Effect of Extraction Solvent on Tannin-Formaldehyde Adhesives for Plywood Production

Abstract

Pine bark is a good source of natural polyphenolic compounds for wood adhesives. The objective of this study was to obtain the most suitable solvent for extracting pine tannins in the preparation of tannin-formaldehyde plywood adhesives. Aqueous acetone, aqueous ethanol, aqueous NaOH and water as solvents were used to obtain crude tannins from pine bark. The tannin content, sugar content and Stiasny number of the extracts were determined. Using the extracts from the solvent extractions, synthesis of tannin-formaldehyde resin was carried out. Plywood panels were made using the synthesized resins and the quality of the resins in plywood application determined. The quality of tannin-formaldehyde resins produced from the tannins were generally in close agreement with the chemical characteristics of the extracts obtained from the various solvent extractions. The aqueous NaOH extraction although gave very high tannin yield (16.1%), its associated high sugar content (33.8%) and very low Stiasny number (49) resulted in poor quality resin. Similarly, although aqueous extraction gave a very high Stiasny number (91), its low tannin yield (8.7%) might not be of commercial interest. The extraction process that gave a high tannin yield (12.9%) and a very good Stiasny number (81.5) with a corresponding good quality resin (shear strength = 1.9 MPa, 22% delamination) was found for 60% aqueous ethanol extraction.