Canada Balsam - distilled sap from the balsam tree - was one of the first materials to be used as an optical adhesive. Whilst of high optical quality, its poor thermal and solvent resistance caused it to be superseded during World War II by more robust materials. To meet the production and performance requirements of optics applications, engineers are often turning to UV curing adhesives. Peter Swanson, Managing Director of Intertronics, explains the factors to consider when choosing an optical adhesive.
Optics engineers commonly require adhesives to bond components together during optical assembly, lens and prism bonding, optical element positioning and fixturing, and fibre-optic assembly.
Every adhesive has its own strengths and limitations, which can make it challenging for the engineer to determine which is the most suitable material for the task at hand.
Selecting an adhesive requires careful consideration, weighing up the properties of the material according to the requirements of the application, balanced against the process demands. Of course, optical transmission and refractive index are often primary considerations, but there are a number of other things to think about.
To some extent, all adhesives shrink in volume during curing. This can cause stress on the parts and can cause focussing and alignment issues during processing. To overcome this, the engineer can opt for a material with low shrinkage. Standard epoxy adhesives typically have a shrinkage of around three to five percent. There are special UV curing optical adhesives that shrink 0.2 to 0.4 percent, whilst maintaining optical clarity. The hardness and modulus of the cured material may be important for the integrity and performance of the structure.
The optics engineer should also consider outgassing - the release of volatile materials both during and post cure - when selecting an adhesive, if the condensing of these volatiles onto optical surfaces will lead to quality issues.
Curing and handling
The optics engineer should consider the curing method of the adhesive and how it will affect the speed and complexity of the project. UV curing adhesives cure in a few seconds, which can be beneficial if speed is a production requirement. This rapid curing is useful during alignment or focussing, as the adhesive is quickly cured once the positioning process is complete, releasing complex jigs and fixtures for the next assembly.
Two part optical epoxies take longer to cure than UV cured adhesives (minutes to days as opposed to seconds); they can be accelerated with heat, although the thermal excursion can induce stress into the parts during and post cure.
The viscosity of the adhesive is also a factor. Will the adhesive be required to fill a gap; wick into a gap; or bridge a gap? If a two-part system is specified, mixing becomes a process concern. Special equipment may be needed to provide a repeatable homogenous mix and degassing may be required if the mixing process induces air that becomes an unacceptable artefact in the light path.
UV curable acrylate adhesives are a popular choice for optics applications because they are easy to use and offer a rapid cure time. For example, low-stress, low movement UV curable optical assembly adhesives, such as from the Dymax OP range, offer optics engineers options that cure in seconds, while still providing strength and clarity. Offering shrinkage as low as 0.1 percent, high-tensile-strength bonds of 3,000psi in a single component, this type of adhesive is a compelling option for many optics applications.
Optical adhesives have come a long way since Canada Balsam and the wide range of options now available can make choosing the right adhesive for your application a challenging task. By working with an experienced adhesives supplier, optics engineers can make informed decisions on the most suitable material and method for their application.