CORRESPONDENCE/REBUTTAL pubs.acs.org/est
Response to Comments on “Sustainability Metrics: Life Cycle Assessment and Green Design in Polymers”
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e thank Carroll, Dale, and Murphy et al. for their interest in the article “Sustainability Metrics: Life Cycle Assessment and Green Design in Polymers”1 and for their insight. We refer the reader to their respective comments. There is no universally recognized set of metrics for evaluating the sustainability of a product or process. Thus, the primary aim of the aforementioned paper was to examine two widely used approaches: Life Cycle Assessment (LCA) and Green Design. Ultimately, conclusions related to which polymer used within the case study is “greener” or more sustainable or more environmentally friendly cannot be drawn from this paper because the polymer case study only evaluates the polymers from cradle to gate. The environmental impacts from polymer end of life (EOL), which was omitted from the study, will likely alter the sustainability or green rankings presented in the paper. The sustainability of any product requires a comprehensive analysis that includes not only the entire life cycle of a product, but an analysis of a wide range of environmental, economic, and social impacts. Many life cycle assessments have been completed for polymers, but most studies are limited in their system boundaries (e.g., cradle to gate) and in the scope of impacts considered. Examples related to biopolymers include refs 2 9. These studies highlight another drawback of existing biopolymer LCAs; they often neglect to evaluate environmental impacts other than global warming potential and life cycle energy consumption. Water quality and eutrophication impacts can be significant particularly since biopolymers are derived from agricultural resources.10 12 When it comes to accurate information on biopolymers’ EOL, more studies are published by the day that evaluate the biodegradation rates of products evolved under different conditions; some examples include refs 13 17. However, these studies are typically not connected to LCAs or green design. A rigorous and unbiased evaluation of the impacts of polymer end of life is essential to understanding their sustainability. Both Dale and Murphy et al. bring up an issue facing all LCA practitioners: How does one choose the best database for the life cycle inventory (LCI)? For this study, we chose one database, ecoinvent, for the sake of consistency. This database is used by many LCA practitioners, and other than a few specific cases (PHA and bPET, products that were not in LCI databases at the time of writing), any user of these LCA software tools and databases would have obtained the same results as presented in the paper. However, different databases often have very different data and thus produce very different results. LCA practitioners might choose LCI data based on other factors such as geographic locale, date published, or worst-case scenarios. Similarly, the choice of the life cycle impact assessment (LCIA) tool can provide results that may also be disparate.18 20 Sensitivity and uncertainty analyses can aid in evaluating these disparities. Carroll expressed concern related to the atom economy calculations for PVC. Within the paper, atom economy is calculated as the molecular weight of the desired product divided by the molecular weight of all reactants used throughout the r 2011 American Chemical Society
synthesis assuming all reactions are run to completion. Our calculation of atom economy for PVC identifies a NaOH outlet stream as waste. However, NaOH is a market driven product and should be a desired product in the atom economy calculation, resulting in a revised 91% atom economy for the synthesis of PVC. However, this result does not change the ranking of polymers presented in the green design dimension. The singlevalue metric for PVC increases from 1.09 to 0.66. This does not change the rankings under either the maximum or the mean normalization. In summary, the primary results of the paper remain the same. First, green design’s atom economy might be a good indicator of LCA environmental impacts. Second, the decision to “choose renewable feedstocks” is not necessarily a good metric of sustainability when not examined in a larger context. Amy E. Landis,* Michaelangelo D. Tabone, James J. Cregg, and Eric J. Beckman
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] .
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(11) Costello, C.; Griffin, W. M.; Landis, A. E.; Matthews, H. S. Impact of biofuel crop production on the formation of hypoxia in the gulf of mexico. Environ. Sci. Technol. 2009, 43 (20), 7985–7991. (12) Selman, M., Greenhalgh, S.; Diaz, R.; , Sugg, Z. Eutrophication and hypoxia in coastal areas: A global assessment of the state of knowledge; World Resources Institute (WRI): Washington, DC 2008. (13) Abou-Zeid, D.-M.; Muller, R.-J.; Deckwer, W.-D. Degradation of natural and synthetic polyesters under anaerobic conditions. J. Biotechnol. 2001, 86 (2), 113–126. (14) Garca de Mara, C.; Hueso Domnguez, K. B. Simultaneous kinetic determination of 3-hydroxybutyrate and 3-hydroxyvalerate in biopolymer degradation processes. Talanta 2010, 80 (3), 1436–1440. (15) Ganjyal, G. M.; Weber, R.; Hanna, M. A. Laboratory composting of extruded starch acetate and poly lactic acid blended foams. Bioresour. Technol. 2007, 98 (16), 3176–3179. (16) Gattin, R.; Copinet, A.; Bertrand, C.; Couturier, Y. Biodegradation study of a starch and poly(lactic acid) co-extruded material in liquid, composting and inert mineral media. International Biodeterior. Biodegrad. 2002, 50 (1), 25–31. (17) Jayasekara, R.; Harding, I.; Bowater, I.; Lonergan, G. Biodegradability of a selected range of polymers and polymer blends and standard methods for assessment of biodegradation. J. Polym. Environ. 2005, 13 (3), 231–251. (18) Dreyer, L. C.; Niemann, A. L.; Hauschild, M. Z. Comparison of three different lcia methods: Edip97, cml2001 and eco-indicator 99 does it matter which one you choose?. Int. J. Life Cycle Assess. 2003, 8 (4), 191–200. (19) Landis, A. E.; Theis, T. L. Comparison of life cycle impact assessment tools in the case of biofuels. IEEE Int. Symp. Electron. Environ. 2008. (20) Pant, R.; van Hoof, G.; Schowanek, D.; Feijtel, T. C. J.; de Koning, A.; Hauschild, M.; Pennington, D. W.; Olsen, S. I.; Rosenbaum, R. Comparison between three different lcia methods for aquatic ecotoxicity and a product environmental risk assessment insights from a detergent case study within omniitox. Int. J. Life Cycle Assess. 2004, 9 (5), 295–306.
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