ABSTRACT
In the current work, a series of black polyimide (PI) films with excellent thermal and dimensional stability at elevated temperatures were successfully developed. For this purpose, two aromatic diamines including 4,4'-iminodianline (NDA) and 2-(4-aminophenyl)-5- aminobenzimidazole (APBI) were copolymerized with pyromellitic dianhydride (PMDA) to afford PIs containing imino groups (-NH-) in the molecular structures. The referenced PI film, PI-ref, was simultaneously prepared from PMDA and 4,4'-oxydianiline (ODA). The introduction of imino groups endowed the PI films with excellent blackness and opaqueness with the optical transmittance lower than 2% at the wavelength of 600 nm at a thickness of 25 µm and lightness (L*) below 10 for the CIE (Commission International Eclairage) Lab optical parameters. Meanwhile, the introduction of rigid benzimidazole units apparently improved the thermal and dimensional stability of the PI films. The PI-d film based on PMDA and mixed diamines (NDA:APBI = 70:30, molar ratio) showed a glass transition temperature (Tg) of 445.5 °C and a coefficient of thermal expansion (CTE) of 8.9 × 10-6/K in the temperature range of 50 to 250 °C, respectively. It is obviously superior to those of the PI-a (PMDA-NDA, Tg = 431.6 °C; CTE = 18.8 × 10-6/K) and PI-ref (PMDA-ODA, Tg = 418.8 °C; CTE: 29.5 × 10-6/K) films.
ABSTRACT
Semi-alicyclic colorless and transparent polyimide (CPI) films usually suffer from the high linear coefficients of thermal expansion (CTEs) due to the intrinsic thermo-sensitive alicyclic segments in the polymers. A series of semi-alicyclic CPI films containing rigid-rod amide moieties were successfully prepared in the current work in order to reduce the CTEs of the CPI films while maintaining their original optical transparency and solution-processability. For this purpose, two alicyclic dianhydrides, hydrogenated pyromellitic anhydride (HPMDA, I), and hydrogenated 3,3',4,4'-biphenyltetracarboxylic dianhydride (HBPDA, II) were polymerized with two amide-bridged aromatic diamines, 2-methyl-4,4'-diaminobenzanilide (MeDABA, a) and 2-chloro-4,4'-diaminobenzanilide (ClDABA, b) respectively to afford four CPI resins. The derived CPI resins were all soluble in polar aprotic solvents, including N-methyl-2-pyrrolidone (NMP) and N,N-dimethylacetamide (DMAc). Flexible and tough CPI films were successfully prepared by casing the PI solutions onto glass substrates followed by thermally cured at elevated temperatures from 80 °C to 250 °C. The MeDABA derived PI-Ia (HPMDA-MeDABA) and PI-IIa (HBPDA-MeDABA) exhibited superior optical transparency compared to those derived from ClDABA (PI-Ib and PI-IIb). PI-Ia and PI-IIa showed the optical transmittances of 82.3% and 85.8% at the wavelength of 400 nm with a thickness around 25 µm, respectively. Introduction of rigid-rod amide moiety endowed the HPMDA-PI films good thermal stability at elevated temperatures with the CTE values of 33.4 × 10-6/K for PI-Ia and 27.7 × 10-6/K for PI-Ib in the temperature range of 50-250 °C. Comparatively, the HBPDA-PI films exhibited much higher CTE values. In addition, the HPMDA-PI films exhibited good thermal stability with the 5% weight loss temperatures (T5%) higher than 430 °C and glass transition temperatures (Tg) in the range of 349-351 °C.
ABSTRACT
Enhancement of flame retardancy of a colorless and transparent semi-alicyclic polyimide (PI) film was carried out by the incorporation of phosphazene (PPZ) flame retardant (FR). For this purpose, PI-1 matrix was first synthesized from hydrogenated 3,3',4,4'-biphenyltetracarboxylic dianhydride (HBPDA) and 4,4'-oxydianiline (ODA). The soluble PI-1 resin was dissolved in N,N-dimethylacetamide (DMAc) to afford the PI-1 solution, which was then physically blended with PPZ FR with the loading amounts in the range of 0-25 wt.%. The PPZ FR exhibited good miscibility with the PI-1 matrix when its proportion was lower than 10 wt.% in the composite films. PI-3 composite film with the PPZ loading of 10 wt.% showed an optical transmittance of 75% at the wavelength of 450 nm with a thickness of 50 µm. More importantly, PI-3 exhibited a flame retardancy class of UL 94 VTM-0 and reduced total heat release (THR), heat release rate (HRR), smoke production rate (SPR), and rate of smoke release (RSR) values during combustion compared with the original PI-1 film. In addition, PI-3 film had a limiting oxygen index (LOI) of 30.9%, which is much higher than that of PI-1 matrix (LOI: 20.1%). Finally, incorporation of PPZ FR decreased the thermal stability of the PI films. The 10% weight loss temperature (T10%) and the glass transition temperature (Tg) of the PI-3 film were 411.6 °C and 227.4 °C, respectively, which were lower than those of the PI-1 matrix (T10%: 487.3 °C; Tg: 260.6 °C).
