Effective process control starts with monitoring and controlling the fabricating environment. All systems involved with ophthalmic laboratory processing are influenced by environmental conditions and variations. Environmental parameters must be optimized to achieve maximum yield, optimized product performance and minimal breakage. This includes the Backside Coating unit and its ultimate performance. We will review the following environmental conditions:
LAB CLEANLINESS –
The very name “laboratory” suggests an extremely clean environment. Historically the typical Ophthalmic Laboratory has been anything but extremely clean. In fact some Labs have been very much the opposite. When backside coating began in the optical industry, many years ago, Lab principles were told they needed a clean room for the unit installation and operation. The cost of such a room was quite expensive and in some cases restrictive of even pursuing a Backside Coater. For this reason coating manufactures developed units which were more self-contained with air, water and chamber filtration. Venting and positive air flow were developed to keep airborne particulates from contaminating the clean/coat chamber. This was all done to eliminate the need for a clean room specific to the coating operation. Purchasing a backside coating system became much more practical from a cost perspective and many Labs acquired the new technology. Backside coating grew and evolved into a standard process of virtually every Lab.
Even with all the efforts to make the units self contained, and impervious to external contaminants, coating particulates remains one of the biggest breakage categories in most labs along with general scratching of lenses prior to packaging. Both of these breakage causes can be greatly reduced by simply paying attention to the overall cleanliness (or lack of) in the lab environment. Generally speaking . . . keep the lab clean and free of unnecessary paper products like cardboard boxes, tissues, etc. Anything that collects dust, dirt, debris, grit, grime should be removed from the Lab and stored (enclosed) away from operating systems. Surfaces (i.e. - floors, table tops, equipment, staging areas, etc.) should be cleaned daily to remove potential containments.
Lens cleaning materials should be micro fibers and disposable. Reusing cloth materials like “diapers” should be banned from labs due to the increased probability of contamination and carry over. This is not an area to be penny wise and pound foolish. Using inferior, reusable cleaning materials will only lead to scratching and costly breakage. To reduce scratching, even with clean materials, eliminate dry wiping whenever possible. Lenses should only be wiped during washing and when submersed under softened water. Lens drying should be done with clean filtered air. Even with clean materials dry wiping carries a very high risk of scratching.
Ensure proper lab ventilation is in place with adequate and maintained filtration to keep airborne particulates to a minimum. DO NOT allow doors and windows to be open during summer months. Free standing fans should be banned from the lab as well, these tend to stir-up particulates that have settled on surfaces and make them airborne. Lens (job) trays are probably one of the most common culprits in labs for contamination. Many labs rarely, if ever, clean their trays. Over time trays become very dirty with debris and dried polish which is easily transferred to clean lenses (especially when lenses are dry wiped creating static surfaces). Trays should be kept as clean as any other aspect of the lab.
LAB CLIMATE CONTROL -
Climate control is one of the most overlooked aspects of the ophthalmic lab. Room temperature and humidity can have a big influence on process stability over time, especially for lab’s located in “seasonal” extremes like northern climates. Any lab operating an AR system can testify to the critical nature of climate control for optimal performance and yield. In fact, lens materials themselves can be influenced by changes in temperature and humidity. This is indicated in the SOP’s of testing facilities for what is called “material conditioning”. Material and/or lens conditioning refers to the need for test samples to acclimate to test lab climate parameters prior to testing. This allows for material stability and consistent repeatable test results. The same is true for the general operation of all systems in the ophthalmic lab, including backside coatings and application units. Room temperature should be maintained at 72 degrees F + 5 and humidity between 35 – 45% (no higher than 50).
LAB SYSTEM AIR –
Air compressors need to have more than adequate capacity to accommodate all participating equipment, even if all equipment is running at once. Otherwise unit failure may occur resulting in breakage. This can affect backside coating units with respect to proper vacuum pressure during lens “chucking”, suction cup operation and post wash drying. Air should also be “dry” and free of contaminants such as dirt and oil. Monitor and maintain all applicable filtration systems.
LAB SYSTEM WATER –
The labs water supply must be monitored and controlled whether it’s in-coming or free standing. Water used for the backside coater should be de-ionized, free of bacteria and kept at room temperature. If recycling/holding containers (buckets) are used, they should be kept clean and dried periodically to remove algae or other forms of bacteria from the water. Monitor and maintain all applicable filtration systems.
The lacquer used in a backside coating system is a complex chemistry designed and formulated to meet specific objectives which enhance the overall durability and performance of an ophthalmic lens. Due to the simplicity of application, and ease of unit operation, many labs consider coatings to be rather generic and simplistic as well. The chemistry and film dynamics of a coating are often overlooked as an important aspect of hard coating performance and the ultimate quality of the final lens product.
This article will focus on the following coating parameters:
Coating viscosity -
One of the most important parameters of a hard coat lacquer is its viscosity. Viscosity is a measure of the resistance of a fluid. In everyday terms viscosity is a fluids "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity. Put simply, the less viscous the fluid is, the greater its ease of movement (fluidity), which contributes to desired flow characteristic’s during application. Application dynamics, lens performance and optical properties are all greatly influenced by the coatings flow characteristics. A coating with the proper viscosity will flow evenly and consistently over the entire surface of the lens from edge to edge. This allows for optimized curing, clarity and film thickness control.
A common way of measuring hard coating viscosity is with a Brookfield device. The device spins a cylinder in the coating liquid and measures its resistance. The viscosity measurement is in centipoise (cps) units. At 20 degrees C water has a cps of about 1 and olive oil is about 81. The optimal range for backside coating viscosity is 50 cps or less. Higher cps levels will likely lead to performance and application issues. The coating solution viscosity should not exceed 50 cps throughout its life in the application unit. Coatings that increase in viscosity over time undermine process stability and quality control. Also keep in mind that temperature can alter a liquids viscosity. Viscosity tends to decrease as temperature rises and increase as temperature falls. Labs with in-house AR systems likely have a Brookfield device for measuring viscosity applicable to the AR process.
Coating film thickness -
The applied film thickness is another important parameter of a hard coat lacquer. In the past “thick” coating film was used to hide surface issues such as scratches and swirls. Thick coating film was also used to increase tintability on polycarbonate lenses. In some cases, very “thin” coating film was used on front surfaces with protruding multifocal segments to reduce visible flow lines around the segments. This led to issues with coating durability and abrasion performance. Therefore proper coating film thickness is very important for optimized lens performance and stability.
Optimal coating performance is achieved with a coating thickness range of 3 – 6 microns. This range is ideal for proper curing, especially UV cured coatings, resulting in performance parameters which meet or exceed the chemical engineering objectives. This is also critical to AR compatibility and performance. The coating thickness should be within 0.5 microns from the center of the lens to the edge. Uneven coating thickness can cause numerous issues ranging from poor adhesion to inconsistent tint.
Lacquer curing technology -
There are two basic methods of curing ophthalmic coatings, thermal (heat) ovens and ultraviolet (UV) light chambers. The original backside coatings were thermal cured requiring refrigeration for storage and ovens for curing. Thermal coatings are commonly used by lens manufacturers for the front side hard coat. Thermal curing tends to produce superior performance due to longer cure times allowing for more effective cross linking. Compared to UV curable coatings, thermal curable coatings usually have a higher concentration of “Collodial Silica” which increases coating hardness. Thermal coatings potentially have more effective curing throughout the coating layer because of the heat distribution in the curing oven.
UV curable coating became popular due to speed of cure and ease of operation. Due to the “fast” cure cycle the coating has a limited time to cross link, this requires complex chemical engineering to provide comparable coating hardness to that of thermal coatings. Also, since the top surface of the coating film layer is exposed to UV light longer than the bottom of the layer, it tends to cure more at the top than the bottom. For this reason cure cycle time and energy output is calculated to minimize inconsistent hardness throughout the film layer. This is critical to proper adhesion and AR compatibility.
Coating tintability –
Tintability in a hard coat creates a significant challenge to the chemical design. In general, if a coating is allowed to absorb tint it means the lacquer is absorbing moisture (H20 & pigment) during a near boiling bath exposure. The best way to degrade coating adhesion is to expose the adhesive surface to heat and moisture. If a lacquer is engineered to absorb tint it will tend to absorb moisture over time (even if it is not tinted) which undermines coating adhesion. In addition, if the lacquer is going to absorb tint it must be limited in its “hardness” to allow for molecular transfer which in turn negatively effects abrasion resistance performance.
It is ironic that for most optical labs tinting is a very small percentage (approx. 5%) of their overall production, but they still feel the need to have a tintable coating for all fabricated lenses. This leads to compromising abrasion resistance and adhesion performance for the majority (approx. 95%) of their production.
In conclusion, not all coating lacquers are created equal. Coating chemistry is very complex with many variables and challenges that require proper chemical balance for effective performance and high production yields. The relationship between lacquer chemistry, curing technology and application mechanics are crucial to the success of the backside coating process.
Understanding lab environmental conditions and lacquer chemistry are the first steps to successful coating application and process control. Anyone who has ever seen an AR application system can understand the production benefits of tight process controls on yield and product quality. This is true of any processing system, including scratch resistant coating systems. Although there are numerous ways of applying lacquers to lens surfaces, the most common in-house lab systems are backside spin technologies.
This article will focus on the following backside spin application dynamics:
Pre-cleaning –
With respect to thin film lacquer application, effective process control begins with strict adherence to proper lens cleaning protocol. An inadequately cleaned lens has a high likelihood of cosmetic and/or adhesion failure post application. Again, look at any AR system and the first thing you will notice is extensive cleaning modules. AR product quality begins with an extremely clean lens prior to the application process. This fundamental step is also critical to backside coating application. In addition to making sure the coating area and unit are clean it is crucial that the lens surface is cleaned according to manufacturer’s recommended procedures. Do not treat this initial step as an inconsequential procedure!
Fountain calibration –
The next fundamental phase to monitor closely is lacquer application. The basic method of applying coating to a lens, in a backside spin process, is by lowering a spinning lens into a “fountain” of coating. The primary mechanics to calibrate and monitor in this process are spindle speed (rpm’s), fountain height and cycle time.
Spindle speed is calibrated by a timing light that reads the rpm’s to assure the lens is spinning at the appropriate revolutions per minute. This is crucial to proper coating flow and ultimate film thickness. Follow application unit manufacturing recommendations and protocol on setting, calibrating and maintaining spindle speed accuracy. Once the spindle speed is established, the next step in application is lowering the lens into the liquid coating. The liquid coating is exposed to the lens surface by creating a “fountain” of coating in the unit. The height of the fountain is set to assure the lens surface is contacted at the center and fully submersed to completely flow across the spinning surface. Once again, follow manufacture recommended procedures for maintaining proper fountain height. The spin-off cycle is determined and set by the manufacture based on coating viscosity, flow characteristics and desired film thickness. DO NOT ADJUST CYCLE TIMES WITHOUT CONSULTING THE UNIT MANUFACTURER.
Lacquer curing – UV systems
This may be the most over looked application parameter in the backside spin coating system. This is ironic since the curing process in a UV system (which is comparatively fast and easy) is the weak link in the process chain when compared to thermal cured coating systems. Thermal coatings are somewhat easier to engineer given the “baking time” of curing the coating. UV coatings require much more chemical engineering to accommodate the quick nature of UV curing. Each UV coating is designed to cure at a specific UV range (nm’s) in a specific exposure time cycle (seconds). This also means the ultraviolet spectral technology of the UV source (lamp) is critical to optimal curing. Lamps not only need to produce the appropriate nm range, but also require the appropriate energy output during the quick exposure cycle.
A UV lamp generates specific wavelengths (usually between 200 – 450nm’s) associated with the materials inside the lamp (i.e. mercury, metal halides, etc.). These wavelength frequencies are very important to the curing process, along with the electrical current supplied to the lamp. The lamps “energy output” intensity is very dependent on the stability of the wavelengths and electrical source (i.e. unit capacitors). A common way of measuring a lamps UV output is with a Dosimeter device. This device measures both wavelength frequency and energy output in Joules per cm2.
All efforts should be taken to monitor and maintain proper energy output. It is also important to allow the application unit time to “warm-up” when power is initiated (usually about 15 minutes), this will create stable wavelength intensity and energy output when the lamp is operating.
Caution is advised when obtaining lamps from “low cost” providers, these lamps may have erratic levels of wavelength frequencies and energy output. Testing should be conducted to obtain performance confidence with after-market lamp suppliers. Suppliers should provide “certification” that the lamp has been calibrated properly at the factory.
Lacquer film (applied coating) plays a critical part in the overall quality, functionality and durability of the ophthalmic lens. Backside coatings not only enhance product scratch resistance, but must also work in harmony with a variety of lens substrates (adhesion), frontside factory coatings (tint stability) and provide a good base for various AR stacks.
From a mechanical standpoint, the thickness (microns) of an applied hard coating is very important to its overall performance. A coating that is too thin, or thick, can lead to abrasion, adhesion and optical performance issues. Generally, a film thickness between 3 – 6 microns is optimal.
Film thickness is controlled by the application unit’s set-up parameters such as spindle speed (rpm’s) and spin-off cycle time. These settings are determined by a coating’s viscosity and designed flow characteristics. ALL COATINGS ARE NOT DESIGNED TO RUN AT THE SAME SETTINGS . . . Always follow the application equipment manufacturers recommendations for process control on specific coatings. Labs that are not equipped to accurately measure film thickness should have lens samples sent to a testing facility and/or backside coating manufacture for film thickness validation.
From a performance standpoint, applied coatings are subjected to a diverse array of abrasion and adhesion tests. Numerous tests have been developed over decades in an effort to qualify and quantify abrasion/adhesion performance in “real-life” terms. Abrasion tests such as Taber, Bayer, Tumble, Steel wool and Eraser are scrutinized as scratch resistance marketing pushes forward. Adhesion tests like Crosshatch, Boiling water, Salt water and Accelerated weathering all attempt to measure the longevity of a coatings life. To date these tests are the best we have, but further research is needed to elevate ophthalmic hard coating to the science and understanding of true thin film technology.
In addition to the variables mentioned above, considerations like digital surfacing, AR enhancement, index matching and improved optical performance are demanding advancements in hard coat lacquer development. These technologies are pushing the hard coating category from the simplicity of the past to the science of the future.