incorporation of metal-ligand complex into epoxy primers successfully and has shown, using electrochemical and salt spray methods, that the corrosion inhibitive properties are comparable with hexavalent chromium containing systems. A stepwise development process will be applied to lower the program risk by incorporating the MOF inhibitors into polysulfide polymer matrices starting with a sprayable sealant of similar viscosity to the epoxy primer and studying the stability and efficacy of the inhibitors in that system. This allows the project team to leverage the successful work of incorporating the MOF inhibitors into primer formulations and serve as a platform for future expansion into more thixotropic sealant systems. Analytical techniques and electrochemical testing will be employed to screen and characterize MOF inhibitors. Material performance testing, analytical techniques and electrochemical testing will be employed to characterize MOF inhibited sealant formulations. Additionally, sustainment analysis will be scoped to help transition and establish this novel technology.
DoD and Boeing will benefit from the MOF inhibited sealants not only from elimination of Cr6+ but also from cost-effectiveness of polysulfide sealant as compared to polythioether sealant. This will benefit scientific community as one of the studies aimed at producing inhibitors that combine low toxicity with high inhibition efficiency in order to comply with environmental protection legislation. MOF inhibited sealants will reduce expenses associated with Environmental Health & Safety due to Cr6+ exposure of personnel and to hazardous waste disposal. In addition, DoD and Boeing will benefit from MOF inhibited sealants with improved performance properties preventing cost of rework and with improved storage life preventing waste generation. Execution of the lifecycle analysis during the development phase will facilitate ease of implementation in manufacturing and in maintenance of defense assets.