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Approved for Ohio Credit by the Ohio Optical Dispensers Board

Lens Hardcoating�Great Expectations, Great Results

By Miriam McGorrin

Release Date: September, 2006
Expiration Date: September 30, 2009

Learning Objectives:
Upon completion of this program, the participant should be able to:

  1. Understand the way that hardcoatings increase the performance of lens materials and treatments.
  2. Know the varieties of hardcoats applied to ophthalmic lenses.
  3. Learn the most common test methods for coatings.
 

Faculty/Editorial Board: 
Mark SachsMiriam McGorrin, product manager, Cytec Engineered Materials, Anaheim, Calif. This CE was written when McGorrin was director of sales and marketing for SDC Technologies, Inc., a major supplier of hard coating chemistry and technology to the optical industry.

Credit Statement:
This course is approved for one (1) hour of CE credit by the American Board of Opticianry (ABO). Course # STWJP018-1 Please check with your state licensing board to see if this approval counts toward your CE requirement for relicensure.


Hardcoating spectacle lenses have grown steadily over the last 15 years and while you may know it primarily for scratch resistance it provides many other functions. They are the ability to:
Differentiate lens materials and designs
� Coatings can help make a commodity material unique by giving it special properties.
Incorporate additional features � Fast tinting, enhanced impact and/or chemical resistance, anti-fog performance or reduced birefringence or rainbow patterns, while also providing abrasion resistance.
Use materials otherwise unsuitable for optical lenses � For example, polycarbonate offers a unique combination of features that make them ideal for use in ophthalmic lenses, however, polycarbonate does not have sufficient abrasion resistance without a hardcoat. The availability of premium hardcoatings have contributed to the growing success of polycarbonate.
Maximize the performance of other coatings � Hardcoatings are used in conjunction with anti-reflective and mirror coatings to optimize durability and abrasion performance. Through the use of the same hardcoat on both the front and back surface of a lens, the abrasion resistance, the optics and adhesion can be optimized and consistent on both surfaces.

As a result, hardcoatings are an inherent part of everyday eyewear.

HOW MANY ARE COATED?

Hardcoated spectacle lenses account for more than 70 percent of lenses sold in the United States. This number has grown because the share of hardcoating increases as the share of premium lens designs increase. Most premium progressive lenses are sold with a premium hardcoat and many new lens materials require the use of a hardcoat. This includes most mid- and high-index materials. Hard resin is one of the few materials that are sold with and without hardcoating. And, AR coatings require hardcoatings to improve durability and scratch resistance so the best AR uses the best combination of hardcoats and AR.

HARD COATING IN PRODUCTS AROUND YOU

Coatings are applied to a variety of surfaces to increase abrasion and scratch resistance. Plastics can be used in many applications where bare substrate alone would not survive. Originally developed in the 1970s, coatings are one of the original nanoscience technologies. First developed to impart glasslike surfaces on plastic materials, the original applications were used in mass transit. In the 1980s, hardcoatings were used on polycarbonate aircraft canopies for fighter planes. Many of the hard-coats that are used on today�s ophthalmic lenses had their origins in these aerospace hardcoats.

Hardcoats are also used in the construction industry for polycarbonate and acrylic glazing, security glazing in architecture and vehicles, window films, headlamps, sunroofs, bus and rail windows, and on many gauges in transportation and industrial equipment. Additionally, features such as impact enhancement and anti-fog performance are used in sports and safety eyewear, as well as in sunglass lenses.

FEATURES AND BENEFITS

Hardcoatings can provide more than enhanced abrasion resistance. A hardcoating is selected by determining what additional features are desired. It is not always possible, however, for one coating to provide all of these features. So, it is necessary to prioritize requirements and select a coating that provides the best balance that answers needs. Lens manufacturers and coating laboratories make selections to improve and balance the properties of the lens substrate materials as well as the anti-reflective coatings that may be applied to the lens afterward.

Coating features can be broken down as follows, refer to the illustration of possible coating layers:

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Durability � Hardcoatings increase the life of a lens. Durability is improved by abrasion, chemical and impact resistance.

Scratch and Abrasion resistance � While the main features of most hardcoatings, scratch resistance and abrasion resistance are often used interchangeably. Technically, scratch resistance refers to the ability to withstand damage from a sharp object that comes in contact with a lens surface. Abrasion resistance, on the other hand, is the ability to withstand wear from the rubbing of an object, such as a cleaning cloth, shirt sleeve or paper towel, across the surface.

Chemical formulation as well as application has an effect on coating thickness and the degree of chemical cross-linking, which in turn has an effect on scratch and abrasion resistance.

AR durability performance � The percentage of AR coated lenses is growing in North America. This is in part due to improvements to the AR coatings as well as improvements to the hardcoats under the AR. It is always recommended to use a good hardcoat underneath an AR coating. AR returns are greatly reduced by the use of compatible hardcoating on both the front and back surface and in general, thermal cure coatings provide the best foundation for an AR coating. Both the adhesion and abrasion resistance of the AR coating are greatly improved through the use of a good hardcoat.

Primers for improved performance � Often, hardcoating performance can be enhanced by the use of a primer applied to the lens surface under the hardcoat. Primers can be used to improve surface adhesion as in the use of a primer to obtain adhesion to polycarbonate. Primers are also used to increase tintability because the primer holds the tint dyes when used in conjunction with a permeable coating. Lastly, primers help to increase the impact resistance of a coated lens by acting as a rubbery material that halts the migration of a crack.

Impact absorption � Hardcoatings and AR coatings typically make a lens more brittle and as a result, decrease the impact resistance of a lens. By using a soft primer, or cushion coat, the energy of impact is absorbed through the primer on the back surface, where cracks are most often initiated preventing cracks from propagating through the lens. The use of cushion coat primers is increasing, as the volume of AR coatings, the use of more brittle substrates and the design of thinner lenses increase.

Tintability � Hardcoatings are used to permit tinting of a non-tintable substrate. Some substrates, such as CR-39, can be tinted without the use of a hardcoat. Polycarbonate requires the use of a hardcoat to tint the lens. Tintable coatings work through both permeation and the ability to hold a dye. Because there is a limit as to how much dye a hardcoat can actually hold, it is often necessary to use a tintable primer with the hard-coat. This allows some dye to be held by the hardcoat and some dye to be held by the primer enhancing final tintability.

Index matching � The demand for thinner and lighter lenses has led to an increase in mid- and high-index substrate usage. Most hardcoats index of refraction closely matches that of hard resin (1.49). When the index of refraction of the coating matches that of the substrate this eliminates the reflections from the light passing through the interface of coating to lens. If there is a mismatch between hardcoat refractive index and lens refractive index, the result is birefringence. Birefringence appears as a rainbow pattern on the lens surface. If the coating and lens have the same index of refraction, the light passing through �sees� the two as one continuous substrate and the reflections at the interface are minimized. As the use of higher index substrates has increased, so has the availability of higher index coatings.

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Antifog � Antifog coatings have become popular on industrial safety and sports eyewear. The antifog performance can be achieved through either surface moisture absorption or sheeting the moisture across the surface. Premium antifog coatings combine abrasion resistance, chemical resistance and antifog performance previously not possible. However, some may still be softer than the hardcoats that have become the norm in the market.

CREATING THE BEST HARDCOATING SYSTEMS
Coating Systems
� Performance and versatility can be greatly improved by using them in combination with a primer. The combination of hardcoat and primer are often referred to as a hardcoating system. The primer provides adhesion to multiple substrates, impact resistance and tintability. The hardcoat provides abrasion and scratch resistance, as well as a host of other features. For the lab and manufacturer, working closely with the hardcoating supplier, one can carefully select both primer and topcoat, and thereby optimize performance of the hardcoating system.

Thermal vs. UV � Most coatings are cured by either UV light or heat (thermal) exposure. The choice of cure method depends on the chemistry of the coating. Each of these two types of coatings has their advantages and disadvantages. The following is a summary of the general pros and cons of each. These are typical attributes of each of these classes of coatings�not all coatings in each class are alike.

Thermally Cured Coatings � Thermally cured coatings were traditionally used only by lens manufacturers and referred to as factory coatings or the front side coating on a semi-finished lens blank. They have good to excellent abrasion resistance and AR compatibility. They allow the option of using primers to achieve better adhesion, tintability and impact enhancement. Most thermally cured coatings are designed for adhesion to a single substrate. This makes them ideal in the lens manufacturing environment. The use of a primer, however, allows the same coating to be used on multiple substrates or even as an overcoat (more details below). This has resulted in increased usage of thermally cured coatings at labs and retailers. Thermally cured coatings are generally thought to be more compatible with AR and mirror coatings. Thermally cured coatings can be spin or dip coated. Thermally cured coatings have longer cure times (typically one to four hours) and have limited substrate compatibility.

UV Cured Coatings � UV cured coatings are traditionally used in the laboratory environment and at some lens manufacturers. The advantages of UV cure coatings are quick cure time and multiple substrate compatibility. This makes them ideal for the laboratory or retailer where multiple substrates are used and quick turnaround times are expected. UV cured coatings can be applied by spin coating, dip coating or in-mold coating. Spin coating is the most typical method of application. UV cured coatings are typically lower in abrasion resistance and often less compatible with AR and mirror coatings. UV cure coatings often have good steel wool abrasion resistance, but do not perform as well as thermally cured coatings in the Bayer Abrasion Test (more on test methods below).

Hybrid Coatings � This is a new category of coatings. The goal is to combine the �best of both worlds.� These coatings are cured by first exposing them to UV light, followed by a short thermal cure. This results in a coated surface with the abrasion resistance and AR compatibility of thermally cured coatings and the quicker cure time of UV cured coatings.

Overcoating � Overcoating is the technique of applying a hardcoat over an existing hardcoat. This is used to apply the same coating on the front and back surface of a lens that already has a factory coating on the front surface or both sides of the lens. This allows the lab or retailer to apply a premium hardcoat with consistent performance on both surfaces and on all substrates. A primer is used to achieve adhesion on a variety of factory coatings, as well as a variety of bare substrates. A thermally cured coating is then applied over the primer. Using this process, the lab or retailer can produce lenses with premium abrasion resistance, optics and AR compatibility on both surfaces and on all substrates. Some coating systems require that lenses have the factory hardcoat etched off with an acid wash before overcoating.

TESTING HARDCOATS
Abrasion and Scratch Resistance
� Although there are several test methods for testing abrasion resistance, there is not one universally agreed upon standard. Abrasion resistance is actually a combination of different factors�adhesion, hardness, flexibility and impact resistance. Each test method measures a different combination of these factors. Since each of these test methods measures a different combination of features, they do not always correlate with each other. It is possible to have excellent steel wool scratch resistance and a low Bayer Ratio and vice versa. Some of the most durable coatings are designed to give the best combination of abrasion resistance as measured by several of these test methods.

Bayer Abrasion � The Bayer test is one of the most often cited test methods for abrasion resistance. This test subjects both a coated lens and an uncoated CR-39 standard to abrasion from oscillating �sand.� The sand is actually alumina zirconia. After a set number of cycles, the haze gain is measured on both lenses. The ratio of haze gain of the uncoated lens to the coated lens is the Bayer Ratio. A Bayer Ratio of �1� means that the coating has equivalent abrasion resistance to uncoated CR-39.

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A Bayer Ratio of �5� means that the uncoated CR-39 standard had five times the haze gain as the coated lens. A common standard Bayer Ratio equal to �4� or greater is considered by the industry to be a premium coating.

Steel Wool Abrasion � Steel wool is a popular test because a version of this test can be performed without any special equipment. There are different ways to conduct a steel wool test. The simplest is to rub the lens by hand with a small piece of steel wool. This method gives a qualitative rating from poor to excellent. There is also equipment available that rubs the lens with steel wool under a specified weight for a specified number of cycles. This type of steel wool test gives a quantitative measurement. The steel wool is of known fiber size and quality.

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Other Abrasion Test Methods � There are several other methods to measure abrasion resistance. They are the Eraser test, Tumble test, Taber test and Nanoscratch test. The two most common methods, however, are the Bayer and the Steel Wool test.

Adhesion � It is critical that any coating exhibits excellent adhesion to the substrate. Poor adhesion results in delamination or peeling of the coating. Adhesion is measured by a Cross Hatch test. A cross hatch pattern is cut into the surface of the lens by using a sharp blade to cut a set of parallel lines through the coating and a second set of parallel lines at 90 degrees to the first set. A piece of tape of a specific grade is then pressed against these lines.

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The tape is quickly lifted off. This is typically repeated three times. The cross hatch is then examined for delamination. To achieve a rating of 100 percent adhesion, there cannot be any areas where the coating is lifted or removed. This test is commonly performed after the coating has been cured. This reading would be the initial adhesion. This test can also be performed after tinting or exposure to boiling water. This would then be called the post-tint adhesion and the boiling water adhesion test. In each of these cases the lenses would also be evaluated for small cracks or crazing.
Coating Thickness � Coating thickness is typically measured by a UV/VIS spectrophotometer or smaller portable device. Coating thickness is important, as too thin a coating will result in decreased abrasion resistance and too thick a coating may result in delamination or poor adhesion.

Tintability � Tintability is measured by subjecting the coated lens to a tint bath. The tint density is then measured as a function of time. Both tintable and non-tintable coatings are tested for tintability. A nontintable coating on the front side of a lens must be tested to ensure that the coating does not delaminate or craze upon exposure to tint bath. Manufacturers perform a second adhesion test after tinting.

In-Process Testing � There are many tests that are conducted during the coating process. These can include viscosity, % solids, temperature and humidity. These values give a good indication of the age of the coating and the processing conditions. As liquid coatings age, the viscosity and other features may change. This will result in changes to adhesion, abrasion resistance, tintability and coating thickness. Processing variables such as temperature and humidity also impact these features. It is important to have a clean and controlled process to achieve a coating with optimized optical properties and mechanical performance.

Who tests? � Not all tests can be performed at all locations due to the need for special equipment. Some tests, such as in-process testing of the coating liquid and environment are performed at the location where the coating is being applied. Tests that require little or no special equipment can be performed at a lab, retailer or office location. These would include handrub steel wool, cross hatch adhesion and tintability. It is important to note, however, that most of the tests are destructive. This means that once the test is performed, the lens is often not usable. Tests that require special equipment (Bayer, quantitative steel wool, coating thickness by spectrophotometer, etc.) can only be performed at laboratories or manufacturing locations with the appropriate equipment and trained personnel. If one desires to run one of these tests and does not have the necessary equipment or personnel, there are test labs that can perform these tests for a fee. Often, it is not important to be able to run these tests onsite. What is more important is to have an understanding of what these tests are and an understanding of the results being looked for to meet performance goals.

TALKING INTELLIGENTLY ABOUT HARDCOATINGS

The best way to talk intelligently about hardcoatings with your laboratory is to have a basic understanding of hardcoatings, their features and the test methods that are used to describe them. Because not one hard-coating can provide every available feature, it is critical to know what features are priorities and be able to communicate them to the lab. There is often a trade-off between features. For instance, coatings that are tintable typically have less abrasion resistance than non-tintable coatings, though recent advancements have helped to minimize this trade-off. The more up-to-date on recent coating advancements, the better able the dispenser is to communicate needs to the lab.

CONCLUSION

Hardcoating is an area of the lens market that has changed significantly in the past decade. Hardcoating companies with research and development departments are continually introducing new and improved coatings. These newer coatings have introduced new features such as index-matching and impact-enhancement while helping to minimize performance trade-offs typical of past hardcoats.

A better understanding of hardcoats, their benefits and methods to test them will enable you to talk more effectively with your laboratory and customers. The hardcoating market has become more complex over recent years. It is now necessary to understand the differences between factory-applied coatings, UV and thermal cure coatings, and the many features available in coatings today.

As the marketplace becomes more and more competitive, advances in hardcoating allow the ECP to offer the latest in technology, provide their customer lenses that are more robust, while continually differentiating themselves.