“In our application we are looking for a light-curable silicone adhesive that cures within seconds, is not acidic, and does not outgas too much. Do you have suggestions? “

There are a few light-curable silicones on the market that are non-acidic. The tensile strength, and cohesion strength of most UV silicones on the market, is very low and although they may stick well to substrates, a bonded joint can usually be taken apart easily. The larger the surface area, the more suitable these products are for bonding or lamination applications.

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“We are using UV light-curable adhesive to bond a polycarbonate hub to the passivated stainless steel hypotube. We encountered a leakage issue during our in-process air-leak test. Confirmation of the leak happened at the interface between the adhesive and the hypotube when injected with a colored dye. Prior to dye injection, “delamination” could be seen between the adhesive and hypotube. We tried appying primer on the hypotube, wiping the hypotube with IPA, and using a longer curing time, but none of these steps helped to eliminate the “delamination” issue.

What do you think is the cause of this problem and the solution?”

If contamination has been ruled out, delamination is most likely related to uneven stresses within the adhesive joint. I suggest looking at the adhesive, viscosity, bond gap, and curing process.

Are the adhesive and the viscosity appropriate for this application and given bond gap? UV light-curable adhesives shrink during the curing process and may pull away from the surface they have less adhesion to. The larger the bond gap, the more likely we will see air bubbles or delamination. In such a case I would try an adhesive with lower shrinkage or a filled, higher viscosity adhesive. Suitable products from DYMAX are 1180-M and 1180-M-T-UR, which are the higher viscosity versions for larger bond gaps.

You mentioned you tried a longer curing time; did you also explore curing with different intensities? I would recommend trying a shorter light exposure at a higher intensity and a longer exposure at a lower intensity.

Do you cure the adhesive from the top or from the side? If you cure from the side and use a UV spot lamp equipped with one or two lightguides, there is a risk of introducing uneven stress due to the shrinkage of the adhesive. The areas not directly exposed to the light often exhibit delamination or airbubbles in the bond joint. In such a case I would recommend using a 3- or 4-pole lightguide instead.

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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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