Q: "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 well for tensile strength, is there any truth that uncured monomers (in very small amount) can cause the adhesive bond to weaken over time?"
A: 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 re-actable 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 re-solvate 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 re-actable 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.