3M™ Dyneon™
PTFE Fine Powder
Processing of Dyneon PTFE
Fine Powder
Contents
1Introduction
4
4
Fundamentals of PTFE Fine Powder Processing
11
1.1
About the Company
4
4.1
Packaging and Storage
11
1.2
About our Flouoropolymer Product Family
4
4.2
Preparation of the Extrusion Mixture
11
4.3
Powder Screening
11
4.4
Mixing with Lubricants
12
2Physical Fundamentals of
PTFE Fine Powder Processing
6
2.1
Production of 3M™ Dyneon™ PTFE Fine Powder
6
2.2
Phenomenology of Paste Extrusion
6
2.2.1 Morphology of the Fine Powder
6
2.2.2 Paste Mixing
6
2.2.3 Preform Fabrication
6
2.2.4 Paste Extrusion in Funnel-Flow Design
7
2.3
7
Morphological Changes during Paste Extrusion
2.3.1 Morphology of the Paste Extrudate
7
2.3.2 Crack-up of the Secondary Particles
8
2.3.3 Reversible Deformation of the Primary Particles
8
3Properties and Handling of
Dyneon PTFE Fine Powder
3.1
Reduction Ratio
3.2
4.5Pigmentation
13
4.6
Maturing of the Extrusion Mix
13
4.7
Preform Compression
13
4.8Extrusion
13
5Fabrication of Films, Tapes and Sealing Cords
14
5.1
14
Profile Extrusion
5.2Calendering
15
5.3
Film and Sheet Drying
15
5.4
Film Stretching
16
5.5Fabrication of Unstretched and Stretched Sealing Cords
16
9
6
Fabrication of Tubing and Hoses
17
9
6.1
Lubricants for Hose Extrusion
17
Extrusion Pressure
10
6.2
Tube Extrusion
17
3.3
Particle Size and Particle Size Distribution
10
6.3
Drying and Sintering of Tubes
18
3.4
Specific Weight
10
6.4
Tube Testing
19
10
6.5
Typical Applications of PTFE Tubes
19
3.5Density
2
7
Fabrication of Thick-Walled Pipes
20
7.1
Specimen Preparation and Lubricant
20
7.2
Carbon Black Pigmentation and Antistatic Treatment
20
7.3 Liner Extrusion
20
7.4 Drying and Sintering of the Liner
21
7.5 Typical PTFE Liner Applications
21
8
Fabrication of Cable Insulations
22
8.1
Preparation of the Extrusions Mix
22
8.2 Cable Extruder
23
8.3 Cable Extrusion
23
8.4 Drying and Sintering of Cables
23
9
24
3M™ Dyneon™ PTFE Fine Powder Compounds
9.1 Property Modification by Means of Fillers
24
9.2 Manufacturing Methods of PTFE Fine Powder Compounds
24
9.3 Typical Applications of PTFE Compounds
24
10 Special Applications
25
11 Trouble Shooting Guide
25
12
27
Compliance and Safety
3
1
Introduction
This brochure provides information on how to process
3M™ Dyneon™ PTFE Fine Powder for the manufacture of a variety of products, such as wire and cable insulation, tubes,
pipes, tapes and sealing cords.
1.1 About the Company
Dyneon is one of the world’s leading fluoropolymer suppliers with decades of experience in developing applications. The company develops
userfocused application possibilities for the entire Dyneon product
range in its own technical service laboratories and research facilities
in Europe, USA and Asia.
1.2 About our Fluoropolymer Product Family
Dyneon offers a broad product family of high-performance plastics that
is divided into three product groups:
Fluoroelastomers
Fluoropolymers
Fluorothermoplastics
PTFE
Figure 1.1: Division of different fluoropolymers
Fluoroelastomers are cross-linkable, amorphous copolymers with a
molecular weight of 5 x 103 - 5 x 104 kg/kmol. They are extruded at
temperatures of <150 °C and then cross-linked in a heated mould. The
resulting vulcanisates lose their formability and weldability.
Fluorothermoplastics are partially crystalline copolymers with a molecular weight of 5 x 104 - 8 x 105 kg/kmol. Due to a modification of their
copolymer composition, they feature a broad melting-point range of
100 - 320 °C. Fluorothermoplastics can be processed at temperatures
of up to 400 °C with conventional thermoplastic technologies.
4
PTFE is a partially crystalline homopolymer with an exceptionally high
molecular weight of 107 - 108 kg/kmol. This high molecular weight makes
melt-processing of PTFE impossible. The product leaves the polymerization process in a highly crystalline form with a crystallization degree of
> 90% providing it with a melting point of 340 - 345 °C. After the first
melt, the crystallization degree decreases to approx. 60% and the melting
point to 327 °C.
Emulsion polymers are converted into water-dispersed latex particles
during polymerization. These so-called primary particles are of spherical
shape with a diameter of 180 - 250 nm. The primary particles are coagulated into so-called secondary particles with a diameter of approx. 400 600 μm and agglomerated into a free-flowing fine powder.
Emulsion polymers are divided into three groups according to their intended use: fine powders, micro-powders and dispersions.
The proven properties of PTFE are:
excellent all-round chemical resistance
widest service temperature range of -200 °C to +260 °C
superior dielectric properties
no embrittlement or ageing
very good non-stick properties
dimensional stability and stress cracking resistance.
The PTFE group is subdivided into suspension polymers and emulsion
polymers (Figure 1.2). Three further product groups evolve from the latter:
micro-powders, dispersions and fine powders. Suspension polymers are
divided into pre-sintered ram extrusion powders as well as non-free-flowing and free-flowing powder grades. PTFE compounds, i.e. blends of PTFE
and fillers, are mostly made of suspension polymers. In addition, Dyneon
has a number of fine powder compounds available.
Suspension polymers leave the polymerization process in the form of
up to 2000 μm polymer particles with irregular shapes, so-called reactor
beads. These are then milled until they are just 10 μm fine and, if necessary, agglomerated in an additional process step, where the fineness can
be adjusted to particle sizes in the range of 100 - 600 μm.
Fine powders cannot be processed from the melt due to their high molecular weight in the 107 - 108 kg/kmol range. A special technology, known
as paste extrusion, has therefore been developed to produce high-quality
finished articles, such as tubes, wire and cable insulations, and liners.
Specific fine powder types have been developed for the various fields of
application, described in the “3M™ Dyneon™ Polytetrafluoroethylene Product Comparison Guide” brochure.
PTFE dispersions with a solid content of up to 65% are used for impregnation and coating of metal surfaces and fibreglass.
Micropowders are used as additives for a variety of applications to improve slip and non-stick characteristics. Their molecular weight, similar
to that of fluorothermoplastics, is in the range 5 x 104 - 106 kg/kmol. In
contrast to fine powders or suspension polymers, micro-powders cannot
be used for the production of semi-finished products, as a result of their
low molecular weight.
The next chapter describes the theoretical and physical background of
the materials, which helps to explain the careful steps that must be taken
during the paste extrusion process.
Agglomerated products are free-flowing. Milled products are non-freeflowing and are also used for the manufacture of PTFE compounds. PTFE
powders are processed by specially developed technologies, such as
pressing and sintering as well as ram extrusion. The products that are
particularly well suited for the ram extrusion process are those that are
non-milled, yet pre-sintered, whose dosing and free-flowing properties
are achieved by screening the rough and fine parts. Suspension polymers
are mainly used for the production of semi-finished products which are
then drilled, turned, planed or milled into a variety of finished articles.
Ram extrusion powders
Suspension polymers
Free-flowing powders
Non-free-flowing powders
PTFE
Fine powders
Emulsion polymers
Dispersions
Micropowders
Fig. 1.2: Various product groups within the PTFE family
5
2
Physical Fundamentals
of PTFE Fine Powder Processing
2.1 Production of
3M™ Dyneon™ PTFE Fine Powders
2.2.2 Paste Mixing
PTFE production starts with the synthesis of the non-natural monomer
(tetrafluoroethylene, TFE) and leads through emulsion polymerization of
the monomer in water to the final polymer. The watery emulsions consist of approx. 180 - 250 nm sized particles that agglomerate during the
precipitation process, leading to the formation of secondary particles of
approx. 400 - 600 μm in size. Then the water is decanted and the still wet
fine powder is dried. The fine powder is highly shear-sensitive because the
agglomerated particle has a very low mechanical stress resistance. The
following pages describe the processes that occur during paste extrusion
with the aim of pointing out those that are rheologically1 relevant in order
to get a better understanding of their impact on the final properties of the
paste extrudates.
2.2 Phenomenology of Paste Extrusion
2.2.1 Morphology of the Fine Powder
As illustrated in Figure 2.1, the fine powder consists of “potato-like” secondary particles with a diameter of approx. 500 μm and a specific weight
of 500 g/l. The space filling degree is 25 vol %, i.e. 1 l of fine powder has
an air-filled pore space of 750 ml.
The secondary particle consists of some 10 primary particles that are
statistically packed in a spherical agglomeration. The packing density is
55 vol %. A statistical spherical packing of spheres with identical size can,
regardless of the sphere diameter, achieve a maximum filling degree of
62 vol %. The spherical primary particles have an extremely tight particle
size distribution. The PTFE they contain is in a highly crystalline form.
10
The potato-like form of the secondary particles ensures the free-flow ability (Fig. 2.1 left). The grainy, island-like structure of the secondary particle,
which can be easily seen, illustrates the statistically packed spherical agglomeration (Fig. 2.1 centre). The particles are “grape-like agglomerates”
of 1010 primary particles that can only be seen at higher magnifications
(Fig 2.1 right).
200 μm
100 μm
By adding lubricants, the pore space of the secondary particle is filled.
Organic PTFE-wetting fluids are used as lubricants, mostly higher boiling
point hydrocarbons (benzenes). In practical use, 20 weight parts of benzene are mixed with 100 weight parts of PTFE. The air in the interior of
the secondary particle is displaced by adding lubricants. The potato-like
shape of the secondary particle is not modified by this. The paste with the
additive still retains its free-flow ability, while the specific weight increases
to about 700 g/l. The air-filled pore space between the secondary particles
is in the range of 500 ml/l.
2.2.3 Preform Fabrication
The air between the secondary particles is removed by compressing
them in a cylinder with a pressure of
approx. 30 to 50 bar, which increases
the density of the lubricant-containing
material to 1650 g/l. The shape of the
secondary particle and the primary
particles is thereby preserved (Fig.
2.2). The cylindrical rod resulting from
this is called the preform or billet. From
the measured density, a filling degree
of approx 63 vol %. From the rheological point of view, the fine powder has
a paste-like state in the billet.
200 μm
Fig. 2.2: Morphology of the fracture
surface of the compressed billet
Rheologically, the paste can be defined as a heterogeneous 2-part system
consisting of an immobilized fluid and a plastically deformable solid. This
system flows when forces exceeding a certain minimum force are applied,
and it is irreversibly deformed. The immobilized fluid thereby becomes a
lubricant (matrix) and the deformed primary particles are a filling substance (islands).
1 μm
Fig. 2.1: Morphology of the fine powder; left: optical-microscope image of a secondary particle; centre and right:
SEM micrograph of the surface of the secondary particle shown in different magnification
1
Rheology, the science of flowing materials and deformation. It was established because the linear elasticity theory with Hook’s law and hydrodynamics with Newton’s law of
friction were not sufficient to describe the deformation behaviour of certain substances.
6
2.2.4 Paste Extrusion
in Funnel-Flow Design
The irreversible deformation during extrusion is
illustrated in Fig 2.3. The billet is transferred to
a cylindrical metallic hopper and then pressed
through a funnel, also made of metal, at a
certain pressure, the so-called paste extrusion
pressure. The narrowing of the cross-section
in the funnel is characterized by the reduction
ratio (RR), which is the ratio of the areas of the
funnel inlet and the funnel outlet. Attached to
the funnel outlet is a very short piece of pipe,
the so-called guide in which the “flowing paste
is calmed”.
In the hopper, the billet material shows plug flow
behaviour, as it does not stick to the metallic
wall. A real flow process, from the rheological
point of view, only starts at the funnel inlet,
shown as flow threads in Fig. 2.3. The flow
velocity increases in direct proportion to the
narrowing of the RR cross-section.
The crowding of the flow threads generates a
shear gradient in the direction of the flow. This
forces the paste material into an irreversible,
plastic deformation. The extruded material
gains mechanical stability from the deformation,
the so-called “green stability”, both in its lubricant-containing, wet state and in the dry state.
The wet extruded material has a density of
approx. 1.8 g/cm3, while the dried extrudate
has a density of 1.6 g/cm3 and a space filling
degree of 70 vol %. This means that the theoretical maximum packing density for spheres
of identical size has nearly been reached. The
next section provides a phenomenological explanation of the processes that are necessary
to achieve a high filling degree.
RV=1
100 μm
Fig. 2.4: The fibrous structure of the extruded material.
SEM micrograph of a split extrudate. The primary particles
in the fibrils are arranged in the direction of flow like beads
on a string
1 μm
Fig. 2.5: Scanning electron-microscope image of a split
extrudate
RV=2
2.3 Morphological Changes during Paste Extrusion
RV=4
RV=40
RV=400
Fig. 2.3: The irreversible plastic deformation of the paste
during extrusion
2.3.1 Morphology of the Paste Extrudate
Fig. 2.4 shows the fibrous structure of an externally smooth paste extrudate that has been split in
the direction of flow. The secondary particles, still intact in the billet (Fig. 2.3), have been irreversibly
deformed into fibrils. The fibrils consist of a “pearlstring-like” alignment of primary particles in the
direction of flow.
The electron-microscope image in Fig. 2.5 of the split extruded material shows that individual primary
particles (200 nm) have been preserved. They are clearly recognizable and survived the high shear
gradient that is necessary for extrusion. The preservation of these primary particles is ensured by
the lubricant.
Two processes during extrusion can be clearly distinguished:
irreversible deformation, so-called crack-up, of the potato-like secondary particle in bead-string-like
fibrils,
the kneading of the primary particles as a reversible deformation.
The specific energy necessary for the two forced processes in relation to the volume corresponds to
the extrusion pressure. The two processes are explained in detail in the following section.
7
2.3.2 Crack-up of the Secondary Particles
The crack-up of the secondary particle, also called paste fibrillation, is illustrated in Fig. 2.3. It shows
the longitudinal section of the funnel cone. The fine powder is mixed with lubricant containing dyed
secondary particles before paste preparation. The dye used is benzene insoluble. It is thus possible to
see how the deformation of the dyed secondary particles into “longitudinal cylinders” increases with
increasing reduction ratio RR.
Deformation is an adaptation to the flow threads (see Fig. 2.3). The crosssection of the cylinder
becomes smaller the higher the reduction ratio (RR) gets. The decrease of the cross-section is
inversely proportional to the RR. It is the result of packet-like regrouping of large primary particle
clusters that are transported in the flow direction to the head of the fibrillating secondary particle. The
transportation of the cluster is inevitably coupled with primary particles changing place. This induces
the pearl-stringlike alignment of the primary particles. The secondary particle is cracked up.
The pearl-string-like alignment of the primary particles should ideally be homogeneous up to ranges
of <10 μm, i.e. larger unaligned “grape-like” clusters should be avoided. Such clusters lead to irregular, unsmooth surfaces of the sintered finished product (orange peel).
2.3.3 Reversible Deformation of Primary Particles
Paste extrusion is accompanied by an enlargement of the extrudate, i.e. the extrudate has a larger
diameter than the guide. This can be proof that an elastic deformation of the primary particles has
taken place, as they are the only ones able to store elastic energy. The shear gradient in the flow
direction deforms the spherical primary particles into ellipsoids. The primary particles are kneaded.
After leaving the guide, the elastic tensions relax and the primary particle returns to its original
spherical shape, as shown in Fig. 2.6. This means that the deformation is reversible. All that remains
is a more compact structure.
Fig. 2.6: Reversible deformation of the primary particles.
Above: before the area reduction in the extruder. Centre:
maximum deformation at the tip of the extrusion die.
Below: relaxation after leaving the extrusion die.
The changed melting behaviour of the paste material after kneading is another major proof that a
reversible deformation has taken place, as illustrated in Fig. 2.7. The DSC diagram shows a uniform
melting peak for the original and a “bimodal” melting peak for the extruded paste material. The
crystalline structure has obviously been changed through the kneading. The original orderly alignment
has been partly destroyed.
Perkin-Elmer Thermal Analysis
Peak = 347 °C
Onset = 340 °C
Peak = 345 °C
Peak = 338 °C
Onset = 335 °C
Onset = 338 °C
Area = 714 mJ
Delta H = 66 J/g
Area = 8.6 mJ
Delta H = 0.8 J/g
290
300
310
320
330
Temperature (°C)
340
350
360
300
310
320
330
340
Temperature (°C)
Area = 662 mJ
Delta H = 64 J/g
350
360
Fig. 2.7: DSC diagram: on the left, unkneaded (original from the drum) and on the right kneaded (extruded) PTFE paste; both samples are unsintered
8
3
Properties and Handling
of 3M™ Dyneon™ PTFE Fine Powders
PTFE fine powder has a very high specific surface with a porous, spongelike structure that is
characterized by a very high absorption capacity for liquid hydrocarbons (lubricants).
The emulsion polymer powders are also often
referred to as powder paste. This is because
they undergo a processing step where the material has a paste-like feel. Its extremely high
molecular weight, however, results in such a
high melting viscosity that processing requires
a special technology, the socalled paste extrusion. The first process step, therefore, is to add
a lubricant to enable processing of the powder
paste. The powder paste with lubricant added is
still powdery with good free-flowing properties.
With special paste extruders (ram extruders),
a variety of profiles, tubes, sealing cords, wire
insulation and even large pipe liners can be
fabricated at slightly elevated temperatures.
The extruded profiles can then be calendered
into tapes and films for use as thread sealing
tapes and electrical insulation films for cables.
If porous structures are required, a subsequent
stretching process can be used. After the drying process in which the lubricant is removed,
a final sintering process with temperatures of
360 - 380 °C gives the products their high mechanical stability and transparency.
The special processing technologies required
for paste extrusion place very high demands on
the raw material. Contaminants such as dust,
lint from garments, or hair would decompose
at the high sintering temperatures, resulting in
discoloration and deterioration of the material’s
properties. The powder paste is soft, deformable and shear-sensitive. Ensuring proper packaging, storage and transportation conditions
is therefore of the utmost importance. The
free-flowability of 3M Dyneon PTFE fine powder
depends on the temperature and humidity.
Temperature
The higher the temperature, the worse the
free-flow ability of the powder becomes. Good
free-flow behaviour is a prerequisite for homogeneous absorption and distribution of the lubricant. The two crystal modification phases at
19 °C and 29 °C (Fig. 3.1) are specific to PTFE.
They are coupled with a significant deterioration
of the PTFE powder’s free-flow ability. PTFE is
prone to clumping. Above 30 °C, it is virtually
impossible to achieve good quality paste processing.
In storage, maximum temperature limits must
therefore be absolutely observed in order to
ensure optimum free-flow ability. Temperatures
below 19 °C (e.g. 15 °C) have proven to be very
effective.
Humidity
To prevent flaws in the finished product, the
residual humidity of the fine powders must be
very low (max. 0.04%). This is taken into account both during the manufacturing process
and in the selection of packaging and storage.
The converter of the raw materials has to keep
these requirements in mind, too. For example,
where the air humidity is very high, cooled
drums should only be opened in a cooled sample
preparation room to prevent condensation of the
humidity from contaminating the raw material.
Peak = 19 °C
Onset = 11 °C
Peak = 29 °C
Onset = 24°C
Area = 3.3 mJ
Delta H = 0.3 J/g
Area = 66 mJ
Delta H = 6.2 J/g
-10
0
10
20
Temperature (°C)
30
40
Fig. 3.1: Determination of PTFE phase transformation with the
help of DSC
3.1 Reduction Ratio
The reduction ratio (RR) is a unit-less number
calculated from the ratio of the cross-sectional area of the extrusion cylinder minus the
cross-section area of the mandrel rod and the
crosssection of the extrusion die minus the
cross-section of the mandrel tip. When extruding full profiles, the crosssections of mandrel
rod and mandrel tip are not taken into account.
Reduction ratio =
EC - M
ED - MT
EC: cross-section of the extrusion cylinder
M: cross-section of the mandrel rod
ED: cross-section of the extrusion die
MT: cross-section of the mandrel tip
Examples of different reduction ratios (RR):
Thick-walled liners:
Thin-walled liners:
Micro-tubes:
Cable insulations:
RR = 10 to 50
RR = 50 to 500
RR = 500 to 2000
RR = 300 to 3000
Tip:
To avoid contamination of the fine powder when opening the drums,
we recommend that the lid and drum be thoroughly cleaned beforehand
(e.g. wiped with a moist cloth and dried).
9
3.2 Extrusion Pressure
As per ISO 12086, 3M™ Dyneon™ PTFE Fine
Powder types are evaluated according to their
extrusion pressure with a reduction ratio of RR =
400 or RR = 1600, which allows a statement
as to their processability with a specific RR.
Extrusion pressure is defined as the pressure
in bar or MPa under standardized conditions
(RR = 400 or 1600) that builds up in the paste
extruder when extruding a mixture of PTFE fine
powder and lubricant, and which is exerted on
the material.
3.3 Particle Size and
Particle Size Distribution
3.4 Specific Weight
The free-flow ability and thus the processability
of the dry powder are determined by the surface
structure of the agglomerate particle and the
particle size distribution. The average particle
size ranges from 500 - 600 μm. The particle
size distribution is determined by means of
fractional screening. When pigments are added
to powder paste that is too coarsegrained, inhomogeneous colour distribution can result.
The extrusion pressure is dependent on the
extrusion conditions, e.g. type of fine powder,
lubricant, extrusion velocity, RR, die geometry
(die angle and land length) and temperature.
The extrusion pressure increases with increasing reduction ratio, as illustrated in Fig. 3.2.
Extrusion Pressure in MPa
3.5 Density
Material for low RR, e.g. liners
Material for medium RR, e.g. large tubes
Material for high RR, e.g. small tubes
The density of the finished parts made from sintered paste PTFE is, according to product type
and processing parameters, between 2.14 and
2.17 g/cm3 and enables evaluation and quality
control of the finished product. Low molecular
weight or slow cooling results in products with
high crystallinity and thus a density that is too
high. High molecular weight or too rapid cooling
results in products with low crystallinity and
thus a density that is too low.
100
80
60
40
20
0
0
200
400
600
800
The specific weight is thus a value to be taken
into account when dimensioning the preform
press. Fine powders tend to compact during
transportation and storage. This tendency is
even greater when the transition point at 19 °C
has been exceeded, so the powder must be
loosened before processing by means of cooling
and screening.
Due to this tendency to clump, even under light
pressure, a special preparation of the specimen
is necessary in order to measure the specific
weight. It should be determined in accordance with ISO 12086 and should be approx.
500 kg/m3.
140
120
The specific weight of PTFE powder is expressed as kg/m3 (mass per volume). Directly
related to this are mass and volume of the preform (also commonly called billet; see Section
4.7 Compression of the Preform).
1000
1200
1400
Reduction Ratio (RR)
Fig. 3.2: Dependency of extrusion pressure on reduction ratio (RR) for different types of fine powder
1600
1800
The density of the crystalline portion is higher
(2.288 g/cm3 ) than the density of the amorphous portion (1.966 g/cm3 ).
The mechanical properties of the PTFE are essentially determined by the amorphous portion
of the product and to a lesser extent by the
crystalline portion. For the reasons given above,
a precise interpretation of the density is only
possible if specimen preparation and processing parameters are precisely defined.
The fabrication of plates, the sintering conditions and density measurement (Standard Specific Gravity, SSG) are described in ISO 12086.
10
Fundamentals of
PTFE Fine Powder Processing
4.1 Packaging and Storage
4.2 Preparation of the Extrusion Mixture
3M™ Dyneon™ PTFE Fine Powders are produced in electronically-controlled processes (process control system) and filled under clean-room
conditions (clean room class 100). They are packed in tightly-closable
plastic drums with a filling quantity of 25 kg.
In order to avoid flaws in the finished product, care must be taken during
processing of the fine powder to avoid all excess mechanical stress of the
powder, as it is highly shear- sensitive. It is recommended to shake the
powder carefully or scoop it out in order to avoid crushing the particles.
The PTFE production facility, as well as the quality of the drum container
with lid and seal, eliminates the need for dry bags, which also avoids possible contamination due to damage to the dry bag. The material is filled and
stored at temperatures below 19 °C. In the hotter months of the year, the
product is usually shipped in refrigerated trucks in order to avoid clumping
due to transportation and/or heat and to maintain the good free-flowability
of the fine powder.
4.3 Powder Screening
In order to preserve these powder properties, it is recommended that
customers store the products in refrigerated rooms; where possible at
temperatures below 19 °C, the point at which crystallite is transformed.
A room temperature of 15 °C is recommended. Should the fine powder be
found to be clumpy or contain agglomerates, despite these precautionary
measures, then the latter can be sieved out (caution: do not apply pressure
to the particles, do not contaminate the powder). The separated agglomerates should be refrigerated for 2 to 3 days at a temperature of between
5 and 10 °C, and then shaken in order to break apart the agglomerates.
They should then be screened at temperatures below 19 °C by which the
agglomerates should fall apart into free-flowing powder.
Before pouring it into the mixing container, the fine powder should be
screened in order to break apart any agglomerates and to loosen it. The
mesh size of the screen should be 3 to 4 mm. The use of riddle sifters
is also possible, which allows harder agglomerates to be broken apart.
Larger clumps that do not fall apart should be removed from the screen
and collected in a separate container. The separated agglomerate particles can be reprocessed through cooling and renewed screening (also see
Section 4.1). Utmost cleanliness is important during the open screening
process. Moisture absorption due to air condensation must be avoided by
maintaining the drum at ambient temperature and reclosing it immediately
after powder removal. PTFE is a good electrical insulator, so when dosing
PTFE it is necessary to avoid high pouring speeds, as the material could
otherwise become charged with static electricity and then explode in combination with the lubricant.
Fig. 4.1 shows the cooling curve (determined by experiment) of a fine powder drum with 25 kg content with a temperature of 30 °C. The ambient
temperature is 5 °C; the temperature sensor is in the middle of the powder
in the fine powder drum. It takes more than 24 hours until the fine powder
material is ready for further processing and approx. 3 days to cool the
material down to 5 °C (also see Section 4.3). A more practical solution
would be a cold room temperature of 15 °C where the cooling of the PTFE
down to 15 °C extends over several days.
Tip:
30
Cooling cure at 15 °C ambient temperature
Cooling cure at 5 °C ambient temperature
25
It is recommended that you earth all
containers coming in contact with
PTFE and PTFE lubricants and use
metallic containers.
20
Powder temperature in °C
4
15
10
5
0
0
10
20
Cooling time in hours
30
40
50
60
70
80
90
Fig. 4.1: Cooling time of a 25 kg powder drum at an ambient temperature of 5 °C and of 15 °C, curves determined by experiment
11
4.4 Mixing with Lubricants
Aliphatic hydrocarbons with different boiling ranges have proven useful as lubricants for paste extrusion. The choice of the lubricant is dependent on the type of extrusion material.
Warning!
Lubricant mixing may cause the formation of ignitable lubricant air mixes in the range of 0.8 vol %
to 6.5 vol % lubricant (also see the material safety data sheet of the respective lubricant as well
as safety regulations concerning the handling of flammable solvents or vapours).
Lubricants with a higher boiling range are usually used for thin-wall applications requiring a calendering process, such as films. Lubricants with a lower boiling range are used for thick-walled extrusion
materials such as liners.
Tip:
The selected lubricant should be well absorbed by the fine powder and equally well removed after the
extrusion. It should also not cause discolorations during sintering. Depending on the application and
the lubricant type, the lubricant content amounts to 17 to 25 parts by weight related to 100 parts by
weight of 3M™ Dyneon™ PTFE Fine Powder. The quantity of the lubricant is stated in parts by weight
for simplicity’s sake. However, it would be more correct to say that the optimum volume of lubricant
is added to the PTFE fine powder, as the void volumes between the primary particles that has to be
filled. Here, the density of the lubricant, that may vary by around 10 - 15 %, plays a role.
Ignitable lubricant/air mixes can be avoided
by using a good suction ventilation system
with a high air-flow rate. The lower explosion limit is not attained.
The lubricant is added to the powder in the centre of the mixing container, not at its edge. Table 4.1
shows a selection of usable lubricants. The mixing procedure should be carried out at a temperature
below 19 °C as the fine powder has a better free-flow behaviour at these temperatures. Depending
on the type of mixer (dolly or tumbling mixer), the mixing time is between 20 and 30 minutes with a
speed set at 20 to 30 revolutions per minute. The powder mix should flow and not splash in the mixing
container. The lubricant is absorbed uniformly by the powder. The mixing containers must be tightly
sealed in order to avoid evaporation losses. The mixing container should be filled to a maximum of
2/3 of its volume in order to attain a good mix.
Earthing is important when mixing the fine powder with the flammable lubricant due to the ignition
risk of the lubricant vapours, e.g. ignition caused by electrostatic charge (also see Section 4.3). The
benzene concentration in the working rooms must be monitored with the help of suitable room air
monitoring devices. Good ventilation should also be provided.
Lubricant
Manufacturer
Boiling range
of the lubricant
Density
Applications
Shell Sol 100/140
Shell
105 - 137 °C
0.730 g/cm3
Isopar E
Exxon
118 - 143 °C
0.724 g/cm
Shell Sol T
Shell
187 - 215 °C
0.761 g/cm3
Shell Sol D70
Shell
190 - 250 °C
0.798 g/cm3
Isopar K
Exxon
181 - 204 °C
0.766 g/cm3
Isopar L
Exxon
189 - 210 °C
0.773 g/cm3
Isopar M
Exxon
206 - 245 °C
0.790 g/cm3
Pipes, cables, tubes
3
Tapes
Table 4.1: Selection of lubricants for use in paste extrusion
12
4.5 Pigmentation
4.7 Preform Compression
The following procedures are recommended
for pigmenting or colouring the powder paste:
When using liquid colour suspensions, add
these to the lubricant before mixing with the
powder paste. If the pigment is to be mixed with
the powder paste in a dry state (e.g. for antistatic applications, carbon black dyeing), the
pigment is screened directly onto the powder
and the mix is then homogenized in dry state
through rolling. After that, the lubricant is added
and handled as described in Section 4.4. This
avoids formation of agglomerates to a large
extent. If any agglomerates persist, cooling as
per Section 4.1 or screening as per Section 4.3
should be carried out.
In this processing step, the mix of 3M Dyneon
PTFE Fine Powder and lubricant is fed into a
preform press where it is compacted into a cylindrical preform.
4.6 Maturing of the
Extrusion Mix
A homogeneous distribution of the lubricant in
the PTFE can be obtained by letting the mix “mature”. This ripening process should take overnight or at best over 24 hours in tightly sealed
containers. Longer times are not necessary.
Tip:
Higher temperatures (e.g. 30 °C) during
the maturing process ensure improved
distribution of the lubricant and provide
more homogeneous extrusion quality.
™
4.8 Extrusion
™
The aim of the compression is to eliminate the
air contained in the mix of powder paste and
lubricant and to bring the mix into a form that
can be fed into the extrusion cylinder without
any problems. The cylinder of the preform press
should be three times the length of the preform,
as the powder is compressed to 1/3 of its volume. The mix of powder and lubricant should be
compacted slowly so as to allow the air to completely escape from the mix in the preform cylinder. This process can be supported by a vacuum placed at the ventilation bores. Pre-pressing
takes several minutes at a pressure of approx.
30 - 50 bar. The quality of the finished products
is, among other things, dependent on a preform
without cracks. The compression pressure is
therefore only slowly decreased and care must
be taken when removing the preform from the
preform cylinder. The compacted part must
then be immediately processed to reduce evaporation of the lubricant from the surfaces to a
minimum.
Inhomogeneous distribution of the lubricant
results in quality and dimensional variations of
the finished product. The preform is fed into the
paste extruder – the cylinder of which should
have a diameter that is 1 mm larger than the
outer diameter of the preform.
Because of the different handling procedures,
the topic of “extrusion” is dealt with in specific
sections titled “Fabrication of Films and Tapes”,
“Fabrication of Tubes”, “Fabrication of ThickWalled Pipes (Liners)” and “Fabrication of Cable and Wire Insulations”. For paste extrusion,
ram extruders with a relatively simple design
are used where the preform is extruded through
the die of a ram extruder.
Tip:
Preforms containing lubricant can be stored
in air-tight, sealed containers. In order to
ensure a uniform lubricant atmosphere in
the storage drum, a cloth impregnated with
lubricant is enclosed in the container. This
prevents the surface of the preform from
drying out. In doing so, the precautionary
measures stated in Section 4.3 must be
observed.
13
5
Fabrication of Films,
Tapes and Sealing Cords
Films, tapes and sealing cords can be fabricated from 3M™ Dyneon™ PTFE extrusion paste in
stretched, unstretched, sintered or unsintered
forms. The following applications are known:
Thread sealing tapes
Flat or plate seals
Electrical insulation tapes for wound
insulations
Tape cable
Varns
a
b
The key processing steps for the fabrication of
films and tapes are shown in Fig. 5.1:
Preparation of the extrusion mix
(Section 4.2 - 4.6)
Preform compression (Section 4.7)
Profile extrusion
Film calendering
Film drying
Mono- or biaxial film stretching
Film sintering, cutting and packaging,
if required
The content of lubricant is 21 - 25 parts by
weight related to 100 parts by weight of PTFE.
In contrast to tube extrusion, higherboiling lubricants (boiling range of 180 - 250 °C, also
see Tab. 4.1) are used in order to avoid lubricant
loss during calendering. This also has a favourable impact on the calendering behaviour (even
edges, splice tendency).
c
d
e
f
Fig. 5.1: Diagram of a tape facility: a extrusion, b calendering, c drying, d stretching, e cutting and f wind-up
5.1 Profile Extrusion
Extruders for profile fabrication can be of relatively simple design. A cylinder
with a nozzle or a ram with a mechanical or hydraulic drive that runs pressureindependently at a constant velocity are basically all that is needed.
The latter is necessary as the pressure can change during extrusion. A
small-dimensioned extruder is normally enough to attain the reduction ratio
of 100:1 that is required for profile extrusion.
It is possible to set preform on preform, in order to minimize loss of material that stays in the cone of the extruder due to the design. The touching
surfaces of the two preforms can be roughened with a fork or similar device
in order to improve the mixing of the two materials and thus avoid a predetermined breaking point in the profile. The forming tool consists of a conical
reducer and a die with parallel guide.
The enclosed angle of the cone is between 20° and 40° (also refer to
Section 6.2, Fig. 6.2, but without mandrel rod). In practical use, the round
extrudate has proven to be a useful solution for extrusion profiles.
For broader tapes, a rectangular or “dog-bone” profile is preferable. Fig.
5.2 shows possible extrusion profiles. The tools or areas coming into contact with the preform should be smoothly polished and made of stainless
steel. Steps or edges at the transition from cylinder to cone should also
be avoided. Profiles of the highest quality are attained when the extrusion
cylinder and die have a temperature of approx. 30 to 40 °C. After extrusion,
the extrudate is wound up and calendered. If the extrudate is to be stored,
it should be placed in a tightly sealed container to avoid lubricant escape.
Tip:
Place a lint-free cloth impregnated with lubricant
into the container to ensure a uniform lubricant atmosphere and to prevent the extrudate surface from
drying out. In doing so, the precautionary measures
described in Section 4.3 must be observed.
a
b
c
Fig. 5.2: Extrusion profiles for tape fabrication: a dog-bone, b rectangular and c round profile
14
Extrudate
Fishtail profile
Fig. 5.3: Feeding of the extrudate through the fishtail profile of a two-roll calender
5.2 Calendering
5.3 Film and Sheet Drying
Two-roll calenders equipped with an appropriate feeding system are used
to calender profiles. The shape of the feeding system is similar to a fishtail
or coat hanger profile, as shown in Fig. 5.3.
The film or sheeting is dried at a temperature of 160 to 200 °C. The line
speed or the length of the oven must be selected so as to enable complete
removal of the lubricant. Calendering and drying should be performed
independently of each other as both steps achieve their optimal results at
different speeds.
The calender rolls usually have a diameter of 300 to 400mm and widths of
approx. 400 mm.
A temperature of around 40 °C is recommended for the tool surface and
a roll speed of approx. 30 revolutions per minute (depending on the roll
diameter). The adjustment of the sheet thickness requires a highly precise
fine adjustment of the roll gap across the width of the film.
Calendering from profile to sheet is usually done in one step. For thickwalled profiles, this process can be performed in several steps until the
required width and thickness are obtained. The calendered width of the
sheets is dependent on both the PTFE type and the following parameters:
shape of the profile
t ype and shape of the fishtail guide and its distance to the roll gap
s heet thickness
lubricant content
roll surface
In general, sheet widths of 240 to 270 mm at a tape thickness of 100 μm
can be achieved by using a dog-bone profile and a calender of the type
described above.
Warning!
The drying process of the sheet may cause the formation of ignitable
lubricant air mixes in the range of 0.8 vol % to 6.5 vol % lubricant (also
see the material safety data sheet of the respective lubricant as well
as safety regulations concerning the handling of flammable solvents or
vapours).
Tip:
Ignitable lubricant/air mixes can be avoided by ensuring a high air
throughput in the drying oven. The lower explosion limit is not attained.
Tip:
The surface of the roll should not have a high-gloss polish but have
a slight coarseness or a coarse polish of the crosssection in order
to ensure enhanced creep of the sheet. This can be achieved by
treating the surface with abrasive with a grit of 200, for example.
15
5.4 Film Stretching
The film or sheet can be stretched up to a ratio
of between 1:10 and 1:15 without formation of
irregularities, with minimal reduction of thickness and width. The material becomes highly
porous and stretching results in a substantial
reduction of the specific weight of the sheet/
film. Fig. 5.4 shows the development of the film
or sheet’s density in relation to the stretch ratio.
The films and sheets are distinguished as either monoaxially and biaxially stretched. Fig.
5.5 shows SEM micrograms of monoaxially
stretched (left) and biaxially stretched (right)
PTFE. The fibres between the PTFE particles can
be seen. Monoaxially stretched PTFE is used for
thread sealing tape. The magnified image part
in Fig. 5.6 shows the transition from an island to
a fibre. Biaxially stretched PTFE is widely used
for breathable, impermeable membranes in the
clothing industry. The space between fibres is
large enough to let water vapour pass through
the membrane (breathable), yet small enough
to keep water drops out (impermeable). Films,
sheets or tapes can also be sintered and then
stretched after the sintering process in order
to produce high tensile-strength films, sheets,
tapes or yarns.
2.0
Tape thickness in g/cm3
The dried films or tapes are stretched for certain applications. In practical use, stretching of
the free-running PTFE sheets at temperatures
of between 280 °C and 300 °C has proven
effective (used for thread sealing tapes). The
sheet is stretched with two different roll systems that are running at different speeds in the
running direction. The sheet is clamped with roll
systems that can be made of steel or combinations of steel and rubber rolls.
1.5
1.0
0.5
0
0
10
Stretch ratio 1 : x
20
30
40
50
60
70
80
90
Fig. 5.4: Dependence of film density to the stretching ratio at a stretching temperature of 300 °C and a stretching speed
of 1.7 m/min
5.5 Fabrication of Unstretched and Stretched Sealing Cords
Sealing cords generally refer to round or rectangular extrudates that can be used in stretched or
unstretched form, depending on their application. The technology used for fabricating sealing cords
is very similar to that used for tape fabrication. As with tape production, the profiles are fabricated by
means of extrusion with appropriate dies being used to correspond to the profile dimension. The extrudate is subsequently dried. For stretched sealing cords, the extrudates are stretched using similar
equipment and conditions as for sheet stretching. The final product should generally have a density of
approx. 0.65 g/cm3, which corresponds to a stretch ratio of around 1:3 to 1:4.
Uniform cord dimensions can be achieved by feeding the extrudates through special calibrating rolls.
During the same process step, adhesive tape can be applied to provide the sealing cords with a
fixation aid for installation.
Fig. 5.5: SEM micrographs of monoaxially and biaxially stretched PTFE sheeting
Fig. 5.6: SEM micrographs of monoaxially stretched PTFE sheeting.
The image shows the transition from an island to a fibre
16
6
Fabrication of
Tubing and Hoses
Tip:
Paste extrusion is used to fabricate extremely thin-walled micro- and spaghetti hoses as well as
thin-walled industrial tubes from 3M™ Dyneon™ PTFE Fine Powder. Dimensions range from approx.
0.1 mm to approx. 25 mm interior diameter with wall thicknesses of around 0.1 to 2 mm.
The extruded tubes can be further processed to fabricate:
shrink tubes
corrugated tubes and
braided tubes
To keep investment costs and the number of extruders low, it has proven useful to apply different
sleeves and rams, thus making it possible to
achieve the optimum reduction ratio of a material.
6.1 Lubricants for Hose Extrusion
The lubricants used for hose extrusion have a lower boiling range (100 to 150 °C) than those used for
tape fabrication to enable easier evaporation of the lubricant during the short time spent in the drying
oven. The following lubricants listed in Table 6.1 can be used:
Suction
Cross-bar
Lubricant
Manufacturer
Boiling range
of the lubricant
Specific gravity Application
Shell Sol 100/140
Shell
105 - 137 °C
0.730 g/cm3
Exxon
118 - 143 °C
0.724 g/cm3
Isopar E
Ram
Hydraulic cylinder
Pipes,
cables,
hoses
Table 6.1: Lubricants suitable for use in tube extrusion
Extrusion cylinder
PTFE filling
Mandrel rod with
mandrel tip
Extrusion die
6.2 Tube Extrusion
Tubes made from paste are fabricated using special ram extruders (paste extruder) with an inner
mandrel that determines the interior diameter of the tube, and the extruders can be set up either
horizontally or vertically. The extrudates are then dried and sintered. Specimen preparation and fabrication of the preforms is described in Sections 4.2 to 4.7.
The extrusion process is discontinuous. Extrusion is stopped after each extruded preform to return
the ram and insert a new preform. Large paste extruders are able to process several preforms with a
total weight of up to 100 kg in one step. Arbitrary enlargement of the preform’s diameter is not possible as this might lead to excess reduction ratios and high local shear resulting in flaws of the extrusion
materials. Limits are imposed by the material here, as high reduction ratios result in high extrusion
pressures. This means that you must have a cylinder with suitable diameter depending on the tube
dimension. Cylinder diameter can vary in a range of 25 mm to 250 mm as the various tube types have
different maximum reduction ratios. The extruder drive has to ensure that the set extrusion speed
stays constant regardless of the changing extrusion pressure. This is necessary to achieve constant
drying and sintering conditions in the downstream drying oven – a precondition for consistent tube
dimensions and quality. The extruder can have either a hydraulic or mechanical drive. Experience has
shown that extruders with hydraulic drives now provide precise extrusion conditions and can be more
easily produced. Fig. 6.1 shows the set-up of an extruder used for processing 3M Dyneon PTFE.
Oven Suction
Chain mechanism
Drying zone (2-3 m)
Sintering zone (2-3 m)
Cooling zone
Wind-up (optional)
Fig. 6.1: Hydraulic paste extruder for tube fabrication
17
a uniform wall thickness of the tube. There have
been good results with flexible mandrels made
of polyacetale or polyimide. The tool consists of
a conic reducer and a die with parallel guide
(land length). The land length L is determined
by the die diameter D. For thick-walled and
large-dimensional extrudates, the ratio is
Fig. 6.2: Extrusion die of a paste extruder for tube
fabrication
Cylinder
Mandrel rod
L >1
D
20°- 40°
L = land length
D = die diameter
and for thin-walled, small-dimensioned tubes
the maximum ratio is
Closing jaws
L =10
D
Mandrel tip,
exchangeable
The enclosed angle of the cone is between
20° and 40°. The higher the reduction ratio,
the smaller this angle must become in order to
keep the extrusion pressure in limits. Fig. 6.2
illustrates the dies of a paste extruder for tube
fabrication.
Die tool,
exchangeable
Die heating
Land length
To ensure a smooth tube surface, the die is
heated to a temperature of approx. 50 - 60 °C.
The tools or areas coming into contact with the
product should be smoothly polished and made
of stainless steel. The necessary extrusion
pressure is mainly dependent on the reduction ratio and the product type. It is also it is
In contrast to profile extrusion, tube extrusion
requires an additional mandrel in the extruder.
This mandrel is fixed to a cross-bar or a frame
plate and runs freely without any fixation in the
die area. During extrusion the ram presses the
material over the mandrel. Precise centring of
the mandrel tip in the die is required to achieve
Processingparameter
Unit
Tool
mm
Extrusions speed
210 : 1
m/min
2.0
Lubricant
6.3 Drying and
Sintering of Tubes
The extruded tube is dried and sintered in a
continuous oven. In the drying zone, the lubricant is evaporated and removed above the boiling point of the lubricant and below the sintering
temperature of the PTFE (150 to 250 °C).
13.3 x 11.4
Reduction ratio
determined by lubricant content, temperature,
die angle, land length and extrusion speed.
Excess extrusion pressure may not only lead to
high extruder stress but also to excess shear
in the tube (cracks, deformations, etc.). If the
extrusion pressure is too low, the extruded
material may have irregularities in the form of
surface coarseness. The relation between extrusion pressure and lubricant content during
tube extrusion with different reduction ratios
is illustrated in Fig. 6.3. The impacts of the
lubricant quantity on the extrusion pressure,
i.e. shrinkage of a selected tube dimension, are
summarized in Table 6.2. The mechanical properties are not affected by the lubricant quantity.
Fig. 6.4 shows the possibility of variations of the
final tube dimensions in relation to the removal
speed of different lubricant contents in parts
by weight (PW). It also provides information on
the relationship between the shrinking properties of the tube and the lubricant content. The
extrusion velocity is dependent on the tube dimension or wall thickness and the length of the
drying and sintering oven. It can be in the range
of 1 to 20 m/min.
Boiling range 100 bis 140 °C
Lubricant quantity
Parts by wgt.
Wgt. %
18
15.3
19
16
20
16.7
21
17.4
22
18
Extrusion pressure
bar
205
161
135
101
91
Outer diameter
mm
12.0
11.7
11.6
11.5
11.3
Interior diameter
mm
10.2
9.9
9.8
9.7
9.5
Shrinkage of outer diameter
%
9.8
12.0
12.8
13.5
15.0
Tensile strength, lengswise
N/mm2
29
30
31
30
31
Tensile strength, crosswise
N/mm2
26
28
26
28
26
Elongation at break, lengthwise
%
315
280
340
300
365
Elongation at break, crosswise
%
625
605
620
615
585
Final tube dimensions
Table 6.2: The impact of the lubricant quantity on extrusion pressure, shrinkage and mechanical properties of a selected
tube dimension
The evaporation speed is dependent on the
oven temperature, the throughput speed, the
wall thickness and the tube dimensions. Complete evaporation of the lubricant at high extrusion pressures requires higher temperatures or
longer ovens. After drying, the tube is sintered
at a temperature of 360 to 380 °C. Sintering
conditions are dependent on
tube dimensions,
extrusion velocity and
temperature and length of the oven.
The drying and sintering process changes the
dimensions of the tube. Crosswise shrinkage of
0 to 15 % and lengthwise shrinkage of 15 to
25 % are expected. This dimensional change
must be taken into account when selecting the
extrusion tool. It is influenced by the lubricant
content, the temperature management and the
tube weight drawing on the extrusion die.
18
6.4 Tube Testing
To monitor tube fabrication it is advisable to carry out regular checks of
the specific gravity and the mechanical properties such as tensile strength
and elongation at break.
Tip:
Determination of the specific gravity
The converter may fine-tune the
final tube dimensions with removal
speed and lubricant content.
The “buoyancy method” has proven useful to determine the specific gravity of the tube specimen. The test method is described in DIN 53479 under
“Method A”.
Determination of Tensile Strength and Elongation at Break
Specimen preparation and test conditions must be precisely defined as
they have an influence on the measurement results. Testing of tensile
strength and elongation at break is described in ISO 12086.
Warning!
Formation of explosive mixes of
lubricant and air can be avoided by
using a proper ventilation system
(also see Section 4.3).
6.5 Typical Applications of PTFE Tubes
Chemical industry: for storage of aggressive media, e.g. use in tank vehicles, sniffer probe lines for operation monitoring, lab devices.
Pharmaceutical and food industries
Biotechnology
E ngineering
Steam lines for laminating and vulcanization presses, extruders, calenders, purification equipment, plastic foaming equipment, spraying and
painting equipment, glue lines for wood processing, hydraulics, air conditioning and cooling equipment
Engine and vehicle construction
E xhaust pipes, fuel lines
Bowden pulleys
Electrical industry and electronics
Insulations for electronic components
3.5
120
Diameter die
RV =1900
3.0
80
60
40
Tube diameter in mm
Extrusion pressure in MPa
100
RV =950
RV =460
Oute
r dia
met
er a
2.5
2.0
Interio
r diam
eter a
t 19
t 19 G
1.5
20
Diameter mandrel
GT
T
at 22 GT
at 22 GT
1.0
0
17
18
19
20
21
22
23
24
Lubricant quantity in parts by weight per 100 weight parts PTFE
25
26
Fig. 6.3: Influence of extrusion pressure and lubricant content in tube extrusion at
different reduction ratios
4
6
Removal speed in m/min
8
10
12
Fig. 6.4: Dependency of the tube diameter on the removal speed and the lubricant in parts by weight (PG)
19
7
Fabrication of
Thick-Walled
Pipes
7.2 Carbon Black
Pigmentation and Antistatic
Treatment
Liners are thick-walled pipes with wall thicknesses of approx. 2 to 15 mm used for corrosion-resistant lining of steel pipes in chemical
plants. This brochure is limited to the description of how seamless liner pipes are fabricated
by means of paste extrusion. It does not deal
with alternative linings implemented with 3M™
Dyneon™ PTFE suspension polymers (isostatic
pressing, ram extrusion, peel films).
The highly conductive, fine carbon black powder must be added in its dry state. The carbon
black is screened onto the PTFE powder. The
mixture is homogenized in mixing vessels with
the help of rolls or tumble mixers. 0.1 to 0.3 %
weight of carbon black have proven efficient for
black coloration, and around 1 to 3 % weight for
antistatic treatment. The lubricant is added afterwards. More detailed information is provided
in Chapter 9, PTFE Fine Powder Compounds.
7.1 Specimen
Preparation and Lubricant
Specimen preparation does not differ from tube
fabrication. The lubricants described in Section
6.1 can also be used for liner fabrication. Liner
extrusion usually has lower reduction ratios in
the range of 20 to 100. They cause low extrusion pressure and low green strength of the liner
pipes. The green strength of a liner refers to the
stability of the extruded PTFE tube right after it
leaves the extrusion die. At this point, the PTFE
still contains lubricants and is very sensitive
to mechanical stress. In order to increase the
extrusion pressure and thus the green strength,
3M Dyneon PTFE liner types have a very high
pressure level of their own. In addition, lubricant
quantities of 17 to 20 parts by weight relating to
100 parts of PTFE by weight are used.
7.3 Liner Extrusion
For liner extrusion the same ram extruders
as described for tube extrusion (see Section
6.2) are used. Due to the heavy weight of the
liner pipes, the extruders are generally set up
horizontally and require substantially larger extrusion cylinders as a result of the large pipe
dimensions. Fig. 7.1 shows how liners are fabricated.
The extruded liner is drawn over an interior
supporting pipe, if required, and put into a supporting half pipe, taking the low green strength
into account. Pipe and half pipe must be cor-
rosion-resistant in order to avoid liner discoloration. Unlike with tube extrusion, the mandrel
diameter may exceed the size of the mandrel
rod in order to enable the large liner dimension.
The mechanical stress may be very high, which
requires the use of large dimensioned mandrel
rods made of high-strength steel. The marked
areas at the mandrel of the tool in Fig. 7.2 show
the spots exposed to the highest stress.
When fabricating thick-walled liners, the phenomenon of orange peel sometimes occurs.
Due to the low reduction ratios, the shear gradient is sometimes reduced to such an extent
that a sufficiently homogeneous crackup of
the secondary particle is no longer ensured.
This problem is obviously not caused by “overshear” as is often assumed, and can therefore
be solved by dramatically increasing the shear
gradient, e.g. through a substantial increase of
the extrusion speed.
Tip:
The inner supporting pipe and the supporting
half pipe have deburred holes to enable easier
lubricant evaporation during drying as well as
reduced drying times. Alloys with high nickel
content or aluminium are considered as discolorationfree materials.
Drying and
sintering oven
Horizontal extruder
2
Liner
Inner
supporting pipe
Supported by holed supporting half pipe
1
3
1 Supporting half pipe (stainles steel)
2 Liner
3 Inner supporting pipe (stainles steel)
Fig. 7.1: Horizontal extrusion of a liner along an inner supporting pipe into a supporting half pipe followed by drying and sintering in the oven
20
7.4 Drying and Sintering of the Liner
The tube sections of up to 10 m length are both
dried and sintered horizontally in the oven. The
pipes are put in metal supporting half pipes with
the interior supporting pipe so as to avoid deformation of the liner pipes during drying and sintering (Fig. 7.1). The drying conditions must be
adjusted to the dimensions of the semifinished
products and the boiling range of the lubricant.
This is necessary to allow complete removal of
the lubricant thus avoiding the sintering process
being impacted by the lubricant.
Closing clamps
Lubricant residues may lead to discoloration,
cracks and bubble formation. The following drying and sintering times and temperatures are
recommended in accordance with wall-thickness and diameter:
Drying: 2 to 3 hours at 150 to 200 °C
Sintering: 1 to 3 hours at 360 to 380 °C
Cooling can be performed quickly or slowly,
depending on the desired crystallinity level. The
speed when passing through the gelling point
at 310 to 320 °C defines the crystallinity. Fast
cooling reduces the crystallinity and enhances
the flexibility, while slow cooling increases the
crystallinity and the specific gravity while reducing permeability.
Mandrel rod
Cylinder
152.4
(6“)
Cylinder
60°
70°
Mandrel
50
30
Die
117
127 (5“)
Fig. 7.2: Head of a liner extruder with extrusion tool
7.5 Typical PTFE Liner Applications
PTFE liners are generally used in chemical plant construction and in the pharmaceutical industry for
pipes, columns, compensators and fittings to provide protection against aggressive media.
Warning!
For safety and health reasons it is important to
ensure good suction ventilation of the vapours
generated during drying and sintering (also see
Section 4.3).
21
Fabrication of
Cable Insulations
The superior dielectric properties of 3M™
Dyneon™ PTFE coupled with high temperature
resistance, all-round chemical resistance and
inflammability under normal conditions (limiting
oxygen index, LOI >95), are the key features
that define 3M Dyneon PTFE as the material of
choice for wire and cable insulations.
vapour extraction system
Similar to tube extrusion, paste extrusion with
special wire cable extruders has established
itself as a suitable processing method. The
individual steps are described in Sections 8.2
to 8.4.
electrical
control cabinet
drying zone
sintering zone
nozzle heating
8.1 Preparation
of the Extrusion Mix
Especially for cable fabrication, a prepared
paste mix already containing lubricant is
screened again into the preform press through
a sifter of 3 to 5 mm mesh sizes. Cable insulations mostly have a low wall strength, which
means that larger agglomerates in the insulation may not lead to flaws. Care must be taken
that the compacting process in the prepress
is slow so that the air can completely escape
from the lubricant-containing powder paste. In
addition, the pressure used to compress the
preform should not exceed 30 to 50 bars and
should be maintained for approx. 5 minutes.
The quality of the cable insulation is highly dependent on the flawless fabrication of the preform which should therefore be given closest
attention.
barrel heating
wire take-up roll
unwind roll
dancer roll
screw drive
wire puller
electrical dreakdown test device
Fig. 8.2: PTFE cable extruder consisting of an unwind roll (a1) and dancer roll (a2), extruder (b), a drying- (c) and sintering oven (d), deflector roll (e), a wire puller (f), electrical breakdown test device (g) and wire take-up roll (h)
The preform is then slowly relaxed in order to avoid cracks. It should either be immediately processed
or stored in an airtight container in order to avoid lubricant loss caused by evaporation (also see
Section 5.1). Lubricants with a low boiling range are preferred as the dwell time in the drying oven
is very short due to the high extrusion speed. The lubricant quantity is variable over a wide range in
order to lower the extrusion pressure when reduction ratios are very high. The range for an optimum
lubricant quantity is, however, very small in order to minimize the number of electrical breakdowns
(Fig. 8.1, also see 6.2 and Fig. 6.3).
Number of electrical
breakdowns
8
Lubricant amount
Fig 8.1: Qualitative representation of the optimal lubricant amount for cable extrusion related to the number of
electrical breakdowns
22
Warning!
8.2 Cable Extruder
Sufficient ventilation must be ensured
(also see Section 5.3).
Fig. 8.2 provides an overview of a cable insulation facility. The extrusion system consists of a wire
unwind roll (a1), a dancer roll (a2), the extruder (b), a drying- (c) and sintering oven (d), a deflector
roll (e), a wire puller (f), the electrical breakdown test device (g) and the wire take-up roll (h).The
cable extruder can be set up either vertically or horizontally. A wire guiding pipe is used instead of
an extrusion dome in the centre of the extrusion cylinder in order to ensure uniform thickness of the
cable insulation. Due to the high extrusion speed, the production of cable insulations requires long
drying and sintering ovens, which are usually arranged parallel to each other to ensure better space
utilization and also require a deflector roll, as shown in Fig. 8.2. The alignment of drying and sintering
ovens as shown here is a compromise solution as the ovens are in vertical position on top of each
other. For construction reasons it is often necessary to deflect the wire by 180 degrees after leaving
the drying oven in order to go through the sintering oven afterwards. Alternatively, the wire can also
be deflected several times in the oven in order to increase the dwell time. After leaving the sintering
route the coated wire passes through a thickness meter and its dielectric strength is tested.
Land length
Distance (a)
Wire guiding pipe
8.3 Cable Extrusion
Die
The preform with inner bore is inserted into the extrusion cylinder of a paste extruder and then
pressed through a die with the help of a ram. The extruded paste material coats the wire that is
guided through the extruder head at the same time. As the extrusion pressure changes during processing, the machine design has to ensure that ram speed and therefore extrusion speed are kept at
a constant level. This is particularly important for high reduction ratios during extrusion (compare Fig.
6.3 of tube fabrication). The diameter of the extrudate after leaving the die is higher than the inner
diameter of the die. This is referred to as the PTFE “swelling rate” explained by the release of the
elastic deformation energy of the particles.
It is also advisable to heat both the extrusion cylinder and the extrusion die (40 to 60 °C) in order
to ensure that the surface of the extrudate is as smooth as possible. For thin wires, a dancer device
is used after unwinding that compensates for pull variations and prevents the wire from breaking.
Apart from the die angle, the die diameter and the land length, extrusion is decisively influenced by
the clearance (“a” in Fig. 8.3) between the upper edge of the wire guide tip and the lower edge of the
cylindrical guiding system (land length).
The clearance “a” is correctly set when the material speeds of the PTFE and the wire are identical
at the exit from the wire guiding pipe. When clearance “a” gets bigger, product quality deteriorates
which is reflected in low insulation strengths. When clearance “a” gets smaller the ring gap immediately in front of the land length can be narrowed to such an extent that it leads to material over-shear
and increase of the extrusion pressure. The adhesion of PTFE to the wire is equally reduced. In
practical use, the optimum clearance “a” should be determined for each type of material and wire and
for each tool. Depending on the reduction ratio, the optimum die angle is between 20° and 30° (Fig.
8.3). In addition, all edges coming into contact with the paste material should be rounded. The extrusion speed is determined by the material’s shear sensitivity and the conditions in the downstream
drying and sintering ovens.
8.4 Drying and Sintering of Cables
Mandrel rod
Wire
Fig. 8.3: Extrusion die of the wire cable extruder
Tip:
It may be advantageous to have a wire speed that
is slightly above the speed of the extrudate as it
allows better coating of the wire and therefore
better insulation properties.
After the extrusion, the insulation must be dried at a temperature of around 150 to 200 °C. Any
remaining lubricant in the extrudate may lead to brownish discolorations, cracks and electrical flaws
during sintering. Sintering takes place at temperatures of above 345 °C, preferably at 360 to 380 °C.
The coated wire must be run at decreasing speed the thicker the insulation material gets. PTFE is a
good thermal insulator and prevents complete drying or sintering of the cable insulation if drying and
sintering times are too short or temperatures are too low.
23
9
3M™ Dyneon™
PTFE Fine Powder Compounds
PTFE offers a variety of excellent properties including:
Combinations of these and other suitable fillers make it possible to tailor
custom compounds for specific applications.
b roadest service temperature range of all plastic materials
lowest coefficient of friction of all known solids
excellent chemical resistance
non-stick properties
superior electrical and dielectric properties
non-flammable under normal conditions
(limiting oxygen index, LOI >95)
9.2 Manufacturing Methods
of PTFE Fine Powder Compounds
However, PTFE does have some properties that restrict its use in applications like seals, self-lubricating bearings, etc:
deformation under load (cold flow)
poor thermal conductivity
high thermal expansion coefficient
limited wear resistance
By incorporating selected fillers into the fine powder, these properties can
be offset. It is therefore possible to create property profiles that are tailored to specific applications.
Property
Increased*
Reduced*
Deformation under load
Wear resistance
Hardness
Thermal expansion coefficient
Thermal conductivity
Electrical conductivity
Tensile strength
Elongation at break
Service temperature range
There are two different methods for producing Dyneon PTFE Fine Powder
Compounds. In the first, the filler is added to the fine powder in the form
of powder or as a suspension together with the lubricant. This process
is often called the dry-mix method. In the second, since Dyneon has the
ability to produce compounds directly from the dispersion, the filler can be
added to the PTFE dispersion and the powder paste is then mixed together
with the filler (wet mix). Which production method is the most appropriate
is mainly determined by the requirements placed on the compound and the
specific application. Dyneon will be glad to help you find the appropriate
compound for your needs.
9.3 Typical Applications
of PTFE Fine Powder Compounds
Bowden pulleys, for example, are applications where low wear and low
friction are important. Here, it is not only important that the PTFE part has
a lower wear but also that the opposing surface is not prematurely worn by
the filler. Ideal fillers here would be carbon, graphite or high-performance
polymers, such as PPSO2. Often, increased pressure resistance is required
in addition to chemical and thermal resistance. This is particularly important for seals. Glass would be an ideal filler for such an application. Carbon
black is used for antistatic treatment of liners and tubes. Here you need a
surface resistivity of ≤ 109 Ω.
Chemical resistance
Coefficient of friction
*depending on filler type and quantity
Tab. 9.1: Property changes of fine powders through filler compounds
9.1 Property Modification by Means of Fillers
3M™ Dyneon™ PTFE Compounds are able to broaden the application scope
of PTFE fine powders even further. Fillers make it possible to optimize
properties thus enabling use of PTFE in applications where pressure- and
wear resistance are of utmost importance. The addition of fillers changes
the properties of fine powders, as shown in Table 9.1. The following fillers
are generally used to reduce abrasion and deformation under load or to
increase thermal or electrical conductivity:
Glass
Bronze
Carbon
High-performance polymers,
such as PEEK, PPSO2 or PI.
Graphite
Carbon black
24
10
Special Applications
3M™ Dyneon™ PTFE Fine Powders are also used as anti-drip agents for thermoplastic plastics. In this
special field of use, the fibrillation properties of the PTFE powder paste are used, in order to prevent
melting of the thermoplastics in the case of fire.
11
Trouble Shooting Guide
Production processes do not always run trouble-free. The reasons for this are manifold. This chapter
therefore provides a number of suggestions on how to find possible causes and solutions for such
problems. As causes can be very complex the following table 11.1 does not claim to be exhaustive.
Problem
Possible cause
Suggested remedy
Contamination of the semi-finished
product
Contaminated lubricant has been added
F ilter lubricant
Change the lubricant batch
During opening of the powder drum
B efore opening, remove dirt particles from the outside
of the drum to avoid contamination
Earth the drum to avoid electrostatic charges
Clean the preparation room
Prior extrusion contained fillers
Clean extruder
Brown coloration of the semi-finished
product
Lubricant has not been fully removed
Increase drying period
Increase drying temperature
Use lubricant with lower boiling point
Improve suction in the oven
Repeat sintering, brown colour will disappear
in most cases
Extrudate is brittle
E xtrusion pressure is too low, green strength is too low
Increase reduction ratio
Reduce lubricant quantity
Use material with higher extrusion pressure
Increase extrusion speed
Semi-finished product is torn in extrusion
direction
Mechanical damage in green state
T reat extrudate more carefully
Use lubricant with higher boiling point
Check die for mechanical damage
Sintered semi-finished product has low
tear resistance but high density and
elongation at break
Sintering of semi-finished product too long or too hot
heck temperature profile of the sintering oven
C
Choose lower sintering temperature (360 to 380 °C)
Check for oven malfunctions
Liner is torn lengthwise and crosswise to
extrusion direction
Inner supporting pipe was too big
Use smaller inner supporting pipe
Irregular cooling after sintering
E nsure uniform cold air distribution
Check for malfunctions of oven or cooling unit
Inner tensions or irregular shrinkage because cooling
was too fast
S low down cooling process
Check for malfunctions of oven or cooling unit
S emi-finished product stuck to contact surface during
sintering
Check contact surface for roughness or flaws
Table 11.1: Problems occurring during paste extrusion, their possible causes and remedy suggestions.
25
Table 11.1: Problems occurring during paste extrusion, their possible causes and remedy suggestions (continued).
Problem
Possible causes
Semi-finished product burst open
Drying temperature is too high
R educe drying temperature to the range between boiling
point of the lubricant and sintering temperature
Check for oven malfunctions
Moisture
ry lubricant
D
Water condensation when opening the powder drum,
bring drum to room temperature
Trapped air during preform fabrication
heck machine parameters (pressure, time, closing
C
speed, etc.)
Drill ventilation bores
Partial tapering of the tube diameter or
wavy extrudate, “snake-effect”
Too much lubricant
Reduce lubricant content
White dots in the semi-finished product
Contamination or PTFE residues from prior extrusions
Clean extruder
Squeezed powder paste
T reat powder more carefully
Check lubricant level
Screen agglomerates
E xcess lubricant
Reduce lubricant level
Squeezed powder paste
T reat powder more carefully
Screen agglomerates
Irregular lubricant distribution
E xtend mixing time
Let lubricant-powder mix rest overnight at 30 °C
Shearing in extrusion die too low
Increase reduction ratio
Increase extrusion speed
Rough tool finish
P olish
If lateral polish is applied, polish longitudinal
L ack of lubricant
Increase lubricant level
L ack of lubricant
Irregular lubricant distribution
Increase lubricant level
Let lubricant/powder mix rest overnight at 30 °C
Inconsistent drying and sintering conditions
Check for oven malfunctions
Filler agglomerates in dry-mixed fine powder compound
R educe filler-particle size
Increase dimensions of semi-finished product
Grind, crush or screen filler
Filler or filler additives not temperature-resistant enough
T rapped air during preform pressing; preform has
expanded
Increase compression pressure suddenly to let air
escape
Partial occurrence of streaks
Scaly surface of the semi-finished product (orange peel)
Irregular surface
Pressed billet does not fit into extruder
Suggested remedy
26
12
Compliance and Safety
If you have any questions regarding compliance with national and international regulations of legislators or associations, please contact Dyneon
GmbH (contacts see last page).
General recommendations on health and safety in processing, on work
hygiene and on measures to be taken in the event of accident are detailed
in our material safety data sheets.
Due to lack of experience, we cannot recommend applications in the
medical field (implants). Applications in this field are therefore the sole
responsibility of the manufacturer.
You will find further notes on the safe handling of fluoropolymers in the
brochure “Guide for the safe handling of Fluoropolymers Resins” by
PlasticsEurope, Box 3, 1160 Brussels, Belgium, Tel. +32 (2) 676 17 32.
27
Technical Information and Test Data
Important Notice
Technical information and guidance provided by Dyneon personnel is based
upon data and testing which is believed to be reliable. Such advice is intended
for use by persons with appropriate technical understanding, knowledge and
skills relating to PTFE compounds. No licence under any Dyneon or third party
intellectual rights is granted or implied by virtue of this information.
All information set forth herein is based on our present state of knowledge and
is intended to provide general notes regarding products and their uses. It should
not therefore be construed as a guarantee of specific properties of the products
described or their suitability for a particular application. Because conditions of
product use are outside Dyneon’s control and vary widely, user must evaluate
and determine whether a Dyneon product will be suitable for user’s intended
application before using it. The quality of our products is warranted under our
General Terms and Conditions of Sale as now are or hereafter may be in force.
General recommendations on health and safety in processing, on work hygiene
and on measures to be taken in the event of accident are detailed in our material
safety data sheets.
You will find further notes on the safe handling of fluoropolymers in the brochure
“Guide for the safe handling of Fluoropolymers Resins” by PlasticsEurope, Box 3,
B-1160 Brussels, Tel. +32 (2) 676 17 32.
The present edition replaces all previous versions. Please make sure and inquire
if in doubt whether you have the latest edition.
Where to go for
more information
Dyneon Customer Service Dyneon GmbH
Dyneon B.V.
3M Advanced Materials Division
Europe
Phone:
Fax:
Italy
Phone:
Fax:
Tunnelweg 95
6468 EJ Kerkrade
The Netherlands
Phone: +31 45 567 9600
Fax: +31 45 567 9619
6744 33rd Street North
Oakdale, MN 55128
USA
Phone: +1 800 810 8499
Fax: +1 800 635 8061
00 800 396 366 27
00 800 396 366 39
800 791 018
800 781 019
Carl-Schurz-Str. 1
41453 Neuss
Germany
Phone:+49 (0) 2131 14 2265
Fax : +49 (0) 2131 14 3857
www.dyneon.eu
Dyneon GmbH
3M Advanced Materials Division
Carl-Schurz-Straße 1
41453 Neuss, Germany
Phone +49 (0) 2131 14 2265
Fax +49 (0) 2131 14 3857
www.dyneon.eu
Dyneon is a 3M company.
Dyneon is a trademark of 3M.
03/2015 All rights reserved.
© Dyneon 2015 | PTFEFP201503EN