Preparation And Tanning Properties Of Laccase Catalyzed Condensed Tannins:A Biomass Tanning Agent

The tanning section of leather is one of the most important sections in leather processing.Chromium salts,as traditional leather tanning agents,play an irreplaceable role in leather and fur processing.However,the pollution caused by traditional chrome tanning cannot be neglected.At the same time,chromium salt is a non-renewable mineral resource,which highlights certain defects in the process of sustainable leather production and cleaner production.So to a certain extent,it can be considered that chromium salts as tanning agents cannot meet the sustainable and clean production of tanning process.Therefore,according to the development status of tanning agent and clean tanning technology in leather industry,this study aims to explore a kind of tanning agent and tanning technology which is consistent with current sustainable and clean tanning,and provide a feasible choice and theoretical basis for chrome-free tanning.Condensed tannins are a kind of plant tannin extract,in which the main components of tannins are different aggregates of catechins.In vegetable tanning chemistry,tannin components of condensed tannins are usually considered to bind with collagen carboxyl,peptide,amino or other non-polar parts through hydrophobic-hydrogen bonds and few covalent bonds or/and other cross-linking forms(e.g.,ionic bond),which makes condensed tannins tanned leather have lower shrinkage temperature(T_s).Therefore,in this study,laccase was used as catalyst to enhance the covalent cross-linking degree of condensed tannins with collagen fibers under the catalytic oxidation of laccase,so as to improve the tanning effect of condensed tannins in the tanning process.Meanwhile,the tanning parameters were studied to obtain the optimum tanning parameters for highly reactive condensed tannins.In order to confirm the feasibility of this project,collagen extracted from bovine tendon was used as collagen fiber simulator for skin collagen fibers and catechins were used as condensed tannins simulator,and under the catalysis of laccase urge catechins and collagen forming covalent cross-linking.The optimum catalytic conditions of laccase catalyzing catechins were researched,and the cross-linking of catechins with collagen was carried out under the optimum catalytic conditions.The cross-linked collagen with catechins catalyzed by laccase was analyzed by infrared spectroscopy(FT-IR),X-ray diffraction(XRD),determination of amino content in collagen and X-ray photoelectron spectroscopy(XPS).It was found that the amino of collagen and catechin successfully formed stable covalent cross-linking.Furthermore,the physical and mechanical properties and microscopic morphology of collagen before and after cross-linking were compared and analyzed.The results showed that 15%laccase catalyzed catechins(LC)cross-linked collagen had the best comprehensive properties,the tensile strength increased from 4.01 MPa to 9.33 MPa,the elongation increased from 41.54%to 57.87%,increased by 56.87%,the swelling rate decreased from 273.82%to 169.91%,the solubility increased slightly from2.14%to 2.87%,the change was not significant,the swelling rate decreased from273.82%to 169.91%,the solubility decreased by 37.95%,and the solubility increased slightly from 2.14%to 2.87%.The denaturation temperature was increased from 57.03?to 74.71?.Through the analysis of micro-structure and macro-physical properties,it can be confirmed that the covalent cross-linking degree between catechin and collagen can be improved by the catalysis of laccase,and the stable cross-linking between collagen and catechin can be formed.This provides a theoretical basis for laccase-catalyzed condensed tannins to improve the covalent cross-linking degree of tannins and collagen fibers of skin.Based on the experimental parameters of laccase-catalyzed catechins cross-linking with collagen,condensed tannins were oxidized directly by laccase to form high reactivity condensed tannins(L-BA).Then the characteristic functional groups and molecular weights of the condensed tannins were detected by UV-vis,X-ray photoelectron spectroscopy(XPS),gel permeation chromatography(GPC)and other modern instruments.The phenolic hydroxyl groups of tannin tannins can be catalyzed to form o-quinones with high reactivity.The tanning process parameters were optimized by using condensed tannins(L-BA)as tanning agent,and the optimum tanning parameters were obtained.By comparing and analyzing the physical properties of the finished leather,it was found that the shrinkage temperature of the leather tanned by L-BA reached80.9?,which increased by 11.25%on the basis of the leather made by condensed tannin(BA)without any treatment.At the same time,the elongation at break,tear strength and softness of L-BA tanned leather were increased by27.99%,7.80%and 7.2%,respectively,on the basis of BA tanned leather.The full,clear and compact leather was observed by ultra-depth microscope.The test of folding fastness showed that the leather tanned by L-BA had good body and no cracks after 10,000 folds.At the same time,the leather tanned by L-BA had a natural dark brown color and could be used directly without dyeing in some areas.In addition,the color of leather tanned by laccase-catalyzed condensed tannin became dark brown.In order to improve the application of leather,aluminum sulfate is used as retanning agent to retanning the main tanned leather,which can reduce the discoloration.Dyeing of aluminium salt retanned leather showed that when aluminium sulfate dosage was 4%,the shrinkage temperature of finished leather could be effectively increased and the efficient absorption of dyes could be guaranteed.T_s was increased by 5.6?,and the absorption rate of dyes reached 83.28%.