Recycling of Polytetrafluoroethylene (PTFE) Scrap Materials

Abstract

Polymers are formed from thermoplastic and thermosetting plastic materials. The binding forces between polymer chains in thermoplastics such as polyethylene are the result of van der Walls forces between the molecules and mechanical entanglement between the chains as shown in Fig.1. Most of the thermoplastics can be reused after melting since the bonds between the molecules are easily broken on heating. However, in thermosetting plastics such as Bakelite various polymer chains are held together by strong covalent bond. They are rigid, strong and more brittle. Due to strong covalent bond and cross-link, they are insoluble in almost all organic solvents. They will not become plastic when heated.

Full text

Chapter 9
Recycling of Polytetrafluoroethylene (PTFE) Scrap
Materials
Arunachalam Lakshmanan and S.K. Chakraborty
Additional information is available at the end of the chapter
http://dx.doi.org/10.5772/59599
1. Introduction
1.1. Plastics
Polymers are formed from thermoplastic and thermosetting plastic materials. The binding
forces between polymer chains in thermoplastics such as polyethylene are the result of van
der Walls forces between the molecules and mechanical entanglement between the chains as
shown in Fig.1. Most of the thermoplastics can be reused after melting since the bonds
between the molecules are easily broken on heating. However, in thermosetting plastics
such as Bakelite various polymer chains are held together by strong covalent bond. They are
rigid, strong and more brittle. Due to strong covalent bond and cross-link, they are insoluble
in almost all organic solvents. They will not become plastic when heated.
1.2. PTFE
Polytetrafluoroethylene (PTFE) was discovered by a research chemist in DuPont in the year
1938. In 1941 it has been patented and got the first brand name as Teflon. It is a fluorinated
polymer obtained from tetrafluroethylene (TFE) monomer through free radical vinyl poly‐
merization. Tetra means four carbon atoms are covalently bonded to carbon atoms. Fluro
means bonded atoms are fluorine. Ethylene means carbon atoms are joined by a double
bond as in the case of ethylene.
In PTFE, carbon to carbon atom double bond becomes a single bond and a linear chain of carbon
atoms are formed with two fluorine atoms covalently bonded to each carbon atom. These fluorine
atoms shield the carbon atoms and hence no solvent can attack the carbon atoms. As a result,
PTFE exhibits extraordinary chemical resistance to acids and alkalis. Carbon to fluorine bonds
have high dissociation energy. Due to the high electronegativity of fluorine,
© 2015 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative
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170 Sintering Techniques of Materials
Figure 1. Typical interwinded PTFE polymer. Due to strong intermolecular forces, the polymer chains are tangled. Due
to its chemical inertness, PTFE cannot be cross-linked.
Figure 2. The molecular structure of PTFE
PTFE repels everything and hence no molecules can stick to the PTFE surface which makes it
slippery (Fig.2). Ice is the only material that is slicker than PTFE. A thin PTFE coating over metal
cooking pans makes them nonsticky with food items. PTFE can withstand a wide range of
temperature (-184°C to 260°C) and is used in cold as well hot environments. It is hydrophobic
(water repellant) and hence is resistant to weathering. It has fantastic chemical resistance and
superb electrical insulation properties. It is the only plastic which can withstand temperatures up
to 300°C. On heating to temperatures above 400°C, PTFE disintegrates with the production of
carbon. Above 500°C, when heated in air, PTFE disappears altogether due to the production and
escape of carbon and fluorine in the form of CO 2
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materials. It has a high dielectric strength and low dielectric loss. Due to high melt viscosity,
injection or blow molding is not possible with PTFE. Only hot sintering or ram extrusion
manufacturing processes which are relatively expensive are being followed for making
PTFE products. In rapidly growing economies like China, the demand for PTFE has grown 5
times over the past 5 years.
2. Filler grade PTFE
PTFE undergoes creep (deformation under loading) which can be reduced with the help of high
shear modulus fillers such as glass. Fillers hinder the relative movement of the PTFE molecules
past one another and in this way reduce creep or deformation of the parts, reduce the wear rate of
parts used in dynamic applications, and reduce the coefficient of thermal expansion. Other
popular fillers used along with PTFE include carbon (improved thermal conductivity and low
deformation under load), graphite (improved lubrication), bronze or stainless steel (excellent
wear resistance) etc. Since PTFE powder is hydrophobic (it floats in water as seen in Fig.3) and
does not flow freely, mixing it will free flowing fillers is a major task. One has to use a cryogenic
medium such as liquid nitrogen to remove the electrostatic forces that hold the PTFE powder
together. This technique is being used to manufacture thermoluminescent material filled PTFE
discs (1:3 weight ratio) which are used for personnel radiation monitoring in India and elsewhere
[2]. After radiation exposures, these discs are usually heated to 300°C during luminescence
measurements and PTFE is the suitable binder for such applications. Organic liquids such as
ethanol can also be used to mix free flowing fillers with non free flow PTFE since they wet PTFE
powder unlike water medium. Alternately, one could use mechanical shearing force to separate
the PTFE particles. The last choice is industrially viable and hence was adopted by us for
manufacturing filler grade PTFE powders (Figs.4 and 5). An overview of different fillers used
along with end use can be had from the brochures supplied by Dupont and other PTFE
manufacturers.

Figure 3. PTFE is hydrophobic – water repellant

172 Sintering Techniques of Materials
Figure 4. A blue pigment (3%) mixed non free flow PTFE powder.
Figure 5. Sintered rods made from blue pigmented PTFE, glass mixed PTFE and carbon mixed PTFE
3. Recycling PTFE scrap material

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plant itself. Dumping of waste will in future be restricted due to nationwide regulations. In
order to avoid this, Environmental Protection Agencies have taken up an action to recycle
all the PTFE materials.
Recycling PTFE involves two different processes with two different end results. One
involves irradiation of the PTFE scrap to heavy dose of ionizing irradiation which will
reduce its molecular weight. On pulverizing, the irradiated PTFE scrap turns into micro
powder which finds certain applications. The second method involves pulverizing the PTFE
scrap without irradiation so that it becomes reusable like virgin PTFE itself.
4. Irradiation stability of PTFE
From Table 1 it is seen that among all plastics, PTFE has the least stability against ionizing
radiation. Heavy gamma or electron irradiation (several kilo Gray) has been found to break
down carbon-carbon bonds in the polymer chain in the PTFE scrap and reduce its molecular
weight which makes it very brittle and the end product is a white, free-flowing PTFE
powder which was found to be useful as additives in other materials or systems (see Fig.6).
While the turnings of PTFE scrap before irradiation are tough and elastic, those after
irradiation in air crumbles into a powdery material. The molecular weight of irradiated
PTFE is in the range of a few tens of thousands to a few hundreds of thousands, compared
to several million for the unirradiated resins. When irradiated in vacuum or inert
atmosphere, the cleavage of the bonds produces highly stable radicals. The recombination of
these stable radicals prevents rapid degradation of PTFE, as the molecular weight rebuilds.
When irradiation is conducted in air, as is the case in the present experiment, the radicals
react with oxygen leading to smaller molecular weight PTFE chains fairly quickly.

Figure 6. PTFE scrap before (left) and after gamma irradiation (right)

174 Sintering Techniques of Materials
Plastic Ionizing Radiation Stability
ABS Fair
Amides
– Aliphatic Fair
– Aromatic Excellent
Cellulosics Fair
Fluoroplastics
– PTFE Poor
– Polychlorotrifluoroethy Fair
lene
– PVF Good
– Polyvinylidene fluoride Good
– Copolymers of ethylene & TFE Good
– Polycarbonate Good
Polyesters – aromatic Good
Polyolefins
– PE Good
– PP Fair
– Polymethylpentene Good
– Copolymers Good
Polystyrene Excellent
Polystyrene acrylonitrile Good
Polysulfones Excellent
Polyvinyls
– PVC Good
– Copolymers Fair
Table 1. Radiation Stability of thermoplastic polymers [3]
4.1. PTFE Micropowder
The irradiated and pulverized PTFE scrap differs from PTFE granular resins and fine powders
because of the very small particle size, typically in the range of 2 to 20 m (and hence the word
micropowder), low molecular weight and the way they are handled and processed. Micro
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Recycling of Polytetrafluoroethylene (PTFE) Scrap Materials
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to four micron range used as additives for Inks, Oils, Lubricants, Paints and Coatings,
Cosmetics and Thermoplastics show enhanced lubrication and wear resistant properties.
Irradiation time in sec Dose, Mrad Average particle size (µm)
2.5 5
11.1
5.0
10
5.3
7.5
15
2.5
10 20
1.5
12.5
25
0.9
Table 2. Effect of electron irradiation dose on PTFE Micropowder Particle Size
70
Volume (%)
100
60
90
80
50
70
40
60
50
30
40
20
30
10
20
10
0
0
0.01
0.1 1.0 10.0
100.0
1000.0
Particle Diameter (µm.)
Figure 7. Particle size analysis of micro powder using laser light scattering.
Table 2 shows that with increasing electron irradiation dose in the range 5 – 25 Mrad, the average
particle size of the PTFE micropowder decreases from 11.1 to 0.9 m. The melt flow index goes up
as the molecular weight of the powder goes down with increasing dose of irradiation. Electron
irradiation is reported to cause cleavage of bonds and generation of gases such as HF acid vapor,
which must be removed by means of adequate ventilation from the processing areas. Electron
irradiation also increases the temperature of the sample which is held below 121°C by
fractionating the irradiation. Both these problems are much less severe

176 Sintering Techniques of Materials
in gamma irradiation as the irradiation rate is several orders of magnitude less than that of
electron irradiator.
Figure 8. Scanning electron micrograph of PTFE micro powder - irradiated and pulverized
Particle size distribution of the micropowder was carried out using a laser light scattering
instrument (Aerosol Dust Monitor Model 1.108 of M/S GRIMM Aerosoltechnik, GmbH,
Germany). The scattering angle by a single particle is inversely proportional to the size of the
particle. By measuring the forward angle of scattering and intensity of the scattering light, both
size distribution and number concentration could be obtained. Fig.7 shows that an average
particle size of 0.26 m was obtained on electron irradiation of the PTFE scrap followed by
milling which was achieved by a jet mill or a hammer mill. In the jet mil particles strike against
each other, causing them to fracture into smaller particles. The flaky morphology of imported
scrap powder seen in Fig.8 offers better lubricant property of the micro powder.
5. Reprocessing of unirradiated PTFE scrap
Unirradiated PTFE scrap can be recycled into many other products and used for rods,
tubing and sheets etc by pulverization followed by suitable heat treatments similar to that of
virgin PTFE. The recycled PTFE is known technically as “Reprocessed” or “Repro” PTFE or
mechan ical grade PTFE. Though off-white in color reprocessed PTFE has certain‐
advantages over virgin PTFE namely low creep and better mechanical strength.
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gaseous components (tetrafluoroethene and hexafluoropropene), cleaned and fed back into
the production of new PTFE. Dyneon GmbH is building a pilot plant in Burgkirchen,
Germany based on the latter technique, to recycle PTFE scrap. It will have capacity to
recycle 500 metric tons of PTFE waste annually.
The most common way is to blend the pulverized scrap fine powder with pure PTFE to be
used either in compression molding or ram extrusion. Before grinding, the scrap is usually
shredded (Figs. 9 and 10) and heated to above its melting point to remove any volatile
contaminants. Once ground, it is treated with acid to dissolve inorganics after which it is
washed
Figure 9. PTFE scrap being shredded

Figure 10. Shredded scrap

178 Sintering Techniques of Materials
Reprocessed PTFE grade powder is manufactured from pre-sintered PTFE shavings, scrap,
etc. It exhibits most of the properties that the virgin grade does but is subject to occasional
contamination within the material. This is the grade of choice when cost is a major concern
and cleanliness is not an issue. When repro grade is mixed with PTFE the cost comes down.
Such mixed grades are used when high purity is not required such as non critical chemical,
electrical and mechanical applications [5-8]. However, virgin grade PTFE is the material of
choice for use in pharmaceutical, food and beverage, and cosmetics industries or for
medical/ electrical applications. Virgin PTFE has better friction characteristics, which may
be important in some applications. Reprocessed grade PTFE is produced for thin sheets with
a maximum thickness of 0.250", For thicker sheets virgin PTFE is used. However, virgin
PTFE is known to undergo creep – deformation under load whereas the compressive
strength and deformation under load for reprocessed PTFE are superior to virgin PTFE.
Reprocessed grade PTFE also has superior wear resistance than virgin PTFE. Reprocessed
grade PTFE rods are available in diameters ranging from 1/8 to 4 inch and lengths of 6 to 12
ft. Reprocessed PTFE is frequently specified for high performance bearings and bushings,
particularly in applications that require resistance to corrosive chemicals.
5.1. PTFE has different grades
Grade A: 100% virgin material.
Grade B: 70% virgin material, with 30% recycled
material. Grade C: 50% - 50%
Grade D: 30% virgin, 70%
recycled. Grade E: 100% recycled
High purity reprocessed PTFE is white in color similar to virgin PTFE and is used for appli‐
cations ranging from extruded billets or molding into tubes, gaskets and ball-valve seats.
Lower grade off-white reprocessed PTFE is blended with pre PTFE and is used for packing
materials for valve stems and other applications (Table 3).
Repro (%) < 5% 10-20% >20%
Visual
No notable change
Off white
Slight off white
Component finish Smooth Rough Rough – with powdery
burrs
Water absorption < 1% > 1% > 2%
Chemical resistance
No notable change
Tensile strength
Slightly reduced Reduced by 10%
Reduced by 20%
Dielectric strength
Slightly reduced Reduced by 10%
Not suitable
Wear resistance
Slightly reduced Slightly reduced
Not suitable

Table 3. ! ) 
$

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5.2. Issues to be tackled in manufacturing reprocessed PTFE powder are the following
i. Difficulties in grinding the scrap into a fine powder
ii. Discoloration due to burning of volatiles and organic materials in ground powder
iii. Unlike virgin PTFE scrap filled PTFE or scrap powder could not be sintered at high
temperatures (350 -400°C) at atmospheric pressure
iv. Agglomeration and Sintering of the scrap powder during thermal anneal caused
cracks in reprocessed PTFE billets during sintering at atmospheric pressure
Efforts needed to solve the above problems are described below.
Figure 11. Charred organic impurities on pre-heating the shredded scrap

Figure 12. Major charred impurities are removed by handpicking

180 Sintering Techniques of Materials
Figure 13. Shredded PTFE scrap after pre-heating treatment
5.3. Grinding PTFE scrap - Kirk-othemer encyclopedia of chemicals
A technique known as Shear Extrusion Pulverisation based on Bridgeman – Anvil was used
for this purpose. This technique is also known as
•Double disc mill
•Solid state pulverization (SSP)
•Pressure shear pulverization (PSP).
It is a physico-chemical process in which cohesive forces within the polymer are broken by
means of mechanically induced stress.
The process is based on “Bridgeman phenomenon” and is realized inside a specially
deigned pulverizer
In PSP, the polymer is subjected to simultaneous action of axial compression and shear stress
between two mirror-like smooth working surfaces in the pulverizer and the pulverized
?$! ! 
:?!

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Figure 14. Pre-sintered scrap powder

Figure 15. Ground scrap powder after pre-sintering

182 Sintering Techniques of Materials
Figure 16. Ground PTFE powder being mixed with virgin PTFE powder.
Figure 17. Mixture being cold pressed into pellet.

Figure 18. Fillers like graphite or pigments normally do not melt or agglomerate nor interact with PTFE during sinter‐
       $ $ 
'$

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Figure 19. Reprocessed PTFE filled (40%) PTFE pellet before (left) and after (right) sintering at atmospheric pressure.
The surface became rough on sintering due to agglomeration of reprocessed particles and their migration to surface of
the pellet. Reprocessed PTFE is amenable to sintering only under pressure or under ram extrusion.
Figure 20. Sintered rods at atmospheric pressure – note pellets from 100% virgin scrap (second from left) crack and
pop out perhaps due to the release of volatile gases whereas the pellet from 100% virgin PTFE (extreme right) are mil‐
ky white in color and exhibit good integrity on sintering at atmospheric pressure.
6. Compression molding
Compression molding is a method of molding in which the molding material, generally
preheated, is first placed in an open, heated mold cavity. The mold is closed with a top force
or plug member, pressure is applied to force the material into contact with all mold areas,
while heat and pressure are maintained until the molding material has cured. Its advantage
lies in its ability to mold large, fairly intricate parts.
Initially it was thought that the die and plunger could be heated during compression in the
hydraulic press. A strip heater was wound around the die to heat the die with the help of a
heating programmer and a thermocouple was inserted into a hole made specifically for this
purpose in the die to measure the temperature during compression, The experimental
arrangement is shown below.

184 Sintering Techniques of Materials
Figure 21. Compression molding
The strip heater wound around the die, however, could not heat it to the desired
temperature even after several hours of operation as the heat capacity of the die made out of
hardened steel was much high and so this attempt was discontinued. Instead, after cold
pressing in the press, the die and plunger itself with the PTFE disc inside it was kept inside
the high capacity air over under mechanical pressure.

Figure 22. 4$?$
 4&

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Figure 23. Die under C-clamp kept inside the air over for sintering near 400°C
Figure 24. Repro filled PTFE Discs when sintered inside the die and plunger under C-clamp show discoloration which
runs through its volume although good surface smoothness is seen and cracks disappeared totally. This was attributed
to carbon production on reaction of volatiles with the die material. Even 100% virgin PTFE pellet show slight discolor‐
ation but only on its surface (both sides) when sintered under C clamp

Figure 25. 10% mixture, mixee mixed, 300 deg C, 2h pre-heated, 2500 psi, No pressure during sintering at 400 deg C in
a furnace. Sample is white in color but surface is rough.

186 Sintering Techniques of Materials
The reason for the discoloration could be due to carbon generation on reaction of volatiles
with the stainless steel die under pressure. This can be avoided if a pathway can be
provided for the escape of volatile gases during sintering under pressure. This will need
fabrication of a new die and plunger with a series of holes. While high pressure (2000 to
3000 psi) is required during cold pressing powder into pellet, a relatively lower pressure
(500 to 1000 psi) should suffice during sintering. Further efforts were made by reducing the
die pressure to 2500 psi and sintering without pressure.
This showed that mild pressure during sintering is a must. High pressures with clamp not
only discolor the pellets but also fuse them with the die. So a compromise in pressure
during cold pressing as well as provision to let out the volatile gases are necessary
6.1. Sintering treatment
Sintering temperatures were varied from 350 to 450 deg C and duration from 15 min to 1h.
From the points of view of polish, smoothness and strength, the best treatment was found to
be 380-400 deg C, 1h which is the same used for sintering virgin PTFE. Lower temperatures
resulted in poor strength due to under-sintering while higher temperatures resulted in poor
strength as it reduced the polymer strength.
7. Ram extrusion
Ram extrusion enables continuous processing of PTFE [9,10]. The PTFE powder (virgin or
reprocessed) is fed into a cylindrical extrusion pipe hydraulically while at the same time
compressing it by means of a ram and transported through the pipe, which is heated up to
sintering temperature in the range 370 to 400°C (Fig.26). The ram is then withdrawn, the die
tube re-charged with powder and the cycle repeated. This way the powder is continuously
fed into the heating section of the die tube where it is sintered and then it passes through a
cooler section from which the finished products (rods, tubes etc) flow out continuously
which are cut into desired lengths. Apart from PTFE materials like ultra high molecular
weight polyethylene as well as their compounds can be ram extruded. Uniform distribution
of powder into the cavity is essential. The powder should exhibit good flow properties. The
extrusion pressure is in the range of 26 to 74 MPa depending on the length of the cold zone
above the heated section of the die tube and the extrusion rate is 3 m/h. The heated length
of the tube can vary from 44 to 90 cm.
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!A-.$!
  -1B.      $  '  $    !    
( 60034     $    
       A>   A -5.
A              $            
''!!
            !          
$

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in the absence of oxygen has deposited within the rod. Acid treatment before sintering has
been found to improve the color further.
Figure 26. A typical ram extruder with 100% virgin extruded PTFE rod.

[Zhejiang, China (Mainland)]
Figure 27. 50% recycled PTFE rod from Deqing VRT Plastic Industrial Co., Ltd.

188 Sintering Techniques of Materials
Figure 28. A 40% repro + 60% virgin PTFE mixture was ram extruded into long rods of diameter 17 mm with the help
of a local industry in India. The rods came out fine from the points of polish and strength but was dark in color with
patches all through the length and volume due to carbon deposition.
8. Conclusions
Among all plastics, PTFE has the least stability against ionizing radiation, a property which
is used to break down carbon-carbon bonds in the polymer chain in the PTFE scrap and
reduce its molecular weight which makes it very brittle and the end product is a white, free-
flowing PTFE powder which was found to be useful as lubricant additive in other materials
or system such as printing ink, thermoplastics, elastomers, coatings and other lubricants.
While the turnings of PTFE scrap before irradiation are tough and elastic, those after
irradiation in air crumbles into a powdery material. The molecular weight of irradiated
PTFE is in the range of a few tens of thousands to a few hundreds of thousands, compared
to several million for the unirradiated resins.
Unirradiated PTFE scrap could be successfully ground with the help of a commercial shredder
and milling machine. A technique based on Shear Extrusion Pulverization based on Bridgeman
– Anvil was successfully used to grind PTFE scrap into a fine powder. Suitable pre-heat
treatments were arrived at to remove organic and other volatile impurities. Since normal
sintering procedures used for molding virgin PTFE did not work with repro filled PTFE, sintering
under pressure and ram extrusion techniques were tried to mold them. Repro filled PTFE Discs
up to 40% tried so far when kept inside the die and plunger and pressurized with a C-clamp
show discoloration which runs through its volume although good surface smooth ness is seen‐
and cracks disappeared totally. This was attributed to carbon production on reaction of volatiles
$  !%$%    $  
  !  $
! $$ C-1000

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to 3000 psi) and hence a solid die and plunger are required during cold pressing powder
into pellet, a relatively lower pressure (500 psi) should suffice during sintering.
A 40% mixture was successfully ram extruded by us into long rods of diameter 17 mm. The
rods came out satisfactorily from the points of polish and strength but was dark in color
with patches all through the length and volume due to carbon deposition. Due to escape of
carbon, the discoloration of ram extruded rod considerably reduced on annealing in air at
400°C in atmospheric pressure as the volatile gases found an easy pathway into atmosphere.
This is a significant result and confirms our view that the discoloration is caused by carbon
production. The fluorine being a gas produced on disintegration of fluorocarbon perhaps
has already escaped during the sintering while carbon in the absence of oxygen has
deposited within the rod. Discoloration could be further reduced with acid treatment so off-
white recycled PTFE rods could be made with ram extrusion.
Acknowledgements
This work was carried out under the TePP-DSIR (New Delhi, India) Techno Entrepreneurship
funded project entitled “Development of Filler Grade PTFE powders and Recycling PTFE Scrap
Materials”. We thank Hindustan Nylons, India for making Ram extruded PTFE rods.
Author details
Arunachalam Lakshmanan1* and S.K. Chakraborty2
*Address all correspondence to: arunachalamlakshmanan@yahoo.com
1 Saveetha Engineering College, Thandalam, Chennai
2 Department of Scientific and Industrial Research, Techno Entrepreneurship, New Delhi,
India
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190 Sintering Techniques of Materials
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