Peer reviewed Grinding and classification of pine bark for use as plywood adhesive filler

Prior efforts to incorporate bark or bark extracts into composites have met with only limited success because of poor performance relative to existing products and/or economic barriers stemming from high levels of processing. We are currently investigating applications for southern yellow pine (SYP) bark that require intermediate levels of processing, one being the use of carefully ground and classified SYP bark as plywood adhesive fillers. Results can be used to select bark fractions having lower levels of ash. In addition, bark fractions rich in periderm tissue appear to perform better as plywood adhesive fillers relative to that prepared from whole bark. INTRODUCTION Since bark contains relatively high amounts of extractives, applications for those extractives have been sought to glean greater value from this biomass resource. For example, condensed tannins from SYP bark have been used to make adhesives for wood composites. Interest in this avenue of bark utilization has waned because of difficulties in competing with entrenched phenolic adhesive systems on both price and performance (1). Promising results have been obtained with substitutes for the more costly resorcinol-based adhesives, however, commercialization still faces barriers (2). An alternative to using bark chemicals for wood composites has been the pressing of bark together to make bark-based composites. Generally, the incorporation of bark results in lower strength relative to that for composite structures made with wood (35). We are investigating applications for bark that require intermediate levels of processing. A few reports have suggested some potential for SYP bark as a filler for plywood adhesives (6). Concerns have been raised about the extractives interfering with the resin cure and high ash contents that would result in higher levels of tool wear (7, 8). The outer bark of SYP is non-living and comprised of obliterated phloem tissue partitioned by periderms (Figure 1). Through specific grinding and classification techniques, we can obtain fractions rich in either of these two tissues. Here we report our results demonstrating the utility of carefully ground and classified SYP bark fractions as plywood adhesive fillers. Figure 1. Periderm and obliterated phloem tissues in SYP outer bark (9) MATERIALS AND METHODS Bark Preparation SYP bark (essentially all Pinus taeda L.) was collected near the debarking station at a local plywood plant. Samples of bark were ground as received with an electric chipper shredder (Echo, Inc., Model SH-5000) and dried under ambient conditions. Additional samples were sorted by hand to obtain samples of large (mostly 2-8 cm wide by 12-15 cm long) and small (mostly 0.4 cm to 2 cm wide by 2-5 cm long) pieces. Samples of unsorted and sorted bark were shaken with a detergent solution (0.05% Triton X-100), rinsed with water, and dried under ambient conditions. All bark samples were subsequently ground in a Wiley mill (10 mesh grinding screen), sealed in plastic bags, and stored in a freezer. Ash contents for the bark samples were determined in a muffle furnace set to 450 oC. Bark Fillers Previously ground bark was ground further in 100 g batches using a blender (Waring Laboratory, Model 36BL23). Bark was ground at high speed for two 1-minute periods, between which, the canister was removed and briefly shaken by hand. The blender-ground bark was then classified for 30 minutes on a sieve shaker (W.S. Tyler, RoTap, Model RX-29) equipped with 20, 35, 80,