"I need a recommendation of a medical grade non-cytotoxic UV adhesive for bonding together clear polystyrene moldings. What adhesive would you recommend? What is a simple but effective method to monitor if the curing reaction has gone to completion? Is there any dosimeter available to accurately measure the UV exposure? Would a post bake after UV curing help? Does anything "outgas" from the UV medical-grade acrylates during curing?"

Two options come to mind when looking for a medical-grade light-curable adhesive for polystyrene – 1201-M-SC and 1120-M-UR from DYMAX Corporation. These materials exhibit excellent adhesion to polystyrene, and are both ISO10993/USP Class VI tested. Product data sheets are available at www.dymax.com. One simple but effective method to monitor if the curing has gone to completion is incorporated into the 1201-M-SC product. This material uses a technology called "See-Cure", where the material starts off with a brilliant blue color, and as it cures changes to clear. This is an excellent visual indicator that complete cure has been achieved in all parts of the bond line. Other methods to determine state of cure include destructive testing of the components to measure tensile force, or a drop of adhesive at the bond-line surface and using this droplet to measure for tack/semi-cure (a go/no-go measurement observed by the presence or absence of adhesive transfer onto a gloved finger). More complex methods include microscope FTIR analysis of the adhesive to identify the presence of the double-bonds peak (on the spectrum) before cure, and the removal of the double-bond peak after cure.

Dosimeters are necessary to accurately measure light exposure, and there are different versions, with different sensors, that measure different parts of the UV and visible light spectrum. While most light-curable adhesives cure with a combination of UV-A, UV-B, UV-C, and visible light, it is often convenient to reference the UV-A light spectrum coming from the light source. UV-A is commonly referred to as 365 nm, but actually covers a range of approximately 320-395 nm. This can be measured with an ACCU-CAL™ 50. If the polystyrene is UV blocking, then you would have to rely on the visible light spectrum of the lamp. The ACCU-CAL™ 50V measures 395-465 nm. Both units can give you average intensity (mW/cm²), peak intensity, and total energy (Joules/cm²). Other options are available like the ACCU-CAL™ 50 LED, which was developed for special lamps (such as LED lights which only emit a single wavelength at either 385 nm or 405 nm) to integrate around the center of the lamp spectrum.

A post bake is not necessary on most adhesives, but there are a few adhesives with a peroxide thermal initiator, which can use heat to cure areas not able to see light.

In regard to the question on outgassing of UV light-curable adhesives during cure, it is sometimes observed that a small amount of smoke comes up from the adhesive surface during the cure step. This is typical, as the adhesive may emit trace amounts of some of the ingredients (or fractions of the ingredients) contained in the formulation while light is shining on the adhesive and cure is taking place. Sometimes this can be overcome by varying the intensity and duration of cure, as well as adhesive choice and light source. This does not happen when the adhesive is used between two surfaces. Proper ventilation can help remove this smoke. If the smoke deposits onto a spot or flood lamp, then periodic cleaning of the end of the lightguide or lamp housing should be done to remove the film that may form there, as this thin film can reduce the intensity at the bond line.

Adhesives, Medical 320-395 nm, 365 nm, 395-465 nm, ACCU-CAL 50, ACCU-CAL 50 LED, ACCU-CAL 50V, adhesion to polystyrene, Adhesives, average intensity mW/cm2, bond line, bonding polystyrene, complete cure achieved, cure lamp, dosimeter, double bond peak, duration of cure, effective method to monitor curing reaction completion, film reduce intensity at bond line, heat cure, intensity, ISO 10993, lamp housing, lamp spectrum, LED cure lamps, light source, light-curable adhesives, lightguide, measure light exposure, measure semi-cure, measure tack, measure uv exposure, medical grade non-cytotoxic uv adhesive, medical-grade light-curable adhesive polystyrene, microscope FTIR analysis, outgas, outgassing during cure, outgassing of uv light-curable adhesives during cure, peak intensity, peroxide thermal initiator, post bake, see-cure adhesive technology, single wavelength 385 nm, single wavelength 405 nm, state of cure, tensile force, total energy Joules/cm2, USP Class VI, uv blocking, uv curing, uv lamp, uv light spectrum, uv medical grade acrylates, uv medical grade adhesive, UV-A, UV-B, UV-C, visible lamp, Visible Light, visible light spectrum, visual indicator

"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.

Adhesives, Medical, Structural accelerated aging, adhesive bond joint, adhesive bond weakening, adhesive cure indicator, adhesive failure, adhesive fully cured state, adhesive tensile strength, cured adhesive, double bond peak, ensure full cure, Evaluate adhesive FTIR, fully cured adhesive, identify presense of uncured monomer, light-curable adhesives, measure crosslink density, monomers dissolve cured resin, monomers soften cured resin, photo-differential-scanning calorimetry, qualification process, resolvating adhesive, See-Cure color change cure indicating adhesive technology, semi-cured adhesive, surface cure, uncured adhesive, uncured monomers, unreacted monomers, uv curing adhesive, UV Light Curing Adhesive