Fiber optic, opto-electronic and photonic applications must have superior transparency, requiring optically clear epoxies, polysulfides, and silicones that exhibit excellent light transmission.
Contributed by Rohit Ramnath, Senior Product Engineer, Master Bond
Optical transmission, the ability for light to be channeled through a material, is particularly important for fiber optic, opto-electronic and photonic applications. As a result, superior transparency is an essential characteristic of an optical adhesive. Optical transmission is measured from 0 to 100% on a wavelength scale of approximately 200 to 3100 nanometers (nm). Another common and interchangeable measurement of wavelength is microns, μm. Ultra violet (UV) light typically falls in the 200-400 nm range. From approximately 400-700 nm (0.4-0.7 microns), there is “visible light.” After that, from about 700 nm to 1 mm, you have “infrared light,” (IR) which can be further broken down into near infrared (0.75-2 μm), middle infrared (2-15 μm), and far infrared 15 μm to 1 mm.
Most optically clear epoxies, polysulfides, and silicones exhibit excellent light transmission across the visible, and near IR ranges, with some specialty systems providing a high transmission in the UV and in the middle IR wavelength ranges.
Optical transmission is required in numerous applications. For example, a manufacturer of IR lasers would inquire about epoxies with a transmission capability in the wavelengths between 800 and 1500 nm range. Other applications include LED lights, display panels, and optical lens coupling, which might need optically clear adhesives, sealants, and encapsulants which transmit in the visible range. There are even rare cases where a manufacturer of night vision goggles may need to block UV light.
To help engineers meet specific requirements for a wide variety of optical related applications, Master Bond has tested several epoxies through independent laboratories, as well as various research organizations at universities over the years. To measure the optical transmission of an epoxy, a cured film, usually about 0.002 to 0.004 in. (2 to 4 mils) in thickness, is prepared. This specimen would then be put through a UV-Vis and IR spectrophotometers to obtain the transmission curve.
Some of the products that Master Bond sent out for independent testing include EP30, EP30-2LB, MasterSil 151, MB600, and UV15. The results and conclusions are reported in the accompanying table* and graphs below. *Disclaimer: Note that the percentages in the table are estimates based on the chart produced by the spectrometer.
The graph in Figure 1 shows the results of EP30-2LB, MasterSil 151 and MB600. EP30-2LB is a two-component room temperature curing, optically clear epoxy. Test results showed that this epoxy transmits well from 450 to 900 nm and above. However, it has the unique capability to block UV light completely from 200 to 400 nm, and is widely used in applications where UV blocking is critical, yet transmission is needed in the visible wavelengths.
MasterSil 151 is noteworthy for its low index of refraction and its ability to transmit light very well from 225-2500 nanometers. It is a silicone compound that combines flexibility, high-temperature resistance, strong electrical insulation, and optical clarity. These properties enable it to be used for coating, sealing, and potting applications involving sensitive optical and opto-electronic type applications.
MB600 retains its optical clarity after curing and has very special light transmission properties, particularly in the UV range of 200-350 nm. It is a one part, aqueous based sodium silicate system widely used for coating applications requiring excessively high temperature resistance, since it is serviceable up to 1,500°F.
The graph in Figure 2 highlights the results of wavelengths ranging from 400 to 3000 nm for these adhesives: EP30, MasterSil 151, MB600, and UV15. EP30 shows very good optical clarity with superior light transmission when compared to many other epoxies. It is a low viscosity, two-part epoxy, widely used for bonding, coating, sealing and casting applications. UV15 is a UV curable epoxy based system, which provides excellent optical clarity and light transmission properties.
To meet ever increasing industrial demands, Master Bond continues to develop advanced systems with unparalleled optical transmittance.