Improved Melt Processabilities of Thermosetting Polyimide Matrix Resins for High Temperature Carbon Fiber Composite Applications

With the goal of improving processability of imide oligomers and achieving of high temperature carbon fiber composite, a series of Thermosetting Matrix Resin solutions (TMR) were prepared by polycondensation of aromatic diamine (3,4′-oxybisbenzenamine, 3,4-ODA) and diester of biphenylene diacid (BPDE) using monoester of 4-phenylethynylphthalic acid (PEPE) as end-capping agent in ethyl alcohol as solvent to afford phenylethynyl-endcapped poly(amic ester) resins with calculated molecular weight (Calc’d M_w) of 1500–10,000. Meanwhile, a series of reactive diluent solutions (RDm) with Calc’d M_w of 600–2100 were also prepared derived from aromatic diamine (4,4′-oxybisbenzenamine, 4,4-ODA), diester of asymmetrical biphenylene diacid (α-BPDE) and monoester of 4-phenylethynylphthalic acid (PEPE) in ethyl alcohol. Then, the TMR solution was mixed with the RDm solution at different weight ratios to afford a series of A-staged thermosetting blend resin (TMR/RDm) solutions for carbon fiber composites. Experimental results demonstrated that the thermosetting blend resins exhibited improved melt processability and excellent thermal stability. After being thermally treated at 200 °C/1 h, the B-staged TMR/RDm showed very low melt viscosities and wider processing window. The minimum melt viscosities of ≤50 Pa·s was measured at ≤368 °C and the temperature scale at melt viscosities of ≤100 Pa·s were detected at 310–390 °C, respectively. The thermally cured neat resins at 380 °C/2 h showed a great combination of mechanical and thermal properties, including tensile strength of 84.0 MPa, elongation at breakage of 4.1%, and glass transition temperature (T_g) of 423 °C, successively. The carbon fiber reinforced polyimide composite processed by autoclave technique exhibited excellent mechanical properties both at room temperature and 370 °C. This study paved the way for the development of high-temperature resistant carbon fiber resin composites for use in complicated aeronautical structures.

Effects of aging process and testing temperature on the open-hole compressive properties of a carbon fiber composite

The properties of T800 carbon fiber–epoxy composite specimens with a hole were studied in terms of mass change, scanning electron microscopy, glass transition temperature ( T g), heat-resistant temperature, Fourier-transform infrared (FTIR) spectroscopy, open-hole compressive strength at different temperatures, and stereomicroscopic observations after being subjected to hygrothermal aging and thermal-oxidative aging processes. FTIR spectra indicated that after hygrothermal aging at 70°C and 85% relative humidity (RH), chemical aging did not occur, whereas after thermal-oxidative aging at 190°C, the specimens exhibited chemical aging. The unaged specimens had a T g of 229°C and an extreme heat-resistant temperature T gmod of 184°C; after hygrothermal aging, the specimens had a T g and T gmod of 207°C and 143°C, respectively; and after thermal-oxidative aging, the specimens had a T g and T gmod of 252°C and 215°C, respectively. The effects of temperature on open-hole compressive strength were evaluated at room temperature of 23°C, 50°C, 100°C, 150°C, and 200°C. The compressive strengths of the specimens decreased after aging and with the increasing test temperature. At the highest test temperature, the unaged specimens, hygrothermal-aged, and thermal-oxidative-aged specimens retained over 73.7%, 65.5%, and 67.9%, respectively, of their compressive strength. Thus, the T800 carbon fiber–epoxy composite evaluated in this study exhibited good resistance to the effects of aging and high temperature. These results should be beneficial to the understanding of the long-term performance of composites.

Effects of thermal–oxidative aging on the mechanical properties of open-hole T800 carbon fiber/high-temperature epoxy composites

T800 carbon fiber/high-temperature epoxy resin composites with holes were subjected to thermal–oxidative aging, and the effects of different aging temperatures and times on the composite properties were investigated. The mass loss, surface topography, open-hole tensile performance, fracture morphologies, dynamic mechanical properties, and infrared spectra were analyzed. The results showed that chemical aging did not occur with thermal–oxidative aging at 70°C and 130°C. However, chemical aging occurred at 190°C. At 70°C, 130°C, and 190°C, all samples showed a slight increase followed by a slight decrease and stabilization in the open-hole tensile strength. The open-hole tensile strength was maximized after 240 h aging at different temperatures; the open-hole tensile strength after 1920 h aging exceeded that of the unaged samples. All composites experienced through-hole failure. With aging, the glass transition temperature ( T g) was gradually increased and then decreased. After 960 h aging at different temperatures, T g was maximized.

A facile synthesis of soluble polyimides with high glass transition temperature and excellent mechanical properties due to intermolecular hydrogen bonds

An unsymmetrical heterocyclic diamine monomer containing proton donor (–NH–), 2-(4-aminophenyl)-5-aminobenzimidazole (PABZ), which can form hydrogen bond interactions with carbonyl functional group, was copolymerized with 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and 2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl (TFMB) through two-step synthetic methods to obtain a series of homo- and co-polyimide (PI). The corresponding homo- and co-PI both exhibited good solubility in aprotic polar solvents, such as N, N-dimethylacetamide, N-methyl-2-pyrrolidone, N, N-dimethylformamide, pyridine, and dimethyl sulfoxide. In addition, the PI-containing PABZ units showed excellent tensile strength ranging from 83 to 164 MPa, 130–260% higher than 6FDA/TFMB homo-PIa films. These PIs, especially PABZ/6FDA, showed very high glass transition temperatures, 430°C.