Do Biopolymers Make Sense?

Trying to determine the environmental benefit of biopolymers can be taxing. The general assumption is that it must be good to make plastics from plants as opposed to oil. Plants sequester carbon dioxide while growing, and can be replenished rapidly.

They also theoretically reduce our dependence on foreign oil. But do they really? There’s no question that farming is  energy intensive, and also requires use of fertilizers that can wash into aquifers. There are very few quality life cycle assessments (LCAs) that engineers can use to make a meaningful evaluation.

Now comes a study conducted at the University of Pittsburgh that may shed some light on the situation.  Researchers at Pitt’s engineering school examined seven petroleum-based polymers, four biopolymers, and one hybrid. They first performed a life-cycle assessment on each polymer’s preproduction stage to gauge the environmental and health effects of the energy, raw materials, and chemicals used to create one ounce of plastic pellets. Then they checked each plastic against a group of  principles of green design, including biodegradability, energy efficiency, wastefulness, and toxicity.

Here’s what they found: The four biopolymers are among the more prolific polluters on the path to production. This is due to fertilizers and pesticides, extensive land use for farming, and the intense chemical processing needed to convert plants into plastic. All four biopolymers are the largest contributors to ozone depletion. The two tested forms of sugar-derived polymer–standard polylactic acid  and the type manufactured by NatureWorks–exhibit the maximum contribution to eutrophication, which occurs when overfertilized bodies of water can no longer support life. One type of the corn-based polyhydroyalkanoate, PHA-G, topped the acidification category. In addition, biopolymers exceeded most of the petroleum-based polymers for ecotoxicity and carcinogen emissions.

Once produced, however, bioplastics rate better, according to the Pitt study.  For example, the sugar-based plastic from NatureWorks jumped from the sixth position under the LCA category to become the material most in keeping with the standards of green design. Oil-based polypropylene is the cleanest polymer to produce, but sank to ninth place as a sustainable material.

The project was supported by the National Science Foundation.

Following is a table of the Pitt findings as published in Environmental Science & Technology:

Is this the be-all-and-end-all analysis? Of course not. It’s a point of view, albeit one that was done without any apparent bias and with technical credibility. But the criteria may be questionable. For example, is biodegradability really a principle of “green design”? I doubt it, given the lack of composting facilities anywhere in the world. And this is only a sampling of four types of biopolymers. There are many, and many more coming. It should also be noted that many biopolymer producers aim to shift from agricultural products to waste biomass. That should have a major impact on this type of analysis.

It’s not clear why PET produced from oil ranks so low on the LCA analysis, or why PET made from sugar can ranks so low on green design. Toyota is making a major investment in bio-PET, and I would guess it disagrees with this assessment.

One of the biggest benefits of the Pitt analysis is that it sheds light on the role of fertilizers and pesticides on the production of biopolymers.

The project will continue with a full LCA, which will also examine the materials’ environmental impact throughout their use and eventual disposal. This is, of course, extremely important. The current widespread recyclability and re-use of PET (both types) will be an important factor in their overall value.

Possibly, the researchers should consider including a third criteria: overall engineering value of the plastic.  The real race isn’t on to develop a material like thermoplastic starch that rates well on just LCA or green design, but on a combination of LCA, green design, and overall performance. That’s where companies like DuPont are betting the ranch, and that’s where the real action will be for design engineers. That’s why materials like the castor oil-derived polyamides are major players.