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
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:
Durability � Hardcoatings
increase the life of a lens. Durability is improved by abrasion, chemical and
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
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.
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
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.
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
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.
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.
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
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
TALKING INTELLIGENTLY ABOUT
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.
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.