Coloration Of Freshwater Cultured Pearls And Its Nano-modification

Pearl is a precious organic gem, and the value of pearl mainly depends upon color and its luster. The person of the different nation, different race may have their distinct favor of different color of pearl. A great proportion of pearls that being sold on the market are cultured pearls. Yellow and write are two kinds of primary color of the cultured pearl, and these colors can not meet customers' needs. On the other hand, the differences of color and luster between distinct pearls are very huge, it is necessary to carry out enhancing and treating on the pearls to make them fit the requirements as gem. The dyeing is an important method to processing of the cultured pearl. But many problems of the colored pearls that were processed by existing technology were appeared after wearing a period of time, the color and luster of the pearl might become unstable. The color of the pearl might fade because the photo-bleaching induced by UV-light in the sunshine, and its luster would become dim because the surface of the pearl was corrupted by the sweat, grease and other matters. So it is necessary to develop a novel processing method to produce high quality colored pearl.In this work, the preparation of chemical colorated freshwater cultured pearl and the mechanism of pearl dyeing processing were investigated;the preparation of nanosized anatase modified aqueous TiO₂ sol was discussed;nanosized TiO₂ particle-polymer co-modified colored pearl was successfully prepared and its protective effect against UV-induced bleaching and corrosion resistant was tested.In the first part of the paper, freshwater cultured pearl was colorated by two steps of chemical pretreatment and dyeing. The colored rose red, orange yellow and black pearls had been obtained by using C₂₈H₃₁CIN₂O₃, C₁₄H₁₄N₃NaO₃S and silver salt respectively. The reflectivity of the pearls was measured by visible spectrophotometer and its morphology was observed by field emission scanning electron microscopy. Then the mechanism of dye processing was discussed. Analyzing the SEM images of the pearl in different chemical treating and coloring time, the results indicated that some nanosized micorpores were generated on the pearl surface during the chemical pretreatment process, and these pores would be repaired by the dye. The quality of colored pearl was directly related with this process.Secondly, nanosized anatase modified aqueous TiO₂ sol was prepared through two steps: preparation of aqueous PTA (peroxo titanic acid) sol by ISG method from TiOSO₄, and then hydro-thermal modified in autoclave to form some nanosized anatase particle in the sol. The XRD and FTIR were used to detect the phase and chemical structure change during the process. Micrographs of TiO₂ particles were observed by TEM. Ti/H₂O₂ molar ratio could play as a very important factor during the preparation of PTA sol, and semi-transparent light yellow sol could be obtained when the ratio is1:3-1:4. TEM micrographs showed that the quantity and size of TiC^ particle increased with the hydro-thermal modification temperature change from 90 °C to 120°C, and prolong the treating time also could promote the formation of nano particle. The XRD pattern indicated that the higher temperature could promote the crystallization of anatase. On the other hand, with the increasing of modification temperature the stability of sol would become worse.Finally, nanosized TiC?2 particle-polymer co-modification technology was inducted to protect the colored pearl from UV-induced bleaching and improve its corrosion resistant. To measure the protective effect of the nanosized particle modified TiC>2 layer prepared in different condition, its UV radiation transmittance was measured by UV-Vis spectrophotometer. The results indicated that the quantity and size of nano particle and the thickness of modified layer were two important factors that could directly affect the protective effects of the modified layer. In order to improve the corrosion resistant of the pearl, a polymer co-modification layer was introduced after processing the nanosized particle modified TiC>2 layer. The heat treatment temperature could affect the reaction in the formation process of polymer. By heat treating in 120°C for 30-40 min a high visible light transmittance and low UV radiation transmittance co-modification layer was obtained. UV ageing test and corrosion test were used to confirm the performance of the nanosized TiC>2 particle-polymer co-modified colored pearls, the results prove that the performance of colored pearl was being markedly improved by the nanosized TiC>2 particle-polymer co-modification method.