Question: We manufacture PTFE and other fluoroplastic parts. We have received a request to make parts that can be bonded with regular adhesives. Is there an additive for the PTFE or a surface treatment that we can use?
Answer: Polytetrafluoroethylene (PTFE) is a high-performance plastic used in a variety of different industries, particularly for its heat and chemical resistance and nonstick properties. However, PTFE has a low surface energy that does not allow adhesives to wet its surface and subsequently bond. In order to achieve adhesion, it is necessary to treat the surface aggressively with chemical agents. The oldest of these is a mixture of sodium metal and ammonia. This mixture significantly improves adhesion, but the surface morphology is considerably changed and chemically damaged by the etching. Furthermore, the surface becomes brown in color. As you can appreciate, this combination of chemicals is very unpleasant, highly toxic and hazardous to use, and poses significant waste disposal problems.
More modern treatments involve using solvent solutions of a complex between sodium and naphthalene. Tetrahydofuran (THF) was the original solvent chosen. (I remember as a student preparing a solution of sodium naphthalene in THF, and it turned the PTFE-coated magnetic stir bar brown!) Glycol ether solvents are more commonly used. You can purchase the surface treatments, or, if you prefer not to handle them, the suppliers frequently offer to treat your parts for you.
Some concerns have arisen over just how long an etched fluoropolymer surface will retain its original surface reactivity. It is now commonly held that any deterioration occurs over a period of weeks or months, as opposed to hours or days, which was the prevailing early belief.