Evaluation of Micron-Sized Wood and Bark Particles as Filler in Thermoplastic Composites

Micron-sized particles, prepared from loblolly pine (Pinus taeda L.) wood and bark, were evaluated for use in wood-plastic composites (WPCs). Particles were also prepared from hard (periderm) and soft (obliterated phloem) components in the bark and compared to whole wood (without bark) filler commonly used by the WPC industry. All bark fillers had different thermal degradation profiles and higher ash contents compared to whole wood and micron-sized wood particle samples. The bark particles had an increased nucleating ability on polypropylene over whole wood filler. However, a drop in modulus and decrease in interaction with matrix were observed by dynamic mechanical analysis. These characteristics of the bark particles impart significant constraints on their processing and utilization in WPCs. Introduction A new class of structural materials has emerged in the past decade based on a composite of thermoplastic and wood (i.e., wood-plastic composite, WPC). The thermoplastic component in the WPC provides improved resistance to moisture and biological attack over traditional wood composites by encapsulating the wood. However, the thermoplastic matrix does not chemically interact with the wood, leading to poor stress transfer and pathways for moisture uptake (Johnson and Nearn 1972). Many possible mechanisms exist for improved performance, ranging from enhanced wood distribution to covalent bonding of the wood fibers and matrix. A recent investigation showed that wood dramatically impacts the crystal morphology of an isotactic polypropylene (iPP) matrix (Harper and Wolcott 2004). However, this effect is physical and closely related to the morphology/roughness of the wood. What is unclear in the literature is how the chemical composition differences among tree parts, other than from the wood in the main stem (e.g., bark, leaves), would impact the interfacial and composite properties. Micron-sized wood and bark particles were investigated as a means to improve the distribution of the filler in the composite. The incorporation of bark in WPCs generally results in lower strength relative to that for composite structures made with wood alone (Muszynski and McNatt 1984, Blanchet et al. 2000). However, in a study on bark-based fillers used in the preparation of plywood adhesive mixes, a bark-based filler exhibited superior performance over standard filler, furfural residue (Eberhardt and Reed 2006). To accomplish this, the outer bark of southern yellow pine (SYP) was partitioned into its hard (periderm, PE) and soft (obliterated phloem, OP) components; in SYP bark, layers of OP tissue are partitioned by layers of PE tissue (Fig. 1). Grinding and sieving operations afforded OP and PE fillers with inferior and superior performances, respectively. It remains to be determined if the improved performance for PE filler would carry over to other applications such as the manufacture of WPCs. In addition to improved biomass utilization through developing markets for bark, it is important to address the impact of bark contamination of wood supplies used for WPCs; the utilization of small diameter trees, with their higher proportion of bark, is becoming increasingly important. Thus, the physical and chemical properties of bark that may be problematic for the WPC industry are of particular interest. Harper: Assistant Professor, University of Tennessee, Tennessee Forest Products Center, Knoxville, Tennessee, USA Eberhardt: Research Scientist, USDA Forest Service, Southern Research Station, Pineville, Louisiana, USA