Strategies for improving the performance of plywood adhesive mix fillers from southern yellow pine bark.

Abstract

Southern yellow pine bark was obtained from an industrial source and subjected to grinding and classification operations to ultimately afford finely ground bark fractions for evaluation as plywood adhesive mix fillers. Specifically, by grinding in a laboratory blender, we

were able to generate a bark fraction rich in periderm tissue with its interlocking spiculate stone cells (sclereids). Another significant bark fraction was comprised of cellular debris from the obliterated phloem tissues in bark that are partitioned by the periderms. Through the grinding and classification operations employed in this study, the filler rich in periderm tissue had superior performance ([greater than or equal to] 90% wood failures) over both the filler rich in obliterated phloem tissue and that prepared directly from the bark as received. This appears to be related to the removal of extractive-rich bark components that likely promote resin undercure. The periderm-rich filler had the added benefit of an ash content (2.5%) that was significantly lower than that for the whole bark filler (9.4%).

**********

Harvested trees are commonly transported to the processing site as bark-covered logs. For southern yellow pine (SYP), approximately 18 percent of the transported load is comprised of bark (Hemingway 1997). Most SYP bark, especially that available at pulpmills, is burned in power boilers where it contributes significantly to the energy demands of this industry sector. In some cases, bark still presents a disposal issue at lumber mills and plywood plants.

Efforts to obtain greater value from such bark resources have generally involved the development of applications for the extractives. For example, condensed tannins from SYP bark have been used to make thermosetting adhesives for wood composite manufacture. While adhesives based on wattle (Acacia mearnsii De Wild.) condensed tannins have been commercialized, efforts with SYP condensed tannins have fallen short because of difficulties in competing with entrenched phenolic adhesive systems on the basis of both price and performance (Kreibich 1989). Promising results were obtained with SYP tannin sulfonates as partial substitutes in phenol-resorcinol-formaldehyde adhesives (Kreibich and Hemingway 1989); however, again, commercialization has not been forthcoming. An alternative to using bark as a source of chemicals has been the pressing of bark fragments together to make bark-based composites (Chow 1975). The incorporation of bark along with wood in particleboards has also been studied. Generally, as bark usage increases, particleboard strength decreases (Muszynski and McNatt 1984, Blanchet et al. 2000).