why do my plastic bowls heat up in the microwave?

[lemur]

I don't know what these bowls are made of except that they are plastic but when they come out of the microwave, the bowl is hotter than the food. I thought I had read that microwaves are only absorbed by water and pass through or are reflected by other materials, which is why they penetrate the food and heat it evenly instead of from the outside inward as with conventional/infrared ovens.

[TonyMcC]

Put simply microwave ovens heat materials by agitating molecules that are not rigidly held in place. In liquid water the molecules are quite free to move and this movement generates heat. In ice and materials such as ceramics the molecules are held rigidly in place and hardly respond to the microwave energy and thus do not heat up to any extent. Plastic usually lies between these two extremes and heats to some extent. The amount of heat generated varies according to the actual plastic formula used.

[Incendia]

I don't know where you read that microwaves are only absorbed by water but that isn't true...Microwaves are absorbed by many things...

[lemur]

I don't know where you read that microwaves are only absorbed by water but that isn't true...Microwaves are absorbed by many things...

 

The main things I remember from the book I read was that microwaves penetrate the surface of food, unlike infrared waves created by conventional ovens, and that they do not penetrate the atmosphere. It seemed like there was also something about water since microwaves are also used for doppler radar to scan for clouds. I also figured this had to do with why they don't penetrate the atmosphere. It's been a while since I read that book, so I thought I'd post my empirical data regarding my hot plastic bowls and see if someone with clearer knowledge could help.

[swansont]

All the materials are going to absorb microwaves to some extent. I think the issues here are that the plastic probably has a smaller specific heat capacity than water, so absorbing the same amount of energy per mass means its temperature rises more than water's would. Also, the plastic in question might have a smaller density than that of water, so there is less mass, requiring less energy absorption. Water's heat capacity is around 4 J/gK, and many plastics have values between 1 and 2. So even if they are half as efficient (per gram) at absorbing the microwaves, you'd expect them to heat up faster.

 

I expect that "microwave safe" plastics are the ones that have higher specific heat capacities and higher densities.

[Newbies_Kid]

I agree with Tony, perhaps that plastic's bowl is a kind of polymer that having a week polymeric chain. The wave radiation might cause the chained monomer to vibrate until it reach the resonance state and weaken the chains ( may be due to elongation or yielding). The monomer will freely vibrated and bump into each other which could produce heat.

Edited December 5, 2010 by Newbies_Kid

[lemur]

All the materials are going to absorb microwaves to some extent. I think the issues here are that the plastic probably has a smaller specific heat capacity than water, so absorbing the same amount of energy per mass means its temperature rises more than water's would. Also, the plastic in question might have a smaller density than that of water, so there is less mass, requiring less energy absorption. Water's heat capacity is around 4 J/gK, and many plastics have values between 1 and 2. So even if they are half as efficient (per gram) at absorbing the microwaves, you'd expect them to heat up faster.

 

I expect that "microwave safe" plastics are the ones that have higher specific heat capacities and higher densities.

 

 

I agree with Tony, perhaps that plastic's bowl is a kind of polymer that having a week polymeric chain. The wave radiation might cause the chained monomer to vibrate until it reach the resonance state and weaken the chains ( may be due to elongation or yielding). The monomer will freely vibrated and bump into each other which could produce heat.

So the same chemical properties of the bowls that cause them to heat up faster than the food also means that the material is breaking down and shouldn't be microwaved? That's annoying. They're my favorite bowls!

 

Back to the real point of my post, though: what is it that causes some materials to absorb microwaves while others do not absorb them?

 

 

[mississippichem]

Microwaves are of energy near the magnitude of vibrations and rotations of molecules. Really closer to the rotation side of that as IR frequencies are more characteristic of molecular vibrations. There is some overlap here.

 

The treatment gets quite mathy, but it basically amounts to a difference between center of mass and center of charge in a molecule. Molecules that are highly polarizable and show a significant permanent dipole moment (asymmetric distribution of charge) often absorb microwaves well but there are many factors to consider including the shape of the molecule itself. Water is a very good microwave absorber, it has a large dipole moment and a small energy requirement to rotate about the O-H bonds.

 

Microwaves aren't energetic enough to break covalent bonds either (there are exceptions), but they have enough juice to disrupt intermolecular forces (van der Waals?) that might hold polymer bulk structures together.

Edited December 5, 2010 by mississippichem

[imatfaal]

Glass will absorb microwaves and eventually melt, iff it is preheated to red hot. If even a tiny portion of the glass in a microwave gets too hot, it will heat the adjacent piece till it will absorb the microwaves and heat up. If you fancy trashing a microwave //DANGEROUS BIT// heat up a bottle with a blow torch till a spot is red and then stick in a full power microwave - after a while you will have a blob of molten glass.

[Mr Skeptic]

So the same chemical properties of the bowls that cause them to heat up faster than the food also means that the material is breaking down and shouldn't be microwaved? That's annoying. They're my favorite bowls!

 

Back to the real point of my post, though: what is it that causes some materials to absorb microwaves while others do not absorb them?

 

Yeah, I've heard bad things about microwaving the non-microwave-safe plastics. Some of the plastics have nasty chemicals and you don't want that breaking down.

 

My favorite dishes/bowls are the corningware ones, they're made of a very thin, white glass-ceramic, they don't scratch nor stain and are lightweight, and my mom has had them from since I can't remember. They're probably older than me

[swansont]

 

Back to the real point of my post, though: what is it that causes some materials to absorb microwaves while others do not absorb them?

 

Sympathetic vibrations. Molecules can bend or rotate, and the oscillating electric field of the microwaves causes this to happen. The efficiency with which this happens depends on how close the resonance frequency is to the frequency of the microwaves, and the resonance frequency depends on the molecular structure. It's not much different from yelling at a bell or tuning fork. It will begin to vibrate when you do so, and the loudness of the ringing depends on how close you are to resonance.

[lemur]

Sympathetic vibrations. Molecules can bend or rotate, and the oscillating electric field of the microwaves causes this to happen. The efficiency with which this happens depends on how close the resonance frequency is to the frequency of the microwaves, and the resonance frequency depends on the molecular structure. It's not much different from yelling at a bell or tuning fork. It will begin to vibrate when you do so, and the loudness of the ringing depends on how close you are to resonance.

Thanks. That is a very good explanation (clear as a bell). Now, what allows one to know which materials/molecules are sympathetic to a particular frequency in a given state?

[swansont]

Thanks. That is a very good explanation (clear as a bell). Now, what allows one to know which materials/molecules are sympathetic to a particular frequency in a given state?

 

Mostly, it's experimental data. You expose the material to the spectrum of EM radiation and see what gets absorbed. There are huge volumes of work showing the resonances of atoms and molecules.

[lemur]

Mostly, it's experimental data. You expose the material to the spectrum of EM radiation and see what gets absorbed. There are huge volumes of work showing the resonances of atoms and molecules.

That makes sense. Do you know if there are any theoretical models attempting to explain why and how specific materials/molecules/atoms absorb specific frequencies and not others?

Edited December 8, 2010 by lemur

[swansont]

That makes sense. Do you know if there are any theoretical models attempting to explain why and how specific materials/molecules/atoms absorb specific frequencies and not others?

 

There are, but I don't know how widespread and/or successful they are. If you Google on spectroscopy models I'm sure you'll get a lot of hits.

[xus]

I don't know what these bowls are made of except that they are plastic but when they come out of the microwave, the bowl is hotter than the food. I thought I had read that microwaves are only absorbed by water and pass through or are reflected by other materials, which is why they penetrate the food and heat it evenly instead of from the outside inward as with conventional/infrared ovens.

 

if the plastick is enviromental fx. made of portatos it can melt i have tryed it on my own it is a lot of mess to clean up

[lemur]

if the plastick is enviromental fx. made of portatos it can melt i have tryed it on my own it is a lot of mess to clean up

Yes, thanks. I've already switched to microwaving ceramic or glass bowls instead of the plastic ones because of this thread.

[curio]

I made solar stills out of 3 types of bowls which were: glass, ceramic and plastic. the plastic bowl distilled the most water, i don't understand how or why...?