| Vat dye is one of old-line dyes with vivid and colorful shades,vat dyes yield coloured fibres of excellent all-round fastness, particularly to light, washing and chlorine bleaching. In the colouration of cellulose fibres, vat dyes now represent a relatively large part of the dyestuff market; among them about 120000 tons of vat dyes are being used annually. They are practically insoluble in water and have no affinity for cellulose fibres in such a state. Thus, these dyes have to be reduced (vatted) before dyeing to be converted into the water-soluble form which have substantivity towards fibres and which, after absorption into the fibres to be dyed, should be re-oxidised to the original water-insoluble dye pigment form in situ in the fibres.A series of anthraquinone vat dyes were calculated using the Hyperchem 8.0 software with semiempirical method PM3.Quantitative structure-activity relationship studies have been approached based on physicochemical, geometrical,electronic structure etc.parameters to interpret the affinity of a series of vat dyes to cellulose fibres.The results show that the affinity will increase due to the larger hydrophobic effects(log P), hydration energy, van der Waals volume and surface area;the lower energy of the lowest unoccupied molecular orbital. Of the molecular orbital descriptors, the LUMO orbital energy is found to yield the most important contribution. The relationships between affinities of these anthraquinone vat dyes and molecular areas, hydrophobicity and the LUMO orbital energy are presented in the form of correlation and multilinear equations.Secondly, the aggregation behavior and the state of the vat dyes was studied with the nano-particle size analyzer in this paper. We measured the size of dye particles,to study various factors affecting the dye aggregation, such as reducing agents, oxidizing agents and salt concentration. The results showed that the vat leuco solution contained dye aggregates in different sizes, the measured dye particle size are mainly distributed in 20-140nm range. Leuco vat dye molecules rarely exist in the single-molecule forms, mostly in the dozens of molecular aggregates, or in the form of hundreds of moleculars, indicating that Leuco vat dye solution is not a true solution, but mainly in the range of colloid. In a certain range of dye concentration, dye particle size will increase with the sodium hydrosulfite, sodium hydroxide and salt concentration increased. In addition, Zeta potential showed that Leuco dye solution was very unstable.Then, Interaction between polyvinylpyrrolidone (PVP) and the reduced form of vat dyes was studied using the results of visible spectrophotometric measurements and particle size determination. The influence of the amount of PVP on the cotton fabric dyeing process with vat leuco dyes was analyzed. Results showed that PVP formed strong complexes with reduced vat dyes which could considerably hamper the dyeing process. Cotton fabric dyeing with reduced vat dyes was found to be retarded by the presence of PVP, the extent of retardation was proportional to the amount of PVP.Lastly,we discuss the pad dyeing procedure,mainly study the impact of factors, including the concentration of alkali and reducing agent, the immersed time, the temperature of dye bath, the concentration of electrolyte and auxiliaries ,aftertreatment, etc.. The experimental results show that the use of vat leuco dyes pad-dyeing technology can not only save time because of its simple technology, but also get the deeper colour under certain conditions.However,there are still some question to be solve.
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