QUESTIONS AND COMMENTS

The non-stick properties of PTFE

When I was writing about PTFE on the page about polymerisation of alkenes, I spent ages trying to find out why PTFE was non-stick - and failed completely. I left a message on the page back in 2003 asking for help from anyone who might know something about it, and then forgot about it.

I then (4 years later) had a query from a lecturer in a German university who was trying to track down the same information, and we had quite a lengthy discussion. As a result of this, I now have some more information, but I'm still not certain about what is happening - although I can make some guesses.

Part of the information I found on the web I know to be untrue or illogical, but there is a mass of stuff which, to be frank, I simply don't understand. Quite a lot of what is out there is written by physicists or other non-chemists who speak a quite different language from me! It also seems to me that there is a reluctance to start right back at the level of the molecules and explain what is happening in terms of molecular interactions. It may well be that someone really understands what is going on at a molecular level, but they aren't telling it in a way that I can get a grip on.

Don't worry about what is on the rest of this page - it is way beyond what you might need for UK A level purposes (or the equivalent 16-18 year old chemistry qualification anywhere else in the world). I'm discussing it in the hope that someone can explain it to me in a way that a simple chemist might understand.

Anyway, this is where I have got to up to now . . .

The structure of PTFE molecules

PTFE poly(tetrafluoroethene) is made by polymerising lots of tetrafluoroethene molecules.

This simple diagram for PTFE doesn't show the 3-dimensional structure of the molecule. In the simpler molecule poly(ethene) the carbon backbone of the molecule just has hydrogen atoms attached to it, and the chain is very flexible - it definitely isn't a straight molecule.

However, in PTFE, the fluorine atoms in one CF₂ group are big enough to interfere with those on the neighbouring groups. You need to remember that each fluorine atom will have 3 lone pairs sticking out from it.

The effect of this is to inhibit rotation about the carbon-carbon single bonds. The fluorine atoms will tend to line up so that they are as far apart as possible from neighbouring fluorines. Rotation will tend to involve a clash of lone pairs between fluorines on adjacent carbon atoms - and this makes rotation energetically unfavourable.

The repulsions lock the molecules into a rod-like shape with the fluorines arranged into very gentle spirals - a helical arrangement of the fluorines around the carbon backbone. The rods will then tend to pack together a bit like long thin pencils in a box.

Intermolecular forces between PTFE molecules

The PTFE molecules are rod-like and can lie closely together, and so there should be very effective van der Waals dispersion forces between neighbouring molecules. PTFE molecules contain a lot of electrons and are very long - that suggests that the dispersion forces should be quite strong. Is there any evidence for this?

The melting point of PTFE is quoted as 327°C. That's quite high for a polymer of this sort - so there must be sizeable van der Waals forces between the molecules. One source says that on melting, PTFE turns into a very viscous liquid. High viscosity is also a sign of strong intermolecular forces (or possibly of tangling - but if PTFE molecules are still straight, the amount of tangling isn't going to be as great as if they were flexible).

However . . . several web sites talk about PTFE having very weak van der Waals forces. That has got to be completely untrue. If it had very weak van der Waals forces, it would be a gas - not a fairly high melting point solid!

Other sources suggest that the van der Waals interactions in PTFE aren't as strong as you might expect given the number of electrons because the C-F bond isn't very polarisable.

What this means is that in the C-F bond the electrons are held well towards the fluorine end because of its high electronegativity. In van der Waals dispersion forces, electrons have to be capable of being moved around in response to temporary fluctuating dipoles in neighbouring molecules. If they are held strongly by the fluorine atoms, then that might not happen as easily as it would otherwise do.

Well - fair enough! The forces may not be as strong as you might expect, but they are nevertheless strong enough to give PTFE a melting point of 327°C. The intermolecular forces in PTFE cannot be dismissed as negligible.

Friction and non-stick properties

PTFE has a very low coefficient of friction. What this means is that if you have a surface coated with PTFE, other things will slide on it very easily.

Suppose you have a block of one material on a flat surface. If you push it very gently sideways, it won't move. Friction is holding it where it is. If you increase the sideways push, it will eventually start to slide.

The coefficient of friction simply measures the amount of friction as the ratio of F/W, where W is the weight of the block. (If the block is on a slope, the whole thing gets mathematically more complicated - but it's irrelevant to our discussion.)

The coefficients of friction for PTFE on various surfaces are very low.

Virtually every site that I have looked at treats the relative lack of friction of PTFE and its non-stick properties as if they were the same effect. I'm not at all convinced about this and will look at them separately.

The low friction

Suppose you have a block of something on a PTFE coated surface.

What appears to happen is that after enough force is provided to get the block moving, a very thin layer of PTFE (less than 10 nm thick) breaks off the coated surface and sticks to the block. After that, you no longer have, say, steel moving against PTFE but one PTFE surface moving against another PTFE surface. Once the movement starts, the force needed to keep it moving falls. This suggests that it is easy to move PTFE layers against each other.

There are two puzzles here. The first thing to note is that the PTFE breaking off the coated surface sticks to the surface of the block. Why - if PTFE is supposed to be non-stick?

I can only suggest that the PTFE surface snags on the uneven surface of the block - in the same way that a piece of sandpaper will snag on a second piece of sandpaper if you press down and rub one across the other. However much you may polish a surface, at the atomic or molecular level there are always going to be hills and troughs. Presumably what happens is that the interlocking of the two surfaces (as long as the weight of the block holds them together) outweighs forces of attraction between molecules in the PTFE.

The other question is why you can get such easy movement between two layers of PTFE. It can't be because intermolecular forces in PTFE are extremely weak - because they aren't! We have already seen that they have to be quite big in order to account for the relatively high melting point of PTFE.

The only thing I can suggest here is that if the PTFE molecules are indeed like long thin rods, and if they are arranged very regularly, then the rods could roll over each other relatively easily. If they can roll, then you aren't breaking the intermolecular forces as much as stretching them slightly, and that will need less energy.

But this is just speculation on my part and could be quite wrong. If you have any reliable information about it (preferably with a reference) could you contact me via the address on the about this site page. But please, I'm not interested in any explanation which doesn't attack it from the molecular level and the intermolecular forces.

The non-stick properties

This is about why things like water and oil don't stick to the surface of PTFE, and why you can fry an egg in a PTFE-coated pan without lots of it ending up stuck to the pan. The mechanism in this case doesn't involve thin layers of the PTFE breaking off and sticking to whatever is in the pan.

Let's concentrate on the water problem. Once that's sorted, we can perhaps make the generalisation that you can extend the same principles to other things as well (or perhaps not!).

Water on a clean PTFE surface doesn't spread out to wet the surface. Instead it stays as small beads of water. That suggests a lack of attraction between the water molecules and the PTFE. Why isn't there an attraction?

The PTFE molecules on its surface are completely encased in fluorine atoms. Those fluorine atoms are very electronegative and so will all carry some degree of negative charge. They also have active lone pairs of electrons sticking out. Those are exactly the conditions needed for hydrogen bonding to be possible between lone pairs on the fluorines and hydrogen atoms in the water. But it clearly doesn't happen - otherwise there would be strong attractions between PTFE molecules and water molecules and the water would stick to the PTFE.

So why doesn't this happen?

You will have to break hydrogen bonds between water molecules, but would replace them by hydrogen bonds of a similar strength between water and PTFE molecules. The enthalpy change of the process is going to be close to zero - so that can't be an explanation.

However, whether a process happens or not depends not only on the enthalpy change, but also on the entropy change. You can think of entropy as being a measure of the amount of disorder in a system. A reaction or other process like mixing is going to be more likely to happen if the disorder of the system increases. This won't necessarily outweigh a large unfavourable enthalpy change, but if the enthalpy change is small, it can make a big difference.

If water molecules attached themselves to the surface of the PTFE, the system becomes more ordered. You have a layer of water molecules fixed (perhaps only temporarily, but nevertheless fixed) into an ordered arrangement, and the entropy of the system falls. That decrease in entropy is enough to make the process energetically unfeasible.

I think you could probably extend this explanation perfectly well to other liquids like oils or melted fats - the difference being that this time you aren't thinking about hydrogen bonds because a typical vegetable oil, for example, doesn't have any suitable hydrogen atoms in it. Instead you would be thinking about dipole-dipole interactions between the PTFE molecules and the oil molecules. But, again, if there was attraction between the PTFE and oil molecules, it would lead to a decrease in entropy because you would again get an ordered layer of molecules on the PTFE surface. Again, it would be energetically unfeasible.

I suspect that you could probably adapt this argument to proteins (as in eggs!) as well - with hydrogen bonds theoretically possible between suitable hydrogen atoms in the protein and fluorine lone pairs on the PTFE. But again, entropy would be against it.

You could, of course, extend this argument to ask why polymers like poly(ethene) aren't used as non-stick coatings. In these cases, you don't have any possibility of hydrogen bonds or strong dipole-dipole interactions, but still have the same entropy argument if any water or oil (or whatever) molecules happened to stick to the surface. Polymers like this should be wonderfully non-stick!

I suspect the reason they aren't used is as a simple as the fact that PTFE has a much higher melting point than poly(ethene) because of its stronger intermolecular attractions. Using a poly(ethene) coating is going to make a sticky mess as soon as you start to heat the pan!

Finally . . .

This is again all speculation on my part. I haven't been able to find a single explanation for the non-stick properties of PTFE at the molecular level which was consistent, logical and understandable (at least by me). I could have got all this entirely wrong! If you know better (or even if you can confirm what I am saying), please get in touch with me.

If you want to write about the non-stick properties of PTFE for some reason of your own, please don't quote any of the attempted explanations from this page as fact. There is a real danger (especially with the internet) that some piece of speculation can rapidly become established all over the web as the definitive truth. This page is just my thoughts about the topic. As new information becomes available, I will keep it up-dated.

Return to questions list . . .

Go to Main Menu . . .