Southern Hardwoods in Wood-Foam Composites

The process of forming composites of specially comminuted wood residues and foamable binders was explored using southern hardwoods growing on pine sites. Three dimensional particles (3-DP) or fibrillated sticks or frazzlings, were originally formed by feeding relatively large pieces of green wood to a hammermill; later it was found that pretreatments such as crushing increased the yield. The 3-DP used in this study were prepared by precrushing, followed by hammermilling and fractionating. Most was crushed in a TVA Log Fiberizer, a machine with a lower drive roll and a pressurized upper roll surfaced with a heavy helix thread spiraling outward from the center; a 5- by 7-inch cant is reduced by the pressure and separating action of the upper roll to a lace-like mat of long interconnected strands; chopped to length, the mat readily fluffs into 3-DP. Appropriate binders may be thermoset and thermoplastic, rigid or flexible resins, as well as mineral base cements; experimental work was done primarily with urethanes. A mat of 3-DP and urethane resin is formed in a mold, the top is clamped on, and consolidation takes place with the self hardening resin. If there has been no compaction, demolding can occur in 5 to 10 minutes. Wood and resin can be blended or the resin can be added to the particles in the mold. Properties of wood-foam composites are determined by density, which can range from less than 6 lbs. per cubic foot to the upper limit of compactability. A trade-off of strength for formability eliminates species as a factor in the properties of the composite. Properties depend on the quality of 3-DP which can be obtained from a wood supply. Three southern hardwoods–oak, red gum, and hickory–were tested; green material was crushed, hammermilled, and used to produced sample composites. The hardwoods tended to produce particles which were more linear, more fibrous, and/or more chunky than desirable; however, further research is needed to determine whether this is due to the slow growth rate of the sampled hardwoods or to overmilling. Potential uses for the molded material include acoustical and insulating products such as speaker cabinets, pallets and containers, furniture, and building products.

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