In the Department of Defense (DoD), organic coating removal operations mainly utilized chemical paint removers qualified for use under the military paint remover specification, MIL-R-81294. MIL-R-81294 paint removers provide superior performance; however, these paint removers contain methylene chloride and phenol that are under various Environmental Protection Agency (EPA) and Occupational Safety and Health Administration (OSHA) regulations. Due to the Environmental, Safety, and Occupational Health (ESOH) reasons, nonchemical based paint removal methods have been developed and implemented. Non-chemical based paint removal methods include water-jet stripping, media blasting, and high intensity light source stripping. Non-chemical based paint removal technologies have been found suitable for certain parts, yet a chemical approach is preferred over non-chemical methods for large scale maintenance work, parts of complex geometry and the paint stripping prior to nondestructive inspection (NDI).
The objective of this project was to determine the depaint mechanisms of methylene chloride based paint removers.
Two pillars of the technical approach used to achieve the objective were (1) a study that monitored the physical changes in the paints exposed to the paint removers and identifies their causes and (2) a spectroscopic study that led to a fundamental understanding of the chemical interactions between the paints and the paint removers at the molecular level.
Physical changes in paints (coatings) were studied by (1) measuring solvent diffusion rates using the diffusion cell and gravimetric solvent sorption test methods and (2) monitoring sorption behavior and visual changes of the coating samples exposed to solvents. The diffusion cell study clearly indicates that methylene chloride diffuses faster than ethanol, phenol, benzyl alcohol, and methylene chloride solutions of ethanol, phenol, and benzyl alcohol. The comparison of these diffusion rates to Hansen hydrogen bonding parameters indicate that the molecules with lower Hansen hydrogen bonding parameters and molar volumes have higher diffusion rates. Another important fact found from the diffusion cell study is that the diffusion rates of ethanol, phenol, and benzyl alcohol are substantially increased when they are mixed with methylene chloride. This suggests that methylene chloride increases diffusion rates of these molecules.
The clear coating samples exposed to the methylene chloride solution of phenol (20% by weight) or ethanol (9% by weight) fractured before their sorption equilibriums were reached while the same samples exposed to methylene chloride did not. It was also found that the sample exposed to the methylene chloride solution of phenol is much stiffer than other samples. No crack developed during the solvent sorption tests for the fully formulated samples. As the case for the clear samples, weight increases of the fully formulated samples exposed to the methylene chloride solution of phenol at their sorption equilibriums were substantially higher than the same samples exposed to methylene chloride only.
Changes to the molecular-level properties of the coatings upon exposure to the components of the paint removers were studied using analytical and spectroscopic techniques; differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), gas chromatography with mass spectrometry (GC/MS), solid-state proton (1H) and deuterium (2H) nuclear magnetic resonance spectroscopy (NMR), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and attenuated total reflectance Fourier transform infrared spectroscopy (FTIR-ATR). DSC results of methylene chloride solutions containing water and phenol show coating degradation. FTIR-ATR and XPS results indicate a hydrolysis reaction occurring in localized regions of the coating exposed to the methylene chloride solution of ethanol and water. The DSC and TGA data show exposure to phenol containing solutions to cause the coating degradation. Deuterium NMR of d2-methylene chloride (CD2Cl2) at various temperatures shows that the methylene chloride molecules present in the polyurethane (MIL-DTL-53039) clear film undergo isotropic rotational tumbling and the rate of tumbling is orders of magnitude slower than that in solvents, suggestive of weak interactions with groups on the polymer, perhaps via electric dipoles. Variable temperature quadrupolar-echo spectra and the transverse relaxation time measurements suggest that the d5-phenol is covalently bonded to the polymer through the substitution reaction of phenol with nucleophilic sites of the polymer chain. Reaction mechanisms for these spectroscopic findings are not yet confirmed.
The coating removal study was performed to verify the supposition that hydrogen bonding between donor hydroxides on the substrate and the epoxy groups of the primer coating is the primary adhesion strength of the substrate-epoxy interface and the hydrogen bond strength can be substantially lowered by adsorbed water working as a lubricant. Tests performed with the fully formulated epoxy primer on chromate conversion coated aluminum panels clearly indicates that water is an important component of the methylene chloride based paint stripper since methylene chloride did not remove the coating at all and the methylene chloride solution of phenol barely removed the coating. These results do not prove the supposition, but support that the water induced interfacial delamination may be the reason for faster removal of coatings.
From the same experiment, the researchers also found that one role of water is to prevent the removed coatings from re-adhesion.
The results from this project will help formulate a new class of paint removers. Once implemented, the process owners will realize advantages of the new paint removers such as (1) lower volatile organic compounds (VOC) emission, (2) less hazardous solid waste disposal, (3) compliance with ESOH regulations, (4) reduced process times, and (5) reduced corrosion-related repair time. As a result, the DoD will realize significant cost savings in the depaint and corrosion control operations.