Water Vapor Transmission Rate (WVTR)

[Christian62]

Hello All.
A supplier of polymer bag is rating their product for WVTR at 0.048 GMS/100SQ INCHES/24 HOURS.

A) What is GMS ?
B) Will this be able to replace the Masson Jar we are using now?

Note:
We keep dry pellets in Masson Jar so the %RH is kept constant. We do this so can measure the relative viscosity at a later date. This method is bulky and we are looking for other containers.

 

[CoryPad]

GMS is supposed grams, so it should read g instead. Obviously you are dealing with someone who is ignorant of SI, and decides to use part SI and part inch-pound units. Not a good situation.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[btrueblood]

The moisture permeation rate through a glass jar is as close to zero as you can get. To answer your question, you need to know: how much moisture is too much, and how long do you plan to store your dry pellets.

 

[Christian62]

Well btrueblood, we test our plastic when they hit below 100ppm RH. They are put in the jar at about 50ppm.
So if I knew what was GMS I could calculate how much moisture can cross this type of plastic (our lab is at 50%RH - 23C)and how long I can keep the product dry (below 100ppm.
If the period is shorter than 24 hrs I would forget about these bags.
We keep 250ml of product.
Thanks

 

[btrueblood]

Well, you are going to need to talk to the manufacturer, or make some assumptions, or test, or better yet all three.

Assuming that GMS means grams, as Cory suggested, I calculate about a 2 ppm per day increase in water content, assuming your pellets have a specific gravity of around 1. I'd also say the numbers could be in error by as much as an order of magnitude, due to the probable large difference between your vapor concentration and the water vapor environment they did their permeation test in. So, a 50 ppm change could take roughly 25 days, or as little as 2.5 days. If those look like workable numbers, get some samples of the bag material, and run some tests.

You could improve on the numbers by putting the bag containing pellets into a second, outer bag, which also contains a (regenerable, indicating) dessicant.

 

[patprimmer]

How is anyone doing the calcs without the surface area of the bag.

A bag inside a bag will help a lot, especially if the outside bag also contains a desiccant. Your dry granules might be a convenient desiccant. ie put your 250 grams in a bag. expel all air or purge with dry gas, seal the bag. Put the sealed bag in another bag with some dry granules in it. Expel all air and seal it. You then have your original barrier plus a second barrier at a much lower concentration gradient for the water vapour.

24 hours sounds fairly doable to me. You might find a better bag, with thicker wall or better resin. The world is full of plastic bags in very many configurations.

Another choice might be a paint pail. That is how we store retained hygroscopic samples. We also have a small vacuum oven in case we do need to dry granules.

Regards

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[btrueblood]

Pat,

You are correct. I made a WAG of 100 sq. inches.

 

[kenvlach]

Don't think the bags will work. To hold 250 ml (~250 g) & have some material for closure, bag dimensions need to be about 5" wide x 8" tall (flat dimensions), so 80 sq. in (2 sides, when flat). If the WVTR was measured at 100% RH (best case) & same temperature as your lab, then at 50% RH (with sample acting as absorbent so ~ 0% RH inside), the bag would transmit

0.048 g/100 sq in/24 hrs x (80/100) x (50/100) = 0.0192 g/24 hrs

0.0192/250 g = 76.8 ppm

So, if started with 50 ppm water, end up with about 126 ppm water (assuming no absorptivity decrease). Need either less permeable or thicker material for the bags.

See how the Water Vapor Transmission Rate (WVTR) is measured per ASTM F1249 at

http://www.ides.com/property_descriptions/ASTMF1249.asp

Also, '4-mil polyethylene will only pass about 0.05 grams/sq. ft./24 hrs at 75° F and 100% RH.'

http://www.drierite.com/catalog3/page17b.cfm

CoryPad: As 0.048 is close to the 0.05 grams in the Drierite catalog example, perhaps this metrification applies for the sq. in. units: 100 sq. in. = 1 sq. ft.???

 

[CoryPad]

Lovely

Regards,

Cory

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