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Tom Kirchner
(ouzel) - MLife

Locale: Pacific Northwest/Sierra
Re: Bags on 08/26/2013 15:19:58 MDT Print View

"Debating things that are obvious isn't interesting to me. Please just read the Loksak website. They explain that Opsaks are made of polyethylene, including this sentence: “These resealable, washable, polyethylene bags are watertight, airtight, and odorproof, so they prevent animals from sniffing out your edibles and toiletries.""

You piqued my curiosity, so I followed your recommendation and did some reading over at loksak.com. Here is what I found: "Our new Odor-Proof barrier bag, OPSAK, has all of the features of the aLOKSAK PLUS A NEW-GENERATION BARRIER FILM THAT IS COMPLETELY ODOR-PROOF."

I capitalized the relevant phrase for the sake of clarity, as it is apparently easy to overlook. The clear implication is that the OPSAK is composed of 2 layers, one polyethylene, and a film made from an undefined(proprietary?) odor-proof material.
This would certainly seem to at least partially explain Samuel's results. This is not to say OPSAK's are the best choice, by any means. I gave up on them last year, as soon as NyloBarrier bags became available, primarily because of the finicky, unreliable zip lock sealing mechanism.

Colin Krusor
(ckrusor) - M

Locale: Northwest US
Material on 08/26/2013 16:10:41 MDT Print View

Tom, the implication you inferred from that quote is not as clear to me as it is to you, but it would be interesting if it came to light that Opsaks are a multilayer film. The Opsak material is soft and subjectively it appears to be a monolithic polyethylene film. So, if it is a multilayer film, it is a pretty safe bet that it doesn't include a PET (polyester) or PA (nylon)layer (which are "hard", crinkly, low elongation materials). The only "soft", high elongation materials I know of that are used in barrier films are PVA and PVDF. PVDF, however, is naturally a milky, translucent color when dry (it turns clear when wet), and Opsaks are clear. That leaves PVA.

As long as we're on trivia, I'd note that even a multilayer film will never match the barrier properties of metallized and foil bags. Small molecule permeance for foil bags is typically 1000x lower than the best multilayer clear films. This is why some barrier bags are metallized on one side and clear on the other, or even metallized all over with a small clear window; companies want buyers to see their product but they want to maximize the proportion of the bag area that is metallized in order to minimize permeation of oxygen, water, and odor molecules.

Tom Kirchner
(ouzel) - MLife

Locale: Pacific Northwest/Sierra
Re: Material on 08/26/2013 17:02:45 MDT Print View

"Tom, the implication you inferred from that quote is not as clear to me as it is to you, but it would be interesting if it came to light that Opsaks are a multilayer film. The Opsak material is soft and subjectively it appears to be a monolithic polyethylene film. So, if it is a multilayer film, it is a pretty safe bet that it doesn't include a PET (polyester) or PA (nylon)layer (which are "hard", crinkly, low elongation materials). The only "soft", high elongation materials I know of that are used in barrier films are PVA and PVDF. PVDF, however, is naturally a milky, translucent color when dry (it turns clear when wet), and Opsaks are clear. That leaves PVA."

Not to be overly disputatious, but when they say "plus" a new generation barrier film(definition: a thin skin or membranous coating) it sure seems to indicate another layer is involved . I suppose the best way to resolve the issue would be to email the question to loksak, but I'm not sure I'm that interested in pursuing the subject further. As I mentioned earlier I'm no longer using OPSAK's, so this discussion is purely theoretical for me. As for the rest of your post, given your use of words like "subjectively" and "it's a safe bet", I'm left to wonder if perhaps there have been advances you are not aware of in the materials you mention that would alter their textural properties. Or perhaps there are even new materials you are not aware of. They did say "new generation" after all.

"As long as we're on trivia,"

The devil, as ever when it comes to the real world, is in the details. As any scientist is well aware ;0)

I'd note that even a multilayer film will never match the barrier properties of metallized and foil bags. Small molecule permeance for foil bags is typically 1000x lower than the best multilayer clear films. This is why some barrier bags are metallized on one side and clear on the other, or even metallized all over with a small clear window; companies want buyers to see their product but they want to maximize the proportion of the bag area that is metallized in order to minimize permeation of oxygen, water, and odor molecules."

A multilayer bag construction involving a film doesn't have to match the barrier properties of a foil/metallized bag, although in the absence of data driven comparisons I'm not inclined to accept your statement as a given. All it has to do is fool the bear. There are probably several ways to skin this particular cat, one of which is no doubt metallized bags, but another of which may well be some form of multilayer bag with an odor proof film. BYOB would seem to be the order of the day.