CORRESPONDENCE/REBUTTAL pubs.acs.org/est
Comment on “Sustainability Metrics: Life Cycle Assessment and Green Design in Polymers”
W
e have several criticisms of this paper and its Supporting Information. These include general conceptual matters and detail aspects of the LCA information which appear to fall short of accepted good practice for LCA research. We are sharing our reservations in this correspondence because publication in ES&T may confer a level of credibility to some of this paper’s content that is undeserved. At least three general conceptual matters are problematic with this paper. First, it provides a ranking of diverse polymers either via their performance under green design metrics or under a cradle-to-gate LCA on a unit volume basis. There is no objective basis for ranking the selected single quantities (volume) of these divergent polymers on a common scale because of their different functional properties, costs and uses. This is particularly inappropriate for LCA-based rankings because the core aspect of a common and meaningful Functional Unit for LCA studies is not met. You would not make a carbonated drinks container from HDPE, just as you would not make a stretch film wrap from PET. Several full LCA studies that conform to the ISO 14040 series address such aspects in detail and show that a high degree of expertise and care is required when selecting the appropriate Functional Unit. For example, a study on clam shell type packaging1 examines four different polymers (PLA, PP, OPS, and PET) and shows that density is only one of several parameters for deriving an adequate Functional Flow for a robust comparative analysis. Similarly, another full ISO 14044 LCA study on plastic bags shows that mechanical properties of different polymers are highly relevant for the final functionality.3 Second, obsolete ISO standards are cited in the paper as the basis for the LCA work (ISO 14041, 14042, and 14043 were withdrawn in 2006 and ISO 14040 and 14044 updated). The ISO standards require that a third party review is conducted by a minimum of three independent reviewers of LCAs for comparative assertions disclosed to the public (as are made in this paper), such a review of the LCA element does not appear to have been done. The work presented also seems to lack attention to other basic LCA requirements, for example, transparency, data uncertainty consideration, sensitivity analysis for key data and methodological choices, etc. Nowhere in the paper or in the Supporting Information is given the normalization and/or valuation/weighting system used as the basis for deriving the single scores for the LCA, yet these exert a critical effect upon the outcomes of single score comparisons (the “rankings” as used in the paper). Valuation or weighting transformations of category indicator values are specifically excluded under the ISO 14040 series for comparative assertions. Third, it is understood that the paper examines if there is a correlation between “empirical” scores for adherence to green design principles and the cradle-to-gate LCA scores for the polymers and manufacture routes. What is the hypothesis for this comparison of metrics? Is there an expectation that the rankings by two different metric systems that are concerned with different things—green design and life cycle environmental impacts—should be “similar”, or should they be expected to be r 2011 American Chemical Society
“different”? Ultimately what do the outcomes of this comparison tell us apart from the fact that scoring systems based on different criteria may yield different rankings? Detail criticisms and reservations about the LCA work are illustrated in the following examples. There is no discussion of the qualitative aspects of the LCI data used—this is required under ISO 14044—nor adequate transparency on the inventory data used (almost nothing of what is normally expected, even in the Supporting Information). Some of the data sets that we understand were used are up to a decade old. The statement that “the environmental impacts of biopolymer disposal have yet to be studied and adequate data on the emissions and energy use of degradation are unavailable” is simply not true.2 The “adjustment” by the Pittsburgh group to the EcoInvent v1.2 sugar cane farming data set by averaging other EcoInvent v1.2 data for different crops is highly inappropriate since the PM10s and methane emissions in the sugar cane data set likely relate to sugar cane crop burning in-field (albeit a practice that is being progressively reduced or phased out in Brazil). Only one Life Cycle Impact Assessment method is used (TRACI) to derive single score results with no sensitivity analysis of its influence on the results (i.e., a comparison with CML or EcoIndicators 99 is warranted). As just one example of the critical effects of data selection, we note that the eutrophication scores for bPET and pPET derived from the EcoInvent v1.2 are 2 orders of magnitude higher than the latest, freely available Plastics Europe PET data set (downloadable on Plastics Europe’s Web site since June 2006), COD is an important driver for this. Within these data sets the respective COD levels are Plastics Europe data 1.4 g COD/ kg bottle grade PET, EcoInvent v1.2 data 107 g COD/kg bottle grade PET. Based on the above we have very serious reservations that the comparison(s) presented in the paper give an accurate representation of these polymers. It seems likely that a detailed critical review of the comparative LCA work under ISO would have been highly beneficial to the manuscript development and publication process. Unfortunately, the many problems severely compromise the paper’s findings. The statements about the environmental impacts of these polymers under LCA are, in our view, at best unreliable and at worst simply incorrect. Richard Murphy,*,† Andreas Detzel,‡ Miao Guo,† and Martina Kr€ uger‡ † ‡
Imperial College London, U.K. Institute for Energy and Environmental Research (IFEU), Heidelberg, Germany
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] Published: May 09, 2011 5055
dx.doi.org/10.1021/es103890v | Environ. Sci. Technol. 2011, 45, 5055–5056
Environmental Science & Technology
CORRESPONDENCE/REBUTTAL
’ REFERENCES (1) Detzel, A.; Kr€uger, M. Life Cycle Assessment of Polylactide (PLA): A Comparison of Food Packaging Made from NatureWorksÒ PLA and Alternative Materials, Final Report, July 2006; IFEU GmbH: Heidelberg, Germany, 2006; Available at www.natureworksllc.com (accessed April 23, 2011). (2) Rosa, D. S.; et al. The biodegradation of poly-b-(hydroxybutyrate), poly-b-(hydroxybutyrate-co-b-valerate) and poly(e-caprolactone) in compost derived from municipal solid waste. Eur. Polym. J. 2003, 39, 233–237. (3) Wellenreuther, F.; et al. Life Cycle Assessment of Waste Bags, Final Report, June 2009; IFEU GmbH: Heidelberg, Germany, 2009; Available on request from www.kunststoffverpackungen.de (accessed April 23, 2011).
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