“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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“What bonding agent would you recommend for bonding soft PVC tubing to a polycarbonate connector? This is a sterile disposable tubing set.”

For this type of application I would recommend a light-curable adhesive. Dymax 1161-M could be a good candidate due to its ability to bond dissimilar substrates like PVC and polycarbonate together. This adhesive is solvent free, resistant to EtO, Gamma or Beta sterilization, and passes ISO 10993 biocompatibility testing. Via UV and visible light, the adhesive cures on demand within seconds using Dymax UV light-curing equipment.

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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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“What are the effects of E-Beam sterilization (54-75 kGys) and accelerated aging (1 year @ 55^°C) on medical device PVC tubing (IV Set)?”

This was a little outside of my area of expertise, but Saint-Gobain Performance Plastics was able to provide information to answer this question.  E-Beam sterilization, as well as other radiation sources like Gamma, are normally run at a level of 25-30 kGys.  Under this condition, the sterilization will have only a minor impact on the properties of the PVC.  At 2x (50 kGys) or 3x (75 kGys) the typical dose, you can expect to start seeing some breakdown of the PVC tubing.  This will be evident by a slight browning of the PVC, or perhaps a drop in tensile strength and elongation, as well as plasticizer exitation/migration to the surface of the PVC.  The variables that can affect the impact of sterilization would be the type and concentration of stabilizers in the PVC, as well as blue colorant levels which may mask the slight browning color.

The effects of accelerated aging, defined in this case as 1 year @ 55^°C, will also show a slight change in performance and characteristics of the PVC tubing.  The plasticizer may also come to the surface, migrating out of the PVC, to form droplets or even volatilize to a gaseous state.  This will leave the tubing harder, less flexible, or even brittle.  You may also experience some browning, depending on the formulation.  Typically, Saint-Gobain uses a standard of 53 days @ 55^°C, which approximates 1 year of actual shelf life, and does not normally recommend going beyond this temperature/time.

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