“We use glass capillary coated with polyamide and insert the glass capillary into a FEP tube. What is the best adhesive that would hold both capillary and FEP tubes?”
In general, FEP (Fluorinated ethylene propylene) is very difficult to adhere to. It has very similar properties to Teflon (PTFE), which is known for its anti-sticking surface. Due to the design of your part, you may achieve sufficient bond strength, especially if the capillary is inserted deep into the FEP tube and the adhesive can flow deep inside the bond gap. If you prefer a light-curable adhesive, I would start trials with DYMAX 1180-M (medical grade) or 3013 (industrial grade).
Before making a final adhesive recommendation, however, a few more aspects need to be considered:
- Length of the bonding area
- Bond gap size (OD capillary versus ID FEP tube)
- Forces / temperature the part will see when in use
- Medical-grade adhesive needed
- Optical properties
If you need higher bond strength, there is an option to chemically etch FEP. By using a Sodium Naphthanate solution, the surface of FEP can be modified so that most common adhesives can be used. This etch, however, creates a darker layer on the surface, so that the clear properties of FEP are no longer given.