Ask the Engineer

Sticky Questions on Adhesives

August 9, 2013


• Categories: Ask the Engineer

“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.

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/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 will 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.

---

“I’m looking for a UV-fixating adhesive to bond a steel cannula onto a polypropylene hub. The hub will be plasma or corona treated prior to gluing to increase the bonding performance and the hub will have annular rings as well. However, since the material is opaque I assume the adhesive has to have a secondary curing mechanism too? In addition to the above, the fixation time for bonding needs to be <10 seconds. Which adhesive might be suitable for this application?”

On occasion full cure via UV/Visible light can be achieved with opaque polypropylene hubs.

Depending on the color and thickness of the polypropylene, some light from the side can transmit through the plastic and polymerize the UV/Visible adhesive. The largest area of the adhesive is usually cured from the top. However, due to limitations in depth of cure of most UV/Visible adhesives, I would recommend keeping the length/depth of the bond joint to a minimum and mold the annular rings near the top of the hub. Suitable UV/Visible materials include medium-viscosity adhesives that won’t flow deep inside the hub or low-viscosity adhesives if the design of the bond joint prevents these materials from flowing too deep inside the hub.

In order to achieve short curing times high-intensity UV lamps are recommended.