Abstract

Phenol-Formaldehyde based resins are widely used in gluing wood panels. Lignin can act as a substitute for phenols in these glues, thereby reducing costs to manufacture these adhesives. Enzymes like laccases are known to modify lignin to make adhesives and can serve as phenol replacements, and this was our objective in using Kraft lignin (KL). Laccases were obtained from 2 fungal isolates, Botryosphaeria rhodina and Trametes svaveolens grown at 28°C/180 rpm. B. rhodina was cultured for 5 days on minimal salts media containing laccase inducers (veratryl alcohol, copper) and glucose (Br-Lac-1), and castorbean meal (Br-Lac-2). T. svaveolens was grown on media containing glucose, yeast extract, peptone, MgSO4 and copper for 10 days (Ts-Lac). The supernatant recovered after removal of fungus was used as laccase source to modify KL. The reaction mixture contained 10 Units laccases (Br-Lac-1, Br-Lac-2), KL (0.5g) in a suspension of 10 mL citrate-phosphate buffer (pH 4.5), and was incubated at 40°C for 2h and 4h with oxygen sparging. In a second experiment, 50 Units laccases (Br-Lac-1, Br-Lac-2, Ts-Lac) was used, but at pH 6.5 for 4h. The modified KL (MKL) was then precipitated until pH 2.0 with 3% H2SO4, and characterized by FT-IR spectroscopy. FT-IR showed that functional groups: methyl, primary alcohols and ether linkages (2935 nm, 1044 nm and 1120 nm, respectively), increased in the aliphatic chain for all enzyme treatments, showing polymerization had occurred in MKL. In 2h reactions with 10-Units laccase, the signals of functional groups increased lesser than in 4h reactions. Higher signal intensities of these groups, and increased degrees of polymerization (DP), resulted in 4h reactions using 50-Units laccase. Differences between the three enzymes were minimal. Highest DP occurred with Br-Lac-2 and Ts-Lac. In all experiments the signals observed at 1510 nm was higher in MKL than all controls. The modified lignin has potential application for gluing wood panels.