epoxy equivelent thickness to poly

Discussion in 'Materials' started by gtflash, Mar 7, 2013.

  1. gtflash
    Joined: Jun 2006
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    gtflash Senior Member

    If a traditonal polysester sport boat deck was layed up in 3x650g cloth.

    Would swapping to epoxy allow a thinner laminate, and what would be equivelent strength in epoxy and bi-axial.

    SP resins appears to indicate epoxy is only 1/3 stronger, therefore could the laminate be 1/3 material??
     
  2. gonzo
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    gonzo Senior Member

    Not really. The substitution is not that simple. Actually, because epoxy is less rigid the laminate may need to be thicker in some cases.
     
  3. rxcomposite
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    rxcomposite Senior Member

    If a polyester laminate is substituted with epoxy, the thickness will remain the same but the laminate will be heavier. Epoxy is a little heavier than polyester.

    The confusion lies in the normal usage of using glass content v.s. resin content by weight, because resin specific gravity varies from 1.2 for polysester to 1.38 for epoxy. Glass is nearly constant at 2.56.

    Also you are confusing yourself. You are talking of cloth (I assume Woven Roving) then you switch to Bi axial. Two different material even of the same class (bi directional) and of the same weight will have different material characteristics. One is stronger than the other but only the material data sheet will tell you. Generally cloth will be thicker because of the undulation due to weaving, Bi ax will be thinner as there is no weave.
     
  4. gtflash
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    gtflash Senior Member

    I agree, didn't ask question well. I wanted to know what layup in epoxy with bi axial fabric would be equivalent to a polyester layup with 3x650 combi. It's probably not as straight forward as that and if epoxy is heavier, it probably wouldn't save weight after all. Would be hand layup.
     
  5. Steve W
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    Steve W Senior Member

    As with most things, the middle of the road approach is usually best, with a hand layup it is easier to achieve a good resin to glass ratio with polyester or vinylester than with epoxy,especially general purpose epoxies but even dedicated laminating epoxies such as Pro set, so a switch from poly to vinyl with biax or all uni would make more sense to me. A lot of folks assume that when you use epoxy you can just leave out some glass and save some weight but it is rarely that simple.

    Steve.
     

  6. rxcomposite
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    rxcomposite Senior Member

    The most popular method of predicting laminate property is by the weight fraction method so we start with it. For a 650 gram/m2 (0.65 kg/m2) biax laminated with polyester (1.2 specific gravity) with a 0.52 glass ratio, 0.48 resin ratio, the thickness of a single ply laminate will be 0.75 mm (complex formula not shown). One square meter of ply will weigh 1.25 kg comprising of 0.65 kg. fiber, 0.60 kg. resin.

    To laminate the 650 gram/m2 biax with a higher performance resin such as vinyl ester or epoxy. The volume fraction method is the only way as the weight fraction method will fail. But we will take a shortcut. The polyester laminate needs 0.60 kg. resin as previously computed. With a specific gravity of 1.20, it needs (60kg resin/1.20 s.g.=) 0.50 liter of resin per square meter.

    To use another resin of a higher density, the liquid volume stays the same to fully saturate the fiber but the weight will increase. Vinyl or epoxy has a density of about 1.4 so, ( 0.50 li. x 1.4 s.g.) = 0.70 kg. resin per one meter square. The new laminate will weigh (0.65 kg fiber + 0.70 kg. resin) = 1.35 kg. Thickness remains the same as the resin volume remained the same.
     
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