Synthesis Of N-Hydroxymethyl-triazole-thione Phthalic Acid Modifier And Its Interaction With Collagen And Chromium Complex

The pollution from the chrome tanning effluent has become increasingly a serious problem that faces leather industry.To solve this problem,one of the routes with good potential for success is to treat collagen with special modifier to implement high exhaustion chrome tannage by simply providing additional cross-linking sites or binding sites for chromium.On the basis of these principles and requirements,in this paper,a new modifier has been synthesized from pyromellitic dianhydride and thiosemicarbazide by the nucleophilic addition of anhydride,the cyclization of acyl thiosemicarbazide in alkaline solution,and the N-hydroxymethylation of the triazole-thione.Its synthesis and structure,its interaction with collagen and chromium complexes,as well as its effect on chrome tanning properties,have been studied.The nucleophilic addition of thiosemicarbazide(TSC)to pyromellitic dianhydride(PMD)in DMF and triethylamine(TEA)produces N-thiouriedopyromellitamic acid(TUPMA)which consists of two isomers.Its yield is influenced by the molar ratio of PMDA to TSC and TEA,the amount of DMF,the temperature and the duration of the reaction.A higher yield(92%)can be obtained when the molar ratio of PMDA to TSC and TEA is 1:2.18:2.4,and the reaction is carried out in DMF 180ml/40g reactant at 5℃-15℃for 48h.The percentage of its main component is more than 76%.Its melting point is at 233.0℃.Its structure, 2,4-di-(thioureido-amino-formoyl)-1,5-phthalic acid,has been confirmed by MS spectra,¹HNMR spectra,IR spectra and elemental analysis.The cyclization of the main component of TUPMA in aqueous NaOH forms 2,5-di-(1,2,4-triazole-5-thione)-1,4-phthalic acid(DTPA)which has a melting point at 165.3℃.The structure of DTPA has been confirmed by MS spectra,¹HNMR spectra and IR spectra.Its yield is influenced by the concentration of TUPMA and NaOH,the temperature and the duration of the reaction.A higher yield(about 80%) can be obtained when 0.20-0.25 mol/1 of TUPMA is refluxed in 0.5 mol/l of NaOH solution at 75℃for 8h. The N-hydroxymethylation of DTPA with formaldehyde in aqueous solution gives 2,4-di-(1-hydroxymethyl-1,2,4-triazole-5-thione)-1,5-phthalic acid(DHTPA) which has a melting point at 172.8℃.The structure of DHTPA has been confirmed by MS spectra,¹HNMR spectra and IR spectra.Whether the molar ratio of DTPA to formaldehyde is 1:2.2 or 1:4.4,a higher yield(74-78%)can be obtained when the reaction is carded out in pH8.2-8.5 at 50℃for 24h.The synthesized modifier(DHTPA)has been reacted with collagen,deaminated collagen,deguanidinated collagen,esterified collagen,deaminated and deguanidinated collagen,and glycine.The interaction of DHTPA with collagen has been studied by shrink temperature determination,isoelectric point determination, acid-base titration,amino acid analysis,DSC analysis,capillary electrophoresis,IR spectra and elemental analysis.It has been found that DHTAP can react with collagen in a wide range of pH value and temperature to increase the shrink temperature and the carboxyl groups of collagen.The increase in shrink temperature and carboxyl groups is influenced by the pH value,temperature,DHTPA dosage,and the duration of reaction.An increase in shrinkage temperature from 53℃to 78℃and an increase in carboxyl groups from 1.27 mmol/g collagen to 1.85 mmol/g collagen can be obtained when the reaction is carried out in pH8.0 with 7%of DHTPA at 33℃for 24 h.The amino groups of collagen are main covalent cross-linking sites and binding sites for DHTPA,and play a dependent role in the increase of shrinkage temperature and carboxyl groups.The guanidino groups and carboxyl groups of collagen are neither covalent cross-linking sites nor binding sites for DHTPA in the neighborhood of pH8.The imidazole groups and phenol groups in collagen can form irreversible covalent binding with DHTPA,which could not be hydrolyzed by acid.However, they scarcely attribute to the increase in shrinkage temperature and carboxyt groups because they are very few in quantity.Each DHTPA molecule can either combine with one amino group by its one N-hydroxymethyl group(binding on single site of collagen),or combine with two amino groups by its two N-hydroxymethyl groups (binding on two sites of collagen).In the present experiment,72.4%of DHTPA has been bound on the collagen by two sites,and 27.6%of DHTPA has been bound on the collagen by one site.The 0.37 mmol of amino groups has been modified,and 0.58 mmol of carboxyl groups has been introduced in 1 g of collagen.The isoelectric point of collagen has been reduced from 5.1 to 4.0.Kinetics studies show that the binding of DHTPA on amino groups of collagen could be described as a second order reaction.The rate constant increases with the increase in pH value and temperature.Activation energy of the reaction of DHTPA with amino groups of collagen is 39.307 kJ/mol at pH8.0,and is lower than the usual. Thus the reaction is easy to take place.The synthesized modifier has been reacted with chromium sulfate complexes, and their interaction has been studied by acid-base titration,ion exchange analysis, DSC analysis,UV spectra and IR spectra.The results show that under certain conditions carboxyl groups of DHTPA can enter into the complexes and take the place of H₂O in the complexes to coordinate with the chromium ions.When the DHTPA dosage is more than a 0.06/1 molar ratio of DHTPA to Cr³⁺,the coordination polymerization between the DHTPA and the complexes would occur,and an insoluble co-polymer with a pale blue green color would be obtained after settlement for a long term.Collagen has been treated with DHTPA and then tanned with chromium complexes,and the effect of DHTPA on chrome tanning properties has been investigated by determining the chrome absorptivity and fixation as well as the physical properties of the crust.The results show that the treatment causes significant increases in the chrome absorptivity and fixation as well as shrink temperature during subsequent chrome tanning.In conventional chrome tanning process,the chrome absorptivity can be increased from 72.70%to 98.91%,and the chrome fixation can be increased from 68.10%to 86.26%by treating collagen with 7%DHTPA.In two-stage chrome tanning process,the chrome absorptivity can be increased from 86.80%to 99.09%,and the chrome fixation can be increased from 76.07%to 90.21%by the same treatment.Because of the enhancement in chromium absorptivity and fixation,the dosage of normal chrome powder can be reduced from 6%to 4%in conventional process,and can be reduced from 6%to 3%in two-stage chrome tanning process,and thus the chrome absorptivity and fixation can be enhanced further.The treatment causes an increase in chrome content of wetblue but could not change the distribution of chrome in various lays.The physical properties of crust leather remain unchanged on the whole after the treatment except for a slight reduction in extensions.The free formaldehyde in crust leather is not nearly increased by the treatment.DHTPA as well as its intermediate TUPMA can also be used for pretreating collagen before aluminium tanning.The shrink temperature and the dye-bath stability as well as the strength of the leather can be significantly improved when collagen is pretreated with DHTPA or a collaboration of TUPMA with glyoxal.Inhibitory and bactericidal activities of DHTPA have been tested against Staphylococcus aureus and Escherichia coli.To a certain degree,DHTPA shows inhibitory and bactericidal activities against the selected microorganisms.The minimal inhibitory concentration and minimal bactericidal concentration of DHTPA for Staphylococcus aureus is respectively 0.25%and 2%,and for Escherichia coli is respectively 0.5%and 4%.All the above results indicate that the chromium absorptivity and fixation can be significantly enhanced by applying DHTPA to the pretreatment of collagen. Therefore it is possible that the chrome tanning effluent can be directly discharged legally without any treatment after dilution with waste water from other processes if the pretreatment with DHTPA or its intermediate is used for two-stage and less chrome tanning processes.The DHTPA as well as its intermediate has a good potential for applying in leather industry.