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How "green" are "green" plastics?

Posted 09/01/00

Summarized from Scientific American, August 2000, Pages 36-41 (archive no longer available online)

Fossil fuels provide both the power and the raw materials that transform crude oil into common plastics - a process that consumes approximately 80 million tons of oil and natural gas a year in the United States alone. As known global reserves of oil are expected to run dry in approximately 80 years, many were delighted by the discovery that plants can be genetically engineered to produce plastics.

Recent studies of this new technology, however, reveal serious energy-consumption obstacles and hidden environmental burdens and highlight the need for not just renewable raw materials, but renewable energy sources as well. These latest findings are described in "Scientific American" (August 2000, Pages 36-41), in an article titled, "How Green Are Green Plastics?" by Tillman Gerngross, Assistant Professor of Biochemical Engineering at Dartmouth's Thayer School of Engineering, and Steven Slater, a senior researcher at Cereon Genomics in Cambridge, Mass.

The ability to grow plastic in plants seems at first a perfect way to reduce our dependency on fossil fuels. Plant-based plastics would not only be made from a renewable resource, but upon disposal would be biodegradable.

Further analysis of the process, however, reveals three main problems:

1) The energy required to extract plastic from plants is currently much greater than anyone expected.

2) All fossil fuel used to make plastic from plants is burned for energy, adding to the problem of greenhouse gas emissions and acid rain, as opposed to oil-based plastic which incorporates a significant portion of its fossil fuel requirements into material for the final product.

3) Biodegradability is not necessarily a good thing because as these plastics break down, they release carbon dioxide and methane - more heat-trapping greenhouse gases.

Gerngross and Slater suggest a way of actually reversing the flux of carbon into the atmosphere by producing non-degradable plastic from resources that absorb carbon dioxide from the atmosphere, such as plants. Burying this plastic after use would then trap the carbon in the ground instead of releasing it back into the atmosphere.

The Gerngross/Slater article clearly shows that, at this point, it is impossible to argue that plastic grown in corn and extracted with energy from fossil fuels would conserve fossil resources. Moreover, production of plant-based plastics inevitably emits more greenhouse gases than its petrochemical counterparts. Successfully making green plastics therefore depends on whether researchers can overcome these energy-consumption obstacles economically, without creating additional environmental burdens.

Using renewable biomass as a primary energy source in the corn-processing industry would be the first step in that direction. Gerngross and Slater's evaluation of plastics processing reveals that it's the use of a plant-based energy source, not the use of plants as a raw material, that stands to provide the biggest environmental benefit. And this benefit can be applied to many industrial processes regardless of the approach to making plastics. However, this shift would require building an entirely new power-generation infrastructure, and current energy-consumption patterns in the primary corn-farming states show the exact opposite trend-namely a disproportional use of fossil energy in power generation.

Renewable energy is the essential ingredient in any comprehensive scheme for building a sustainable economy, and as such , it remains the primary barrier to producing truly "green" plastics.

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