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Impact resistance of polycarbonate

  1. Dec 1, 2009 #1
    So I'm working on an article about the imapact resistance of ophthalmic lens materials. The two main lens materials used are those of a hard resin called CR-39 and the other is polycarbonate. I have been looking for something that describes the difference in their ability to withstand certain degrees of impact. All I could find is that when CR-39, or other hard resins. are cured the molecules cross link. I'm assuming that this is what makes the lens very hard and also some what brittle. Where as polycarbonate molecules do not cross link and instead create long chains. And this is what gives the lens it's impact resistance and flexability. Am I correct in my assumptions?
     
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  3. Dec 1, 2009 #2

    chemisttree

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    Not completely. Toughness is a property that tends to follow trends in crystallinity with more crystalline polymers being less tough. Glass transition point (Tg) also tends to influence toughness with materials having lower Tg having greater toughness. Polycarbonate is completely amorphous, it has a high Tg and yet it is still very tough! Very unique behavior for a plastic. Best not to use polycarbonate as an example to classify materials in general.
     
  4. Dec 1, 2009 #3

    turbo

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    Polycarbonate is very shatter-resistant, but more prone to scratching than CR-39. It also costs a bit more as eyeglass lenses.

    The shatter-resistance is very important for children and for physically-active persons. If anyone here has children that need corrective lenses, protect their eyes with polycarbonate. As a nice side-benefit, polycarbonate has good UV protection without additional optical coatings.

    I had a nice simple test machine in my optical lab. It consisted of a target area into which a lens could be inserted, and a vertical metal tube, and a steel ball bearing to drop onto the lens. If a customer was wavering about what lens material to buy for their child, I'd pop into my lab, get the tester and some inexpensive 0-diopter blank lenses and demonstrate for the customer. Glass blanks always shattered, CR-39 would usually shatter (and produce some very pointy shards) and polycarbonate would come out whole. After a while, I made up a simple display of blanks/shards that anybody in the optical shop could point out to illustrate the same point without destroying blanks.
     
  5. Dec 2, 2009 #4
    OK I'm trying to understand what your saying. Is there any way you could "dumb" that down for me? :)
     
  6. Dec 2, 2009 #5

    chemisttree

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    I'm saying that toughness isn't as simple as crosslinked vs non-crosslinked. Crosslinking can aid in toughness to a point but if a system is highly crosslinked it can become brittle. Toughness is a property where the energy of the impact is spread about through the material rather than being focused to a point, say a crack tip. How that spreading out of the impact energy is accomplished can vary from material to material. Some materials will have areas along the backbone that can stretch and deform under stress and then relax back to their original shape like non-crosslinked polycarbonate. Other materials cannot stretch so much before bonds break and the stress is passed along to the next bit of material as is seen in a crosslinked polycarbonate like CR-39.

    The notched http://en.wikipedia.org/wiki/Izod_impact_strength_test" [Broken] is one measure of toughness. In that test a notched specimen is held and a weighted bar on an arm is raised to a certain height and released so the bar breaks the specimen. The energy remaining in the bar after striking the sample is measured and the energy lost to the sample is determined.

    Mechanical testing of an http://www.gallinausa.com/polycarbonate.tech.html" [Broken]

    Of course a visual like Turbo's shattered blank CR-39 is much more impressive than these numbers!
     
    Last edited by a moderator: May 4, 2017
  7. Dec 2, 2009 #6

    turbo

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    Probably the method of manufacture contributes to polycarbonate's toughness. The lenses are injection-molded and chilled quickly, so there is not as much time for orientation and other formation of internal structure as there is in plastics that are hot-molded. BTW, the polycarbonate lens blanks that went through my torture-test were entirely unusable afterward - they just didn't break into nice sharp shards.

    Years back, I was seen by a neurologist who wore semi-rimless glasses with plastic lenses. There was a chip in the edge of one lens, creating a weak point (and internal stresses that you can see with a polarimeter if you have one) so I asked her what her hobbies were, and the first one was horse-back riding. I told her that the next time she got slapped in the face by a branch while trail-riding, she could end up with a lacerated eye, and possible blindness. After I explained why, she promised to get poly replacement lenses. I didn't have my shards of plastic, but having worked for a large team of ophthalmic surgeons, I had seen a LOT of severe eye injuries, so I was able to be sufficiently descriptive to scare her.
     
  8. Dec 2, 2009 #7

    chemisttree

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    Yes, the method and rate of cooling have large influence. Also, the descriptor "polycarbonate" is not enough information to form definite opinions. Quality, molecular wt, polydispersity, etc... all vary among suppliers. Injection molding actually helps to orient polymer chains and contributes to crystallinity as well as slow cooling can. The extrusion temperature and the viscosity are big factors as well. The more viscous the extrusion the more highly oriented as you might expect. Kind of like strain hardening in metals.
     
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