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From Science & Research

From Science & Research PLA Composites with Field Crop Residues Tensile strength, MPa Article contributed by Calistor Nyambo Amar K. Mohanty Manjusri Misra University of Guelph, Guelph, Canada Figure 1: Field crop residues (a) Corn stalks; (b) Soy stalks and (c) Wheat straws Figure 2: Tensile properties of (a) PLA with (b) 30 wt % of agricultural residue, (c) 30 wt % hybrid fibers (i.e. 10 wt % each of wheat, corn and soy stalks) and (d) 30 % fiber compatibilized with PLA-g-MA. 80 70 60 50 40 30 20 10 Tensile strength Tensile modulus 8 6 4 2 Tensile modulus, GPa Field crop residues such as, cereal straws, corn and soy stalks are widely available in large quantities and are normally discarded as waste or used as animal feed.These field crop residues contain cellulose based fibers [1] and their utilization in ‘green’ composites has potential for generating extra revenue for farmers. Using field crop residuesmight be another way of making affordable injection molded biocomposites with specific desired mechanical performance. Our recent studies have focussed on the use of field residues, shown in Figure 1 (i.e. wheat, corn and soy stalks), and their hybrids as a principle source of fibers for making affordable and sustainable bio-based polylactide (PLA) composites [2]. We estimated the cost of the field crop residues to be around ###COLUMNCONTENT###.15/kg. Varying amounts of ground fibers from 10 to 40 wt % were successfully incorporated into the PLA matrix. It was found that the addition of the field crop residues slightly reduces the tensile strength and significantly increases the elastic modulus. The mechanical performance of the different types of these fibers and their hybrids at 30 wt % loading in PLA were similar as shown in Figure 2. This is an important finding since it may mean that agricultural residues can be substituted for each other without compromising mechanical performance in the event of fiber shortages. Automotive part makers have raised some concerns regarding the supply chain of natural fibers. Therefore, the development of multiple compositeformulations using hybrid fibers might be another important way of reducing concerns from automotive part makers since many formulation options will be available in the event that one type of fiber is temporarily out of supply. The hydrophilic nature of agricultural fibers presents another problem in natural fiber composites. Natural fibers tend to agglomerate as the loading is increased and this may lead to poor dispersion in bioplasticthereby decreasingthe mechanical performance of the composite. Various fiber surface treatments techniques and coupling agents have been developed for improving the fibermatrix adhesion [3]. The use of maleic anhydride grafted polymers like polypropylene-grafted with maleic anhydride (PP-g-MA) is one of the best example. Unfortunately, PLA grafted with maleic anhydride (PLA-g-MA) is not yet commercialized but synthetic routes have been reported 0 a b c d 0 52 bioplastics MAGAZINE [01/11] Vol. 6

(a) From Science & Research (b) in literature [4]. Upon, the additionof 5 wt % of PLA-g-MA, which was prepared via reactive extrusion, the tensile strength of wheat straw increased by about 20 % matching that of the neat PLA as shown in Figure 2. This is a good result because compatibilized composites have low cost since they are filled with 30 wt% inexpensive fibers; whilst having better stiffness than the neat PLA and comparable tensile and flexural strength. Figure 3: SEM images for PLA with (a) 30 wt % biomass and (b) with 30 % biomass compatibilized with PLA-g-MA Scanning electron microscopy (SEM) images presented in Figure 3 showed less evidence of fiber fracture and pull-out in the compatibilized composites than in the uncompatibilized composites which suggest good fibermatrix adhesion. The PLA composites were found to have low densities (1.3 g/cm 3 ) and no enhancements in the heat deflection temperature, (HDT) were observed. Stereocomplexation (blending the two different stereoisomers of PLA i.e. D-PLA, and L-PLA) is one of the most promising techniques that has been developed for improving the heat resistance of PLA. One of the advantages of using PLA is that it is 100 % biodegradable and recyclable. The biodegradation of PLA is influenced by several factors such as moisture level, temperature and pH. Since the fibers are hydrophilic, they tend to absorb water which is essential for the hydrolysis of the ester groups on the PLA chains to form oligomers which can easily be attacked by bacteria. It was found that the PLA/agric residues composites biodegrade faster than the neat PLA [5]. This result is also important since it may mean that the PLA composites can alleviate shortages of landfills since they can easily biodegrade. Prototype composite panels are presented in Figure 4. It was observed that the PLA/agro residue fibers can easily be tinted with a pigment to give certain desired colour. We estimated the costs for these composites to be around ###COLUMNCONTENT###.95/lb and this is lower than our estimate cost for polypropylene/glass-fiber at .10/lb. Acknowledgements Financial support from 2008 Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) – University of Guelph Bioproducts program, NSERC- Discovery grant program individual (Mohanty) is greatly appreciated. The authors gratefully thank Elora Farms in Guelph for kindly providing all the agricultural residues. Figure 4: Prototypes plaques of the PLA/30 % wheat straw composite panels prepared via extrusion followed by compression molding (a) without (b) with pigment. References [1] US Department of Energy http://www.eere. [2] Nyambo, C.; Mohanty, A. K.; Misra, M.Biomacromolecules. 2010, 11, 1654 [3] Mohanty, A. K.; Misra, M.; Drzal, L. T. Compos. Interfaces2001, 8, 313. [4] Carlson, D.; Nie, L.; Nayaran, R.; Dubois, P. J. Appl. Polym. Sci.1999, 72, 477. [5] Pradhan, R.; Misra, M.; Erickson, L.; Mohanty, A.K. Bioresource Technology.2010, 101, 8489. bioplastics MAGAZINE [01/11] Vol. 6 53

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