Bioplastics and Global Warming
Fred Michel, PhD
Associate Professor of Biosystems Engineering
The Ohio State University
Biodegradable bioplastics are meant to be products that are environmentally friendly relative to plastics made from fossil fuels. To assess environmental acceptability, however, many aspects of a materials life cycle must be considered. These include biodegradability, durability, litter potential, recyclability, feed stock source, energy use for production, how it is ultimately disposed of, and greenhouse gas emissions. A material can be beneficial from one respect, but detrimental from another. Such is sometimes the case with biobased plastic alternatives.
Plastics are made of polymerized repeating units, or monomers, derived largely from fossil sources such as natural gas or crude oil. However the repeating units can also be made from renewable resources. For example lactic acid derived from corn fermentation can be made into polylactic acid or PLA. Another example is the conversion of sugar cane to ethanol and then to ethylene, the building block of one of the most widely used plastics; polyethylene. Although made from a biomass feedstock, this type of polyethylene is still essentially not biodegradable and like all polyethylene persists in the environment for centuries. On the other hand, petroleum can be used to make plastics that are biodegradable. For example lactic acid used to make polylactic acid (PLA) is produced both from the fermentation of starch and synthetically from petroleum, and both are biodegradable. On this basis, plastics can be classified into four types; plastic, bioplastic, biodegradable plastic and biodegradable bioplastic (see table). Understanding how these four classes of materials affect atmospheric CO2 accumulation can be confusing and is not always straightforward.
THE FOUR CLASSES OF PLASTICS...
| ||PETROLEUM/NAT GAS ||BIOMASS |
|NOT BIODEGRADABLE ||PLASTIC |
e.g. Polyethylene from petroleum
e.g. Sugarcane Polyethylene
|COMPOSTABLE ||BIODEGRADABLE PLASTIC |
e.g. Polylactic acid from petroleum
|BIODEGRADABLE BIOPLASTIC |
e.g. PLA from starch
As recent studies have shown, global oil production is at or past its peak. Worldwide oil discoveries peaked nearly 40 years ago, and today production outpaces new discoveries by a wide margin. In another 40 years, oil will be scarce, very expensive and nearly completely depleted. Because of this, alternatives to petroleum products must be developed soon, including plastics. Eventually the feed stocks for nearly all plastics will be renewable sources such as wood, corn, soybeans, or other types of biomass.
Plastics made from petroleum, such as polyethylene, have a well defined lifecycle. Of the carbon required to produce 1 kg of polyethylene from petroleum, 75% is embodied in the plastic itself and the remaining is energy used to make the plastic. When landfilled, the carbon in the plastic will be sequestered and not contribute to global warming. Recycled polyethylene would contribute less fossil CO2 to the environment. This is because less energy is used to recycle polyethylene than is used to make it in the first place. So landfilling may not be such a bad fate for petroleum based plastics that are recycled and not biodegradable.
For some of the reasons presented above, more and more plastics are being made from biomass feedstocks. A second class of plastics are called “Bioplastics”, and defined as traditional non-biodegradable plastics made from renewable feedstocks. On balance this type of plastic offers the greatest potential to reduce greenhouse gases in the atmosphere by sequestering carbon. This is because atmospheric CO2 is fixed into carbohydrates used as their feed stock. If the plastic is eventually landfilled, this carbon will become locked for millennia within the landfill and on balance reduce atmospheric CO2.
The third class of plastics, biodegradable bioplastics, are made from biomass and are also designed to be compostable and/or biodegradable. These types include PLA and PHB made from corn. This class of polymer is carbon neutral from the standpoint of the carbon in the plastic, but a substantial amount of fossil energy is used to produce the plastic and the biomass feedstocks.
The class with perhaps the greatest potential to contribute to green house gas emissions is biodegradable plastics made from petroleum. This is because not only is fossil energy needed to produce them in the first place, but even more carbon is released when the material ultimately biodegrades. If this biodegradation occurs in a landfill, then it usually will generate methane, which is a greenhouse gas with 21 times the warming potential of CO2. Most landfills do a poor job of capturing this methane, even those with methane recovery systems. So ultimately methane, whose carbon was originally derived from a fossil fuel, will end up in the atmosphere.
This is where composting plays an important role. When biodegradable plastics are used it is important that they be composted and not landfilled. That way they ultimately will be converted to CO2 and not methane. Since this CO2 originated from biomass, on balance it will not increase atmospheric CO2.
In the end, all the aspects of a products impact on the environment must be considered, of which greenhouse gas emission is only one. But knowing how different classes of plastics may impact greenhouse gas emissions is an important piece needed to complete the complex sustainability puzzle.