Polytetrafluoroethylene (PTFE) is low-surface-energy plastic that is very difficult to bond. PTFE exclusively comprises fluorine and carbon atoms with no polar atoms such as oxygen or nitrogen. Most adhesives are very polar materials. The secret to bonding PTFE is to modify the surface, known as etching. Only very aggressive chemicals can achieve this. The oldest surface treatment involves hazardous mixtures of sodium metal and ammonia and is very effective.

Etching has a big effect on the surface chemistry of PTFE. Etched surfaces are composed of carbon and oxygen with some unsaturation and with very little fluorine. Somewhat less hazardous treatments use sodium naphthalene solutions, originally using tetrahydrofuran solvent but more recently bis(2-methoxyethyl) ether (diglyme), which has a higher flash point. All these treatment change the color of the surface to yellow, brown or even black, but give high adhesive strengths.

Note that manufacturers of the plastic say that media to avoid when using PTFE include “metal hydrides, e.g., boranes (B2H6).” In recent years, two-part acrylic adhesives incorporating organoboranes have been developed for the bonding of low-energy materials like polyethylene and polypropylene. These adhesives use an amine-complexed organoborane in one component and a decomplexing agent (usually an acid) in the second component. What seems to be very significant is that patent literature from the companies clearly shows outstanding adhesion to PTFE. This may be a way to avoid the hazardous etching treatments, but does not seem to have been commercially exploited with the patent holders concentrating on applications on polyolefins such as in the automotive industry.

Any views or opinions expressed in this column are those of the author and do not represent those of ASI, its staff, Editorial Advisory Board or BNP Media.