Since
its discovery nearly 80 years ago, polytetrafluoroethylene (PTFE), has become
one of the world’s most versatile and useful materials. Originally used for
military applications, it can now be found in many other applications, such as
a non-stick coating for cookware. Approximately half of the PTFE resins
produced today go into aircraft wiring and computer-related products such as
semiconductor fabrication equipment and printed circuit boards. It is also used
extensively in the pharmaceutical and valve industries due to its chemical
inertness and thermal properties.
PTFE
is constructed of carbon and fluorine molecules which, when combined, produce a
compound with a high molecular weight that possesses excellent insulating
properties, resists moisture, can withstand both hot and cold temperature
extremes and have an extremely low coefficient of friction. These properties
make it suitable for numerous applications in the medical device, aerospace,
oil and gas, electronics and chemical processing industries.
Types of PTFE
There are four
general types of granular PTFE material: virgin, modified, reprocessed and
filled. Virgin PTFE, as the term implies, is the purest form of the material.
Modified PTFE is a copolymer resin manufactured by adding a small percentage of
a melt processable fluoropolymer to enhance the final product. Reprocessed PTFE
is made from reclaimed virgin PTFE scraps that have been chopped, cleaned and
pelletized for reuse. Filled or compounded PTFE is virgin or modified material
blended
with various types and proportions of fillers to add strength, abrasiveness,
lubricity, color or other desired characteristics
Virgin
PTFE is excellent for applications requiring high purity, mechanical
performance or superior electrical properties. Premium virgin PTFE resins are
used to make products such as semiconductor equipment components,
pharmaceutical valve liners, gaskets for the chemical industry and components
subject to FDA compliance such as gas-line manifolds, filtration housings and
others. Reprocessed PTFE is typically used for cost savings in applications not
requiring the proprieties associated with a premium PTFE such as flange gaskets.
Modified PTFE offers weldability and improves deformation characteristics. It
also has greater resistance to permeation of chemicals and exhibits a higher
dielectric breakdown voltage.
Fillers
There
are a number of standard fillers for enhancing the performance of PTFE while
maintaining some of its basic properties. Glass fiber is added to increase
compressive strength, rigidity and wear and reduce creep and cold flow in
sealing applications. There is minimal effect on chemical and electrical
properties.
Carbon
increases compressive strength, hardness, wear and load properties, and
provides good chemical resistance. Often, carbon and graphite are combined as
fillers to further increase strength and reduce friction and initial wear.
Carbon fillers can also be added in various loadings to make the product
conductive or static dissipative.
Molybdenum
sulfide increases hardness, rigidity and wear, and like glass fiber has little
effect on chemical and electrical properties. Bronze fillers are often added to
increase hardness, wear resistance, compressive strength and dimensional
stability required for bushing or bearing applications. However, it is not
recommended for corrosive or electrical applications. In addition to these
fillers that enhance mechanical properties, pigments can be added for
identification, visibility or branding purposes.
Molding methods
PTFE
material and products are produced by a variety of molding methods, including
compression, isostatic, automatic compression and granular ram and fine powder
extrusion. Compression molding and extruding granular PTFE are the most common
methods used in forming component parts for machining. Sheets, rods and
cylinders are typically processed using a mold that is close to the finished
part dimensions, minimizing clean-up during the machining process. Cylinders
and rods also can be customized in length to meet component dimensions or to
maximize yield.
In
compression molding, granulated PTFE is poured into a mold and compressed to
yield a sheet, solid rod, cylinder or tube. After removal from the mold, the
material is sintered or cured in an oven. In isostatic molding, granulated PTFE
is poured into a near-net mold surrounded by a rubber bladder. Unified air or
water pressure is then applied to the entire mold to form the near-net shape.
The shape exits the mold in a green state and too must be cured. Unified
pressures applied to all areas of the mold result in highly uniform and
consistent density throughout, which is important when tight tolerances and consistency
are required. This process lends itself to producing more complex components
for machining in the form of tapered sleeves, closed-end cylinders or buckets
and even parts with appendages. Because this process molds parts to near-net
shape, it uses significantly less material than the more conventional use of
blocks and cylinders.
Automatic
compression or auto molding involves a press with a custom mold configuration
to form finished or near-net-shape parts requiring a secondary machining
operation. Granulated PTFE is placed into a single- or multi-cavity mold,
compressed to produce a part and released. Finished and semi-finished parts
produced by this method likewise require curing. This process allows a wide
variety of shapes and geometries to be produced in volume where tight
tolerances are not required, thus minimizing or eliminating the need for
machining. This process also reduces stress defects in parts as well as machine
scrap.
In
granular ram extrusion, granular PTFE powders are poured into a hopper then
extruded through mold with an OD (outside diameter) to produce a rod or an
OD/ID (inside diameter) pin to produce a tube. As the material is forced
through the extruder die, heat and pressure are applied simultaneously, so it
exits the extruder completely cured.
In
fine powder extrusion, the PTFE is blended with a surfactant and compressed
into a small billet or charge, which is then forced through a small orifice and
extruded into PTFE tape, film, tube or custom shape. The material exiting the
die can be either sintered or unsintered depending on the final product or
application.
Sintering/annealing
As
noted above, sintering cures the PTFE, converting it from a compressed or
“green” state to a solid state. Sintering cycles are customized depending on
the thickness and length of the material, type of material (virgin, filled,
modified, etc.) as well as the overall size. The product is typically placed
into an oven in a freestanding state. The oven is heated in a controlled manner
to ensure even transition of the polymer through the melt point of 621°F/327°C.
The maximum temperature is held for a prescribed length of time to ensure
complete bonding of the polymer. The temperature is then brought down to
ambient by a controlled cool down which controls the crystallinity of the
molding. These steps are just as important as the initial compression molding
as they affect the microporosity and crystalline structure of the finished
article.
If
extremely tight tolerances are required for the finished component, annealing
(also known as stress relieving), is often required to stabilize the material.
This process involves placing the sintered material into an oven and applying a
controlled heat to the molding that exceeds the service temperature of the finished
part. The molded shape is held at this temperature for a calculated period of
time, after which the temperature is slowly brought back down to ambient. The
material is not heated past the melt temperature of the polymer during this
process.
Products/applications
There
are many applications in multiple industries for the various forms of PTFE.
Valve manufacturers use machined virgin and filled PTFE valve seats, seals and
O-rings. Depending on the type of valve, the manufacturer may also use machined
virgin or filled PTFE valve liners.
Chemical
companies use molded and skived virgin PTFE sheet to line tanks containing
corrosive materials. Chemical plants also use large quantities of PTFE-based
gaskets.
Semiconductor
equipment manufacturers use virgin PTFE machined components in the equipment
that processes silicon wafers prior to becoming computer chips. These include
gas line manifolds, specialty parts exposed to corrosive materials and seals,
among others.
The
aerospace industry as noted uses large volumes of virgin and pigmented PTFE
tape for insulating aircraft wiring. The pigmentation is primarily for
identification purposes for various types of wiring construction. The
pharmaceutical industry uses large amounts of thin-wall extruded tubing, thin films,
fiber and machined parts made of different forms of PTFE.
In
addition to these products, PTFE is used in the production of flange gaskets
for industrial piping systems, envelope gaskets, machined O-rings and
spring-loaded seals. It is also used for manufacturing films for preserving
historical artifacts and fibers for everything from dental floss to
architectural membranes for stadium roofs.
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