From Science & Research
From Science & Research Novel Bioplastic Blends and Nanocomposites Article contributed by John R. Dorgan, Department of Chemical and Biochemical Engineering, Colorado School of Mines, Golden, CO 80401 USA Birgit Braun and Laura O. Hollingsworth , PolyNew Inc., Golden, CO 80401 USA Figure 1: TEM of cellulose nanowhiskers derived from acid hydrolysis of cotton linters. 200 nm The prospect of a hot, flat, and crowded [1] planet earth requires greater technological efforts in meeting the challenges of creating industrial sustainability. The triple technological convergence of industrial ecology, biotechnology, and nanotechnology offers promise of being able to deliver such sustainability. Industrial ecology uses the quantitative tools of Life Cycle Assessment to consider impacts like the generation of green house gases (GHGs) when renewables are substituted for fossil resources. Biotechnology is providing efficient biochemical conversions and nanotechnology is having big impacts both in catalysis and in materials sciences. Here it is argued that the convergence of these technologies is defining a new field of inquiry which can be referred to as ecobionanotechnology. Within this context a new class of green materials, ecobionanocomposites, is being developed. The now rapidly developing field of degradable bioplastics and plastic materials based on renewable resources, provides tremendous opportunities to sustain and enhance the domestic plastics industries, the fourth largest manufacturing sector. Growth in the use of these new, greener plastic is proceeding rapidly, however, there are a number of cases in which bioplastics lack the properties needed to compete with petroleum based materials. Drawing on scientific knowledge about the new emerging field of polymer nanocomposites, these property limitations can be overcome. In this article, the development of novel polymer nanocomposites based on renewable cellulosic nanowhiskers combined with polylactide is described. The fossil energy requirement for the PLA production process as implemented by NatureWorks is substantially less than for other commercially produced polymers as shown by life cycle assessment [2]. Significant increases in the heat distortion temperature of polylactides (PLA) have been achieved using these nanowhisker fillers. Prototypical thermoformed trays have been fabricated from first generation nanocomposites and shown to be suited for use as microwaveable frozen food packaging. Second generation nanocomposites have been shown to maintain transparency while having higher use temperatures. The use of cellulosic nanowhiskers means that the resulting nanocomposites maintain the desirable feature of biodegradability. Experimental Materials. Commercial-grade PLA (2002D, melt-flow index 4-8 g/10 min, < 4% D-lactide) was supplied by NatureWorks LLC. L-lactide was also obtained from Natureworks. PLA resin was recrystallized at 110°C for 24 hours prior to compounding. Cellulosic nanowhiskers (CNW) were prepared via acid hydrolysis of cotton linter using hydrochloric acid as described in reference [3]. An impact modifier ‘Biomax Strong‘ was obtained from DuPont. 32 bioplastics MAGAZINE [03/09] Vol. 4
From Science & Research Methods. For the melt mixing procedure PLA resin was dried for 24 hours at 80°C under 23 inHg (3,0 Pa) vacuum. The melt mixed samples were prepared in a Haake RheoMix 3000. PLA was fully melted at 180°C and 0. wt% tris(nonylphenylphosphite) (TNPP) was added as a stabilizer. The required amount of CNWs and impact modifier were added and mixed at 0 rpm for 2 minutes. Composite samples were vacuum/compression molded into rectangular bars, crystallized at 110°C for three hours, and physically aged for 24 hours. Mechanical properties were determined through dynamic mechanical thermal analysis (DMTA) using an ARES-LS rheometer with torsional rectangular fixtures. The testing was carried out at 0.0% strain, 1 Hz, with a temperature ramp from 30°C to 10°C at °C/min. The DMTA data was used to calculate the heat distortion temperature (HDT) via the methodology of Takemori [4]. Results Figure 1 is a transmission electron micrograph of the cellulose nanowhiskers (CNW) derived from cotton. Evidence of aggregation is clearly present which is usually present but which becomes more severe upon isolation and drying [3]. Figure 2 presents the data on tensile properties for the melt mixed nanocomposites. A typical tradeoff between modulus and strain at break is observed. PLA already suffers from relatively low impact properties so the decrease in impact which is associated with the decreased strain at break would preclude the use of these materials for most practical applications. Figure 3 presents the improvement in the impact strength associated with the addition of wt% DuPont Biomax. The simultaneous addition of both reinforcing CNWs and the Biomax impact modifier produces a material with both improved modulus and toughness compared to the base PLA. Finally, in Figure 4 it is shown that the nanocomposites have improved HDTs. While the addition of Biomax Strong decreases the extent of the HDT it is still possible to reach for example, an HDT above 90°C while simultaneously improving the impact properties. Conclusions Substantial challenges exist regarding developing a truly sustainable plastics industry. The judicious selection of combined technological platforms can assist humankind in meeting this important goal. In this study, elements of industrial ecology, biotechnology, and nanotechnology are combined to create a new largely renewable and largely degradable polymer nanocomposite with improved thermophysical properties. These Ecobionanocomposites are one example of a larger trend towards the triple technological convergence of these areas of inquiry. Modulus [ksi] 450 400 350 300 250 200 Tensile Testing Data vs. Cellulose Loading 0% cellulose 10% cellulose Modulus Strain @ break Figure 2: Ultimate mechanical properties of melt mixed nanocomposites. Izod Impact [J/m] 600 500 400 300 200 100 0 PLA PLA (5% Biomax + CNW) Figure 3: Impact properties of nanocomposites with impact modifying agent addition. Heat Distortion Temperature [°C] 130 120 110 100 90 80 70 0% Dupont Biomax Strong 120 2% Dupont Biomax Strong 120 5% Dupont Biomax Strong 120 25% cellulose PLA (5% Biomax) 0 10 20 30 Cellulose Loading Level [wt%] Figure 4. Heat distortion temperatures of degradable ecobionanocomposites. Acknowledgements This article was previously published at SPE‘s GPEC 2009, Orlando, Florida, USA, Feb. 2-2, 2009. This research was supported by the National Science Foundation through an SBIR grant to PolyNew Incorporated. References [1] Friedman, Thomas, Hot, Flat, and Crowded: Why We Need a Green Revolution - And How it Can Renew America (Farrar, Straus & Giroux, New York, NY) 2008. [2] Vink, E. T. H.; Rabago, K. R. ; Glassner, D. A.; Gruber, P. R. Poly. Deg. Stab. 2003, 80, 403. [3] Braun, B.; Dorgan, J.R.; Chandler, J.P. Biomacromolecules, 2008 9(4), 12. [4] Takemori, M.; Polym. Eng. and Sci. 199 19(1) 1104. 5 4 3 2 1 0 Strain @ break [%] bioplastics MAGAZINE [03/09] Vol. 4 33
