"I am trying to determine the proper cure time for the Loctite 3106 using a Dymax PC-5 Light Welder. Can you help?"

To determine the proper cure time of any light-curable adhesive when exposed to light from any light source, there are a couple of different approaches that can help. The greatest tool is a radiometer, which will tell you how much intensity you have at the bond line. The PC-5 is an older model flood lamp, with an intensity of 50-150 mW/cm² over a 5" x 5" area. The different approaches depend on how you are using the adhesive. If you are using the adhesive between two substrates in a bond-line thickness of 0.002-0.006 inches, then measuring the fixture time should be sufficient. Per the Loctite TDS, fixture time at this intensity should be <5 seconds. If you are potting a deeper section, then depth of cure is important, and you can reach a depth of 2 mm in approx 12 seconds. The Loctite TDS plots the depth of cure at an intensity of 50 mW/cm². If the adhesive bond line has some squeeze out, or has a surface exposed to air, then a tack-free surface cure may be important. Tack-free time is the point when the adhesive is sufficiently cured that you will not get smearing or residue transfer onto a gloved finger.

With any of the three described situations, measuring this yourself is the best way to figure out the proper cure time, whether looking at fixture time, depth of cure, or tack free time. Set the bond line up at the lowest intensity you can use – say 50 mW/cm². Do this by increasing the distance away from the lamp until the radiometer measures 50 mW/cm². (You will want to manufacture your parts at a higher intensity to start, and within a window of intensity and time. This will control your process.) After setting a constant intensity, cure the adhesive for different times. You will see the tensile strength, burst pressure, tack-free time, depth of cure, durometer, or other datapoint climb to a max value and then plateau. Once you have identified the start of the plateau, add a safety margin, and you have the foundation for your process. You can also set the time constant, vary the intensity, and record the same datapoints. You want to define your process by knowing the minimum and maximum intensity and time needed to cure the adhesive.

Adhesives, Coatings, Curing Equipment, Medical, Structural 150 mW/cm2, 50 mw/cm2, adhesive, adhesive between substrates, adhesive bond line, bond line, burst pressure, control process, cure adhesive, cure time, depth of cure, fixture time, flood lamp, high intensity, inteisity and time, intensity, Light Curable Adhesive, light source, loctite, lowest intensity, maximum intensity and time, minimum intensity and time, potting, proper cure time, radiometer, residue, residue transfer, smearing, squeeze out, surface exposed to air, tack free time, Tack-Free, tack-free surface cure, tensile strength

"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