Comparison of strandboards made with phenol-formaldehyde resin and resins modified with TVA acid-hydrolysis lignin

Lignin was extracted from four wood hydrolysis residues with a yield of 10 to 37 percent, dry residue weight basis. The lignins varied in number average molecular weight from 550 to 2,400 and had polydispersity ranging from 2.0 to 5.0. Although the various lignin extracts had moderate to high formaldehyde- lignin reactivity, only 0.47 moles of formaldehyde to 100 g of lignin were used in the resin formulations. Phenol-formaldehyde (PF) resins, which are commonly used in the manufacture of oriented strandboard, were formulated with the lignins at a 30 percent phenol substitution level. The lignins blended well into the PF resin synthesis and were comparable to the commercial control in storage stability and application (spray) properties. The nonvolatile solids of the modified resins ranged from 52 to 56 percent, in comparison to 50 percent for the control resin. Other modified resin physical properties were similar to the control resin. In the laboratory, homogeneous strandboards were fabricated using strands from a regional oriented strandboard mill. The strandboards were bonded with 4 and 5 percent resin solids on a furnish dry-weight basis. The 12.7-mm-thick panels were hot-press cured at 4 and 5 minutes at 205?C. The panels were evaluated for density, dry- bond strength properties (internal bond and modulus of rupture), and wet-bond strength retention (after an accelerated-aging six-cycle American Plywood Association test procedure). In some minor ways, the adhesive performance of the lignin-modified resins differed among themselves and from the control PF resin in bonding strandboard. At application rates of 4 percent resin solids (dry wood weight basis), there were no statistically significant differences in properties of panels bonded with lignin-modified resins and those bonded with the control resin, and all panels met industry standards. At application rates of 5 percent resin solids, the panels bonded with the control resin had a statistically higher internal bond strength than panels bonded with the lignin-modified resins; otherwise the panel properties of the two types were the same. Due to the limited amount of lignin available and lack of repeat resin synthesis, these experimental results are considered indicative but preliminary.

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