Total phenolic compounds and free phenol in softwood structural plywood

Construction-grade plywood panels manufactured at five plywood mills were analyzed for total phenolic compounds and free phenol detection. Small samples of plywood were ground into <1-mm-size powders. The samples were subjected to an ambient temperature, methylene chloride extraction, and tested for free phenol content by a gas chromatography-mass spectrometry method. The plywood samples were also analyzed for total phenolic compounds by a distillation-colormetric method. The range of total phenolic compounds was 6.8 to 2 5.3 mg/kg (parts per million), and the range of free phenol was 0.090 to 0.73 mg/kg (parts per million). The sources of phenolic compounds in plywood are wood components (lignin and extractives), the phenol-formaldehyde resin adhesive, and the ligno-cellulosic adhesive fillers (bark tannins, furfural residue lignins, etc.). The source of free phenol in structural plywood is presumably the phenol-formaldehyde resin adhesive. The extraction procedures used in this study were extreme and are not typical for plywood in service. Yet the levels of phenolic compounds and free phenol detected were so low that they most often were beyond the quantitative accuracy of the test methods and instruments, requiring extrapolative techniques. The low levels are supportive of the fact that structural wood composites bonded with phenol-formaldehyde resins (such as plywood) have been found to be very safe environmentally for multiple uses.

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