Technology Research Of Inorganic Coating On The Surface Of TiO₂ And CaCo₃

Most of the inorganic fillers have hydroxyl group on the surface,which result to the poor compatibility with the matrix material.Therefore,their filling effect is poor,application and development of inorganic fillers are also limited.Inorganic coating of the inorganic filler can not only improve its surface properties,but also improve the compatibility between the inorganic filler and the matrix material.According to the physical and chemical properties of TiO₂ and CaCO₃ and their application fields,four core-shell structures were prepared in this paper as the follows:TiO₂@SiO₂ core-shell structure,TiO₂@Al₂O₃ core-shell structure,TiO₂@SiO₂@Al₂O₃ core-shell structure,and CaCO₃@SiO₂ core-shell structure.Ultraviolet shielding performance,water dispersibility and dispersion stability of the TiO₂@SiO₂ core-shell structure,TiO₂@Al₂O₃core-shell structure,TiO₂@SiO₂@Al₂O₃ core-shell structure were tested,and the coating kinetics of CaCO₃@SiO₂ was researched.The main research work and conclusions are as follows:?1?A rutile TiO₂ core and sodium silicate as a coating agent were used to prepare a core-shell structure TiO₂@SiO₂ by cocurrent method.The effect of pH,temperature,stirring rate,coating time,initial concentration of TiO₂,coating amount and aging time on the acid solubility of the product were sudied.Using response surface experiments to optimize the preparation process and further study the process parameters.The products were characterized by SEM,TEM,XRD,XPS,FT-IR,etc.,and the UV shielding performance of the final product was evaluated.The results show that when the fixed stirring rate is 300 r/min,the coating time is 60 min,and the initial concentration of TiO₂ is 200 g/L,the order of the factors affecting the acid solubility rate of each factor is:coating dose>coating temperature>pH>aging time.The preferred proc-esss is as follows:the amount of coating agent remained at 4.41%,the coating temperature came to 84°C,the pH fixed at 9.59,the aging time went to 25.62 h,and the acid solubility rate reduced to 3.43%.After optimizing the process conditions,the coating effect is greatly improved.Through various characterization methods,it was determined that the coating was an amorphous SiO₂ with a thickness of about 5 nm,bonding to the surface of TiO₂ in the form of Ti-O-Si,which enhanced the stability of the core-shell particles.The surface-coated SiO₂ layer can obviously inhibit the ultraviolet absorption ability of TiO₂ nanoparticles,and give TiO₂@SiO₂ core-shell structure particles a higher ultraviolet shielding ability.?2?The core-shell structure of TiO₂@Al₂O₃ was prepared through a cocurrent method,in which rutile TiO₂ as core and sodium metaaluminate as coating agent.The effect of pH,temperature,stirring rate,coating time,initial concentration of TiO₂,coating amount and aging time on the acid solubility of the product were sudied.Using response surface experiments to optimize the preparation process and further study the process parameters.The products were characterized by SEM,TEM,XRD,XPS,FT-IR,etc.,and the UV shielding performance,water dispersibility and dispersion stability of the samples were evaluated under the optimal conditions.The results show that the order of the factors affecting acid solubility which was determined by single-factor experiments and response surface optimization process is:coating amount>temperature>aging time>pH.The better technology can improve the coating effect is:stirring rate stayed at 300 r/min,coating time remained 60 min,initial concentration of TiO₂ went to 200 g/L,pH=10.05,temperature fixed at83°C,silicon content came to 4.46%,aging time remained 27.86 h,and the value of acid solubility rate reduced to 17.50%.Various characterization methods show that the coating is a layer of boehmite Al₂O₃ with a thickness of about 6nm,and is bonded to the surface of TiO₂in an Al-O-Ti manner,which enhances the stability of the core-shell particles.The coating layer on the surface can suppress the ultraviolet absorption ability of the TiO₂ nanoparticles,and improve the dispersibility and dispersion stability of the TiO₂ nanoparticles in water.?3?Core-shell structure TiO₂@SiO₂@Al₂O₃ was formed by coating nanofilms of SiO₂ and Al₂O₃ on the surface of rutile TiO₂ by a co-current method,in which rutile TiO₂ is the main body,sodium silicate is the silicon source,sodium metaaluminate is the aluminum source,and sodium hexametaphosphate is the dispersant.The stirring rate of 300r/min,coating time of 60 min,initial TiO₂ concentration of 200 g/L,coating temperature at 84°C,pH=9.59,and aging time of 25.62 h are the preparation conditions for silicon coating.The stirring rate of 300r/min,coating time of 60 min,initial TiO₂ concentration of 200 g/L,pH=10.05,coating temperature at 83°C,and aging time of 27.86 h are the preparation conditions for aluminum coating.The effects of silicon-aluminum ratio on acid solubility,water dispersibility and dispersion stability were investigated.The products were characterized by SEM,TEM,XRD,XPS,FT-IR,etc.,and their UV shielding performance was evaluated.The results show that when the ratio of silicon to aluminum is 1:1,the acid solubility of TiO₂ after composite coating decreases to 3.05%.The cladding layers were determined to be an amorphous SiO₂ film of about 5 nm and a boehmite Al₂O₃ film which formed Al-O-Si bonds on the surface of TiO₂ and enhanced the stability of the core-shell particles.Surface-coated SiO₂ and Al₂O₃ layers can significantly inhibit the ultraviolet absorption capacity of TiO₂nanoparticles which give TiO₂@SiO₂@Al₂O₃ core-shell structure particles higher ultraviolet shielding capabilities.The water dispersibility and dispersion stability of TiO₂ after composite coating were obviously improved.?4?Core-shell structure CaCO₃@SiO₂ was prepared by co-current method,using CaCO₃ as core and sodium silicate as coating agent.The effects of single factor pH,temperature,stirring rate,coating time,initial concentration of CaCO₃,coating amount and aging time on the acid resistance of the product were studied.Using kinetic analysis,the growth kinetics of the cladding layer under different pH conditions was explored.The results showed that the optimal conditions were agitation rate of 250r/min,coating time of 120 min,initial CaCO₃ concentration of 100 g/L,coating agent dosage of 6%,coating temperature of 90°C,pH=9.5-10,aging time of 36 hours and the acid resistance dropped to about 18%.The products were characterized by SEM,TEM,XRD,XPS,FT-IR.Finally,it was determined that the coating was an amorphous SiO₂ nanometer film,and was bonded to the surface of CaCO₃ by Ca-O-Si,which enhanced the stability of the core-shell particles.The relationship between the thickness of the silica film?d_s?formed on the surface of CaCO₃ and the drop time of the sodium silicate solution at pH=10 and 7 was:d_s=0.8793t-4.1014;d_s=0.4554t-0.9772.It can be seen that the pH of the system has a great effect on the growth of the silica coating on the surface of CaCO₃.The crystal growth rate at pH of 10 is about twice that at pH of 7.