SYNTHESIS, CHARACTERIZATION AND PERFORMANCE OF BIO-PHENOL FORMALDEHYDE ADHESIVES FROM Detarium senegalense J.F. Gmel BARK IN PARTICLEBOARD PRODUCTION

Abstract

This study explored the production of phenolic bio-oil from the bark of Detarium senegalense (forest biomass) via the solovolysis liquefaction method. The bio-oil was used as a partial substitute of commercial phenol with bio-phenol: 25, 50 and 75 % bio-phenol, respectively, in the synthesis of bio-based phenol formaldehyde (BPF) resin. The performance of BPF resins was subsequently evaluated in the production of laboratory scale particleboard. Bio-oil with molecular weight (Mw = 990 g/mol and Mn = 310 g/mol) was obtained from direct liquefaction of Detarium senegalense bark, using 50/50 (v/v) co-solvent of ethanol/water as the liquefaction medium at 300 oC. The Functional group characterization and chemical compositional analysis of the obtained bio-oils confirmed the presence of primary phenolic compounds and their derivatives. The result of molecular weight suggested that hot-compressed ethanol, as liquefaction solvent, favoured lignin degradation into monomeric phenols as indicated by its narrow molecular weight distribution curve. The phenolic bio-oil was successfully used to partially substitute petroleum-based phenol up to 75 wt% in the synthesis of bio-oil phenol formaldehyde resins (BPF). All the synthesized BPF resins displayed similar physical and chemical properties to the commercial and laboratory synthesised pure (PF) resole resin. The viscosities of the 25 wt%, 50 wt% and 75 wt% BPF resins were 380 cP, 420 cP and 640 cP respectively and was higher than that of the commercial and laboratory synthesised pure PF resins which were 280 and 200 cP respectively. The molecular weight of the resins ranges between 1492- 1746 g/mol. The molecular weight of the BPF resins were all greater than those of the commercial and laboratory synthesised PF resins, the molecular weight distribution curves of the BPF resins were wider than that of the commercial and laboratory synthesised PF resin. The BPF resins exhibited different thermal stability and thermal degradation kinetics than commercial and laboratory synthesized pure PF resins. All the BPF resins were cured at temperatures between 144.5 oC and 157 oC. The resins containing bio-phenol replacement at 75 wt% have a higher curing temperatures at the different heating rates and in particular at 10 oC (157.0 oC) than the laboratory synthesised pure PF resins (152.2 oC) and commercial PF resin (150.0 oC). However, at bio-phenol replacement of 25 wt% and 50 wt%, their curing temperatures were 144.5 oC and 145.8 oC respectively and were remarkably lower than that of commercial and laboratory synthesised pure PF resins. The curing reaction for all the resole resins proved to be approximately 1st order, and the activation energies of the BPF resins at 25 wt% and 50 wt% bio-phenol replacement were 132.2 kJ/mol and 131.6 kJ/mol. These were comparable to that of the laboratory synthesised pure PF resin 131.0 kJ/mol but also remarkably lower than the commercial PF resin (138.1 kJ/mol) respectively. The result of the thermal degradation kinetics showed that the post-curing thermal stability of the 25 and 50 wt% BPF resins were similar to that of the commercial and laboratory synthesised pure PF resin but, that of 75 wt% differed from the commercial or laboratory synthesized PF resin. 3 The synthesised BPF resins were applied as an adhesive in the production of laboratory scale particleboards to evaluate their performances in relation to the following physical and mechanical properties internal bond (IB) strength, flexural strength (MOE, MOR) and dimensional stability test (TS, WA). The experimental design adopted was a one factor experiment in a Completely Randomized Design (CRD), and data collected were analysed using the analysis of variance, (ANOVA), followed by application of the LSD at the 95% confidence level. The IB for the 25 wt%, 50 wt%, and 75 wt% BPF bonded particle boards were 0.52 N/mm2, 0.78 N/mm2 and 0.35 N/mm2 respectively while that of commercial PF and laboratory synthetized pure PF were 0.46 and 0.48 N/mm2 respectively. The IB for the bonded particleboards were significantly different from each other at (p ≤ 0.05) probability level. The MOE for the 25 wt%, 50 wt% and 75 wt% BPF bonded particleboards were 2660 N/mm2, 2435 N/mm2 and 2300 N/mm2 respectively, while the MOE for the commercial and laboratory synthetized PF were 2050 and 2200 N/mm2 respectively. The MOE for the bonded particleboards were significantly different from each other at (p ≤ 0.05) probability level. The MOR for the 25 wt%, 50 wt% and 75 wt% BPF bonded particleboards were 16.15 N/mm2, 19.85 N/mm2 and 14.12 N/mm2 respectively, while the MOR for the commercial and laboratory synthetized PF were 14.25 and 14.00 N/mm2 respectively. The MOR for the bonded particleboards were significantly different from each other at (p ≤ 0.05) probability level. The TS for the 25 wt%, 50 wt% and 75 wt% BPF bonded particleboards were 20.85 %, 18.50 % and 15.00 % respectively, while the TS for the commercial and laboratory synthetized PF were 22.65 and 21.10 % respectively. The TS for the bonded particleboards were significantly different from each other at (p ≤ 0.05) probability level. The WA for the 25 wt%, 50 wt% and 75 wt% BPF bonded particleboards were 21.61 %, 19.86 % and 19.00% respectively, while the WA for the commercial and laboratory synthetized pure PF were 25.35 and 22.00 % respectively. The WA for the bonded particleboards were significantly different from each other at (p ≤ 0.05) probability level. Holistically, when the properties tested were assessed across board, the BPF resin-bonded particleboards with 50 wt% bio-oil replacement gave comparable strength and stability values to those of the commercial and laboratory synthesized PF resin. It therefore suggests that the use of bio-phenol as partial substitute for petroleum-based phenol up to 50 wt% bio-phenol replacement would produce bio-based resole resins useful as particleboard binder.