|Fig. 1: Does corn plastic take less energy to create than PET? And does it really compost into soil after use? (Source: Wikimedia Commons)|
I was recently eating at a picnic and grabbed a corn derived fork out of its box. The box caught my eye with language like "Certified Compostable," "We Turn into Soil, not Waste," "Converts to Soil in 3-6 Months*," and "Takes less energy to make than traditional plastic." All of this sounded very good, not to mention the very earthy, natural packaging. Now, everything on a package aimed at selling has to be taken with a grain of salt, so I decided to dig into the numbers and facts on corn-derived plastics.
The first claim I examined was the composting claim. I noticed that asterisk on the 3-6 months, and it read "In a commercial composting facility." This was intriguing, and apparently it is the case that PLA does not degrade at all in a regular compost site.  In fact, it sticks around just as well as PET, which can be up to 1000 years.  So what does it need to decompose? The plant for composting must be maintained at 140 degrees Fahrenheit, and the PLA only decomposes with a large supply of Oxygen.  Collection for these facilities rarely exist, because of the relatively small use by consumers. Commercial composting occurs from large stadiums, where all of the items are known to be PLA, and truckloads can be removed at once. And since PLA is not recyclable, as a consumer buying PLA instead of PET (which is readily recyclable), basically destines the plastic to end up in the landfill, where it will sit for hundreds of years.
The second claim I looked at was the energy cost of producing PLA, or PolyLactic Acid, the type of plastic that is made from corn.  The claim is that it only takes 65% of the energy to make PLA as it does to make PET.  Looking into the production of PLA first, the energy cost of producing the corn that is used to make the PLA is reported as 5.4 MJ/kg of PLA by Cargill Dow.  Now Cargill owns NatureWorks, a huge producer of PLA, and Cargill itself is the largest merchant of corn in the world, so while they should know what they are talking about when it comes to growing corn, they are also invested in marketing PLA as an outlet for their corn. Looking at USDA numbers, it takes 42,000 BTU/bushel or 3.8 MJ/kg of corn produced.  However, this number needs to be weighted by the amount of corn that is needed to create the PLA, which is about 1.75 kg corn/kg PLA, again according to Cargill.  Therefore, using the USDA numbers, we get 6.6 MJ/kg of PLA to produce the corn. The number from Cargill is a little low, but both of these numbers are still a small percent of the energy required to then take the corn and convert it to PLA, 48.8 MJ/kg.  Therefore, the total energy to create the PLA, including corn growing, is between 54.1 and 55.3 MJ/kg.
How does this compare to the production of a competitor plastic, PET? Looking at a report from Environmental Research Letters, the energy required to produce PET is 70 MJ/kg.  Now we should add the cost of acquiring the oil, which is about 1/25 of the energy of the oil.  So adding this in, we get 71.7 MJ/kg. Now if we divide the energy to create the PLA by the energy to make the PET (both of these numbers are to make the pellets of the plastic, not a finished product), we get about 0.77. So not surprisingly, the marketed number of 65% seems a bit low, but if all of this data is correct, it is in the ballpark.
However, there is one more caveat to the number for PLA. The 55 MJ/kg isn't actually the entire energy that is needed to create the PLA. There is another 28 MJ/kg of energy in the corn used.  Now this is "renewable" energy, but energy that is being produced on arable land that is being used to produce plastic instead of feed people isn't free. So if this is added to the calculus, then it is 83 MJ/kg for PLA, or 115% of the PET number.
Analyzing the claims on the package revealed some truth and some very stretched truth. While it is true that less petroleum energy is consumed to create PLA than PET, when the energy of the corn is added into the calculus, more energy is needed for PLA than PET. However, in the "We turn into soil" claim, a larger divergence from the truth is found. It is next to impossible that the forks from that package will ever make it to a large-scale private composting site, and thus they are non-recyclable expensive landfill.
© Elliot Hawkes. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 E. Royte, "Corn Plastic to the Rescue," Smithsonian Magazine, August 2006.
 E. Vink et al., "Applications of Life Cycle Assessment to NatureWorks™ Polylactide (PLA) Production," Polymer Degradation and Stability 80, 403 (2003).
 H. Shapouri et al., "2008 Energy Balance for the Corn-Ethanol Industry," U.S. Department of Agriculture, Agricultural Economic Report Number 846, June 2010.
 P. H. Gleick and H. S. Cooley, "Energy Implications of Bottled Water," Environ. Res. Lett. 4, 014009 (2009).
 J. D. Hughes, "Drill Baby, Drill: Can Unconventional Fuels Usher in a New Era of Energy Abundance," Post Carbon Institute, February 2013.