"Heat resistance temperature" and melting point?

[kenny1999]

I've got a kitchen bowl made of polypropylene, it states that the "heat resistance temperature is 90 degree cel", while on Google, I found that the melting point of PP is 160 degree cel.

I understand that melting point is the temperature in which the atoms/molecules change its state from solid to liquid. How about the term "Heat resistance temperature"?

 

[Borek]

I doubt it is something precisely defined in terms of physics. At best it looks like kind of an industrial standard saying "we guarantee it won't deform if you don't heat it above 90°C".

Also note that MP is a property of crystalline substances. PP is an amorphous substance that doesn't have a sharp melting point, it will slowly get softer and less viscous, to the point where it will just freely flow. Whole process will take place over a range of temperatures (that is, assuming it won't decompose first).

 

[kenny1999]

I doubt it is something precisely defined in terms of physics. At best it looks like kind of an industrial standard saying "we guarantee it won't deform if you don't heat it above 90°C".

Also note that MP is a property of crystalline substances. PP is an amorphous substance that doesn't have a sharp melting point, it will slowly get softer and less viscous, to the point where it will just freely flow. Whole process will take place over a range of temperatures (that is, assuming it won't decompose first).

Hi, does "deform" mean "decompose" ? What is the difference between "Deform/Decompose" and "Melt" ??

 

[mjc123]

No, "deform" means to change shape under an applied stress. PP will not decompose chemically at 90°C - if it did, there would be no sense in talking about a melting point at 160°C. There will be a test for heat distortion temperature (it may go by various names), which is defined as the temperature at which a test piece distorts by this much under this stress. This is somewhat arbitrary since, as @Borek says, it softens gradually over a wide temperature range.

PP is, however, a partially crystalline polymer - polymer chains will line up in a regular array in crystalline domains. These domains are embedded in regions of amorphous polymer; polymers are never fully crystalline.
The melting point is where these crystalline domains lose their crystalline order and become amorphous.

For PP, this is complicated by the fact that you have different stereochemical forms, where the stereochemistry (R/S) is the same from one propylene unit to the next (isotactic), or alternates (syndiotactic), or is random (atactic). Atactic PP is amorphous, and the other two forms have different melting points.

 

[kenny1999]

No, "deform" means to change shape under an applied stress. PP will not decompose chemically at 90°C - if it did, there would be no sense in talking about a melting point at 160°C. There will be a test for heat distortion temperature (it may go by various names), which is defined as the temperature at which a test piece distorts by this much under this stress. This is somewhat arbitrary since, as @Borek says, it softens gradually over a wide temperature range.

PP is, however, a partially crystalline polymer - polymer chains will line up in a regular array in crystalline domains. These domains are embedded in regions of amorphous polymer; polymers are never fully crystalline.
The melting point is where these crystalline domains lose their crystalline order and become amorphous.

For PP, this is complicated by the fact that you have different stereochemical forms, where the stereochemistry (R/S) is the same from one propylene unit to the next (isotactic), or alternates (syndiotactic), or is random (atactic). Atactic PP is amorphous, and the other two forms have different melting points.

Hello. Thank you for the detailed explanation.

I have a new polypropylene kitchen bowl in which it states that the "Heat resistance temperature = 90°C", I wasn't aware of that and poured some freshly boiled hot water into it (should be around 100°C). In this case, would it cause any damage to the structure of the bowl and give out dangerous chemicals?? Is the bowl still safe for using?

 

[mjc123]

Shouldn't do, but the bowl might have deformed if you'd put stress on it.

 

[kenny1999]

Shouldn't do, but the bowl might have deformed if you'd put stress on it.

Would "Deform" leach any chemicals? Or just a change of shape only?

 

[Borek]

Would "Deform" leach any chemicals? Or just a change of shape only?

At these temperatures it is only about shape change.

 

[kenny1999]

At these temperatures it is only about shape change.

Could it lose/leach any atoms/particles/compounds during the process?? Thanks

 

[Borek]

Could it lose/leach any atoms/particles/compounds during the process?? Thanks

It always does, question is: how much and how fast. And yes, the higher the temperature, the faster these processes are. Still, around boiling point they are negligible, personally I wouldn't care.

 

[kenny1999]

It always does, question is: how much and how fast. And yes, the higher the temperature, the faster these processes are. Still, around boiling point they are negligible, personally I wouldn't care.

Does "How much" and "how fast" also depend on the material? Is it safer to use food-grade stainless steel container compared to Polypropylene? It feels like stainless steel is stronger and their particles/atoms/compounds won't give out, true or not??

 

[Borek]

Does "How much" and "how fast" also depend on the material?

Yes.

Is it safer to use food-grade stainless steel container compared to Polypropylene? It feels like stainless steel is stronger and their particles/atoms/compounds won't give out, true or not??

More or less yes to both. Matter of price and ease of use, you don't want to drink from a metal cup as it burns. Still, PP is safe enough as long as you don't heat it above water BP.

 

[snorkack]

No, "deform" means to change shape under an applied stress. PP will not decompose chemically at 90°C - if it did, there would be no sense in talking about a melting point at 160°C.

There would be some sense of talking about melting point at 160 degrees, though.
"Deform" means change shape.
"Decompose" means change composition. Whether the substances are given off or stay in the material.
Like deforming, decomposing takes time and is slower at lower temperature. Therefore, on rapid heating of a substance, you could have a modest amount of decomposition below the melting point which nevertheless has little effect on the melting point.

 

[kenny1999]

It's said that when Polypropylene is exposed to sunlight / UV, free radicals could be given out because of UV degradation. Where will those free radical end up in the air?? Otherwise than free radicals, any other chemicals would be given out when stressed by UV?? Are those free radicals and chemicals harmful to us?