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FIFTY years ago, a young chemist was working one Saturday, in the laboratories
of the large American chemical company, du Pont, when he spotted something
curious about one of the gas cylinders he was cleaning out. Roy Plunkett
had been working with an unusual gas called tetrafluoroethylene. This is
an organic compound with a difference. Like ethylene (C₂H₄),
it contains two carbon atoms, but instead of having four hydrogen atoms
linked to them, it has four fluorine atoms. Fluorine is an element similar
to chlorine but even more reactive. Du Pont used tetrafluoroethylene to
make new refrigerants, such as the now notorious chlorofluorocarbons (CFCs).

Plunkett noticed a white solid at the bottom of the cylinder that had
previously contained the gas. This substance turned out to be a material
that was to become a household name. Molecules of tetrafluoroethylene had
joined together, or polymerised, to form the plastic, polytetrafluoroethylene
– better known as PTFE, or Teflon. This consists of long chains of carbon
atoms each with two fluorine (CF₂)_n atoms attached. Polyethylene (polythene)
is the same but with hydrogen atoms in place of fluorine (CH₂)_n.

Fluorine endows polyfluorocarbons, such as PTFE, with some remarkable
chemical properties . Bonds between the carbon and fluorine are very strong,
so these compounds are extremely unreactive. Although PTFE has been around
for 50 years, it is only recently that chemical companies have started developing
other plastics containing fluorine. These new materials combine inertness
with novel optical and thermal properties. PTFE is so inert that even fluorine
gas, one of the most reactive chemicals known, does not affect it. In fact,
this turned out to be the key property that ensured the commercial development
of PTFE. During the Second World War, scientists working on the atom bomb
needed fluorine to make uranium hexafluoride, which they used to separate
the radioactive isotopes of uranium. The hexafluorides diffuse at different
rates depending on the weight of the isotope. PTFE protected the walls of
the vessels and pipes containing the fluorine.

Chemists soon discovered that PTFE was not only inert, but also actively
repelled other molecules, such as water and oil. The plastic is famous for
its nonstick properties; pans with a coating of Teflon certainly made omelettes
and pancakes easier to fry. One type of waterproof clothing, successfully
tested in the Falklands war, has a layer of expanded PTFE between the outer
fabric and the lining. Today, chemical companies produce more than 50 000
tonnes of PTFE a year.

The latest use for PTFE in the home is to line the automatic bakers
that make bread overnight. In other less familiar ways, PTFE is making buildings
weather resistant, chemical plants safer and bearings almost frictionless.
Surgical implants for replacing ligaments are made of an expanded form of
PTFE. The army is even coating tanks with it so that chemical warfare agents
will not stick to their surfaces.

Although PTFE may seem a perfect plastic, it has some drawbacks. It
is difficult to process, it cannot be moulded by heat, it is opaque and
expensive. Chemical companies are developing other polyfluorocarbons that
overcome some of these problems.

One way of tailoring a plastic so that it has particular characteristics
is to combine two monomers to form a copolymer. Polyethylene (polythene)
comes from ethylene; unlike PTFE, it is cheap and flexible, but not very
strong and it burns easily. Make a polymer from a mixture of the ‘monomers’
ethylene and tetrafluoroethylene, and something interesting happens: the
product retains the best characteristics of both polyethylene and PTFE.
It is strong and flexible. The copolymer also adds a few new characteristics
of its own. Two companies make the plastic: Hoechst, the West German company,
produces it under the name Hostaflon; du Pont calls its copolymer Tefzal.

Hostaflon, or Tefzal promises to be an ideal material for conservatories,
canopies and greenhouses. It is transparent to ultraviolet and visible light,
so whereas glass screens out the tanning rays from the Sun, you can actually
get a tan under a sheet of Hostaflon. Plants grown in greenhouses made of
the material will also benefit because the ultraviolet radiation penetrating
the plastic walls kills bacteria that flourish in conventional hot-houses.

Temporary greenhouses and cloches made of polythene sheeting do not
last long because the plastic is so weak. Strong winds and sharp objects
tear it easily. Hostaflon, on the other hand, is remarkably strong. Samples
of the plastic film left to weather in various climates, in India, Europe
and the US, show no weathering or discoloration even after 10 years or more.
Dirt does not cling to the film, and hailstones and sandstorms do not dent
it. The film does not tear even when punctured.

Hostaflon can be sewn, welded, painted, covered with metal and moulded
using heat. It is also fire resistant. It keeps its strength and useful
properties over a range of temperatures, from -50 °C to +150 °C
. This made Hostaflon the ideal material for the parabolic reflectors of
the new radio telescopes at the Plateau de Bure in France and Mount La Silla
in Chile, both situated 2500 metres above sea level. These reflectors consist
of a film of the fluoropolymer only 30 micrometres thick, coated with a
film of aluminium and supported on a framework reinforced with carbon fibres.
Such radio dishes perform much better than conventional aluminium or steel
structures, and have withstood buffeting from winds blowing at 200 kilometres
per hour and temperatures down to -30 ° C without deforming.

Many copolymers grown from a mixture of two monomers have chains of
units arranged randomly. But Hostaflon grows in a strictly regular way with
units of ethylene (CH₂CH₂) alternating with units
of tetrafluoroethylene (CF₂CF₂). The curious thing about this copolymer
is that it maintains this regular arrangement even when the percentage of
the two monomers varies from, say, between 25 and 65 per cent tetrafluoroethylene.

A more typical, random, copolymer is that made from vinylidene fluoride
(CH₂CF₂) and hexafluoropropylene (CF₂CFCF₃). This is a rubbery
material, a ‘fluoroelastomer’, which its manufacturer, du Pont, calls Viton.
High temperatures and corrosive chemicals that destroy ordinary rubbers
do not affect the fluoroelastomer, so the chemical industry employs it where
such conditions prevail. The 3M company makes a similar rubber, which it
calls Fluorel. These elastomers will be ideal for seals, hoses and gaskets
in the cars of the 1990s because they withstand the heat and corrosive conditions
associated with unleaded petrol and fuel injection.

As well as being incorporated in copolymers, fluoropolymers are also
being exploited in composites, with graphite, for example. For more than
30 years, industry has used graphite to make heat exchangers for corrosive
liquids, such as concentrated sulphuric acid or sodium hydroxide, as well
as organic solvents. Graphite conducts heat twice as well as steel and more
than 1000 times as efficiently as a plastic such as PTFE. The problem is
that graphite is weak. If, however, you mix graphite and a fluoropolymer
together to form a composite, you obtain a material that can be easily moulded
and welded and conducts heat almost as well. Hoechst has made a product
that can withstand up to 3000 hours of corrosive conditions.

Most fluorocarbon polymers are tough solids because the chains of carbon
atoms are very long, which is why they are so useful in composites. PTFE,
for example, can have up to 200 000 units of CF2 in each chain. Polymers
with short chains are just as stable as PTFE but are liquids. Fully fluorinated
hydrocarbons, the so-called perfluorocarbons or PFCs (the Latin prefix ‘per’
means ‘completely’), with between 6 and 20 units in the chain have boiling
points between 56 and 260 °C. The short-chain polymers have some remarkable
properties. For instance, they do not mix with either water or hydrocarbon
oils. A subsidiary of RTZ, Imperial Smelting Corporation Chemicals, makes
perfluorocarbons, selling them under the name of Flutec. The company 3M
calls them Fluorinert, and Hoechst uses the trademark Hostinert.

You can obtain a more versatile range of fluorocarbon oils by reacting
tetrafluoroethylene or hexafluoropropylene with pure oxygen gas and irradiating
the mixture with ultraviolet light at -40 °C. This forms a crude mixture
of peroxide compounds, which when treated with fluorine gas at 200 Degree
C gives perfluoropolyethers. These oils have a backbone of carbon atoms
interspersed with oxygen atoms (the name ‘ether’ refers to the chemical
grouping C-O-C). All other chemical bonds are between carbon and fluorine
atoms – the carbon compound contains the maximum number of fluorine atoms
possible. The resulting oil is a sort of liquid Teflon. It is chemically
very stable but has a fluidity that comes from the less rigid ether links,
which behave like molecular hinges.

The perfluoropolyethers have a mixture of long and short chains with
molecular weights between 500 and 20 000. The lighter oils can be distilled
off under reduced pressure. Montedison, the Italian chemical company, sells
these as Galden, while the less volatile oil is called Fomblin. Chemists
discovered these oils 20 years ago but they were too expensive for all but
a few exotic uses, such as lubricating oils and greases in space vehicles.
Today, manufacturers see perfluoropolyethers as replacements for silicones.
Like these materials, made from silicon and oxygen, they will soon appear
in household products such as soaps and shampoos. Ken Johns of Montedison
says that the company is already testing these toiletries on consumers,
with excellent results.

Why are these oils so interesting? Again, they are extremely unreactive,
and at the same time have unusual physical properties. Perfluoropolyethers
are not attacked by strong acids or alkalis, oxygen or chlorine gas, all
of which present problems for conventional oils and greases. Fomblin is
nonflammable, so it works well at high temperatures. It has a low surface
tension, which means that it spreads quickly and evenly over the surfaces
to be lubricated. It does not impregnate other plastics, which makes it
an ideal lubricant for them, especially if these are in the form of thin
membranes, such as video tapes and condoms. Fomblin is also suitable for
magnetic and video discs. The newest development is to swop the carbon atoms
at the end of the polymer chains, which have three fluorine atoms attached,
for something more chemically reactive, such as carboxylic acid groups (COOH).
In this way, chemists can use the carboxylic acid group to bind the oil
permanently to the surface to be lubricated. The surface will then not need
any further lubrication.

The lighter oils, Galden, Fluorinert, Flutec and Hostinert, with their
large heat capacity, can absorb or release heat evenly without conducting
electricity. They provide an ideal way of warming delicate materials such
as the plastic on printed circuit boards. The monomer of the plastic is
polymerised directly on the circuit board by heating. Conventional ‘curing’
using infrared lamps is uneven and takes about half an hour. Passing the
printed circuit board through the vapour of boiling Galden cures the polymers
evenly in about a minute. Stuart Briggs of Montedison, who has been developing
the technique, says that the first European plant to use the method will
come into operation in Britain later this year. Electronic companies can
test whole appliances, such as televisions, by immersing them in a bath
of the perfluorocarbon and switching them on. This prevents overheating
while revealing faults, such as imperfect seals. Fluorinert is also used
to cool super-computers such as the mighty CRAY.

The most public use of Fomblin is as a protective layer for buildings.
The fluorocarbon provides an ideal barrier against rain, dirt and grease
while allowing stonework to ‘breathe’. Acid rain and atmospheric pollution
threaten many of the world’s architectural treasures. Attempts to protect
them with hydrocarbon or silicone films have not been successful because,
after a few years, they discolour, crack and flake off, creating a further
problem of how to remove the films.

Before you can apply any coating to an ancient monument it must meet
some stringent tests. It should be chemically inert; remain stable in the
coldest winters and hottest summers; have a low surface tension so that
it will penetrate even the smallest crevices; repel water and oil; be colourless
and not discoloured by sunlight; be biologically inert so that it does not
encourage the growth of bacteria and other organisms; and, of course, be
safe to apply. Fomblin is the only transparent coating that conforms with
this formidable list of requirements.

A 70-per-cent solution of Fomblin in a volatile solvent sprayed on to
the stonework dries within minutes. The Italian government has given this
technique its seal of approval for preserving ancient buildings. Various
types of marble and stone have been coated with no adverse effects, and
recently restorers have treated several complete buildings, such as the
cathedrals at Syracuse in Sicily and Lucca in Tuscany and the Palazzo Pitti
in Florence.

Fomblin can also protect the human body in the same way. Cosmetic companies
will soon be incorporating Fomblin into face creams, sunscreen lotions,
shampoos, foam baths and soaps. Fomblin has many features that make it ideal
as a cosmetic material: it is odourless, colourless, transparent, nontoxic,
nonirritant and gives a satin-like feel to skin and hair. Sunscreen lotions
containing Fomblin will be on sale this summer. Tests show that they are
not greasy like normal suntan oils, nor are they washed from the skin by
perspiration or swimming.

Many cosmetic formulations are emulsions, which are two-phase systems
of a suspension of tiny oil droplets in water. Fomblin has a curious effect
on such emulsions. It mixes with neither the oil nor water phases, but forms
a third phase coating each tiny oil droplet. This not only causes smaller
droplets to form but prevents them from coalescing again, giving emulsions
a longer shelf life. It also inhibits discoloration and stops the oil going
rancid.

Although perfluoropolyethers are completely stable and fire-resistant
up to 300 °C, they do have one curious property. They dissolve oxygen
from the atmosphere. This can be a disadvantage in lubricants for jet engines,
for example, where the oxygen can lead to corrosion. On the other hand,
this tolerance of oxygen makes Fomblin oils ideal as pump oils. Oxygen degrades
most hydrocarbon oils used in pumps within days, whereas Fomblin is unaffected
for weeks. Although it is much costlier to use Fomblin at $300 a litre,
this investment should be worthwhile. Engineers are designing oil pumps
especially for perfluoropolyethers. In research that requires ultra-high
vacuums, Fomblin is turning out to be an ideal pump oil.

Recently, Sun Tech of Philadelphia has taken out patents for using PFCs
or Fomblin as markers and tracking agents. If you add PFCs to the emissions
from power plants, you can trace the progress of the waste gases as they
disperse into the atmosphere. Add PFCs to crude oil and you can follow the
way in which oil slicks at sea break up. Once incorporated into plastic
explosives such as Semtex, it will be possible to detect them by analysing
the tiny amount of PFC vapour given off.

These and other uses for polyfluorocarbons will come in the next decade.
They will be certainly common by the next century. But can the environment
cope with them? When the CFCs were introduced in the 1930s, people hailed
them as the perfect gases for refrigeration and aerosols; they seemed to
be completely inert and perfectly safe, being nonflammable and nontoxic.
For 40 years, their use increased year by year until the effect of their
chlorine atoms on the ozone layer halted their success.

Could we repeat the same mistake with the PFCs? The answer is no, for
three reasons. First, PFCs do not contain any chlorine, so the volatile
ones do not threaten the ozone layer. Secondly, as liquids they are twice
as dense as water and do not mix with it, so their ultimate destiny, should
they escape into the environment, will be the bottom of the deepest oceans.
Thirdly, and most compelling, they are too expensive to waste, so major
users will be recycling PFCs rather than throwing them down the drain.

A system of reclaiming used PFCs from industrial users already exists.
Because they are so stable, it is relatively easy to ‘clean’ them by distilling
them. Any PFC that escapes into the environment poses no harm to living
things, as far as we know, and chemical companies have already tested PFCs
extensively to see if they are hazardous to health. They appear to be perfectly
safe, indeed so safe that one product, perfluorodecalin, is used in artificial
blood.

So what can we expect in the 1990s from fluoropolymers? Because their
chief asset is their ability to protect, we can expect to see them in paints,
polishes and skin-care products. Further research will improve what PFCs
can do by putting reactive chemical groups of atoms at the ends of the fluoropolymer
chains, so that they can be permanently attached to the surface that they
are protecting. In a world of depleting resources, the time will soon come
when ‘disposable’ will be a dirty word, and once again people will regard
‘long-lasting’ as a desirable feature. By the year 2000, buildings, furniture,
cars or computers will all last longer thanks to fluoropolymers.

* * *

THE ACTIVE POWER BEHIND FLUOROPOLYMERS

THE ESSENCE of the chemistry of carbon is that carbon atoms combine
readily to form rings and chains. Polymers consist of long chains of carbon
atoms. The energy needed to break the bonds between carbon atoms is large,
348 kilojoules per mole. Polymers also have other atoms attached to the
carbon atoms of the chains; these might be hydrogen atoms as in hydrocarbons
and the polymer polyethylene. Bonds between carbon and hydrogen atoms are
stronger than those between carbon atoms, and have bond energies of 413
kilojoules per mole. The bond that carbon forms with fluorine is even stronger,
485 kilojoules per mole.

The strength of the carbon-fluorine bond is the secret of polyfluorocarbons:
when you replace all the hydrogen atoms of a hydrocarbon chain with fluorine
atoms, you effectively clothe the molecule in an impenetrable sheath. The
fluorine atoms are big enough to hide the backbone of carbon atoms and yet
not too big to stress the molecule and thereby weaken it. The fluoropolymer
is now almost impervious to heat, strong acids, concentrated alkalis, radiation
damage, enzymes and electric currents. The result is that although fluorination
(introducing fluorine into a compound) is expensive, it is usually well
worth the cost in terms of improved performance.

There are difficult technical problems when it comes to preparing fluorinated
hydrocarbons. Fluorinating agents are not the easiest materials to make
or to handle. The best known of all is fluorine gas itself (F2), first isolated
by Henri Moissan in 1886. Fluorine reacts with almost everything, which
is why it was so difficult to obtain in the first place. Humphry Davy tried
to isolate fluorine as long ago as 1813, but all he got was sore fingers
and eyes from the hydrogen fluoride gas, HF, given off when he passed an
electric current through hydrofluoric acid.

Chemists had known about this acid for a century before Davy’s experiment.
Glaziers used it to etch patterns in glass. They made it from calcium fluoride
and sulphuric acid. Most calcium fluoride mined in Britain today is treated
with sulphuric acid to turn it into hydrogen fluoride. Some companies use
this to make certain CFCs. Hydrogen fluoride is also used to make fluorine
gas by electrolysing a mixture of potassium fluoride and hydrogen fluoride.

Fluorine is extremely reactive. It will readily etch glass, and contact
with an organic compound can lead to a violent reaction as the fluorine
rips the molecule to pieces. It reacts like this because the bond holding
the two fluorine atoms together is weak (155 kilojoules per mole) and splits
into two fluorine atoms, F, which are free radicals and as such are extremely
reactive. Attacking a hydrocarbon produces a fluorocarbon and a lot of heat:

C-H + F₂ → C-F + HF

You can tame the reaction by diluting the mixture with nitrogen gas,
which absorbs the excess heat.

The search for new ways to make fluorocarbons is a very active branch
of chemistry. Most fluorinating agents are corrosive and dangerous, but
not so vicious as fluorine. ISC Chemicals use cobalt trifluoride, which
works well and is safer. Passing a hydrocarbon through a layer of cobalt
trifluoride completely fluorinates the hydrocarbon.

Research groups around the world are looking for the ideal fluorinating
agent: something that is easy to make and readily forms carbon-fluorine
bonds safely and at normal temperatures. Eric Banks and his group at the
University of Manchester Institute of Science and Technology have discovered
fluorine compounds that behave as if they were positively charged fluorine,
F+. This is quite a feat considering that fluorine atoms usually attract
electrons to form negative ions, F-. Removing an electron gives a fluorine
radical, F, which is extremely reactive. To obtain F+, yet another electron
has to be pulled off. When this is achieved almost anything is possible.
So far, this work is only of academic interest. Industry works with more
conventional fluorinating reagents.

Even fluorine gas has become more manageable. Once rarely encountered
outside special industrial plants, it is now available commercially as a
mixture diluted with nitrogen gas. This makes it as easy to handle as chlorine
gas. With care you can even use it in glass apparatus. Two companies, Bettix
in Bolton, Lancashire, and Air Products in Crewe, Cheshire, are already
using mixtures of fluorine and nitrogen to fluorinate polythene containers.
These then become impervious to solvents such as paint thinners and cleaning
fluids, and solvent-based products, such as insecticides and toiletries.
Until now, companies could not sell these products in polythene bottles
because the solvents leak out. The fluorine reacts with the polythene and
converts carbon-hydrogen bonds on the surface of the polythene to carbon-fluorine
bonds. This produces a protective layer of polyfluorocarbon less than a
millionth of a centimetre thick.

Volkswagen and Peugeot are making petrol tanks from surface-fluorinated
polyetheylene, and most car firms will follow suit in the 1990s. The advantages
are that you can mould such tanks to fit the most protected space in the
car. Moreover, if they rupture in a crash they do not cause a fire ball,
as sometimes can happen with steel fuel tanks.