Etching: How to Bond PTFE
PTFE is a critical material used in countless applications across a number of industries. Its unique physical, chemical, and dielectric properties make it ideal for use in chemical processing equipment, aerospace electronics, semiconductor processing, medical and pharmaceutical equipment, and other applications. However, the lubricity of PTFE makes it difficult to bond—either to itself or another surface.
Despite this challenge, most industrial grade adhesives will readily adhere to PTFE after the material has been chemically etched. Chemical etching fundamentally alters the surface of the material to permit adhesion without sacrificing PTFE's most valued characteristics.
- Alkali metals
- A group of elements consisting of lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr). Molten alkali metals are one of the few substances that react with PTFE. The chemical reaction between PTFE and alkali metals defluorinates the PTFE.
- Unit of measure equivalent to one ten millionth of a millimeter.
- Coefficient of friction
- A dimensionless number representing the lubricity of a material. A lower coefficient of friction means that a material is more slippery. Usually measured against polished steel.
- The removal of fluorine from a substance. In the etching process, defluorination specifically refers to fluorine atoms being removed from PTFE molecules, which leaves a surface composed mostly of electron-deficient carbon atoms.
- The slipperiness of a material. Materials with higher lubricity are more slippery than others. In the plastics industry, this specifically refers to the material's coefficient of friction.
One of the Slipperiest Substances on Earth
The most well-known property of PTFE is its slipperiness, which is referred to as lubricity or low coefficient of friction in technical literature. There's good reason for this, given that PTFE is one of the slipperiest substances yet discovered, with a static coefficient of friction ranging from 0.08–0.10 and a dynamic coefficient of friction ranging from 0.06–0.08. In fact, PTFE is so slippery that insects cannot climb it and PTFE is the only known surface to which geckos cannot stick.
PTFE's low coefficient of friction is due to its chemical structure. PTFE molecules are composed entirely of carbon and fluorine atoms linked together in very strong covalent bonds. The backbone of the PTFE molecule is a chain of carbon-carbon bonds and carbon-fluorine bonds. Due to the size of fluorine atoms, the carbon chain backbone of the molecule is completely covered in a fluorine sheath. It is the fluorine sheath that is responsible for the lubricity of PTFE.
Applications Requiring a Low Coefficient of Friction
Many applications rely on PTFE's low coefficient of friction. Perhaps the application that first comes to mind is the use of PTFE in non-stick coatings for cookware. Of course, PTFE is not only useful in the home—it is often the material of choice for industrial food processing equipment due to its non-stick properties. Aside from the food industry, PTFE is a common material for low-friction bearings and seals. Its low coefficient of friction also makes PTFE an excellent material for fluid handling systems, where it provides increased flow rates.
Although the lubricity of PTFE is beneficial in a wide range of applications, it poses a challenge when the PTFE needs to be adhered to another surface. Simply put, adhesives cannot form a strong, lasting bond with an untreated PTFE surface.
Other Properties of PTFE
In addition to its well-known property of lubricity, PTFE has several other properties that make it valuable in various applications. Many of these applications require the PTFE to be bonded to itself or another substrate. In these applications, lubricity is less important and can be sacrificed (at least on one surface) to take advantage of PTFE's other properties.
PTFE is resistant to most chemicals, with the notable exception of alkali metals, which defluorinate the material. Additionally, exposure to chlorine trifluoride, oxygen difluoride, gaseous fluorine, and halogens can cause chemical reactions, especially at high temperatures and pressures. Aside from these exceptions, PTFE offers exceptional chemical resistance and is often used as a liner for pipes and tanks in applications involving acids and alkalis.
The chemical inertness of PTFE lends to its biocompatibility. High-purity medical grade PTFE is used in a variety of medical applications, including stents, grafts, implants, catheters, surgical instrument coatings, centrifuges, and laboratory equipment. PTFE is also valuable in pharmaceutical applications, where it is used to line vessels, mixer shafts, chutes and other processing equipment.
PTFE exhibits excellent dielectric properties, including high dielectric strength and a low dielectric constant. These properties, combined with PTFE's resistance to relatively high working temperatures, good fire performance, and resistance to property changes over time, make PTFE a preferred electrical insulator in many applications. PTFE is particularly valued as an electrical insulator in the aerospace industry, where the chemical resistance of PTFE makes it impervious to hydraulic fluid and other chemicals it may encounter.
High Working Temperature
In addition to its other properties, PTFE offers excellent performance at high temperatures. The maximum continuous working temperature of PTFE is 260° C (500° F), but the material can withstand even higher temperatures for limited periods. Because of this, PTFE is a common material choice for food processing equipment (for example, oven conveyors) and other applications that require high heat resistance.
In light of the above, it is clear that although the lubricity of PTFE is prized in many applications, it is completely unnecessary in many others. In these applications, lubricity is more of a challenge to overcome than a valuable feature of the material.
How to Bond PTFE
The two most common methods of bonding PTFE are the use of contact adhesives and chemical etching. Each method has its advantages and disadvantages, but ultimately the application will determine which method is suitable.
The simplest means of bonding PTFE is with contact adhesives—most often in the form of pressure-sensitive adhesives. Proper contact adhesives will adhere to the surface of the PTFE, but the bond is very weak and often short-lived. Roughing up the surface of the PTFE may slightly enhance adhesion, but not to any appreciable extent. Nevertheless, pressure-sensitive adhesives (usually in the form of PSA-backed tape) are suitable for certain applications where load and wear are not critical concerns.
Since the lubricity of PTFE is the result of its chemical structure—a fluorine sheath covering a chain of carbon atoms—an alteration of this chemical structure is necessary to attain true adhesion with a strong bond. This can be achieved by chemically etching one or more surfaces of the material. In chemical etching, the PTFE is brought into contact with sodium naphthalene (or another etchant), by either dipping the PTFE into a tank or applying the etchant to the surface of the PTFE. As sodium naphthalene is derived from sodium, an alkali metal, it chemically reacts with the PTFE, removing the fluorine atoms from the PTFE molecules. (See Chemical Resistance above.)
Because the fluorine sheath is responsible for the lubricity of PTFE, removing most of the fluorine from the surface essentially removes this property, allowing the surface to be adhered to virtually any substrate.
Does Etching Alter the Properties of PTFE?
Given that etching fundamentally alters the chemical composition of PTFE, it is reasonable to ask how this will affect the properties of the material. However, chemical etching only affects the surface of PTFE to a depth of a few angstroms. For those readers who are unfamiliar with this unit of measure, an angstrom is one ten millionth of a millimeter, so the layer of material affected by etching is incredibly shallow. This means that the bulk of the material is unaffected by etching and therefore retains its normal properties.
Appearance of Etched PTFE
Once PTFE has been chemically etched, the etched surface will appear light tan to dark brown in color. The coloration of the etched surface may not be uniform. For example, it may have a streaked or blotchy appearance. This does not indicate that the chemical etching process was ineffective or poorly performed. In fact, visual inspection of etched material is not a reliable indicator of adhesion strength after bonding. The preferred method of determining etch quality is through a contact angle test.
Selecting an Adhesive for Etched PTFE
There is no universally recommended adhesive to use on etched PTFE. Instead, we generally recommend using whichever adhesive is recommended for use on the non-PTFE substrate. Most industrial grade adhesives, including epoxies, polyurethanes, and cyanoacrylate, will bond well with the etched surface of the PTFE.
Proper Storage of Etched PTFE
The etched surface of PTFE is susceptible to degradation over time—particularly when exposed to UV light—and only has a shelf life of a few months. For this reason, etched PTFE should be stored in a dark place or in UV-blocking bags. Etched PTFE should also be kept away from air and moisture as much as possible prior to bonding it.
Availability of Etched PTFE
Etched PTFE is available both as a stock item and etched-to-order. Due to the relatively short shelf life of etched PTFE, we generally only stock a limited quantity of the most popular sizes of etched sheets. For all other etching requirements, Vital Polymers sends material to a contract etcher for surface treatment. The turnaround time for etching varies, but is generally only a few weeks.
Etched PTFE Sheets
PTFE sheets can be etched on one or both sides, depending on the requirements of the application. Vital Polymers stocks a selection of etched sheets from .030–.250 inches thick in both virgin and 25% glass-filled PTFE. Other thicknesses of etched sheet are available upon request.
Etched PTFE Film
Like PTFE sheets, PTFE film can be etched on one or both sides. Due to the short shelf life of etched material and the wide variety of film thicknesses and custom-slit widths available, Vital Polymers does not keep any etched films in stock. However, we are able to fulfill most custom etched film orders within a few weeks.
Etched PTFE Tube and Rod
Etched PTFE tube and rod are less common than etched sheet and film, but are available for custom order. Generally, only a small area around the end of the tube is etched so that it can be bonded to a part or vessel. Of course, other applications may have different etching requirements and these can be accommodated as well.
Etched PTFE Parts
Sometimes etching is required on one or more surfaces of PTFE parts so that the part can be bonded to other pieces of an assembly. Lead times for etched parts are highly dependent on the complexity of the part and the quantity ordered.
While untreated PTFE can be difficult or impossible to bond, chemical etching is a readily available process that enables strong adhesion, broadening the number of applications in which PTFE can be used. If you have any questions about etching or other technical issues related to PTFE, Vital Polymers is always happy to assist.
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