"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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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?

  
  

  

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