CHARACTERIZATION OF WATTLE TANNIN-BASED ADHESIVES FOR TANZANIA

Wattle tannin-based urea formaldehyde (UF) adhesives for exterior grad e plywood panels were formulated and tested. Laboratory work was condu cted in Canada and plant trials in Tanzania. Tests showed that tannin- based adhesives gave quite good plywood which mel the Canadian Standar d Association (CSA) exterior grade requirements under press conditions similar to those currently employed for UF (control) adhesives. Formu lations containing formaldehyde and oil instead of paraformaldehyde we re found to be slightly less reactive under mill conditions. For parti cleboard, hydrolysis of the tannin was required for producing adhesive with acceptable viscosity and ''pot lift'' for commercial application s. The hydrolysis of tannin also improved mechanical properties of par ticleboard. Relatively long press cycles or high press temperatures we re required to produce particleboard panels with aged modulus of ruptu re (MOR) strength above the CSA exterior grade requirements. Prior to particleboard malting, size exclusion chromatography (GPC) and differe ntial scanning calorimetry (DSC) were performed on tannin, hydrolysed tannin, tannin-UF adhesive and commercial PF and UF adhesives. GPC ind icated, 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 a re activation energy, enthalpy of cure and reaction order, were determ ined to be comparable with those of UF adhesives. Tannin hydrolysis lo wered the activation energy for cure and this resulted in lower press times, as shown in test results.