CORRESPONDENCE/REBUTTAL pubs.acs.org/est
Comment on “Sustainability Metrics: Life Cycle Assessment and Green Design in Polymers” abone et al.1 present a useful template for evaluating polymers from cradle to resin. It is necessary to point out one error and one significant caution associated with this methodology. In Table 2, overall Atom Economy associated with polyvinyl(chloride) (PVC) is given without reference as 55%. This is incorrect. The process of making PVC involves (1) addition of chlorine to ethylene over an iron catalyst to produce 1,2dichloroethane (EDC), (2) dehydrochlorination of EDC to vinyl chloride, (3) recycling coproduct HCl with ethylene and oxygen over a copper catalyst to generate more EDC, and (4) free-radical polymerization of vinyl chloride with recycling of any unpolymerized material. The atom efficiency of processes 1, 2, and 4 are 99%, 98%, and 99þ%, respectively. Process 2 is carried to 5060% conversion to avoid side product formation; however, unreacted EDC is purified and recycled in the cracking process. Side product formation is about 2%, and conversion on a chlorine basis of EDC to VCM is about 97% for ultimate atom economy of about 94%.2 Omission of the recycling process in the calculation may be the source of the error. Treating material from cradle to resin, while a useful step in life cycle assessment, can be misleading. Differences in physical properties can distort the assumption that equal volumes of materials will be used to produce fungible articles. As an example, a significantly larger mass of high-density polyethylene (HDPE) than PVC must be used to manufacture pipe with the same pressure specifications. This difference is significant and may impact the decision on the “greenest” raw material for an application, but by design is not captured by the methodology in this article. This comment should not detract from what is a good attempt to put green chemistry metrics in the context of real product choices. At the same time, the error and caution must be taken into account if correct choices are to be made on that basis.
T
William F. Carroll, Jr. Occidental Chemical Corporation, 5005 LBJ Freeway, Suite 2200, Dallas, Texas 75244, United States
’ AUTHOR INFORMATION Corresponding Author
E-mail:
[email protected].
’ REFERENCES (1) Tabone, M. D.; Cregg, J. J.; Beckman, E. J.; Landis, A. E; Sustainability metrics: life cycle assessment and green design in polymers. Environ. Sci. Technol. 2010, 44, 82648269; DOI 10.1021/ es101640n (2) PVC Handbook; Wilkes, C. E., Summers, J. W., Daniels, C., Eds.; Carl Hanser Verlag: Munich, 2005. Published: May 09, 2011 r 2011 American Chemical Society
5054
dx.doi.org/10.1021/es103760e | Environ. Sci. Technol. 2011, 45, 5054–5054
