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Using PLA for Long-Term Outdoor Applications

Eh….probably not….but maybe.

Some of the structural components we have designed for the weather station have been developed on a 3D printer that uses PLA.  PLA is great as the warping is less than with ABS.  This is particularly important for larger pieces or those with extremely thin walls, such as the Stevenson screen.  Typically corners are printed more sharply as well, and PLA is comparably more rigid than ABS.  All of this is good for the end ‘fit and finish’ of the station, or case components for something like a Chatty Beetle.

Unfortunately, PLA is not really intended for outdoor use.  One of the great environmentally friendly things about PLA is that it is biodegradable, but I am not so sure we will want this material property for a weather station.   Some of the research looking at the properties of PLA note that biodegradation will occur in a matter of months, but this is with powder or small fibers maintained at a high temperature and moisture in a controlled composter.  A larger piece undergoing natural temperature and humidity fluctuations, such as for the project weather station, might not biodegrade as quickly.

Despite many stating that PLA is not appropriate for outdoor use, which may be true for structurally critical components, the survivability may just be fine for light structural applications.  The components on our weather station, for instance, are largely to hold sensors and wiring that is feather weight.  In Biodegradable Poly (Lactic Acid): Synthesis, Modification, Processing, and Applications, author Jie Ren notes:

To biodegrade within 90 days, as described, the products have to reach 140 F for 10 consecutive days.  This requires a special facility, which few consumers have access to.  If your PLA products end up at the landfill, they will not degrade any faster than a petroleum-based product.

The above mentioned reference does, however, state that PLA will degrade in high humidity and temperatures above 110 F.  In all likelihood then, PLA does degrade when outside, but the rapid biodegradation often discussed requires a specific set of conditions, which are unlikely to naturally occur.   As a side note, PLA is referenced as considerably UV resistant.

So the question is will a weather station printed with PLA degrade considerably over a one year period, or will it be able to remain largely intact up to a three year period, when replacement of structural components makes sense in the normal life cycle of the station?  Based upon some of the references I have mentioned, I do not think PLA would rapidly deteoriate as shown in a composting situation, but it is not clear if some level of degradation would still occur, which would make a component unusable or affect the readings taken by a sensor.  Of course the durability will be affected by local environmental conditions, to which the PLA printed structures are exposed.

To attempt to figure this out, we are placing test blocks of ABS and PLA in various outside conditions, however, I suspect in the end, we will initially need to print in ABS, as we do not want to delay the project simply to test structural components over a multiple year period.

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