"I am using some UV curing adhesives and was told that there is a risk of leaving uncured monomers in the adhesive that could cause adhesive failure long term (like 6+ months) where the monomers dissolve or soften the cured resin. Assuming my cured adhesive is very hard and tests good for tensile strength is there any truth that uncured monomers (in very small amount) can cause the adhesive bond to weaken over time?"

If a material is fully cured, there is no risk of re-solvating the adhesive due to uncured monomers left behind since everything that could react has been reacted. However, it is our experience that many people who use a light-curable adhesive do not actually reach a fully cured state. Instead of reaching a fully cured state of 96-100% conversion of reactable materials, sometimes a particular process or part configuration will only reach 75-80% conversion. If a material only reaches semi-cured status, it could appear to be cured, and give good tensile strength and a cured surface, but have unreacted monomers at some level within the adhesive - which can then resolvate or attack the surrounding adhesive, thereby weakening the adhesive and the bond joint. This would be noticed with accelerated aging or within 1-6 months. A good qualification process will eliminate this risk.

• Evaluate various safety factors (cure time or intensity at 1.3x, 1.5x, 2.0x, 3.0x) to verify that the adhesive strength and properties have reached a plateau
• Run accelerated aging at a moderate temperature to verify long-term stability
• Evaluate the adhesive in a process by FTIR to identify the presence of uncured monomer (a skilled analytical chemist can identify a double bond peak, indicating the presence of uncured adhesive, and the lack of a double bond peak indicating that all reactable materials have been reacted), or use photo-differential-scanning calorimetry to measure the change in crosslink density.

Building a process to ensure that you reach a fully cured state, and have a good safety margin is the key to successfully using a light-curable adhesive.

See-Cure Technology available from DYMAX has a color indicator that changes from a blue color to clear when full cure has been reached. This helps to identify when you have reached a fully cured state.

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"We currently use a light-curable acrylated urethane adhesive to bond PVC tubing to a part molded from TPE. We are seeing the adhesive turn yellow and tacky after gamma sterilization and accelerated aging. We also observed the PVC tubing becoming harder in the bond area. These conspire to cause bond failure. The suspect is plasticizer (DEHP) leaching out of the PVC and entering the adhesive. In your opinion, is this the likely cause? Once cured, I would have expected the adhesive to be impervious to DEHP."

I agree that the suspect is the plasticizer migrating during the sterilization and accelerated aging process. Plasticizers like DEHP and BOP will often migrate with heat and time from areas of high concentration to areas of low concentration. It does not matter if the adhesive is cured or uncured. Plasticizers will in effect solvate the adhesive, and migrate into it - often causing it to change color and become gummy or tacky. Just like the plasticizers keep PVC nice and flexible in the cured state, they still migrate away from the PVC under the right conditions. In this case, they migrated into the adhesive, eventually leading to bond failures. This can be tested by subjecting the PVC tubing by itself to the same heating and accelerated aging conditions, and wiping the surface periodically throughout the process. Testing the wipe media for contamination like DEHP or BOP can give an indication of the process step that causes this migration, and how much. Instead of wiping, you can “chemically wash” the part with a proper solvent, collect the solvent, and run it through Gas Chromatography to have it analyzed. To fix the problem, we would recommend trying different PVC tubing with a less mobile plasticizer, or switching to a comparable polyurethane tubing with similar physical properties, but without the need for plasticizer. Changing the chemistry of the adhesive is possible, but a last resort in most cases.

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