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.
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.
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.
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.
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.
In summary, different types of PTFE are available to meet the performance and economic requirements of a wide range of products and applications. Its unique properties can be enhanced with the addition of fillers, and it can be molded and machined into precision components. In addition, the material has been reformulated to make it more environmentally friendly while maintaining its basic characteristics — the characteristics that made it a miracle material when it was discovered in 1938 and still make it one today.
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