"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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"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 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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"I need to bond ABS to ABS in a pure-water environment (50° to 180° F). Concern for leaching chemicals into the pure water is high. What FDA-approved solvent choices do I have?"

One website that I found listed various solvents like Cyclohexanone, Cyclohexanone/THF, Cyclohexanone with various medical-grade acrylic polymers dissolved in the solvent for added strength, and various other combinations. www.ineos-nova.com. The grade of solvents are typically not listed as Medical Grade or FDA approved, but based on the level of purity of the solvent. Obtaining the solvent of choice with the highest purity (99.9% or higher) would limit the potential leachables into the water. Sigma Aldrich or Alpha Aesar both carry small quantities of these solvents in various grades for evaluation. Methyl ethyl ketone (MEK) is an alternative solvent system. Using a solvent requires special handling due to the smell, flammability, and explosive storage requirements, and is carefully monitored by the EPA. Dispensing systems like those from Technoideal are options to limit operator exposure . If you want to consider a solvent-free adhesive, you might look at a 1-part, light-curable adhesive like DYMAX 1161-M if you can get visible light to the bond line (non-opaque parts), or a 2-part urethane or epoxy from companies like Epoxy Technology or 3M (to name a few) may be considered. These alternatives to solvent can provide a bond almost as strong as solvent, fill gaps in the molded ABS bond lines, and are much more environmentally friendly.

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A recent question that came through:

“Can you recommend an adhesive that will adhere to inorganic salts?  I have used UV-curable adhesives in the past, including some of your products, and had good results in bonding poly-blend substrates.  But I’m not sure if that type of adhesive would work with the salts.”

Response:

Our experience has been that adhesives that bond to other inorganic materials like glass and metal will bond to inorganic salts.  We have seen applications where inorganic salt plates/crystals were bonded together with very good results.  We have also had success with salt-like crystals that are deposited onto a thin film of adhesive, imbed themselves into the coating, and then are cured in place.  It comes down to selecting an adhesive with the right properties important to your application, such as durometer/hardness and viscosity.

Structural

“Our application requires a watertight seal between FEP tubing (0.8 mm OD, 0.2 mm ID) and a borosilicate glass capillary (0.17 mm OD, 0.10 mm ID) with an overlap of 1-3 mm. This is part of a one-time use, disposable cartridge. We are currently using a 5 minute epoxy because the zero shrinkage is advantageous. The cure time, however, is not. We have tried UV-cure epoxy in the past for this joint but found that the epoxy did not cure inside the FEP tubing. Any suggestions?”

UV light-curable epoxies typically cure with the UV spectrum from 300-390 nm and do not make use of visible light to cure. With the semi-hidden bond described in the application above, switching to a visible-light-curable, acrylated-urethane adhesive would be worth trying. A visible-light-curable adhesive will allow more of the available light to hit the adhesive and cure deep within the FEP tubing.

An important issue with this application is that the borosilicate glass capillary will act like a light fiber.  It will take the light, carry it like a fiber-optic cable, not allow it to get to the adhesive (as it is bouncing the light internally within the glass), and move it past the bond area. The visible-light-curable adhesive should be exposed with high-intensity light. The adhesive should allow the light to penetrate into the gap.

One alternative to a visible-light-curable adhesive is a cyanoacrylate adhesive that could cure deep within the FEP tubing without light.  Due to the deep overlap area in this application, only the top surface would be exposed to water and could provide enough protection to create a water-tight seal.  Another alternative is a new product, DYMAX 9440 A/B, which is a light-curable silicone adhesive.  This material is unique in that you can expose the adhesive to light during dispensing and assembly and still have enough time to assemble the part before the material starts to set up.  DYMAX Applications Engineers can work with you one-on-one to discuss exact options and materials.

Medical, Structural Adhesives, CA's, Cyanoacrylates, Epoxies, FEP Tubing

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?”

Response:

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 200^th 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.

Structural Acrylic, Plastic Bonding, Plastics, Polycarbonate