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I know that polyethylene's melting point is between 115–135 °C, however I would like to know if can it start dissolving between 50-60 °C?

In a real example, if I put warm food (straight from the oven, I think it can be max. 50-60 °C) into a re-closable polyethylene plastic bag and close it, will any chemical reaction affect the food (from the polyethylene's side)? Or is polyethylene safe at that temperature, and if the bag is warm it doesn't mean that any chemical reaction is started? I didn't experienced any alteration, but I'm not an expert and don't know much about chemistry yet, so I would appreciate some guidance.

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There may be better answers forthcoming, but a chemical does not need to melt (i.e. be in the liquid phase) to be reactive. For example, your food very likely has some liquid component, and that could be enough to react with some solid. Now polyethylene is used all over the place and is very nonreactive in many non-rigorous situations (like food service), so you are probably just fine. But I wanted to make the more general point that you don't have to be a liquid or gas to be reactive.

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    $\begingroup$ This should be a comment. $\endgroup$
    – LDC3
    Jun 15, 2015 at 1:33
  • $\begingroup$ @LDC3: No, it is a perfectly fine answer. It may not be the best answer, as I noted, but it is an OK answer. $\endgroup$
    – user467
    Jun 15, 2015 at 1:35
  • $\begingroup$ Am I right that you are pointing that if something (polyethylene in this case) doesn't melt or smokes it doesn't means that there is no chemical reaction (releasing any type of gas or something)? As an instance if I put ketchup/soups into a heated plastic bag it may be reactive because it's liquid and it can be mixed up with the molecules we get during the chemical reaction - despite the change is not apparent -, and it won't if I put something that contains much less or not contains water like dried spices etc.. ? $\endgroup$
    – cimp23
    Jun 15, 2015 at 15:18
  • $\begingroup$ I'm saying that lots of chemical reactions are possible besides liquid-liquid. Now PE is quite chemically resistant, to the point that it is used for lots of food containers, even for hot food. So I'm guessing you have no issues. But the premise of your question seems flawed. $\endgroup$
    – user467
    Jun 16, 2015 at 1:06
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Though polyethylene (polythene in the UK) food bags and containers are quite safe to use even with hot food, oils, fats and other materials do penetrate polyethylene. For example, lycopene in tomato sauce or paste is absorbed so that the container is stained orange in a few days at room temperature, or in minutes when microwaved.

Whether this is true solution of the lycopene and/or oil carrier into the polyethylene, or penetration of microscopic pores, I do not know. This patent claims "staining may occur because the relatively non-polar nature of these commodity resin materials allows greater amounts of diffusion of non-polar substances (such as oils), which may contain dispersed pigments (e.g., lycopene), into the resin."

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In addition to the answeres from @trb456 and DrMoishe, please be careful if you talk about PE as a homogene material. There a quite a lot different "types" of PE, mostly categorised by chain lenght and side arms.

LLD-PE (linear low density Polyethylene) can start to melt at temperatures as low as 45°C. Containers for food normaly have a stamp on the bottom to indicate if they can be used for microwave ovens or hot food/liquids. Same should be there for bags sold for storing food. They should have a lable if you can use them for hot food (at least in EU and US).

From a reactivity standpoint, PE is mostly unreactive due to being composed of saturated hydrocarbon chains. Therefore, you can safely eat normal PE from a chemical health perspective. As long as you can't cut or otherweise harm yourself with it, it should leave your body in a state similiar to how you've eaten it (excluded are nanosized particles, which are still in discussion). Health risks from (mostly cheap) PEs normally come from additives like colours and softeners.

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  • $\begingroup$ So if it's a normal PE without any additives it can't release toxic gases inside the plastic bag when heated, like chlorine and benzene gas? (Of course, I'm talking about normal use and not burning it) Or if it does the amount of these chemicals won't be more harmful than smoking a tobacco? $\endgroup$
    – cimp23
    Jun 15, 2015 at 15:19

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