 Nadine Blaesing Dymax Corp. Market Segment Manager Visit Website |
Nadine Blaesing is the market segment manager, Europe, for Dymax. She has more than 12 years of experience selling adhesives and providing technical assistance to the medical device market. Although she began her career as an applications engineer in Europe, and later in the United States, Blaesing eventually transitioned into marketing while keeping her focus on the medical device industry. Dymax specializes in the manufacture of light-curable adhesives, cyanoacrylates, epoxies, light-emitting curing equipment, and dispensing systems.
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Adhesives and Epoxies Q&A
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I am using a wicking cyanoacrylate adhesive to bond a stainless-steel hypotube to a polycarbonate cylinder. The stainless steel is roughened and cleaned using alcohol before bonding. The outer diameter of the hypotube is 0.065 in., and the inner diameter of the cylinder is 0.067 in. After initial production, the bond holds around 500 N of load. After EtO sterilization, the average load at break is around 370 N, so we do see some weakening. However, the bonds are still sufficiently strong. Also, after sterilization, due to property changes in the rest of the device, there is an approximately 3.4-N load on the joint, which remains as the product sits in its packaging. The product undergoes accelerated aging, in which it is placed in a 50°C temperature for 52 days in a low-humidity environment (typically, 5–15%). After this conditioning, the bonds have weakened significantly, with an average load at break of only 70 N. Product that is not accelerated-age conditioned but has sat on the shelf for four months after sterilization shows an average load at break of around 260 N. It has also shown some weakening over time. In all of the above tests, when the bond is broken, it appears that the cyanoacrylate is breaking off of the stainless-steel hypotube and staying with the polycarbonate. The polycarbonate is clear, annealed, and vapor polished. Is this weakening over time something you would expect to see in a cyanoacrylate bond between stainless steel and polycarbonate? If not, are there particular factors in our conditioning that could be at fault—the temperature, the residual load on the joint, etc.? The tech data sheet indicates that the adhesive should not lose strength retention at 50°C.Answered July 12th, 2010 by Expert:There are a couple of things going on here that are contributing to the failures under accelerated aging but that are not seen after product sits on the shelf and undergoes real-life aging. Cyanoacrylates are susceptible to a chemical reaction called hydrolysis. When placed in a hot and humid environment, the chemical polymer in the cyanoacrylates tends to break down. EtO sterilization uses steam or humid water vapor as the carrier to bring the EtO into the chamber, so we typically view this as a moisture-rich environment. A slight decrease in bond strength with EtO is expected.
It is also important to realize that cyanoacrylates are very brittle adhesives. If you have ever stuck your fingers together accidentally with a cyanoacrylate and then moved them together in a rubbing motion, the adhesive will crack and eventually enable you to separate your fingers. As you put the parts into a 50°C oven, you correctly lower the humidity, but there is still a concern, since the temperature remains fairly high, which helps drive chemical reactions. The length of time involved is fairly long, and with even a low amount of humidity, additional hydrolysis reactions may be occurring.
Another issue relates to the coefficient of thermal expansion (CTE) differences between the stainless steel, the polycarbonate, and the adhesive. Materials expand at different rates at a given temperature. The metal expands at one rate, the polycarbonate expands at another, and the adhesive at yet another. This puts additional stress on the bond line, a phenomenon that does not occur when product undergoes aging in real life. This interaction may be enough to crack or break enough of the cyanoacrylate bonds to cause your bond-strength test to fail.
Between the two substrates, the cyanoacrylate is most similar to the polycarbonate. Therefore, the weak bond will most likely be to the stainless steel, which matches your observations. In addition, adhesives such as cyanoacrylates can penetrate slightly into the surface of the polycarbonate and become chemically embedded or intertwined with the polymer. In such cases, the only bonds available on the stainless steel are weaker van der Waals forces and mechanical interlocks into the microscopic nooks and crannies on the metal surface. These bonds are typically the first to break.
Other adhesive options such as urethanes, acrylates, light-curable acrylated urethanes, heat-curable acrylated urethanes, or epoxies can have a lower modulus value, which will be able to absorb the stresses of the CTE differences. Even rubber-toughened cyanoacrylates may be an option. These adhesives are able to withstand shock or impact better than cyanoacrylates because they are not as brittle. This makes them better suited to survive much higher temperatures and stresses.