Improving UV Resistance And Surface Modification For Aramid Fibers By Synthesizing TiO₂ Using Supercritical Carbon Dioxide

Aramid fiber has properties of low density,high strength,high modulus and high heat resistance.Aramid/resin composites are widely used in transportation,bullet-proof vest,cable and national defense.In recent years,with the destruction of the ozone layer and the increase of the UV radiation to the ground,the long-term UV radiation will degrade the fiber,affect the life of the fiber and restrict its application,so it is necessary to improve the anti-photodegradation ability of aramid fiber.Because oxygen atoms are easier to penetrate into the fiber/microfiber interface and more difficult to penetrate into the crystalline region,photodegradation generally occurs at the fiber/microfiber interface.At the same time,the high crystallinity,smooth surface,low roughness and surface inertia of aramid fiber prevent the interface of aramid fiber and resin.Therefore,it is very important to improve the interfacial adhesion between aramid fiber and resin by surface modification.In this study,the aramid fiber was modified by tetrabutyl titanate?TBT?in the presence of supercritical carbon dioxide?sc CO₂?,then TiO₂was prepared by hydrolysis of the fibers containing precursor in sc CO_2.The effects of sc CO₂ treatment on the aramid fiber structure,morphology,surface composition,thermal stability,photostability,mechanical properties and Interface performance were investigated using Fourier transform infrared spectroscopy,X-ray diffraction,scanning electron microscopy,X-ray photoelectron spectroscopy,thermogravimetric analysis,ultraviolet–visible spectroscopy,single-fiber test,contact angle test and microsphere debonding test.The main results are as follows:?1?The influence of pressure and temperature on the surface morphology and the size of nano-TiO₂was investigated.The results of XRD and SEM show that TiO₂ prepared by sc CO₂ is amorphous,and TiO₂ is synthesized not only on the surface,but also on the interface between fiber/microfiber.The effects of supercritical pressure and temperature on the content and morphology of TiO₂ were studied.The density of sc CO₂ increases with the increase of pressure,and the content of TiO₂ increases.When the pressure is too high,TiO₂agglomerates under the action of intermolecular force and forms large particles,which adhere to the surface of the fiber unevenly and are easy to fall off under the action of external force,and the content of TiO₂ decreases.At the same time,the density of sc CO₂ decreases with the increase of reaction temperature,and the agglomeration between particles is reduced.With the increase of temperature,the solubility of TBT in CO₂ increases and the content of TiO₂ increases.When the temperature is too high,the density of CO₂ decreased leading to the TBT content reduced,so the content of TiO₂ is decreased.The results show that when the supercritical pressure is 15MPa and the temperature is 100°C,the content of TiO₂ is the most,the average diameter of particles is small,the O/C ratio increases from 0.23 to 0.35,and the content of Ti is the highest,up to 2.53%.Therefore,the optimum supercritical conditions for TiO₂ preparation are 15MPa and 100°C.?2?The effects of different sc CO₂ pressure and temperature conditions on the mechanical properties and light aging resistance of aramid fibers were studied.In the meantime,the modulus and strength of modified aramid fiber are improved.The UV resistance property of the modified aramid fiber was studied by simulated ultraviolet aging experiment in different time.The tensile strength and modulus retention of the untreated aramid fiber were 81.48%and 51.41%respectively after 168 hours of UV irradiation.The strength and modulus of the monofilament decreased seriously,and the surface of the fiber showed obvious etch pits,grooves and stripes,which resulted in serious damage.After supercritical carbon dioxide treatment,the strength and modulus of the sample monofilaments decreased very little,and the retention of the fiber at 15MPa,100°C was higher than that of other samples.The strength and modulus of the sample monofilaments were 86.34%and 65.03%,respectively,compared with the untreated aramid fiber,which was improved by 5-14%.The results show that sc CO₂ assisted preparation of TiO₂ on the surface of aramid fiber and fiber/microfiber interface can effectively improve the anti-aging properties of aramid fiber.?3?The effect of sc CO₂assisting TBT and TiO₂ on the surface properties of aramid fiber was studied.Compared with the untreated aramid fiber,the number of oxygen-containing polar groups of modified aramid fiber increased.When the supercritical pressure and temperature are 15MPa and 100°C,the contact angle of TBT and TiO₂ modified aramid fiber are the lowest,the values are 83.18°and 77.6°respectively,which shows that the aramid fiber and resin have the best wettability under these conditions.At the same time,the IFSS values of TBT and TiO₂ modified fibers were the highest at 15MPa and 100°C,and the IFSS was 57%and 80.42%higher than that of untreated aramid fibers.The results show that sc CO₂assisted TBT and TiO₂ modification is an effective way to improve the surface properties of aramid fibers and has a broad application prospect in the field of composites.