The Depolymerization Of Polyimide And The Stability Of 4,4'-diaminodiphenyl Ether In Sub-critical Water

Depolymerization of polyimide (PI) and stability of 4,4'-diaminodiphenyl ether (4,4'-ODA) in sub-critical water were studied in a batch autoclave reactor under different conditions, respectively. The experimental products were analyzed qualitatively and quantitatively by Gas Chromatography and Mass Spectrometry (GC-MS), High Performance Liquid Chromatography (HPLC) and Fourier-transform Infrared Spectroscopy (FT-IR). The visual observation of phase behavior changes about depolymerization of PI and stability of 4,4'-ODA in sub-critical water was also carried out in fused silica capillary reactor (FSCR) under a microscope. The study can provide foundation data for the further study of conditions optimazation and mechanism of PI depolymerization in sub-critical water.PI was efficiently depolymerized to 4,4'-diaminodiphenyl ether (4,4'-ODA), 4,4'-oxydibenzoic acid, aniline and hydroquinone under the conditions of temperature (320 ~ 360℃), pressure (11.5 ~ 20.0 MPa), reaction time (30 ~ 90 min), ratio of water/PI (12.0 ~ 24.0). It was found that PI could be degraded to monomer completely, the PI depolymerization rate and products yields increased with the reaction temperature and time, and had little relationship with ratio of water/PI. Over 99% PI was converted, producing yields of 81% for 4,4'-oxydibenzoic acid, 34.93% for aniline, 24.13% for hydroquinone and less than 10% for 4,4'-ODA, when the reaction was conducted with a reactant ratio of water to PI at 20:1 for 30 minutes at 350℃and 17.5 MPa.Further stability tests of 4,4'-ODA were carried out in batch autoclave reactor at temperature 270 to 330℃under the reaction time range of 15 to 120 min. It revealed that 4,4'-ODA was unstable and partially converted to aniline with the reaction temperature and the time increased, reducing its yield. Additional kinetic analysis suggests that 4,4'-ODA in sub-critical water follows a first-order reaction kinetics with an activation energy of 73.59 kJ·mol-1. Effect of catalytic on the stability of 4,4'-ODA in sub-critical water was also studied under the conditions of temperature (270 ~ 330℃), pressure (5.5 ~ 13.2 MPa), reaction time (15 ~ 120 min), ratio of water/4,4'-ODA (24.0). Compared to the results that absence of the catalytic, the stability of 4,4'-ODA was weaken with the addition of 4,4'-oxydibenzoic acid. The recovery of 4,4'-ODA was decreased by 16.18% and yield of aniline increased by 14.39% in the presence of catalytic of 0.01 g at 330℃with 30 min. It was identified speculate that the unstable of 4,4'-ODA and the catalytic reaction of 4,4'-oxydibenzoic acid leading to its low yield in sub-critical water.Upon examination in hot water inside FSCR under a microscope, phase changes were observed when PI and 4,4'-ODA was heated, respectively. It was found that 4,4'-ODA started to melt at a low temperature, then dissolved completely above 199℃and eventually became an aqueous fluid coexisting with vapor phases. The video images showed phase changes of PI in water during heating and reacting. The system consisted of PI solid, water and steam composition at room temperature, then PI shrank and became to spherules during the heating process, which was not hydrolyzed completely at 330℃for 30 min. The reaction system was composed of PI spherules, aqueous fluid and vapor phases.There is no phase change during the cooling process.