An article on the spin application of thermally cured coatings, which was published in the September 2007 issue of Optical Lab Products, a supplement to Vision Care Product News.
The Spin Application of High Performance Thermally Cured Coatings
For many years, optical laboratories within the United States have successfully spin applied various coatings to ophthalmic lenses of assorted materials. These spin applied coatings provide such benefits as mar resistance, uniform tintability, and a suitable base for a subsequently applied antireflective coating or mirror coating. Almost without exception, these spin applied coatings have been radiation curable materials as they are cured with ultraviolet light, although other frequencies of light, such as visible or infrared, could be used to cure the coating, provided that the appropriate photoinitiator has been incorporated into the coating, as is done in other industries.
There are essentially two coating application methods that are widely employed in coating ophthalmic lenses. These two methods are spinning and dipping. Of these two methods, the spin application method has historically been by far the most popular.
There are also two types of curing techniques that are employed with lens coatings. One technique is to irradiate the coating with ultraviolet light and the other technique is to heat the coating. The former is done using with an arc lamp or some other source of ultra violet light. The latter may be accomplished via conduction, convection, radiation, which is known as radiant heat or infrared, or some combination of these three means of heating.
Crossing the two application methods with the two curing techniques results in four possible permutations. Of these four permutations, only three have been widely used in the ophthalmic lens industry by either the lens manufacturers or the optical laboratories. The remaining permutation, specifically, the spin application of thermally cured coatings, has been largely overlooked, yet, in many instances, this may very well be the best combination of application method and curing technique.
Because the traditional practice of using ultraviolet light to cure spin applied coatings has been so widespread and well established, somewhat of a mindset has developed in which it is thought that it is either not possible or not practical to spin apply thermally cured coatings. Somewhat of a false dichotomy has developed which says that all spin applied coatings are cured with ultraviolet light and all dip applied coatings are cured with heat, thus dip applied coatings always have better properties. This is certainly no longer the situation. Due to a number of advances in technology, it is now possible to successfully spin apply high performance, thermally cured coatings, and, in many situations, it is advantageous to do so. It is this integration of the spin application method with the thermal cure technique that is the focus of this article.
Advantages of Spin Application
The method of spin applying a coating to an ophthalmic lens has many advantages, most of which are well known to the skilled personnel who are working in the optical laboratories as they routinely employ this method. However, there may be other benefits
that are not so well known, which benefits may therefore be underappreciated, but which benefits may be rather important, especially in the context of spin applying a thermally cured coating. These benefits include more uniform coating thickness, decreased coating consumption, increased yield, lower operating costs, and other benefits.
The benefit of more uniform coating thickness results in reduced coating consumption, greater reproducibility in performance, and interference fringes that are less visible. For spin coating, these interference fringes are less visible since they are in a symmetrical pattern of a few smooth, concentric circles as opposed to numerous jagged lines that have been called tiger stripes, zebra stripes, or worse, and which seem to become even more noticeable after the application of an antireflective coating.
The benefit of decreased coating consumption results from several factors, including a more uniform coating thickness, as was previously mentioned. In a spin application process, the lens is held by a suction cup on one side of the lens. In a dip application process, significant quantities of coating are applied to the fixturing that holds the lenses. This fixturing must be critically cleaned before each use and remains a source of contamination as the lenses move from bath to bath. The spin application process eliminates drag out, which can be a source of considerable coating loss in dip processes. Furthermore, by spin applying the coating, it is possible to coat only one side of a lens, which is desirable when one side of the lens already has an excellent factory applied coating from the lens manufacturer. This benefit also results from the absence of a large buildup or pooling of coating at the bottom of a lens when a lens is spin coated. The amount coating that ends up on the edges of the lenses can be reduced in a spin application process. The excess coating that is spun off of the lenses can, in some cases be reclaimed, reconstituted, and reused.
The benefit of increased yield also results from several factors. One such factor is the centrifugal pattern of coating flow that occurs with spin coating. The centrifugal force that results from spinning the lenses aids in the removal of any particles that may present and dislodged by the solvents that are in the coating. But there is another important yet little known advantage to a centrifugal pattern of coating flow. Edge imperfections are common on both cast and injection molded lenses. While these edge imperfections themselves are not a concern since they will later be removed by in either a cribbing or edging process, they will, however, be the source of numerous coating headaches such as runs and drips if the coating were to flow from one edge of the lens across the center to the other side of the lens as is the case in a dip coating process. These edge imperfections are usually of little or no concern in a spin application process by reason of the centrifugal pattern of coating flow. The yield can also benefit from the entire absence of a dip tank or coating bath, which is effectively the last wash station as the organic solvents in the coating are often able to dislodge particles that aqueous cleaning baths are not able to remove. Furthermore, dip tanks, with their relatively large surface area, are effectively collection sites for airborne particles. Undissolved particulate material can have a surprisingly long residence time in the coating bath before being removed by a filter, or a lens, in spite of what may be
considered a high recirculation rate. This theoretical average residence time can be calculated based on the volume of the dip tank, the recirculation rate, as well as other factors, and might surprise you. However, the calculation goes out the window when particles adhere to the sides of a coating bath, as they often do, and do not pass through the filter but become dislodged from the sides of the dip tank later when a batch of lenses are immersed. Of course, any dissolved contaminants that might be in the coating bath are not removed by the filters but are removed on the lenses that are dip coated until the coating bath is replaced, which can be an expensive endeavor in terms of downtime and materials. In a spin coater, there is no chance for contaminated coating to flow over a lens since all coating is filtered immediately prior to being applied to a lens.
The benefit of lower operating costs is a consequence of such things as faster process feedback. Lenses are spin coated individually or in pairs such that when defects do occur, the problem is detected sooner, before a large quantity of scrap or rejects have been produced. Other contributors to lower operating costs include the elimination of the ongoing large scale cleaning of fixturing that is required by dip coating processes. Solvent evaporative losses are greatly reduced for a spin application process due to the elimination of the dip tank with its large surface area. This is solvent that would need to be purchased in order to maintain the composition of the coating but it also represents skilled labor to constantly monitor the composition of the coating that is in the dip tank as evaporation rates are not always constant and the ratio of solvents can be a delicate balance requiring sophisticated analytical instrumentation. Water consumption is considerably less with a spin coating operation. It may be possible to coat 10,000 lenses with as little as five gallons of water. In some instances, even the detergent itself can be eliminated due to the effective use of a high pressure water jet to clean lenses in a spin application process. This high pressure wash is an extremely effective mechanical means of removing many particles that are not removed by aqueous ultrasonic detergent baths or even solvent baths. The intense ultrasonic energy coupled with the heat that is present in these cleaning baths can cause materials such as an internal mold release to exude or migrate from the interior to the surface of a lens as fast as it is removed by the detergent bath thereby effectively recontaminating the surface of a lens, especially in an ultrasonic rinse tank that has no detergent or other means of dissolving this material, which can adversely affect adhesion. Lastly, the energy consumption for environmental control inside a spin coater is greatly reduced relative to a dip coater since even the large, high volume spin coaters are more compact than dip coaters of comparable capacity and, water, with its very high specific heat, does not need to be heated for use in cleaning lenses in a spin coater.
The next time you pick up a CD or DVD, all of which are made of optical grade polycarbonate, notice that the interference fringes are in a pattern of concentric rings, a dead giveaway that the coating has been spin applied, and for good reasons. These combined advantages make for a compelling case to use the spin application method in conjunction with thermally cured coatings on ophthalmic lenses.
Benefits of Thermally Cured Coatings
The benefits of thermally cured coatings are many and well known. These benefits include, among other things, increased mar resistance, greater potential for higher refractive index, and, very importantly, better adhesion to antireflective coatings, regardless of their method of application.
These benefits are largely the result of the high inorganic, siloxane, metal oxide, or ceramic, content of the coating that forms the very basis of the thermally cured coatings. These coatings are more like glass in their composition and therefore they are more like glass in their performance properties, perhaps most notably, mar resistance. The antireflective and mirror coatings are also largely inorganic, hence the better adhesion to the thermally cured coatings with which they can react and bond to some degree.
The coatings that are cured with ultraviolet light are composed largely of organic materials, which in turn largely contain the elements carbon, hydrogen, oxygen and nitrogen, not the oxides of metals or metalloids as do the thermally cured coatings. Attempts to enhance the performance of ultraviolet light cured coatings by incorporating glass, silicate nanoparticles, or other inorganic particles into these coatings via simply mixing these materials together has met with poor results. It was hoped that this approach might provide performance properties close to those of some of the better thermally cured coatings. However, it appears that the properties of such mixtures generally take on the characteristics of the organic materials into which the particles are dispersed. This can be explained by the illustration of incorporating sand into a cake batter before it is baked. The cake batter forms what is called a continuous phase that surrounds and covers all of the particles of silica or sand, which would form what is called the discontinuous or dispersed phase. Even at high concentrations of sand, the surface of the cake would still feel soft or perhaps even a little bit sticky and the cake could still be easily cut with a knife or broken into crumbs.
Another important albeit little known benefit of using a thermally cured coating is that, in many instances, if a coating defect is discovered, the thermally cured coating may be readily removed immediately after the precure, which is the initial, partial cure to a tack free state. This allows coating defects to be removed from a lens such that the lens may be recoated, thereby saving time and other resources.
Equipment is available that can be used to spin apply thermally cured coatings of the siloxane or other varieties. Some of this equipment is designed with the ability to apply both ultraviolet and thermally cured coatings in the same unit. Such equipment has both an ultraviolet light and a heat source. Changing from one type of coating to another can be as simple as the flick of a switch. Short cycle times of ten seconds, perhaps less, can be obtained for thermally cured coatings.
Small, automated, compact coating and curing units are available, having been designed for use in the smaller surfacing laboratories that may be doing a higher percentage of custom jobs or special orders. This equipment is economical, easy to
use, durable, and flexible enough to allow for some process variation so that various coatings can be used in conjunction with a variety of lens types.
Larger, high capacity spin coaters are also available for larger wholesale surfacing laboratories and lens manufacturers. One such thermal curing spin coater has a foot print of only six feet by six feet and stands about six feet tall yet is able to coat well over 8,000 acceptable lenses per day due to a short cycle time and very high yield. Not only is this equipment affordable in terms of initial outlay, but, and perhaps more importantly, it is also affordable when considering the complete scenario, including all of the important factors. These important factors include such things as capacity, coating consumption, energy consumption, durability, labor, yield, and other considerations. This equipment also has sufficient processing latitude such that it may be possible to spin coat even segmented lenses with good cosmetics. Numerous safety features are designed into this equipment to ensure employee safety. Lastly, this equipment is readily integrated into highly automated, inline, continuous processes.
New advancements in technology have given us thermally cured coatings that can be applied in various types of spin coating equipment. These new coatings exhibit a remarkable combination of excellent properties.
We now have the ability to spin apply a highly mar resistant, economical, stable, higher refractive index, primerless coating to polycarbonate or other lens materials and precure the coating to a tack free state in well under one minute. For some coatings, even the ultraviolet lamp of commonly available spin coaters will generate enough radiant and convection heat to effect an adequate precure. It should be noted that, after the precure, the lenses will need to be placed into a forced air oven or other suitable heat source in order to complete the cure of the coating so as to develop full properties.
Although reasonably stable at room temperature, these coatings will cure more rapidly and at lower temperatures to better properties than were ever possible before, thus reducing the long cure times at elevated temperatures that have traditionally come to be associated with thermally cured coatings. The ability to cure in less time and at lower temperature allows better properties to be imparted to lenses containing heat sensitive materials without the risk of warping or other damage.
Some of these coatings are able to bond tenaciously to preexisting coatings that may be present on the front surface of a lens as they have been applied by the lens manufacturer. Good adhesion to even ultraviolet light cured coatings is possible without the use of primers or corrosive pretreatment solutions.
The concept of spin applying thermally cured coatings is a combination that, heretofore, has been largely unknown and therefore little used in the ophthalmic lens industry. However, this combination of spin application and thermal cure offers numerous
advantages. This combination is now a practical approach to the better utilization of resources as well as higher yields for both optical laboratories and lens manufacturers.
Many changes are affecting our interesting industry. Among them are advancements in automation engineering and coating chemistry. Look for continued advances in these areas of technology to allow for further improvements in performance through the spin application of high performance thermally cured coatings and seriously consider incorporating these advances into your operations