Study On Functional Modification Of Polysaccharides For Cleaner Leather Tanning

The leather industry is to use skins or hides,the by-product of animal husbandry and food industry to make leather products needed by people.Skins and hides are renewable and degradable,and the leathers could give people a comfortable feeling.Although the leather industry has a long history,it still keeps a good momentum for development.In leather making,the by-product of hides/skins from animal husbandry and food industry are transformed into leathers,which are popular in People's Daily life.With the development of leather industry,huge economic and social benefits are achieved,while some environmental pollution is caused.There are usually three stages in leather making,such as preparation,tanning and finishing.Preparation is the foundation,and tanning is the key.At present,90%of leather tanning is done with trivalent chromium(Cr³⁺)with plenty of residual Cr³⁺in the tanning waste water,which may be easily oxidized to more harmful hexavalent chromium(Cr⁶⁺).Therefore,chromium pollution from tanning wastewater,solid tannery waste and leather products has been an urgent problem facing the leather industry.In order to meet the needs of clean production and sustainable development of leather industry,the response surface methodology was used to optimize the preparation of dialdehyde soybean polysaccharide for green tanning agent.Soybean polysaccharide oxidized by hydrogen peroxide was used as novel masking agents in zirconium tanning.Polyfunctional polysaccharide-based high chromium absorbent additives with both groups of aldehyde and carboxyl were prepared for tanning sheepskin,providing a new material for the preparation of ecological leather.The main research contents are as follows:(1)With soybean polysaccharide(SSPS)as raw material and sodium periodate as oxidant,dialdehyde soybean polysaccharide(DPA)was prepared.The effects of oxidation temperature,oxidation time and sodium periodate dosage on the shrinkage temperature(Ts)of DPA-tanned leather were studied.The results showed that Ts is closely related to oxidation conditions.Response surface methodology(RSM)was used to establish the Ts model of leather,and the optimal oxidation conditions were obtained.At the oxidation time of 0.53 h,the oxidation temperature of 20?,and the mass ratio of soybean polysaccharide to oxidizing agent of 1:1.9,the Ts(78.62?)was the highest.The R² of 0.9980,indicated that the predicted model value was in good agreement with the experimental.The Ts model is a direct and effective method to predict the interaction between the Ts of DPA tanned-leather and the oxidation conditions.The structure analysis of DPA showed that the appropriate amount of aldehyde group and molecular weight had better permeability and crosslinking in collagen fiber,which was more suitable as green tanning agents.(2)Soybean polysaccharide(SSPS)was modified with hydrogen peroxide to obtain oxidized soybean polysaccharide(HPS).FT-IR results showed that the HPS contained carboxyl group,and particle size analysis showed that the particle size of HPS was smaller.Compared with zirconium tanning leather,the shrinkage temperature of HPS-60 and zirconium combination tanned leather was increased by 5?,the tensile strength increased from 13.23 MPa to 18.90 MPa,and the filling effect was good and the sensory properties were better.Furthermore,SEM results showed that the introduction of HPS helped to better disperse collagen fibers,the tanning mechanism was explored by FT-IR and XRD methods.(3)Polysaccharides(DCA)with both groups of aldehyde and carboxyl were prepared from sodium alginate(CA)with carboxyl group as raw material and sodium periodate as oxidant.Before chrome tanning,DCA was added as pre-tanning agent,which reacted with the amino group of collagen to form a cross-linking structure and initially fixed the collagen fiber.Meanwhile more carboxyl groups that could be combined with chrome tanning agent were introduced to help increase the absorption and fixation of chromium later.Compared to the 8%chrome tanned leather,1%DCA and 6%chromium combination tanned leather shrinkage temperature increased by10?,with the thickening rate increased by 98.6%.The EDS results show that the chromium content is higher,the distribution more uniform,the COD and BOD of tanning wastewater were decreased,which meets the requirements of leather industrial production and is conducive to promoting the sustainable development of leather industry.