How long do your PLA parts last?

[Ryan Carlyle]

Anyone out there printing PLA and have your parts degrade before you wanted them to? What's the shelf life of a printed part indoors, outdoors, etc?

[Richard]

I haven't tested my PLA parts outdoors, but did some research awhile back. PLA is biodegradable but from what I read will take a very long time to degrade. When they talk about composting it, they are talking about industrial composters at 200+ degrees with oxygen pumped in.

[Richard]

Actually, it looks like 140f for six days.

I did a quick search...
PLA PLASTIC:
PLA is a bioplastic made from starch; specifically it is being manufactured from starches derived from genetically modified corn (GMO food). This technology and supporting organizations such as BPI (Biodegradable Products Institute) claim that PLA biodegrades. However, this claim is confusing because they are using the term “biodegradable” extremely loosely. PLA is a “compostable plastic” in that it goes through “degradation” to break down and is therefore not true “biodegradation”. PLA does not break down or biodegrade in a landfill and will only begin to “degrade” after being exposed to heat (specifically 60° C over a five day period). This kind of environment can only be found in a commercial composting facility, NOT in the domestic composter in your garden. We find that many of the articles and organizations who support PLA are greatly contributing to the confusion by not using correct standards based definitions of that technology. Once PLA composts the remnant is CO2 and because professional composting facilities are not currently capturing the gas it is usually released into the atmosphere.
PLA and things they don't always tell you

[Dan Newman]

See Section 6.3 of http://jimluntllc.com/pdfs/polylactic_fibers.pdf . Towards
the end of that section are some environmental considerations. I was surprised
that it has good UV resistance (since generally polymers do not).

Dan

P.S. One of the co-authors of that paper is at NatureWorks, a major supplier
of plastics for 3D printing.

[Ryan Carlyle]

140F is hot, but not really out of the question for a large domestic compost pile. Around 120F is pretty typical for a home compost pile in summer. I suspect a large domestic compost pile would easily break down PLA -- it would just take longer than a week.

140F is also attainable for a dark material sitting in bright sunlight. That might be why so many people say it breaks down in the sun.

[Jay]

I don't know about outside but I have an indoor shelf made from PLA that gets 2-3 hours of direct sunshine every day....and it's still hard as a brick and showing no ill effects.

Jay

[Ryan Carlyle]

You know, dishwashers easily reach 140F. Which means you might be able to just leave the PLA parts in the bottom tray of your dishwasher and let them slowly melt away down the drain...

Lactic acid is often used as a food-safe cleaner, so I really don't see any big downside to this.

[Richard]

You would need to run our dishwasher for 5 days straight.  It is not 5 total days, it is 5 sustained days at that temp.

As far as your compost pile, the one in the backyard probably won't do it, a commercial one that turns trees into topsoil probably could.  Everything I have read on the environmental sides is pointing out that people probably think biodegradable cups (PLA) are better than "plastic" because they will break down quickly in the rain, or in a landfill...apparently they don't.  Part of it may be that landfills are oxygen pour after they are compressed and things have rotted a bit and used up the available oxygen.  Just guessing here.  If there was more oxygen, people would not be able to dig up 30 year old hotdogs out of landfills as shown in a National Geographic article a number of years back.

[Ryan Carlyle]

Yes, exactly. Material in landfills -- even something as flimsy as newspaper -- is essentially immune to decomposition. There isn't enough oxygen. We seal landfills with plastic sheets and impermeable clay to prevent liquids leaching into the ground, and that also keeps oxygen out. This is why throwing away organic matter (and PLA) is actually a form of carbon sequestration. PLA sent to landfills has a negative carbon footprint for about a thousand years.

The exception is in methane recovery landfills, where the pile is deliberately watered and aerated so the decomposition will generate methane for electricity production. I don't know how hot these get.

I think a dishwasher with a hot "sanitize" cycle would work because they get up to ~165F or so. That should more than double the decomposition rate of PLA at 140F. I would expect the part to lose a small amount of surface material on every wash cycle and eventually disappear. I'm not saying I plan on trying it, just that it would be an interesting experiment.

Turning 3d printer waste back into more filament is definitely a superior disposal method. 

[Ryan Carlyle]

Remember, PLA decomposition is a hydrolysis reaction. It's a surface effect that requires water to work. That means it's likely to progress from the outside in. PLA decomposing in a compost pile requires 5 straight days to fully reach biodegradable status throughout, but I bet the outer layers are degraded much faster than that.

[Eighty]

It's been a while since I read about it, but I recall that biological activity was a part of the breakdown process for PLA (along with heat). That's really what makes a compost heap do it's job. So I don't think the dishwasher would do anything.

[Ryan Carlyle]

From what I've been reading, water/ heat breaks the long polymer chains into short, digestible polymer chunks. Bacteria turn the chunks into lactic acid and then into CO2. So I guess it would depend on whether short-chain PLA is soluble in water.

[DHeadrick]

I have a wedge I printed in PLA to mount a convex mirror to a wall outside at the correct angle.  It has been stuck outside throughout the winter and hasn't fallen apart yet.  I printed it in the fall (maybe September) and it is still up.  It isn't in direct rain (it is under a small roof) but it will be seeing the temperature cycles throughout the day.  I'll let you know how it fared in the spring :)