Sustainability of Polymers from Renewable Sources or Fossil Fuel. Who wins?

Guest Authored with Zoltan Mester, Ph.D,

Common thinking would dictate that polymers from renewable sources would be more sustainable and environmentally friendly.  But is this really the case?  For polymers, LCA and Green Design evaluations were presented in an outstanding recent paper titled “Sustainability Metrics: Life Cycle Assessment and Green Design in Polymers,” by Amy Landis, et. al., Environmental Science and Technology, September 2, 2010.  The work assessed both fossil fuel based synthetic polymers and biopolymers.  The fossil fuel based polymers included: polyethylene terephthalate (PET), high density polyethylene (HDPE), low density polyethylene (LDPE), polypropylene (PP), polycarbonate (PC), polyvinyl chloride (PVC), and general purpose polystyrene (GPPS). The biopolymers included polylactic acid via general process (PLA-G), polylactic acid by Nature Work (PLA-NW) (Nature Work is wholly owned company by Cargill Co.), polyhydroxyalkanoate derived from corn grain (PHA-G), and polyhydroxyalkanoate (PHA-S) derived from corn stover.  The study also included a hybrid fossil fuel based and bio-based polymer biopolyethylene terephthalate (B-PET). 

The LCA evaluations were based using the following parameters:

• Acidification
• Carcinogenity
• Ecotoxicity
• Eutrophication
• Global warming
• Non-carcinogenetic
• Ozone depletion
• Respiratory impact
• Fossil fuel depletion

Green Design was evaluated based on the following parameters:

• Atom economy
• Carcinogens (kg benzene equivalent)
• Non-carcinogens (kg toluene equivalent)
• Respiratory effect (kg PM-2.5 equivalent/liter)
• Ecotoxicity (kg benzene equivalent/liter)
• Cumulative energy use (MJ equivalent/liter)
• Percent renewable material
• Percent recovery
• Biodegradation

The table below depicts the results for polymer rankings by LCA and Green Design evaluations:

The ranks represent low (worse) to high (better) performance ranking.

The figure below shows the tabulated ranks plotted according to decreasing Green Design performance and the associated LCA performances:

These results highlight an important finding: LCA ranks and Green Design ranks do not necessarily correlate. For example bio-based polymers have generally the best Green Design performance but they are not the best LCA performers.  The reasons for this include that bio-based polymers may need significant process energy to manufacture (fossil fuel energy needed for grinding, distillation, extraction, etc.), fertilizer needs, pesticide and herbicide use, water supply and potential for causing ecological depletion of resources. 

Considering the LCA ranks, traditional fossil fuel based polymers PP, HDPE and LDPE were the best performing polymers.  Remarkably for HDPE and LDPE, atom economy is 100 percent since they are manufactured by addition reactions and for this specific parameter they are good green chemistry polymers. 

It should be emphasized that the results of LCA and Green Design studies depend on the specific parameters chosen for the evaluation.  For example fossil fuel energy requirements can be reduced if renewable energy sources are available (solar, hydroelectric, wind, biomass), atom economy can be improved by newly discovered catalytic conversions, and for bio-based materials genetic engineering, although not without controversy, can reduce pesticide and herbicide use.  The very notion that a chemical substance, including a polymer, is labeled “green” “organic” or “natural” also depend on industry accepted ratings (such as the product 95 percent naturally derived) and these ratings are constantly evolving and not without controversy. 

For more details on the study methods, interested readers are encouraged to get “Sustainability Metrics: Life Cycle Assessment and Green Design in Polymers,” by Amy Landis, et. al., Environmental Science and Technology, September 2, 2010.