“We use two-part epoxies and UV-curable cements to seal Viton- or PVC-bending rubber onto the distal end of flexible endoscopes. Our current epoxy works well when it is sterilized using Steris or glutaraldehyde solutions. With the introduction of Sterrad H2O2 plasma processes, the epoxy fails after 20 to 30 cycles. What do you recommend that will hold up to more than 100 cycles?”

When ASP introduced Sterrad low-temperature hydrogen peroxide gas plasma, they tested many UV-curable adhesives and epoxies from different suppliers. The results showed that many adhesives were compatible and did not exhibit any material damage after 200 cycles. But some products were not compatible with this sterilization method. The article with the results can be viewed here: http://www.mddionline.com/article/compatibility-medical-devices-and-materials-low-temperature-hydrogen-peroxide-gas-plasma.

Since this article is more than 10 years old, some products may not be available anymore, but it is a helpful guide to identifying the most suitable product.

Adhesives Adhesives, bond failure, bonding rubber, epoxy, light-curable adhesives, PVC-bending rubber, Sterrad H202 plasma processes, uv-curable adhesive, Viton-bending rubber

"We use a lot of RTV adhesives. Every lot is tested by QA. Last week we found out that in one of our components the adhesive did not cure at all. The RTV is one component and it adheres metal to ceramic. It has holes for air to penetrate. Even after opening the bond and exposing it for one week it still did not cure. What could be the reason for not curing?"

RTV silicone adhesives rely on moisture and humidity in the air to cure properly. Generally the conditions have to be 40-60% RH, but can extend down to 20%, and up to 70% in certain cases. The moisture in the air reacts with the stabilizer in the RTV, and once the stabilizer is removed, the adhesive can cure fully. In a very high-humidity environment, the humidity in the air can saturate the surface of the RTV, and effectively seal it off, limiting the penetration of the humidity to deeper levels. If you have a thick bond line or cross section of material, it may take longer than one week to cure fully. The silicone manufacturers generally set a 5-7 day cure schedule for RTV’s before they can test the physical properties in a thick slab of material, and that’s with the condition of 40-60%. If you have a 70% RH condition during the summer time, it may take longer, or disrupt the cure enough to appear gummy or semi-cured. Acidic surfaces may also cause problems with the cure mechanism.

Another avenue to explore: Was this failure linked to just one lot of material? And was it 100% failure for this lot, or 1% failure of one tube within the lot? These answers can lead the manufacture of the RTV to help determine the root cause of the failure.

Adhesives, Medical 40-60% RH, acidic surfaces cure mechanism, adhere metal to ceramic, bond, bond failure, fully cure, gummy, high-humidity, humidity cure, moisture cure, one component, RTV adhesive, RTV silicone adhesives, RTV stabilizer, semi-cured

"I am trying to understand the UV-curing process. How can I determine when an adhesive is fully cured? What are the critical parameters that I need to control in order to gain good consistency for the curing? Also, I was trying to cure some adhesive on a piece of stainless steel coupon. One small drop of adhesive was placed onto the coupon and formed a kind of round shaped droplet. I am wondering if the curing is more efficient on the surface of the droplet or on the inside of the droplet. Thanks a lot for your help!"

Very good question! Light-curable adhesives (whether it is by UV light, visible light, or a combination of UV and visible light) cure from the surface closest to the lamp, and then cure to depth. If you have a droplet, the surface will cure first, and then the rest of the dome will follow. The last area to cure would be against the substrate, so this leads us to the question:

How do you know when the adhesive is fully cured?

• Adhesion to the substrate is one way to evaluate the full cure
• A simple test is to try and use a tool to get underneath the droplet. If there is liquid at the interface, then it is not fully cured. You would need to increase either the intensity of the lamp, or increase the amount of time of exposure.
• Most applications have a minimum energy needed to achieve good cure. The energy, or Joules/cm^2, is a multiplication of the intensity (Watts/cm^2) x dose (seconds). You want to build a process around the total amount of Joules needed to reach full cure, so you can vary either the intensity or time needed to cure, and as long as you reach the minimum energy for a given lamp, then you should have a robust process.

The best way to determine if you have a robust process would be to:

• Run adhesion strength tests (bond laps or components together to see when full or maximum strength is achieved) or physical characterization (i.e. durometer, elongation, tensile, or modulus) at different conditions. When full strength is reached, additional energy (intensity or time) does not lead to an increase in properties.
• Compare the results in your process to the manufacturers data sheet. The manufacturers data sheet may indicate that the material will ultimately reach a specific durometer (i.e. A-40, D-60, D-90). Under most conditions, if you were plotting durometer/hardness for example, the hardness will build (incomplete cure) and then plateau (complete cure).
• Build in enough time to add a safety margin

It is important to have a radiometer as this device will tell you the intensity in Watts/cm^2 or mW/cm^2, which will be critical in the application.

The ability to cure on the surface can be affected by a phenomenon called oxygen inhibition. Some older adhesive technologies may be affected by oxygen during the cure process, which leaves a slightly tacky residue on the surface. The best way to overcome this issue is to start with a higher intensity, which would allow you to cure for a shorter time. New materials are being designed to overcome this issue, but lamp selection and bulb spectrum are important when developing a new process.

DYMAX has a new technology to help you define the parameters of a robust process, and ensure that during production the material is fully cured. See-Cure Technology is a patent-pending adhesive technology available in many DYMAX products that allows the adhesive to appear bright blue in the uncured state. Upon reaching full cure under a light source, the blue color will disappear, leaving a colorless clear adhesive in the bond line. It will only go clear when it has reached enough energy to be fully cured. This adhesive color-changing technology was designed to incorporate a safety margin before the color change happens, so is a great way to not only build a process, but have a quality inspection system within the adhesive to tell you if you have reached full cure.

Adhesives, Coatings, Curing Equipment Adhesive Cure, Adhesive Cure Parameters, Adhesives, bond failure, color change adhesive, cure indicator, Curing Equipment, durometer hardness, elongation, Light Curable Adhesive, Light Curing Adhesive, Light Curing Process, modulus, Oxygen Inhibition, radiometer, See-Cure, Tack-Free, tensile, UV Curing Process, UV Light Curing Adhesive, UV/Visible Light Curing Adhesive

"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.

Adhesives, Medical accelerated aging, Adhesives, bond failure, BOP, DEHP, gamma sterilization, gummy, leaching, Plastic Bonding, plasticizer, PVC Tubing, Tack-Free, tacky, TPE, yellowing

The answer is a little more complex than most people realize. In the world of adhesive bonding, most failures can be categorized into three distinct groups. They are:

• Substrate Failure: Substrate fails before the adhesive.
• Cohesive Failure: Adhesive fails down the middle, adhesive remains on both substrates.
• Adhesive Failure: Adhesive fails from one substrate. Usually a customer would say, "The adhesive failed from the cold rolled steel but remained on the glass."

  In many cases, however, customers experience a combination of all three types of failure.

  When asked what would be the preferable mode of failure, most customers will answer substrate failure. Is this an accurate answer?

   

  My response to this question is that while the mode of failure is important, what should drive the adhesive manufacturer and the customer’s selection is choosing the adhesive that will meet the customer’s specific requirements. For example, if a company wants their assembly to withstand rigorous environmental testing while being subjected to 2500 psi of pressure, then the adhesive also needs to meet this requirement. Whether the customer experiences cohesive, adhesive, or substrate failure, for the most part, is irrelevant if the adhesive does not meet their requirements.

   

  Is there any situation when the mode of failure is important? Yes, when everything is equal during the qualification process, it may be helpful to review the failure modes to narrow the field of adhesive candidates down. Also, in the case of failure analysis, the mode of failure is important to help determine the root cause. This information can be used to determine why a particular adhesive is or is not bonding to one or more of the substrates.

   

  Ultimately, identifying the mode of failure is only important once the customer’s requirements have been met. And even then, does it really matter that the adhesive failed from one of the substrates if the customer’s requirements have been exceeded?

  
  

  

  Adhesives adhesive failure, Adhesives, bond failure, Cohesive Failure, substrate failure