"Hello, currently I am working with 2 Cal Poly professors to make monitoring operating room packs easier. Our goal is to be able to attach micro RFID tags to surgical tools so that they can be scanned and bookmarked. What type of adhesive would work best to attach these tags to surgical tools? The adhesive needs to be biocompatible and be able to undergo sterilization."

Assuming most surgical tools are stainless steel, and need to survive repeated autoclave, I would recommend looking at 2-part epoxies as your base chemistry. Options are available from Loctite, 3M, and Epoxy Technology, to name a few, and some have biocompatibility certificates on file. If the surgical tools are disposable or plastic, and only need to withstand a single autoclave cycle, EtO, or Gamma sterilization, then a light-curable acrylated urethane like the DYMAX 1120-M-UR light-curable medical device adhesive would be my first choice. Acrylated urethane light-curable adhesives have excellent adhesion, are simple to apply as a 1-part material, and cure in less than a second.

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"In my application I have a process where I apply UV adhesive between two pieces of plastic and I am seeing a short contraction period followed by a longer expansion period. Is it possible for UV adhesive to behave this way? How much does UV adhesive shrink during cure? Could this cause a pulling force between two plastic materials? If under an opposite force could the UV adhesive relax and expand somewhat?"

When light-curable adhesives cure, whether curing with UV light or visible light, crosslinks are forming between polymer chains. This pulls the chemical chains closer to each other very rapidly. We typically see a 1-2% linear shrinkage, which could translate into a 2-5% volumetric shrinkage. This may stress some plastics or optical components. There is a relaxation effect, usually over the next few hours or overnight, where the chains relax slightly as they rotate into an optimum alignment. In the spirit of valentine’s day – polymer chains like to spoon together and snuggle. If they are at odd angles to each other, they are still touching, but want to find that alignment where they are in the same direction and bending the same way. Chemical bonds can stretch and spin around their axes and allow for this relaxation. Also good to note, a product with a low modulus will stretch easier under stress, and a product with a very high modulus will not stretch much at all. A silicone (on one extreme) can have a modulus as low as 300 psi, whereas an epoxy can have a modulus as high as 2,000,000 psi. Many UV-curable adhesives are urethane acrylates and can vary in their modulus’ over a very wide range. The product data sheet should list this value.

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"Are methanol, ethanol, acetone, or acetonitrile FDA approved?"

These solvents are generally not FDA approved. These particular solvents bond plastic together by melting the plastic, and then allowing the plastics to intermingle. As the solvent evaporates, the plastics harden to form a strong plastic weld between the plastics. The choice of which grade of solvent you buy is up to the medical device manufacturer (higher purity equals higher price). Since solvents evaporate and do not remain in the bond line, they are not normally tested for biocompatibility.

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"I have an application bonding opaque parts with a Cyanoacrylate (Super Glue Type). There is much surface area and I have a very good bond. Will Gamma Sterilization effect my bond strength?"

Most cyanoacrylates will survive 1x, 2x, and sometimes even 3x gamma sterilization without a significant impact on bond strength. Repeated Gamma sterilization will start to add additional crosslinking, which will start to reduce elongation. As most cyanoacrylates are already brittle materials (depending on the grade), the adhesive may become even more brittle. Drop tests, impact testing, or tensile testing may be a good indicator of the final impact on your bond strength.

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"I need an epoxy to join two BK7 glass parts together. Gap is around 0.2mm. Light will cross the interface. Reasonable index match to the glass required. Low stress/shrinkage so it doesn’t distort the parts please. Viscosity not too runny, as we want it to stay in place prior to cure."

Optical glass-bonding adhesives are available that have good adhesion to BK7 Glass, a close refractive-index match to the glass, low shrinkage (low modulus), and moderate viscosity to avoid running. Light-curable adhesives like OP-29, which is a one-part adhesive from DYMAX, are available exclusively through Fiber Optic Center. FOC also carries various 2 two-part epoxies or 1 one-part frozen epoxies which should meet your requirements. An alternative source would be to visit the Edmund Scientific website and review their adhesive selections. Epotek 353-ND and 353-T epoxy are also widely used in the glass-bonding/optical adhesives market.

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"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.

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"Is there a temporary adhesive to bond glass and withstand 1400°F?"

Before this question can be answered, we need to ask for the definition of temporary. Only some materials will withstand up to 1400°F. Companies like Cotronics Corp and Aremco (to name a few) offer high-temperature adhesives. These adhesives are often modified with fillers like Alumina, Zircon, mica, ceramics, etc. that allow the adhesive to withstand higher-temperature environments. Some materials, including liquid materials or tape-type products, have a continuous service temperature of 1500°F, and a melting point of 2800°F. So in one sense, it is permanent at 1500°F, but temporary as you raise the temperature. Some epoxy systems can withstand up to 400°F and 500°F, so the question becomes - how long does it need to survive at 1400°F? If the answer is minutes, then finding a material that can survive short bursts of high temperature for only a little while may be sufficient. There are a number of different chemistries available such as silicone or epoxy. Typically, unfilled organic adhesives like acrylics and urethanes will break down long before 1400°F.

Other questions to ask are about which properties are required, such as: Does it (the adhesive) need to be clear or is opaque okay, or how do you want to remove the material? Unfortunately there isn’t a simple answer to this question. Some of the technical support groups at these companies may be able to help further, or recommend other options for you to consider.

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"I am using some UV curing adhesives and was told that there is a risk of leaving uncured monomers in the adhesive that could cause adhesive failure long term (like 6+ months) where the monomers dissolve or soften the cured resin. Assuming my cured adhesive is very hard and tests good for tensile strength is there any truth that uncured monomers (in very small amount) can cause the adhesive bond to weaken over time?"

If a material is fully cured, there is no risk of re-solvating the adhesive due to uncured monomers left behind since everything that could react has been reacted. However, it is our experience that many people who use a light-curable adhesive do not actually reach a fully cured state. Instead of reaching a fully cured state of 96-100% conversion of reactable materials, sometimes a particular process or part configuration will only reach 75-80% conversion. If a material only reaches semi-cured status, it could appear to be cured, and give good tensile strength and a cured surface, but have unreacted monomers at some level within the adhesive - which can then resolvate or attack the surrounding adhesive, thereby weakening the adhesive and the bond joint. This would be noticed with accelerated aging or within 1-6 months. A good qualification process will eliminate this risk.

• Evaluate various safety factors (cure time or intensity at 1.3x, 1.5x, 2.0x, 3.0x) to verify that the adhesive strength and properties have reached a plateau
• Run accelerated aging at a moderate temperature to verify long-term stability
• Evaluate the adhesive in a process by FTIR to identify the presence of uncured monomer (a skilled analytical chemist can identify a double bond peak, indicating the presence of uncured adhesive, and the lack of a double bond peak indicating that all reactable materials have been reacted), or use photo-differential-scanning calorimetry to measure the change in crosslink density.

Building a process to ensure that you reach a fully cured state, and have a good safety margin is the key to successfully using a light-curable adhesive.

See-Cure Technology available from DYMAX has a color indicator that changes from a blue color to clear when full cure has been reached. This helps to identify when you have reached a fully cured state.

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"Our research lab is looking for a medical-grade adhesive to bond:
1.) Dacron cloth to silver plate and
2.) Glass and PMMA optical cylinder in a silver tube.
It needs to be moisture resistant and should last for a long time (20 years).
Please suggest a product."

Dacron is a commercial name for PET, or polyethylene terephthalate. Bonding cloth to a rigid substrate like a silver plate can be done in a few different ways. Bonding to cloth is mostly a mechanical lock that forms by encapsulating strands of the cloth and then locking them to the rigid substrate. The viscosity of the adhesive will play a role, as the thinner the viscosity, the more it will wick into the cloth. A very high viscosity will not wick very far into a cloth. A 2-part epoxy, such as found from Loctite or 3M, or a 2-part urethane, such as found from Lord Corporation, are just two products that you might want to explore. A silicone adhesive may also do the trick, and would suggest contacting representatives of Dow Corning, Momentive Performance Materials, NuSil Technology, or any of the other silicone manufacturers.

To bond glass and PMMA you may be able to use the same adhesive, but the application might require a lower-viscosity material, depending on the gap between the parts and method of assembly. The epoxy and silicone systems will be moisture resistant and have good usage life, but most manufacturers will not warrantee a 20-year usage lifetime. A 1-part, light-curable urethane acrylate, like 203A-CTH or 209-CTH from DYMAX, are options if you can get light to the adhesive.

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