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Is this a good way to reduce the bubble content of acrylic?

  1. Oct 30, 2013 #1
    Suppose I want to make a good acrylic lens, with very few bubbles. Could my eyes tell the difference between perfection and standard optical quality acrylic? Would getting it very hot and low viscosity, and then centrifuging it, be a good way to separate out the bubbles and inclusions? How do glass makers improve their quality? I would think getting it hot and full of convection currents would be a good way to get it more homogenous, which is also important.

    If I grind it in water to prevent dust and overheating, will the acrylic absorb the water fast enough to have optical problems when the lens is done?

    What if I make a glass lens, then a mold, then try to inject the low bubble acrylic into the mold. Does injection usually reintroduce new bubbles? Maybe a hot syringe and vacuumed mold would be in order.

    If I have a perfect acrylic lens, which can absorb water from the air, would a 1/4 wavelength AR coating of MgF seal it so that no water can get it?

    Would the large thermal coefficient of thermal expansion (about 70x larger) vs the low coefficient of glass mean that the AR coating would flake off if ever exposed to hot car temperatures or freezing overnight temperatures?

    And how smooth do you have to get an acrylic lens before fire polishing it can take it the rest of the way? I would probably have a concave similar but negative shaped iron which I heat read hot (or maybe lower, whatever is best) and lower to within 1/4 inch of the lens. I'd watch through the glass window and lift the iron off as soon as the lens looked shiny from surface tension. Then it would air cool. Do you think that would cause enough thermal expansion difference between the surface and core to crack the lens?

    I also looked at some custom lens sites, and they reported their scratch/dig ratios and surface tolerances, and I was surprised that even though they offered fire polishing as a surface, they still report having scratches and digs. I would think the surface tension would fix all that.
  2. jcsd
  3. Oct 30, 2013 #2


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  4. Oct 31, 2013 #3
    I would try heating it and putting it inside of a vacuum chamber if available. The low pressure would make the bubbles expand and pop.
  5. Oct 31, 2013 #4
    The acrylic concrete floor sealer is different from what I'm doing. When a thick enough layer hardens on top first, bubbles from evaporating solvent can form. I did not know that concrete is very porous and breaths. That makes me reconsider making molds.

    As for the laser to fire at bubbles, I think that is outside my budget unless it is very effective. Also, since I don't know why it would work, I would be taking a gamble to try it. I'd rather stick to something less conceptually sophisticated unless it is well proven by others besides the patent owner. The only way a laser could work is by heating the bubble or glass, which I'm already doing the old fashioned way.

    The low pressure sounds like a good idea. I can easily see raising a plunger cap on a centrifuge vial and locking it in place before centrifuging. I assume acrylic would not evaporate under low pressure.
  6. Oct 31, 2013 #5


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    If you are getting bubbles with acrylic paints you simply add more solvent ... and stir less.

    It is the stirring of the acrylic which entrains the air; the air then is trapped as the acrylic solidfies, generating the bubbles. So stir less in your preparation stage.

    The bubbles will flow out of the acrylic solution while it is in liquid form ... slowly moving upwards. They become trapped when the upper surface solidifies. So add more solvent so that the entrained air has time to "float to the top".

    I don't see how pressure exterior to the acrylic will have any effect upon the speed of the bubble flowing up through the acrylic - this rate is determined by the viscosity of the acrylic. You can reduce the viscosity by adding more solvent.
  7. Oct 31, 2013 #6
    reducing the pressure will make the bubbles grow bigger. Bigger bubbles pop faster.
  8. Oct 31, 2013 #7


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    The bubbles are _inside_ the acrylic. You are reducing the atmospheric pressure, not the pressure inside the acrylic.
  9. Oct 31, 2013 #8
    The pressure inside the acrylic will reduce as well because it is in hydrostatic equilibrium, being in a liquid state.
  10. Oct 31, 2013 #9
    I'm not using acrylic paint. I plan to buy a block of optical quality acrylic for making lenses. To possibly increase the optical quality, I want to melt it and remove any tiny bubbles or unevenness. This depends of course on how far standard optical quality acrylic already is from perfect, and how much it costs to buy a higher grade.

    I suspect that it is easier to get high quality acrylic than glass because it melts at a lower temperature and probably had a lower viscosity at a reasonable temperature.
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