PTFE Paste Extrusion - Zeus Industrial Products, Inc.

TECHNICAL NEWSLETTER
PTFE Paste Extrusion: The Basic Process
Introduction
Some time ago we looked at extrusion and discussed the basics of the extrusion process. The
previous Newsletter though focused on 'screw extrusion', a process similar to that used in a meat
grinder where a rotating screw reduces the size of the meat, mixes it all up, and then extrudes the
result out as a fine stream through the die at the face of the grinder. The other main type of
extrusion is 'ram extrusion' where the best analogy is to that of a tube of toothpaste where
pressure applied to the tube forces the toothpaste out of the hole and onto the toothbrush.
Disc
extrusion
Extrusion
Screw
extrusion
Paste
extrusion
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Ram
extrusion
The basic types of extrusion
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Solid state
extrusion
While the majority of polymer extrusion is carried out using screw extruders, there are some
plastics that simply cannot be extruded using this method. For polymers that have very low
coefficients of friction or are otherwise difficult to extrude because of their high melt viscosity,
screw extrusion is not possible and ram extrusion is used instead.
Plastics such as PE-UHMW (ultrahigh molecular weight polyethylene) and PTFE
(polytetrafluoroethylene) are typical of these non-melt processable polymers and are generally
processed by ram extrusion where the pressures attained are much higher. The basic layout of a
ram extruder is shown below.
Ram extruder
Schematic of a ram extruder
Extrudate
PTFE can be processed in a conventional ram extruder as a granular powder that is compacted
by the ram and forced through a heated die to give the required extrudate. A more common
method of producing PTFE also uses ram extruders but extrudes a PTFE paste as the required
extrudate. This is PTFE paste extrusion and this Newsletter focuses on this process.

Paste extrusion
Paste extrusion of polymers is a fairly simple process and an outline of the PTFE paste extrusion
process is shown.
PTFE granular
powder
Measure and
mix
Pre-form billet
Paste extruder
Dry and sinter
in furnace
Take-off and
winder
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Lubricant
Schematic of the paste extrusion process
PTFE granular powder from the emulsion polymerization process is sieved to remove any large
granules or clumps and to ensure that the particle size is both uniform and small. This granular
powder is spongy as a result of the emulsion polymerization process and this aids with the uptake
of the lubricant.
The lubricant for the paste extrusion is a hydrocarbon extrusion aid, and is tumble blended with
the PTFE granules and any other process additives such as colorants. The amount of lubricant
used will vary by weight depending on the resin and the specific extrusion die. These
components are accurately weighed and blended by a simple mechanical tumbling process (i.e. it
is not carried out under heat). The blended paste is finally pressed into a pre-form billet of the
same dimensions as the extruder barrel and then loaded into the ram extruder barrel.
The ram extruder applies direct pressure to the pre-form billet by mechanical or hydraulic
methods to force the paste through the desired conical die/mandrel combination to form the
desired extrudate. Unlike conventional extrusion, which takes place at high temperatures and
uses a polymer melt, ram extrusion takes place at slightly above room temperature, generally in
the region of 40 o C to 60 o C.
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After extrusion, the extrudate is drawn through a drying/sintering furnace where the processing
aid is driven off by heating the extrudate above the volatilization temperature of the lubricant. The
final extrudate is then coiled, cut or otherwise treated by the necessary downstream equipment.
Paste extrusion is a discontinuous or batch process, and the amount of material that can be
produced is limited by the size of the pre-formed billet which in turn is limited by the size of the
ram portion of the extruder.
Variations on the process
Wire coating
If the application requires wire coating then the same basic process is used except that the wire
to be coated is fed through the billet, into the ram area, and drawn out of the wire coating die as
the extrudate coats it. The complete coated wire is then drawn through the drying/sintering
furnace to complete the process.
Tapes
PTFE insulation and thread sealing tapes are made using the paste extrusion process to produce
an unsintered rod that is calendared and then dried to produce a thin PTFE tape. These products
are not generally sintered after production.
Products, applications and materials
Paste extrusion can be used to produce similar products to those produced by screw extrusion.
Typical products are:
Thin tubing
Wire insulation
Biaxially unsintered stretched films and tapes
Rods
Sections and hollow sections
The applications for paste extruded products are wide and some typical applications are:
Hoses with diameters between 1 and 50 mm for use in aggressive environments
Medical products (hoses and tubes)
Pipe and tube linings
Wire insulation and coatings
Insulation and thread sealing tapes
While pure PTFE is the main material processed by paste extrusion, it is also possible to process
granular PTFE by this method to extend the range of applications.
The last ‘black art’
Paste extrusion is a simple process in concept, but the low volume of material processed in
comparison to screw extrusion means that the science of paste extrusion is not as well
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developed. As a result, the successful operation of the process demands careful processing
technology and experience in the art.
One aspect of paste extrusion that is of more than academic interest is how the final extruded
product stays complete. The extrusion of granules at just above room temperature does not melt
the granules (although it does produce considerable shear forces in the paste), therefore there is
no mixing into a continuous melt as there is with screw extrusion. The mechanism that holds the
paste granules together is thought to be due to the unwinding of PTFE crystallites at the surface
of the granules during extrusion to create fibrils of sub-micron dimensions. The entanglement of
these fibrils effectively creates the solid structure of the paste extrusion. 1 The number and
properties of these entangled fibrils determines the final strength of the extrudate and this is a
delicate balance to be achieved during processing. High pressures are necessary to unwind the
fibrils and to orient them in the extrusion direction. Research continues into the basic mechanism
and the optimum combination of die dimensions, lubricant content, and molecular weight of the
pre-form paste.
Summary
Paste extrusion is a vital part of polymer processing technology. It is very different than
conventional screw extrusion, but it allows for materials that are non-melt processable to be
extruded and is an essential technique in polymer processing. Zeus has been doing PTFE paste
extrusion since its founding in 1966. For over 40 years, the process has been perfected and
honed making Zeus the leader in the fluoropolymer tubing extrusion industry.
How Zeus Can Help
With a technical inside and outside sales force backed up with engineering and polymer experts,
Zeus is prepared to assist in material selection and can provide product samples for evaluation. A
dedicated R&D department staffed with PHD Polymer chemists and backed with the support of a
world-class analytical lab allows Zeus an unparalleled position in polymer development and
customization.
Zeus was built upon the core technology of precision extrusion of high temperature plastics.
Today, with a broad portfolio of engineered resins and secondary operations, Zeus can provide
turnkey solutions for development and high-volume supply requirements.
1 Properties of polytetrafluoroethylene (PTFE) paste extrudates. A. B. Ariawan, S. Ebnesajjad, S.
G. Hatzikiriakos. Polymer Engineering & Science, Volume 42, Issue 6 (2002), Pages 1247 - 1259
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Contact Us
Additional technical support is available by contacting a Zeus technical account manager at:
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Orangeburg, SC 29118 United States
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