“We have recently taken on some medical tubing device-assembly work from another company but the information supplied seems to be missing some crucial points. We are having bond-strength issues when fixing a flexible PVC tube into the female port of an ABS connector.

The instruction we had was to use 100% cyclohexanone, but my feeling and internet research seems to show some form of cyclo/THF and pellet combination would perform better. Would you agree or is there a ready-made alternative available?”

In general, adding pellets (PVC) to the solvent mixture can help avoiding stress cracks and allows more flexibility in the gap design due to its gap-filling properties. So far, we have not come across a readily available mixture.

If you consider moving away from using solvent, I would suggest looking at light-curable adhesives. Dymax offers a variety of products suitable for bonding PVC to ABS. Medical grade adhesive Dymax 1161-M for instance provides strong bonds to a variety of substrates and cures within seconds upon exposure to UV and/or visible light.

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“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.

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