Embed Size (px)
Citation preview
1
SILESIAN UNIVERSITY OF TECHNOLOGY
MATERIAL SELECTION OF TEFLON
LEVEL 2
Dr inz. Zbigniew BRYTAN
Institute of Engineering Materials and Biomaterials
Prepared by
Efekan BAKIR
2
CONTENTS
1.INTRODUCTION
2.HISTORY OF TEFLON
3.PROPERTIES
3.1. Characteristics Of Teflon
3.2. What Properties Make Teflon Non-Stick?
4. MANUFACTURING OF TEFLON
4.1. Raw Materials
4.2. The Manufacturing Process
4.2.1. Making the TFE
4.2.2. Suspension Polymerization
4.2.3. Dispersion polymerization
4.2.4. Nonstick cookware
4.3. Quality Control
4.4. Byproducts/Waste
5.TYPES OF TEFLON COATINGS
5.1. Teflon PTFE
5.2. Teflon ETFE
5.3. Teflon FEP
5.4. Teflon PFA
5.5. Teflon-S One Coat
5.6. Silverstone SUPRA
6.MATERIAL SELECTION
7.CONCLUSION
8.REFERENCES
3
1.INTRODUCTION
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. [1]
Figure1:Polytetrafluoroethylene
2.HISTORY OF TEFLON
PTFE was accidentally discovered in 1938 by Roy Plunkett, in New
Jersey while he was working for Kinetic Chemicals. As Plunkett was
attempting to make a new chlorofluorocarbon refrigerant, the
tetrafluoroethylene gas in its pressure bottle stopped flowing before the
bottle's weight had dropped to the point signaling empty. Since Plunkett was
4
measuring the amount of gas used by weighing the bottle, he became
curious as to the source of the weight, and finally resorted to sawing the
bottle apart. Inside, he found it coated with a waxy white material which was
oddly slippery. Analysis of the material showed that it was polymerized
perfluoroethylene, with the iron from the inside of the container having acted
as a catalyst at high pressure. Kinetic Chemicals patented the new
fluorinated plastic (analogous to known polyethylene) in 1941, and registered
the Teflon trademark in 1945. DuPont, which founded Kinetic Chemicals in
partnership with General Motors, was producing over two million pounds
(900 tons) of Teflon brand PTFE per year in Parkersburg, West Virginia, by
1948. An early advanced use was in the Manhattan Project as a material to
coat valves and seals in the pipes holding highly reactive uranium
hexafluoride at the vast K-25 uranium enrichment plant at Oak Ridge,
Tennessee.[2]
Figure2:Shape of PTFE
In 1954, French engineer Marc Grégoire created the first pan coated
with Teflon non-stick resin under the brand name of Tefal after his wife
Collete urged him to try the material he had been using on fishing tackle on
her cooking pans. In the United States, Kansas City, Missouri resident
Marion A. Trozzolo, who had been using the substance on scientific utensils,
marketed the first US-made Teflon coated frying pan, "The Happy Pan", in
1961.In the 1990's, it was found that PTFE can be radiation cross-linked
above its melting point and in an oxygen free environment. Electron beam
processing is one example of radiation processing. Cross-linked PTFE has
improved high temperature mechanical properties and radiation stability.
This is significant because for many years irradiation at ambient conditions
has been used to break down PTFE for recycling. The radiation induced
chain scissioning allows it to be more easily reground and reused. [3]
5
3.PROPERTIES
PTFE is a thermoplastic polymer, which is a white solid at room
temperature, with a density of about 2200 kg/m3. According to DuPont, its
melting point is 600 K (327 °C; 620 °F). Its mechanical properties degrade
gradually at temperatures above 194 K (−79 °C; −110 °F).PTFE gains its
properties from the aggregate effect of carbon-fluorine bonds, as do all
fluorocarbons. The only chemicals known to affect these carbon-fluorine
bonds are certain alkali metals and most highly reactive fluorinating agents.
[4]
Table1:Properties about Teflon
3.1. Characteristics Of Teflon
Below you can find the most uprising characteristics of this material
which makes it useful for tons of industrial and domestic applications in our
daily life:
6
- Resistant to many chemicals
This includes ozone, chlorine, acetic acid, ammonia, sulfuric acid and
hydrochloric acid. The only chemicals known to affect these coatings are
molten alkali metals and highly reactive fluorinating agents.
- Weather and UV resistance
- Non stick
Very few solid substances will permanently adhere to a Teflon coating.
While tacky materials may show some adhesion, almost all substances
release easily.
- Excellent optical properties
- Outstanding performance at extreme temperatures
In fact it can temporarily withstand temperatures of 260C and
cryogenic temperatures of -240C and still have the same chemical
properties. It has an initial melting point of 342C (+- 10C) and a secondary
melting point of 327C (+- 10C).
- Low coefficient of friction.
It is the ratio of the force required to make two surfaces slide over each
other. A low number equals low resistance and smooth operation. This
indicates the difficulty in sliding one surface against another. The coefficient
of friction is generally in the range of 0.05 to 0.20, depending on the load,
sliding speed, and type of Teflon coating used.
- Non wetting
Teflon finishes are both hydrophobic and oleophobic, cleanup is easier
and more thorough.
- Exceptional dielectric properties
Teflon has a high dielectric strength over many different frequencies,
low dissipation factor and high surface resistivity. Dielectric strength is the
high voltage that the insulating material can withstand before it breaks
down. In addition it has a low dissipation factor; this is the percentage of
electrical energy absorbed and lost when current is applied to an insulating
material. A low dissipation factor means that the absorbed energy dissipated
as heat is low.The high surface resistivity refers to the electrical resistance
7
between opposite edges of an unit square on the surface of an insulating
material.[5]
3.2. What Properties Make Teflon Non-Stick?
Teflon is DuPont’s Tradmarked brand name for PTFE
polytetrafluoroethylene, a polymer. It is a long chain polymer, one of the
biggest known to science. It is a solid fluorocarbon, and is hydrophobic. This
means that it ‘rejects’ water. It is very stable and inert, due the high strength
of the carbon-fluorine bonds, but its properties do degrade with temperature.
Slipperiness’ is measured by a material’s coefficient of friction. It has a
‘waxy’ feeling to the touch. Only two other solids are more slippery –
aluminium magnesium boride, and diamond. Both these are very hard
materials, unlike PTFE.
Stickiness’ is due to what are known as Van Der Waals forces. These
forces operate at a molecular level between materials. They give the Gecko its
ability to climb walls – unless of course they are coated with Teflon. Teflon is
unique in this respect. The pads on a Gecko’s feet have a relatively high Van
der Waals force (actually a sum of three different forces), but Teflon’s are
very low. So, there is little attraction – or ‘stickiness’.
Figure3:Temparature-time graphic
8
Its other properties include creep. This is the ability to stretch without
relaxing to its previous dimensions when strain is removed. It makes PTFE
ideal for seals. Hence, it is loved by plumbers.It is subject to wear where
surfaces have regular relative movement, despite its slippery nature. It is
now being combined with lubricants such as molybdenum disulphide which
result in advanced composites which are ideal for hi-tech bearings typically
used in aerospace applications.[6]
4. MANUFACTURING OF TEFLON
4.1. 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.[7]
Figure4:Raw Material
9
4.2.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.
4.2.1. Making the TFE
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.
Figure5:Teflon being process
4.2.2. Suspension Polymerization
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.
10
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.
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.
Figure 6:Shaping the materials
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.
11
4.2.3. Dispersion polymerization
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.
4.2.4.Non-stick Cookware
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.
Figure7:Coating of Teflon
12
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.
4.3. 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.
Figure8:Laser Controling
13
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.
4.4.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.[8]
5.TYPES OF TEFLON COATINGS
DuPont Teflon coatings are just one of the many industrial coatings
solutions offered at Plas-Tech Coatings.Teflon is DuPont's registered
trademark for its non-stick coatings. Teflon coatings are specially formulated
finishes that are based on PTFE, PFA, FEP, and ETFE fluorocarbon resins.
Teflon-S is a related family of fluorocarbon coatings containing binding
resins which provide increased hardness, abrasion resistance, and other
desirable properties.
5.1. Teflon PTFE
PTFE (Polytetrafluoroethylene) nonstick coatings are typically two-coat
(primer/topcoat) systems. These products have the highest operating
temperature of any fluoropolymer (290° C/550° F), an extremely low
coefficient of friction, good abrasion resistance and good chemical resistance.
14
5.2.Teflon ETFE
ETFE is a copolymer of Ethylene and Tetrafluoroethylene, and is also
sold under the Tefzel trademark. Although not fully fluorinated, ETFE has
excellent chemical resistance and can operate continuously at 150° C/300°
F. This resin is the toughest of the fluoropolymers and can be applied at film
builds up to 1,000 micrometers (40 mils) to provide a highly durable finish.
Figure9:Tefzel(ETFE)
5.3. Teflon FEP
FEP (Fluorinated Ethylene Propylene copolymer) nonstick coatings
melt and flow during baking to provide non-porous films. These coatings
provide excellent chemical resistance. In addition to low friction, FEP
coatings have excellent nonstick properties. Maximum use temperature is
205° C/400° F.
5.4.Teflon PFA
Like FEP, PFA (Perfluoroalkoxy) nonstick coatings melt and flow during
baking to provide non-porous films. PFA offers the additional benefits of
higher continuous use temperature (260° C/500° F), film thicknesses up to
1,000 micrometers (40 mils) and greater toughness than PTFE or FEP. This
combination of properties makes PFA an excellent choice for a wide variety of
uses, especially those involving chemical resistance.
5.5. Teflon-S One Coat
These solvent-based liquid coatings are formulated with special blends
of fluoropolymers and other high-performance resins to improve toughness
and abrasion resistance. Because the film components stratify during
baking, most of the fluoropolymer properties (such as low friction and
nonstick) are retained. The resins provide adhesion and abrasion resistance.
15
These products can sometimes be applied to smooth, clean metal. Bake
requirements vary, depending on the specific coating, from 165° C/325° F to
370° C/700° F.
5.6. Silverstone SUPRA
Silverstone is a specialty line of superior nonstick finishes produced by
DuPont. Silverstone coatings are three-coat (primer/midcoat/topcoat)
systems formulated with PTFE and PFA. Characteristics of Silverstone
coatings are similar to other PTFE coatings, however durability is greatly
increased. A reinforced version with higher scratch and abrasion resistance
is also available. Maximum continuous use temperature is 290° C/550°F. [9]
6.MATERIAL SELECTION
How to choose Teflon? Here is table for answering this question and
decide which kind of teflon is better then others. (Most common of 4 teflon)
Properties Points Teflon
PTFE Teflon (FEP) Teflon PFA Tefzel(ETFE)
Specific
gravity 1point
2.13-
2.22(0.30p) 2.15(0.30p) 2.15(0.30p) 1.70-1.78(0.10p)
Tensile
Strength
Mpa (psi)
2points
21-35 Mpa
(3,000-
5,000 psi)
(0,60p)
23 Mpa(3,400
psi)(0.20p)
25 Mpa(3,600
psi)(0.20p)
40-47Mpa
(5,800-6,700
psi) (1,0p)
Flexural
Modulus
Mpa (psi)
3points
500 Mpa
(72,000psi)
(0.50p)
600
Mpa(85,000psi)
(0.60p)
600
Mpa(85,000psi)
(0.60p)
1,000
Mpa(145,000psi)
(1,30p)
Impact
Strength
J/m
(ft.lb/in)
4points
189 J/m
(3.5ft·lb/in)
(0.60p)
No
Break(1,20p)
No
Break(1,20p) No Break(1,20p)
Melting
point
°C(°F)
5points 327 (621)
(1,40p)
260 (500)
(1,10p)
305 (582)
(1,30p)
245-280 (473-
536) (1,20p)
Total
Points 12,4points 12,8points 13,8points 16,8points
Table 2 : Choosing material which is according to Specific gravity, Tensile Strength,
Elongation, Flexural ModulusImpact Strength, Melting point.
Teflon PTFE is always choosen because of price and easy workability
but if we analyze whole properties such as specific gravity,tensile
16
strength,flexural modulus,impact strength,melting point etc. Tefzel is the
best option. New uses are still being found for it, and its utility is being
extended by combining it with other chemicals.China is now the world’s
biggest producer (and consumer). DuPont produced 900 tons a year in 1948
and worldwide production is forecast to reach 240,000 tons by 2017. So,
despite its properties, Teflon is likely to stick with us for many years to
come. [10]
7.CONCLUSION
Making the final decision on whether you want Teflon PTFE, Teflon
(FEP), Teflon PFA, Tefzel(ETFE) is really a personal decision based on how
much time you spend cooking, your expertise in the kitchen and your
budget. You should check information from Table 2. Which has lots of
information and opinion, also when you look up you will see some points for
helping the people who wants to use better teflon.
One of the common justifications for the space race was that mankind
benefited from the technology spin-offs. ‘Teflon is the usually quoted as an
example.Teflon is a proprietary brand name. It actually pre-dates the space
race by almost 30 years. It was invented as long ago as 1938 by Roy Plunkett
at DuPont (which owns the brand name). Invented is perhaps the wrong
word – it was observed during some other research, in much the same way
that penicillin was discovered.
The word Teflon has passed into our everyday lexicon and is used to
describe anything slippery – especially politicians, of whom Ronald Reagan
was the first to enjoy the dubious distinction.Carcinogenic by-products can
be released when it is heated beyond 260 Centigrade. This rarely happens in
practice.DuPont’s original production process used a toxic chemical known
as PFOA but alternatives are now available and the use of the chemical is
being phased out.
REFERENCES
17
[1] http://en.wikipedia.org/wiki/Polytetrafluoroethylene
[2]http://www2.dupont.com/Teflon/en_US/products/safety/what_is_it.html
[3] http://www.coolquiz.com/trivia/explain/docs/teflon.asp
[4] http://whatscookingamerica.net/LindaPosch/ToxicCookware.htm
[5] http://www.lenntech.com/teflon.htm
[6] http://composite.about.com/od/Plastics/a/What-Is-Teflon.htm
[7] http://www.wisegeek.org/what-is-teflon.htm
[8] http://www.madehow.com/Volume-7/Teflon.html
[9] http://www.plastechcoatings.com/teflon_coating.html
[10] ] http://www.lenntech.com/teflon.htm
Table1- http://www.omega.com/techref/images/teflon3.gif
Figure’s References
Figure1- http://en.wikipedia.org/wiki/File:Teflon_structure.PNG
Figure2-http://www.800mainstreet.com/7/teflon-ball-stick-polymer%20fragment1.jpg
Figure3-http://www.mindfully.org/Plastic/Teflon/Canary-Teflon-ToxicosisAug03a.GIF
Figure4- http://www.youtube.com/watch?v=itqTL3knVeM
Figure5- http://www.youtube.com/watch?v=itqTL3knVeM
Figure6- http://www.youtube.com/watch?v=itqTL3knVeM
Figure7- http://www.youtube.com/watch?v=itqTL3knVeM
Figure8- http://www.youtube.com/watch?v=itqTL3knVeM
Figure9-http://www.microwavejournal.com/legacy_assets/FigureImg/AR_3331_F1_L.jpg
