How teflon is made – material, manufacture, making, history, used, processing, parts, steps, product

Table of Contents1 Background 2 History 3 Raw Materials 4 The Manufacturing Process 4.1 Making the TFE 4.2 Suspension Polymerization 4.3 Dispersion polymerization 4.4 Nonstick cookware 5 Quality Control 6 Byproducts/Waste 7 Where to Learn More 7.1 Books 7.2 Periodicals Background Teflon is the registered trade name of the highly […]

Background

Teflon is the registered trade name of the highly useful plastic material
polytetrafluoroethylene (PTFE). PTFE is one of a class of plastics known
as fluoropolymers. A polymer is a compound formed by a chemical reaction
which combines particles into groups of repeating large molecules. Many
common synthetic fibers are polymers, such as polyester and nylon. PTFE is
the polymerized form of tetrafluoroethylene. PTFE has many unique
properties, which make it valuable in scores of applications. It has a
very high melting point, and is also stable at very low temperatures. It
can be dissolved by nothing but hot fluorine gas or certain molten metals,
so it is extremely resistant to corrosion. It is also very slick and
slippery. This makes it an excellent material for coating machine parts
which are subjected to heat, wear, and friction, for laboratory equipment
which must resist corrosive chemicals, and as a coating for cookware and
utensils. PTFE is used to impart stain-resistance to fabrics, carpets, and
wall coverings, and as weatherproofing on outdoor signs. PTIZE has low
electrical conductivity, so it makes a good electrical insulator. It is
used to insulate much data communication cable, and it is essential to the
manufacture of semi-conductors. PTFE is also found in a variety of medical
applications, such as in vascular grafts. A fiberglass fabric with PTFE
coating serves to protect the roofs of airports and stadiums. PTFE can
even be incorporated into fiber for weaving socks. The low friction of the
PTFE makes the socks exceptionally smooth, protecting feet from blisters.

History

PTFE was discovered accidentally in 1938 by a young scientist looking for
something else. Roy Plunkett was a chemist for E.I. du Pont de Nemours and
Company (Du Pont). He had earned a PhD from Ohio State University in 1936,
and in 1938 when he stumbled upon Teflon, he was still only 27 years old.
Plunkett’s area was refrigerants. Many chemicals that were used as
refrigerants before the 1930s were dangerously explosive. Du Pont and
General Motors had developed a new type of non-flammable refrigerant, a
form of Freon called refrigerant 114. Refrigerant 114 was tied up in an
exclusive arrangement with General Motor’s Frigidaire division, and
at the time could not be marketed to other manufacturers. Plunkett
endeavored to come up with a different form of refrigerant 114 that would
get around Frigidaire’s patent control. The technical name for
refrigerant 114 was tetrafluorodichloroethane. Plunkett hoped to make a
similar refrigerant by reacting hydrochloric acid with a compound called
tetrafluoroethylene, or TFE. TFE itself was a little known substance, and
Plunkett decided his first task was to make a large amount of this gas.
The chemist thought he might as well make a hundred pounds of the gas, to
be sure to have enough for all his chemical tests, and for toxicological
tests as well. He stored the gas in metal cans with a valve release, much
like the cans used commercially today for pressurized sprays like hair
spray. Plunkett kept the cans on dry ice, to cool and liquefy the TFE gas.
His refrigerant experiment required Plunkett and his assistant to release
the TFE gas from the cans into a heated chamber. On the morning of April
6, 1938, Plunkett found he could not get the gas out of the can. To
Plunkett and his assistant’s mystification, the gas had transformed
overnight into a white, flaky powder. The TFE had polymerized.

Polymerization is a chemical process in which molecules combine into long
strings. One of the best known polymers is nylon, which was also
discovered by researchers at Du Pont. Polymer science was still in its
infancy in the 1930s. Plunkett believed that TFE could not polymerize, and
yet it had somehow done so. He sent the strange white flakes to Du
Pont’s Central Research Department, where teams of chemists
analyzed the stuff. The polymerized TFE was curiously inert. It did not
react with any other chemicals, it resisted electric currents, and it was
extremely smooth and slick. Plunkett was able to figure out how the TFE
gas had accidentally polymerized, and he took out a patent for the
polymerized substance, polytetrafluoroethylene, or PTFE.

PTFE was initially expensive to produce, and its value was not clear to
Plunkett or the other scientists at Du Pont. But it came into use in World
War II, during the development of the atomic bomb. Making the bomb
required scientists to handle large amounts of the caustic and toxic
substance uranium hexafluoride. Du Pont provided PTFE-coated gaskets and
liners that resisted the extreme corrosive action of uranium hexafluoride.
Du Pont also used PTFE during the war for making nose cones of certain
other bombs. Du Pont registered the trademark name Teflon for its patented
substance in 1944, and continued to work after the war on cheaper and more
effective manufacturing techniques. Du Pont built its first plant for the
production of Teflon in Parkersburg, West Virginia in 1950. The company
marketed Teflon after the war’s end as a coating for machined metal
parts. In the 1960s, Du Pont began marketing cookware coated with Teflon.
The slick Teflon coating resisted the stickiness of even scorched food, so
cleaning the pans was easy. The company marketed Teflon for a variety of
other uses as well. Other related fluoropolymers were developed and
marketed in ensuing decades, some of which were easier to process than
PTFE. Du Pont registered another variant of Teflon in 1985, Teflon AF,
which is soluble in special solvents.

Raw Materials

PTFE is polymerized from the chemical compound tetrafluoroethylene, or
TFE.

A non-stick pan is composed of varying non-stick layers.

TFE is synthesized from fluorspar, hydrofluoric acid, and chloroform.
These ingredients are combined under high heat, an action known as
pyrolosis. TFE is a colorless, odorless, nontoxic gas which is, however,
extremely flammable. It is stored as a liquid, at low temperature and
pressure. Because of the difficulty of transporting the flammable TFE,
PTFE manufacturers also manufacture their own TFE on site. The
polymerization process uses a very small amount of other chemicals as
initiators. Various initiators can be used, including ammonium persulfate
or disuccinic acid peroxide. The other essential ingredient of the
polymerization process is water.

The Manufacturing

Process

PTFE can be produced in a number of ways, depending on the particular
traits desired for the end product. Many specifics of the process are
proprietary secrets of the manufacturers. There are two main methods of
producing PTFE. One is suspension polymerization. In this method, the TFE
is polymerized in water, resulting in grains of PTFE. The grains can be
further processed into pellets which can be molded. In the dispersion
method, the resulting PTFE is a milky paste which can be processed into a
fine powder. Both the paste and powder are used in coating applications.


Making the TFE

  • 1 Manufacturers of PTFE begin by synthesizing TFE. The three ingredients
    of TFE, fluorspar, hydrofluoric acid, and chloroform are combined in a
    chemical reaction chamber heated to between 1094-1652°F
    (590-900°C). The resultant gas is then cooled, and distilled to
    remove any impurities.

    Teflon con be used on a wide variety of cookware.

    Teflon con be used on a wide variety of cookware.


Suspension Polymerization

  • 2 The reaction chamber is filled with purified water and a reaction
    agent or initiator, a chemical that will set off the formation of the
    polymer. The liquid TFE is piped into the reaction chamber. As the TFE
    meets the initiator, it begins to polymerize. The resulting PTFE forms
    solid grains that float to the surface of the water. As this is
    happening, the reaction chamber is mechanically shaken. The chemical
    reaction inside the chamber gives off heat, so the chamber is cooled by
    the circulation of cold water or another coolant in a jacket around its
    outsides. Controls automatically shut off the supply of TFE after a
    certain weight inside the chamber is reached. The water is drained out
    of the chamber, leaving a mess of stringy PTFE which looks somewhat like
    grated coconut.
  • 3 Next, the PTFE is dried and fed into a mill. The mill pulverizes the
    PTFE with rotating blades, producing a material with the consistency of
    wheat flour. This fine powder is difficult to mold. It has “poor
    flow,” meaning it cannot be processed easily in automatic
    equipment. Like unsifted wheat flour, it might have both lumps and air
    pockets. So manufacturers convert this fine powder into larger granules
    by a process called agglomeration. This can be done in several ways. One
    method is to mix the PTFE powder with a solvent such as acetone and
    tumble it in a rotating drum. The PTFE grains stick together, forming
    small pellets. The pellets are then dried in an oven.
  • 4 The PTFE pellets can be molded into parts using a variety of
    techniques. However, PTFE may be sold in bulk already pre-molded into
    so-called billets, which are solid cylinders of PTFE. The billets may be
    5 ft (1.5 m) tall. These can be cut into sheets or smaller blocks, for
    further molding. To form the billet, PTFE pellets are poured into a
    cylindrical stainless steel mold. The mold is loaded onto a hydraulic
    press, which is something like a large cabinet equipped with weighted
    ram. The ram drops down into the mold and exerts force on the PTFE.
    After a certain time period, the mold is removed from the press and the
    PTFE is unmolded. It is allowed to rest, then placed in an oven for a
    final step called sintering.
  • 5 The molded PTFE is heated in the sintering oven for several hours,
    until it gradually reaches a temperature of around 680°F
    (360°C). This is above the melting point of PTFE. The PTFE
    particles coalesce and the material becomes gel-like. Then the PTFE is
    gradually cooled. The finished billet can be shipped to customers, who
    will slice or shave it into smaller pieces, for further processing.


Dispersion polymerization

  • 6 Polymerization of PTFE by the dispersion method leads to either fine
    powder or a paste-like substance, which is more useful for coatings and
    finishes. TFE is introduced into a water-filled reactor along with the
    initiating chemical. Instead of being vigorously shaken, as in the
    suspension process, the reaction chamber is only agitated gently. The
    PTFE forms into tiny beads. Some of the water is removed, by filtering
    or by adding chemicals which cause the PTFE beads to settle. The result
    is a milky substance called PTFE dispersion. It can be used as a liquid,
    especially in applications like fabric finishes. Or it may be dried into
    a fine powder used to coat metal.


Nonstick cookware

  • 7 One of the most common and visible uses of PTFE is coating for
    nonstick pots and pans. The pan must be made of aluminum or an aluminum
    alloy. The pan surface has to be specially prepared to receive the PTFE.
    First, the pan is washed with detergent and rinsed with water, to remove
    all grease. Then the pan is dipped in a warm bath of hydrochloric acid
    in a process called etching. Etching roughens the surface of the
    metal. Then the pan is rinsed with water and dipped again in nitric
    acid. Finally it is washed again with deionized water and thoroughly
    dried.
  • 8 Now the pan is ready for coating with PTFE dispersion. The liquid
    coating may be sprayed or rolled on. The coating is usually applied in
    several layers, and may begin with a primer. The exact makeup of the
    primer is a proprietary secret held by the manufacturers. After the
    primer is applied, the pan is dried for a few minutes, usually in a
    convection oven. Then the next two layers are applied, without a drying
    period in between. After all the coating is applied, the pan is dried in
    an oven and then sintered. Sintering is the slow heating that is also
    used to finish the billet. So typically, the oven has two zones. In the
    first zone, the pan is heated slowly to a temperature that will
    evaporate the water in the coating. After the water has evaporated, the
    pan moves into a hotter zone, which sinters the pan at around 800°F
    (425°C) for about five minutes. This gels the PTFE. Then the pan is
    allowed to cool. After cooling, it is ready for any final assembly
    steps, and packaging and shipping.

Quality Control

Quality control measures take place both at the primary PTFE manufacturing
facility and at plants where further processing steps, such as coatings,
are done. In the primary manufacturing facility, standard industrial
procedures are followed to determine purity of ingredients, accuracy of
temperatures, etc. End products are tested for conformance to standards.
For dispersion PTFE, this means the viscosity and specific gravity of the
dispersion is tested. Other tests may be performed as well. Because Teflon
is a trademarked product, manufacturers who wish to use the brand name for
parts or products made with Teflon PTFE must follow quality control
guidelines laid down by Du Pont. In the case of nonstick cookware
manufacturers, for example, the cookware makers adhere to Du Pont’s
Quality Certification Program, which requires that they monitor the
thickness of the PTFE coating and the baking temperature, and carry out
adhesion tests several times during each shift.

Byproducts/Waste

Though PTFE itself is non-toxic, its manufacture produces toxic
byproducts. These include hydrofluoric acid and carbon dioxide. Work areas
must be adequately ventilated to prevent exposure to gases while PTFE is
being heated, or when it cools after sintering. Doctors have documented a
particular illness called polymer fume fever suffered by workers who have
inhaled the gaseous byproducts of PTFE manufacturing. Workers must also be
protected from breathing in PTFE dust when PTFE parts are tooled.

Some waste created during the manufacturing process can be reused. Because
PTFE was at first very expensive to produce, manufacturers had high
incentive to find ways to use scrap material. Waste or debris generated in
the manufacturing process can be cleaned and made into fine powder. This
powder can be used for molding, or as an additive to certain lubricants,
oils, and inks.

Used PTFE parts should be buried in landfills, not incinerated, because
burning at high temperatures will release hydrogen chloride and other
toxic substances. One study released in 2001 claimed that PTFE also
degrades in the environment into one substance that is toxic to plants.
This is trifluoroacetate, or TFA. While current levels of TFA in the
environment are low, the substance persists for a long time. So TFA
pollution is possibly a concern for the future.

Where to Learn More


Books

Ebnesajjad, Sina.

Fluoroplastics.

Norwich, NY: Plastics Design Library, 2000.


Periodicals

Friedel, Robert, and Alan Pilon. “The Accidental
Inventor.”

Discover

(October 1996): 58.

Gorman, J. “Environment’s Stuck with Nonstick
Coatings.”

Science News

(21 July 2001): 36.

Source Article

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