Building & Construction
Building & Construction Biodegradable green foam: the building material of the future by Srikanth Pilla 1,2 Shaoqin Gong 1 Lih-Sheng Turng 1 1 Wisconsin Institute for Discovery, University of Wisconsin-Madison, USA 2 Department of Automotive Engineering & International Center for Automotive Research, Clemson University, USA Figure 1: Temporary housing made from wood plastic composites [Source: www.upm.com]. A 3000 Strength / Moduli (MPa) 2500 2000 1500 1000 500 0 B 3000 Strength / Moduli (MPa) 2500 2000 1500 1000 500 20 18 16 14 12 10 8 6 4 2 PLA PLA-10% Flax PLA-20% Flax 0 Figure 3: Mechanical properties and weight reduction of (A) PLA–FF and (B) PLA–SF biocomposite foams. Pure PLA PLA–10% Flax PLA–20% Flax Weight Reduction (%) Weight Reduction (%) 0 0 PLA PLA-10% SF PLA-30% SF Weight Reduction Tensile Strength Youngs Modulus 12 10 8 6 4 2 Natural calamities, such as the tsunami in Southeast Asia, the earthquake in Haiti, and Hurricane Katrina in the US, wreak havoc on communities and leave families homeless. To aid in such emotional disasters, the governments of the respective countries provide those affected with temporary shelter, food, and clothing. Among these forms of assistance, temporary housing is a major investment. It provides a long-term housing arrangement, often for 2 years or more, for the displaced families until permanent housing can be secured (Fig. 1). Thus, temporary houses must not only meet current needs, but must also provide durability, strength, and stiffness for the duration of their use. Most temporary housing is made from engineered plastic composites wherein the plastics—such as polyvinyl chloride, polyethylene, and polypropylene—are derived from petroleum, which is not biodegradable. This poses a huge environmental burden when these units are no longer needed, and must be disassembled and disposed of. In addition, the non-renewable and limited nature of petroleum resources results in variable costs for the various products derived from it, including polymers. Hence, it is critical that new materials be developed, especially from renewable resources, which not only provide in-service functionality but also out-of-service biodegradability thereby supporting environmental sustainability. An interesting route to create new material variations (e.g. for us in the contruction of such temporary housing applications) and additionally lower the material cost is through the development of foamed composites. On this front, the authors have made substantial innovations, especially through the development of biopolymer–natural fiber biocomposite foams using an environmentally benign supercritical fluid (SCF)- assisted microcellular processing method [1-4]. Commonly referred as gas-assisted injection molding/extrusion, the SCF processing method employs nitrogen or carbon dioxide in a supercritical state to foam the polymer melt. SCF effectively reduces the viscosity of the polymer melt thereby enabling the polymer to be processed at lower temperatures and pressures. Figure 4: Morphological observations of PLA–flax biocomposite foams. 18 bioplastics MAGAZINE [04/13] Vol. 8
Building & Construction Cavity cross section Supercritical N 2 or CO 2 Rapid pressure drop in nozzle triggers cell nucleation Special reciprocating screw Single-phase polymer-gas solution Figure 2: Schematic of the microcellular injection molding process. Higher back pressure (80-200 bar) This is a very desirable feature for bioplastics, some of which are moisture- and heat-sensitive. This article summarizes a few studies related to PLAbased natural fiber biocomposite foams [1-4]. Presented here are the mechanical and morphological properties of the engineered biocomposite foams, which form the basis for assessing the durability of fabricated structures, as well as the percentage of weight reduction achieved. Polylactic Acid–Natural Fiber Engineered Biocomposite Foams PLA was compounded with natural fibers such as flax fiber (FF) and shopping bag fiber (SF) using a twin-screw extruder at various fiber ratios. Shopping bag fibers are cellulosic fibers from recycled paper shopping bags. Since natural fibers are hydrophilic and PLA is hydrophobic, in both of the studies, γ-methacryloxy propyltrimethoxy silane was used as a coupling agent to ensure strong interfacial adhesion between the dissimilar surfaces. The compounded formulations were then fabricated into dog-bone shaped test specimen using an Arburg Allrounder 320S injection molding machine with a 25 mm diameter screw equipped with microcellular injection molding (MuCell ® ) technology (Trexel, Inc., Woburn, MA). A schematic of the microcellular injection molding process is shown in Fig. 2. Fig. 3 shows the mechanical properties and weight reduction of (a) PLA–FF and (b) PLA–SF biocomposite foams. As can be observed in Fig. 3, the moduli of the PLA–FF and PLA–SF biocomposite foams was significantly enhanced without compromising the tensile strength. This was due to favorable interfacial adhesions between the natural fibers and the PLA achieved through silylation as well as the favorable cell morphology (i.e., small cell size and high cell density) of the foamed biocomposites. Figure 4 shows the morphology of PLA–FF biocomposite foams. As shown in the figure, the cell size decreased as the cell density increased with an increasing flax fiber loading level. This can be attributed to several factors: (1) the natural fibers may have served as nucleating agents for the cell/pore formation, and/or (2) the addition of natural fibers could have increased the melt viscosity and induced strain hardening which could have hindered cell growth and coalescence [4]. Overall, the employment of the microcellular foam processing technology resulted in a weight reduction of 10 to 18%. Conclusions With stringent regulations set forth by environmental protection agencies across the world, as well as the rapid advancement of research and development into the field of biobased polymers, composites, and foams, it is foreseeable that these novel materials will enter into structural and nonstructural parts (both exterior and interior) for the building industry far earlier than anticipated. These materials are 100% biobased, largely biodegradable (even compostable, when chopped into small chips), and exhibit a low carbon footprint, thus providing much needed sustainability to the building industry. The foamed biocomposites, which result in a further weight reduction of 10 to 18% (or more with advances in foaming technology), will be the materials of the future since they provide a significant step forward for realizing truly affordable (low-cost) structural materials with reduced densities for the building industry, especially for the temporary housing market. It must however be kept in mind, that for a disposal via biodegradation/composting a composting infrastructure must be in place. References: 1. Pilla, S., Kramschuster, A., Lee, J., Auer, G.K., Gong, S., and Turng, L-S., “Microcellular and Solid Polylactide-Flax Fiber Composites,” Compos. Interfaces, 16(7-9), pp. 869-890 (2009). 2. Pilla, S., Gong, S., O‘Neill, E., Rowell, R.M., and Krzysik, A.M., “Polylactide-Pine Wood Flour Composites,” Polym. Eng. Sci., 48(3), pp. 578-587 (2008). 3. Kramschuster, A., Pilla, S., Gong, S., Chandra, A., and Turng, L-S., “Injection Molded Solid and Microcellular Polylactide Compounded with Recycled Paper Shopping Bag Fibers,” Int. Polym. Proc., XXII (5), pp. 436-445 (2007). 4. Lee, L.J., Zeng, C.C., Cao, X., Han, X.M., Shen, J., and Xu, G.J., Compos. Sci. Technol., 65, pp. 2344-2363 (2005). bioplastics MAGAZINE [04/13] Vol. 8 19
