cross/uncross linked

Discussion in 'Materials' started by Matthieu, Feb 26, 2012.

  1. Matthieu
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    Matthieu New Member

    Hi

    Does anyone know what is the structural difference between cross linked and uncross linked PVC foam ? Is it just the molecular bonding which changes ? and what are the issues of using a cross linked foam instead of an uncross linked, or the contrary ?

    Thanks
     
  2. Herman
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    Herman Senior Member

    cross linked gives more stiffness, lineair gives more flexibility, and impact resistance.

    To gain approx the same values for stiffness, Core-Cell sells their 95 kg/m3 foam as "M80" which suggest an 80 kg foam.

    Check the datasheets of:

    Airex C70.75
    Divinycell H80
    Core-Cell M series

    to learn more.
     
  3. Matthieu
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    Matthieu New Member

    yep thanks for the answer

    However, my question was more from a chemical point of view : how they are made, polymer bonding and all that stuff...and does it changes something in the resin they can absorb during an infusion process for example, or in their moisture absorption after laminated ect...
     
  4. Eric Sponberg
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    Eric Sponberg Senior Member

    Matthieu,

    I am not a chemist, but I will give you some background from what I know. Foams are made in a baking and rising process much like baking bread. Large "buns" of foam are mixed chemically as liquids, and then they foam, rise, and cure. These buns are then cooled, removed from their containers, and cut into sheets and shapes, and the sheets are later processed with kerfs, bleed holes, etc.

    The earliest foams were polyurethane and cross-linked PVC. Cross-linked PVC has much better mechanical properties than polyurethane. Both are available in different densities, and strength and stiffness are directly proportional to density--the higher the density, the more strength and stiffness. Also, the higher the cost. One concern with cross-linked PVC foams is that they can shatter if hit by a hard object, like a boat going against a dock or a reef, or another boat. Polyurethane foams are particularly bad in this regard.

    Then came Airex, a linear PVC (not cross-linked). It's advantage is that it will not shatter under impact. It's drawbacks are that it is available in only a few densities, and in hot weather it can deform under stress. For example, if a dark colored boat is sitting in boat stands on a hot day, and the stands are not located in way of bulkheads but rather between bulkheads, the hull can sag and deform around the stand pads under the high heat of the day. The same is true for dark colored decks on a hot day--Airex can deform slightly.

    Then later came SAN foam (styrene acrylionitrile), i.e. Core-Cell. It is kind of the best of both worlds--it comes in various densities, doesn't shatter, and doesn't deform. The process is much the same--chemicals mixed, poured into pans, allow to rise into buns, and processed accordingly.

    Those are the basic features in a nutshell. Most foams used in boatbuilding are in the 4 to 6 pound per cubuc foot density, and this gives adequate shear strength and stiffness, their primary desired quality. Higher density foams are used where hardware may be attached, where the bolting compression strength demands a denser foam. Balsa core, by comparison, is usually in the 9 to 10 pound per cubic foot density, and overall is much stronger and stiffer as a result, and also is much cheaper. Some hand select 6 pound per cubic foot balsa is available, but is more expensive because it takes time and labor to select and process it uniformly.

    All cores have to be laid up properly so that water does not get into the core. Almost all water ingress into core comes from poor lay-procedures, or because of cutting through the laminate and core to install fittings, and not properly sealing the core and laminate afterwards.

    That's Core 101. I hope that helps.

    Eric
     
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  5. Matthieu
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    Matthieu New Member

    That helps great ! thanks a lot

    Matt
     

  6. Herman
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    Herman Senior Member

    In the 102 page I like to write that Airex does not only produce lineair foam, but also cross linked foam and a whole range of other foams, which can be of use to the boating industry. (their T90 foam is great below fittings)

    On the same page please also that ATC Core-Cell made A foam at first, which still had a relatively low heat distortion temperature. During the life cycle they ended up with M-foam nowadays, which indeed has best of both worlds, but at a financial premium over PVC foam.
     
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