Ask Dr. Dave
Bonding Fluoroplastic Parts to Metals
We manufacture PTFE and other fluoroplastic parts. We have been requested to make parts that can be bonded to metals. Is there a good adhesive, some sort of additive to the PTFE, or a surface treatment that we can use?
Polytetrafluoroethylene (PTFE) is a high-performance plastic used in a variety of industries, particularly for its heat and chemical resistance and “non-stick” properties. However, PTFE has a low surface energy that does not allow adhesives to wet its surface and subsequently bond.
Some modern acrylic structural adhesives will give moderate adhesive strength. In order to get better adhesion, it is necessary to etch the surface aggressively with chemical agents. The oldest of these is a mixture of sodium metal and ammonia. This significantly improves adhesion, but the surface morphology is changed considerably and chemically damaged by the etching. Furthermore, the surface becomes brown in color, although this is usually not an issue in an adhesive.
As you can appreciate, this combination of chemicals is unpleasant, highly toxic, and hazardous to use, and it poses significant waste disposal problems. I do know a company that routinely uses this treatment to bond PTFE to steel, however.
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, but suppliers often offer to treat your parts for you if you prefer not to handle them.
There have been some concerns 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 the prevailing early belief of hours or days.
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