Sustainable development is a great goal - especially if we can substitute renewable resources for finite ones. Unfortunately an analysis presented to the American Chemical Society, indicates that that in some (but not all) cases, "green" manufacturing may be more energy-consuming and polluting than traditional methods. The study by Assistant Professor Tillman Gerngross of Dartmouth College examined the process by which maize, or corn, is turned into plastics called polyhydroxyalkanoates, or PHAs. Currently petroleum, a finite resource is the basis for most plastics. This dependence on fossil fuels has prompted exploration of manufacturing processes that substitute renewable resources, such as plants for petroleumn (plants can be turned into plastics through fermentation).
PHA-based plastics have similar properties as polystyrene that is used extensively in the food packaging and fast food industry. Amongst a group of alternative polymers, PHAs are considered one of the leading candidates to replace conventional plastics on a large industrial scale. This is because in addition to being synthessed biologically, the final products are also fully biodegradable.
Unfortunately according to this research the process used to make PHA's consumes nineteen times more electricity, twenty two percent more steam, and seven times more water than the chemical method of manufacturing polystyrene. Professor Gerngross notes that, "Using plants as the basis of polymer production sounds like a sustainable solution, but in this particular case the increased energy consumption has offset any benefits from switching to a renewable raw material. By focusing on the origin of the raw materials we have lost track of the energy it takes to move materials through a process."
In their work the research team calculated the indirect use of fossil fuels for generating the power used in manufacturing, the net energy required to produce the fertilizer, insecticide and herbicides used in growing maize as well as the energy required to harvest and process the crop. They found the total energy needed to produce one pound of PHA is equivalent to the consumption of 2.39 pounds of fossil resources. Producing the same amount of polystyrene using chemical manufacture requires only 2.26 pounds of oil. And the difference doesn't stop there. In the fermentation process, the entire 2.39 pounds would have to be burned to produce energy; whereas in the chemical process only 1.26 pounds would be burned. So the polluting effects of the 'green' approach are also greater.
Does this spell the death knell for the use of plants in the production of plastics? According to Professor Gerngross, " It would be most unfortunate if this study were viewed as a general indictment of biological processing. We now have the tools to look at the environmental impact of biological processes. We expect some processes to fail this analysis but we anticipate others will demonstrate superior performance. We will be looking for both."
