Controlled Synthesis And Tanning Application Of Calcined Layered Double Hydroxides Nanomaterials

Currently,although traditional chrome tanning can produce leathers with excellent overall performances,there is still environmental pressures from chromium-contained waste disposal in leather industry.Therefore,the development of environmentally friendly chrome-free tanning materials and related tanning technologies have became the main way to effectively solve the environmental problems from the source.Layered double hydroxides(LDHs)is a kind of typically layered nanomaterials,which is mainly composed of positively charged bimetallic hydroxide nano sheets and interlayers with exchangeable anions.Due to its nontoxicity,exchangeability of layered elements and anions within interlayers,and structure memory effect,it has been widely concerned because LDHs can confer synthetic or natural polymers excellent thermal stability,mechanical properties and barrier properties.In this dissertation,combined with the co-precipitation and calcination treatment,a series of LDOs has been synthesized by adjusting the type and molar ratio of metal elements in the laminate.The tanning properties and effects of leather tanned by these LDOs on the absorption of dyes and fatliquoring agents in post-tanning procedures have been also studied.Herein,the following work has been carried out:(1)Mg2Al-LDH was synthesized using co-precipitation method by optimizing temperature and precipitation pH value.The morphologies and surface charges of Mg2Al-LDH were analyzed by X-ray diffraction(XRD),Fourier transform infrared spectroscopy(FT-IR),Scanning electron microscopy(SEM)and Zeta potential.The results showed that the positively charged Mg2Al-LDH had a typical layered structure and a size of～60 nm.The particle size of the Mg2Al-LDH decreased and its crystallinity increased with increasing the precipitation pH value from 8.0 to 12.0.When the pH increased from 2.0 to 13.0,Zeta potential of the Mg2Al-LDH gradually decreased.Moreover,the results of shrinkage temperature(TS)indicated that the crust tanned by Mg2Al-LDH with a highest Ts when using 3%Mg2Al-LDH at initial pH 3.0 and final pH 4.0 with tanning time of 2 h.(2)The optimized Mg2Al-LDH was further calcined to prepare Mg2Al-LDO.SEM and N2 adsorption and desorption analysis were used to analyze the morphology and specific surface area of the Mg2Al-LDO,and structure characteristics of the Mg2Al-LDO and Mg2Al-RLDH after calcination and restoration were also analyzed by XRD and FT-IR.The results showed that Mg2Al-LDO was prepared by the calcined method without changing its morphology and size,but increasing its specific surface area.When the pH value of the solution was 7.0,the Mg2Al-LDO could be successfully restored into the Mg2Al-RLDH structure.Moreover,the Mg2Al-LDO was used to goatskin tanning,and optimized the Mg2Al-LDO dosage,initial and final pH in the leather tanning process.It was clear that the optimized tanning system of 5%Mg2Al-LDO at initial pH value 3.0 and final pH value 6.0 could give the highest TS to the crust.Compared with the crusts tanned by Mg2Al-LDH,the TS and thickening rate of the crusts tanned by Mg2Al-LDO were enhanced.Furtherly,SEM observations revealed that the calcination was favorable for improving the disperse uniformity of Mg2Al-LDO within collagen fibers without changing the periodic structure of collagen molecules which is alternating light and shade.Combined with XRD and FT-IR results,it could be speculated that Mg2Al-LDO could bind to the collagen fibers mainly through hydrogen bonds and electrostatic interaction without destroying the triple-helical conformation of collagens.(3)By the combination of co-precipitation and calcining method,the metal ion of Zn2+and Fe3+with tanning properties were introduced into the laminate to synthesize a series of layered double oxides(LDOs)with different metal elements.Their morphologies and tanning properties were also analyzed and tested.The results showed that Mg2Fe-LDO,Zn2Al-LDO and Zn2Fe-LDO were prepared successfully,and Mg2Al-LDO exhibited better tanning properties.(4)MgAl-LDO with different metal molar ratios was prepared by tuning the molar ratio of n(Mg)/n(Al),and their morphology and structure were also analyzed by XRD,FT-IR and SEM-EDX.Furtherly,the MgAl-LDO were used in goatskin tanning and post-tanning procedures.The results indicated that MgAl-LDOs with different n(Mg)/n(Al)molar ratios were synthesized.The effect of n(Mg)/n(Al)molar ratio on the Ts and thickening rate of the crusts tanned by MgAl-LDO were not significant.With decreasing n(Mg)/n(Al)molar ratio of MgAl-LDO,the dyestuff concentrations,chemical oxygen demand(COD)and total suspended solids(TSS)could decrease in the spent liquors after post-tanning procedures.This phenomenon indicated that the addition of Mg2Al-LDO could facilitate the absorption of dyestuff and fatliquoring agents by the tanned crust leathers.