A Division of EMS Acquisition, Inc.
PO Box 380 - 1560 Industry Road
Hatfield, PA 19440
Tel #: 215-412-8380 - Fax#: 215-412-8450
E-Mail: [email protected]
|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.
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.
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.
|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.
|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
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.
|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.
|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.
|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.
|Small shops and cementing departments in general use what ever they mixed
the cement to apply it as well. When large numbers of doublets are being bonded the
technician may choose to use a syringe or automated dispensing system to reduce waste or
insure consistent amounts of cement to each element. When such instruments are used the
technician should be especially careful of materials compatibility and the absence of any
lubricants the manufacturer may have put on the moving parts that contact the cement.
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.
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A Division of EMS Acquisition, Inc.
PO Box 380 - 1560 Industry Road
Hatfield, PA 19440
Tel #: 215-412-8380 - Fax#: 215-412-8450
E-Mail: [email protected]
|Most companies involved in manufacturing optical adhesives today have been doing so for some time now. Some for as long as 45 years. Their cements have been extensively tested by optical manufacturers and/or the military. When bond failure occurs it is more likely an incorrect choice of cement type, an anomaly in substrate preparation or incorrect interpretation of the manufacturers instructions than a question of the quality of the cement. The following will be descriptions of some of the common bond failures, their probable causes and suggestive corrective measures.
Strain, Distortion and Bond Surface Breakage
Most optical adhesives have a nominal shrinkage on cure of 5%. Add this to their strong adhesive quality and bending, slight distortion or even tearing of bonding surface can occur. As a doublet approaches hemispherical, perpendicular shrinkage forces act on the system. While the convex element is pulled into the concave, the shrinkage on the sides pulls outward. Thin elements, meniscus lenses and especially double concave lenses can be exceptionally prone to distortion. Ring configurations metal or glass as in early design ruby lasers are prone for the same reasons as described in the hemispherical lens situation.
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
An element shifting off optical axis during pre-cure or shortly after is not uncommon with the faster room temperature two-component cements or when high temperature oven cure adhesives are being used. As cross-linking can sometimes occur unevenly, shifts occur. The UV cements can fall victim as well, especially the very rapid curing varieties.
Edge Pinch, Perimeter Separation, Reticulation
Thin flats with 90 degree ground edges can show a distortion after cure around the perimeters of their bond surfaces. Beam splitter blocks and corner cubes can show this same distortion, however, it is usually along the line of the acute angle. This is pinching of the edges and is caused by cement shrinkage. Occasionally this shrinkage will cause bond separation, evidenced by mirror like reflection or interference rings around the perimeter. Reticulation is a term used to describe a "cobwebbing" radiating from the edges of the doublet inward. A look at this under magnification will show microscopic bubbles caused by air being drawn in during the curing because there was not enough or no cement around the bond perimeter as the cement contracted during cure.
Uncured Leneses, Elongated cure Times, Bond Failure After Cure
As the title implies, many bond failures after "cure" can be the result, in fact, of partially cured or uncured cement. This can occur with all cements, anaerobic, non-anaerobic, two-component as well as UV curing. Failures during environmental or mechanical testing can be caused by testing too soon or the cement taking too long to cure. The most common causes of curing problems with two-component cements are improper catalyst rations, incomplete mixing, reactive mixing containers and improper temperatures. Ultraviolet curing cements are sensitive to temperatures, transmission of substrate, wavelength and intensity of light source, and the distance of light source to substrate.
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
Discoloration or haze that occurs during or immediately after curing is almost always a sign of contamination of the cement. There are many ways a cement can become contaminated on its way from the manufacturers package to the cemented lens. The catalyst can react with a metallic or polymeric mixing container or mixer. Cleaning an element with any volatile solvent in a room with high humidity can cause water condensate. Transferring mixed cement into a disposable syringe without thoroughly cleaning every component of the syringe can cause lubricant contamination. Although some manufacturers supply their cements in a dispenser package, the dispenser tip should always be inspected and kept clean. As previously mentioned, if the cement is stored under refrigeration, it should not be used until it is allowed to return to room temperature.
There are times when users of cements will attempt cementing and collimating at the same time. This might be by choice to save time or by necessity in aligning centers in a prism train. When such an exercise calls for having the bond surface out of the level horizontal plane, cement wedge occurs. In the time that it takes to adjust the piece and then collimate, the cement will flow to the low side of the plane causing the wedge. Wedge can also occur when attempting to bond two bond surfaces such as in a triplet or the hypotenuse of a corner cube while the cube is resting on one of its legs.
Failures Under Moderate Stress or Weak Chemical Attack
When sporadic bond failures occur during production runs that have been bonded by the same cement, it is logical to assume that since the cement is constant and each element is the variable, something is preventing the cement from bonding to particular elements. Most manufacturers are employing some type of mass cleaning process, pressure sprays, centrifical systems or ultrasonics but occasionally a few elements by their locations in the baths are incompletely cleaned. Some polishing compounds such as cerium oxide if not cleaned immediately can leave a very stubborn film if it hardens. Acrylic elements may not permit the use of solvents, therefore, thorough cleaning can be sporadic.
The Cementing Department
|Upon reviewing what has been covered, it is
obvious that the cementing department must in some ways be a mini laboratory containing
the appropriate non-reactive mixing and measuring equipment when two-component cements are
used, proper light sources for UV curing adhesives, an assortment of solvents for cleaning
and even an ultrasonic cleaner. Since some lenses will have to be decemented, a hot plate
and containers for that procedure will be needed. These are in addition to a collimator,
blocking tools, oven, and a clean, humidity controlled environment.
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.
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