Epoxy and water absorption

Discussion in 'Materials' started by Cacciatore, Mar 20, 2020.

  1. Cacciatore
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    Cacciatore Junior Member

    Hi Guys, i have seen some epoxy datasheet of water absorption around 1% after 8 days at 24 deg Celsius. So how is possible to keep impermeability? Some gelcoat? I believed that epoxy was the best in the matter.
     
  2. fallguy
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    fallguy Senior Member

    Link to the datasheet
     
  3. Cacciatore
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    Cacciatore Junior Member

  4. Cacciatore
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    Cacciatore Junior Member

  5. jehardiman
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    jehardiman Senior Member

    All epoxies absorb some water. Epoxy is not a molecular solid, but rater a porous network of cross linked long chain polymers. Don't think of it as liquid water, think of it as individual dihydrogen monoxide molecules "trapped" in the spaces between the polymer chains. The epoxy is not "wet", it just got heavier. Water absorption is more marked with increasing water pressure. Items exposed to deep submergence pressures gain even more weight. The same thing happens with almost all polymer/plastic materials...some even swell.
    Water Absorption 24 Hour - (ASTM D570) Test of Plastics https://omnexus.specialchem.com/polymer-properties/properties/water-absorption-24-hours
     
  6. rxcomposite
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    rxcomposite Senior Member

    1. Do not worry about water absorption of the polymer. That is a cast resin and laminate is a mixture of glass fibers and resin. Glass do not absorb moisture.
    2. Water absorption follows the Fickian's diffusion curve.It is linear at the initial stage (about 60%) but takes a long time for it to reach a "saturation point" or plateau. It is because the initial layer has to be saturated first before it reaches the second layer.
     
  7. Cacciatore
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    Cacciatore Junior Member

    So can i have directly a lamination without epoxy gelcoat? The water doesn t decay the mechanical propriety of the lamination stack?
     
  8. fallguy
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    fallguy Senior Member

    Water is not the enemy of epoxy, but the sun will destroy it. It is not UV stable. This is why it must be coated with something that is more UV resistant.
     
  9. rxcomposite
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    rxcomposite Senior Member

    Yes you can. You will be worrying about blister problem. Polyurethane top coat over epoxy works as good as epoxy top coat. I have a graph somewhere. Will search for it.
     
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  10. mastcolin
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    mastcolin Senior Member

    All epoxies will absorb small amounts of moisture and as someone said it diffuses in...and out all the time when it reaches saturation point. Don’t worry if it good epoxy well cured at correct mix ratio. The point is that is isn’t hydroysed chemical by the water. If you chuck a piece in water it will still be their in a million years. Polyesters? Gone. Ps glass doesn’t absorb water. The wetting agents/size will.
     
  11. gonzo
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    gonzo Senior Member

    Polyesters absorb more water, but does not make them decay. In short, they are not dissolving in water. Epoxies, in general, absorb a much lesser percentage of water. However, the effect in mechanical properties is minimal and usually disregarded.
     
  12. mastcolin
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    mastcolin Senior Member

    Polyesters are chemically unstable. They will to varying degrees (speed) all hydrolyse ie break down. Probably not in your life time (but that would depend on how well it was cured and with what base components). I was perhaps being overly dramatic but if you leave them in water they will effectly unzip themselves. I’d have to dig out my university books to give you thermodynamics behind it all.
     
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  13. fallguy
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    fallguy Senior Member

    can you explain the blister point better?

    i.e.
    If an epoxy locker is left uncoated; will there be a blister potential?
     
  14. rxcomposite
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    rxcomposite Senior Member

    Water/moisture absorption causes a relaxation of the matrix properties. This process is very slow and I remember the process takes about 20-25 years.

    What is immidiate is in the curing process. During curing, water and alcohol is given off as a byproduct by the polymer. You can observe this when laying up thick laminates. After the peak exotherm, while it is still hot to touch, you can see water droplets forming on the outer layers. You have to let it dry before any additional layer is added.
     

  15. rxcomposite
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    rxcomposite Senior Member

    Blister is a trapped moisture or uncured chemicals In between Layers. When additional moisture gains entry, it will expand and form a blister. Care must be observed during the in between laminating process. Improperly mixed chemicals, improper mix ratio, or too much humidity in the shop floor can cause problem later on.
     
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