Preparation and properties of light-colored and transparent semi-alicyclic polyimide films with enhanced flame retardancy from alicyclic dianhydrides and aromatic diamine containing phenolphthalein unit

Two semi-alicyclic polyimide (PI) films with light colors, good thermal stability, and enhanced flame retardancy were successfully developed from an aromatic diamine containing phenolphthalein unit, 3,3-bis[4-(4-aminophenoxy)phenyl]phthalide (BAPPT) and alicyclic dianhydrides, including 1,2,4,5-cyclohexanetetracarboxylic acid dianhydride (HPMDA) and 3,3',4,4'-bicyclohexanetetracarboxylic acid dianhydride (HBPDA). For this target, two organo-soluble PI resins, including PI-1 (HPMDA-BAPPT) and PI-2 (HBPDA-BAPPT) were first synthesized by a high-temperature polymerization route. The derived preimidized PI resins were soluble in N-methyl-2-pyrrolidinone (NMP), N,N-dimethylacetamide (DMAc), and other polar aprotic solvents. Corresponding PI films with pale-yellow appearances at the thickness of 25 mu m were prepared by casting the PI solutions on glass substrates, followed by thermally baking at elevated temperatures up to 280 degrees C in nitrogen. The obtained PI films exhibited the cutoff wavelengths (lambda(cut)) lower than 300 nm, the optical transmittances as high as 84.5% at the wavelength of 450 nm, the yellow indices (b*) as low as 3.10, and the haze values as low as 3.71%. Both of the PI films exhibited good thermal stability with the 5% weight loss temperatures (T-5%) higher than 480 degrees C in nitrogen, char yields higher than 40% at 700 degrees C, and glass transition temperatures (T-g) as high as 318.1 degrees C. In addition, the semi-alicyclic PI films showed acceptable high-temperature dimensional stability with the linear coefficients of thermal expansion (CTE) values of 52.3 x 10(-6)/K for PI-1 and 60.4 x 10(-6)/K for PI-2 in the temperature range of 50-200 degrees C, respectively. At last, the PI-1 film showed good flame retardancy with the limited oxygen index (LOI) of 32.8%, the flame retardancy class of UL94 VTM-0, and good smoke suppression properties.