Fast-Cure Silicone Adhesives for Medical Devices

Originally Published MDDI May 2004

Adhesives



In an industry where customers demand more products in less time, fast-cure silicone adhesives may help achieve those goals.

Bill Riegler, Rob Thomaier, and Henry Sarria

Everything in our world today revolves around speed and completing tasks faster, because speed means better and more-efficient use of time. Speed is even beneficial for silicone adhesives used in medical devices. For the purposes of this article, a fast-cure adhesive achieves a full cure in less than 10 minutes or a partial cure (sufficient for handling) in less than 5 minutes when exposed to heat, and achieves a full cure at room temperature in 8 hours.

While silicone adhesives have been around for more than 60 years, silicone is actually a misnomer.¹ The suffix -one indicates that a substance contains a double-bonded oxygen in its chemical backbone. Early scientists thought that silicones did contain a double-bonded oxygen, hence the name. However, polysiloxane is the correct term for silicones, as they are inorganic polymers and contain no carbon atoms in their backbone, as seen in Figure 1.

Polysiloxanes offer excellent elastomeric properties, a phenyl-substituted temperature range of –115° to 260°C, optical clarity with a refractive index as high as 1.54, 2% shrinkage, and low shear stress.² They are also inert to the body and used in a variety of medical devices.³

With these multiple properties and applications has come the desire to decrease the cure time of silicone adhesives for a variety of reasons. Electronic potting components in devices made in high volumes cannot use a silicone that cures slowly, because that extra processing time means higher costs. In the medical device industry, parts assembly is the norm. Fabricators cannot afford to wait for an adhesive to cure before sending the joined parts to the next assembly phase. In addition, long cure times can alter schedules and incur costly delays for device repair.

Manufacturing Advantages 

Speeding up cure times offers many advantages, including

• Increasing production rates.
• Saving space.
• Lowering labor costs.
• Eliminating scheduling problems.
• Shortening customer response time.
• Improving quality control.
• Eliminating odors.
• Lowering capital costs. 
• Optimizing cure.

Cure Systems

One-part adhesives are the most common silicone adhesives used in medical devices. They have a broad range of applications, from caulking medical equipment housings to adhering pacemaker leads. These one-part adhesives are based on acetoxy (alkyltriacetoxysilane) or alcohol (alkoxy) cross-linked cure systems. The cure reaction, which requires water, is outlined in Figure 2.

One-part adhesives are very effective when cured in thin sections, and provide excellent adhesion to most substrates. One-component systems are composed of a hydroxyl-terminated silicone polymer, an alkyltriacetoxysilane or tetraalkoxysilane cross-linker, and a tin catalyst. Together, they vulcanize on exposure to ambient humidity. 

The cure liberates acetic acid or an alcohol; therefore, the process requires a controlled-humidity environment and exposure of the adhesives to the air, which is often difficult in some applications. Full cure will occur in 3 to 7 days, depending on thickness, and cannot be accelerated with heat. Unfortunately, 3 to 7 days is prohibitive in the current work environment.

UV curing has been touted as a solution to speed up silicone cure time. However, UV is not the only way to a very fast cure, nor it is it problem free. UV-curable adhesives are solvent-free systems that cure when exposed to radiant energy. As Figure 3 shows, these adhesives need a photoinitiator to activate the cure. 

Light energy in the ultraviolet range of the spectrum (200 to 500 nm) is absorbed by the photoinitiator, causing a photochemically driven cure that is completed in seconds throughout the bond.⁴

Components of UV-light curing systems include a light source (usually a quartz lamp), a power supply, reflectors to focus or diffuse the light, cooling systems to remove heat, and a conveyor to move coated substrate to and from the light source.

Not all of the UV-curable silicone adhesives available in the market are fast-cure systems. Alcohol-cure systems can be partially cured in 20 seconds, but take 72 hours to cure fully.5 Other systems can take as long as 20 minutes for full cure because they have to go through the light tunnel repeatedly. Other problems include line-of-sight curing limitations (not curing in shadowed areas), and costly capital expenditures, qualification, and maintenance. All these factors minimize the potential benefits.

Fast-cure adhesives, based on a two-part platinum catalyst system commonly called addition cure, do not require moisture or open air. The two parts generally both contain a vinyl functional silicone polymer with the platinum catalyst added to Part A and a hydride functional cross-linker and an inhibitor added to Part B. Also often included in both parts are reinforcing fillers, pigments, and special additives such as barium sulfate for radiopacity.

For these adhesives, the cure involves the direct addition of the hydride functional cross-linker to the vinyl functional polymer, forming an ethylene bridge cross-link, as demonstrated in Figure 4. Unlike one-part systems, this mechanism involves no by-products, allowing the systems to be cured in closed environments.

Most platinum systems can fully cure at room temperature in 24 hours, or the cure can be accelerated with heat. They can be partially cured with heat until tack-free and then packaged; curing will continue in the sealed package with no adverse effects. However, it may be necessary to take special care in eliminating the presence of contaminants that might impair the catalyst.

Adhesive Options

Several adhesives are suitable for medical product applications, depending on which properties will best suit the device. For example, a silica-filled, addition-cure general adhesive is useful in device electronics, balloon adhesion, and subassembly (see data for Material 1 in Table I). Such an adhesive obtains a fast cure at a temperature of 65°C and can be fully cured in 1 minute. For adhering temperature-sensitive substrates, this system would be a good choice. It also would be effective in packaging an assembled part after a quick partial cure, as two-part, addition-cure systems can be cured in closed environments.

Another kind of adhesive has an extended work time of 6 hours, and must be cured with heat (see data for Material 2 in Table I). Although it needs a higher temperature to obtain a fast cure, it fully cures in 2 minutes at 100°C. This adhesive can be useful because of its extended work time. In device electronic assembly, the same mix tip could be used throughout an entire shift.
Finally, a high-durometer, tough adhesive would be ideal for high-strength applications like feeding or trachea tubes (see Material 3 in Table I). This type of adhesive would prove a good choice for a high-temperature cure at 185°C. It can achieve a 300-psi lap shear in 30 seconds. 

For many applications, this is a good partial cure. For device repair work, the adhesive could be applied, heated with a hot heat gun for 30 seconds, and then fully cured at room temperature over time. 

Conclusion

Manufacturing industries generally employ processes that speed production. Speed means more-efficient use of time and space, improved scheduling, and improved customer response. Faster cure times also improve quality control and lower capital costs. While medical device production is a high-end manufacturing industry, it takes advantage of many techniques favored in more mature, commodity industries, especially those that increase production speed. 

The fast-cure silicone adhesives described in this article not only offer time-efficient and production-friendly curing, they also feature additional properties that make them suitable for a wide range of applications. Based on convenient, addition-cure systems, fast-cure silicone adhesives can provide many material solutions for medical device manufacturers.

References

1. W Noll, Chemistry and Technology of Silicones (New York: Academic Press, 1968).
2. K Rhodes, “Adhesives Deliver Low Shrink, Low Stress Bonds and Fast UV Cure,” Optomechanical Engineering 2000 4198 (November 2000).
3. National Academy of Science, Institute of Medicine silicone study, June 23, 1999.
4. C Bachmann, “UV Structural Adhesives and Sealants,” Adhesives Age (April 1999): 24.
5. C Salerni, “ Selecting Engineering Adhesives for Medical Device Assembly,” Medical Device & Diagnostic Industry 22, no. 6 (2000): 90.  

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