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Need physics explanation for defects after epoxy cure

  1. Nov 26, 2008 #1
    Qn:Why do bubbles like this form?
    -My guess is that intially air trapped in the liquid before cure expands in all direction(to maintain equal pressure) during curing, as curing shrinks the epoxy, causing bubbles to form. (Or is it the heat which expands it?)

    Microscope picture of bubble:

    Qn: Why do cracks like this form?
    -My guess is that there is uneven contraction, 1 side contracting faster than other, causing a crack.

    Qn: Why do voids form?
    -My guess is that there is contraction, leading to voids.

    Qn**:Why do "funnels" form?
    -I dont know, the funnels form at both ends of the capillary tubes, only at both ends, nowhere else, why? My guess is as it contracts, a drop in pressure causes air to rush in, forming a "funnel" shape.

    I performed these experiments in a capillary tube (not me in one, but the epoxy)

    Looking forward to lucid, physics explanations if possible (this is for my engineering project)
     

    Attached Files:

  2. jcsd
  3. Nov 26, 2008 #2
    Cracks:
     

    Attached Files:

  4. Nov 26, 2008 #3
    " Funnels " :
     

    Attached Files:

  5. Dec 9, 2008 #4

    chemisttree

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    Dissolved gases and moisture in the uncured resin can cause this. Were your samples degassed (along with the capillary) before you loaded them?
     
  6. Dec 9, 2008 #5

    chemisttree

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    These look like shrinkage cracking. There is a strong bond between the epoxy and the walls of the capillary so when the resin shrinks, you get weird looking cracks in the center where the stresses are the highest.
     
  7. Dec 9, 2008 #6

    chemisttree

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    If you were to silanize the interior of the tubes (trimethylsilyl chloride or preferrably a perfluoro analog) the resin could shrink away from the wall and the funnels might not form. Your epoxy plug would pull away from the walls and shrink within the tubing most likely.
     
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