AskPablo: Glass vs. PET Bottles

PET.jpgChris McLaren, owner of Lush Beverages of Ottawa, Canada, sent this week’s question. He changed his packaging from glass to PET this year, partially for environmental reasons. But to date no one has been able to quantify those reasons for him. I will give it the old Boy Scout try to see if I can substantiate his decision numbers.

Here is what we know:

  • The bottles contain 12 fl. oz. (355 ml)

  • 12 fl. oz. weighs 355 g (1 ml of water weighs 1 g)
  • An empty glass bottle weighs 250-280 g ( we will use 265 g)
  • An empty PET bottle weighs 28-32 g (we will use 30 g)
  • PET bottles are always made from virgin materials and never recycled into new bottles
  • Transportation distance is hard to estimate since there are many points of sale

It is difficult for a company to know what happens with their packaging after the product leaves their facility. We can encourage consumers to reuse and recycle but ultimately it is out of our control. This is why I encourage companies that I work with to minimize the ecological impact of their packing production since that is something that they do have control over. And this is exactly what Chris is trying to do. Clif Bar did this by making their foil wrappers thinner, probably saving tons of polypropylene plastic each year.
Reducing packaging weight is also a great way to save on shipping costs. Chris tells me that, due to the lightness of the PET bottles, he is able to get 72 cases on a standard 48″ x 48″ pallet, instead of the 60 cases with glass bottles. This not only increases the amount of product per truckload but also economizes on fuel use.
The semi trucks used to transport Chris’ product probably get around 6.1 mpg (2.6 km/l) and can carry up to 45,000 pounds (20,412 kg). Over 260 km the truck would use 100 liters of diesel fuel, emitting 301 kg of CO2 (for a derivation of this see “AskPablo: The Tailpipe Mystery”). Transport emissions are often based on units of tkm (weight transported x distance). In this case we are looking at 301 kg of CO2 emissions and 5307.12 tkm (20.412 t x 260 km), or 56.7 g/tkm. The Wuppertal Institute’s MIPS Data Tables actually put the figure at 102.00 g/tkm, but that is for an “8 t articulated lorry” in Europe.
Before we look any more into transportation let’s look at the material intensity of making the bottle. Material intensity is a measure of all material inputs related to product outputs and provides a measure of material use efficiency. Glass uses 3.04 g/g of abiotic materials (minerals and fossil fuels), 17.1 g/g of water, and emits 0.716 g/g of GHGs. PET uses 6.45 g/g of abiotic materials (minerals and fossil fuels), 294.2 g/g of water, and emits 3.723 g/g of GHGs. The material intensity, in terms of gram per gram, is great for PET but keep in mind that the PET bottle also weighs quite a bit less. Next we will see how that factors in.
By multiplying the weight of the empty PET bottle by the material intensity factors we determine that one bottle represents 193.5 g of abiotic material (mainly oil), 8826 g of water (mainly in power-plant cooling water), and 11.7 g of GHGs. The glass bottle is almost nine times as heavy at 265 g and is responsible for 805.6 g of abiotic material (mainly sand and oil), 4532 g of water (again, from power-plant cooling water), and 189.7 g of GHGs (that’s about 15x more than the plastic bottle).
Focusing more on the GHG emissions, let’s look at trucking emissions. Each bottle contains the same amount of product, 355 g, but the glass bottle weigh 335 g more than the PET bottles. To find out how much GHGs that is over 1000 km we first calculate our tkm (335 g x 1000 km = 0.335 tkm) and multiply it by the trucking emission factor from earlier (56.7 g/tkm) to get 18.99 g (0.335 tkm x 56.7 g/tkm). So, each PET bottle that is shipped 1000 km saves 19 g of CO2 over the equivalent glass bottle, that is 456 g per case, or 19 tons per million bottles. Now that is just the difference between the two bottles, but what is the total GHG impact for 1000 bottles?
One thousand PET bottles represent 11.7 kg of GHG emissions for manufacture. Once filled, they collectively weigh 385 kg (385 g x 1000). When shipped 1000 km this equals 385 tkm (0.385 t x 1000 km), which equals 21.83 kg of CO2 (385 tkm x 56.7 g/tkm). This adds up to 33.5 kg (11.7 kg + 21.83 kg) of CO2 for every 1000 PET bottles shipped 1000 km. Compare this with 224.9 kg for 1000 glass bottles shipped 1000 miles, and these calculations don’t even take into account the delivery of the bottles from the bottle factory, which would add a few more kg.
Chris, consider your decision justified and thank you for Asking Pablo!
Pablo Päster, MBA
Sustainability Engineer
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