Preparation of Polyimide Powders via Hydrothermal Polymerization and Post-Heat Treatment for Application to Compression-Molding Materials

Aromatic polyimides (PIs) are classified as high-performance polymers owing to their excellent thermal, mechanical, and chemical resistance, and they are used widely in aerospace, membranes, microelectronics, and optoelectronics industries. To synthesize aromatic PI powder in an environmentally friendly manner, a hydrothermal polymerization (HTP) method of heating dianhydride and diamine in distilled water in an autoclave has recently been suggested. However, the application of the hydrothermally synthesized PI powder has been very rare. In the present study, we first report the synthesis of hydrothermal PI powders and their application as compression-molding materials. A mixture of unreacted monomer salt (MS), oligoimide, and PI (MS/PI) powders was obtained in distilled water in an autoclave and thermally imidized to obtain a post-heat-treated polyimide (T-PI) powder with improved sintering behavior. The inherent viscosity, degree of crystallinity, and specific surface area of the T-PI powder were investigated for their suitability for compression molding. The mechanical properties of compression-molded specimens were observed to be affected by the properties of the T-PI powder. When the autoclave temperature was 140 or 160 °C, the T-PI powder showed effective sintering behavior. A tensile strength of 57.2–57.9 MPa, an elongation at break of 6.8–7.4%, and a density of 1.35–1.37 g/mL were achieved with the T-PI powder. Compression-molded specimens prepared with the T-PI powders showed excellent thermal properties and sufficient mechanical properties for use as thermally stable parts and shapes.