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Foam Biodegradable

Foam Biodegradable PLA/PBAT Foams Volume Expansion Ratio Open Cell content (%) Article contributed by Srikanth Pilla, George K. Auer, Shaoqin Gong University of Wisconsin, USA Seong G. Kim, Chul B. Park, University of Toronto, CA Figure 2: Volume Expansion Ratio vs Temperature 1.8 1.6 1.4 1.2 1 60 50 40 30 20 10 0 PLA Ecovio PLA+55%PBAT Figure 3: Open Cell Content vs Temperature PLA 130 140 150 Ecovio PLA+55%PBAT PLA+0.5%Talc Ecovio+0.5%&Talc PLA+55%PBAT+0.5%Talc Die Temperature (°C) PLA+0.5%Talc Ecovio+0.5%&Talc PLA+55%PBAT+0.5%Talc 125 130 135 140 145 150 155 Die Temperature (°C) In this study, a unique processing technology viz. microcellular extrusion foaming, was used to produce biodegradable foams that could potentially replace existing synthetic foams thereby reducing carbon footprint and contributing towards a sustainable society. Introduction As a biodegradable and biobased polymer, polylactide (PLA) has attracted much interest among researchers world-wide in recent times; however, its commercial application is still limited due to certain inferior properties such as brittleness, relatively high cost, and narrow processing window. Certain drawbacks can be overcome by copolymerizing lactide with different monomers such as ε-caprolactone [1-4], trimethylene carbonate [5] and DL-β-methyl-δ-valerolactone [6] and by blending PLA with poly(butylene adipate-co-terephthalate) (PBAT) [7], poly(εcaprolactone) (PCL) [8-12] and many other non-biodegradable polymers [13-19]. Though the blended polymers exhibited certain improved mechanical properties compared to non-blended parts, immiscible polymer blends may lead to less desirable properties that were anticipated from blending. Thus, compatibilizers are often used to improve the miscibility between the immiscible polymer blend. Foamed plastics are used in a variety of applications such as insulation, packaging, furniture, automobile and structural components [20-21]; especially, microcellular foaming is capable of producing foamed plastics with less used material and energy, and potentially improved material properties such as impact strength and fatigue life [22]. Also compared to conventional foaming, microcellular foaming process uses environmentally benign blowing agents such as carbon dioxide (CO 2 ) and nitrogen (N 2 ) in their supercritical state [23]. Microcellular process also improves the cell morphology with typical cell sizes of tens of microns and cell density in the order of 109 cells/cm 3 [23]. Additionally, compared to conventional extrusion, the microcellular extrusion process allows the material to be processed at lower temperatures, due to the use of supercritical fluids (SCF), making it suitable for temperature- and moisture-sensitive biobased plastics such as PLA. Solid PLA components processed by various conventional techniques such as compression molding, extrusion and injection molding have been investigated by many researchers [24-25]; however, foamed PLA produced via microcellular technology has been a recent development. Pilla et al. [26-29] and Kramschuster et al. [30] have investigated the properties of PLA based composites processed via microcellular injection molding and extrusion foaming. Mihai et al. [31] have 36 bioplastics MAGAZINE [01/11] Vol. 6

Foam investigated the foaming ability of PLA blended with starch using microcellular extrusion. Reignier et al. [32] have studied extrusion foaming of amorphous PLA using CO 2 ; however, due to very narrow processing window of the unmodified PLA, a reasonable expansion ratio could not be achieved. In this study, PLA/PBAT blends have been foamed by the microcellular extrusion process using CO 2 as a blowing agent. Two types of blend systems were investigated: (1) Ecovio ® , which is a commercially available compatibilized PLA/PBAT blend (BASF); (2) A non-compatibilized PLA/PBAT blend at the same PLA/PBAT ratio (i.e., 45:55 by weight percent) as Ecovio. The effects of talc,compatibilization and die temperature on the cell size, cell density, volume expansion and open cell content were evaluated. Effects on Cell Size and Cell Density Representative SEM images of the cell morphology of different formulations are shown in Figure 1. From the figure, it can be noted that the addition of talc has decreased the cell size. This shows that talc has acted as a nucleating agent thereby reducing the cell size. Thus, as more cells started to nucleate, due to excess nucleation sites provided by talc, there was less amount of gas available for their growth that lead to reduction in cell size. Also, the addition of talc significantly increased the melt viscosity, which made it difficult for the cells to grow, leading to smaller cell sizes [33]. Also, from Figure 1 it can be observed that the cell size of the compatibilized blends (both Ecovio and Ecovio-talc) is much less than that of the non-compatibilized ones (PLA/PBAT and PLA/PBATtalc). Thus it can be concluded that compatibilization has reduced the cell size. This might be due to increase in the melt strength of the blend as a result of the compatibilization [34]. In general, as shown in Figure 1, the addition of talc has increased the cell density because of the heterogeneous nucleation. In a heterogeneous nucleation scheme, the activation energy barrier to nucleation is sharply reduced in the presence of a filler (talc in this case) thus increasing the nucleation rate and thereby the number of cells [35]. While comparing the compatibilized and non-compatibilized samples, it can be observed that the cell density 500 μm PLA PLA + 0.5% Talc Ecovio Ecovio + 0.5%Talc PLA + 55% PBAT PLA + 55% PBAT + 0.5%Talc Figure 1: Representative SEM Images of Various Formulations Temperature Increase 130°C 140°C 150°C bioplastics MAGAZINE [01/11] Vol. 6 37

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