Q: 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.
A: 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.
For assistance with your bonding process, contact Dymax Application Engineering.