PHYSICOCHEMICAL AND STRUCTURAL CHARACTERIZATION OF ALKALI LIGNINS FROM ABACA FIBER

Abaca fibre was treated with 1, 2.5, and 5% sodium hydroxide at 25, 35 , and 50 degrees C for 0.5-5 h. The dissolved alkali lignins LA were s eparated from the solubilized polysaccharides using a two step precipi tation method. The chemical and structural compositions of the pure li gnin fractions LA were determined by UV, GPC, FT-IR C-13-NMR spectrosc opy and nitrobenzene oxidation. All of the lignin fractions LA are fre e of associated polysaccharides. Their molecular-average weights range d from 1960 to 2640. The results obtained by alkaline nitrobenzene oxi dation showed that all the lignin fractions LA contain large amounts o f non-condensed syringyl units, together with fewer non-condensed guai acyl and p-hydroxyphenyl units. beta-O-4 and beta-5 ether bonds are fo und to be the major linkages between the lignin structural units. The less common beta-beta, beta-5, and 5-5' carbon-carbon linkages are als o identified to be present between the lignin structural units. p-Coum aric acid and ferulic acid are linked to lignin molecules by ester and ether bonds, respectively.