P2 Audit
lthomas1020
Grocery bags 195
•• Eco-attributes of the materials
Attribute Polypropylene Polycarbonate Cardboard
Embodied energy, virgin material (MJ/kg)* 95 110 28
Carbon footprint, virgin material CO2,equiv (kg/kg)*
2.7 5.6 1.4
Molding energy (MJ/kg)* 21 18.5
Molding carbon footprint, CO2,eQUiV (kg/kg)* l.6 l.4
Oil-equiv. energy for single wash, 1,000 cups (MJ)
195
*From the data sheets of Chapter 15
Suppose the reusable cups are used, on average, n times. Then they become more energy efficient when
n Edisposable >Ereusable + (n -1)Ewash
Inserting the data and solving for n reveal that reusable cups are more energy efficient only if reused at least 15 times.
This might be an achievable target in a restaurant, but for outdoor events it is totally unrealistic. Garrido and del Castillo, investigating a major outdoor event at the Barcelona Universal Forum of Culture in 2004, report the gloomy statistic that only 20% of reusable cups were actually returned. The rest ended up in trashcans or simply disappeared.
Further reading Garrido, N. and del Castillo, M.D.A. (2007), "Environmental evaluation of single-
use and reusable cups," Int. f. LeA, 12, pp. 252-256. Imhoff, D. (2005), Paper or plastic: searching for solutions to an overpackaged
world, University of California Press, Berkeley, CA. ISBN-13: 978-1578051175. (What the title says: a study of packaging taking a critical stance)
8.3 Grocery bags
Few products get a worse press than plastic grocery bags. They are distributed free, and in vast numbers. They are made from oil. They don't degrade. They litter the countryside, snare water birds, and choke turtles. Add your own gripe.
Paper bags are made from-natural materials, and they biodegrade. Surely it's better to use paper! And come to think of it, why not use bags made out of jute-it's a renew- able resource-and use them over and over? That must be the best of all?
196 CHAPTER 8: Case studies: ceo-audits
A lot of questions. Let's see what answers the data sheets of Chapter 15 can give, using CO2 footprint as the measure of goodness or badness. First we must look at some real bags (Figure 8.2 and Table 8.3). The function of an eco-bag is "low-carbon containment," but there is more to it than that. 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 standing of the store from which it comes (it is a bookshop). It is attractive and strong, too good to throwaway, at least not straight away.
~ Carrier bags. The lightest weighs 7grams, the heaviest weighs 257 grams .
•• The characteristics of the carrier bags shown in Figure 8.2
Bag Material Mass (g) Material CO2 footprint (MJ/kg)*
CO2 footprint, How many 100 bags (kg) reuses?
l.5 1
4.3 3·
6.4 5
7.6 6
20.3 14
28.3 19
Polyethylene (PEl 7 2.1
2 Polyethylene (PEl 20 2.1
3 Paper 46 1.4
4 Paper 54 l.4
5 Polypropylene (PPl 75 2.7
6 Juco-75% jute 25% cotton 257 l.l
*From the data sheets of Chapter 15.
496 CHAPTER 15: Material profiles
Polypropylene (PP) The material. Polypropylene, PP, first produced commercially in 1958, is the youn- ger brother of polyethylene-a very similar molecule with similar price, processing methods, and application. Like PE it is produced i~ very large quantities (more than 40 million tons per year in 2010L growing at nearly 10% per year, and like PEl its molecule lengths and side branches can be tailored by clever catalysis, giving precise control of impact strength, and of the properties that influence molding and drawing. In its pure form polypropylene is flammable and degrades in sunlight. Fire retardants make it slow to burn and stabilizers give it extreme stability, both to UV radiation and to fresh and salt water and most aqueous solutions.
Composition (CH2-CH(CH3l)n
General properties Density Price
Mechanical properties Young's modulus Yield strength (elastic limit) Tensile strength Elongation Hardness-Vickers Fatigue strength at 107 cycles Fracture toughness
Thermal properties Melting point Maximum service temperature Thermal conductor or insulator? Thermal conductivity Specificheat capacity Thermal expansion coefficient
Electrical properties Electrical conductor or insulator? Electrical resistivity Dielectric constant Dissipation factor Dielectric strength
890 1.85
910 2.05
kg/m" USDIkg
0.9 1.55 GPa 21 37 MPa 28 41 MPa 100 600 % 6.2 11 HV 11 17 MPa 3 4.5 MPa·m1f2
150 175 DC 100 115 DC Good insulator 0.11 0.17 W/m·K 1,870 1,960 JIkg.K 122 180 ustrain/rC
Good insulator 3.3 X 1022 2.1 3 X 10--4
22.7
3 X 1023 2.3 7 X 10-4
24.6
uohrrr-cm
Polymers 497
Polypropylene is widely used in household products.
Eco properties: material Global production, main component Embodied energy, primary production CO2 footprint, primary production Water usage Eco-indicator
44 X 106 75 2.9 189 254
Eco properties: processing Polymer molding energy Polymer molding CO2 footprint Polymer extrusion energy Polymer extrusion CO2 footprint
20.4 1.5 5.9 0.44
End of life Embodied energy, recycling CO2 footprint, recycling Recycle fraction in current supply Heat of combustion Combustion CO2 Recycle mark
45 2.0 5 44 3.1
metric tonlyr 83 MT/kg 3.2 kgIkg 209 Ukg
millipoints/kg
22.6 MT/kg 1.7 kglkg 6.5 MT/kg 0.49 kgIkg
55 MT/kg 22 kglkg 6 % 46 MT/kg 3.2 kgIkg
&pp Typical uses. Ropes, general polymer engineering, automobile air ducting, parcel shelving and air-cleaners, garden furniture, washing machine tank, wet-cell battery cases, pipes and pipe fittings, beer bottle crates, chair shells, capacitor dielectrics, cable insulation, kitchen kettles, car bumpers, shatter proof glasses, crates, suit- cases, artificial turf, thermal underwear.
498 CHAPTER 15: Material profiles
Polyethylene (PE) The material. Polyethylene, (- CH2 -)ll/ first synthesized in 1933, looks like the simplest of molecules, but the number of ways in which the-CH2-units can be linked is large. It is the first of the polyolefins, the bulk thermoplastic polymers that account for a dominant fraction of all ,polymer consumption. Polyethylene is inert, and extremely resistant to fresh and salt water, food, and most water-based solutions. Because of this it is widely used in household products, food containers, and chopping boards. Polyethylene is cheap, and particularly easy to mold and fabricate. It accepts a wide range of colors, can be transparent, translucent, or opaque, has a pleasant, slightly waxy feel, can be textured or metal coated, but is difficult to print on.
Composition (-CH2-CH2-)n
General properties Density Price
960 1.9
kg/m" USD/kg
939 1.7
Mechanical properties Young's modulus Yield strength (elastic limit) Tensile strength Elongation Hardness-Vickers Fatigue strength at 107 cycles Fracture toughness
0.62 0.86 CPa 18 29 MPa 21 45 MPa 200 800 % 5.4 8.7 HV 21 23 MPa 1.4 1.7 MPa·m1l2
Thermal properties Melting point Maximum service temperature Thermal conductor or insulator? Thermal conductivity Specificheat capacity Thermal expansion coefficient
125 132 °C 90 110 °C Cood insulator 0.4 0.44 W/m·K 1,810 1,880 J/kg·K 126 198 ustrain/rC
Electrical properties Electrical conductor or insulator? Electrical resistivity Dielectric constant Dissipation factor Dielectric strength
Good insulator 3.3x 1022 2.2 3 x 10-4 17.7
3 X 1024 2.4 6 x 10-4 19.7
uohm vcm
Further reading 197
Bags 3 and 4 are made of paper. Paper bags suggest a concern for the environ- ment, a deliberate avoidance of plastic, good for company image. But there is more mass of material here-about seven times more than that of Bag 1.
And finally, reusable bags-"Bags for life" as one supermarket calls them. Bag 5 is an example. It is robust and durable and looks and feels as if it is made from a, woven fabric, but it's not-it's a textured polypropylene sheet. The color, the "Saving Australia" logo, and the sense that it really is green propelled this bag into near-universal popularity there. Here is one Aussi paper: "Forget the little black dress. The hot new item around town is the little green bag."
But isn't murmuring "Green" and "Save the planet" a little bit, well, yesterday? Today is Bag 6. Discrete, understated, almost-but not quite-unnoticeable. Those who have one have the quiet satisfaction of knowing that it is made of [uco, a mix of 75% jute and 25% cotton. But it uses a great deal of material-36 times more than Bag 1.
So you see the difficulty. We have wandered here into a world that is not just about eontainment but also about self-image and company branding. Our interest here is eeo-analysis, not psychoanalysis. 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 eeo-terms? The data sheets of Chapter 15 help here. The fourth column of Table 8.3 lists their values for the carbon footprints of PE, PP, paper, and jute (that for jute includes spinning and weaving). If you multiply these values by the masses, taking 100 bags as the unit of study, you get the numbers in the fifth column. Divide these by the value for the single-use bag, and you get the number of times the others must be used to provide containment at lower carbon per use than Bag I-last column.
Now you must make your own judgment. Would you re-use a paper bag six or more times? Unlikely-they tear easily and get soggy when wet. If you don't, Bag 1 wins. Would you use the green Bag 5 more than fourteen times? I have one and it has already been used more than that, so it looks like a winner. Finally Bag 6, the thinking person's eco-bag, is less good than plastic until you've used it 19 times. Not impossi- ble, 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.
Further reading Edwards, C. and Fry, J.M. (2011), "Life-cycleassessment of supermarket carrier bags,"
Report: SC030148, The Environment Agency, Bristol, UK. www.environment- agency.gov.ul<lstaticidocumentslResearchiCarrier _Bags_final_18-02-11.pdf. Accessed January 2011. (An exemplar of LCA at its most LCA-liRe)