A typical response to the previous article on why PLA filament is so darn brittle. This has led some people to not use PLA filament at all, while others promote using PLA only for prototyping and throw-away parts, especially in light of PLA being compostable under the right conditions. For many mechanical parts, people turn to PETG.

Much like the PET polymer used for everything from food containers to drink bottles, PETG is durable, more resistant to degradation through mechanisms like hydrolysis and its filament form doesn’t need to be coddled like PLA does. PETG, on the other hand, tends to come from crude oil and shrugs at industrial composting conditions.

In terms of durability, degradation mechanisms and recyclability, is PETG the basic FDM filament which we should all just be using?

Polyethylene Terephthalate

PET (backbone: C10H8O4) is a polyester, just like PLA (backbone: C3H4O2). Due to how common PET is in both packaging and textile applications it’s usually just called “polyester” when used for textiles, where it is often blended with cotton and other fibers. When used in packaging PET has the sometimes rather unfortunate property of being permeable to oxygen and carbon dioxide, so that it may have to be combined with an additional oxygen-blocking layer.

This aspect of using PET for packaging is detailed for example in a 2017 review paper by Youri Michiels et al. in Applied Sciences. The main drive for using polymers like PET in packaging is due to these being more economical than traditional materials like glass and aluminium, both of which are highly impermeable to oxygen and carbon dioxide.

Consequently, a number of active and passive barriers were developed, something that is also essential in organic electronics like OLED displays when the traditional glass layers are replaced with polymers for making something like flexible displays. Active barriers can be oxygen scavengers, while passive barriers would be the application of additional materials to the basic PET film or bottle, usually in the form of a coating. This also helps to prevent the infusion of PET with colorants and aromas from food or drink, which is a common issue with recycling PET bottles.

Naturally, this can pose issues when trying to use PET materials for 3D printing, as you cannot be certain what percentage of PET waste is actually PET and what other polymers make up the remainder. This was studied by Mikołaj Garwacki et al. in a 2024 paper in Materials, with a PET-PETG blend containing PET film waste. They found it necessary to add an elastomer additive (IM) to prevent brittleness.

And this is how we get the “G” in PETG.

Triple Copolymer Structural formula of PETG, with glycol and CHDM monomer sections. (Credit: CLauterb, Wikimedia)

The name polyethylene terephthalate (PET) already suggests that it’s more than just a polymer of purified terephthalic acid (PTA) monomers. Thus, in addition to said PTA we also see ethylene glycol (EG) monomers. This creates the basic PET copolymer as we know and love it. When we talk about PETG (polyethylene terephthalate glycol-modified), we thus do not mean the addition of glycol, but rather the replacement of said EG monomers with another monomer.

Thus PETG has a third monomer added, usually in the form of cyclohexanedimethanol (CHDM), which replaces some of the EG monomers in the resulting copolymer. In addition to PETG the ratio of CHDM monomers can create additional copolymer types:

  • PET    -> PTA + EG.
  • PETG -> PTA + EG + <50% CHDM.
  • PCT    -> PTA + CHDM.
  • PCTG -> PTA + EG + >50% CHDM.

Typically PETG is chosen for its high transparency and toughness properties, which is a property that’s rather useful in packaging like bottles. Although PETG FDM filament is basically PETG, something like a plastic ‘PET’ bottle can thus be PETG as well, and likely is due to the aforementioned properties.

Stability Degradation routes of PET. (Credit: Ali Chamas et al., ACS Sustainable Chem. Eng. 2020)

One of the complaints with PLA filament is generally that it’ll become brittle no matter what, owing to the combination of increasing crystallization reducing movement within the polymer matrix, and hydrolysis shortening the polymer backbones. PETG filament has a much better reputation here, with it usually considered to be basically immune to brittleness.

The bad news here is that PETG still attracts moisture from the environment and – it being a polyester – still has a backbone that is susceptible to hydrolysis. Another common degradation mechanism is that of photo-oxidation, along with thermal degradation.

This is detailed in this 2020 review paper by Ali Chamas et al. in ACS Sustainable Chemistry & Engineering on the degradation mechanisms of various plastics within a range of environments.

As noted in the paper, hydrolytic cleavage of PET chains is very slow under neutral pH, but strongly enhanced in acidic environments. Fortunately carbonated drinks stored inside PET(G) bottles aren’t at too much of risk here, as the carbonic acid from dissolving CO2 in water is only a mild acid and thus shouldn’t hasten degradation too much.

In order for thermal degradation to occur, fairly high temperatures are required, above that for PLA, while exposure to UV light can cause photodegradation as is typically observed with many plastics. Overall long-term stability of PET polymers is not a commonly cited concern, with the cynical take of discarded PET single-use bottles potentially outliving human society being not too far from the truth.

Barring PET polymers ending up in a particularly acidic, UV-irradiated and high-temperature environment, it’s remarkably stable, even if not as much as polypropylene or polyethylene. While there are now enzymes that can dispose of PET polymers, such as PETase and MHETase, these probably are no real risk factors to your spools of PETG filament and rather just convenient ways to dispose of misprints and waste in the future.

Sample Size Of One A pleasant sight to behold when printing with an old spool of filament.

In addition to the above theory and the perusal of the scientific literature on the topic, I figured that I would also try printing with a rather old spool of PETG filament. It dates back to the time when I was still printing on the Creality Ender 3 v2, with receipts showing a purchase date of March 2023. This spool has spent considerable time just lounging about on the FDM printer’s spool holder before being stuffed back into a sealed plastic bag and forgotten about until recently.

Digging this Reprapper-branded spool of clear PETG filament out of storage, I was momentarily dismayed at it having been stored by past me in just a resealable bag with no attempt at a vacuum, just with a probably expired baggy of forbidden silica candy tossed inside the bag. Let’s consider this a worst-case scenario.

Feeling particularly adventurous, I decided to not even dry this much-abused spool of PETG filament and just see what happens when printing on my Neptune 4 bed slinger. After some initial fiddling with loading issues I fixed this by snipping off of the dodgy tip of the filament, presumably due to its mangling by the Ender 3 v2’s extruder gear and hotend.

I still had my worries about the extruder teeth marks from the previous printer and the presence of a few bends in the filament that made me suffer PLA-related flashbacks, but after slicing up a cable chain model that I had printed many times in PLA before, the printing was very much uneventful.

As can be seen in the photo, I was greeted by a happy print result. While keeping an eye on it for the first layers I did notice a bit of oozing, but after a few minutes it normalized and the cable chain elements were printed cleanly, including the big overhangs. I was able to clip the elements together as well without any brittleness or other issues.

I probably should have turned on the auxiliary cooling fan on the Neptune 4 as PETG prefers things cool unlike PLA, but even with this YOLO printing attempt with a neglected spool of PETG I was left impressed. After my recent experiences with PLA I was bracing for at least a few hours of troubleshooting, but this was almost boring.

Vibe Check

With this printing experience and the scientific literature put side by side, I think that they concur. Despite me taking no precautions at all, even a more than three-year old neglected spool of PETG filament printed just fine. There were no signs of stringing or other issues that would prevent me from just queueing up another print. Although I’d still advocate at least for keeping PETG spools in at least a sealed bag or container, it’s not nearly as fragile as PLA.

Since my previous article on PLA’s brittleness, I have opened its still factory-sealed twin in the form of black PLA and found that although it’s not nearly as brittle as its white sibling, it too began to snap off if left at anything beyond a gentle curve. Clearly this puts a pretty big expiration data on PLA filament, but it’s still an open question whether it was just this particular batch of Sunlu PLA filament.

In light of all this, I think it is fair to say that PETG is by far the most ‘no fuss’ filament for general use, with PLA only having its theoretical composting advantage. Yet when taking into account that this is only guaranteed for pure PLA without additives, and PET-degrading enzymes exist, that theoretical advantage doesn’t seem so convincing anymore.