Controlling Heat Curing Adhesive Processes Using Infrared Spot Curing
- Sunday, 01 January 2012
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.
The 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
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.
The 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.
The 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
The 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.