"I have an application where we would like to cover the edge of a silicon die (that has been first tacked in place with epoxy that has been cured to a Valox/PBT base) with an uncured epoxy. Then add additional epoxy to a second bond location that ultimately bonds the Valox base to a Valox cover that has a slight press fit, covering the silicon die. After the press fit assembly, I would like to light or UV cure through the cover plate, causing both applications to cure. By both applications I am referring the bond line between the Valox cover plate & Valox base as well as the edge coating around the periphery of the silicon die, bonding it to the Valox base.
Are there any polymers that can transmit UV or light sufficiently to allow these types of cures? Is there a special light source required? Can the polymer be anything other then clear? Lastly, is there a medical-grade polymer that can achieve this?"
There are a couple of ways to handle this application.
There are a number of polymers that can be used for a cover plate that will let light through, whether UV or visible light. One way is to use a tinted plastic like polycarbonate or acrylic. Tinting the plastic with a dark blue dye will create the "illusion" of being opaque, but will let the proper wavelengths through the plastic to allow it to cure. Finding the right balance of dye will be critical. One way to judge the impact of the dye in the plastic, or the transmission through the plastic, is to measure the intensity of light coming through the plastic from the light source. You can calculate a % loss of transmission through the plastic in both the UV and visible regions. Most adhesives want to see a minimum of 200 mW/cm2, and some of the light sources on the market are emitting light of 20,000 mW/cm2. Even if the plastic blocks off 90% UV light, and 70% visible light, this might be enough to allow it to cure. If the plastic compounder uses a pigment to augment the tint, make sure the level of pigment is low, and they use a white pigment like Titanium Dioxide. A good field test – if you hold the plastic up to the light and can see a shadow as you wave your hand behind it – you might have enough light coming through to cure the adhesives. Stay away from yellow-orange-red colors. Whites, blues, and greens are better. There are a number of medical grade adhesives that can be used for this application, and various curing lamps. I might recommend contacting DYMAX Application Engineering to arrange for some free samples and to discuss our Trial Rental/Lease Equipment program. www.dymax.com.
“I’m looking for a UV adhesive that will bond a PET or Acrylic lens to a photo print such as those you get from a professional photo lab.”
DYMAX light-curable dome coatings have crystal-clear coating/adhesive properties, and stick well to plastic lens, thin films, and professional photo printing paper, without wrinkling or distorting the image. I recommend a soft, flexible A-70 durometer material with a viscosity of 1,500 cP that it is easy to apply and roll onto the printing paper surface, so the entrapment of bubbles is eliminated. It’s important to use a low-medium intensity light to reduce stress during cure. Other coatings are available that provide a glossy, scratch-resistant coating over the photograph, usually in applications without a covering of PET or acrylic lens.
A recent question that came through:
“Do you have an adhesive recommendation for gluing polycarbonate to acrylic? We are currently using 3M Marine Adhesive/Sealant 5200.The adhesive holds initially but starts to “weaken” after some time. The adhesive seems to be sticking well to the acrylic (Plexiglas) but not to the polycarbonate. We tried sanding the polycarbonate surface before gluing, but the adhesive still did not hold. Any suggestions?”
Typically, bonding to polycarbonate and acrylic is a very feasible application for many adhesives. If you want to try a light-curable, acrylated-urethane adhesive, DYMAX 3099 or 3025 adhesive might be a good option.
The comment about the adhesive weakening after achieving good results the first time is concerning. I recommend that you speak to your polycarbonate supplier/molder to discuss why this might be happening. For instance, the raw resin supplier may be adding a mold release to improve molding with polycarbonate and allow the polycarbonate to release from the metal mold easier. If a monomeric mold release is used, it can sometimes migrate to the surface and push the adhesive away from the bond line, causing interference with adhesion over time. If they are using a polymeric mold release, it cannot migrate as easily and should not have a negative effect on adhesion. Or, maybe the polycarbonate molder is spraying a Teflon® mold release every 200th shot to help the part release from the mold.
I also suggest asking the polycarbonate supplier/molder about the levels of stabilizers being used. Most stabilizers in the plastic are at acceptable levels and do not interfere with bonding. On occasion, however, the supplier/molder adds additional stabilizers to give light or heat resistance and stability to the plastic. These stabilizers can also migrate to the surface over time and destroy a bond line. To be sure, you can submit samples of the polycarbonate before and after heat aging to an analytical laboratory to run a solvent extraction on the surface of the plastic, to see if there is a contaminant at the surface, and to identify the contaminant. I have even seen contaminants like finger oil at the surface of the plastic migrate along the bond line and eventually degrade the bond strength.