Preparation And Tanning Properties Of Epoxidized Biomass-Based Materials

Chrome tanning has a history of more than 160 years.Currently,nearly 90%of the leather in the world is produced by chrome tanning,and the resulting chromium-containing wastewater and chromium-containing solid waste cause serious pollution and impact on the environment,as well as seriously restrict the sustainable development of the tannery industry.Therefore,the development and promotion of chrome-free tanning agents are imperative.Organic chrome-free tanning agents are an important part of a chrome-free tanning system.However,organic chrome-free tanning agents usually use highly toxic reagents and solvents in the preparation process.The use of these reagents and solvents will pose a pollute to the environment.In addition,the leather tanned with current organic chrome-free tanning agents also has problems such as poor hydrothermal stability(low shrinkage temperature),poor physical and mechanical properties,the release of free formaldehyde,poor yellowing resistance,easy mildew,and low absorption on traditional anionic wet finishing materials.In this work,three kinds of epoxidized biomass-based organic chrome-free tanning agents were prepared by oxidation and grafting using two typical biomass materials(starch and collagen polypeptide)as base materials.The structures of the three tanning agents were characterized and identified by modern analytical instruments;the tanning properties of the three tanning agents were compared with those of commercially available organic chrome-free tanning agents;and the mechanism of action of the three tanning agents on collagen fibers was studied.The research contents are summarized as follows:(1)Aiming at the problems of low shrinkage temperature(Ts)and poor physical and mechanical properties of leather tanned with organic chrome-free tanning,in this work,corn starch was oxidized by sodium periodate to introduce aldehyde groups into corn starch to prepare dialdehyde corn starch(DCST)with a degree of oxidation of 91%.Then,the C6-OH of DCST was grafted with an ecofriendly epoxy monomer(ethylene glycol glycidyl ether,EGDE)to prepare an organic chrome-free tanning agent(DCST-EGDE)with an epoxy value of 0.284 mol/100 g.The results of Fourier infrared spectroscopy(FTIR),nuclear magnetic resonance spectrum(NMR),x-ray photoelectron spectroscopy(XPS),x-ray diffraction(XRD),scanning electron microscope(SEM),and gel permeation chromatography(GPC)analysis show that DCST-EGDE has been successfully prepared;its relative molecular mass is between 447 Da and 3391 Da,and its appearance is needle-like.Under the optimal process,DCST-EGDE was applied to the tanning process of pickled sheepskins.The application results showed that DCST-EGDE-tanned leather was white,and the Ts(85.2℃),and physical and mechanical properties(tensile strength(TSS)=10.3± 0.7 MPa,tear strength(TS)=27.8±1.9 N/mm)of the leather were better than those of commercially available leather(F-90-and TWS-tanned leather).Among them,the Ts of DCSTEGDE-tanned leather were 1.8℃ and 7.6℃ higher than those of F-90-and TWStanned leather,respectively;the TSS increased by 3.4 MPa and 2.7 MPa respectively;and the TS increased by 6.8 N/mm and 2.9 N/mm,respectively.(2)Aiming at the problems of leather tanned with organic chrome-free tanning agents,which have poor yellowing resistance and low absorptivity to traditional anionic wet finishing materials,in this work,collagen polypeptide(CP)was extracted from chromium-containing waste leather by in-situ dechroming and enzymatic hydrolysis,and its free amino group was grafted by EGDE to prepare an amphoteric organic chrome-free tanning agent(CP-EGDE)with an epoxy value of 0.316 mol/100 g and pI of 5.16.The results of FTIR,NMR,GPC,and Zeta potential analysis showed that CP-EGDE was successfully prepared and its relative molecular mass was distributed between 592 Da and 25095 Da.Under the optimum process,CP-EGDE,F-90,and TWS tanning agents(the dosage of F-90 and TWS is the same as that of CP-EGDE)were applied to the pickling-free and salt-free tanning process of deacidified sheep skin.The application results showed that the Ts(84.9℃),physical and mechanical properties(TSS=11.4±1.3 MPa,TS=23.5±3.6 N/mm),organoleptic properties(thickening rate(TR)=68.3±0.3%,softness(Sof.)=7.9±0.01 mm),yellowing resistance,and absorption of anionic fatliquors(88.4%)and dyes(95.3%)of CP-EGDE-tanned leather were better than those of F-90-and TWS-tanned leather.Among them,the Ts of CPEGDE-tanned leather were 3.9℃ and 2.7℃ higher than those of F-90-and TWStanned leather,respectively;the TSS increased by 3.8 MPa and 4.5 MPa respectively;and the TS increased by 0.6 N/mm and 1.6 N/mm,respectively;the TR increased by 51.3%and 57.6%respectively;and the Sof.increased by 4.3 mm and 3.9 mm,respectively;the absorptivity of anionic fatliquors was increased by 12.2%and 28.2%,respectively;and the absorptivity of anionic dyes was increased by 19.1%and 11.6%,respectively.In addition,the preparation process in the CPEGDE tanning agent does not use toxic reagents and organic solvents,and the tannery wastewater of the CP-EGDE tanning agent has good degradability(BOD5/CODCr=0.46 ±0.02),which is more eco-friendly than the F-90 and TWS tanning agents.(3)Aiming at the problem that crust leather is susceptible to bacterial and fungal erosion during preparation,storage,and processing,in this work,an ecofriendly broad-spectrum antibacterial agent was used as the raw material,and the hydroxyl group of HHTT was grafted with EGDE to prepare an HHTT-EGDE intermediate with an epoxy value of 0.319 mol/100 g.Then HHTT-EGDE was used to graft and modify the free amino groups of CP to prepare an amphoteric organic chrome-free tanning agent(HHTT-EGDE-CP)with antibacterial properties,for which the epoxy value was 0.267 mol/100 g and the pI was 5.21.The results of FTIR,NMR,GPC,and Zeta potential analysis showed that the tanning agent was successfully prepared and that its relative molecular mass was distributed between 418 Da and 10819 Da.Under the optimum process,CP-EGDE,F-90,and TWS tanning agents(the dosage of F-90 and TWS is the same as that of CP-EGDE)were applied to the pickling-free and salt-free tanning process of deacidified sheep skin.The application results showed that the Ts(78.8℃),physical and mechanical properties(TSS=13.5±1.0 MPa,TS=54.2±4.2 N/mm),organoleptic properties(TR=51.9%±3.5%,Sof.=5.5 ± 0.05 mm),and absorption of anionic fatliquors(93.1%)and dyes(97.5%)of HHTT-EGDE-CP-tanned leather were better than those of F-90-and TWS-tanned leathers.Among them,the Ts of HHTT-EGDE-CP-tanned leather were 2.4℃ and 3.1℃ higher than that of F-90-and TWS-tanned leather,respectively;the TSS increased by 6.7 MPa and 5.6 MPa respectively;and the TS increased by 31.4 N/mm and 30.3 N/mm,respectively;the TR increased by 36.7%and 41.9%respectively;and the Sof.increased by 2 mm and 1.3 mm,respectively;the absorptivity of anionic fatliquoring agent increased by 40.3%and 31.5%respectively;and the absorptivity of anionic dyes increased by 44.7%and 16.1%.In addition,HHTTEGDE-CP-tanned leather showed a good inhibitory effect on Escherichia coli and Staphylococcus aureus,and the inhibition rates were 99.93%and 99.99%,respectively.HHTT-EGDE-CP-tanned leather was stable in 90 days of storage experiments and still showed good inhibition effects on Escherichia coli,Staphylococcus aureus,Aspergillus flavus,and Aspergillus niger after fatliquoring and dyeing processes.In addition,the preparation process in the HHTT-EGDE-CP tanning agent does not use toxic reagents and highly toxic solvents,and the tannery wastewater of HHTT-EGDE-CP tanning agent has better degradability(BOD5/CODCr=0.41±0.01),which is more eco-friendly than the F-90 and TWS tanning agents.(4)The mechanisms of action of DCST-EGDE,CP-EGDE,and HHTTEGDE-CP on collagen fibers were studied and clarified.This work takes collagenI and raw hide powders as the research object and explores the interaction between three tanning agents and collagen-I and raw hide powders,as well as the relationship between the pI of leather tanned with different tanning agents and their absorption effect on anionic wet finishing materials,thus revealing the following mechanism:a.The three tanning agents can penetrate into the threedimensional network structure of raw hide,disperse between collagen fibers,and use their respective active functional groups(the active functional groups of DCST-EGDE are aldehyde groups and epoxy groups;the active functional groups of CP-EGDE and HHTT-EGDE-CP are epoxy groups)to form covalent crosslinks with the free amino groups of collagen fibers without destroying the triple helix structure of collagen fibers.b.The tanning agents with high pI(CP-EGDE and HHTT-EGDE-CP)can compensate crust leather for the depressed positive charge due to the consumption of free amino groups on collagen fibers by tanning agents,thereby improving the absorption effect of crust leather on anionic wet finishing materials.