Chemical paint strippers based on methylene chloride and phenol historically were widely used to remove polymeric coatings. These strippers were highly effective, inexpensive, and exhibited minimal impact on the substrate. However, environmental and health concerns suggested the need for replacements. Replacement attempts have led to more environmentally friendly alternatives at the cost of performance, price, and substrate damage. The objective of this project was to elucidate the mechanism of action of methylene chloride and phenol based paint strippers on polymeric coatings in order to lead to efficient, cost-effective, and safe alternatives. This was accomplished by monitoring the changes in physical and molecular-level properties of polymeric coatings after exposure to the components of organic solvent based paint strippers.
This project used wet organic chemistry, thermal analysis, visible microscopy and spectroscopy, and vibrational spectroscopy to understand how methylene chloride based paint strippers remove polymeric coatings. Two component polyurethane MIL-PRF-85285 and single component polyurethane MIL-PRF-53039 topcoats were made into three formulations: a clear coat, partial formulation and a full formulation. These coatings were exposed to the different control paint stripper solutions and then examined using Confocal Microscopy, Scanning Electron Microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS), Fourier Transformed Infrared Spectroscopy-Attenuated Total Reflectance (FTIR-ATR), Raman, Near Edge X-ray Absorption Fine Structure (NEXAFS), Thermogravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC), and contact angle.
The analysis of the microscopy showed that the solutions containing phenol caused significant degradation and that sporadic film deposition was occurring on the surface of coatings exposed to solutions containing methocel. To help analyze methocel’s role in the degradation of the coatings, two new solutions were made—methylene chloride/ethanol/phenol and methylene chloride/ethanol/phenol/methocel. Spectroscopic techniques were used to examine the degradation and confirm the presence of methocel. FTIR-ATR and Raman allowed for identification of methocel on the surface of the coatings exposed to solutions containing methocel. XPS confirmed methocel on the surface of the coatings and showed that degradation is occurring in the paint stripper solutions with phenol. NEXAFS analysis further confirmed that methylene chloride solvates and reorders the coating and that phenol degrades the coating systems. Contact angle of water and surface free energy of the coatings was determined before and after exposure. The contact angles varied for the different formulations of MIL-PRF-85285 and MIL-PRF-53039, indicating the effect that different polymer backbones, pigments, and flatteners have on the coating system. While the data were sporadic, the surface free energy and water contact angle of the coatings exposed to the solutions with phenol and the solutions without phenol grouped together, another indicator that phenol is the main agent of degradation. The surface free energy for the coating containing methocel was similar in value to the surface free energy of methocel, further proving that the film on the surface of the coating is methocel. Roughness analysis was conducted using the confocal microscope. The paint stripper solutions of methylene chloride and methylene chloride/ethanol caused small sporadic changes with no clear trend for both MIL-PRF-53039 and MIL-PRF-85285. The coatings exposed to methocel and/or phenol all decreased in surface roughness. This indicates that methocel deposition smoothes out the coating by filling in the valleys on the surface and that the degradation of the coating also results in a smoothing effect on the surface. The TGA curves and glass transition (Tg) data of the exposed coating further confirm that phenol degrades the coating. Comparing the methylene chloride/ethanol/phenol and methylene chloride/ethanol/phenol/methocel exposed coatings shows that the addition of methocel slightly increased the degradation. Comparison of the methylene chloride/ethanol/phenol/methocel and the methylene chloride/ethanol/water/ phenol/methocel exposed coatings indicates that water acts synergistically with phenol to degrade the coatings. The wetting behavior of the paint stripper solutions was examined on unexposed coatings and coatings exposed to solutions containing methocel. The results of this project showed that the methocel deposited on the surface of the coating slowed the wetting or ingress of the solution into the coating. This implies that the deposited methocel slows the egress of the paint stripper solution from the coating, thereby increasing the degradation.
The results of this project show that methylene chloride solvates the coating and phenol is the main agent of degradation. The project showed water’s importance in degradation and how methocel not only emulsifies the solution but increased the degradation by depositing on the surface and trapping the solution inside. This improved understanding of how methlyene chloride and phenol based paint removers function will aid in the development of new, environmentally benign materials that maintain performance.