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.
Several surface modification techniques have been developed to promote adhesion with PTFE and increase the strength of bonds. 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.[1] 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.[1]
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 sodium naphthalene (a sodium 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.[1]
Plasma etching
In plasma etching (corona treatment, plasma activation), the PTFE is exposed to plasma, an electrically charged gas. Various gases may be used to generate the plasma.
Like chemical etching, plasma etching also defluorinates the PTFE, though not to the same degree. F/C ratios drop from 2.0 to 1.4 with an argon plasma, and to 1.8 with an oxygen plasma,[6] and to 0.7-0.8 with an ammonia or hydrogen plasma.[8][10]
Topographically, plasma treatment changes the surface morphology, with different morphologies resulting from different plasma gases used.[6]
Contact angle decreased with treatment by some, but not all, plasma gases – in one study, argon plasma decreased the contact angle from about 105 degrees to 30 degrees after 1 hour of treatment, but oxygen plasma did not affect the contact angle.[6]
Comparison between chemical etching and plasma etching
Despite similar failure mechanisms in both sodium-etched and plasma-etched samples, sodium etching produces much higher bond strengths than plasma etching. Sodium-etched samples exhibited 4 to 5 times the strength of plasma-etched samples when tested in tension per ASTM D4541.[8][10] When tested in peel, sodium-etched samples exhibited 3 to 12 times the peel strength of plasma-etched samples, depending on the type of plasma used.[6]
One proposed explanation for the large difference in bond strengths is that chemical etching modifies the PTFE to a greater depth than plasma etching, increasing the tortuosity of the fracture path through the etched layer during adhesion testing.[7] Another explanation for the large difference in bond strengths is that, in addition to defluorination, sodium etching results in cross-linking which may stabilize the modified PTFE interface, while plasma etching may cause chain scission (breakage of the PTFE polymer chain), since the C-C bond is weaker than the C-F bond.[10]
Other PTFE surface treatments
Ion beam and laser treatments have also been studied as methods to improve PTFE adhesion. However, neither of these treatment modalities appears to be in use commercially.
Ion beam-treated PTFE exhibits significantly greater surface morphology changes than either chemical etching or plasma etching.[6] Ion beam treatments with pure argon or pure oxygen result in minimal defluorination as determined by F/C ratio. Contact angle actually increased with ion beam treatment.[6]
Peel strength with ion beam treatment increased as a function of the ion beam dose, achieving higher peel strengths than plasma-treated samples at doses above 5E15 ions/cm2.[6]
The primary effect of ion beam treatment therefore is morphology modification, with little chemical effect. Longer ion beam treatment time is assumed to increase surface area for bonding, which in turn increases peel strength.[6]
Laser surface roughening of PTFE has also been studied as a potential method for increasing bond strength to PTFE.
References
- Sina Ebnesajjad. Fluoroplastics, Vol. 1: Non-Melt Processible Fluoroplastics Elsevier, 2015^
- Bonding PTFE Materials for Microwave Stripline Packages and Other Multilayer Circuits 2003^
- Material Safety Data Sheet, Poly-Etch 2009^