Preparation Of Al³⁺/TiO₂ Nanosol And Its Application In Decelerating UV-Photodegradation Of Aramid Fibers

Aramid fiber is a kind of photosensitive material and it is prone to degrade under ultraviolet radiation. Wavelength of 280～320 nm, corresponding to UV radiation, is most sensitive to aramid fibers. Therefore, aramid fibers can be protected from photodegradation and prolonged their service life by addition of proper anti-UV agers capable of inhibiting light within the wavelength. Studies have discovered that TiO₂ of nano size can provide good protective means by reflecting, scattering or absorbing harmful UV rays. Recent years, studies are focusing on adding TiO₂ to spinning solution to produce anti-UV fibers or dispersing in certain medium with dispersants as coating to protect fibers. However, it is difficult to add inhibitor in the spinning process of aramid fibers. Therefore, it has to use finish method to form a coating in protecting aramid fibers against UV irradiation. It is known that nano TiO₂ is hard to disperse in the medium and easily congregate, even with a proper dispersant. To form a proper coating on the surface of aramid fibers, sol-gel, a wet chemical method, is used in the present study to make nano particles disperse evenly in the coating to sheathe the surface of aramid fibers as an anti-ager. This method has solved certain problem, but brings also some other problems. High costs are expected to obtain the sol with metal organic alkoxide. In addition, although anatase structure TiO₂ can guard against ultraviolet ray, it can accelerate aramid fibers aging because of the strong photochemical catalysis function.The present study concentrates the preparation of Al³⁺/TiO₂ nanosol, the characterizations of Al³⁺/TiO₂ sol and coating prepared by this method. The change of mechanical properties and the surface structure of Al³⁺/TiO₂ coated aramid fibers are investigated under accelerated UV aging treatment.Some conclusions are summarized as following:1. Stable Al³⁺/TiO₂ sol has been prepared by using inorganic titanium compound as precursor and water as solvent, while adding hydrochloric acid as peptizator. The effects on the sol particle size distribution and the properties of Al³⁺/TiO₂ nanosol are studied. The TiO₂ sol is prepared with HC1 as a catalyst. When the percentage of Al³⁺is 0.07, TiCl₄ (3.2 mol/L) used is 1 ml, hydrochloric acid solution (1.6mol/L) used is 6 ml, water consumption is 30 ml and the time of peptization 3 h, the sol particle size is small and the pellet distribution is even. 2. The uniform transparent coating is prepared on glass substrates. The influence of the sol particle size distribution on the properties of Al³⁺/TiO₂ nanosol and Al³⁺/TiO₂ thin coating UV shielding effects are studied. Al³⁺/TiO₂ thin coating shows the best shielding effects on UV radiation when the percentage of Al³⁺ is 0.07, TiCl₄ (3.2mol/L) is 1 ml, hydrochloric acid solution (1.6mol/L) is 6 ml, water consumption is 30 ml and the time of peptization is 3 h.3. It is shown that 100 % anatase of Al³⁺/TiO₂ coating on the aramid fibers has been observed when the sample is treated at 80°C for 1 h and Al₂O₃ is crystal structure. When the treatment is at 120℃for 1 h, Anatase and rutile of Al³⁺/TiO₂ have been observed and Al₂O₃ is a crystal structure.4. Because Al³⁺ can cause the distortion and contraction of crystal lattice, the lattice imperfection can become the catalyst active center, which is advantageous to the photochemical catalysis response. With an increase of Al³⁺ doping ration, an excessive lattice imperfection can become an electronic-hole deathnium center, so that photochemical catalysis activeness drops. Along with the enlargement of Al³⁺ doping quantity, part of Al₂O₃ causes the pellet surface with the crystal structure, separated from TiO₂. More and more small granule of Al₂O₃ can open up the light absorption scope of Al³⁺/TiO₂ powder, so that light degeneration performance increases. 5. The anti-aging effect of the Al³⁺/TiO₂ coating is studied by accelerated aging test with a UV lamp. During the period of aging, the mechanical properties of the coated fibers with Al³⁺/TiO₂ are studied in comparison with uncoated ones. The properties are tensile strength and extension. The micro fractures of the fibers are also analyzed by SEM, ATR and XPS. The results show that the TiO₂ coating decreases the intensity of UV light on aramid fibers resulting in the deceleration of photodegradation of the fibers.- In the process of UV degradation, the tensile strength and elongation rate of the uncoated aramid fibers are decreased significantly. While for the coated ones, the mechanical properties are decreased but with less degree due to the protection effects of Al³⁺/TiO₂ coating. Verifying the data of the tensile properties by means of statistics. The results show that the maximum confidence interval range of tensile strength and elongation at rapture between the coated fibers and uncoated ones reach to (-10.79cN, -9.19cN) and (-1.17%, -0.97%), respectively.- The analytical results of SEM and XPS show that the Al³⁺/TiO₂ coating on aramid fibers can protect amide bonds against photocleavage by means of UV absorption, preventing or decelerating the photodegradation of the fibers.- The condition of preparing Al³⁺/TiO₂ coating and the different rations of Al³⁺ doping on Al³⁺/TiO₂ coating have certain effects on the properties of inhibiting UV radiation. The Al³⁺/TiO₂ coating on aramid fibers is characterized with better improvements of the properties of resistance to UV irradiation, which is provided by 0.07% of Al³⁺ doping ration and via heat treatment at 80℃.