Extraction,Purification,Modification And Tanning Mechanism Of Coriaria Nepalensis Bark Tannin

As an important natural product,tannin has been widely used in industrial production and daily life for its functional diversity.During tanning process in leather making,plant extracts which are rich in tannins are commonly used as renewable vegetable tanning agents,occupy a vital position in the leather industry.However,due to the shortage of vegetable tanning agents in China,the development of vegetable tanning in leather industry has been limited to a great extent.Therefore,it is particularly important to find more exploitable vegetable tanning agents.Coriaria nepalensis Wall.is a deciduous shrub of the genus Coriaria and the family Coriariaceae.It has strong environmental adaptability,rapid growth,rich natural resources,and is widely distributed in China.Coriaria nepalensis contains a variety of chemical components,among which its abundant tannins are in urgent need to be investigated.In this paper,Coraria nepalensis bark(CNB)was used as the raw material to extract the tannins,and the tannin extracts were used as the vegetable tanning agent for leather tanning,and the preliminary tanning effect was researched.In order to clarify the relationship between the chemical structure of tannins and the tanning effect,the extracted tannins from CNB were purified to analyze its structural characteristics.Subsequently,in order to improve the tanning effect,the acylation modification based on gallic acid was further explored,and the performance of acylation modified gallic acid crosslinked gelatin was studied.On the basis of gallic acid modification theory,the acylation modification of CNB tannins was studied,and the acylation modified tannin was used in tanning sheepskin clothing leather,with discussing the tanning effect and mechanism.The results of this study are as follows:(1)The response surface method was used to optimize the tannin extraction conditions from the CNB on the basis of the single factor experiment,and the optimal extraction condition was obtained as follows:Extraction solvent was sodium hydroxide with a concentration of 0.22%,and liquid-solid ratio was 23 mL·g-1.Under the extraction temperature of 82?,and lasting for 63 min,the extraction yield of CNB tannin can reach 16.02%.(2)The macroporous adsorption resin was used to purify the CNB tannin extracts.Among different types of resins,D101 macroporous adsorption resin reflected the appropriate adsorption and desorption effect toward CNB tannin.The optimal purification condition was as follows:The concentration,pH,injection flow rate and injection volume of tannin solution were 2.41 mg·mL-1,3,2 BV·h-1 and 2.4 BV,individually.Ethanol(70%)was used as eluent with the eluent flow rate of 3 BV·h-1,and the volume of 6 BV.The effect of purification effect was so obvious that the purity of CNB tannin was 70.6%.The relative molecular mass of CNB tannin was between 424 Da and 1464 Da.Triphenol groups,ester groups and polyols can also be detected in its structure,indicating it is a hydrolysable tannin that mainly composed of galloyl groups as structural units.Further research found that the main structural components of CNB tannin include Tri-O-gallolylglucose,Tetra-O-gallolylglucose,Penta-O-gallolylglucose and Hexa-O-gallolylglucose.(3)After protecting the carboxyl group of gallic acid,it can be acylated under the action of hexamethylenetetramine(HMTA)and trifluoroacetic acid(TFA)to introduce an aldehyde group into its structure.The optimal modification condition was as follows:Added 10 mmol TFA and 2 mmol HMTA into 1 mmol methyl gallate,and heated at 90? for 80 min.Under this condition,the yield of the acylation reaction can reach to 47%.The modified gallic acid crosslinked gelatin film was prepared with modified gallic acid as the crosslinking agent.Compared with the blank gelatin film and the gallic acid composite gelatin film,the tensile strength of modified gallic acid crosslinked gelatin film was increased by 175%and 47%,respectively,and the elongation at break was increased by 182%and 61%,respectively.It can be assigned to the modified gallic acid structure,which can react with the gelatin molecule to form a stable Schiff base structure.Under the force of hydrogen bond and covalent bond,the crosslinking degree of modified gelatin film can be greatly increased,as well as the performance of the film,which lays a theoretical foundation for the subsequent tannin modification.(4)On the basis of the modification theory of gallic acid,the acylated modified tannin with an aldehyde group content of 2.33 mmol·g-1 was prepared by adding CNB tannin to the reaction system of HMTA and TFA,and reacting at 100? for 1.5 h.The results showed that the relative molecular masses of modified tannins were mainly distributed between 653 and 1675 Da,and the inherent phenolic hydroxyl structure was not destroyed.In particular,the modified tannin was used for sheepskin clothing leather tanning.Under the condition of 20%tanning agent dosage,the penetration pH of 4.5,and the combination pH of 4.0,the shrinkage temperature of tanned leather reached 85.3?.The BOD5 and CODcr of the waste liquor after tanning with acylated modified tannins was 2780 mg·L-1 and 13140 mg·L-1,respectively,which were all reduced compared to the waste liquid produced after unmodified tannin tanning.It was also noticed that after tanning with modified tanning agent,the leather becomes firmer,with a thickness increase of 71.3%,and a softness of 5.7 mm.The physical and mechanical properties of the leather have been significantly enhanced.The tear strength,tensile strength and elongation at break reached 47.7 N·mm-1,10.2 MPa and 97.9%,separately.(5)Taking gallic acid as the structural unit of CNB tannin,modified gallic acid as the structural unit of modified CNB tannin,and glycine as the structural unit of leather collagen fiber,a tanning mechanism model of modified CNB tannin and leather fiber was established.By comparing the difference of the NMR spectra before and after the mixing of gallic acid and glycine,and the 1H NMR spectra tested at different temperature,hydrogen bond was detected in the mixture of gallic acid and glycine,which can further confirm the hydrogen bond crosslinking theory between the tannins in vegetable tanning agents and leather collagen fibers during the tanning process.Further research found that the carbonyl group was more likely to form hydrogen bonds with the amino group in the glycine structure.It showed that when CNB tannin extract was used as a tanning agent for leather tanning,the carbonyl group in the tannin structure was more likely to generate hydrogen bonds with the amino group of the leather collagen fiber.Comparing the changes of NMR spectra before and after mixing modified gallic acid and glycine,as well as the 'H NMR spectra tested at different temperatures,it can be detected that not only hydrogen bonds were existed in the mixture of modified gallic acid and glycine,but also the covalent bonds formed by the reaction between the aldehyde group in modified gallic acid and the amino group in glycine.After which,the tanning mechanism of the modified CNB tannin was summarized.