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Odds are, you probably have a roll of cling wrap tucked away in a kitchen drawer, ready for wrapping up your leftovers.
你家厨房的抽屉里一定藏着一卷保鲜膜用来包裹你吃剩的食物。
For a lot of people, it's a kitchen staple.
对于大多数人而言，这是厨房订书机。
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.
但是如果这些年你一直在使用保鲜膜的话，你或许会发现这些保鲜膜的效果没以前那么好了。
And for once, you're not imagining it.
就那么一次，你就没往下想了。
The gold standard for cling wrap used to be the Saran Wrap brand, made by the company SC Johnson.
保鲜膜的黄金标准曾是莎伦食物包装膜，这种保鲜膜的生产公司是庄臣公司。
But around the early 2000s, the company actually changed their formula to a version that was less sticky and generally less effective.
但是早在2000年，该公司就将保鲜膜的配方换成另一个版本：粘性更小、效果也没那么好了。
Thankfully, they at least had a good reason for it.
不过至少，他们有更改配方的充分理由。
Saran Wrap was originally made out of a chemical called polyvinylidene chloride, or PVDC.
莎伦食物包装膜起初是有一种被称为聚偏二氯乙烯或PVDC的化学物质制成。
It was actually discovered by accident by a chemist at another company, Dow Chemical, in 1933.
这种物质事实上是1933年另一家公司陶氏化学的一位化学家意外发现的。
It formed as a sticky residue in beakers that had been used to develop a chlorine-based dry-cleaning chemical.
它是由烧杯中的粘性残余物形成，烧杯中的物质被用来开发氯基干洗化学制品。
And not that dry-cleaning isn't fascinating or anything, but PVDC turned out to have some interesting properties of its own.
并不是那种干洗不够惊艳，只是PVDC本身就有些有趣的特质。
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.
这是一种特别的化合物—聚合物，在分子内一些相似且更小的分子相连形成一个长链。
There are all kinds of these, but PVDC was especially useful because it forms a nearly impenetrable barrier to odors,
还有很多其他物质，但是PVDC尤其有效，因为它能形成近乎不可逾越的屏障抵御气味，
making it ideal for wrapping and preserving food.
使之成为包裹和储存食物的理想材料。
That's because it has a super-low permeability: the rate at which it lets liquid or gases pass through it.
这是因为，它有着极低的渗透性：液体和气体都无法穿过。
Scientists think this is probably thanks to the densely packed, unbranched, stable structure of its molecular chains.
科学家认为这可能是由于分子链密集包裹、无分支且稳定的结构。
Dow first sold this super-effective Saran Wrap in 1953, and SC Johnson bought the formula and the brand name in 1998.
陶氏于1953年首次出售这种高效莎伦食物包装膜，1998年，庄臣公司购买了其配方和品牌名称。
But around that time, concerns started to surface about chlorine-containing compounds like PVDC.
但是大约在那个时期，人们开始担心含氯的化合物，比如PVDC。
It turns out that when they're thrown away and burned in municipal trash incinerators, they can release toxic chemicals.
原来当这种材料被丢弃并扔进市政垃圾焚烧炉时，它们会释放出有害的化学物质。
When this came to light, SC Johnson made the responsible decision to eliminate PVDC from all of its products by 2004.
当危害曝光后，庄臣公司决定截止2004年底将消除产品中的PVDC。
After some research, they made a new cling wrap formula that used chlorine-free polyethylene.
经过一些研究后，他们制成了一种新的保鲜膜配方—使用不含氯气聚乙烯。
It's another polymer that's okay, but definitely not as good at trapping odors and keeping food fresh.
这是另一种聚合物，不会产生危害，但是却无法有效阻挡气味和保持食物新鲜。
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.
这取决于温度，一平方米的PVDC薄膜一天渗透1-5克水汽。
But for low-density polyethylene, that ranges from 16 to 31 grams, a big difference.
但是低密度聚乙烯会渗透16-31克水汽，差别还是很大的。
And the difference in their oxygen transmission rates is even more impressive,
而它们之间的氧气透过率差异甚至更大，
jumping from about 2 to 16 cubic centimeters per day for PVDC to almost 8,000 for polyethylene.
PVDC为每天约2-16立方公分，而聚乙烯将近8,000立方公分。
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.
数据很多，但是结果就是新品保鲜膜没有之前的有效。
And maybe even worse, this polyethylene mix doesn't even seem to be quite as clingy.
甚至更糟糕，这种聚乙烯混合物甚至都没什么粘性。
But if the switch means putting less toxic pollutants into the atmosphere, I think we can all probably make do.
但是如果这种转变意味着给大气制造的有毒污染物更少的话，我觉得我们还是能接受的。
Thanks for asking, and thanks to all of our patrons on Patreon for keeping the questions coming.
感谢提问，感谢Patreon上一直提问的赞助人们。
If you'd like to submit a question or vote on which one we'll answer next, you can go to patreon.com/scishow.
如果你想提问或给问题投票，请登录patreon.com/scishow
