Polytetrafluoroethylene (PTFE), better known by its trade name Teflon, has many desirable properties which make it an attractive material for numerous industries. It has good chemical resistance, a low dielectric constant, low dielectric loss, and a low coefficient of friction, making it ideal for reactor linings, circuit boards, and kitchen utensils, to name a few applications. However, its nonstick properties make it challenging to bond to other materials or to itself.
A number of adhesion promotion methods have been developed to enhance PTFE bond strength. The primary methods currently used in industry are sodium etching and plasma etching. Results of ion beam treatment and laser surface roughening have also been reported in the literature, but do not have a significant presence as commercial processes.
Sodium etching
Wetting the surface of PTFE with commercially available solvents and liquid adhesives is virtually impossible. The exception to this is with special halogenated solvents that have a surface energy lower than PTFE, such as 3M's FC series solvents. These 3M solvents are, however, toxic and expensive. Additionally, even if wettability is acceptable, the PTFE will not form hydrogen bonds which are the primary source of adhesion strength. The PTFE surface therefore must be chemically modified to produce a surface which is capable of forming hydrogen bonds.
Early sodium etching solutions
Sodium etching of fluoropolymers has been used for decades to enhance bondability of PTFE. It is performed by immersion of the PTFE in a solution containing sodium followed by rinsing in alcohol and water. The process was originally performed by dissolving sodium metal in liquid ammonia. An alternative method was to form a complex with naphthalene, which was then dissolved in an ether such as tetrahydrofuran (THF). Both types of solutions carry risks to the user – both ammonia and THF are irritants, and both are flammable. At higher concentrations, THF is also a central nervous system depressant. In rats, the inhalation LC(50) (Lethal Concentration which kills 50% of test subjects) is 21,000 ppm for 3 hours. In humans, chronic effects have not been reported, but researchers using THF have developed severe occipital headaches and marked decreases in white blood cell counts.
Newer sodium etching solutions
More recently, glycol ethers (known as glymes) have come into use as carriers for the sodium naphthalene complex for PTFE etching. These glymes are ethylene glycol dimethyl ether (monoglyme), diethylene glycol dimethyl ether (diglyme), and tetraethylene glycol dimethyl ether (tetraglyme). Glymes pose minimal or no health risks to the user, and the solutions do not require special storage conditions. When using glyme-based etchants, it is recommended that the etching process be performed at moderately elevated temperatures, about 50 °C. The elevated temperature causes the etchant to release more active sodium. It also lowers the viscosity of the etchant which enhances wetting of high aspect ratio features such as plated through-holes in printed circuit boards. Tests of diglyme-based etchants used at 50 °C have shown bond strength increases of 50% or more over room temperature etching.
Commercially available etchants today are primarily glyme-based. Rogers Corporation, a manufacturer of PTFE printed circuit board laminates, refers to Poly-Etch and FluoroEtch etchants in its Fabrication Guidelines, "Bonding PTFE Materials for Microwave Stripline Packages and Other Multilayer Circuits". Poly-Etch is a sodium naphthalene complex in tetraglyme, while Fluoro-Etch is a sodium naphthalide complex in diglyme Matheson, the manufacturer of Poly-Etch, also manufactures a monoglyme-based etchant called Poly-Etch W. Fulcrum Chemicals manufactures three different etchants called Natrex25, NatrexHighFp and Natrex64.
Sodium etching mechanism
The main effect of sodium etching is defluorination of the PTFE, stripping the fluorine molecules from the carbon backbone of the polymer. The fluorine-to-carbon atomic ratio (F/C ratio) is reduced from PTFE's theoretical ratio of 2.0 to 0.2 or less, after exposure to sodium naphthalene for 1 minute. The fluorine atoms are replaced with hydroxyl, carbonyl, and other functional groups which can form hydrogen bonds.
Topographically, chemical etching of PTFE with sodium results in a highly porous defluorinated layer. Superficially, it displays a characteristic "mud crack" appearance.