Effects of bark extraction before liquefaction and liquid oil fractionation after liquefaction on bark-based phenol formaldehyde resoles

• Bark extraction and HTL-oil fractionation led to more free formaldehyde in BPF resoles.
• HTL-oil introduction into PF resole promoted the curing of BPF resole.
• All the BPF resoles met the JIS standard as plywood adhesives.
• Bark extraction reduced the water resistance of BPF resole.
• HTL-oil fractionation improved the water resistance of BPF resole.

Liquid product from white birch bark liquefaction in water/ethanol (50:50, v/v) mixture was successfully applied in substituting 50 wt% of phenol in the synthesis of bark based phenol formaldehyde (BPF) resole (i.e., 50% HTL-oil BPF resole). Besides, eHTL-oil from liquefaction of the bark extracted in 70 vol% aqueous acetone, and fHTL-oil obtained after HTL-oil fractionation in water, were also used to substitute 50% of phenol for BPF resoles synthesis. The results showed that all the three BPF resoles contained more free formaldehyde than the neat PF resole, and bark extraction before liquefaction and HTL-oil fractionation after liquefaction led to higher free formaldehyde contents in the resulted 50% eHTL-oil BPF resole and 50% fHTL-oil BPF resole. Furthermore, bark extraction before liquefaction retarded the curing of the resulted 50% eHTL-oil BPF resole, while HTL-oil fractionation after liquefaction slightly promoted the curing of the resulted 50% eHTL-oil BPF resole. All the three BPF resoles displayed less thermal stability than the neat PF resole, but the effects of bark extraction before liquefaction and HTL-oil fractionation after liquefaction on the thermal stability of the resulted BPF resoles were negligible. All the three BPF resoles could meet the bond strength requirements as adhesives for plywood in accordance to the JIS standard. Bark extraction before liquefaction led to less water resistance for the resulted 50% eHTL-oil BPF resole, while HTL-oil fractionation after liquefaction improved the water resistance of the resulted 50% fHTL-oil BPF resole.