Information Requirements

Prior to 1946 optical elements were bonded with purified, filtered Canada balsam. Balsam was easy to apply and in most cases an optically compatible bonding medium except that it had little thermal or solvent resistance. The war and advances in aviation underlined these limitations and so a synthetic resin adhesive was developed. This adhesive required very high temperatures and long curing times, so research was conducted by DRs. Souren Sadjian and Marco Petronio to develop a low or room temperature, catalyst cured adhesive. The research was done under the auspices of the U.S. Government at the Frankford Arsenal in Philadelphia. The result of that research was a two component polyester resin based cement that many optical companies use today.

Since then, private adhesive manufacturers have developed a myriad of polyester, epoxy and urethane based single and two component adhesives for bonding optical elements. All of these adhesives are more complex than balsam and demand a thorough knowledge of what the finished optics will experience in order to choose the correct one. Before choosing, the technician must take into account physical aspects of the elements as well as the optical properties and the environmental conditions the finished optics will be expected to withstand.

Listed below is some but perhaps not all of the information that must be known to correctly choose the adhesive to bond an optical element.

PHYSICAL

1. Do the elements have chamfers?
2. Depth and radius of curvature.
3. Will there be post cementing processing? (Cutting or Grinding)
4. Types of materials to be bonded.
5. Coefficients of thermal expansion.
6. Bond line configuration.
7. Surface area vs cement viscosity.

OPTICAL

1. Refractive indices of the elements.
2. Transmission of the elements.
3. Tolerance of internal reflection and absorption.

ENVIRONMENTAL

1. Working temperature extremes.
2. Mechanical Shock requirements.
3. Chemical resistance requirements.
4. Pre-bonding chemical and substance exposure.

All of the items listed must be known by the bonding technician and perhaps communicated to the adhesive manufacturer in order to correctly choose the cement to be used. However, even with this done, witness pieces or samples of the elements should be tried and tested prior to production. It will be made evident in the following sections why they are important and how they can adversely affect the finished optics if overlooked.

We will first discuss bonding a simple crown and flint doublet with a two component adhesive in order to establish the proper preparation of this type of adhesive and also to establish a technique for applying a cement whether it be of the single or two component variety. We will also discuss the preparation of the elements themselves. A section is devoted to UV curing adhesives to show their advantages as well as some of the problems that can arise with their use. Because of the difficulty in trying to itemize all preventative measures that should be taken to avoid bond failures, a section on bond failures themselves is discussed including the causes and preventative measures. Finally an overview on perhaps some of the equipment a cementing department should have to insure successful bonding.

Substrate and
Cement Preparation

Preparation of
the Optical
Elements

One cannot bond optical elements that are not clean. Residues left from the manufacturing and handling of the elements will place a barrier between the bond surface and the adhesive. In most cases a light wiping with reagent acetone just prior to the application of the adhesive will be sufficient if the elements have gone through several cleaning stages during their manufacturing phase. However, the technician must be aware that through the physical location of the elements in any bulk cleaning bath or because of the pitch or polishing compounds used in manufacturing, some residue that is resistant to a drag cleaning process might still exist. There is also a possibility that the element is of a material that precludes the use of solvents i.e. plastic elements. Therefore, an immersion or ultrasonic cleaner should be part of the cementing department. Usually a few minutes in a mild acid solution followed by a deionized water rinse and then a mild alkaline solution and then again a deionized water rinse will remove most common soils. The drying can be by solvent if the element material will tolerate it. If not, hot, filtered nitrogen can be used on acrylics.

In final analysis it is incumbent on the technician to know the chemicals and materials that come in contact with the element, the cleaners or solvents that will remove them, and the process that will do it most effectively.

Properly orienting the elements prior to bonding may seem a rather elementary step. However, it is very important to know that the surface you place down will match with the surface containing the cement. It is very easy to forget what surface is what. If done incorrectly, the elements must be taken apart, cleaned and the process started over. There is also the possibility of scratching the mismatched surfaces. Leaving some reminder may prevent loss of time and even elements.

Preparation of
the Optical
Adhesives

Since we have decided to conduct this particular exercise with a two component cement and have already been warned to follow the instructions exactly, we will assume that the manufacturers suggested catalyst to cement ration will be used. We will also assume that we will not mix too much more cement than we can use in the manufacturers stated pot life. Do not mix such a small quantity that proper mixing or accurate ratios are jeopardized. The next area to consider is the mixing container and mixer. Small amounts of cement can bond a relatively large number of lenses. Do not use a 30cc container to mix 3cc of cement and do not use a metallic container when the catalyst or hardener is an oxidizer such a methyl ethyl ketone peroxide. If the container is too large in comparison to the quantity of the cement, you will not be able to accurately assess whether the two components have reached homogeneity. Compatibility is also important. A metallic stirrer or mixing container could react with either of the components causing a runaway polymerization or worse, discoloration or hazing of the cement upon cure. For our purposes let us choose a 10cc test tube and a polypropylene stirring rod to mix 3cc of cement. We accurately measure the cement into the test tube and drop in the catalyst according to the manufacturers suggested ratio. We will mix the cement for approximately 60 - 90 seconds or until the mixture is visibly homogeneous. In many cases the hardener or catalyst will have a different optical density from the cement and this will aid in checking the homogeneity. The cement will be left to deaerate for 3 - 5 minutes depending on the manufacturers pot life time. Since we are only bonding a single doublet we will use the stirring rod to apply the cement. The use of other applicators for production quantities will be discussed in a later section.

A Cementing
Technique

Most manufacturers give a "pre-cure time" in their instructions. This time is given to tell the technician when the cement is sufficiently firm to permit removal of any holding devices. This not only frees holding devices for more bonding but also prevents holding devices from being bonded to the perimeter of the lenses. So attention should be paid to these pre-cure times. The doublet can now be left to cure either at room temperatures or placed in an oven depending on the manufacturers instructions.

Something should be said at this time about cure speeds. Some lenses, especially those with deep curvatures or those that will see extreme temperature changes, may require slow curing adhesives.

Shrinking during cure can set perpendicular stress forces in lenses with 90 degree or greater radius of curvatures and elements of different coefficients of thermal expansion will incur strain while curing at oven temperatures. Manufacturers have attempted to alleviate these problems by introducing plasticizers and flexibilizers into their adhesives but these are compromises at best. These additives can increase outgassing or reduce the cohesive strength of the cement. Slowing the cure of an unmodified cement in many cases yields the desired strength without the incurred stresses.

After the lenses are cured, the remaining run-off can be cleaned with a damp tissue of acetone and if the elements permit, a razor blade. Attention should be paid to whether the cement used is anaerobic. If so, caution should be paid to keep too much acetone out of the chamfer area. The cement in this area may still be soft allowing solvent migration into the bond surface.

Ultraviolet Curing
Adhesives

Two types of ultraviolet curing adhesives predominate in optical cementing; urethane based copolymers and polyester resin based copolymers, each has its advantages and disadvantages.

Urethane based copolymer UV curing optical adhesives are generally water white and some are non-anaerobic. They exhibit faster cures and are more wavelength specific in the cross-linking.

The water white color and the non-anaerobic qualities are excellent when embedding or using the cement to tack the perimeter of a lens during centering operations but when bonding a thin lens or one with a sharp radius the non-anaerobic aspect along with what at present appear to be higher shrinkage rates can cause considerable strain or distortion. As previously mentioned, additions of plasticizers or flexibilizers will reduce the problem but generally increase outgassing results which are quite low in the unmodified adhesive.

The shorter pre-cure times allow increased productivity during collimating; however, the time saved there is lost to the time when the elements can be subjected to hostile environmental testing. Manufacturers of these adhesives generally instruct users to wait 3-5 days before temperature shock tests can be conducted.

Wave length specific cross-linking is an advantage because some doublets require considerable time to center. In a normally fluorescent lighted room these cements give much more time before stray UV light can bring them to pre-cure.

Polyester resin based copolymer UV curing optical adhesives have color to them which is a draw-back to embedding. Their anaerobic quality leaves a surface tack for some time after cure. This prevents "edge-pinch" and distortion in optics but requires additional time and in some cases heat to alleviate in embedments. Polyester based adhesives are slower curing than urethane adhesives but still exhibit speeds that can incur strain resulting in the same problems discussed in urethane adhesives but to a much lower degree. Polyester based UV curing adhesives show lower shrinkage, higher chemical resistance, and better thermal shock resistance after cure than urethane based adhesives. thermal shock tests show better results sooner after cure due probably to their cross-linked chain length.

Ultraviolet cured optical adhesives create their own set of demands on the technician. He must be sure that the elements transmit a high percentage of light at the wavelength required to cure the cement.

BK7 optical glass has a transmission curve rising at precisely the wavelength that most UV curing cements require for cure. Test results show bonded BK7 elements have lower resistance to heat, humidity, and mechanical stress than fused silica or quartz when using the same light source, length of exposure, and distance from substrates. Some acrylics transmit very low percentages of long wave ultraviolet light.

If the doublet is on a dark background or in a collimator, this can effect curing time. A radiometer is a necessary tool since UV light sources degrade with age. The technician must keep in mind that light intensity decreases with distance and also full range (UV-A, B&C) lamps cure cements faster than filtered sources. The speed of cure can adversely affect the mechanical aspects of adhesion in that the wetting of the surface of the substrates can be incomplete when pre-cure is too rapid.

In summary, the application of UV curing cements in bonding doublets is the same technique as two component adhesives but without mixing. Application, rotating the center axis of the upper element around the center axis of the lower, allowing run-off and filling the chamfer are all the same. Power and distance of the UV light source determine speed of cure. UV cements have been a great help in increasing efficiency of the cementing departments but where lens design or operating conditions increase the probability of strain, slower curing adhesives have performed better.

Applications in
Production Bonding

Automated dispensing syringes contain supply lines from the cement container to the syringe. It should be checked that these lines will not dissolve or impart any contaminant into the cement. When using an ultraviolet curing cement, these lines must be impervious to UV light.

Hand operated disposable syringes seem to cause the most problems for cement shops. It must be remembered that manufacturers of these devices almost always lubricate the plunger and/or the O-ring with some lubricant. The lubricant is usually silicone. Silicone is incompatible with most optical adhesives and imparts a haziness in thefinished doublet. A thorough washing with a suitable detergent and then a solvent that will dissolve any residue must be used on each and every part of the syringe prior to using it for cement dispensing. When dispensing UV curing cements the syringe should be opaque to UV light and should not be used after being left uncovered during long breaks in production such as lunch breaks. If the syringes are pre-loaded and stored under refrigeration to extend shelf life, the technician must allow the entire package, unopened, come to room temperature before using.

Disposable pipettes made of polyethylene are very good applicators for moderate size production runs, they can be easily rinsed with acetone and air dried in a dust or lint free atmosphere. The cementing technician can use these right in his/her mixing container and dispose of them at each break or pause in production without incurring any appreciable cost.

Cleanliness is the key when deciding on applicators. Whatever is chosen, automated, semi-automatic or even a glass or poly-rod, they must be cleaned before use and continually checked during use for contaminants.

Strain, Distortion and Bond Surface Breakage

REMEDY:
Cements can be modified with plasticizers or flexibilizers. Most manufacturers have available softer cements off the shelf. However, as has been mentioned they can exhibit higher outgassing figures. In the case of hemispherical doublets, the convex lens should have a slightly shorter radius to allow an equal space around the entire arc that will be filled with cement. A larger chamfer on the concave element to hold a larger quantity of cement to be drawn in during cure is of considerable benefit. Ring configurations should be designed with very tight tolerances so that although the percent of shrinkage may be the same, the mass is much smaller thereby reducing the overall change.

All the configurations described above have been bonded with unmodified cements by reducing the cure speed. By using a very slow cure speed, cross-linking is done gently and the adhesive is permitted to conform. It must be noted that optical adhesives are not good fillers. Combined surface match should be within eight fringes.

Decentration During Pre-Cure

REMEDY:
When using a long curing two component adhesive it may be possible to use a non-anaerobic UV curing adhesive on 3 locations of the outer perimeter of the lens and when center is attained, precure the UV cement to hold the elements while the two-component cement passes precure. In the cases where rapid UV curing cements show shift it could be the use of a lamp that does not irradiate the entire bond area. A light source that only covers a small area of the bond surface will precure only the area covered, which might also lead to precured cement tears.

Edge Pinch, Perimeter Separation, Reticulation

REMEDY:
Although both anaerobic and non-anaerobic cements can result in edge pinch, perimeter separation and reticulation; the non-anaerobic cements will cause edge pinch and separation more often. This is because the outer perimeter cement is curing at the same speed as the cement between the elements. As the cement cures the adhesion and shrinkage combine to pull down and in, resulting in pinch, distortion and in some cases bond failure and separation. Reticulation is more common with anaerobic adhesives and is primarily due to lack of cement at the perimeter of the bond. It can not be over emphasized that the chamfer must hold enough cement to be drawn in during cure. Elements without chamfers exhibit a high percentage of perimeter difficulties so chamfers should be designed in to eliminate this problem. Another cause of reticulation is insufficient degassing of the mixed cement. The technician should read carefully any manufacturers instructions to eliminate entrapped air and also take care that there are no air bubbles in the cement left around the perimeter of the bond surface.

Uncured Leneses, Elongated cure Times, Bond Failure After Cure

REMEDY:
Problems in curing two-component cements can be avoided by carefully reading and following the manufacturers instructions. Logical deviations such as increasing the specified curing temperature or increasing the catalyst ratio above recommended amounts will not always yield faster cures. Sometimes the inverse will occur. Clean non-reactive mixing apparatus should always be used. If the cement is stored at refrigerated temperatures to elongate shelf life, it should always be left unopened for enough time to allow it to come up to room temperature.

If the lenses are to be cured in an oven, the technician should be advised that the oven should be up to temperature prior to inserting the lenses. He should also note how the temperature drops during loading and not start his curing time until the oven has returned to the original temperature. When especially thick or large lenses are being cured, some additional time must be allotted for the internal surfaces of the lenses to come up to temperature. Users of single component UV curing adhesives also prefer to store at lower temperatures so the previous admonition also applies to them. Often users of UV curing adhesives irradiate their lenses for longer than the instructions call. Although this has no effect on the cement, the working life of the lights is expended quicker. Spare light sources or a radiometer are suggested. The technician should always be aware of the recommended distance between substrate and light source. Before choosing a UV curing cement, the user must know the percent of transmission of the elements, primarily between 325nm and 375nm. If the elements do not transmit above 75% at these wavelengths, expect much longer cures.

Above all, the user must be aware that full cure times quoted by most cement manufacturers reflect 90-95% cure. In the case of anaerobic and two-component adhesives this is because of the slight tackiness around the perimeter of the bond surface. Different cross-linking structure and end speed is the cause in UV curing cements. Hostile environmental testing and severe mechanical stresses should not be conducted on cemented doublets for at least 24-36 hours after the manufacturers stated full cure time. The test results are appreciably different when this rule is applied.

Haze, Fog or Discoloration of Bond Layer

REMEDY:
Manufacturers sometimes assume that the technician will know that organic peroxides used as catalysts in two component cements will react with metallic substrates such as aluminum mixing pans or stirrers. This oversight can result in hazy doublets. Glass, polyethylene, and polypropylene should be the materials of choice for mixing and applying two-component optical cements. When polyethylene or polypropylene syringes are used to apply cement the user should be aware that these syringes contain plunger O-rings lubricated with silicone which can haze the cement. Each component of the syringe should be separately wiped and then rinsed with the appropriate cleaner to remove any lubricant that might be on it. A final wipe with acetone will assure no residual contaminant. The cement and the elements must be at room temperature. Cold cement on a warm element or vice versa will result in condensation and haze will result.

Cement Wedge

REMEDY:
When bonding any element an attempt should be made to keep the surface horizontal and level. When bonding prisms, corner cubes or beam splitter blocks, if the hypotenuse is the bond surface; inexpensive holding blocks can be fashioned from styrofoam cut to match the 90 degree intersection angle of the two legs thereby holding the hypotenuse level and horizontal. In the case of triplet bonding, one simply should not attempt bonding all three elements. With the availability of very rapid precure UV adhesives and moderately fast two-component room temperature cure adhesives, very little extra time is involved in bonding one surface at a time even if critical centering is involved. Remember that it takes a great deal more time and effort to decement and recement that it would to do the cementing correctly the first time. It is almost impossible to address a situation where multiple prisms must be cemented and centered at the same time and where one of the bond surfaces will be off level when the others are level. If instruments can be rigged to hold one surface lever, so be it. The others will have to be bonded later.

Failures Under Moderate Stress or Weak Chemical Attack

REMEDY:
It would be most advantageous for the cementing department to be equipped with an ultrasonic cleaner with several compartments so that suspect elements can be recleaned with a mild alkaline solution and then a deionized water rinse. In the case of particularly stubborn films it may be necessary to have a mild acidic and then deionized water rinse prior to the alkaline bath. Because of environmental and governmental concerns, solvent cleaning will be kept to a minimum. Therefore, aqueous cleaning solutions in ultrasonic baths seem to be the most efficient alternative.

The Cementing Department

All of the tools and apparatus mentioned will be of no avail unless the cementing technicians are completely familiar with the physical, and optical properties of the elements to be bonded. They also must be aware of the environmental and mechanical demands that will be placed on the finished lens. Knowledge of the chemicals and compounds that come in contact with the elements during grinding and polishing as well as the production cleaning procedure will warn them of potential films or residual materials that could prevent adhesion.

In conclusion, it is suggested that prior to any production run, witness pieces of the same materials or samples of lenses should be carefully bonded and tested to the specifications that the production items will be required. From this procedure a production protocol should be established and overseen by quality control. It should be emphasized to all personnel that even minor deviation from the protocol can have major effects on the finished optics.