Study On The Modification And Properties Of Layered Double Hydroxide Used As UV Blocking Materials

Exposure to ultraviolet（UV）rays（λ=200–400 nm）can produce accelerated photodegradation of organic polymers,pigments,and dyes.Furthermore,such rays are harmful to human health causing sunburn,photoaging and skin cancer.Consequently,protection of human health from solar ultraviolet radiation has been a complex problem and advanced materials are in demand for the effective treatment and protection of human health.Sunscreens are widely used to protect human skin against the harmful effects of UV radiation.Based on their protection mechanism,sunscreens are classified as organic absorbers and inorganic UV blocking materials.Organic absorbers may pose safety problems when used at higher concentrations.Generally,inorganic UV blocking materials(TiO₂ and ZnO ultrafine particles)would induce photodegradation of organic substances and damage the living organism simultaneously,which may be a serious problem because it takes place on the skin.Therefore,it is necessary to search for novel anti-ultraviolet materials with high safety stability and excellent light shielding behavior.As a UV blocking material,Layered double hydroxide（LDH）draw our attention.LDH is a class of anionic clays,the tunable metal ions in a wide range without changing the structure of the material and anion exchange properties of LDH materials make them receive much attention.Therefore,it can be anticipated that the incorporation of the organic UV absorbent into an LDH matrix will give rise to one kind of novel sunscreens with the following advantages.Firstly,the supramolecular layered host-guest structure of LDH enables both physical shielding（by LDH host layers）and chemical absorption（by interlayer guest）of UV light.Secondly,the absence of a close contact between skin and organic UV absorber could increase the safety of the cosmetic formulations.In this dissertation,ZnTi-CO₃-layered double hydroxides have been synthesized by coprecipitation of zinc and titanium salts from homogeneous solution.ZnTi-CO₃-LDH show stronger UV absorption than that of Mg Al-CO₃-LDH and ZnAl-CO₃-LDH due to the incorporation of the titanium element in the layers.We have systematically explored the UV absorbing and screening properties of ZnTi-LDH with different amounts of titanium cations in the layer.Based on the conception of interealation chemistry,the hydrophilic and hydrophobic organic UV absorbent,p-aminobenzoic acid（PABA）and cinnamic acid（CA）were intercalated into Zn-Ti layered double hydroxide（LDH）by anion-exchange reaction.The new organic-inorganic nanocomposites ZnTi-CA-LDH and ZnTi-PABA-LDH have been synthesized.The samples are formulated in a sunscreen cream to study the matrix protective effect towards UV rays.The detailed contents of this paper are listed as follows:（1）Study on the UV blocking mechanism of ZnTi-LDHZnTi-LDHs with varying Zn²⁺/Ti⁴⁺molar ratio in the layers were successfully synthesized by the method of co-precipitation.As a contrast,MgAl-LDH and ZnAl-LDH were also prepared and investigated.The UV-vis spectra show that the UV absorbing properties of ZnTi-LDHs is stronger and broader than both MgAl-LDH and ZnAl-LDH due to the existence of Ti.Moreover,the UV absorption property increased with the content of Ti.Both the experimental and theoretical results show that the introduction of Ti element is an effective way to tune the electron structure,band gap,and the resulting UV-shielding properties.Besides,compared with common inorganic UV blocking materials TiO₂ and ZnO,ZnTi-LDH generates remarkably lower active radicals,implying ZnTi-LDH is more suitable as a safe UV-shielding material.（2）Fabrication of Zn-PABA-LDH and its application as UV absorbentAn organic ultraviolet（UV）ray absorbent,p-aminobenzoic acid（PABA）was intercalated into a Zn-Ti layered double hydroxide（LDH）precursor by an anion-exchange reaction to obtain ZnTi-PABA-LDH,a new organic-inorganic nanocomposite.Markedly enhanced thermal stability of PABA exhibited after intercalation into ZnTi-LDH.The UV-vis-NIR spectrophotometric and ESR data show excellent UV ray resistance and greatly decreased photocatalytic activity when PABA is intercalated into the interlayers of the ZnTi-LDH.Furthermore,the sample was formulated in a sunscreen cream to study the matrix protective effect towards UV rays,the resut show that ZnTi-PABA-LDH can availably improve the UV resistance of sunscreen.（3）Synthesis and characterization of ZnTi-CA-LDH for cosmetic applicationAn organic ultraviolet（UV）ray absorbent,cinnamic acid（CA）was intercalated into Zn-Ti layered double hydroxide（LDH）by anion-exchange reaction.ZnTi-CA-LDH,a new type of host-guest UV-blocking material has been synthesized.ZnTi-CA-LDH exhibits a superior UV blocking ability compared to pure CA and ZnTi-CO₃-LDH.The thermal stability of the intercalated ZnTi-CA-LDH was investigated by TG-DTA,which showed that the thermostability of CA was markedly enhanced after intercalation into ZnTi-LDH.The EPR data showed greatly decreased photocatalytic activity compared to common inorganic UV blocking agents TiO₂ and ZnO.Furthermore,when the UV absorbent-intercalated LDH was introduced into a cream formulation the UV resistance capability of sunscreen was significantly enhanced.