Adhesives formulated from bark bio-crude and phenol formaldehyde resole

Rather than replacing phenol in the synthesis of PF resole, bark-derived bio-crudes were used as an adhesive constituent by simply formulation with neat PF adhesive in this study. Bio-crude derived from white birch bark (WBB)—a typical hardwood bark and white spruce bark (WSP)—a typical softwood bark were blended with neat PF adhesive to prepare bark bio-crude formulated bio-phenol formaldehyde (BPF) adhesive (designated as bio-crude formulated BPF adhesive). It was discovered that the viable formulation ratio between bark bio-crude and neat PF adhesive depended on the bark species. WBB bio-crude could be formulated with neat PF adhesive at a formulation ratio of 50:50 (wt/wt), while WSP bio-crude can be formulated with neat PF adhesive at a higher formulation ratio up to 75:25 (wt/wt). The peak curing temperatures of the bio-crude formulated BPF adhesives were lower than that of neat PF adhesive. Activation energy for the curing of WBB bio-crude formulated BPF adhesives was higher than that for the curing of neat PF adhesive, while WSP bio-crude formulated BPF adhesive required less activation energy for curing than neat PF adhesive. Condensation between bio-crude and neat PF adhesive occured during the pre-curing process at 125 °C and was confirmed by FTIR spectra. Interestingly, the introduction of bio-crude enhanced the thermal stability of the bio-crude formulated BPF adhesives at low temperatures, but as expected, thermal stability of the bio-crude formulated BPF adhesives reduced at higher temperatures and a higher formulation ratio led to lower thermal stability. At a same formulation ratio, WSP bio-crude formulated BPF adhesives showed better thermal stability than WBB bio-crude. More interestingly, the free formaldehyde emission level of 3-ply plywood bonded by bio-crude formulated BPF adhesives, in particular with the WSP bio-crude, was lower than that of the neat PF adhesive bonded plywood. Free formaldehyde emission levels from 3-ply plywood bonded with WSP bio-crude formulated BPF adhesives at the formulation ratios of 50:50 (wt/wt) and 75:25 (wt/wt) reached JIS F*** level. However, bio-crude formulated BPF adhesives reduced the tension shear strength of bonded 3-ply plywood, in particular at higher formulation ratios. At the same bio-crude formulation ratio, WSP bio-crude formulated BPF adhesives gave better tension shear strength for 3-ply plywood than the WBB bio-crude formulated BPF adhesives. Nevertheless, 3-ply plywoods bonded with bark bio-crude formulated BPF adhesives at a ratio up to 50:50 (wt/wt) still met the JIS standards with respect to tension shear strength. © 2015 Elsevier B.V.