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

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 ( 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). We are currently investigating applications for SYP bark requiring intermediate levels of processing that fall between the isolation of chemical constituents and the fabrication of bark-based composites. One option under evaluation is the use of SYP bark in plywood adhesive mix fillers. For plywood manufacturing, fillers are added to the adhesive mix to improve its workability. Specifically, these finely ground organic and/or inorganic materials promote bonding by holding the adhesive on the veneer surface where it is needed (Sellers 1985). Aside from performance, desirable filler features include low cost, consistent quality, and sufficient supplies. Commonly used plywood adhesive mix fillers include furfural residue, alder bark, and nutshell flours. Elimination of doThe authors are, respectively, Research Scientist and Physical Science Technician, USDA Forest Serv., Southern Research Sta., Pineville, LA ([email protected]; [email protected]). This study was made possible through the expertise shared by David Perry, Bates and Co., Inc. Assistance with plywood manufacture and testing was provided by Dale Huntsberry. The authors especially appreciate the materials provided by Boise Cascade Corporation and Hunt Forest Products, Inc. This paper was received for publication in September 2005. Article No. 10108. ©Forest Products Society 2006. Forest Prod. J. 56(10):64-68.