The Fabrication Of Layered Double Hydroxides Used As UV Blocking Materials

Ultraviolet radiation has tremendous destructive effect on biology and materials on earth, and causes large amounts of recourse and economy losses every year because of the UV aging of the materials. Layered double hydroxides (LDHs) has been widely used in the fields of medicine materials, flame retardant materials and catalytic materials as a kind of inorganic supramolecular functional material. In recent years, LDHs has shown good performance in the application of anti-UV aging asphalt. However, the UV blocking mechanism of the supramolecular structure of LDHs and the factors that affect the UV blocking properties of LDHs still need further exploration and research, based on which the UV blocking properties could be improved by the structure design and adjustment.In this work, in order to investigate the UV blocking mechanism of LDHs, improve the UV blocking property and promote its industrial application in the field of polymer materials, we studied the supramolecular structure, energy level characteristic, defect structure and host-guest interaction of the layered double hydroxides based on the principle of structural chemistry and supramolecular chemistry. Taking into account the UV aging of the polymer materials and the unresolved problems of most UV blocking materials, the supramolecular structure and morphology of LDHs has been studied in detail and tuned in a few aspects to improve the UV blocking performance. A series of supramolecular structured UV blocking materials were prepared, and added to the asphalt and polypropylene to test the anti-UV aging properties through accelerated UV aging test.The study of the UV blocking mechanism of LDHs and the UV accelerated aging test of LDHs modified asphalt and polypropylene has provided theoretical basis and technical support for its future application in the field of anti-UV aging polymers. At the same time, this work also provides some experimental and theoretical research for the development of novel supramolecular structured UV blocking materials. The main research contents and results are as follows:1. Study on the UV blocking mechanism of LDHs and the influence of size effect and layer composition of LDHs on its UV blocking performance.LDHs with different layer composition and different particle size distributions have been successfully synthesized using a method involving separate nucleation and aging steps. The UV blocking properties of the LDHs depends on both layer composition and particle size distribution. The extent of UV absorption increases with the increasing amount of zinc in the layers; this can be attributed to the decreasing band gap energy, as confirmed by density functional theory calculations. LDHs show increasing UV screening performance with increasing particle size, which can be explained by the Mie scattering theory. Moreover, accelerated UV aging tests of actual asphalt samples containing LDHs confirms the excellent UV blocking properties, as shown by the significantly lower VAI values and softening point increments. Addition of only 3 wt.% Zn4Al2-CO3-LDH affords excellent UV aging resistance, highlighting the potential practical applications of this material as an additive for asphalt, as well as in a variety of organic coatings which are also prone to UV aging.2. Effect of surface morphology and the defect structure of LDHs on its UV blocking properties.The UV blocking properties of LDHs were optimized according to the aging process and mechanism of polymer materials. The oxygen vacancy defects were produced through chemical surface modification, and the effect on the UV absorbing performance of LDHs was studied. On one hand, the oxygen vacancy defect structure affected the band gap of LDHs and optimized its UV absorbing performance; On the other hand, the positively charged oxygen vacancy defect structure on the LDHs layers could absorb the active free radicals produced in aging process of polymer materials, thus restraining the aging process. The modified LDHs was added into polypropylene and asphalt to test the anti—UV aging properties of the polymer material, and the results showed that both of the two modified polymer materials exhibit excellent anti-UV aging properties, indicating the potential applications of modified LDHs.3. Fabrication and evaluation of a novel host-guest supermolecular structured UV-blocking material.Based on the principle of energy conversion, a fluorescent guest was intercalated into the LDHs layers to fabricate a new type of host-guest structured ZnAl-CPBA-LDH, which enables both physical shielding and absorption of UV light. Furthermore, in contrast to conventional UV blocking materials—which convert UV light into thermal energy—the CPBA anions in the LDH interlayer galleries convert UV light into lower energy fluorescence emission, thus reducing the thermal aging of the polymer composite materials. In addition, the fluorescence lifetime and quantum yield of ZnAl-CPBA-LDH are greatly improved because of the spatial confinement on the interlayer guest of LDHs layer, indicating the high transformation efficiency of UV light. The density functional theory has been used to conduct the research on the effect of host-guest interaction on the UV absorption properties. In polypropylene (PP) aging performance tests, after adding 1 wt.%ZnAl-CPBA-LDH to PP the resistance to UV degradation of the resulting ZnAl-CPBA-LDH/PP composite is higher than that of pristine PP or a CPBA/PP composite. Therefore, this work provides a facile approach to construct new types of host-guest layered materials for UV-blocking applications.