Characterization of Wattle Tannin Based Adhesives for Tanzania

Abstract

Wattle tannin-based urea formaldehyde (UF) adhesives for exterior grade plywood panels were formulated and tested. Laboratory work was conducted in Canada and plant trials in Tanzania. Tests showed that tannin-based adhesives gave quite good plywood which met the Canadian Standard Association (CSA) exterior grade requirements under press conditions similar to those currently employed for UF (control) adhesives. Formulations containing formaldehyde and oil instead or paraformaldehyde were found to be slightly less reactive under mill conditions. For particleboard, hydrolysis of the tannin was required for producing adhesive with acceptable viscosity and «pot life» for commercial applications. The hydrolysis or tannin also improved mechanical properties of particleboard. Relatively long press cycles or high press temperatures were required to produce particleboard panels with aged modulus or rupture (MOR) strength above the CSA exterior grade requirements. Prior to particleboard making, size exclusion chromatography (GPC) and differential scanning calorimetry (DSC) were performed on tannin, hydrolysed tannin, tannin-UF adhesive and commercial PF and UF adhesives. GPC indicated, possibly due to aggregation, that the tannin average molecular weight increased upon heating in the presence of alkali. The thermal or kinetic cure characteristics of the tannin-based adhesives, which are activation energy, enthalpy of cure and reaction order, were determined to be comparable with those of UF adhesives. Tannin hydrolysis lowered the activation energy for cure and this resulted in lower press times, as shown in test results