I was once told that if a piece of plastic gets oily, it's harder to "de-oil" than other substances because plastics are actually made from a derivative of oil. Is this so?

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    it's certainly true that plastics are made from crude oil, but cooking oil are not made from crude oil, nor is crude oil made from cooking oil. While it's true that there are some chemical similarity between crude and cooking oil as both are made of Carbon and Hydrogen; but so is human body is made of Carbon and Hydrogen. Even if it is true that plastics are harder to "de-oil", it is certainly not because plastic is made from oil. – Lie Ryan May 11 '11 at 12:53
  • Yes, plastics are made basically from oil. No idea about the washing part. – Martin Scharrer May 11 '11 at 12:53
  • You should be more concrete about 'harder to wash than ...'. Wood, glas, metal, hair, silicon? My mother used a brush to put butter into backing forms, which was pretty hard to wash. And plastics isn't plastics - is it? But I don't know about the details, especially not about common plastics used in kitchens. Does silicon count as plastic too? From my experience, it is easily cleaned. It shouldn't just be a matter of material, but of the structure of the surface too. – user unknown May 11 '11 at 15:58
  • Well, all you have to do is put some strong salsa in a plastic bowl and a glass bowl overnight, wash both bowls and have a good whiff, to know that something is left behind in the plastic bowl, but not in the glass one. – Robert Harvey May 11 '11 at 16:37
  • @Robert Harvey - But that could be because the plastic, despite being de-oiled (to the point where you can't tell it's still oily), is more porous than most other materials so some of the aromatic hydrocarbons in the salsa diffuse inside. I'm not sure, actually--but I am sure that "feels greasy" and "smells like garlic" are potentially different issues. – Rex Kerr May 13 '11 at 2:25

Plastic (e.g. polyethylene) is made mostly of hydrocarbon chains. So is oil (both lipids (organic) and inorganic alkanes (paraffin, etc.)). Both of these substances are hydrophobic, so it's hard to get them to mix into water--and you are washing your plastics with water, not DMSO or something, right? But long hydrocarbon chains are happy to stick to each other, as you can tell by looking at melting and boiling points of long alkanes (the more you want to stick, the hotter you have to be before you melt/boil).

On the other hand, it's not as though oil somehow knows that plastic was made from oil. It's simply a property of the chemical interactions. Other surfaces that are lipophilic will have the same hard-to-wash property.

Incidentally, there is research (example) into materials that are both lipophobic and strongly hydrophobic, because almost nothing likes to stick to these and thus the surfaces are "self-cleaning".

  • This only explains why you cannot wash oil with water. It hadn't explained whether it is significantly more difficult to wash off oily plastic compared to washing off oily <insert-non-plastic-materials>. Oil is hard to wash with water on any materials because oil itself is hydrophilic; but give it a little carbon chain that was engineered to be lipophilic on one side and hydrophilic on the other side and you can wash oils off relatively easily. – Lie Ryan May 11 '11 at 16:15
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    @Lie Ryan - Plastic is lipophilic and hydrophobic (and these things usually go together). Maybe I wasn't clear enough. Most surfaces are less hydrophobic. You can test this yourself by looking at contact angles between a small drop and the surface (difficult) or measuring the angle at which the drop will roll off the surface (easier). The drops need to be small and of reproducible size for this to work. Anyway, if the surface is more lipophilic, detergents are less effective at pulling oils away (since they have a stronger interaction to overcome). – Rex Kerr May 11 '11 at 16:59

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