This writing is an attempt to prevent many of the common errors and failures that occur when an optician or technician begins bonding optics with synthetic adhesives. In many places the experienced optical technician will be strained to endure what to him seem only common sense. In these instances patience is requested. It is experience that has prompted this writing. Many duplications of errors have occurred over the years because what was taken for granted by some was totally new and unexpected by others. There is also the chance that there are a few new aspects to be learned by the experienced. Designers may after reading, be able to design out some of the problems mentioned before they reach the cementing department. 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 Do the elements have chamfers? Depth and radius of curvature. Will there be post cementing processing? (Cutting or Grinding) Types of materials to be bonded. Coefficients of thermal expansion. Bond line configuration. Surface area vs cement viscosity. OPTICAL Refractive indices of the elements. Transmission of the elements. Tolerance of internal reflection and absorption. ENVIRONMENTAL Working temperature extremes. Mechanical Shock requirements. Chemical resistance requirements. 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 Correct preparation is the key to the successful bonding of optical elements. Thorough cleaning of the elements and careful, accurate preparation of the cement is absolutely essential.

Preparation of

the Optical

Elements The two major considerations in preparing elements for bonding are proper cleanliness of the elements as well as the equipment to be used in cementing, and orienting the elements so that the matching surfaces go together the first time without having to remove the upper element because it was mistakenly inverted during the final cleaning process. 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 Read the manufacturers instructions carefully. Follow the instructions exactly or your only source of technical help, the cement manufacturer, will be lost to you because you may have stepped beyond the limits of their experience with the cement. 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 The doublet we will join is a double convex element to a concave-plano element. After the acetone wipe, we will apply the prepared cement to the center of the concave surface of the concave-plano element. Three or four drops off of the end of the stirring rod will create a puddle in the center of the concave surface. Now we will place the double convex element onto the concave-plano element containing the cement. Some attempt should be made to keep the element bond interfaces parallel while placing the upper element onto the lower. After the two elements are together, slight downward pressure in the center of the upper element will begin the procedure of working the cement and any incurred air bubbles out to the perimeter. With the index finger or the eraser end of a pencil, rotate the center axis of the upper element around the center axis of the lower element. Do not spin the upper element. This will only create an orbit for any entrapped air bubbles and will not work them to the perimeter. Do not use excessive pressure when following this procedure. We want to eliminate any air but leave sufficient cement to create a .0003" to .005" bond layer thickness. Following this procedure there should be a fillet of cement filling the chamfered area plus some run-off down the edge of the lower element. This should exist around the entire perimeter of the doublet. The run-off can be cleaned off with a dry cotton swab or lens tissue. Do not clean out the chamfer area. The cement in the chamfer area is drawn back in between the elements during the curing cycle as the cement contracts slightly. Side holding devices can now be applied to hold center until the cement reaches pre-cure. Side holding devices such as v-blocks are the only type we can suggest. Clamps or other devices which press the two elements together are not advisable. Most optical cements polymerize creating their own heat of reaction. This internal heat thins the cement and wherever there is a point of pressure, the cement layer will be pressed out leaving an uneven bond layer or worse a cement void. Any attempts to equalize the pressure across the entire bond surface only creates too thin a bond layer that results in lessened thermal, mechanical, or chemical resistance. Therefore, side holding devices are best. 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 In the mid '60's, single component ultraviolet cure optical adhesives were introduced to alleviate mixing and to speed pre-cure times on doublets requiring critical centering. The optical industry was quick to see the advantages of no mixing, reduced technical training, and reduced equipment needs; however, as time has passed certain difficulties have surfaced that have shown single component cements have problems of their own. 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.