Research On Lignin Structural Characterization And Degumming Process Of Jute And Kenaf Fiber

Jute and kenaf are renewable, cheap, and easily grown annual plant. The lignin content of jute and kenaf fiber is high which resulted in the coarse and rigidity handle of the fiber, therefore, jute and kenaf fiber are mainly used to make the low grade products such as package fabric and bag. At the same time, the existence of lignin in jute and kenaf fiber have more disadvantages to the performances of fiber obtained after fiber preparation and to the fiber post-processing. Actually, the predominant task aiming at getting finer and soft fiber in degumming process is to remove lignin with as less as possible damage to the fiber cellulose. In the textile industry, jute and kenaf fiber are processed as the same fiber.Lignin is a three-dimensional polymer that basically consists of numerous, various substituted phenylpropane units linked by carbon-carbon and ether bonds. This skeleton bears a wide variety of functional groups affording reactive centers for chemical and biochemical modification. There are many kinds of function groups in the side chain of lignin. The changes of linkages among units and the differences of function groups in lignin structure will affect the mechanical character and chemical reactivity of fiber. Therefore, an understanding of the chemical composition of jute and kenaf fiber lignin gives insight into the preparation to get finer fibers and application of kenaf fiber as material for clothing and other high grade products.In this paper, milled wood lignin (MWL) were isolated and purified from jute and kenaf fiber, the lignin obtained was characterized and compared. Linkage style among lignin structure units and content of functional group were quantitatively analyzed for further research on the chemical structure of lignin between jute and kenaf fiber, which provided reliable theoretical basis for illuminating the various reaction mechanisms, and furthermore, kenaf fiber was degummed by the biological enzyme and chemical treatment, the effect of lignin removal was evaluated and the changes of lignin structure during the degumming process was explained. Results were illustrated as follows: (1) Milled wood lignin (MWL) of jute and kenaf fiber were characterized by elemental analysis, FTIR, 1H-NMR and 13C-NMR spectroscopy. The C9 formula were calculated for MWL from jute and kenaf fiber as C9H9.27O3.52(OCH3)1.26 and C9H9.32O3.69(OCH3)1.30. The spectra of FTIR,1H-NMR and 13C-NMR indicated the jute and kenaf fiber lignin to be of the G/S type with high proportion of syringyl (S) unit. It is evident that theβ-O-4 structures mainly linkaged in the MWL of jute and kenaf fiber which contain more erythro stereochemistry type than thero stereochemistry type. In general, the characteristics of lignin of jute and kenaf fiber were similar to that of hardwood. Comparing with lignin of jute fiber, lignin of kenaf fiber contained more condensedβ-5 structures.(2) The chemical method and instrument analysis method were adopted in the determination of three types basic lignin unit ratio. The HPLC results of the products of alkalinous nitrobenzene oxidation about the jute and kenaf fiber showed a predominance of syringyl(S) units over guaiacyl(G), the uncondensed lignin of jute contained 7.56% uncondensed guaiacyl units and 92.44% uncondensed syringyl units, the uncondensed S/G value was 12.22; the uncondensed lignin of kenaf contained 10.6% uncondensed guaiacyl units and 89.29% uncondensed syringyl units, and few 4-hydroxyphenyl units were found in lignin of kenaf fiber, the uncondesnde S/G value was 8.4; Furthermore, the MWL of jute and kenaf fiber were analyzed by Py-GC-MS, the molar ratio of H:G:S in jute lignin was 0:26.08:73.92, the S/G value was 2.835, the molar ratio of H:G:S in kenaf lignin was 4.59:34.11: 61.30, the S/G value was 1.797. It was concluded that the lignin structure in kenaf fiber was more difficult for delignification in degumming process than jute fiber.(3) The structure of MWL from jute and kenaf fiber were studied by using XPS. The binding energy of C1s was assigned to 284.6 eV(C-C or C=C),285.2 eV (C-H), 286.5 eV (C-O) and 289.27 eV (O-C=O). The binding energy of O1s was assigned to 532.3 eV(O1) and 533.3 eV (O2). Comparing with lignin of jute fiber, the lignin of kenaf fiber contained more carbon atom in C-C or C=C bonds, less carbon atom in C-H bonds and ether bonds, more carbon atom in carboxyl group. The ratio O1/O2 of jute fiber lignin was higher than kenaf fiber lignin indicating more single O bond in lignin of jute fiber.(4) Laccase and laccase/mediator system were employed in kenaf fiber degumming process. The effects of single laccase and combined with ABTS were studied. The effects on the properties of the degummed fiber such as fiber fineness, residual lignin content and tensile property were tested by the designed experiments. Meanwhile the degumming effects were analyzed by XRD, IR and HPLC of the products of alkalinous nitrobenzene oxidation about the degummed kenaf fiber. The results showed that the effect of laccase used in degumming of kenaf fiber was not obvious, and its effect could be strengthened with the existence ABTS as mediator and the process with pre-treatment. IR indicated that the intensity of characteristic peak of lignin and hemicellulose decreased, while the intensity of characteristic peak of cellulose increased. The HPLC results showed that it helped to remove the uncondensed G and S units of lignin by using laccase/ABTS degumming process.(5) Seven chemical treatments were employed in kenaf degumming process. The properties of degummed fiber such as fiber fineness, residual lignin content were tested, and the degumming effects were analyzed by XRD, IR and HPLC of the products of alkalinous nitrobenzene oxidation about the degummed kenaf fiber. The crystallinity of kenaf fiber increased after being treated. Adding the strong oxidizing agents including potassium peroxymonosulfate and peracetic acid (plan CT2 and CR2) in the later process of caustic soda and sodium percarbonate methods (plan CT1 and CR1), then after scouring using the sulfate method (plan CT3 and CR3), the relative integral area value Aj/As of the characteristic peek of the cellulose in IR gradually increased, while the sum of those of the lignin and hemicellulose gradually decreased, and the content of lignin gradually decreased, while the finess gradually increased. The results indicated that the seven chemical treatments adopted in degumming had obvious effects. As a new oxidant, the mixed liquid of potassium peroxymonosulfate and peracetic acid was effective on delignification. The HPLC results showed that it helped to remove the uncondensed G and H units of lignin by using plan CT1, it helped to remove the uncondensed G unit of lignin by using plan CT2, and it helped to remove the uncondensed H and S unit of lignin by using plan CT3.