Is there anything better than a fresh, juicy
omelet served piping hot? Is there anything worse than the frazzled
remains of burnt egg that's stuck to the pan and just won't budge? If
your cooking's as clumsy as mine, your favorite frying pan is made of
a slippery material called PTFE (sold by Chemours™ under the brand name
Teflon®) that's virtually impossible for food to stick to. Like
many other miracle materials, PFTE doesn't occur naturally: it has to
be specially made in a chemical plant. Its discovery was a wonderful
fluke—a laboratory accident that happened back in the 1930s. Since
then, PTFE has been used on everything from
waterproof clothing and
Moon-landing spacesuits to replacement body parts and stadium roofs.
Let's take a closer look at what it is and how it works.
Photo: This nonstick pan is coated with a thin layer of DuPont's PTFE, better known as Teflon®. You can see how slippery a nonstick pan is if you pour a little water into it and rock it back and forth.
Notice how the water barely wets the surface of the pan? That's because the cohesive forces holding the
water molecules together into droplets are far greater than the tiny adhesive forces between the water molecules and the nonstick surface. Empty the water out and you'll find the pan is practically dry
to the touch straight away.
What makes an egg stick to a pan? If the pan is
old and well used, chances are you've washed it hundreds of times,
probably with a pan scourer or scrubbing brush. Every time you clean
the metal surface, you scratch it slightly. The scratches make the
surface rougher and, ironically, make it even easier for eggs to
stick to its surface. It's probably true to say that the more you
scrub the pan, the more you'll have to scrub it next time.
It's a horrible vicious circle!
Photo: 1) Routinely cleaning an ordinary steel plan like this scratches it and makes food even more likely to stick. 2) From a distance the pan looks clean and shiny, but in this digitally enhanced closeup photo, you can see how very badly scratched it is.
Why is a rough surface easier for an egg to stick to? Think of rock climbers
hauling themselves up the vertical face of a cliff. It's the
scratches, bumps, and holes that make it possible to climb straight
upward: if the cliff were perfectly smooth, climbing would be
impossible.
Why don't things stick to a nonstick pan?
Short answer: The sticking forces between PTFE (a long-chain molecule or polymer) and things that
come into contact with it are virtually non-existent.
Longer answer...
When we say PTFE is "nonstick," we really mean
there's very little friction between the PTFE and anything that comes
into contact with it. But why? PTFE's full name is
polytetrafluoroethylene. It's a plastic polymer, which means its
molecules are made from an incredibly long string of atoms bonded
together. In fact, there are just two kinds of atoms in PTFE: carbon
and fluorine. The carbon atoms form a kind of central backbone and
the fluorine atoms are bonded onto it on either side, like this:
Artwork: The structure of PTFE.
The red blobs are carbon atoms; the blue blobs are fluorine.
A carbon atom can make bonds (links) to as many as four other atoms.
In PTFE, each carbon atom is connected to two other carbon atoms with two fluorine atoms above and below.
This is the basic repeating unit (chain link) or "monomer" in the polymer PTFE.
PTFE is chemistry's equivalent of an introvert. It
likes its own company, but it doesn't like other molecules all that
much. Put a load of PTFE molecules together and they clump and stick
together like spaghetti. In physics terms, we say there are strong
cohesive forces between them. When you see a drop of rain
sticking to a pane of glass, hanging in mid-air like a climber on a
cliff, what you're seeing is cohesive forces in action. There are
strong forces of attraction inside the water tugging its molecules
inwards into the shape of a drop. But you're also seeing another kind
of force too. There are adhesive forces holding the water
molecules to the molecules in the glass. These are
called van der Waals forces (pronounced:
'van dur varls') forces, named for the Dutch physicist
Johannes Diderik van der Waals (1837–1923) who discovered them (and
earned himself a Nobel Prize in the process). On the microscopic
scale, van der Waals forces are what cause friction. Imagine a legion
of invisibly tiny fingers helping the water molecules cling to the
glass. That's what van der Waals forces are like.
Photo: Sometimes rough surfaces are very handy.
Rock climbers certainly couldn't climb a cliff coated with Teflon®!
In PTFE, the cohesive forces are strong, but there
are virtually no adhesive, van der Waals forces between the PTFE
polymer molecules and anything that comes into contact with them. Try
putting a single drop of water on a nonstick pan. Now turn the pan
vertically so the drop has to cling like a climber on a cliff. How
long does it stay there? No time at all: it cannot stick to the
surface because the weight of the water is many times greater than
the frictional, van der Waals forces—so the drop screeches
immediately to the bottom of the pan. And notice that it doesn't
smear like a raindrop falling down glass. The frictional forces are
so low that it stays as a drop as it falls. It's still a drop when it
gets to the bottom.
Chart: PTFE is one of the slipperiest things in our world—PTFE on PTFE
technically, has a very low coefficient of friction—but, as always,
nature does it better. Cartilage (containing a natural lubricant
called proteoglycan 4) is even more slippery.
Sources of data: [1]
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What makes Teflon stick to the pan?
This raises an obvious question. If you want to
make a nonstick frying pan, and you have a lump of round metal in one
hand and a splodge of slippery PTFE in the other, how do you get the
two to stick together? How do you get the nonstick surface to
stick to the pan? It turns out that you have to make the pan
super-rough to start with by blasting it with grit. If the pan is
extremely rough, the PTFE will bind to it like a rock climber
clinging to the face of a cliff. Then you just bake the pan in a
really hot oven and the PTFE will permanently lock itself to the
metal underneath. Job done.
How nonstick pans are made
A process for Teflon-coating aluminum pans was patented by Atlas Copco in the 1960s, and goes something like this:
Two-stage power-spray wash and rinse: The uncoated aluminum pans ("aluminum blanks") are sprayed with a mild alkaline cleaner
using a high-pressure hose, then sprayed again with high-pressure hot water. This initial step removes oil and dirt
so the blanks are perfectly clean and ready for blasting.
Rinse and dry: The blanks are rinsed with hot water and air dried to prevent any marking.
Grit blasting: Hard, sharp iron grit is blasted across the aluminum to roughen its surface, using a centrifuge to disperse it evenly.
Air blasting: This step removes any loose grit.
Washing and rinsing: This step is very similar to the initial two-step cleaning process (stage 1), using the same mild alkaline cleaner, followed by a hot-water rinse.
Deoxidation cleaning: Acids and other cleaning agents restore the bright and shiny appearance of the aluminum.
Drying and Teflon coating: Once the blanks are dry, they're coated with Teflon by spraying, brushing, or flowing it across their surface, then air dried.
Sintering: The Teflon coating is "baked" onto the pan at a temperature of around 370–385°C (700–725°F) for
about five minutes.
Photo: You can tell nonstick is a coating, rather than an inherent property of your
pan, because it will scratch and wear away. Where does the coating go? Well... into the food you're eating.
If your pan looks as bad as this, it's probably time to throw it away.
There are various alternative methods (for example, patented by Collette Gregoire
in 1960
and 1961
and Reynolds Metals Co in 1963),
but also seem to involve attacking the metal to be coated either physically (with blasting), chemically (with acid),
or both.
What is PTFE used for?
PTFE's not just in your frying pan—you can find
it in all kinds of unexpected places. The world's biggest dome
building, London's once-controversial
Millennium Dome
(now called the O2 Arena), has a huge white
tent roof made out of PTFE-coated fiberglass to keep the rain off. PTFE is also used in
bionic body parts such as replacement hips, where the artificial
joint is made from a strong and lightweight titanium or steel ball
that swivels effortlessly inside a PTFE socket.
It's also used in spray lubricants
of various kinds, first launched in the early 1980s.
[2]
The very same chemical that's lubricating your legs could be
keeping you clean and dry too. Clothing fabrics are often treated with
a PTFE-based stain repellent to keep oil, grease, and stains at bay.
Since PTFE is a plastic, it repels water, so you'll often find it on umbrellas
and wet-weather clothes. What works for soft textiles works just as well
on hard buildings; PTFE additives are also used in wipe-clean paints and woodcare products.
And if you're got leaky pipes or faucets in your home, a plumber might use thin PTFE tape
(which looks a bit like a wider version of dental tape) to make a quick repair.
Photo: Many buildings have Teflon-coated roofs. This is the tent-like roof of Denver International Airport in Denver, Colorado, United States. Detail of a photo by Carol M. Highsmith courtesy of Gates Frontiers Fund Colorado Collection within the Carol M. Highsmith Archive, Library of Congress, Prints and Photographs Division.
Many people think PTFE was invented as part of the
1960s Apollo space program, but it actually dates back to 1938. That
was when DuPont™ chemist Roy Plunkett (1910–1944) made some PTFE
completely by accident when an experiment he was doing on
refrigerator chemicals went badly wrong. Like many of the
word's best inventions, PTFE started life as a hopeless mistake—a
glob of white goo no-one knew what to do with. It wasn't until the
1940s and 1950s that nonstick "Teflon®," as DuPont called it,
really took off as a low-friction coating. During the 1960s, NASA's
space scientists realized that PTFE, which has high resistance to
heat, doesn't react with other chemicals, and is slippery enough not
to cause abrasion, was a perfect protective material to use
inside space suits. Thus was born the myth of PTFE as a spinoff from
the space program. Truth is, PTFE was invented over 30 years before
astronauts went anywhere near the Moon!
Photo: Teflon® was used in space suits like this, but it wasn't invented
during the space program. Ed White, the first American spacewalking astronaut, relied
heavily on protective materials in his suit and helmet.
Photo courtesy of NASA on the Commons.
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Lubricants: Friction-reducing chemicals used in engines and other machines.
On other sites
Teflon®: Official Chemours website. Explains the many different uses for Teflon and goes into some detail to explain why "...Teflon® nonstick coatings are safe for consumer use."
Articles
Popular science and news
The Lawyer Who Became DuPont's Worst Nightmare by Nathaniel Rich, The New York Times, 6 January 2016 and The Teflon Toxin by Sharon Lerner, The Intercept, 11 August July 2015. Two detailed articles investigating the potentially harmful effects of Teflon and chemicals used in its manufacture.
"Material of the
month—Teflon" by Anna Ploszajski. Materials World, Feb 1, 2015. A great introduction to how Teflon was developed and how it's become an indispensable modern material.
A non-stick panacea by Tim Hayward, The Guardian, 2 July 2010. A food writer asks whether cooks really need to use nonstick pans.
'Virtually non-stick' gum created by Rebecca Morelle and Liz Seward, BBC News, 13 September 2007. Is it possible to make an environmentally friendly chewing gum that doesn't mess up our streets?
US Patent 2,230,654: Tetrafluoroethylene polymers by Roy J Plunkett, Kinetic Chemicals Inc, 4 February 1941. This is Roy Plunkett's original Teflon patent. Although short on details, it describes the chemical composition of PTFE.
US Patent 2,401,897: Preparation of tetrafluoroethylene by Anthony F Benning, Frederick B Downing, and Roy J Plunkett, Kinetic Chemicals Inc, 11 June 1946. This follow-up patent goes into more detail about the practical preparation of PTFE.
US Patent 3,243,321: Method of Teflon coating of metals by Ronald P Rowand, Atlas Copco, 29 March 1966. How do you get Teflon to stick to a pan? This patent explains the process by which aluminum cooking pan blanks are nonstick coated with Teflon.
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