Assessment of Safer Polysulfide and Polythioether Sealant Removers for Aerospace/Defense Industry Applications
This assessment identifies and evaluates potentially safer alternatives for sealant removal applications. The sealant removal performance of the alternatives was evaluated using a designed experiment.
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Dr. Gregory Morose, Toxics Use Reduction Institute, University of Massachusetts Lowell
Dr. Jason Marshall, Toxics Use Reduction Institute, University of Massachusetts Lowell
Dayna Lamb, Raytheon Company
Chiagoziem Uzor, College of Health Sciences, University of Massachusetts Lowell
Polysulfide and polythioether based sealants are widely used in the aerospace/defense industry to seal and protect components of aircraft, missiles, and other products. Maintenance, repair, and overhaul (MRO) operations require the removal of sealant from coated surfaces, mostly with the aid of sealant removal chemicals. In the aerospace/defense industry, commonly used sealant removers include hazardous chemicals such as toluene, methylene chloride, and N-propyl bromide.
This assessment identifies and evaluates potentially safer alternatives for sealant removal applications. The sealant removal performance of the alternatives was evaluated using a designed experiment. The environmental, health, and safety impacts of the alternative removal products were evaluated using the Pharos Chemical and Material Library chemical hazard assessment tool and the Toxics Use Reduction Institute’s Pollution Prevention Options Assessment System (P2OASys).
For the technical performance of the sealant removers, the PolyGone 310-AG product had the best results for all the sealant removers evaluated. The Diestone DLS, SkyKleen 1000, and Soy Safe Graffiti Remover products exhibited sealant removal performance comparable to that of toluene.
For environmental, health, and safety considerations, the SkyKleen 1000, PolyGone 310-AG, and Diestone DLS products had safer chemical profiles than toluene, nPB, and methylene chloride. The Soy Safe Graffiti Remover could not be fully evaluated because of limited information on the MSDS and the non-disclosure of the actual ingredients of the product.
Polysulfide and polythioether sealants are widely used in the aerospace industry for sealing structures and components of aircrafts such as the windows, access doors, windshield, fuselage, fuel tanks, joints in the wings, and small rivets and bolts. Repair operations require efficient removal of the sealants from different surfaces. Chemical solvents are often utilized for their ability to be isolated to the area of the sealant, and increase the efficiency of the removal process. Hazardous chemicals such as toluene, methylene chloride, and N-propyl bromide are used for this purpose. However, while these chemicals meet the required performance objectives, they are hazardous to the environment and to the health of repair workers. This research was focused on identifying and evaluating potentially safer sealant removers.
The main goals of this evaluation were to accomplish the following:
- Identify potential safer alternatives for the removal of polysulfide and polythioether sealants in aircraft repair operations
- Screen out potential alternatives that do not meet the minimum performance and safety requirements
- Evaluate the chemical safety of the alternatives
- Evaluate the technical performance of the alternatives
Sealant is primarily removed in the following two methods: 1) smaller parts are immersed in liquid sealant removal products for a pre-determined time, after which the sealant dissolves or is physically removed using a cutting or scraping instrument, 2) larger parts require an application of the sealant removal product directly onto the sealant area for a pre-determined time, after which the sealant either dissolves or is physically removed. This method typically requires the sealant removal product to be in a gel or paste state.
Some of the commonly used solvents for sealant removal are toluene, methylene chloride, and n-propyl bromide. These solvents are preferred because of their proven effective performance in weakening the strength of the sealants and therefore increasing the efficiency of the removal process over manual removal. There are significant human health hazards associated with exposure to such chemicals. The harmful health effects of these solvents are compounded by the potentially high occupational exposure potential for workers removing sealant. The workers are in close proximity to the solvents and therefore there is a high risk of inhalation exposure especially in poorly-ventilated areas, as well as dermal exposure potential.
In addition to the health effects, there is an increase in operation costs for companies who seek to protect workers from these hazardous solvents. Expenditures are often required for employee safety training, personal protective equipment, additional ventilation and other engineering controls, and the safe use and handling of hazardous materials and its subsequent disposal.
Identification of Alternatives
The first step of this evaluation was to identify potential candidates to replace the use of the following hazardous sealant removal materials: toluene, methylene chloride, and N-propyl bromide. Potential safer alternatives were identified by utilizing the professional judgment of Toxics Use Reduction Institute (TURI) Cleaning Laboratory personnel, reviewing previous sealant removal test reports, using the TURI CleanerSolutions database, reviewing trade journals, and accessing manufacturer’s websites. The TURI CleanerSolutions database contains extensive information about the performance and safety of various industrial cleaners and solvents. (TURI, 2014a) As a result of this investigation, eighteen different sealant removal products were identified.
Aluminum Plates for Screening and Performance Testing
The performance screens and testing for this evaluation required the use of sealant strips applied to aluminum plates. There were two different aluminum plates used for this testing: 1) large aluminum plates and 2) small aluminum plates.
Large Aluminum Plates
TURI provided eighteen large aluminum plates made from 6061 and 7075 aluminum alloys with the following dimensions: 2” wide x 4.5” long x 0.25” thick. Each aluminum plate had four strips of sealant applied at the Raytheon facility in Tucson, Arizona. The sealant strips were applied to the top surface of the large aluminum plates and were approximately 0.5” wide by 2” long by 0.005” thick. The following six types of sealants were applied to the aluminum plates: 1) PPG Aerospace PR-1440, 2) 3M AC-735, 3) PPG Aerospace P/S 870, 4) PPG Aerospace PR- 1775, 5) Flame Master CS-5500, and 6) PPG Aerospace PR-2870. The large aluminum plates were used for the foam swab screen, the immersion screen, and the sealant removal performance testing.
Small Aluminum Plates
Raytheon provided eighteen small aluminum plates made from 2024 aluminum alloy with the following dimensions: 1” wide x 4” long x 0.075” thick. Each aluminum plate had three strips of sealant applied at the Raytheon facility in Tucson, Arizona. The sealant strips were applied to the top surface of the small aluminum plates and were approximately 1” wide by 1” long by 0.005” thick. The following six types of sealants were applied to the aluminum plates: 1) PPG Aerospace PR-1440, 2) 3M AC-735, 3) PPG Aerospace P/S 870, 4) PPG Aerospace PR- 1775, 5) Flame Master CS-5500, and 6) PPG Aerospace PR-2870. The small aluminum plates were used for the sealant removal performance testing with the sonic bath.
Screening of Alternatives
Due to the limited time and resources available to conduct the sealant removal performance testing, there were three screens applied to the 18 identified sealant removal alternatives to narrow down the number of alternatives to undergo performance testing. There were three different methods used for the screening of the sealant removal alternatives: 1) swab screen, 2) immersion screen, and 3) TURA screen. The first two screens (swab and immersion) are considered performance screens, and the third screen (TURA) is considered an environmental, health, and safety screen.
The first screening method was to apply three drops of sealant remover to a sealant strip for a duration of two minutes, after which the strips were rubbed for one minute using foam swabs to determine if any sealant material was removed. Sealant removers that generated removal of some sealant material during the swab test were considered to pass this screen. Therefore, if there was any sealant residue that could be visually identified on the surface of the foam swab after the rubbing process was completed, then the sealant removal passed this screen. If there was no sealant residue that could be visually identified on the surface of the foam swab after the rubbing was completed, then the sealant removal failed this screen. Figure 1 shows the sealant strips and foam swabs used for this screening effort. These screens were conducted at the TURI Cleaning Laboratory using the large aluminum plates.