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I have some electronics housed inside a sealed plastic enclosure, which will be used in a very humid environment. The environment can be assumed to be 100% humidity at a constant temperature, all the time. Plastic is permeable to water, so over time water diffuses through the plastic and the humidity inside the enclosure gradually increases. Within a few weeks the humidity inside the enclosure exceeds 80%.

Can I slow or stop the permeation of moisture by pressurizing the air inside the plastic enclosure? Is there a relationship describing how the rate of moisture transmission changes with pressure?

Nick
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    Use a better enclosure, some are designed for high external pressure without leaking. But often price limits performance. – Solar Mike May 01 '21 at 06:34
  • @SolarMike It's not about seals leaking. The water vapor permeates through the plastic. A metal enclosure would fix the problem, but unfortunately that's not an option for my application. – Nick May 01 '21 at 12:11
  • Good question... Dont know but alternatives: Is the container sealed? Can you get a different plastic? Some are better. You could also reduce local RH enough to prevent condensation, with mild heating, if the device is usually-plugged in. – Pete W May 01 '21 at 13:48
  • You have the freedom to add a valve to pressurize the enclosure but not the freedom to just use a different kind of plastic? What kind of plastic is being used now? – DKNguyen May 02 '21 at 01:36
  • @DKNguyen I can change the type of plastic. I can already quantify what improvement I would see by changing to, say, HDPE. I would like to do the same for a pressurized enclosure and compare the options. I may have to do implement multiple solutions as changing plastic alone probably won't be enough. – Nick May 02 '21 at 02:49
  • Have you also considered conformal coatings? – DKNguyen May 02 '21 at 03:46

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Consider the hard drives of yester-year and more recent. They had a hole for pressure equalization (to control the bulging of pivot fix-points) with a patch of Gortex(R) covering the hole to control liquid transfer. Water vapor could come and go with temperature and pressure variation and they held some of our most precious data.

WVTR (water vapor transmission rate) for plastics is well understood.

Additionally, once you get moisture in a sealed container: at that temperature the partial pressure of the saturated water vapor becomes significant (about 5-psi at 60-deg C, that means the dry gas partial pressure of a mono-gas is only 9.7-psi while it is 14.7-psi dry gas mono-gas outside (60-C and dry)); this increases the dry gas partial pressure differential of the gas components and this drives up gas transmission rates for plastic. Consider a sealed baggie with a moist towel enclosed (put it in a dry 60-deg C oven). Over time (1-day is noticeable, keep it going for longer to see a major effect) the baggie inflates because of the differential dry gas partial pressures inside and outside according to the gas diffusion rates (for that plastic, that too is well know). The baggie inflates until the internal pressure, 19.7-psi, produces a drygas partial pressure equal to the outside, (19.7-psi combined dry gas and water vapor - 5-psi water vapor) inside = 14.7-psi dry gas outside, or the baggie ruptures.

Modern Plastics Encyclopedia (which was a monthly journal) compiled data for this phenomena, and how to use it, back in the 80's through 90's (maybe even before).

Jim Clark
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  • My understanding is that Gortex blocks liquids but allows water vapor to pass. If I put a gortex patch on my enclosure, the inside will reach 100% humidity very quickly. The points you make about partial pressures and bags expanding sounds like it is on the right track. But I need a better understanding of this before I can apply it to my application to quantify what effect pressurizing the air in the enclosure will have. – Nick May 02 '21 at 03:01
  • If the outside is 100% rh, then yes, the inside will eventually equilibrate to 100% rh. If the outside becomes less, the inside will decrease. If the outside is 50% rh nominally with spikes to 100% rh for short duration and back to 50% rh, then the inside will change slowly through the Gortex, and will more likely oscillate say 50% to 55%. Alternatively, you could add dehumidification coils inside to cool and condense inside moisture then pump it to the outside and actively control the inside humidity. PCTFE is great with WVTR, but eventually will equlibrate to 100% rh. This is only plastic. – Jim Clark May 02 '21 at 14:22
  • I wanted to characterize water-fraction loss of our ink distribution system for our product, I had to characterize the losses of the individual components myself and create a model. This equation was integrated to greate a general solution given time, temperature, and humidity to predict losses over time. Does internal pressurization reduce wvtr, great question. These sounds like experiments you may have to conduct using "Design of Experiments" to determine for yourself, or search the web. Discusses losses, https://www.pnnl.gov/main/publications/external/technical_reports/PNNL-26070.pdf – Jim Clark May 02 '21 at 14:41
  • Alternatively, you could create a support plan for your product. The user pays so much a month or year and you send them a new (or refurbished) product at a time frame before the product is expected to saturate (100% rh) internally. They return the older product and they're credited for the return. – Jim Clark May 02 '21 at 14:46