When light-curable adhesives cure under either UV or visible light, cross-links form between polymer chains. This pulls the chemical chains closer to each other very rapidly. We typically see 1 to 2% linear shrinkage, which could translate into 2 to 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. Polymer chains like to spoon together and snuggle. If they are at odd angles to each other, they still touch, but you 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 axis, allowing for this relaxation. It is also good to note that products with a low modulus stretch easier under stress than products with a very high modulus, which do not stretch much at all. Silicones on one extreme can have a modulus as low as 300 psi, whereas epoxies can have a modulus as high as 2,000,000 psi. Many UV-curable adhesives are urethane acrylates, which can have a wide range of modulus values. Your product data sheet should indicate this value.

There are a couple of ways to handle this application. A number of polymers 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 such as polycarbonate or acrylic. Tinting the plastic with a dark blue dye will create the illusion of opaqueness but will let the proper wavelengths through the plastic to allow it to cure.

Finding the right balance of dye is 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 percentage 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 emit light of 20,000 mW/cm². Even if the plastic blocks off 90% of the UV light and 70% of the visible light, this might be enough to allow the adhesive to cure.

If the plastic compounder uses a pigment to augment the tint, make sure that the level of the pigment is low and that it contains a material such as titanium dioxide.

A good field test is to hold the plastic up to the light. If you 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. A number of medical grade adhesives and various curing lamps can be used for this application. I might recommend contacting Dymax Application Engineering to arrange for some free samples and to discuss the Try/Rent lamp program.

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. You can use a light-curable adhesives such as OP-29, which is a one-part adhesive from Dymax that is exclusively available through the Fiber Optic Center (FOC; New Bedford, MA). The FOC also carries various two-part epoxies or one-part frozen epoxies that should meet your requirements. An alternative source would be to visit Edmund Scientific (Tonawanda, NY) and review their adhesive selections. Epotek 353-ND and 353-T epoxy are also widely used in the glass bonding and optical adhesives market.

Most cyanoacrylates will survive 1×, 2×, and sometimes even 3× gamma sterilization without a significant impact on bond strength. However, repeated gamma sterilization will add additional cross-linking, 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.

Methanol, ethanol, acetone, or acetonitrile are generally not FDA approved. These solvents bond plastics together by melting them and allowing them to intermingle. As the solvent evaporates, the plastics harden to form a strong weld. The choice of which grade of solvent to use is up to the medical device manufacturer. Higher-purity grades are more expensive than lower-purity grades. Since the solvent evaporates and does not remain in the bond line, it is not normally tested for biocompatibility.

Before this question can be answered, we need to ask how you define “temporary.” Only some materials will withstand temperatures up to 1400°F. Companies such as Cotronics Corp. (Brooklyn, NY) and Aremco (Valley Cottage, NY)—to name just a few—offer high-temperature adhesives. These adhesives are often modified with fillers such as alumina, zircon, mica, or ceramics 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, these adhesives are permanent at 1500°F but temporary as you raise the temperature. Some epoxy systems can withstand temperatures up to 4000° and 5000°F.

Thus, the question becomes: How long should the adhesive survive at 1400°F? If the answer is minutes, finding a material that can survive short bursts of high temperatures for only a little while may be sufficient. For this purpose, a number of different chemistries, such as silicone or epoxy, are available. Typically, unfilled organic adhesives such as acrylics and urethanes will break down long before they reach 1400°F.

The last question is: What are the properties needed? Does the adhesive need to be clear, or is opaque acceptable? How do you want to remove the material? Unfortunately, there is no simple answer to these questions. The technical support groups at the companies mentioned above may be able to help further or recommend options for you to consider.

An information sheet from Ineos Nova lists various solvents such as cyclohexanone, cyclohexanone/tetrahydrofuran, cyclohexanone with various medical-grade acrylic polymers dissolved in the solvent for added strength, and various other combinations.

Solvent grades are typically not listed as medical grade or FDA approved but rather by the purity level of the solvent. Obtaining the solvent of choice with the highest purity (99.9% or higher) would limit the potential of leaching chemicals into the water. Sigma Aldrich or Alpha Aesar both provide small quantities of these solvents in various grades for evaluation purposes. Methyl ethyl ketone is an alternative solvent system. However, because of their odor, flammability, and explosive-material storage requirements, using solvents requires special handling and is carefully monitored by the EPA. Dispensing systems such as those offered by Tecnoideal are options to limit operator exposure.

If you want to consider a solvent-free adhesive, looking at a one-part light-curable adhesive such as Dymax’s 1161-M is possible if you can get visible light to the bond line (the nonopaque parts). Alternatively, a two-part urethane or epoxy from companies such as Epoxy Technology or 3M (to name two options) can be considered. These alternatives can provide a bond almost as strong as that provided by a solvent and with fill gaps in the molded ABS bond lines. In addition, they are much more environmentally friendly.

Dacron is a commercial name for polyethylene terephthalate (PET). Bonding cloth to a rigid substrate such as 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, since the thinner the viscosity, the more it will wick into the cloth. A very-high-viscosity adhesive will not wick very far into the cloth. A two-part epoxy such as that offered by Loctite or 3M and a two-part urethane such as that offered by Lord Corp. are two avenues to explore. A silicone adhesive, offered by such companies as Dow Corning, Momentive Performance Materials, NuSil Technology, or other silicone manufacturers, may also do the trick.

Bonding glass and PMMA may also work with the same adhesive you use to bond Dacron cloth to silver plate, but depending on the gap between the parts and method of assembly, it may require a lower-viscosity material. The epoxy and silicone systems will be moisture resistant and have good usage life, but most manufacturers will not warrantee an adhesive for 20 years. If the bond area can be subjected to light, one-part light-curable urethane acrylates such as 203A-CTH or 209-CTH from Dymax are options.

If a material is fully cured, there is no risk of resolvating the adhesive because of uncured monomers left behind, since everything that could react has been reacted. However, it is our experience that the light-curing adhesives used by many people do not actually reach a fully cured state. Instead of reaching a fully cured state in which 96 to 100% of reactable materials have been converted, a particular process or part configuration will sometimes reach only 75 to 80% conversion. If a material reaches only a semicured state, it could appear to be cured, providing good tensile strength and a cured surface, but have unreacted monomers at some level within the adhesive. These unreacted monomers can then resolvate or attack the surrounding adhesive, thereby weakening it and the bond joint.

This problem will be noticeable with accelerated aging, or within one to six months. A good qualification process will eliminate this risk. There are several qualification options:

● Evaluate various safety factors (cure times or intensity at 1.3×, 1.5×, 2.0×, 3.0×) to verify that the adhesive strength and properties have reached a plateau.

● Run accelerated aging tests at a moderate temperature to verify long-term stability and evaluate the adhesive in a process using Fourier transform infrared spectrometry to identify the presence of uncured monomers. A skilled analytical chemist can identify a double-bond peak, indicating the presence of uncured adhesive, or the lack of a double-bond peak, indicating that all reactable materials have been reacted.

● Use photodifferential 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 Adhesive Technology,” available from Dymax, has a color indicator that changes from blue to clear when full cure has been reached. This tool helps users to identify when the adhesive has reached a fully cured state.

Very good question! Light-curing adhesives—whether they are cured using ultraviolet (UV) light, visible light, or a combination of UV and visible light—cure from the surface closest to the lamp. If you have a droplet, the surface will cure first, and then the rest of the dome will follow. The last area to cure is against the substrate, so this leads to the question: How do you know when the adhesive is fully cured?

Adhesion to the substrate is one way to evaluate full cure. A simple test is to use a tool to get underneath the droplet. If there is liquid at the interface, then the adhesive is not fully cured. In such cases, you would need to increase either the intensity of the lamp or the exposure time. Most applications require a minimum amount of energy to achieve a good cure. Energy, or Joules/cm², is arrived at by multiplying intensity (watts/cm²) by dose (seconds). Since you want to build a process around the total amount of Joules needed to reach full cure, you can vary either the intensity or the time needed for curing. And as long as you reach the minimum energy for a given lamp, you should have a robust process.

The best way to determine whether you have a robust process is to run adhesion strength tests by bonding laps or components together to see when full or maximum strength is achieved or to perform physical characterizations for such parameters as durometer, elongation, tensile, or modulus under different conditions. When full strength is reached, additional energy (intensity or time) does not lead to an increase in adhesion properties. Compare the results of your process to the specifications in your manufacturer’s data sheet. The data sheet may indicate that the material will ultimately reach a specific durometer, such as A-40, D-60, or D-90. For example, under most conditions, if you were plotting durometer/hardness, the hardness will build (incomplete cure) and then plateau (complete cure). You should also build in enough time to add a safety margin, and then you will achieve a robust process. It is important to have a radiometer at hand because this device will tell you the energy intensity in watts/cm² or milliwatts/cm², which will be critical to 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 allows you to cure the adhesive for a shorter period of 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 the material will cure fully during production. See Cure Technology is a patent-pending adhesive method available in many Dymax products that allows the adhesive to appear bright blue in its uncured state. Upon reaching full cure under a light source, the blue color disappears, leaving a colorless clear adhesive in the bond line. The adhesive becomes clear only when it has reached enough energy to be fully cured. This product was designed to incorporate a safety margin before the color change happens, so it is an effective way not only to build a process but also to have a quality inspection system within the adhesive to tell you if you have reached full cure.