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.