Light Cure adhesives have become the assembly method of choice in high volume, automated manufacturing lines. They cure in seconds on exposure to UV or visible light wavelengths in the 200nm to 500nm range, offer infinite open times, and allow manufacturers to reposition parts as necessary before cure begins.
These adhesives adhere to many substrates including glass, metals, and a wide range of plastics. Light cure adhesives process rapidly, cure on demand, dispense easily without mixing, and fill gaps larger than 0.5 inches. Adhesives are available with temperature resistance up to 350°F and offer very good resistance to polar and non-polar solvents.
A simple and immediate in-line process, the Loctite AssureCure Systemincludes a new adhesive technology, a fiber optic light source, a light detector unit, and software that ties into the user’s existing PC or PLC.
Proven applications for light cure adhesives include handheld electronics, medical devices, appliances, optical equipment, speakers, and screens/displays.
To date, the greatest challenge for manufacturers using light cure adhesives has been easily verifying the adhesive's degree of cure. Even the most conscientious manufacturer can experience incomplete cure processes if:
An inappropriate or degraded light source is generating insufficient irradiance to complete cure
Rapid processing speeds result in insufficient light exposure times, or
Parts of the bondline are inaccessible to light due to opaque, shadowed, or shielded areas.
While the adhesive may appear solidified and the assembly may seem strong, without extensive offline testing that involves either physical destruction of an assembly or time-consuming analytical quality testing, manufacturers have had no way to easily and conclusively confirm the degree of cure online.
Physical or destructive testing processes will confirm cure, but just for that tested assembly. As the procedure cannot be integrated inline, detection is not immediate, and a time lag occurs between assembly of the defective part and detection of the problem. During this time, manufacturing continues, generating an inventory of defective parts. This same scenario holds true for analytical testing processes that must also be completed offline.
Manufacturers have employed two other techniques in an effort to confirm the quality of bonded assemblies. Fluorescent adhesives confirm that adhesive is present in the required areas of the bondline, but give no indication that the curing process has occurred. Color-changing adhesives prove that the adhesive has been exposed to light, but do not confirm conclusively that the curing process has initiated and progressed to completion.
naperlou - The manufacturer does mention a good correlation between their technique and FTIR. My consternation involved a fiber optic detection probe. I'm familiar with reflective FTIR techniques such as Attenuated Total Reflection (ATR) and Diffuse Reflection (DRIFTs), but I've only seen them used with sample stages and compartments having very known and calibrated sample geometries and optics. My suggestion of Raman was that it is a sister vibrational technique that often employs fiber optic probes as part of the detection optics. I don't know if Henkel is keeping the information close to the vest to simplify the technology or to keep it proprietary. I would love to learn more about it. -Bill =]
William, you may have already thought of this, but since the article mentions a FTIR measurement, I expect that they had identified a signature of cured material in Fourier space.
As a kid, mr orthodontist used a "new" adhesive to adhere my braces to my teeth. It was a quick procedure that avoided the long time spent in the chair. It worked well and was quick. This is something that could really help manufacturing items that need quick adhesion.
A number of years ago, a version of some of this tech was used by my
dentist, who put a UV cured compound that remained white on some of
my teeth in lieu of metal fillings. The newer cure times are wicked fast. I wonder if this technology has or will soon reach medical and dental applications?
A visit to Henkel's AssureCure(TM) website contains a tab for Adhesives. It displays an image of Henkel LOCTITE 3924AC, which I assume contains the AC suffix for use with the AssureCure system. Their technology overview does not give specifics but it appears that it is a reflective or emissive technology that utilizes LEDs or Diode Lasers for illumination and fiber optics for collection. The displayed response signal is broad like an IR spectrum but from the configuration it looks like it may be based on RAMAN scattering. Changes to the RAMAN features exhibited between the cured/uncured adhesive would probably not be very large. Henkel provides a technical paper in which they mention a "specially developed algorithm" that calculates a direct correlation to the adhesive's degree of cure. I am definitely interested in learning more about their technology.
If Henkel isn't providing an adhesive along with this new testing system, it seems you might have identified some good partnership opportunities, Rob. It would seem that the light cure technology and test system would need to part of the same package/installation. I would hate to be one of those manufacturers that invested in the light cure technology only to find later on that the lightening source didn't curate the bond and I was stuck with a sizeable inventory of defective parts. Even the threat of such a scenario would quash any interest in the bonding technology, I would think.
Is Henkel providing a new adhesive as well as a new testing system? Is the testing based on light? Does the adhesive change color on curing. Is that how the test equipment determines a sound cure?
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