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Thread: Impact resistance vs Shatter resistance of various non-glass lenses

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    Impact resistance vs Shatter resistance of various non-glass lenses

    I'm comfortable with using Trivex on kids glasses and other safety challenged applications, but can't seem to find much info on mid and high index materials. Would appreciate seeing a table of results and am interested in shattering vs just cracking or breaking.

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    Quote Originally Posted by Dr. Bill Stacy View Post
    I'm comfortable with using Trivex on kids glasses and other safety challenged applications, but can't seem to find much info on mid and high index materials. Would appreciate seeing a table of results and am interested in shattering vs just cracking or breaking.
    http://www.youtube.com/watch?v=knnSK...eature=related


    I hopr this is somewhat useful to you.

    My understanding is that mid index is the weakest optical material, then CR39, then 1.74. The strength of 1.6 or 1.67 depends on which monomer is used, but they are generally very strong. I'm sorry that I don't have a table of results as you asked for; hopefully someone can help on that front.

    I do know that all lens materials will shatter with enough force except for poly and trivex.

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    Thanks. I think the industry should drop the term "mid-index" and just name the lens or it's index, as both poly and trivex are actually mid-index materials. BTW how does 1.7 stack up?

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    Again, just my understanding, but I believe 1.7 is a lot stronger than 1.74 and about the same as 1.67 but not as strong as 1.6.

    With regard to the term 'mid-index', you're right. It was lazy of me. I meant 1.56.

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    Thanks again. One reason for this question has to do with what we should put on a kid's SRx. I hate writing "poly only" when I really mean something like "no glass, CR-39 or 1.54 index materials". Still pondering the best wording.

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    You can save a whole lot more kids eyes outlawing bottle rockets. But then I don't think we should do even this.
    Do we really feel we are saving the world? Do we think the world want's us to save it?
    Not allowing children to have sissors would save quite a few. Should we not allow them to learn to use them?
    Stopping them from throwing sticks and rocks at each other would help too. I would like to see this happen.
    Statisticly (and I don't trust this word either) how many kids have eye injuries from broken lenses? I suspect that any lens would provide some degree of protection, at least much more than the emetropes without glasses have. As a youngster I remember that every time I played basketball, I either got hit straight on with the ball (felt like my nose was broken) or hit from an angle and getting my temple broke. But in all my miss-spent youth I never saw or heard of anyone being injured by broken spectacle lenses.
    I did see a basket ball coach that lost an eye from being hit by a basketball (and several that did so from baseballs) but they were all emetropes not wearing glasses.

    Chip

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    Well I admit partly it's for my own (legal) protection, but I think you're kindof taking libertarianism to the extreme. I suspect lots of kids get injured from broken lenses. Do you make your kids buckle up? Wear a helmet when cycling? Yeah, I never lost any IQ from riding my bike without a helmet, but then my ex-sister-in-law lost part of her brain doing it. She cost Calif. hundreds of thousands of dollars. A little libertarianism goes a long way.

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    It seems to me that any impact hard enough to shatter a plastic lens would either push the lens out of the frame or push the frame out of the way anyway. So I agree with you Chip. However, I'm probably wrong.

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    You may notice some sports glasses frames with a little lip on the back of the eyewire groove designed to prevent the lens from being so pushed through. Usually a lens that dislodges like that causes eye injury even if the lens doesn't shatter. Worse injuries when they do shatter, because CR-39 shards can be way sharp.

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    Master OptiBoarder Darryl Meister's Avatar
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    I'm comfortable with using Trivex on kids glasses and other safety challenged applications, but can't seem to find much info on mid and high index materials.
    As Robert_S noted, popular 1.60 and 1.67 polyurethane materials are more impact-resistant than standard hard resin or CR-39. Nevertheless, most trade organizations advocate the most impact-resistant lens materials available for children, typically polycarbonate or Trivex, NXT, and similar materials.

    This also means that you may run the risk of being accused of professional negligence by failing to adhere to the accepted standard of care, should you dispense an alternative lens material to a child who later suffers an eye injury, whether that injury could have been prevented with a more impact-resistant lens material or not.

    As Chip noted, there are several potential causes of injury to the eye and adnexa. Some can be prevented with more impact-resistant lenses, some not. Some materials are better for certain types of impact and vice versa. And some injuries could even be worse without eyeglasses. But, in a lawsuit, I don't know that any of that really matters.

    Further, while certain high-index lens materials are much more impact-resistant than hard resin, these lenses are typically also surfaced or cast to relatively thin center thickness in minus powers, which can significantly reduce impact resistance. And the application of various coatings may also reduce impact resistance.

    Yeah, I never lost any IQ from riding my bike without a helmet, but then my ex-sister-in-law lost part of her brain doing it. She cost Calif. hundreds of thousands of dollars. A little libertarianism goes a long way.
    I suspect that the libertarian would argue that it was her brain to lose, but not California's money to spend. Still, it is a tragedy, either way.

    Best regards,
    Darryl
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    ps,

    I should also add that generally only polycarbonate and Trivex-like materials will satisfy the impact-rated protector requirements of the current ANSI Z87.1 safety standard.

    Best regards,
    Darryl
    Darryl J. Meister, ABOM

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    Quote Originally Posted by Darryl Meister View Post
    As Robert_S noted, popular 1.60 and 1.67 polyurethane materials are more impact-resistant than standard hard resin or CR-39.
    Holy Cow! Are you telling me there are benefits to hi-index lenses-vs. CR-39 over and beyond the ability to be a much thinner lens!!! I'm sold! Will be recommending hi-index to all from this point forward!

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    Master OptiBoarder Darryl Meister's Avatar
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    Holy Cow! Are you telling me there are benefits to hi-index lenses-vs. CR-39 over and beyond the ability to be a much thinner lens!!!
    Common 1.60 (MR-6 and MR-8) and 1.67 (MR-7 and MR-10) high-index materials have higher tensile strengths than hard resin, which is also why these lens materials are better suited for rimless frames.

    Best regards,
    Darryl
    Darryl J. Meister, ABOM

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    Quote Originally Posted by Dr. Bill Stacy View Post
    Thanks again. One reason for this question has to do with what we should put on a kid's SRx. I hate writing "poly only" when I really mean something like "no glass, CR-39 or 1.54 index materials". Still pondering the best wording.
    Can you put "Impact Resitant Material only" and allow the optician to use their best judgement? You would have done your duty to the patient and then a competent optician can work out which is the best material/options for the child. The only crappy part is a HUGE chunk of the country still believes poly is the only lens out there regardless of the childs prescription and/or life's daily needs. I have run into a few people who tell me that their business doesn't even offer Trivex as an option, oh well. Good luck Doc!


    A man went to an eye specialist to get his eyes tested and asked, "Doctor, will I be able to read after wearing glasses?"
    "Yes, of course," said the doctor, "why not!"
    "Oh! How nice it would be," said the patient with joy, "I have been illiterate for so long."


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    What has tensile strength to do with impact resistance. Tensile strength is the measurement when you pull it apart. I have seen poly lenses that did not break on impact but they did "turn wrongside out." They did not injure the one-eyed woman wearing them. However I can see a senario where they could have impacted the globe rupsuring it even though the lens didn't break.

    Chip

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    there was no other choice than glass..............................

    Quote Originally Posted by chip anderson View Post

    I never saw or heard of anyone being injured by broken spectacle lenses.


    That same goes for me.................and I grew up in a period where there was no other choice than glass.........not even made shatterproof.

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    Remember when I relate my observations, I have made aproximately 7500 prosthetic eyes (and yes, I mean custom made with the patient present, not working in some lab). I don't doubt that someone somewhere out there has lost an eye from a broken lens (it's a very big world out there) but I also don't think it's enough of a danger for the world or ourselves to worry about except from a standpoint of sales of protective lenses.

    Chip

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    Master OptiBoarder OptiBoard Gold Supporter Judy Canty's Avatar
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    The overarching concern with "Duty to Warn" is that the US in particular is extremely litigious. Our court system is clogged with lawsuits over injuries, real or imagined. The process is slow and expensive, so we try to protect our businesses and our personal lives in as many ways as can be possibly devised.

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    Master OptiBoarder Darryl Meister's Avatar
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    What has tensile strength to do with impact resistance
    First, for plastics, tensile strength is indicative of overall "toughness." Second, and more specifically, when plastic spectacle lenses shatter upon impact the fracture typically initiates as a result of tensile forces at the rear surface.

    Essentially, when the lens is struck from the front, the front surface is put into a state of compression and the rear surface is put into a state of tension as the lens attempts to flex. Once the force at the back surface exceeds the ultimate tensile strength of the material, a crack forms that propagates through the lens.

    I have seen poly lenses that did not break on impact but they did "turn wrongside out."
    In the case of polycarbonate, because the material is so ductile, it is entirely possible for it to "oil can" or buckle inside out, if struck hard enough.

    Best regards,
    Darryl
    Darryl J. Meister, ABOM

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