Bonding PTFE

Using epoxy cement or adhesive for sealing polymer to glass and metal and to itself.

The resistance of polytetrafluorethylene (PTFE ) to any sort of adherence is legendary and often very useful as is its chemical inertness.  But these qualities do have one significant drawback, that polymer is very difficult to bond reliably to other materials or to itself.  There is a solution to this problem brought to the author’s attention by a company called Reltek. (More about them below).  The solution to the problem is Sodium Naphthalide dissolved in 2-Methoxyethyl Ether or in Tetrahydrofuran.  This is used as an etchant to replace fluorine atoms on the surface with a more reactive and bondable molecular species.  After etching the polymer surface is much more accepting of various cements, including epoxy resin adhesives such as Araldite, or the more specialised Reltek Epoxy Resins.  The etching is straightforward and involves exposing the roughened and degreased polymer surface to a solution of Sodium Naphthalide for about 2 minutes and then washing the etched surface with Isopropanol.  The etched surface is stable and mechanically resistant and bondable.  The only difficulty is the making and handling of the Sodium Naphthalide complex.

Sodium Naphthalide is an extremely powerful reducing agent, and it reacts very rapidly with atmospheric Oxygen and with water.  Any significant exposure to the atmosphere will quickly deactivate it.  It is thus best made and used in a glove box under nitrogen or using Scklenk technology.

In the preparation employed by the author, 2.5 gm of Naphthalene (GPR Rectapur)was dissolved in 25 ml of Tetrahydrofuran in a Schlenk tube. To this mixture 0.2 gm of freshly cut sodium metal was added.  The air-space in the Schlenk tube was purged with Argon and sealed with a Subaseal stopper.  The side tap was closed and the schlenk tube was partially immersed in a water-bath held at 45C for twelve hours.  

In use, Sodium Naphthalide is best handled in a Schlenk tube or in a glove box and a supply of Nitrogen or Argon is advantageous to prevent reagent oxidation during manipulations.  An arrangement for etching thin polymer tubes for sealing into stainless steel or glass tubing.

The polymer tube to etched is sealed at the far end to prevent aspiration of the etchant, and inserted through the bore of a hypodermic needle which pierces the subaseal.   The Subaseal is a versatile closure in that it can carry multiple tubes of varying diameters.  This means that quite wide tubes (up to 6mm od) can be etched in this way.