“How can I reverse adhesive crystallization? Will the adhesive be compromised in any way?”

Only a very small number of adhesives are susceptible to crystallization and the likelihood of this phenomenon actually occurring is slight. If you do experience crystallization, heat will reverse the process and restore the adhesive to its liquid state with no loss in performance. There is no effect on strength, viscosity, or any other adhesives property. Tests performed at Dymax indicate that heating the original container of crystallized adhesive (in an oven or in a warm-water bath) to 100°F (adhesive temperature) will return the adhesive to its uncrystallized, liquid state. The adhesive will reach 100°F in approximately 2 to 3 hours depending on container size. For consistent dispensing, the adhesive should be allowed to return to room temperature. We would also recommend that you purge any dispensing reservoirs, lines, valves, and needles before any prolonged periods of production shutdown (i.e., weekends, shutdowns, etc.). This will help prevent adhesive crystallization.

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“Will overexposure during curing have any effect on the adhesive?”

The effect of underexposure is obvious… incomplete cure. The effects of overexposure are more complex. Double and triple exposures (two to three times the dosage required to cure) typically have little effect on light-curable materials. However, significant overexposure to UV light with attendant heat may age DYMAX materials and some substrates (especially plastics). Severe aging may appear as cracking, physical distortion, changes in color, or chalking. Some physical properties such as an increase in hardness or decrease in elongation may also change. The degree of aging will depend upon several factors including intensity of the lamp, the wavelengths transmitted to the resin, temperature, exposure time, substrates, and specific formulations.

Aging from UV light is not the only concern associated with extra long exposures. Parts may get hot under UV lamps with extended exposures. Thermal aging can exhibit the same effects as UV aging. Some types of plastics may warp, scorch, or decompose from excessive heat absorption. A fan in the curing area may help keep parts cooler.

Significant overexposure of a resin to UV-curing light is unlikely to occur in a properly controlled curing process. End users should always test and validate their assembled device at the upper and lower limits of their process against the lifetime use of their device.

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“We need to glue an epoxy-coated magnet to a copper-clad FR4 PC board. The UV-cure glue needs to resist mechanical shock without cracking. Any Ideas?”

DYMAX offers 6-625-SV01-Rev-A, a flexible light-curable adhesive with secondary heat cure and activator cure capabilities for shadowed areas. Its high viscosity of 10,000 cps also enables it to be dispensed from the outside and act as an edge-bonding material.

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"Is there a UV-curable adhesive that when cured conforms to USP Class VI and passes ISO 10993 requirements for permanent implant?"

Unfortunately, not that I am aware of. Technically, most light-curable adhesives are acrylated urethanes or epoxy-based systems, and would not survive permanent implantation. There are other hybrid light-curable technologies, but as far as I know none have been released technically for long-term implantation. In addition, the legal liability is too high for most applications. Perhaps something from the dental cement industry might be a suitable option.

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"I have an application where the cuff is glued to the tube and I require a smooth, tapered transition between the cuff edge and the tube.
Writing will take place over this area; that is why I need the smooth, tapered glue connection."

It is very common to use an adhesive to make a nice, smooth, tapered transition between cuff edges and the tube, or marker bands, or transitions where there is a jump from one size tube to another. A low-viscosity adhesive in the 200-600 cP range, which cures rapidly to a smooth tack-free finish, is ideal. DYMAX light-curable materials like 1120-M-UR or 204-CTH-F are often used in these types of applications. Techniques include applying the adhesive in either a vertical or horizontal position, or even at a slight upward angle to achieve the taper. If it is a horizontal or tilted orientation, rotating the shaft during application and curing with a spot system for 1-8 seconds (typical), will allow the material to cure in the proper profile without risk of the adhesive slumping or running. Typically these transitions have a max height of 2-5 mils or less. Lower viscosities will have a sharper taper, and higher viscosities will have a shallower taper. Selecting the right needle dispense tip will help control adhesive quantity and position on the catheter shaft.

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In the uncured state, humidity under most normal conditions of 0-70% Relative Humidity (RH) has no impact on a light-curable adhesives. It does not affect the curing time, chemical properties, or adhesion to the substrate. At 70% RH or higher, we would want to watch for condensation on the surface of the substrate that is being bonded or coated, as this would effectively act as a barrier between the adhesive and the substrate, and may reduce the bond strength localized around those droplets.

In the cured state, high levels of humidity for extended periods of time at high temperature may affect some acrylated urethanes by causing hydrolysis in certain formulations, which may decrease the bond strength and cohesive strength of the light-curable material. Applications that require exposure to these conditions should be tested thoroughly to determine suitability for the application. An indication of this effect can be seen by looking on the data sheet to determine the % water absorption, but this is only an indicator, and testing in the actual application should be done. Limiting the exposure to the adhesive bond line through proper design can reduce the impact of humidity on 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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