Preparation,Properties And Tanning Mechanism Of Sulfonated Calixarenes

The chrome tannage is very important in tanning process due to its outstanding tanning properties.However,the utilization rate of chrome tanning agent is low in the traditional chrome tanning process,leading to a large amount of Cr(?)remaining in the effluent which can easily be oxidized into carcinogenic Cr(VI)under certain conditions.Meanwhile,as chrome is a scarce resource in China,the development of chromium-free or chrome-less tanning agent is a hot research area in leather industry.The structure of water-soluble calixarenes is similar to that of commonly used syntan.Both of them contain large amounts of phenolic hydroxyl,sulfonic acid,and carboxyl groups,which can form hydrogen bonds,electrovalent bonds with amino,hydroxyl and peptide groups on leather collagen.The calixarenes can also coordinate with metallic ions and its cavity morphology facilitate?-?stacking interaction with metal ions.In this study,three kinds of sulfonated calixarenes with different structures were designed and synthesized,including tetrasulfonic-tetrahydroxy calix[4]aren(SCA),sulfonated tetramethylene tetraphenyl calix[4]resorcinarene(STTCR)and sulfonated tetraphenyl calix[4]resorcinarene(STCR).Their structures were characterized by FT-IR and ~1H NMR.Taking shrinkage temperature as indicator,the tannage dosage,the permeation pH and the combination pH as factors,three sulfonated calixarenes were applied to goat skin tanning process respectively to obtain an optimum tanning process.The relationship between the structure and properties,as well as the tanning mechanism,was studied and discussed.The research results are shown as follows:(1)The p-tert-butylcalix[4]arene was synthesized by one-step method using p-tert-butylphenol and formaldehyde,and the concentrated sulfuric acid was used as sulfonating agent to prepare SCA.The sulfonation degree of SCA was the highest when the molar ratio of calixarene to concentrated sulfuric acid was 1to 72 under 90?for 4 h.The application results showed that the shrinkage temperature of the crust was 65.1?after tanned by SCA,and the physical and mechanical properties and the thickening rate were better than that of the conventional chrome tanned leather.(2)Calix[4]resorcinarene was synthesized from resorcinol and benzaldehyde catalyzed by concentrated hydrochloric acid,and sodium sulfite was used as sulfonating agent to prepare STTCR.The yield of STTCR was the highest when the molar ratio of calix[4]resorcinarene and sodium sulfite was 1 to 4 under 80?,while the sulfonation time was 4 h.The structure of calix[4]resorcinarene and STTCR were characterized by FT-IR and ~1H NMR.Application results showed that the shrinkage temperature of the crust was 66.9?after the tanned by STTCR,and the physical and mechanical properties and the thickening rate were better than the conventional chrome tanned leather.(3)Using concentrated sulfuric acid as sulfonating agent,the sulfonation degree of STCR was the highest when the molar ratio of calix[4]resorcinarene and concentrated sulfuric acid was 1 to 75 under 80?with a 3 h sulfonation time.Application results showed that the shrinkage temperature of the crust was77.2?after tanned by STCR,and the physical and mechanical properties and the thickening rate were better than that of the conventional chrome tanned leather.To increase the shrinkage temperature of the crust,the STCR and zirconium were combined for goat skin tanning.The shrinkage temperature of the leather tanned by 5%STCR and 5%zirconium tanning agent was 88.9?,and the thickening was obvious.Hence,this combination tanning system can be used as chrome-free tanning agent.(4)The tanning properties of three sulfonated calixarenes were compared by tanning the different part of the same goat skin.In this process,shrinkage temperature,physical and mechanical properties of the crust were used as tanning property indicator.The structure-effect relationship of three sulfonated calixarenes was studied.Application results showed that larger tannage molecule and more phenolic hydroxyl groups endow better tanning property.The best tanning property was observed when the sulfonic acid group was attached to the side chain of calixarene,followed by that the sulfonic acid group was attached to the calixarene by the methylene group,while the worst tanning property was observed when the sulfonic acid group was attached to the calixarene main chain.That is the order of the tanning performance is STCR>STTCR>SCA.The SEM and AFM observations of the grain surface demonstrated that the SCA and STTCR tanned leathers were rough while the STCR tanned leather was uniform.The EDS and fluorescence spectrums analysis of the tanned leather showed that the three sulfonated calixarene tannages were uniformly distributed in the longitudinal section of the leather.Furthermore,the biodegradation of STCR was superior to the other two sulfonated calixarenes and conventional chrome tanning,indicated by wastewater treatment assay.(5)Polycaprolactam and polyvinyl alcohol were selected as the collagen model analogue to interact with three sulfonated calixarenes.The interaction mode and degree between the tanning agent and the collagen were investigated by using binding amount as an index.The results showed that the amino group on the collagen was the main binding group with the sulfonated calixarene tannage,following is the hydroxyl group and the peptide group.Collagen type I and raw skin were used to reacted with three sulfonated calixarenes respectively.The results showed that synthesized sulfonated calixarenes could penetrate and distribute in college fibers evenly,whose phenolic hydroxyl and sulfonic acid groups was hydrogen and ionized bonded with the collagen amino group on collagen.Hydrogen bonds and electrovalent bonds can be formed between sulfonated calixarenes and skin collagen,however,the collagen fibers and the triple helix structure of collagen were not destroyed after tanning.Based on the above studies,we have proposed the tanning mechanism model between sulfonated calixarenes and collagen.