Selecting the right optics adhesive: A guide to what sticks
Optical grade epoxies, silicones, and UV curable compounds provide solutions to engineers for bonding, sealing, coating, and encapsulating in fiber optic and optoelectronic applications, as well as in other demanding areas such as medical, military, and aerospace systems. But, as always, it’s important to pick the right “tool” for the job.
For instance, adhesive systems can vary from transparent to opaque. For optical devices, the tendency is to think that you always need optical clarity, but this isn’t necessarily so. There is no hard and fast rule for many optical applications, and the requirements can vary from case to case.
Some applications may require a transparent product with high light transmission, whereas other optical assemblies require an opaque compound with light-blocking properties. In many optical devices, the adhesive may not even be in the optical path.
A one-component, 80-degree C curable, exceptionally low viscosity epoxy for bonding, coating, and sealing is Master Bond’s EP4CL-80. It is clear and is a reliable electrical and thermal insulator.
In general, optical adhesives must work well on diverse substrates, and offer excellent temperature resistance, stress relief, and protection from moisture and other corrosive elements. Systems can be formulated with additional performance properties like high mechanical strength. Handling and processing parameters such as mix ratios, viscosity, working life, and cure time play a vital role in the adhesive selection process.
Master Bond’s MasterSil 151 is a two-component, highly flexible silicone with superb optical clarity and a refractive index of around 1.405. It offers very low stress, and is suitable for use in various potting, sealing, and bonding applications involving substrates with different coefficients of thermal expansions.
The specifics of any optical adhesive application must be examined, including the geometry of the parts and their dimensions. For example, in a fiber in a ferrule-type application, the compound should be very thin or have an ultralow viscosity, so that it can wick into very narrow gaps.
In other situations—for example, mounting optics on various substrates or sealing—a more moderate viscosity or even a paste is desirable. Ultimately, the choice depends upon a host of factors including dimensions, the nature of the application, how the material will be applied, and what properties are required after cure.
EP42HT-2LTE is a two-component epoxy for bonding, sealing, coating, and select casting applications featuring an ultralow coefficient of thermal expansion. Photo credit: Master Bond
After polymerization, salient features for epoxies include dimensional stability, high modulus, low shrinkage upon cure, and a low coefficient of thermal expansion. The stress-reduction properties of adhesives are vital in many applications. Stress can arise at the bonded interface due to differences in the coefficient of thermal expansion (CTE) of the components. Over time, temperature variations can induce stress, separation, and eventual failure of the bond if large differences in CTE exist between the bonding material, and the components.
Polymer systems are engineered such that they can address these concerns across a wide range of CTE requirements. The use of additives such as aluminum oxide or negative CTE fillers such as zirconium tungstate can help achieve low to ultra-low CTEs. On the other hand, engineers can choose an optically clear silicone for applications requiring a flexible compound. Some silicone compounds offer excellent stress relief with their very low modulus of elasticity and low shore hardness levels after cure.
UV or dual-curing chemistries offer ultrafast fixturing and the ability to offer fast alignments in certain bonding situations. UV-curing adhesive systems cure with simple exposure of light of the required wavelength. In fact, dual-cure compounds cure on exposure to UV and to heat, simplifying manufacture of assemblies where UV illumination would be unable to reach concealed areas within complex geometries.
Low outgassing is another consideration when choosing optical adhesives. This feature is highly desirable in applications involving lenses, semiconductors, optical components, and vacuums. Many epoxies pass the NASA low outgassing test based on the ASTM E-595 standard, which is tested under vacuum conditions. Specific specialty silicones and UV-curable products also pass this stringent test. Military and aerospace applications may impose NASA low outgassing requirements, combining the need for optical transparency with the ability to withstand exposure to harsh environmental conditions and extreme mechanical stress. Adhesives can provide optical systems vital resistance to high-stress events like sudden acceleration, vibration, and mechanical shock.
Master Bond’s EP42HT-2LTE is a two-part opaque epoxy, which offers the lowest coefficient of thermal expansion of 9 to 12 ppm. It meets NASA low outgassing standards and withstands 1,000 hours at 85 degrees C and 85 percent relative humidity. It is a great fit for optical bonding and mounting applications where high levels of dimensional stability are needed.
Another product, EP30-2, is a two-part, clear, low-outgassing epoxy which was qualified for use in critical bonding applications for the Laser Interferometer Gravitational-Wave Observatory, a.k.a. LIGO. The main applications involved bonding prism elements, and bonding acoustical dampeners to optical components.
Optical devices in the medical industry may require nontoxic transparent adhesives that bond tightly to a variety of surfaces while remaining immune to radiation, chemical, sterilization, and biological agents. Medical device manufacturers may impose additional requirements such as the ISO 10993-5 standard for cytotoxicity and/or USP Class VI specifications. Medical applications also present unique requirements for safe and reliable curing methods.
Master Bond’s UV22DC80-1MED is a nano-silica filled, UV plus heat (dual) curable, USP Class VI and ISO10993-5 certified biocompatible and noncytotoxic adhesive that features excellent optical clarity and dimensional stability. It cures upon exposure to a UV light source emitting at 320–365 nm wavelengths with an energy output as low as 20–40 mw/cm2 and shadowed-out areas can be post-cured at 80 degrees C for around one hour.
Finally, for optical device reliability, adhesive compounds may be specially formulated to withstand environments of aggressive 85 degree C heat with 85 percent relative humidity, to ensure the adhesive remains physically and chemically sound.
The right adhesive allows peak performance for laser systems, optical devices, and fiber optics. Be it temperature resistance, stress relief, or protection from moisture or other corrosives, adhesives are critical partners for a wide range of optics and photonics applications.
Venkat Nandivada is Master Bond’s Manager for Technical Support.
Rohit Ramnath is a Senior Product Engineer at Master Bond.
