Map of recent plastic bag bans (click on the image to open the map; photo credit: Factory Direct Promos)
This past August the town that I live in, Evanston, Illinois, banned the distribution of single use plastic bags. Large stores in Evanston can now only provide paper bags at checkout. With its approach, Evanston is following the example of Los Angeles and many other communities in California and elsewhere in the U.S. in an attempt to reduce municipal plastic waste (check out the case study in the Plastic // Recycle module for more background on this). Similar legislation has become popular throughout the world in the last 10 years as a way to make cities and countries more sustainable. In many cases plastic bags are banned, but stores can still distribute paper bags. This got me thinking about how sustainability is assessed. What factors are considered when determining how green a product is? Is paper really more sustainable than plastic?
Think in life cycles
When I initially think about choosing between paper and plastic bags my mind goes to the photos of plastic pollution in our waterways. Plastic is slow to break down when it ends up in the environment and plastic products are often swallowed by birds and other animals with fatal consequences. However, when we assess the sustainability of a product, or how green a product is, we have to consider its entire life cycle, not just what happens to it after use. If we only consider one part of a product’s life cycle, such as the fate of plastic bags in our oceans, we may not have an accurate description of how green a product really is.
General life cycle of a product.
(photo credit: )
The life cycle of a plastic bag
The life cycle for a plastic bag actually begins with oil (for more on the life cycle of a plastic bag after it is used check out this video). Crude oil, the unprocessed oil that comes out of the ground, contains hundreds of different hydrocarbons (molecules made from hydrogen and carbon), as well as small amounts of other materials. The job of an oil refinery is to separate these materials and also to break down (or "crack”) large hydrocarbons into smaller ones. Next, a petrochemical plant receives refined oil containing the small hydrocarbons, including ethylene and propylene, which can be used like building blocks to make long chain molecules called polymers. A plastics factory buys these polymers in the form of resins and introduces additives to modify or obtain desirable properties, then molds or otherwise forms the final plastic products. This video on how plastics are made explains this process in more detail.
The molecule ethylene, a monomer, is the building blocks put together to form the polyethylene polymer that is used to make polymers.
Even though plastics are made from oil, a natural resource that cannot be replenished, the production of the polymers used to make plastics is a fairly efficient process through the use of catalysts, molecules that help to speed up a chemical process. The discovery of catalysts for the polymerization of plastics by Drs. Karl Ziegler and Giulio Natta led to a shared Nobel Prize in Chemistry in 1963. Today, Ziegler-Natta catalysts are used throughout the world to produce a variety of polymers.
And paper bags?
On the other hand, paper bags are manufactured using energy-intensive methods. To make paper, trees must be cut down, de-barked and chipped, then, the remaining material must be pulped and refined. This process is energy intensive, consumes tons of water, and introduces pollutants to the atmosphere. Pollutants from paper manufacturing include greenhouse gases, including carbon dioxide, as well as methanol and sulfur, which as you can imagine makes for a pretty smelly process.
So, which one is 'greener'?
To determine this, we need to look at the entire life cycle of shopping bags. Life cycle assessments that involve quantitative (that is, frequency-based or numerical) information need to be gathered and studied, for example, how much carbon dioxide the manufacturing of a bag produces. Typically, when comparing statistics for the production of shopping bags, bags made from plastic are found to have a lower carbon footprint (less energy input, fewer green house gases out) than bags made from paper or cloth as seen in the chart.
Comparison of existing life cycle analysis of shopping bag alternatives (click for higher resolution image).
(photo credit: Copyright 2007, State of Victoria, Australia)
If the production of single use plastic bags actually has a lower carbon footprint, then why are cities targeting plastic bags? You’ve probably figured this out already: the environmental consequences of improperly disposed plastic bags and bottles can still be devastating, and this is primarily what these cities are trying to avoid. In other words, it's not the production, but the disposal of plastic bags that creates so many problems.
What can we do to merge low carbon footprint manufacturing with minimizing plastic waste?
You’ve probably guessed this as well… a reusable grocery bag. If a reusable plastic bag is made from recycled plastic it has a low carbon footprint, and potentially never needs to be thrown away. If consumers understand the science behind the paper vs plastic question we can all make environmentally friendly decisions, which include recycling plastic properly and using reusable bags that are manufactured from plastic. While many cities with a plastic bag ban offer paper bags, the hope is for people to choose reusable bags. In Evanston for instance, our plastic bag ban came with a campaign called Think Outside the Bag, which kicked off with an event of reusable bag giveaways and started a reusable bag sharing program that allows people to donate and pick up bags.
The bag problem is relatively simple, and many of us already knew the answer before we started this discussion, but it is a good example of how you have to look deeply at a product’s many environmental impacts in order to judge its sustainability and it gave us a chance to learn a bit of chemistry.
Julianne Troiano is a graduate student at the Center for Chemical Innovation on Sustainable Nanotechnology (CSN) at Northwestern University in Evanston, IL. Julianne is interested in environmental science and has experience as a blogger (www.sustainable-nano.com). She recently travelled to Iceland to study glaciers and alternative energy and will share her experiences with us.