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Thread: Scratch resistant coating and AR question.

  1. #1
    What's up? drk's Avatar
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    Scratch resistant coating and AR question.

    When providing AR coating, including the Zeiss Foundation coat or Crizal, are factory coated SRC lenses the starting point of manufacture, or are uncoated lenses the starting point?

    In other words, should I be specifying, being charged for, and charging for UNCOATED CR-39 when ordering a lens with AR, or should I add the price of a factory-SRC to the lens and then add the price of the AR?

    I believe the former, because VSP does it that way.

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    Manuf. Lens Surface Treatments
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    Scratch Resistant Coatings ...........................

    AR coatings will NOT stick to plastic lenses without a hard coat. The hardcoat ionly the means to give the AR coating the proper adhesion. It does not serve any other purpose and will not protect your lens as it is below the AR coating.

    Therefore all AR coating companies will use hardcoated lenses or if not hrad coated they will coat them because they have to, for quality purposes.

    I am not aware that they charge you for hard coating because they can not do without it. It is a must for them or the AR coating would delaminate in no time.

  3. #3
    OptiBoard Professional RT's Avatar
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    Advanced coats such as Foundations, Crizal, HiVision, et al, use uncoated lenses, and put a lab-applied hardcoat onto the lens to act as the base for the AR coat. Typically, this is a harder coat than factory-applied scratch coats (and much harder than lab-applied UV cured backside coats), and is designed specifically for improved adhesion to the AR coat stack. Labs typically do not charge for the scratch coat, as it is considered to be an integral part of the branded coating.

    Generic, or in-house, AR coats typically put the AR coating on top of factory-applied scratch coats. Since there are many types of factory scratch coats out there, adhesion to the AR stack can be an issue. There can be a difference between the front side of a poly lens (factory hardcoat) and the back side (UV cure lab hardcoat). And some generic in-house AR coats do not put any hardcoat on the back side of CR39 lenses, since the UV cure backside hardcoats tend to have equal abrasion resistance to uncoated CR39. For generic AR coats, many labs DO charge for the hardcoat on CR39 (except for progressives). These coats typically do not carry as long a warranty period as coats where the hardcoat and AR are considered to be integral.
    RT

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    Generic .....................??????????????

    Quote Originally Posted by RT

    Generic, or in-house, AR coats typically .......................................... And some generic in-house AR coats ........................................For generic AR coats,


    .
    What is a generuc hard coat ??

    What is a non generic hard coat ????? Can you specify?

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    What's up? drk's Avatar
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    Muchas gracias.

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    Quote Originally Posted by Chris Ryser
    AR coatings will NOT stick to plastic lenses without a hard coat. The hardcoat ionly the means to give the AR coating the proper adhesion. It does not serve any other purpose and will not protect your lens as it is below the AR coating.
    Hardcoats under AR do provide abrasion resistance. An AR stack is so vanishingly thin that it provides little or no abrasion resistance in and of it self. If you put a great AR on a junk hardcoat then the abrasion resistance is junk, plain and simple. If you put an AR directly on a soft lens material (e.g. poly) and make it stick it doens't matter how good the AR it will scratch early, often and deeply. The only good AR coated product is one coated on a strong, stiff hardcoat.

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    Love it ..............................

    Quote Originally Posted by coda

    Hardcoats under AR do provide abrasion resistance. An AR stack is so vanishingly thin that it provides little or no abrasion resistance in and of it self. If you put a great AR on a junk hardcoat then the abrasion resistance is junk, plain and simple.
    I love above argument and would like to compare it to the real world:

    It actually says that the same case would be if you would implant a sheet of steel underneath your skin, and then believe that your skin is now protected against abrasion, cuts and other damages.

    Whatever materials are in direct contact with matters of abrasion would need protection in between the the two and not underneath.

    Plain common sense .....and logic.

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    Rising Star
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    Chris,

    As you know, the top layer of most A/R coatings applied to plastic lens materials is SIO2(Glass)

    Glass is hard and durable but also very brittle. A good hardcoat which provides the base to allow the A/R coating to become very abrasion resistant also alllows the SIO2 not to deform under pressure from cleaning which can cause the A/R coating to fracture and then evenutally fail.

    A soft hard coat will allow to much movement of the A/R coating and cause problems.

    The best hardcoat for A/R coating is usually a thermal cure nano-composite hardcoat which has a reasonally high content of Silca (Sio2) in it.

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    Quote Originally Posted by Chris Ryser
    I love above argument and would like to compare it to the real world:

    It actually says that the same case would be if you would implant a sheet of steel underneath your skin, and then believe that your skin is now protected against abrasion, cuts and other damages.

    Whatever materials are in direct contact with matters of abrasion would need protection in between the the two and not underneath.

    Plain common sense .....and logic.
    Interesting hyperbole of my statement Chris. If abrasion of lenses was simply a matter of rubbing or scraping off the top atomic layer you might have something resembling an argument but it isn't. Abrasion of lenses is always a complicated mechanism which includes simple abrasion (top 10 atomic layer disturbances), what is known as crushing abrasion where the lens is deformed elastically during abrasion and what is essentially cutting.



    I'll give you the argument on the first mechanism, though it's actually much more complicated, and take up the second two. First however let's describe the three potential materials in the ophthalmic lens product. There's the lens which may depending on material range from a fairly hard plastic (CR-39) to a very soft plastic (poly) all of which have a medium stiffness and high elasticity (meaning they can flex then return to their original shape fairly well). The next is a good (factory applied or thermally cured in lab) hardcoat. These are much stiffer (orders of magnitude) than the lens material but still retain a decent elasticity though nowhere near as good as the lens. Finally there’s the AR coating which is made up of very, very thin layers of what is essentially glass. These are very, very stiff and not elastic in a meaningful way, in fact these materials are generally considered brittle which is the opposite of elastic. We should also understand that a hardcoat is about 25 times thicker than the total thickness of an AR coating (5 micrometers vs.



    So now lets think about a crushing abrasion. What happens in this case is that a significant load is moved across the surface of the lens. If an AR coat was applied directly to a lens surface then as the lens deforms elastically (meaning it will spring back to it's original shape) under this load the AR must either deform with the lens or delaminate (come off) the surface. If the AR deforms it will fracture exposing sharp edges to the thing rubbing the surface. These sharp edges will be caught by the abrading object and in turn fracture, what you end up with is a significanly crazed surface with each of those cracks having broken and missing edges. If the width is small this looks like a wide, diffuse scratch, if the area is large it appears to be a scuff. If you have a decent hardcoat things happen very differently. Because of the significantly increased stiffness (provided by the hardcoat) the AR isn’t forced to flex and therefore doesn’t fracture and you don’t get the same cracking and end up with an area of crazing and broken AR.



    Now lets consider a cutting type abrasion. In cutting abrasion the lens is impacted with a sharp object. This sharp cuts into the lens deforming it plastically (meaning in such a way that it can’t return to it’s original shape). The plastic lens material is particularly prone to this type of abrasion and the softer the material the worse this is (which is why poly scratches so easily). The varnishingly thin AR coating can’t do a thing to stop this type of abrasion because, even though it’s very hard, it’s so thin that it is unable to resist this cutting and simply fractures in front of the onslaught of this cutting object. Now a good hard coat, because of it’s much greater thickness, is able to provide a significant resistance to this type of attack and therefore an AR coated lens with a decent hardcoat will fare better than one with a junk hardcoat.



    Now, there are a bunch of other mechanisms by which a lens can be scratched some of which are more or less well understood but I think these examples provide enough insight into the value of a good hardcoat when the final product is AR coated.



    Now as to why a sheet of metal under our skin would and wouldn’t provide abrasion resistance in the same way a hardcoat does. First it wouldn’t provide a complimentary interface between the skin and the structure beneath it (muscle and fat) so crushing abrasion resistance isn’t provided. However it would provide the underling ‘meat’ great abrasion resistance to cutting abrasion which is why our brains are encased in bone and why, when the skull is damaged, they put a metal plate in your head.



    Hope that shines some light on the subject using logic, sense and science.



    -Matt

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    Yikes! Who was that masked man?

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    You got a point.............


    As you know, the top layer of most A/R coatings applied to plastic lens materials is SIO2(Glass)

    Glass is hard and durable but also very brittle. A good hardcoat which provides the base to allow the A/R coating to become very abrasion resistant also alllows the SIO2 not to deform under pressure from cleaning which can cause the A/R coating to fracture and then evenutally fail.

    Peter Z,

    You are in the AR coating business. I am in the SIO2 and hard coat business developing and manufacturing these types of products for 24 years. I am well aware what is involved.

    There is a very fine line when talking about hard coats. We can make hard coats that are as hard as glass. Hard coats applied on a lens average a thisckness of 2 microns. Your limit in hardness and thickness is the flexibility of the substrate. You make it too hard and to thick it will craze and chip when the lens is too flexible. contracts or shrinks with heat and cold conditions.

    The more tintable a hard coat, the softer it becomes until it does not provide a normal scratch resistance.

    Thermo cure hard coat materials containing siloxanes provide the best adhesion for AR coatings, but they do not provide scratch resistance for the AR layer being situated underneath.

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