Global trends in allowable formaldehyde (CH2O) emissions from nonstructural wood-based composites require a renewed consideration of biogenic CH2O from wood. Increment cores from living Virginia pine (Pinus virginiana), yellow-poplar (Liriodendron tulipifera), and radiata pine (P. radiata) trees were used to measure CH2O and CH2O generation due to heating (200 °C, 10 min). Significant variations within and between trees of the same species were observed. Tissue types (juvenile/mature, heartwood/sapwood) sometimes correlated to higher CH2O contents and greater heat-generation potential; however, this did not always depend upon species. Heating increased CH2O levels 3–60-fold. Heating with high moisture levels generated more CH2O than that generated from dry specimens. Radiata pine generated extraordinarily high CH2O levels when heated, far exceeding the other species. It was suggested that pine extractives might catalyze CH2O generation, perhaps in lignin. Regarding wood-based composites, findings suggested that compliance with emissions regulations may be complicated by CH2O generated in the hot press. If we could reveal the precise mechanisms of CH2O generation in wood, we could perhaps manipulate these mechanisms for beneficial purposes.

Lignocellulose naturally contains formaldehyde, and generates much more when heated. A simple quantitation of such biogenic formaldehyde is desirable for the analysis and utilization of lignocellulose. Heretofore, a boiling toluene extraction (the perforator method) was the best known technique to determine biogenic formaldehyde in wood. Described here is a simple milligram-scale water extraction that avoids specimen heating. This method was validated by comparison to a laborious extraction using poly­(allylamine), PAA, beads that strongly sorb formaldehyde. The PAA-based extraction and the water-only extraction were found to be effectively equivalent, recovering about 94% of wood formaldehyde. The incomplete formaldehyde recovery is offset by experimental simplicity, and suitability for large sampling. For instance, the new method was applied to the analysis of tree increment cores; formaldehyde levels measured in never-heated Pinus virginiana ranged from 1 to 5 μg/g dry wood, and were comparable to published values using the perforator method. Heating at 200° for 10 min generated about 10–20 times more biogenic formaldehyde. This simple extraction is useful to document biogenic formaldehyde levels in wood, and the formaldehyde generation potential associated with heating, as in the manufacture of wood-based composites.

No abstract

Wood processing typically involves heating that generates lignin-borne formaldehyde, a well-known aspect of lignin acidolysis that played an important role in early efforts to elucidate lignin structure. Previously, we found that lignin-borne formaldehyde accounts for a small percentage of lignin acidolysis. This is explained by two competing lignin acidolysis pathways, C2 cleavage producing formaldehyde, and C3 cleavage (no formaldehyde). Here, the topic was studied with industrial-scale thermomechanical refining of Douglas fir wood, seeking correlations between refining energy and fiber chemistry and rheology. Refining caused substantial polysaccharide degradation accompanied by lignin acidolysis, the latter determined by nitrobenzene oxidation, titration of free phenols, and determination of formaldehyde captured in dried fiber. Formaldehyde generation (C2 cleavage) accounted for only 1% of the total loss of β-aryl ethers (C2 + C3 cleavage). This could imply that lignin-borne formaldehyde always results from lignocellulose thermal processing, raising possibilities for industrial process control using in-line formaldehyde monitoring. That requires future verification and correlation of formaldehyde generation with biomass properties. In this case, measured formaldehyde levels correlated with refining energy, reductions in the in situ lignin glass transition temperature, and reductions in lignin oxidative decomposition temperature.