Physicochemical investigation of chemical paint removers: Interactions of methylene chloride with polyurethane coatings

A variety of thermal and spectroscopic techniques have been used to investigate interactions of the widely-used paint-stripping solvent methylene chloride upon model polyurethane coatings. Thermal analysis reveals that methylene chloride penetrates and swells the polymer film and leads to a slight depression in the glass transition temperature (T_g) after drying. The ¹H NMR spectra and T₁ and T_1ρ relaxation times show that methylene chloride is responsible for increased polymer segmental motion in the polymer due to swelling, and indicate intimate contact between the methylene chloride molecules and the polymer, with no liquid-like pools of the solvent observed. The quadrupolar-echo ²H NMR spectra of CD₂Cl₂ in the polyurethane over a temperature range of 24 C to -27 C reveal a lengthening of the rotational correlation times of the methylene chloride by over four orders of magnitude compared to the neat liquid, indicating restricted mobility due to an interaction with the polymer. Although this interaction is likely due to the electric dipoles in the solvent and the polymer backbone, the absence of significant residual nuclear quadrupole couplings due to a high degree of ordering or solvent immobilization shows that the interaction strength is weak compared to thermal energy (kT). Raman spectroscopy indicates that methylene chloride causes swelling by interacting with the carbonyl group responsible for inter-chain bonding, thus permitting dilation. FTIR and Raman spectroscopy demonstrated that methylene chloride leads to no irreversible chemical changes in the coating. A common cellulosic chemical stabilizer used in commercial paint removers has been found by XPS to deposit as a thin conformal but heterogeneous coating on the surface of the polymer, suggesting a possible important function to retard evaporation while allowing some permeation. Purpose Commercial military paint stripper is effective because it contains methylene chloride; in order to replace it, understanding its mechanism of action/interaction with the coating is crucial.