Kevlar, Water Absorption report

Discussion in 'Boat Design' started by brian eiland, Mar 1, 2015.

  1. brian eiland
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    brian eiland Senior Member

    An interesting observation by some friends who recently hauled there 65 foot catamaran vessel and found significant water absorption by the Kevlar material utilized in some of the construction
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  2. upchurchmr
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    upchurchmr Senior Member

    I was living in Japan and had a neighbor who worked for Dupont.
    He said he was the initial person responsible for kevlar use in tires.
    I asked him if Kevlar soaked up water and he said it was such a miniscule amount (in the fiber) that he could never understand the "reports".

    I don't understand the one above.
    Why in the world will kevlar not accept resin?
    If there was no epoxy around the kevlar, then the layup must have been bad.

    Any more information?
  3. DavidJ
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    DavidJ Senior Member

    Did the fibres themselves actually absorb water? I would be highly skeptical that they did.

    How was this Kevlar originally installed in the vessel? The more likely explanation is that initial construction was not well done. Kevlar is very difficult to work with and requires high quality shop practices to achieve the best results.

    When I was involved in composite construction and repair we never used Kevlar on hand lay-ups. We only used Kevlar in vacuum bagged installations. The reason being that Kevlar had a tendency to "float" in the resin. It did not stick down to the mold the way fibreglass does. If you vacuum bagged or resin infused the Kelvar it always bonded beautifully.

    The other reason for not using Kevlar (or carbon fibre for that matter) with hand lay-ups is that it is a high performance material. What's the point of using a lightweight high performance material in a crappy resin rich hand lay-up?
  4. sdowney717
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    sdowney717 Senior Member

    The space between the fibers soaked up the water.
    And maybe the water the boat sits in dissolved some of the resins.
    And I have read wetting out kevlar with resin is not easy?

    Maybe a hydraulic pressure erosion of the bond as hull moves caused water to get behind the kevlar.

    It is not the resin giving the strength of the kevlar, so maybe an entirely different 'resin' maybe use a polyurethane rubber to embed the kevlar into?
    Then put more of same polyurethane on top of the kevlar.
  5. upchurchmr
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    upchurchmr Senior Member

    Nothing is going to dissolve a laminating resin.
    There should be no spaces between the fibers

    What hydraulic pressure erosion could you be talking about ???

    Polyurethane is an elastomer - flexible- if you want the fibers to provide some strength, the resin needs to be not actually flexible like rubber.

    David J - sounds good to me.
  6. sdowney717
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    sdowney717 Senior Member

    Oh, ok, I was thinking of blisters, they hydrolyze and dissolve styrene.
    For all I know that was what happened, how else will water get behind the Kevlar laminate? If the resin saturation was poor, and the hull works, it will suck water into the strands.
    I have read of hulls pumping and failing cracking their laminates on another boat survey site..
    And hydraulic erosion of cored hulls, pulverizes the hull.

    I know kevlar in a poly would have flex, but kevlar I thought was for preventing hull penetration and sinking of the boat. So then in a poly it would be fine. Unless the kevlar IS the HULL.

    And I think if it had been embedded in a poly it would not have cracked loose from the hull and soaked up water. Granted this is just my own opinion. i have coated my own 37 foot wood boat with various polyurethanes, coated as in coated thickly. And embedded the seams to make a watertight hull, since 2004 it has been fine.

    I will bow out now and not respond again.
  7. Canracer
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    Canracer Senior Member

    They mentioned that repairs were being made after a scrape with a reef. Maybe the kevlar broke free after doing it's job, protecting the hull in a reef collision. Hard to know exactly what happened.
  8. Karsten
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    Karsten Senior Member

    The tricky part with composites is that there are many possible combinations of fibres and resins and some combinations work better than others.
    Kevlar is particularly tricky. Resin has a hard time to bond to the fibre. Under compression the strength is only about 1/10 of the tensile strength. It only makes sense to use Kevlar where tensile loads are expected, weight is critical and epoxy resin is used. Polyester is not going to bond to the fibre. Even in impact or abrasion critical areas glass can be used for a fraction of the cost. Additionally it is possible that the fibres were sized for weaving bullet proof vests and not for laminating. Sizing is a process where the fibres are coated with a thin surface layer to enhance manufacturing processes. Weaving of vests will require different sizing compared to laminating boats. Unless you know what you are doing stick to glass fibres.

  9. Kojii
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    Kojii All is remodelling

    Orca hull and deck and mast was laid up with high-performance resin - thixotropic, isothalic Cargill poly product with greater elongation and fire resistant qualities. No sign of any water in the layup at twenty years or since (splashed in 1990). I put on vinylester resin last time we took it down to bare hull as precaution. Tales of water in kevlar seem to be short on details. Photos really tell a lot. I did a new hatch with carbon and kevlar. Tough to work with and, done right, very tough.
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