A scientific look at the value proposition of biobased plastics

ASTM standard D6866 helps determine the amount of CO2 removed from the environment by kilogram of material, says MSU professor

ASTM logoASTM International (West Conshohocken, PA) recently announced a revised standard (D6866) used by the U.S. Department of Agriculture’s (USDA) bio-preferred program and the Environmental Protection Agency (EPA) to help improve the sustainability of plastics. The revised standard will help these organizations determine which bioplastics are truly “greener” and which fall short in the effort to be sustainable. In addition to being used by the USDA and EPA, D6866 is referenced by many large corporations throughout the world that are increasingly using biobased products.

Michigan State University Distinguished Professor Ramani Narayan forwarded me a paper in response to my questions regarding the revised standard. Narayan is a member of the ASTM’s Committee on Plastics. In his discussion paper around biobased polymers, Narayan states that “replacing petro/fossil carbon with biobased carbon (from plant-biomass feedstocks) in plastics and industrial products offers the value proposition of removing carbon present as CO2 in the environment and incorporating it into a polymer molecule via plant-biomass photosynthesis in a short time scale of one (agricultural crops, algae) to 10 years (short rotation wood and tree plantations) in harmony with nature’s biological carbon cycle.”

Contrary to plant biomass cycles, Narayan notes that plastics made from petro/fossil resources (like oil, coal and natural gas) “are formed from plant biomass over millions of years and so cannot be credited with any CO2 removal from the environment, even over a hundred-year time scale (the time period used in measuring global warming potential, GWP100).

I would like to question Narayan about the idea of global warming—now called simply “climate change”—given that projections based on computer models done a number of years ago predicted a warming trend that, in fact, did not occur, according to a number of climatologists.

Determining the “biobased carbon content of products” using “radio carbon analysis as codified” in ASTM D6866 and “applying fundamental stoichiometric calculations” (the calculation of the quantities of reactants and products in chemical reactions) means that “one can readily calculate the amount of CO2 removed from the environment by 1 kg of material.” For example:

  • 1 kg of biobased polyethylene (PE) containing 100% biobased carbon content would result in removing 3.14 kg. of CO2 from the environment.
  • 1 kg of PLA (100% biobased carbon content) would remove 1.83 kg of CO2 from the environment.
  • 1 kg of the current bio PET (20% biobased carbon content) results in 0.46 kg of CO2 removal from the environment.
  • 1kg of the 100% biobased carbon content PET results in 2.29 kg of CO2 removal.

In contrast, the petro-fossil carbon-based products result in zero CO2 removal from the environment.

In other words, if I understand Narayan correctly, we can “capture” CO2 in plastic materials, thus preventing it from escaping into the atmosphere where it can potentially cause harm. However, notes Narayan, eventually at the end of life of these plastics, “the carbon will be released back into the environment as CO2 through waste-to-energy systems, incineration, composting or anaerobic digestion (if the plastic has a biodegradability/compostability feature built into it). However, the CO2 released will be captured by the next season’s crop or biomass plantation, resulting in a net zero material carbon footprint, in harmony with nature’s carbon cycle. In contrast, non-biobased PE or PP will contribute a net 3.14 kg of CO2 into the environment for every 1 kg of PE used. 1 kg of PE will contribute 2.29 of CO2 to the environment.”

Narayan points out that while some proponents of biobased polymers say that only 100% biobased polymers are acceptable—“an all or nothing option”—this is based on the "analogous requirements of ‘complete biodegradability in a targeted disposal environment in a short defined time period." This thinking is the result of studies that show that “degraded” or “partial biodegraded fragments left in the environment could have environmental consequences.” However, even “partial substitution of the petro-fossil carbon by biobased carbon results in a positive environmental value attribute—removing CO2 from the environment.

He also notes that the “biobased carbon content has zero impact on the end of life of biodegradable plastics. The molecular structure of the plastic and the availability of its carbon for transport into the microbial cell and subsequent utilization for energy drives the microbial assimilation (percent of biodegradability) of carbon substrates like plastics—the availability of carbon in a molecule to the microbes and not the source of the carbon is the key learning.”

There are some who would disagree with Narayan on whether or not fossil-fuel based polymers are biobased. Critics such as Professor Emeritus Igor Catic of the University of Zagreb in Croatia argue quite convincingly that anything that is created by nature and comes from the natural sources of the earth is a biobased material.

What Narayan has done, I believe, is develop a way to find value in the sequestration of CO2 in polymers and the end-of-life strategy. Several years ago, at an SPE Thermoforming Conference, Narayan commented in a panel discussion on this topic: “Biodegradable is a misused and abused term. What we need is an end-of-life strategy.”

He even asked, “Why is replacing petro-carbon with bio-carbon better? Carbon is carbon. There is organic carbon and inorganic carbon. It takes 10 years to turn an inorganic carbon into an organic carbon through biodegradability.”

And I believe that he would agree with Catic when he stated: “There’s nothing un-natural about oil or coal. The rate and time scales for carbon is millions of years (to turn organic matter into oil or coal) vs. 10 years to turn inorganic matter into organic matter.

“There are 10 carbons in PET—if I replace two of them with bio, what is the value?”

Perhaps Narayan has found that value, or at least he continues to search for that value and an end-of-life strategy that will provide some answers.          

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