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My wife prepares lunch for me every morning. After eating it on my desk, I typically wash the lunchbox and utensils in the company's kitchen. I have noticed that it is very difficult to remove the oiliness from the plastic containers (think Tupperware, but also harder plastics that look like polycarbonate): I need to rub the detergent-soaked sponge extensively and rinse with very hot water, several times. This does not happen with glasses, porcelain dishes or metal utensils like forks.

In other words, all the plastic containers I have come in contact with seem to be quite lipophilic.

Is this because edible oils and plastic are chemically "similar" (i.e., both contain long-ish chains of hydrocarbons), or is there another reason that I am missing?

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  • $\begingroup$ I suspect it's not that plastics are very lipophilic (maybe a little), but that it's more hydrophobic than metals etc. $\endgroup$ – iammax Jul 28 '17 at 4:22
  • $\begingroup$ So, metals and glass are lipophobic and that makes oil harder to stick (easier to remove)? $\endgroup$ – Nicolas Miari Jul 28 '17 at 5:59
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    $\begingroup$ Yes @Nicolas Miari. With a lot of differences between them, you can still say common plastics and oils have similar "hydrocarbon like" fragments. Note that what is lipophilic is hydrophobic. Ergo we need a detergent (a molecule with both parts). $\endgroup$ – Alchimista Jul 28 '17 at 10:32
  • $\begingroup$ I really hate that phenomenon too. I think if the plastic surface could be made more hydrophilic I see a good business... But surely it has been already considered and probably overall it does not make sense for household applications. I want to add that, hands being wetted, I think that we feel the plastic to remain more greasy even when is already washed because of its hydrophobicity. $\endgroup$ – Alchimista Jul 28 '17 at 10:37
  • $\begingroup$ @Alchimista I don't know about the greasiness of hands; I judge both by touch and look. The still-oily surface looks very different, reflects light according to the oil pattern. When clean, the reflection is even. $\endgroup$ – Nicolas Miari Jul 28 '17 at 10:39
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First, many plastics are somewhat porous. Since plastics are permeable to water vapor and oxygen, coffee and other food products are sometimes packed in two-layer bags of polyethylene (for strength) laminated to aluminum foil (to reduce permeability).

Second, lipids and other non-polar substances can dissolve in and and migrate through common plastics used in food containers, such as polyethylene and polypropylene, as you and @Alchimista note. This is why carotene, lycopene and other chemicals in tomatoes eventually stain poly containers permanently: they've dissolved into the plastic.

In a similar way, mercury dissolves into gold (e.g. jewelry) in a matter of seconds, leaving a silvery stain. That stain then disappears in a week or two as the mercury migrates throughout the object.

The reverse phenomena also occurs, where substances in plastic packaging can migrate out of the packaging.

As the Bard might have put it, "Will all the water in the ocean wash this food from my cans?"

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  • $\begingroup$ Thank you; this is the kind of in-depth, additional information I was looking for beyond my superficial observation/guessing. $\endgroup$ – Nicolas Miari Jul 31 '17 at 6:30
  • $\begingroup$ I'm not sure this actually answers the question which is why plastics are lipophilic. $\endgroup$ – matt_black Jul 31 '17 at 13:59
  • $\begingroup$ @matt_black : True, the question contained the answer: the molecules are similar, i.e. non-polar compounds. $\endgroup$ – DrMoishe Pippik Jul 31 '17 at 17:54

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