It is common practice throughout the aerospace industry to apply protective coatings to finished parts, components, and even entire aircraft. These coatings provide environmental protection, reduce visual, infrared, or radar signature, and are used to control the visual appearance of an aircraft or subsystem. Throughout the useful life of a system many of these coatings must be periodically removed to conduct inspections, repairs, or the coating itself needs to be replaced. Historically, coatings have been removed through the use of solvents that aggressively interact with the coating, but not with the substrates, allowing the coating to be easily removed without damaging the underlying surface. Unfortunately, the solvents used in these operations often include components that are considered hazardous air pollutants (HAPs) as defined under the 1970 Clean Air Act (and as amended in 1990) and subsequently regulated through such standards as the Aerospace National Emissions Standards for Hazardous Air Pollutants (NESHAPS). Despite the problems associated with meeting strict air emission standards, chemical stripping remains one of the most favored options for removing coatings due in large part to its familiarity, effectiveness, and simplicity.
The objective of this project was to obtain a sound understanding of how methylene chloride/phenol-based (MC/P) paint strippers function by understanding the specific roles of the primary paint stripping components; methylene chloride, phenol, ethanol, and water.
This study was accomplished through a series of tasks including sample selection, conceptual and computational molecular modeling, infrared spectroscopy, measurements of volume swell, the extent and rate of debonding and the analysis of the solvent absorbed by a model coating system.
The results show that while methylene chloride is a major component of many paint stripping formulations, its interactions with the coatings themselves is relatively weak. However, its small size, its weak interactions with the bulk solvent and its ability to form weak hydrogen bonds with the coatings combine to make this an efficient penetrant that rapidly diffuses into the coatings, causing them to swell and soften, and as a carrier for other solvent components, most notably phenol. Phenol is a very unique molecule that is relatively small and capable of forming exceptionally strong hydrogen bonds with the coatings making it a powerful penetrant. However, unlike methylene chloride, phenol is a solid under normal conditions and requires a carrier solvent to be effective. The unique properties of phenol arise from a very special relationship between the aromatic ring and its hydroxyl group. Briefly, this group would ordinarily by itself form strong hydrogen bonds with the coatings, but it would also form strong hydrogen bonds with other phenol molecules in the solvent resulting in a relative poor penetrant. However, in phenol the hydroxyl oxygen shares electrons with the aromatic ring, delocalizing the oxygen’s electronegative charge, and making phenol a strong hydrogen bond donor, but a comparatively weak hydrogen bond acceptor. The result is that it penetrates the coatings with a solubility that is approximately 7 times higher than methylene chloride. However, this by itself is insufficient to effectively remove the coatings. For this phenol has another unique characteristic. The same molecular structure than makes phenol an efficient penetrant also makes it a weak organic acid. Specifically, the water present in the solvent reacts with the phenol to produce phenoxy (PhO-) and hydronium (H3O+) ions. These can then react with hydrogen bond donor and acceptor sites in the coatings, physically fracturing the coatings and cleaving the intermolecular bonds holding the coatings to the surface. Ethanol itself does not seem to participate in the paint stripping process, but instead serves to increase the solubility of water in the solvent phase.
This study has shown that the two most significant functions of a paint stripping solvent are to penetrate the coating to deliver a weak organic acid to the bonding interface. In the near term it suggests that the performance of some alternative paint strippers may be improved by including a weak organic acid. In the long term, this provides a framework to developing new paint removal systems. Specifically, methylene chloride could be eliminated if an alternative means of accessing the bonding interface could be devised. This in turn would allow the use of a weak organic acid other than phenol, resulting in an environmentally acceptable paint stripping method.