Carbon with polyester

Discussion in 'Materials' started by zerogara, Jan 4, 2006.

  1. zerogara
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    zerogara build it and sail it

    One of my suppliers calls the necessity of epoxy or vinylester resin use on carbon is a myth. He says that if carefully impregnated polyester works very well with carbon. He does sell all three types of resins, so the suspicion of trying o push the only resin he has (polyester) would be false.
    I've actually bought several types of all 3 resins with good results.

    I don't mind trying it in a small piece but at the improved mechanical abilities of non-polyester resins and at the cost of carbon it seems counter productive to save on resin.

    Other than this obvious disadvantage what other problems are there in impregnating carbon with polyester?
     
  2. marshmat
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    marshmat Senior Member

    It's a waste of carbon, for one- poly isn't strong enough to take full advantage of the stiffness of carbon. Keeping water out is another thing. A composite is only as good as its weakest link- using cheap resin on a high-tech fabric isn't much better than cheap resin with a cheap fabric.
     
  3. AVMan
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    AVMan Junior Member

    Carbon and Polyester

    The main issue with using polyester with carbon fiber is that the resin does not bond very well to the fiber. In general, resins have a hard time bonding well to carbon fibers, and to date carbon fiber manufacturers (one of which I work for) have developed surface treatments and sizings that are only compatible with epoxies or high temp resins like polyimides, BMI's, etc. Over the past several years, carbon fiber manufacturers have developed sizings that work well with vinyl esters and consequently should work with polyesters.

    The main point I should make is that carbon fiber can be used with polyester, but your application must be suitable for the composite properties. For carbon/poly composites, the best applications are those that are lightly loaded (maybe just stiffness critical) and/or in pure tension (compressive and shear properties will be comprimised when using polyester).

    One application that uses carbon and polyester is the new Corvette Z06, so it is possible to do it, it just depends on the situation.
     
  4. zerogara
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    zerogara build it and sail it

    I understand that in shear loading the medium holding the fibers in place plays a big role. But on compressive loads I thought it is meant to keep the fibers straight and along the axis of the load (mast). That shouldn't be much of a force in comparison, unless the resin is more elastic and deforms much faster than the fiber. I guess in this case epoxies are again superior.
    Are those assumptions correct?
    In real world situations it is hard to find any place of a composite structure that has one of the three types of load and doesn't have the others. It is usually a combination.
     
  5. AVMan
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    AVMan Junior Member

    The fiber-resin interface (or adhesion between the two) does play a significant role in compression strength of a carbon fiber composite. Carbon fibers will have a tendency to buckle in compression if not properly adhered to the matrix (load transfer between fibers is not high).

    As for "pure" tensile/compressive/shear loads, there are many cases were this is seen. In a sandwich composite, the outside skin can be said to be in pure tension when bending is induced, and the inside skin in pure compression. When carbon fiber is used in stringer/stiffener caps, the main laoding is in tension.
     
  6. SamSam
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    SamSam Senior Member

    Does this mean, in a sandwich hull banging into something, the outside skin is in compression while the inside is in tension.?
    Aside from epoxy adhering to the material better than polyester, isn't poly more suited to carbon fiber because the flexibility characteristecs of the material and the resin are more in line? Poly is stiff and brittle compared to epoxy.(Or vinylester) Carbon fiber is stiff and brittle compared to kevlar. Kevlar with poly doesn't work because when the flexible fabric flexes and stretches, the stiff, brittle resin breaks. Kevlar with epoxy works so well because the resin/ material matrix (?) work more or less the same. Wouldn't the opposite more or less happen with a carbon/epoxy mix, where the material wouldn't be reinforced by the resin, the material would break before the resin? By the way, I'm no expert and I didn't sleep in one of those motels last night, this is just hypothetical blathering on my part. Sam
     
  7. marshmat
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    marshmat Senior Member

    "outside" and "inside" are relative to the direction of bending, not to the object. The outside skin is the one under tension; the inside, the one in compression. Bend a piece of thin plywood in an arc and you'll see.
    Part of the basic idea of composite construction is that the component materials each have different properties appropriate to their job. With CF, it is the carbon that provides most of the stiffness; the epoxy's role is mainly to bond the fibres to each other. Using a stiffer resin that doesn't bond as well just allows the fibres to slide past each other and the material fails sooner.
     
  8. zerogara
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    zerogara build it and sail it

    Samsam you're confusing, I think, the shape, the direction of the impact, and the actual bending moment induced. A totally flat panel is very sensitive in staying flat when a load is induced. A curved panel is much more resistant. A precurved surface that has a load applied to it
    ) <---
    still locally the outside surface will compress and inside arc will be in tension, a flat
    | <--
    the same and a negatively arched as well
    ( <---

    I believe marshmat is correct a he describes it, based on the current way those materials interact. One issue that comes out of this is bonding of resin and fibers in their longitudinal axis. In other words if carbon fibers had an improved friction coeficient against the resin their properties may be improved.
    Which brings me back to the question that if the cheapest type of polyester resin deforms and shrinks the most of all resins used, doesn't that help in clinking to the fibers and keep them together? In doing so it may distort the fibers.
    The deformation rates of resin and fibers as mentioned by marshmat makes a lot of sense (same logic with concrete rebar structures, an other composite) and neither is any good separate but together they are magic.
    But wouldn't there be the perfect matching resin for each type of fiber and panel/load type? What a tube/spar does with its fibers and resin is very different than a flat panel on the deck of a boat, or a foil (rudder/board).
    The Bethwaites in Australia seem to have mastered aluminum or carbon masts with above shroud section (for gust response) made of glass fibers for their elastic properties.
    It seems there is much more to composites than what we utilize in boat building. Maybe custom made fabrics that vary in various spots can be fabricated and eliminate half or more of the fabrics used in the boat and more weight yet be much more effective. Cost will sky rocket!
     

  9. D'ARTOIS
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    D'ARTOIS Senior Member

    Carbon, just like some other fibers lose a lot of their calculated strength when becoming wet.

    All polyresins are away of being impermeable, so the obvious conclusion is ready at hand.
     
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