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Plastic food grade storage box

I have recently purchased a 10 litre semi-opaque (grey) plastic container which appears to be ideal for sous vide cooking. It was sold as food grade, freezer & microwave safe, and has all the relevant marks on the bottom to confirm this rating. It is manufactured from polypropylene and is (PP5) graded.

Looking online, all the other sous vide baths I see are manufactured out of much more expensive polycarbonate, and this container was a fraction of that cost. I assume here that polycarbonate has been chosen because it is more rigid than polypropylene.

I have tested this up to 85 Centigrade and the walls of the container flex and bow only slightly more than they do when cold. I doubt if I will sous vide beyond 75 Centigrade, am I taking a risk using this product here?

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  • I don’t know if all polypropylene is the same, but I did find mention that 180°F / 83°C is the maximum recommended temp from one supplier (they said strength decreases above 180°F). I couldn’t find any strength vs temperature charts, though
    – Joe
    Sep 26, 2022 at 1:22

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Polypropylene is fine for use up to (or even past) boiling temperatures. I scald milk for yogurt by steaming it in used PP5 yogurt containers.

Indeed, this list rates PC as 10°C lower service (90-125°C) than PP (100-130°C), or basically equivalent for "high heat" PC at 100-140°C).

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If you intend on using it as a container for your bath water, then the concern is mainly for long-term durability compared to polycarbonate. Often part of the added cost for 'nicer' polycarbonate containers is NSF/other certifying body testing and validation for commercial use, including durablity and ease of cleaning - liability control in industry, more peace-of-mind for home cooks..
If the flexing is acceptable to you within the temperature ranges you cook at then there shouldn't be a problem for flexibility.

Polypropylene has a melting point above 100C/212F and can be used above this temperature[1,2]. Microwave heating is very uneven and it's typical to develop boiling or even superheated hot spots along edges and corners - you may see some small melted rough patches on old microwaved Ziploc etc. food containers. In contrast, normal sous vide temperatures are very evenly below boiling, let alone polypropylene melting temperature. The risk of plasticizer/additive leaching from container > water > bag membrane > food is even lower.

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  • Non-thermoset plastics lose strength well before they get to their melting point. If it starts to deform, you can get necking (reduction of cross section), which can cause rapid failure as the item can no longer support the same maximum force. (I’m working from memory: I took materials science 25+ years ago)
    – Joe
    Sep 26, 2022 at 1:28
  • @Joe that's true - though at sous vide temperatures polypropylene can still support hundreds of cycles of strain stress on the scale of MPa's, while the hydrostatic pressure in the container is orders of magnitude lower - doi.org/10.1002/app.1981.070261141 , doi.org/10.1002/pen.11131
    – borkymcfood
    Sep 26, 2022 at 7:19

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