The objective of this project was to conduct a multi-service demonstration/validation (Dem/Val) study of commercial-off-the-shelf (COTS) environmentally-friendly chemical removers for removing military specification (MIL-SPEC) sealants from the external and internal surfaces of military aircraft structures. This effort was accomplished through a methodology that included baselining current practices, pre-screening and down-selecting vendor products, laboratory testing of down-selected vendor products, and then subsequent Dem/Val activities on active and out-of-service U.S. Air Force (USAF) and U.S. Navy (USN) aircraft component parts.

Technology Description

This project focused on defining an array of acceptable (COTS, environmentally-friendly) sealant removers that could be successfully applied depending on the specific situation encountered. This “toolbox” approach (i.e., targeted removers and removal processes for multiple types of sealants and specialty coatings) was designed to provide the end user with the most efficient and effective means of removing aged sealants.

The COTS sealant removers surveyed for this project differed from traditional removers in that they do not rely on a solvent-based methodology to accomplish chemical de-bonding of the sealant or adhesive. Traditional removers use some type of solvent (usually a hazardous air pollutant [HAP] or toxic release inventory [TRI] chemical) to cause the chemical bonds to deteriorate, and thus allow the sealant to de-bond to the substrate. The down-selected COTS chemicals (i.e., PolyGone [PG] 300/310AG, SkyKleen [SK] 2000, and SkyRestore LM 306) introduce an active, environmentally compliant chemical compound that chemically digests the sealant, causing the chemical de-bonding of the sealant with the substrate.

Demonstration Results

The project was performed in two Phases; each Phase consisted of two parts. Phase I focused on qualifying compliant sealant removers that were effective on polysulfide and polythioether sealants on various substrates. Phase II focused on qualifying compliant sealant removers that were effective on polythioether and polyurethane sealants on various substrates. Part 1 of each Phase included the development of a detailed program plan, and the qualification of several COTS chemical products capable of removing polysulfide, polythioether, and silicone sealants from aircraft structures in a production environment. Part 2 focused on demonstrating/validating the qualified and test-certified products on condemned and serviceable aircraft structures at Department of Defense (DoD) maintenance, repair, and overhaul (MRO) facilities and preparing the preliminary cost-benefit analysis and interim final report with recommendations for technology transfer and transitioning across DoD.

Results from this investigation indicate that the subject sealant removers down-selected during the program perform adequately and reinforce the “toolbox” approach to solving the problem of removing sealants in an environmentally-friendly manner. Laboratory testing, performed by University of Dayton Research Institute (UDRI), confirmed the efficacy of the down-selected sealant removers (SK 2000 and PG 300 for polysulfide; SK 2000 and SkyRestore LM 306 for polythioether) in removing sealants from metallic and composite test panels.

Battelle identified several advantages over the current baseline desealing operations during Dem/Val studies at Hill Air Force Base (AFB), Fleet Readiness Center Southeast (FRCSE)-JAX, Florida; and Marine Corps Air Station (MCAS) Cherry Point, North Carolina.  Efficiency gains were realized while removing polysulfide sealants at Hill AFB on components of the F-16 and C-130 weapon system platforms, giving the workers at this location greater flexibility in scheduling and completing tasks on both platforms. Similarly, evidence indicates that efficiency gains can also be realized in stripping the polysulfide sealant from the outer moldline (OML) of the P-3 aircraft, although dwell time required by the sealant removers affects the throughput of the aircraft in the maintenance cycle. Efficiency gains were also noted when stripping polythioether sealants from AV-8B Harrier wings at MCAS Cherry Point.

Cost analyses performed for the P-3, F-16, and C-130 weapon systems platforms confirm economic and environmental benefits associated with usage of the compliant chemical sealant removers for sealant removal from the OML of full-scale aircraft (P-3) and the inner moldline (IML) of specific component parts (F-16 and C-130). The magnitude of economic savings varies from platform-to-platform and application-to-application. One conclusion reached is that the utilization of chemicals does not always prove to be cost-effective.

Implementation Issues

No end-user or original equipment manufacturer (OEM) issues are anticipated for transition of these environmentally-friendly sealant removers to be used on DoD assets; neither are any other regulatory issues anticipated. Due to the environmentally-friendly nature of the sealant removers qualified in this test, no environmental permits are anticipated for transitioning this technology for use on DoD assets. Further, it is anticipated that implementation of these sealant removers will result in the need for fewer environmental permits and fewer worker exposure issues.

In addition, technical transition efforts are promising.  Maintenance personnel at the Tucson Air National Guard have expressed interest in the PG 300 remover for use on an F-16 inspection port.  Additional interest has been expressed by Hill AFB (F-16 components), Tinker AFB and Boeing (B-52 integral fuel tank), U.S. Coast Guard (fuel tank repair), and USN Cherry Point (helicopter components). RPM Technology, LLC (RPM), the manufacturer of the PG 300 remover, provided a mildly reformulated product, PG 310AG, to address paint softening, hydrogen embrittlement, stress corrosion, and sandwich corrosion issues which surrounded the PG 300 material.  The project team, working with Naval Air Systems (NAVAIR), provided additional corrosion testing to facilitate transition of the remover to these locations and weapon systems.