[♪ INTRO]
Odds are, you probably have a roll of cling wrap tucked away in a kitchen drawer,
ready for wrapping up your leftovers.
For a lot of people, it's a kitchen staple.
But if you've been buying the stuff for years, you might've thought that it just
doesn't work as well as it used to.
And for once, you're not imagining it.
The gold standard for cling wrap used to be the Saran Wrap brand,
made by the company SC Johnson.
But around the early 2000s, the company actually changed their formula
to a version that was less sticky and generally less effective.
Thankfully, they at least had a good reason for it.
Saran Wrap was originally made out of a chemical called polyvinylidene chloride, or PVDC.
It was actually discovered by accident by a chemist at another company,
Dow Chemical, in 1933.
It formed as a sticky residue in beakers
that had been used to develop a chlorine-based dry-cleaning chemical.
And not that dry-cleaning isn't fascinating or anything, but
PVDC turned out to have some interesting properties of its own.
It's a special kind of chemical compound called a polymer, a molecule in which a
bunch of identical, smaller molecules are linked to form a long chain.
There are all kinds of these, but PVDC was especially useful
because it forms a nearly impenetrable barrier to odors,
making it ideal for wrapping and preserving food.
That's because it has a super-low permeability:
the rate at which it lets liquid or gases pass through it.
Scientists think this is probably thanks to the densely packed,
unbranched, stable structure of its molecular chains.
Dow first sold this super-effective Saran Wrap in 1953,
and SC Johnson bought the formula and the brand name in 1998.
But around that time, concerns started to surface about
chlorine-containing compounds like PVDC.
It turns out that when they're thrown away and burned in municipal trash incinerators,
they can release toxic chemicals.
When this came to light, SC Johnson made the responsible decision to
eliminate PVDC from all of its products by 2004.
After some research, they made a new cling wrap formula that used chlorine-free polyethylene.
It's another polymer that's okay,
but definitely not as good at trapping odors and keeping food fresh.
See, depending on the temperature, one square meter of a thin film of PVDC
will let about 1 to 5 grams of water vapor pass through it in a day.
But for low-density polyethylene, that ranges from 16 to 31 grams, a big difference.
And the difference in their oxygen transmission rates is even more impressive,
jumping from about 2 to 16 cubic centimeters per day for PVDC to almost 8,000 for polyethylene.
That's a lot of numbers, but the bottom line is that this new cling wrap just
doesn't have the same odor-trapping, freshness-preserving magic that it once did.
And maybe even worse, this polyethylene mix doesn't even seem to be quite as clingy.
But if the switch means putting less toxic pollutants into the atmosphere,
I think we can all probably make do.
Thanks for asking, and thanks to all of our patrons on Patreon
for keeping the questions coming.
If you'd like to submit a question or vote on which one we'll answer next,
you can go to patreon.com/scishow.
[♪ OUTRO]
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