Plastic cling wrap or aluminum foil?
Impermeable to what? The answer will differ if you’re asking about air molecules, visible light, water, etc.
Per the internet, Aluminium foils thicker than 0.025 mm (0.001 in) are impermeable to light, gases (including oxygen) and water vapor. Foils thinner than this become slightly permeable due to minute pinholes caused by the production process.
As for plastic wrap, it depends on the type of plastic it’s made of. Saran wrap used to be made of polyvinylidene chloride, which is about the least permeable plastic wrap. It is now made of low density polyethylene which has a great deal higher gas permeability. Polyvinylidene chloride has an oxygen permeability of 0.6 cm3 μm m−2 d−1 kPa−1 while low density polyethylene under the same conditions has an oxygen permeability of 2000 cm3 μm m−2 d−1 kPa−1, or a factor of over 3,000 times more permeable.
So aluminum foil is more impermeable than plastic wraps.
Regarding impermeability to visible light, I was going to make a joke about transparent aluminum but Googling, there is apparently a real ceramic made of aluminum, oxygen and nitrogen that is transparent.
If you want impermeable plastic then Mylar is very good, and even better if it’s aluminized.
If I wrap a sandwich in each, which will keep it fresher longer?
Now the question becomes not the impermeability of the wrap, but how air-tight you can make the seals of a 3D package composed of a folded 2D film.
I think operator technique will outweigh any inherent difference in the material. With enough effort you can make a water-tight seal in aluminum foil.
For casual non-OCD wrapping, I’m going to bet you’ll have better success with plastic cling wrap. I know I would.
Rubies and sapphires, too.
If wrapping a sandwich is the aim, both aluminium and cling wrap will seal a sandwich well enough if you do it carefully to let it go mouldy before it becomes dry.
Request for nitpicking:
Is “more impermeable” correct, or is “less permeable” more accurate?
The correct answer is “which is impermeabler”, of course. The opposite question is “which is permeabler?”
Of Saran Wrap and aluminum foil, Saran Wrap is permeablest.
ETA: On the question of sealing a sandwich, plastic wrap would do a better job, I bet.
Please don’t bring religion into this.
(plus letters)
And I find smoked salmon sandwiches irresistible. So what happens when an irresistible sandwich meets an impermeable wrapper?
Well, I guess there are more variables than I thought. What started this in my brain was cat food. I often have wet cat food (or tuna) left in the tin, I store it in the fridge - which my wife is not crazy about. I have been storing 1/2 cans in a baggie. But they are only good for about 5 uses before they get grungy. To save, I thought maybe aluminum foil might be better (this is strictly a guess, I haven’t run the numbers). So I used some and she complained about smell. Not sure it had anything to do with the foil as we have lots of potentially smelly things (to her) in the fridge.
That’s what made me think of the question in the first place. So… all things being equal (wrapping prowess, etc) which material as it is sold commonly and commercially, would form the better barrier against air and smell transfer?
The problem with aluminum foil is that it is easy to puncture. Plastic wrap is stretchier, so less so, although something can catch on it.
Why not use zip-lock type plastic bags? Those are thicker and therefore less susceptible to nicks and cuts. And the seal is more likely to be airtight than a wrapping, just because people don’t not always get a true tight wrapping in the first place, and even less likely if they open the package more than once.
Not a bad suggestion to the ‘storing the cat food’ problem, but like I said, it was this issue that made wonder about the permeability of each material in the first place. However clunky I’ve made it, that’s the question I’ve tried to ask.
IMO the real problem with aluminum foil is how you seal all the edges in an airtight fashion. A square or rectangle set atop an open tuna can then folded downwards around / across the sides is wide open to air.
A multiply-folded seam of flat unwrinkled foil will be pretty air tight. But will take using WAG 18" x 12" of foil to seal a small tuna can. And result in a large unweildy package.
They make re-usable caps for the purpose (they’ll fit any can of the size type, not just cat food cans.) I have several of them.
Here’s an example. – several examples actually, if you scroll down the page.
If you’ll accept a compound of aluminum with other elements, it’s even easier: Sapphire is just aluminum oxide.