Understanding Full Cure in Adhesives
Q: 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.
A: 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/cm2, is a multiplication of the intensity (Watts/cm2) 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 which 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/cm2 or mW/cm2, 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 technology to help you define the parameters of a robust process and ensure that during production the material is fully cured. See-Cure is a patented technology formulated into many Dymax products that allows the adhesive to appear in a bright color in the uncured state. Under sufficient exposure to light energy, the adhesive transitions to colorless, indicating full cure. This adhesive color-changing technology incorporates 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 indicate it has reached full cure.