Isolation, characterization and preparation of spent coffee ground tannin as eco-friendly wood adhesives

Abstract

The objectives of this study are to investigate the recovery and application of spent coffee ground (SCG) tannin as wood adhesive, meanwhile to discover the properties of tannin / lignin adhesive, which are currently underexplored. The modelling and simulation of analyses revealed that the optimum extraction conditions were; 5 % sodium hydroxide concentration, 100 °C extraction temperature, 30 minutes of extraction time and 8.23 liquid to solid ratio, so that tannin with a high extraction yield (21.02 %) and high reactivity (29.69 %) could be recovered. The structural characterization by attenuated total reflectance-Fourier transform infrared (ATR-FTIR) and solid state cross polarisation-magic angle spinning carbon-13 nuclear magnetic resonance (CP-MAS 13C NMR) as well as matrix-assisted laser desorption / ionization time-of-flight (MALDI-TOF) mass spectrometric analyses concurrently agreed that the SCG tannins comprised of condensed (catechin, fisetinidin, gallocatechin and gallocatechin-3-O-gallate) and hydrolyzable (chebulic acid) tannins with a degree of polymerization up to dimer. On the other hand, the development of tannin / lignin wood adhesives based on the extracted SCG tannin and kraft lignin demonstrated a decreasing trend in solid content, density and viscosity as well as an increasing trend in hydrogen ion concentration (pH) and thermal stability with the elevation of SCG tannin content in the formulations. The adhesion of tannin / lignin adhesives primarily relied on the dispersion forces and to a lesser extent on polar forces. By formulating the adhesive at 60 % of SCG tannin and 40 % of kraft lignin, the bonding effectiveness, namely work of cohesion and work of adhesion were achieved at 65.12 mN/m and 64.62 mN/m respectively. These findings were equivalent to 55 % and 70 % of the work of cohesion and work of adhesion delivered by the commercial wood adhesive. In conclusion, SCG has proven its potential as a renewable tannin resource. Structural information and the properties of the developed adhesive confirmed the potential use of extracted SCG tannins for adhesive applications.