# Optical Forums > Ophthalmic Optics >  How is scratch resistance relevant to Anti-Reflective coat?

## eekthorp

Hi again tech gurus, 

Some current gen AR's have hard scratch coats (SC) (super hi vision/alize/etc). However, the SC is under the AR.. 

Is the "10 bayer" (or whatever) hardness SC actually relevent to protecting the AR?  How carefull do you have to be?

Thanks, 
 eekthorp

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## LKahn

AR coatings by themselves tend to be prone to cracking, scratch easily and have adhesion problems. Many "optical scratch resistant coatings" are applied between the plastic substrate and the AR coating. Many of the coatings applied as an intermediate layer are there for adhesion and added characteristics in a drop ball test. Technically this is a hard coat and not a scratch coat. We manufacture several intermediate hard coating and scratch coatings for the optical industry. 

Most vendors use some version of a silicone based formulation. Our formulation is based on entirely different science and is water based, faster, easier, uses less energy to apply, has a longer shelf life and costs less. Contact me I can send you a technical bulletin which goes into detail.

When you look up "scratch resistant coating" on this board you will see most do not understand the technical difference between a hard coat and a scratch coat. As a chemical vendor this was at first puzzling to us. We learned to adapt our marketing language to fit the optical industry.

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## Chris Ryser

> *Some current gen AR's have hard scratch coats (SC) (super hi vision/alize/etc). However, the SC is under the AR..* 
> 
> *Is the "10 bayer" (or whatever) hardness SC actually relevent to protecting the AR? How carefull do you have to be?*


In the old days of glass lenses to adhere the SIO2 which makes the AR coating was no problem an both materials were silicone relared and in the same family.

With the introduction of plastic lenses the old way did not work anymore. The SIo2 would not stick to these plastic surfaces and the AR coatings would not last very long on the lenses. Lots of delaminations 29 years ago.

Then came the hardcoats to protect the lens surface with a harder coat to reduce the ratio of scratched lenses. Factory coated lenses are usually hadcoated with thermocure polisiloxanes which are again compatible with the SIO2. Some of the large corporations are using a japanese made hard coat and claim that their lenses have no delamination problems.

*To sum it up: The scratch coat on AR coated lenses has nothing to do with protection against scratching, it is purely a mean to help and promote good adhesion of the SIO2 layer. It is always the outermost layer that is prone to scatching.*

check: http://optochemicals.com/ARcoating_principle.htm

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## Judy Canty

So Real Life Simulation testing, such as done by Colts, is a more accurate reflection of the quality and durability of an A/R coating than a Bayer rating?

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## Chris Ryser

> *So Real Life Simulation testing, such as done by Colts, is a more accurate reflection of the quality and durability of an A/R coating than a Bayer rating?*


*When you buy yourself the most expensive new mattress..............put some gravel on your bedsheets............you will be uncomfortable and scratch your back................even by having the best rated mattress in the wolrd. :finger:* 

It is the last layer that countsm and that is the one that has to be tested by whoever with whatever means that are acceptable.

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## Judy Canty

The outermost layer is what the Real Life Simulation testing addresses.

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## LKahn

Judy

Colts does a Bayer test to determine abrasion resistance. A Bayer test is performed by using a specific grade of steel wool, at a specific pressure for a specific number of strokes. Then the amount of haze on the surface is evaluated. A Bayer test is a common industrial test for abrasion. An intermediate layer does add some strength to the AR coating. But, ultimately the top layer determines abrasion resistant characteristics. 

When you add a coating on top of the AR you need to take into consideration its affect on the AR performance, refractive index, optical clarity, among others. For example a coating with a refractive index different from the plastic substrate creates a new set of issues such as a rainbow effect. This effect can be annoying to some customers. We have invested a good deal in coating technology as a manufacturer of coatings. 




> So Real Life Simulation testing, such as done by Colts, is a more accurate reflection of the quality and durability of an A/R coating than a Bayer rating?

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## FCChemist

The variations in color, rainbow appearance, that one might observe in a optical coating is due to variations in coating thicknesses across the surface.

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## LKahn

The rainbow effect is often caused by an intermediate layer between the plastic substrate and the AR coating having a different refractive index than the plastic substrate. One of the intermediate coatings we manufacture has the same refractive index as polycarbonate which eliminates this effect.

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## eekthorp

> Colts does a Bayer test to determine abrasion resistance.


Ok, so I've seen Alize hardness number quoted at both 3.66 and 5.2. Is this because the intermediate hard-coat is 5.2 bayer, so some marketing may emphesize this, but the actual surface will scratch at 3.66 bayer (using Colts "Real Life Simulation")?






> The rainbow effect is often caused by an intermediate layer.


Are you refering to the Chromatic abberation _through_ the lens, or the 'oil slick'/thincoat ranbow you see by looking _at_ the surface of the lens?

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## FCChemist

> The rainbow effect is often caused by an intermediate layer between the plastic substrate and the AR coating having a different refractive index than the plastic substrate. One of the intermediate coatings we manufacture has the same refractive index as polycarbonate which eliminates this effect.


 
If the hardcoat is applied with a uniform coating thickness then a rainbow effect, interference, will not be observed even if the refractive indices of the substrate and the coating do not match.  Overall color and % reflectance can be impacted.  One can reduce the risk of interference through refractive index matching as you describe or precision coating.

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## HarryChiling

no longer playing in this sand box

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## LKahn

The rainbow effect is also caused by the hard coat having a different refractive index than the plastic substrate. Vendors that manufacture lens with AR coating ask about this specific issue frequently. Our hard coat formulation resolved this issue. Silicone based hard coats have a refractive index which is different than polycarbonate.

Many in the optical industry use hard coat and scratch resistant interchangeably. A hard coat is not a scratch resistant coating. Back in December we did a thread asking folks to "describe scratch resistant coating". I was amazed by the confusion. Hard coats are used to improve the adhesion of the AR coating, to add strength to the AR coating, to reduce penetration in a drop ball test and a few additional features which tend to be trade secrets.

Something can be hard, but easy to scratch. The classic example is steel. Hard to penetrate, but easy to scratch.

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## FCChemist

> Many in the optical industry use hard coat and scratch resistant interchangeably. A hard coat is not a scratch resistant coating. Back in December we did a thread asking folks to "describe scratch resistant coating". I was amazed by the confusion. Hard coats are used to improve the adhesion of the AR coating, to add strength to the AR coating, to reduce penetration in a drop ball test and a few additional features which tend to be trade secrets.


Larry, where did you learn that the purpose of a hardcoat is to improve the adhesion of the AR coating? I agree that the terms scratch coat and hardcoat are used loosely. But their purpose with respect ophthalmic lenses and other plastic substrates (plastic sheeting, plastic film, plastics on automotives, etc) as I understand it is solely for abrasion resistance...making a soft material more glass like and less likely to scratch to maintain appearance. Though an AR is on top of the abrasion resistant coating, its performance against abrasion is based on the performance of the underlying coating.

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## LKahn

FCChemist

I acquired a company about 18 months ago that manufactured a lousy scratch resistant coating. I would not sell it. We went to work on a new scratch resistant coating. Accidentally, we came up with a primer and an intermediate coating which are both excellent hard coats. Since then we finished work on a real scratch resistant coating with the characteristics of a hard coat and true abrasion resistance. Our family of coatings are sold under the LensArmor brand. 

As to your adhesion question. The customers that want this stuff in large quantities had among there first questions, adhesion performance. LensArmor has amazing adhesion. The lens manufacturers ranked this feature among top 5 requirements. The formulation of our product is uniquely different from other products. Not silicone based.

Call me. I will get you a technical bulletin.

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## Julian

> The variations in color, rainbow appearance, that one might observe in a optical coating is due to variations in coating thicknesses across the surface.


It is not only due to variations in coating thicknesses. The index of lacquer comparing to index of substrate is important as well.

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## Julian

> Larry, where did you learn that the purpose of a hardcoat is to improve the adhesion of the AR coating? I agree that the terms scratch coat and hardcoat are used loosely. But their purpose with respect ophthalmic lenses and other plastic substrates (plastic sheeting, plastic film, plastics on automotives, etc) as I understand it is solely for abrasion resistance...making a soft material more glass like and less likely to scratch to maintain appearance. Though an AR is on top of the abrasion resistant coating, its performance against abrasion is based on the performance of the underlying coating.


I agree with you that a hardcoat is for abrasion resistance. It is a fact, but we should remember that Ar, hardcoating and substrate works together and sometimes happens that in some combination it is problem with an adhesion.

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## FCChemist

Julian, I agree. Abrasion resistant coatings can serve purposes beyond their primary purpose of scratch resistance. In the company I work for, we devote considerable R&D resources towards development of new additives to improve the attributes of the hardcoats we use.

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## LKahn

FCChemist 

What characteristics are you seeking? Contact me off line for a technical bulletin. We might be able to be of assistance

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