"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.

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"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.

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"We currently use a light-curable acrylated urethane adhesive to bond PVC tubing to a part molded from TPE. We are seeing the adhesive turn yellow and tacky after gamma sterilization and accelerated aging. We also observed the PVC tubing becoming harder in the bond area. These conspire to cause bond failure. The suspect is plasticizer (DEHP) leaching out of the PVC and entering the adhesive. In your opinion, is this the likely cause? Once cured, I would have expected the adhesive to be impervious to DEHP."

I agree that the suspect is the plasticizer migrating during the sterilization and accelerated aging process. Plasticizers like DEHP and BOP will often migrate with heat and time from areas of high concentration to areas of low concentration. It does not matter if the adhesive is cured or uncured. Plasticizers will in effect solvate the adhesive, and migrate into it - often causing it to change color and become gummy or tacky. Just like the plasticizers keep PVC nice and flexible in the cured state, they still migrate away from the PVC under the right conditions. In this case, they migrated into the adhesive, eventually leading to bond failures. This can be tested by subjecting the PVC tubing by itself to the same heating and accelerated aging conditions, and wiping the surface periodically throughout the process. Testing the wipe media for contamination like DEHP or BOP can give an indication of the process step that causes this migration, and how much. Instead of wiping, you can “chemically wash” the part with a proper solvent, collect the solvent, and run it through Gas Chromatography to have it analyzed. To fix the problem, we would recommend trying different PVC tubing with a less mobile plasticizer, or switching to a comparable polyurethane tubing with similar physical properties, but without the need for plasticizer. Changing the chemistry of the adhesive is possible, but a last resort in most cases.

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Tackiness or stickiness may be noticed on the surface of some ultraviolet (UV) light curable adhesives and coatings. This phenomenon, known as oxygen inhibition, is the result of atmospheric oxygen inhibiting the cure on the surface layer of the polymerizing material. This condition is present anytime free radical polymerization occurs. However, the ability of a UV resin to be cured "tack-free or to a slick, dry finish" is dependent on the composition of the adhesive or coating formulation and the intensity and wavelength of the UV light.

Overcoming the effects of oxygen inhibition and producing a tack-free surface cure is dependent upon several factors:

• Heat generated by the UV curing system
• Intensity generated over the entire UV spectrum (200-390 nm)
• Exposure time
• Specific formulation of the adhesive or coating

Typically, short and medium wavelength (220-320 nm) UV light generated by mercury vapor lamps achieve more efficient surface cures. Short and medium wavelength curing systems, however, do have depth of cure and safety issues associated with them making them an undesirable option for many UV curing applications. Longer wavelength (320-390 nm) systems, which usually emit a small fraction of UV light in the lower wavelengths, will provide fast, tack-free curing while achieving better depth of cure.

Time to cure “tack free” should not be confused with full cure time. It is only an indication of the material’s ability to overcome oxygen inhibition, at the surface of the curing material, when the material is exposed to a given level of light intensity for a specific period of time. It has been demonstrated that the higher the intensity of the UV light the lower the total energy level needed to achieve a "tack free" surface. For example, to produce a tack-free surface cure of a DYMAX conformal coating (984-LVUF) using a 200 mW/cm² light source, the coating should be exposed for 20-30 seconds. This equals approximately 7 J/cm² of energy. This same coating cures in 1-2 seconds when exposed to 2500 mW/cm² equaling 2.5 J/cm² of energy.

Even though removing oxygen from the surface will also work at achieving a tack-free surface, this tends to be the least desirable method since it can be logistically challenging to implement.

Adhesives, Coatings, Conformal Coatings, Dome Coatings, Electronic Adhesives, Conformal Coatings, Dome Coatings, Oxygen Inhibition, Tack-Free

I was recently asked to respond to a question that I thought would make a good blog post. The question was:

“In the past, we have seen that some light curing adhesives are impacted by some type of inhibition, leaving a tacky residue at the surface.  What causes this, and how can we avoid this?”

 Here’s how it works…

Light curing acrylate adhesives cure by exposing photoinitiators to certain wavelengths of light, which break apart the photoinitiators into radical species, which react with the oligomers to create long chains and crosslinks. (When you shine a light on the adhesive, it turns into a solid).  Usually, by picking a high intensity light of >1 W/cm2 (keep in mind the sun is 0.002-4 W/cm2), and across a broad spectrum range from 300-450 nm, there is so much energy that the adhesives crosslink extremely fast, leaving a firm, tack free surface.  However, once in a while, some monomers & oligomers, which are the building blocks of these adhesives, may be susceptible to “oxygen inhibition” during the cure process.  Oxygen inhibition occurs during the curing step, whereby if there is oxygen present at the surface, the oxygen can penetrate into the surface and interfere with the radical polymerization, leaving unreacted monomers and oligomers at the surface. This is the tack that some people feel as they rub a finger across the surface, and may get traces of wet residue on their gloved hand.

Many of the newer adhesives are designed to go tack free under medium & high intensity conditions and when using the proper light wavelength. Generally, the higher the energy of light (lower wavelength (200-300 nm)) the better surface cure you get, but you limit the depth of cure. The lower the energy of light (higher wavelength (400-500 nm)), you get great depth of cure, but may get more oxygen inhibition.  The UV/Visible light spectrum in the 300-450 nm range seems to have the best blend of surface and depth of cure.  Now you have to consider the intensity of the lamps.  Higher intensity lamp systems can put out massive amounts of intensity, measured in mW/cm2.  Some spot lamp systems emit up to 15-20 Watts/cm2 (as measured at 365 nm).  It will be very easy to cure most adhesives to a tack free state with such a high intensity light emitting both UV and visible light.

We have observed that most adhesives have a minimum intensity threshold where they will go tack free within a specific time, as well as a minimum total energy threshold to get full cure.  Let’s look at the equation ”Joules/cm2 = Watt/cm2 x seconds”.  You can vary the intensity in Watts against the time of exposure in seconds to get the same amount of Joules/cm2.  2 Joules/cm2 = 2 Watts/cm2 x 1 second, OR  2 Joules/cm2 = 0.02 Watts/cm2 x 100 seconds.  While it may seem like you get the same amount of total energy in both situations (high intensity for short time, or low intensity for long time), the minimum intensity threshold may show that if you fall below 0.1 Watts/cm2 - you will never get a tack free state. 

Sometimes, some people find it beneficial to tackle the problem a different way, and flood the curing area with nitrogen or argon gas during the cure.  The nitrogen molecules are larger, and are not able to penetrate into the surface as easily, therefore give a tack free surface even with low intensity light sources. This is helpful in situations where a substrate is very heat sensitive or UV sensitive.  

Adhesive selection also plays a part.  The black magic happening in the chemistry labs allows the chemists to create new adhesives that go tack free at lower intensities.  Many of these find roles in potting and coating applications, with all different properties.  It used to be that only high durometer materials went tack free (D60-80).  Now the chemists have found ways to get soft yet tack free adhesives with low durometer ranges down into the A40-60 range. 

It all boils down to:  With the proper adhesive, process, light wavelength, time, and lamp intensity, you should be able to get a tack free surface.

Adhesives Adhesives, Oxygen Inhibition, Tack-Free