Abstract:

Objective As a representative of advanced polymer materials, polyimide fibers show irreplaceability in extreme service environments such as military and aerospace with their unique molecular structure and performance synergy. However, the outstanding properties of polyimide will inevitably be affected by prolonged exposure to extreme conditions such as high-energy irradiation and atomic oxygen. This research is an attempt to explore in-depth the aging mechanism of polyimide fibers and fabrics under multiple environmental factors, so as to further improve the stability and reliability of the materials for the intended applications.

Method The aging test chamber was used to evaluate the radiation resistance, corrosion resistance and other properties of materials by simulating and accelerating environmental factors, from which the performance degradation of products in extreme environments was predicted. According to the GJB 150.10A-2009 standard, polyimide fabrics were placed in an aging test chamber with different experimental conditions to separately simulate mold, xenon lamp and salt spray aging environment before they were cleaned to remove surface contaminants by washing. In addition to the environmental aging tests, the fabrics underwent another abrasion aging test using a Martindale abrasion tester, referring to the GB/T 21196.2—2007 standard. The properties and structure of fabrics were analyzed by mechanical property testing, thermos gravimetric analysis (TGA), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM).

Results The initial polyimide fabric showed a warp tensile strength of 705.66 N/cm and an elongation at break of 9%. After undergoing mold, xenon lamp, abrasion, and salt-spray aging respectively, the tensile strength and elongation at break of the fabric decreased to various degrees. The retention rates of fabrics warp tensile strength were 91.44%, 89.35%, 87.48%, and 85.29%, respectively, still remaining at a relatively high level. The elongation at break of the different aged samples all decreased to around 7%, indicating that the flexibility of the fabric has declined. The thermal decomposition temperatures of the polyimide fabrics decreased after aging. The correlation between mechanical properties and heat resistance of fabric demonstrated that the experimental aging schemes indeed caused damage to the actual performance of the polyimide fabric to some extent. Subsequently, the chemical structures of the fabric before and after aging were investigated. The positions of different characteristic absorption peak in the FT-IR spectra of these samples did not change. In the characteristic XPS spectra of C 1s for the initial fabric, the proportions of C—O and C—N were 4.19% and 5.05%, respectively, and after different aging treatments, the proportions of both decreased. In contrast, an increase in the peak area of the C=N and the emergence of C^*—C=O peak were observed, confirming that all three aging conditions affected the chemical structure of the polyimide. Mould, xenon lamp, and salt-spray aging treatments have no significant effect on the overall macroscopic morphology of the fabrics. The original surface morphology of the fiber was smooth, but the surface roughness increased after the above three aging treatments, displaying obvious etching grooves and partial peeling. After abrasion and stretching, the regularity of fabrics in warp direction deteriorated significantly. For the sample after abrasion aging, fibers showed obvious bending, the smooth surface structure was completely destroyed, and a structure similar to microfibrils emerged. For the fabric after warp tensile test, the longitudinal fibers exhibited obvious layered fracture morphology and “V-shaped” fracture notches. Finally, the possible aging mechanisms of polyimide fabrics under different environmental conditions were explored.

Conclusion The research results show that mold, xenon lamp, salt spray and abrasion aging negatively affected the mechanical properties and heat resistance of the high-strength polyimide fabrics to various degrees, and salt spray aging appears to be the most significantly damaging factor which reduced the tensile strength and elongation at break of the fabrics by 14.71% and 28.44%, respectively, and caused decrease in thermal weight loss temperature. After aging under different environmental conditions, the surface smoothness of fibers decreased, and microscopic defects such as grooves and peeling appeared. The chemical structure of fiber before and after experiment was analyzed, and the aging mechanism may be related to the destruction of chemical bonds such as C—N and C—O, as well as long molecular chains or conjugated structures in the polyimide macromolecular chain.

Key words: polyimide, mould, xenon lamp, salt-spray, aging, service performance, high performance fiber