| Aramid fibers/epoxy resin composite is a preferred candidate material for outer shell of rocket motor. Unfortunately, the adhesion between fibers and the resin matrix is poor because the surface of aramid fibers is chemically inert and smooth. This adhesion problem has to be solved so that the composites can be widely applied in aircraft, aerospace and missile fields. In this thesis, Co60γ-ray irradiation and irradiation grafting techniques were used to modify aramid (Armos) fibers in order to improve the adhesion between aramid fibers and epoxy resin. Irradiation parameters were studied and various grafting monomers were tested to achieve desired modifications of Armos fibers without lowering the strength of fibers. The overall properties including the interface adhesion of aramid fibers composites were significantly improved after modification.The interlaminar shear strength (ILSS) test, interfacial shear strength (IFSS) test, flexural strength test and nanohardness test were adopted to study the effects of irradiation and irradiation grafting on the properties of Aramid fibers/epoxy resin composites. The optimal irradiation doses in various mediums were determined as 600kGy in N2, 600kGy in air, 500kGy in 5% epoxy resin/acetone, 500kGy in 1.5% phenol-formaldehyde resin/acetone, 200kGy in epoxy chloropropane. The quasistatic tensile test of single Armos fiber showed thatγ-ray radiation and irradiation grafting didn't significantly reduce tensile strength of Armos fibers.X-ray diffraction (XRD) and carbon nuclear magnetic resonance spectroscopy (13C-NMR) were adopted to study the structure change of Armos fibers after irradiation. The crystallinity of Armos fibers after irradiation varied only slightly and the interplanar distance was smallest at 600kGy in N2. 13C-NMR results showed the contents of C=O and C=N groups in Armos fibers were lowered after irradiation. It was found that the cross-linking reaction occurred in the bulk of Armos fibers during irradiation. The in-depth cross-linking reaction was evidenced by higher viscosity of fibers/concentrated H2SO4 solution, smaller indentation diameter and higher nanohardness together with modified fractography of fibers. Therefore, it was the structure change that accounts for the improvement of fiber properties. The loop force test with two single fibers showed that the compression resistance of fibers was increased. The thermogravimetic analyses indicated that the thermal stability of Armos fibers was improved by irradiation and irradiation grafting modification.The effects of irradiation and irradiation grating on the surface properties of fibers have been investigated by X-ray photoelectron spectroscopy (XPS), capillary method, scanning electron microscope (SEM), atomic force microscope (AFM), Fourier transform infrared (FT-IR) technique and viscosity test. FT-IR showed epoxy chloropropane, epoxy resin and phenol-formaldehyde resin all grafted to fibers via active functional groups on the surface of fibers helped by radiation. XPS indicated the atomic ratio of O/C and the contents of polar groups on the surface of fibers were enhanced after irradiation. After modification the surface of Armos fibers was rougher as examined by SEM and AFM. The surface energy increased and the wettability of fibers improved through modification. The higher roughness of fibers'surface led to mechanically stronger interfacial interlocking between the irradiated the fibers and the matrix. Large amount of active functional groups cause chemical binding between fibers and the matrix. Therefore, the improvement of the Armos fibers'wettability in the epoxy resin led to stronger interfacial binding between the fibers and the matrix.The aging test of Armos fibers afterγ-ray irradiation activation was carried out. The tensile strength of fibers and ILSS test of composite showed that the modified fibers were stored in the desiccator for 6 months, the strength of fibers and the ILSS of composites didn't significantly change. |
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