Study On The Modification Of Aramid Surface And Its Composite Interface By Air Dbd Plasma Treatment

Aramid fiber reinforced high-performance thermoplastic composites have excellent heat-resistance and mechanical properties. When compared to traditional thermoset resin-matrix composites, they have the outstanding advantages of high toughness, damage tolerance, impact resistance and excellent processability, which can be used in extreme conditions with high loads and high temperatures, for example, aeronautical and astronautical field, national defense and military industry. However, surface of aramid fiber is smooth and chemically inert due to the high orientation and crystallization of their molecular chains. Therefore, surface modification of aramid fiber is necessary, in order to enhance the interfacial adhesion and improve the whole performance of their composites.In this thesis, aramid fiber (Twaron and Armos) was treated by air dielectric barrier discharge (DBD) plasma at atmospheric pressure, and the relation between interfacial adhesion in aramid reinforced thermoplastic poly(phthalazinone ether sulfone ketone)(aramid/PPESK) composites and this surface treatment had been established and discussed. X-ray photoelectron spectroscopy (XPS) was used to analysis fiber surface elements and functional groups, attenuated total reflectance infrared spectroscopy (ATR-IR) was selected to confirm XPS results, scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to observe the changes of fiber surface morphology and roughness, dynamic contact angle analysis (DCAA) was used to measure contact angles and surface free energy, and single fiber tensile strength (SFTS) was used to detect changes of fiber tensile property before and after the plasma treatment. The interfacial adhesion properties of aramid/PPESK composites were determined by interlaminar shear strength (ILSS) measurement, failure mode and water absorption testing.Firstly, this thesis reported the influence of cleaning treatment before plasma modification on interfacial properties of Twaron fiber reinforced PPESK composites and effectiveness of air DBD plasma treatment on Twaron fiber surface. Results show that ILSS of composites reinforced by Twaron fiber with oily finish on the surface is lower (42.9MPa) than that after the removal of surface finish (46.0MPa) because of the potential weak boundary layer, and meanwhile the plasma modification on fiber surface will be weakened by the cleaning effect. Therefore, aramid fiber needs to be rinsed by acetone before plasma treatment in order to insure the modification effectiveness on fiber surface and composite interface properties.Secondly, influence of atmospheric air DBD plasma discharge parameters, including sample treatment time and discharge power density, on surface composition, physical topography, surface wettability, tensile property of Twaron fiber and ILSS, failure mode, water absorption of Twaron/PPESK composites was studied. Results show that the moderate (for example12s-55.2W/cm3) plasma treatment is responsible for the increase of O/C from0.177to0.233and the introduction of oxygen-containing groups, such as C-O, C=O and O=C-O, and for the increase of surface roughness and specific surface area. Both the chemical and physical changes result in the noticeable enhancement of fiber wettability with surface free energy increasing from50.6to71.0mJ/m2, and the improvement of wetting behavior of PPESK resin on plasma treated Twaron fiber; the ILSS of plasma treated Twaron fiber reinforced PPESK composites increases by34.1%, the failure mode changes, and meanwhile the water absorption of composites declines sharply. Note that the decrease of SFTS is only2%and the bulk property is not affected. Results also show that extending treatment time and increasing power density can both enhance plasma modification effectiveness; however, excessive treatment will decrease the incorporated polar groups and damage the fiber surface, impairing the properties of fiber surface and Twaron-PPESK interface.Then, this thesis reported the influence of different storing environment on aging effect of Twaron fiber after the air DBD plasma modification. Results show that ILSS of composites reinforced by Twaron fiber stored in air for48h declines from58.0MPa to47.3MPa and water absorption increases evidently while the ILSS of composites reinforced by Twaron fiber stored in ambiance including O3active particle only decreases to51.4MPa and water absorption has a less increase. The main reason is the changes of oxygen-containing polar groups introduced to Twaron fiber surface by the plasma treatment and the fiber wettability in different storing ambiance. Meanwhile the results indicate that aging effect after the plasma treatment might be weakened by the oxidising storing environment.Finally, the thesis further discussed the influence of atmospheric air DBD plasma treatment on wettability of aramid with better properties, Armos fiber, and interfacial adhesion of Armos/PPESK composites. Then, the changes of surface chemical composition, morphology and roughness, tensile property of Armos fiber were analyzed. Results show that after the moderate (for example18s-27.6W/cm3) plasma treatment, surface free energy of Armos fiber increases form50.6to71.0mJ/m2, ILSS of Armos/PPESK composites increases from60.9to71.4MPa, failure mode changes from interface debonding to matrix fracture, indicating the enhancement of composite interfacial adhesion, which results from the increase of O and N, i.e. the generation of C-O, C=O,O=C-O and some nitrogen-containing polar groups, and the increase of surface roughness. More important, the tensile strength of plasma treated Armos fiber is not affected, indicating that the atmospheric air DBD plasma is an effective technique for aramid surface modification.