Preliminary evaluation of an H 2SO 4-wood saccharification lignin for use in phenolic resins

Abstract

The solid residue from a two-stage, sulfuric acid-based wood hydrolysis process was tested for its utility in phenol-formaldehyde resins. It yielded between 5 and 60 weight % of the residue in the form of a homogeneous fraction when extracted with different organic solvents or alkali, and thus varied in relation to molecular weight. A 40% fraction soluble in aqueous ethanol was structurally more condensed than comparable hardwood lignin isolated by a kraft or organosolv process. This fraction proved to be relatively unreactive with formaldehyde and phenol, and this was attributed to its hardwood and chemically condensed nature. Phenolic resins were formulated at a 45% solids content in a two-stage resin cook procedure with >50% replacement of phenol by lignin. These resins were evaluated by thermal analysis with regard to cure, and by shear testing of laminated blocks with regard to dry strength. Results with the best resins revealed minor differences as compared to control phenol-formaldehyde resins in terms of viscosity, cure rate, activation energy of the curing reaction, and dry shear strength performance. Dry strength characteristics were found to be more variable than those obtained using phenol-formaldehyde resins, and network formation characteristics were inferior to other lignin products. The significance of these differences needs to be evaluated under commercially realistic conditions usually employed in plywood manufacturing, and in larger scale tests.