Top 5 Benefits of Fluoropolymer Tubing
...and fluoropolymer chemistry you can impress your boss with
When choosing the appropriate material for fluid transfer, the material choices can be overwhelming. From flexible PVC to silicone rubbers to fluoropolymers, each material has distinct characteristics and advantages. But fluoropolymer tubing has five advantages unique to this family of materials.
Advantage #1: Excellent Chemical Resistance
Polytetrafluoroethylene (PTFE), fluorinated ethylene propylene (FEP) and perfluoroalkoxy alkane (PFA) fluoropolymers are fully fluorinated and generally considered to be chemically inert except in a few extreme situations. At a basic level this is because the carbon-fluorine bonds that make up perfluoropolymers (aka fully fluorinated polymers) are very difficult to cleave by either thermal or chemical methods. This property results in a product impervious to strong acids, strong bases, organic solvents, d-limonene, gasoline…you get the picture.
Exceptional chemicals which attack fluoropolymers under standard conditions include alkali metals such as elemental sodium, potassium and lithium, and aggressive oxidizers such as fluorine gas. At extreme temperature and pressure conditions other reagents such as concentrated sodium or potassium hydroxide, ammonia, metal hydrides, aluminum chloride and some amines and imines have been shown to react with fluoropolymers. However, these temperatures are at or close to the upper continuous use temperature and are unlikely to be an issue in the majority of applications.
In some cases, the reaction of fluoropolymers with various reagents may be beneficial. For example, elemental sodium is often used in combination with ammonia or naphthalene to etch the surface of fluoropolymers to promote adhesion. The etch solution is used to treat the surface and does not affect the bulk properties of the fluoropolymer in these highly controlled commercial reactions.
Partially fluorinated polymers such as ethylene tetrafluoroethylene (ETFE) and polyvinylidene fluoride (PVDF) are more complicated when it comes to chemical resistance. The incorporation of hydrogen into the backbone results in weaker carbon-hydrogen bonds and increased polarity which is also impacts chemical resistance. While ETFE and PVDF are isomers (same atoms in a different arrangement), there is a drastic difference in polarity due to structure of the alternating C-H2 and C-F2 groups. In ETFE, this alternation occurs every two carbons (-CF2-CF2-CH2-CH2-), while in PVDF it occurs every carbon atom (-CF2-CH2-CF2-CH2). This results in PVDF being highly polar and susceptible to dissolution in common ketones. Beyond ketones, PVDF is chemically resistant under ambient and elevated temperatures to many common solvents. It is important to understand the conditions, including temperature, concentration and continuous use versus short-term use, when considering PVDF in an application.
On the other hand, ETFE is not impacted by ketones and is considered inert to strong mineral acids, inorganic bases, halogens and metal salt solutions. Classic polymer solvents such as ethers, alcohols, aromatic and aliphatic hydrocarbons and others have very little effect on ETFE polymers. ETFE is impacted by very strong oxidizing acids at high concentration as well as strong organic bases to varying degrees. It is important to follow guidelines for maximum use temperature for a given chemical with ETFE.
Advantage #2: Wide range of operating temperatures
Perfluoropolymers have excellent thermal stability due to their high degree of crystallinity and their comparably high molecular weights. This results in higher melt temperatures and thus higher continuous service temperatures. For example Versilon™ FEP tubing has a maximum recommended operating temperature of 200oC, whereas Versilon™ C-210-A polyurethane tubing is only recommended up to 93oC. Both Versilon™ PTFE and Versilon™ PFA tubing have operating temperatures up to 260oC, even higher than silicone tubing such as Versilon™ SPX-50 at 204oC.
Perfluorinated polymers also have great low temperature performance, including in cryopreservation applications. The majority of thermoplastic tubing becomes brittle and cracks between 0°C and -100°C, but fully fluorinated fluoropolymers maintain flexibility and impact resistance down to liquid nitrogen storage temperatures of -196°C. Versilon™ FEP has a minimum operating temperature of -250°C, well below cryogenic storage temperatures.
Advantage #3: Excellent resistance to permeability
Permeability is a critical property for a wide variety of applications across a broad range of materials and penetrating molecules. Fluoropolymers have very low water vapor transition rates and are less permeable to other gases such as CO2 and oxygen when compared to many thermoplastics on a thickness for thickness basis. Because the vast majority of fluoropolymers are semi-crystalline, the individual polymer chains tend to be more organized and more tightly packed in the crystalline regions. The organization of the denser crystalline regions prevents even small molecules such as helium, water or carbon dioxide from passing through. Permeation is limited to the less dense, randomly organized amorphous regions.
Polymer permeability can be impacted by a number of factors including: the size and physical state of the penetrating molecule (liquid or gas), morphology and other polymer properties, solubility of the permeant in the polymer, humidity, and the presence of fillers and plasticizers in the polymer. Permeability will also be impacted by temperature, pressure, contact area and thickness of the tubing. A higher temperature and pressure, as well as larger contact area, will result in higher permeability. In addition, thicker tubing will be less permeable than thinner tubing.
Advantage #4: High Purity
Additives and processing aids are an important factor to consider when considering chemical resistance. The interaction of additives with chemical reagents can result in leachables from polymer tubing which can change the polymer properties as well as contaminate a process. Because many fluoropolymers are inherently high in purity, extractables and leachables in chemically aggressive environments are extremely low.
Advantage #5: Low Friction
The shielding of the polymer backbone by the fluorine atoms directly contributes to the lowest coefficient of friction of any polymer and very low surface energy. This means that fluids pass through fluoropolymer tubing and hoses more easily, requiring less energy to pump. Below is a table summarizing the dynamic coefficients of friction for various fluoropolymers compared to steel.
So we see that fluoropolymers have several unique properties that make them uniquely suitable for demanding applications. But as the old adage goes, you can have anything but not everything you want. This applies to polymers as well. Here are the main disadvantages of fluoropolymer tubing.
Disadvantage #1: Cost
Fluoropolymer tubing tends to be more expensive than other thermoplastic tubing. This is due to higher raw material costs and complex processing conditions. Therefore, fluoropolymer tubing is generally selected for use in applications where multiple extreme properties are required and no other polymer will meet the performance standards. For example, fluoropolymer tubing is frequently used in semiconductor fabrication because both chemical resistance and high purity are required.
Disadvantage #2: Mechanical Properties
For some applications, rigidity is a problem. Fluoropolymers are often quite quite rigid when compared to more flexible thermoplastics and silicone rubber. Versilon™ FEP has a Shore D hardness of 55-66 whereas Versilon™ SPX-50 silicone has a hardness of 50 Shore A, which is much softer and more flexible. Reduced flexibility translates into a higher minimum bend radius and stiffer handling. Single-layer fluoropolymer tubes do not work well in peristaltic pumping applications due to the high force required to compress the tubes and the risk of tube rupture due to this rigidity.
In other applications, the perfluoropolymers have lower strength than engineering polymers and partially fluorinated polymers because their relatively nonpolar nature limits the attraction between individual polymer chains. For this reason, the partially fluorinated polymers have better mechanical properties but reduced chemical resistance. The addition of the hydrogen functionality increases polarity because hydrogen is less electronegative than fluorine, so individual chains are more attracted to each other. This serves to increase stiffness and tensile properties and often even translates to better barrier properties. It is vitally important to understand the trade-off of improved mechanical properties at the cost of chemical resistance, flammability and surface energy in order to select the best material for the application.
With those advantages and disadvantages listed, we can summarize the pros and cons of each polymer family.
PTFE Tubing Characteristics
PTFE chains form rod-like helical structures, resulting in a polymer that is highly crystalline with a fluorine sheath surrounding the polymer backbone. The strength of the carbon-fluorine bond results in a polymer with excellent stability at high temperatures and also prevents long-chain branching, allowing polymerization of a linear polymer with very high molecular weight. The very high molecular weight results in reasonable mechanical properties despite the lack of chain entanglement or polar interactions between chains.
PTFE can be polymerized with less than 1% of a comonomer modifierand still be classified as PTFE under ISO 12086. The addition of comonomers allows for improved physical properties at a lower molecular weight and improves processing in some applications. Fine powder PTFE is used to manufacture tubing by combining with a lubricant and paste extruding at low temperatures. The lubricant is evaporated and the PTFE sintered to form the final tubing product.
Comparison of Versilon™ fluoropolymer products
PFA Tubing Properties
Perfluoroalkoxy alkane (PFA) is a comonomer of TFE and a perfluoroalkoxy vinyl ether (PAVE), typically perfluoropropylvinyl ether (PPVE). Perfluoromethylvinylether (PMVE) is copolymerized with TFE to make fluoropolymers classified as methylfluoroalkoxy polymers or MFA . The addition of a higher level of comonomer (>1 % by weight) allows the PFA to be melt processed, while still retaining the chemical resistance and thermal stability of PTFE. PAVE comonomers are more efficient at breaking up crystallinity than other fluoromonomers such as HFP, so less is required to achieve the desired physical properties. .
PFA offers the heat resistance of PTFE combined with the clarity and strength of FEP tubing, perhaps making it the highest performing perfluoropolymer. PFA is one of the more expensive fluoropolymer resins due to the cost of the PAVE monomer as well as small batch production
High purity PFA is typically used in the manufacturing process of silicon wafers in the semiconductor industry. Semiconductor processes are sensitive to contamination from metal ions, so high purity PFA with very low metal content is ideal for this application. The high purity PFA grades are treated via a fluorination process after polymerization to remove reactive end groups that may still remain on the polymer. Versilon™ HP PFA 400 is available for high purity requirements such as semiconductor manufacturing.
Comparison of Versilon™ fluoropolymer products
FEP Tubing Properties
Fluorinated ethylene-propylene (FEP) tubing is among the most popular fully fluorinated material with application in a wide range of industries. FEP is a melt processible copolymer of TFE and hexafluoropropylene (HFP), with comparatively good flexibility and clarity at a lower price than PFA. However the tertiary carbon at the branch point in HFP is less stable than other carbons due to steric effects which reduces the upper continuous use temperature relative to PTFE and PFA. FEP melts around 260°C and can be used continuously in applications up to 200°C. FEP maintains the excellent low temperature properties of other fluoropolymers, maintaining flexibility at temperatures as low as -250°C. For this reason, FEP is used in cryopreservation applications with liquid nitrogen at -196°C.
Comparison of Versilon™ fluoropolymer products
But what gives fluoropolymers unique properties? Let’s start with a history and delve deeper into particular polymer properties.
Overview of Fluoropolymers
The unique properties of fluoropolymers arise from the nature of the carbon-fluorine bond. The extreme difference in electronegativity between carbon and fluorine results in the formation of a strong chemical bond that is extremely difficult to sever. The electron withdrawal towards the fluorine atom in the C-F bond has the added benefit of strengthening the carbon-carbon bonds that make up the polymer backbone by pulling the electron density away from the C-C bonds Additionally, the fluorine electronegativity compels the atoms to repel one another which in turn forces the polymer configuration into a rod-like helical structure. Because the fluorine atoms are smaller than carbon atoms, this results in a fluorine sheath uniformly surrounding the C-C backbone. Fluoropolymers are therefore hydrophobic (insoluble in polar solutions), non-stick and lipophobic (insoluble in non-polar solutions).
PTFE was accidentally discovered in 1938 by DuPont chemist Dr. Roy Plunkett. Plunkett was experimenting with gases for new fluoro-refrigerants when he discovered a white waxy substance in a canister of frozen, compressed tetrafluoroethylene (TFE). The TFE had spontaneously polymerized into what is now known as polytetrafluoroethylene or PTFE. The Teflon™ brand name was registered in 1945 by DuPont and early uses included pipe and seal coatings to protect against aggressive chemicals. Teflon™ quickly became a household name with the advent of non-stick Teflon™-coated pots and pans. . Melt processible fluoropolymers started with the commercialization of fluorinated ethylene propylene (FEP) in 1960, quickly followed by ethylene tetrafluoroethylene (ETFE) in 1970 and the perfluoroalkoxy (PFA) resins in 1972.
The chemistry of commercial fluoropolymers ranges from fully fluorinated to partially fluorinated varieties. Partially fluorinated polymers are produced by incorporating hydrogen- or chlorine-containing monomers into the polymer chain. Replacing fluorine with hydrogen or chlorine results in some improvement in properties relative to perfluoropolymers including stiffness, tensile strength and barrier performance but at the expense of other properties. Fully fluorinated polymers have better chemical resistance, electrical properties, flammability and non-stick properties when compared with partially fluorinated polymers.
Fluoropolymers may be blended with other materials to provide additional functionality. Fluoropolymers are commonly filled with carbon in applications where antistatic properties are required. PTFE in particular is filled with glass in valve and seal applications to increase hardness and wear resistance. Blends with various metals such as bronze or stainless steel can be made to increase hardness and thermal conductivity while still maintaining chemical resistance.
Monomer Production – precursors to the plastic
Production of fluoropolymers begins with the monomer tetrafluoroethylene (TFE), the basic component of most commercial perfluorinated polymers. TFE is synthesized by pyrolysis (heating) of chlorodifluoromethane (CHClF2 aka R-22 refrigerant) in which hydrochloric acid (HCl) and CF2 radicals are produced. The CF2 radicals combine to produce TFE (C2F4). The major fluoropolymer manufacturers are fully integrated and use fluorspar (CaF2), hydrofluoric acid, and chloroform as the starting ingredients. Fluorspar is used to generate HF and chloroform is the basis for R-22 preparation. The synthesis of TFE from fluorspar follows the reaction scheme below.
Polymerization of TFE to high molecular weight, particularly in PTFE, requires extremely pure monomer, so TFE monomer is typically purified to 99.99995%. Before polymerization, TFE is inhibited by a selection of oxygen scavengers to prevent autopolymerization in the presence of oxygen.
Hexafluoropropylene (HFP) is one such monomer copolymerized with TFE. This is the basis for production of FEP. HFP monomer (CF3CF=CF2) is produced as a byproduct of the pyrolysis of R-22 in TFE production when TFE combines with CF2 radicals.
PAVEs are a family of monomers that when copolymerized with TFE at sufficient levels, result in PFA polymers. PAVEs are generally produced from hexafluoropropylene oxide (HFPO) which is generated from HFP (the comonomer used in FEP production). A perfluorinated acyl fluoride (RfCOF) is reacted with HFPO to produce PAVE monomers. The selection of the “Rf” group determines the PAVE monomer side chain length, and when incorporated into a fluoropolymer chain, different side chain length can have a significant impact on polymer properties, particularly stress crack resistance. PAVE monomers are historically much higher price than HFP due to the number of processing steps required to produce and purify the monomer, resulting in PFA being more expensive than FEP or PTFE.
The most commonly used PAVE is perfluoropropyl vinyl ether ( PPVE) with a three-carbon perfluorinated side group. Methyl-, ethyl- and butyl- PAVEs have also been used. Increasing the length of the side chain has more impact on the crystallinity for a given amount of comonomer. For these reasons, PFA polymers are less modified than FEP and retain higher melt and continuous use temperatures.
Polymerization of Fluoropolymers
TFE is the main monomer for PTFE production. In some cases, less than 1 wt% of other monomers can be added to the PTFE structure to modify properties and processing. Commonly used comonomers in PTFE include HFP, PPVE, and perfluorobutyl ethylene (PFBE). TFE is polymerized in water with an initiator and surfactant plus other additives depending on the process and manufacturer. There are two routes to commercial production of PTFE: suspension polymerization and emulsion polymerization. Suspension polymerization yields granular PTFE through aqueous polymerization with little to no surfactant and vigorous agitation. Emulsion polymerization on the other hand produces fine powder and dispersion PTFE through mild agitation with significant surfactant levels and paraffin wax as a stabilizer. Fine powder PTFE grades are used in the production of PTFE tubing.
Melt processible fluoropolymers such as FEP and. PFA incorporate higher percentages of comonomers with bulky side chains that significantly reduce the crystallinity as well as the melting temperature. The reduction in crystallinity allows for good physical properties to be achieved at molecular weights one to two orders of magnitude lower than PTFE. This correlates to significantly lower viscosity, allowing for processing via typical polymer melt extrusion methods.
PTFE Fine Powder
Polytetrafluoroethylene (PTFE) is a homopolymer of tetrafluoroethylene (TFE) that is available in three forms – fine powder, dispersion and granular. PTFE homopolymers are composed exclusively of TFE monomer and have to be polymerized to very high molecular weight to achieve reasonable physical properties. Low levels of fluorinated comonomers are introduced into the PTFE backbone (<1wt%) to achieve reasonable physical properties at lower molecular weight. PTFE cannot be processed using convention polymer melt extrusion techniques due to the high melting temperature and extremely high viscosity (1012 Poise at 380°C).
Aqueous dispersion polymerization is used to produce both PTFE dispersion and PTFE fine powder. In general, the dispersion polymerization process involves large quantities of surfactant with mild agitation at elevated temperature and pressure. In this case, polymerization of the TFE occurs in the aqueous emulsion media, and the polymer remains stable in the emulsion. The aqueous phase contains water, surfactant, initiator and wax. Polymerization is typically conducted at temperatures ranging from 50°C to 85°C at high pressure. Relatively slow agitation speed is maintained throughout the polymerization to keep the aqueous phase saturated with TFE.
Lower molecular weight PTFE fine powders can be produced by adding chain transfer agents such as methane or incorporating a comonomer modifier, i.e. HFP or PPVE. Modifiers may be introduced to the polymerization process at any time to control molecular weight and generate different final properties. For example, a comonomer can be added after 70% of the TFE is consumed during a reaction, generating PTFE particles with high molecular weight PTFE cores and low molecular weight PTFE shells. In this process a very small percentage of the modifier is added, but this can have a dramatic impact on the polymer properties.
Fine powder is produced from the dispersion polymerization process by coagulation of colloidal particles, separation of agglomerates from the aqueous phase and drying the agglomerates. Coagulation is achieved by diluting raw dispersion to 10 to 20% solids by weight and adjusting pH to neutral or basic. A coagulating agent such as a water soluble organic compound, inorganic salt or inorganic acid may be added to boost the coagulation process. PTFE agglomerates formed from primary particles are isolated by skimming or filtration, then gently dried at 100 to 180°C to yield fine powder PTFE. This fine powder PTFE is then processed via paste extrusion to generate final products such as PTFE tubing.
When comonomers such as HFP and PPVE are added in higher concentrations, in the range of 3 to 12% by weight, melt processible fluoropolymers such as FEP and PFA are synthesized. PFA polymers are produced by copolymerization of TFE with PAVE monomers, typically PPVE. Several percent of PPVE is incorporated to produce a melt-processible copolymer. Typical PFA manufacturing processes use an aqueous process with TFE and PPVE copolymerized in water with a water-soluble initiator and surfactant.
Generically, temperature controlled reactors are charged with water and heated with agitation.The reactor is pressured with TFE and evacuated to remove oxygen. A chain transfer agent such as ethane is then added to the reactor in addition to PAVE monomer and surfactant. The temperature is then increased to the reaction temperature at this point, and the reactor is pressurized with TFE. Once TFE is added, the reaction is ready to “kick off.” This process is started by adding an initiator to the kettle. TFE pressure is maintained through the reaction by continually pumping TFE, and the PAVE monomer is also pumped in for the duration of the reaction.
The final product of this reaction is a dispersion of PFA particles in aqueous media. In order to obtain PFA “fluff”, the dispersion is coagulated using chemical or mechanical means. The PFA solids are separated and excess water is removed by pressing and drying. This PFA powder is then melt extruded to produce the PFA pellets that Saint-Gobain then converts to tubing through the melt extrusion process.
FEP is a copolymer of TFE and HFP that is produced by a free-radical polymerization process similar to that described for PFA. The emulsion polymerization process often uses a surfactant to combat the insolubility of FEP and requires an initiator to kick off the reaction. The resulting FEP dispersion is then coagulated and dried, and the FEP fluff is melt extruded to generate the FEP pellets used by Saint-Gobain to produce tubing.
End group stabilization
“End groups” are naturally created by the polymerization process resulting from the initiator, chain transfer agent, solvents, contaminants or post-polymerization handling. The instability generated by these end groups is not noticeable in PTFE where both the viscosity and molecular weight are extremely high. However, the perfluoro copolymers have much lower molecular weight and as such many times more terminal groups. These groups can cause issues during high temperature melt processing between 300 and 400°C, resulting in the formation of gasses and bubbles. FEP and PFA are often stabilized by the manufacturer through various post-processing methods to impart improved thermal stability, particularly for high purity and wire and cable applications.
Processing and shaping of fluoropolymer tubing
Fluoropolymer tubing is manufactured using two different methods depending on the polymer properties: melt extrusion and paste extrusion.
Melt extrusion is used for polymers such as fluorinated ethylene propylene (FEP) and PFA which are melt processed. The pellets of FEP or PFA resin are melted in a single-screw extruder and then pumped through a die that forms the material into its final shape. The high processing temperatures used in fluoropolymer extrusion result in an environment that can be quite corrosive, so processing machinery in contact with the fluoropolymer is commonly made of high-nickel alloys such as Inconel® or Hastelloy®.
One advantage of melt extrusion is that the continuous processing allows for tubing of any length to be produced without splicing.
Because of the high melt temperature and extremely high molecular weight and viscosity of PTFE, it cannot be processed using conventional polymer melt extrusion processes. Processing of PTFE more closely resembles ceramic processing with cold pressing, paste extrusion and sintering techniques. First , PTFE resin is blended with a lubricant and compressed into a billet or a preform. The preform is then loaded into a ram extruder and pushed out through a die that determines the PTFE product geometry. The extruded tubing is considered “green” until it is passed through ovens to drive off the lubricant and then sintered well above the melting temperature to fuse the PTFE particles. Because paste extrusion is a batch process (each batch requires one discreet billet of material), lengths of PTFE tubing are limited based on the preform size, the diameter of the tubing, and the stroke length of the extruder. If longer lengths are desired, shorter tubes must be spliced together.
Coextrusion - optimization of properties
Because fluoropolymers are typically stiffer and more expensive than other polymers, multi-layer tubing products may be beneficial in some applications in which flexibility is required and the aggressive fluid only contacts the inner surface of the tube. During manufacturing, a polymer layer is extruded on top of a fluoropolymer inner liner. The more flexible outer jacket of the tube allows the tube to be easily connected in tight spaces while the inner fluoropolymer liner continues to provide chemical resistance. Two examples of multi-layer fluoropolymer products are Versilon® SE-200, a clear and flexible multi-layered tube and Versilon™ Duality, a translucent multi-layered tube with higher pressure ratings. Because of the inert nature of the fluoropolymer liner, the inner layers of these products are not chemically bonded to the outer jacket so some care must be taken in choosing appropriate fittings.
The general stiffness of fluoropolymer tubing can also be overcome by adding corrugations or helical convolutions to the tube. These shape modifications allow greater flexibility while maintaining the chemical resistance and heat resistance expected of fluoropolymer tubes. The disadvantage of this approach is that the inner surface is no longer continuously smooth. Examples of fluoropolymer products whose shapes are modified for flexibility are Versilon™ CT-Flex corrugated tubing, Versilon™ Convoflex helically convoluted tubing, and Versilon™ CON-T helically convoluted tubing with a wire reinforcement for added pressure and vacuum capabilities.
Bonding to Fluoropolymers
Because fluoropolymers are often bonded to other materials such as metal rolls or other thermoplastics, it is often beneficial to chemically modify or “etch” one surface to allow better adhesion to a substrate. Because one of the few chemicals that attack fluoropolymers are highly reducing agents such as elemental metals, anhydrous solutions containing elemental sodium are commonly used to etch fluoropolymers. The etch solution attacks only the surface of the fluoropolymer, leaving the bulk fluoropolymer unaffected. During the etching process on an extruded tube the fluorine ions are removed from the polymer backbone and replaced with more reactive or “sticky” species. In some applications, such as roll covers, the inner surface of a tube is etched. In other applications, the outer surface is etched so that another material may be bonded to the tube exterior.
Saint-Gobain has a selection of standard fluoropolymer tubing in inventory, ready to ship. We also specialize extrusions which are made with reasonable minimum order quantities. Contact Us today to discuss your application requirements!
Inconel® is a trademark of the International Nickel Company, Inc.
Hastelloy® is a trademark of Haynes International.