From a Coke Bottle to 3DBenchy: How Plastic Waste can be Reused for 3D printing
In a world where waste and pollution threaten almost all life on Earth, it’s very important that we find new and innovative ways to reduce our impact on the environment. One solution is the recycling of PET (polyethylene terephthalate) bottles into 3D printing filament. This process not only helps to reduce the amount of plastic waste in our landfills, but it also provides a sustainable alternative to traditional 3D printing materials. As we get closer and closer to irreversible climate change, it’s crucial that we take steps to ensure that our progress is not at the expense of our planet.
Approximately 25 000 tonnes of PET plastic is put in landfills in the US each year. In 2021, 2.2 million 3D printers were sold. That means that each new 3D printer could use an average 11 kilograms of recycled PET plastic as filament over its lifetime. That is a pretty reasonable amount of filament to be used. The recycling process of PET is quite simple, and there is even the potential to turn PET into PETG, which is a better material for 3D printing.
Recycling PET bottles into 3D printing filament is a process that involves several steps. The basic process includes:
- Collection and sorting: The first step is to collect and sort used PET bottles. This can be done through recycling programs or by individuals who collect and sort the bottles themselves.
- Shredding: Once the bottles are collected and sorted, they are then shredded into strips of plastic. This makes it easier to use for the next step.
- Extrusion: Next, the strips of plastic are fed into an extruder. This machine melts the plastic and forces it through a small hole, called a die. This creates long, thin strands of plastic that can be used as filament.
- Cooling and cutting: As the filament comes out of the die, it is cooled by passing through water or air. The filament is then cut into the desired lengths.
- Winding: Finally, the filament is wound onto spools for use in 3D printing.
One of the most significant benefits of recycling PET bottles into 3D printing filament is that it reduces the amount of plastic waste in landfills. It is estimated that the world consumes around 1 million plastic bottles per minute. With so many bottles being used and discarded, it’s essential to find ways to recycle them. By turning them into 3D printing filament, we can reduce the amount of plastic waste in landfills and make the most of the resources we have. This not only helps to protect our environment but also reduces the cost of landfills and the pollution caused by the decomposition process of plastic. Plastic in oceans can also kill zooplankton, which are instrumental in the carbon-removing process currently fighting to delay climate change.
There have been a number of solutions for consumers to recycle their own PET, such as the PETamentor, which allows you to turn a plastic bottle into a spool of PET filament. This is great, but it does require you to buy some electronics (about $150 CAD total when buying from Amazon), and it does require you to use a substantial amount of filament to print the parts required to get it running. However, it can pay itself off pretty easily with the filament it produces.
An even more interesting solution is to turn PET into PETG (a common printing filament.) PETG has better heat resistance, less crystallization, and can be sterilized. It’s great for food storage, objects that will be exposed to heat, and more. PETG is similar to PET, but instead of using only ethylene glycol in the production, cyclohexanedimethanol (CHDM) is also used. This makes it better for 3D printing.
To turn PET into PETG, the process is not very complex. First, we would need to break the PET down into its base components. We can do this using hydrolysis, as shown in this study. The acid catalyst would be terepthalic acid (TPA), which is a major component in the PET/PETG production process. We would then have TPA and ethylene glycol seperately.
Another way (as shown in this study) of separating the PET into TPA and ethylene glycol would be to turn the PET into disodium terephthalate and ethylene glycol using alkaline hydrolysis. The alkaline hydrolysis is already performed on wastewater, which can allow us to use plastic in the sea for recycling without having to do this step. We would then use the acidification process, combining the disodium terephthalate and sulfuric acid to get an output of TPA and sodium sulfate. The end result leaves us with TPA and ethylene glycol, which is exactly what we need. However, this requires more material, which means more cost.
To turn the TPA and ethylene glycol into PETG, we would add CHDM and add some of the ethylene glycol back to the TPA to get PETG.
Separating the TPA and ethylene glycol using hydrolysis has to be done under specific conditions. The study used 2.5 grams of PET, a TPA concentration of 0.1 g/mL, a mass ratio of PET:H2O of 1:8, a temperature of 220°C, and 180 minutes of time. When adding the ethylene glycol and CHDM, there should be between an 80–20 mol% and 50–50 mol% ratio of ethylene glycol to CHDM in the PETG. This study used a 70–30 split, and they achieved great results.
The end product is a clear filament that can be left clear, or colored using dye. This solution is very cool, and can change the entire landscape of recycling plastic. It would also be very cheap. According to this patent, the best ratio of ethylene glycol to terephthalic acid is from 8/1 to 12/1, so I will go with a ratio of 10/1 for PET. If we replace 30% of the ethylene glycol with CHDM, we get a ratio of ethylene glycol to TPA of 7/1 and a ratio of CHDM to TPA of 3/1. This means we would have a ratio of CHDM to the total of 3/11. We only need to buy the CHDM, since we get the ethylene glycol and TPA from the PET we recycle. Buying CHDM in bulk (17 kg) can get us a price of around $13 USD ($799/17 kg = $47/kg. $47 * (3/11) = $12.78). 1 kg of clear PETG on Amazon is around $20 USD. We could realistically set our prices as low as $15.
Since the PETG can be dyed, the recycled filament can be sold in any color. This means that consumers wouldn’t have to choose between aesthetics and saving the planet. The excess ethylene glycol can be sold too, resulting in even more recycling. If you’re interested in recycled filaments, check out Zack Freedman’s video about this topic (none of the filaments in the video cover PET to PETG recycling, but it’s still very interesting).
This process can be used to create similar plastics like PCT (PET with CHDM completely replacing ethylene glycol). It can also be used to recycle those plastics into PETG, which is great.
Recycling PET into filament is a great way to recycle, but turning it into PETG is even cooler and an even better way to encourage people to use it. The process isn’t very complicated, and the cost is low enough that it even beats out normal PETG offerings. This process can result in a lot of plastic being saved from the landfill and turned into something useful. I hope you learned something from this, because it’s a very cool topic that I hadn’t heard almost anyone talk about. I’m not sure how long this process will take until a PETG company notices it, but I hope it will be soon. This technology has the potential to delay or even completely stop climate change.
Want to read similar stories? Follow me to know when my next article gets posted.
