Biobased Phenol Formaldehyde Resins Derived from Beetle-Infested Pine BarksStructure and Composition

In this study, two types of biobased bark-derived phenol formaldehyde (PF) resins, namely, liquefied bark-PF and bark extractive-PF, were synthesized from acid-catalyzed phenol-liquefied bark and bark alkaline extractives, respectively. The biobased resins were characterized for their chemical compositions and molecular structures using the liquid-state 13C nuclear magnetic resonance (NMR) technique. The results indicated that the introduction of bark components (either as liquefied bark or as bark extractives) to the phenolic resin synthesis affected resin structures and curing performance. Methylene ether bridges were found in the bark-derived PF resins. Bark components made the formation of para–ortho-methylene linkage more favorable in bark-derived PF resins than in lab PF resins. Molecular structures of the liquefied bark-PF resin differed significantly from those of the bark extractive-PF resins. The liquefied bark-PF resin showed a higher ratio of para–para/ortho–para-methylene link (−CH2−), a higher unsubstituted/substituted hydrogen (−H/–CH2OH) ratio and a higher methylol/methylene (−CH2OH/–CH2−) ratio than the bark extractive-PF resin. The tannin components of the bark extractives accelerated the curing rate of the resulting bark extractive-PF resin. The bark extractives made the ortho position of phenol react more favorably with formaldehyde than the para position. The liquefied bark with phenolated structures had more reactive sites toward formaldehyde than the bark extractives and accelerated the curing rate of the resulting liquefied bark-PF resin.