[shvcko99]

What happens when commonly available polypropylene become brittle? I mean, what will it look like when it becomes brittle? Will it break as tiny insignificant powders or torn into smaller part but easily observable?

I'm asking this question because I have been freezing foods in freezer with PP containers probably well below 0ºC but I only happen to know that Polypropylene will become brittle below 0ºC. If it will become powder than it is very dangerous to the food but if it will be just torn into parts then I'll keep freezing food with PP without too much worries

[Borek]

It won't turn into a powder just because, and it is safe for food storage.

[shvcko99]

It won't turn into a powder just because, and it is safe for food storage.

so it will just break into significant parts in case of being brittle, am I correct

I know that it will not leach, but if it could break into powders then chances are I'll not be able to clean it up and have it eaten with normal food

[Borek]

so it will just break into significant parts in case of being brittle, am I correct

Yep, that's what I would expect.

Not that I had it ever break, I believe I saw PP container with a small crack, but still in one piece.

[Enthalpy]

Here you mean "brittle" due to cold? Neither do I expect anything to happen. This embrittlement occurs over a huge temperature range, so some arbitrary limit must tell a definite temperature limit. Time doesn't make it worse.

Just try it: take a bag from the freezer, swiftly fold it, it doesn't break. It would need rather liquid nitrogen (-196°C) to break when bent because of cold.

Embrittlement due to sunlight, as opposed, does reduce plastic bags to tiny chips when you touch them. I saw it in an attic after 3 decades.

[Corribus]

A good metric to use would be the glass transition temperature, which marks the point where a polymer's amorphous phase transitions from a glassy state (at lower temperature) to a rubbery state (at higher temperature). Polymers tend to be more brittle below the T_g. The T_g is usually between 0 and -20 C for polypropylene (PP), and it can depend a lot on the type of PP and any additives/plastizers the plastic contains. Thus it may range from above to is slightly below the temperature of most household freezers.

Most freezer bags are made out of polyethylene (PE), which usually has a much lower T_g than PP. So while some PP products may get brittle in a household freezer, most PE products will be fine. Because of this, for applications at temperatures at or around the freezing point of water, PP is not usually a preferred material.

For more information see this article about the link between glass transition temperature and mechanical properties of polymers.

This of course, as Enthalpy mentions, does not account for changes due to chemical or photo aging, to which PP is particularly susceptible.