Few products get a worse press than plastic shopping bags. They are distributed free, and in vast numbers. They are made from oil. They don’t degrade. They litter the country side, snaring water-birds and choking turtles. Add your own gripe.
Paper bags are made from natural materials, and they bio-degrade. Surely it’s better to use paper? And come to think of it, why not bags made out of jute – it’s a renewable resource – and use them over and over? That must be the best of all?
A lot of questions. The function of an eco-bag is “containment with minimum carbon footprint”. So perhaps we should start by using CO2 footprint as the measure of ‘goodness’ or ‘badness’, and examining some real bags with approximately the same capacity (Figure 1 and Table 1).
Polyethylene Bags: Bag 1 is a typical one-use supermarket container. It is made of polyethylene (PE) and it weighs just 7 grams. Bag 2 is also PE but it is 3 times heavier and the designer graphics tell you something else: the bag is a statement of the cultural and intellectual self-image of the store from which it comes (it is a bookshop). It is attractive and strong, too good to throw away, at least not straight away.
Paper Bags: Bags 3 and 4 are made of paper. Paper bags suggest a concern for the environment, a deliberate avoidance of plastic, good for company image. But there is more mass of material here – about 7 times more than that of Bag 1.
Reusable Bags: And finally, re-usable bags – “Bags for life” as one UK supermarket calls them. Bag 5 is an example. It is robust and durable and looks and feels as if it made from a woven fabric, but it’s not – its polypropylene. The color, the “Saving Australia” logo and the sense that it really is green propelled this bag into near-universal popularity in Australia. Here is how one Aussi paper put it: “Forget the little black dress. The hot new item around town is the little green bag”.
But isn’t shouting “Green” and “Save the planet” a little bit, well, yesterday? Today is Bag 6. Discrete, understated, almost – but not quite – unnoticeable. And those who have one have the quiet satisfaction of knowing that it is made of Juco, a mix of 75% jute and 25% cotton. But it uses a great deal of material – 36 times more than Bag 1.The material facts
Table 1. The characteristics of the shopping bags
|Bag||Material||Mass (g)||Material CO2 footprint (MJ/kg)*||CO2 footprint, 100 bags(kg)||How manyre-uses?|
|6||Juco(75% Jute,25% cotton)||257||1.1||28.3||19|
* From the material records of CES EduPack
From what we’ve found out already, you will see the difficulty. We have wandered here into a world that is not just about containment, it is also about company image and self-image. Our interest here is eco-analysis, not psycho-analysis. So consider the following question. If the 7 gram plastic bags are really used only once, how many times do you have to use the others to do better in eco-terms? The fourth column of Table 1 lists its values for the carbon footprints of PE, PP, paper, and juco (that for jute includes spinning and weaving). Multiplying these values by the masses, taking 100 bags as the unit of study, gives the fifth column. Dividing these by the value for the single-use Bag 1 gives the number of times the others must be used to provide containment at lower carbon per use than Bag 1 – last column.
Now you must make your own judgment. Would you re-use a paper bag five, six, or more times? Unlikely, they rip quite easily and get soggy when it rains, and if you don’t, the familiar supermarket plastic bag (bag one) wins over paper (bags 3 and 4).
What about the reusable bags? Would you use the green PP bag (Bag 5) more than 14 times? I have one and it has already been used more than that, so it looks like a winner. Finally Bag 6, the thinking-peoples eco-bag, is less good than plastic until you’ve used it nineteen times. Not impossible, provided nothing leaks or breaks inside it causing terminal contamination.
So from a carbon and energy point of view, single-use bags are not necessarily bad – it depends how meticulous you are about reusing any of the others. The real problem with plastic is its negligible value (so people discard it without a thought) and its long life, causing it to accumulate on land, in rivers and lakes and in the sea where it disfigures the countryside and harms wildlife.
Shen L. and Patel, M.K. (2008) “Life Cycle Assessment of Polysaccharide Materials: A Review” J. Polymer Environ., vol.16, pp. 154–167. (A survey of the embodied energy and emissions natural fibers.)
González-García, S., Hospido, A., Feijoo, G. and Moreira, M.T. (2010) “Life cycle assessment of raw materials for non-wood pulp mills: Hemp and flax Resources” , Conservation and Recycling vol. 54, pp. 923–930
CES EduPack supports and enhances teaching at over 800 universities and colleges worldwide, providing resources and software to support university-level education in science, engineering, and design. It allows students to quickly browse and search for materials information (such as the carbon footprint used here), as well as perform material selection projects, chart material properties, and assess the environmental impact of different material choices within a design project. CES EduPack also provides access to a growing library of teaching resources on Granta’s Teaching Resource Website.