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Controlling Heat Curing Adhesive Processes Using Infrared Spot Curing

Novel infrared fibers provide precision heating and curing of glues in medical device assemblies, improving workflow and design.

Adhesives are often used as the joining compound between substrates in the medical device industry. Typical applications for adhesives include tube-to-connector bonding, steel-cannula-to-hub bonding, and any other joining process. Adhesives work particularly well in the assembly of dissimilar materials where traditional solvent-welding methods are being eliminated due to workplace safety legislation and where other joining methods such as ultrasonic welding and laser welding are inadequate.

altThe medical device industry has migrated in many of its adhesive joining processes to ultraviolet/visible curing adhesives for applications where exposure to the light source is possible and towards engineering epoxies, poly urethanes, and silicones, where opaque substrates demand alternate reactive chemistry. These reactive adhesive chemistries are often structured in the form of solvent-free catalyst/resin mixtures that can also be accelerated through the application of heat. The need to maintain components aligned during the slower polymerization processes of these reactive chemistries has required the use of expensive fixtures or heating sources and the use of batch operations outside the normal assembly flow of the medical device. In addition, the integration of microelectronics and optomechanical elements in medical devices makes their design and assembly more complex and increases the requirement for rapid online fixturing or curing.

The solution to the controlled cure of thermally cured adhesives comes from the advances in photonic technology and innovations in glass fiber technology. These technological advances have led to the creation of novel optical fiber light bundles coupled to the packaging of an intense photonic radiant energy source to provide a very powerful punctual source of heat that can be integrated easily into assembly processes: the iCure.

altThe principle of photonic radiant heat used by the iCure works within the physical laws of thermodynamics, where every surface that is irradiated will absorb, transmit, or reflect photonic energy that impinges on it. The amount of radiant energy that is absorbed creates very fast molecular movements and ultimately heat that either transmits through the substrate in a controlled manner or causes an adhesive to accelerate its reaction once it reaches its set catalytic temperature.

The iCure Thermal Spot Curing system has been designed to provide a point source of photonic energy along a wide range of wavelengths (from the UV to the mid infrared), through a uniquely transmitted medium (the iCure fluoride fiber optic light guide) and through a set of optics within a system that not only provides controllable energy (wavelength and irradiance controlled) but also allows the user to create heating profiles according to their own particular design needs. See Fig. 1 to view the desktop footprint of the iCure and how its small footprint allowed it to be easily integrated into medical device assembly lines. No other controlled spot heating system like the iCure currently exists due to its proprietary use of mid- IR optical fiber technology to remotely deliver the power in the unit. The iCure joins photonic technology, material innovations with optical fibers, and the integration with adhesive curing processes to provide solutions to design and manufacturing engineers.

altThe power distribution along the spectrum of emission as shown by Fig. 2 is particularly interesting for the medical device industry, as different segments of this radiant energy can be responsible for generating heat using simple thermodynamic principles. For example, wavelengths below 750 nm (composed of ultraviolet and visible energy) absorb into colored plastics preferentially, but also absorb well into opaque surfaces commonly found in micro-optic or microelectronic medical assemblies. The transformation of light into heat is particularly rapid due to the power of low wavelengths, but paradoxically, this heat generation is concentrated on the surface layers of the substrates or adhesive. However, wavelengths above 750 nm transmit particularly well through deeper layers of substrates and adhesives. This region of the spectrum is the near and midinfrared region, which is also absorbed and transformed into heat very quickly by many of the fillers and form modifiers used in adhesive formulations such as silica, silver, and aluminum, some of which are also used in solder pastes.

In addition, wavelengths in the midinfrared above 2500 nm are particularly important for their heating effect resulting from absorption and vibration energy of the C-H and O-H bonds in the typical organic adhesive formulations. The rapid rise in heat also controls excessive flow-out of the adhesive by driving the cure to the gel point more rapidly and reliably. Controlling the spectral emission (bandwidth), the power output, and the length of time at a set temperature, a temperature profile can be generated that cures the adhesive in a lowstress environment.


altThe innovative photonic radiant manner of curing has not extensively been compared to traditional curing methods by specialty glue manufacturers. This is typical of emergent technologies as adhesive formulators are currently trying to develop curing profiles that integrate the particular high irradiance found in the iCure and how this can benefit their curing requirements. Although comparisons to conventional curing methods are application-specific, there are some basic elements that can be benchmarks of performance after cure. For example, the degree of cure of a thermally curing adhesive cured with the iCure can be determined through differential scanning calorimeters (DSC). Other analytical techniques are available to examine various properties of cured adhesives that are beyond the scope of this technical brief.