PETG longevity underwater?

[macado]

Does anyone have any real-world experience with PETG longevity for parts underwater? I'm mostly concerned about cave markers (arrows / cookies) that live permanently in the cave system.

A friend and I pulled out some old 3D-printed arrows/markers out of a cave system (that were approximately 4-5 years old). They were completely deteriorated / crumbled. I believe they were PLA (polylactic acid) which makes sense since that is supposed to be 100% bio-degradable plastic and made of corn or sugar cane.

The question remains since all mine are printed with PETG 100% infill. Will PETG suffer the same fate? I'm really hoping these will be stable and not completely break down.

 

[Badger7]

Not from scuba, I'm just a casual rec diver, but I could comment with some personal experience around saltwater reef tanks and 3D printing. Generally speaking, none of the common filaments seem to negatively affect water chemistry, but unless the PLA is in the display tank and gets encrusted with coralline algae, it does break down a little bit over a year+ timeframe. Using PLA in a sump with no lighting for mounting pumps or plumbing, it can get chalky and can break down between layers, some brands of PLA more than others. I suspect that has more to do with swelling and delamination more than breakdown of the polymer itself. PETG and ABS haven't seemed to affect water chemistry either, but as long as they are printed with good layer adhesion initially, haven't broken down much over year+ timeframe, even when used to mount a pump with 24/7 vibration. Might suggest PETG when slight flexibility is desired and ABS when you want something more stiff? In a cave environment, you couldn't care less about how the material handles UV exposure (which is where ABS really shines over both PLA and PETG).

 

[DiveDiversions]

PLA is made from corn starch and will eventually break down (they promote it as environmentally friendly since it breaks down but takes a long time to actually break down, as you are probably witnessing.)

Both ABS and PETG should by stable in a cave environment as there is a constant temperature and no UV light.
PET is what water bottles is made out of, PETG just has glycol added to give it greater strength and durability.

Zack Freedman just released a Youtube video going through several types filament types, very informative.