27 декабря, 2015 
Pokraskin It is of utmost importance that the adhesive has good mechanical properties when optical lenses and prisms are bonded to mechanical fixtures and mounts. As the bonding area is usually smaller than in the case of optical cementing, the mechanical strains are much higher. The dimensions ofthe elements, the temperature range and the coefficients of thermal expansion of the substrates to be bonded determine how flexible the adhesive must be. A key characteristic of these adhesives is elasticity to prevent strains from occurring within the optical component over as large a temperature range as possible, without impairing placement accuracy through creeping effects of the adhesive. Strains caused by thermal expansion can be minimized not only by a highly elastic adhesive, but also by sticking to a predefined thickness of the bond-line. The optical system must be designed to prevent any liquid adhesive from penetrating into thin centering gaps. At a bond-line thickness of a few micrometers, even elastic adhesives will cause strains in the optical components at temperature changes.
Black or dark adhesives are well suited for the mounting of lenses and prisms when light scatter must be prevented. Bubble-free application of the adhesive is a key requirement for process stability, as it improves the tightness and moisture aging stability of the bond.
To allow for good adhesion of the adhesive on the glass surface, the hydrated glass surface must be cleaned immediately prior to bonding. For industrial applications, the glass elements are cleaned with aqueous tenside solutions in special ultrasound equipment, where alkali metal ions as well as particles and impurities are reliably removed from the surface. In spite of this cleaning process, following moisture aging, the adhesive does not adhere sufficiently to the glass surface, and this results in an inadequate long-term stability of the bonding. Special glass primers, mainly derived from silanes, sustainably improve the resistance to moisture aging.
Besides the capillary forces that play a pivotal role, a key characteristic of automatically applied adhesives in the bonding of optical components is the viscosity. Selecting the correct viscosity of the adhesive and choosing the correct design allows the bond-line thickness to be kept constant, resulting in a tight, strain-free bonding.
When UV-setting adhesives are employed, an optical alignment tool can be used to accurately place the optical components, and the mechanical tolerances of the elements can be leveled out by a variation of the bond-line thickness. However, as it is not always possible to irradiate the optical elements from every side after joining, a combination of setting mechanisms (hybrid of UV light and heat, or UV light and anaerobic conditions) offers a solution to this problem. These systems, however, have limited temperature stability due to the high coefficients of expansion of the flexible adhesive systems. Owing to thin bond-lines, the placement accuracy of optical microcomponents is better than that of optical components with larger dimensions.
The key requirements of adhesives used for the bonding of lenses and prisms to fixtures and mounts are resistance to fungal attack and low outgassing figures, and this makes epoxies, polysulfides, polyurethanes, urethane-modified acrylics and silicones (for special applications, e. g. in space) the first-choice adhesives. Unfortunately, silicones have a poor resistance to fungi, and the surfaces may be contaminated by dimethyl siloxane spreading out onto the surface; the surface can then no longer be cemented, bonded or coated unless a special surface preparation is carried out. Hence, the processing of silicone-based adhesives is subject to special guidelines on the handling of this material to prevent surface contaminations.
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