This invention relates to a solution of a substantially fully-imidized polyimidesiloxane in a substituted pyrrolidone solvent and to a method of coating substrates. In particular, it relates to solutions of polyimidesiloxanes in solvents containing N-cyclohexyl pyrrolidone (CHP), which do not develop a white crust when the solution is applied.
A coating of a polymer can be made by dissolving the polymer in a solvent, spreading the solution over a substrate, and evaporating the solvent. When solutions of certain polymers in some solvents are spread and the solvent is evaporated, an objectionable white crust is formed. The whitening of the coating is a problem because it affects the flow characteristics of the solution in such a way that it sometimes does not uniformly coat the substrate.
For example, solutions of polyamic acids or polyimides in N-methylpyrrolidone (NMP) can develop this problem. The problem has been overcome for particular polyamic acids or polyimides by mixing the NMP with various co-solvents such as cyclohexanone (EP Application 364791 A2), tetramethylbenzene (EP Application 363737 A2), and butyl cellosolve acetate (Japanese Application 83-76689). While those solutions to the problem may be adequate for the particular polyimide being coated, they are not adequate for polyimidesiloxanes, because polyimidesiloxanes are less soluble than polyamic acids and may not dissolve in those mixtures of solvents. Also, whitening could still occur and there may be other reasons for not using those particular solvents. Until now a general solvent system for polyimidesiloxanes that does not produce whitening when a coating is formed has not been found.