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Thread: Mechanical Properties of Cured Slurry, Wet Paste and Dry Paste

  1. #1

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    Mechanical Properties of Cured Slurry, Wet Paste and Dry Paste

    I need the mechanical properties of post-cured slurry, wet paste and dry paste. The system I have in mind is MGS epoxy resin (certainly with the hardener) and 3M glass bubble. I can find the cured MGS resin mechanical properties from resin specification, but how about after mixing with glass bubble? Is there a datasheet or a formula (for example, based on the mixture ratio of resin and bubble)? Any pointer would be highly appreciated! I can't do experiments at this stage.

  2. #2

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    Glass balloons do not make a structural filler. Chopped cotton flox or silica make a structural filler. The tone of your post reads like you are in trouble. The good news is that if you used the wrong stuff you can grind it away and redo with good stuff.

    Best of luck,

    Wes
    N78PS

  3. #3

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    Thanks for reply, that is an important point. I have not started anything yet, so I am not in touble. However, I am equally desperate in getting the necessary mechanical properties that would allow me to design a composite airplane. So how can I get the mechanical properties of cotton flox/resin or silica/resin pastes?

  4. #4

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    There are differences between a laminating resin system and a structural paste adhesive; each used for a specific purpose. A laminating resin(epoxy) has low viscosity to allow saturation into fiber reinforcements. A structural adhesive has high viscosity to maintain bondline thickness in a joint and additives to provide high shear and peel strength. You normally use each as intended and don't try to make one do the job of the other. You can take the laminating resin and add fillers (balloons, chopped fibers) to increase viscosity but not for structural bonding. This combination is used for making a fillet or smoothing a surface.
    One of the best structural paste adhesives is Hysol 9640 because it is thixotropic and has high peel and good shear strength. This is used for bonding hard details intended for shear loading. The key to a strong bond is surface preparation and bondline thickness control.
    So don't mix up material systems and expect to make a better product.

  5. #5

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    Quote Originally Posted by Bob H View Post
    There are differences between a laminating resin system and a structural paste adhesive; each used for a specific purpose. A laminating resin(epoxy) has low viscosity to allow saturation into fiber reinforcements. A structural adhesive has high viscosity to maintain bondline thickness in a joint and additives to provide high shear and peel strength. You normally use each as intended and don't try to make one do the job of the other. You can take the laminating resin and add fillers (balloons, chopped fibers) to increase viscosity but not for structural bonding.
    Bob, thanks for your post. However, I have seen epoxy resin (used for laminating carbon and glass fabrics) mixed with cotton flock in bonding a left half fuselage with a right half fuselage in mass production of a certified composite GA airplane. The addition of cotton flock makes the mixture much thicker so the bondline thickness can be maintained.
    Last edited by wantobe; 05-07-2012 at 10:16 AM.

  6. #6

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    According to Burt Rutan, if you read his book on building composite airplanes and then build a sample, a mix of epoxy and cotton flox or carbo-sil is good enough for use in structural joints for at least generalo aviation airplanes. Are their newer mixes with even better physical properties? Absolutely! But Aeropoxy or West System can be mixed up with structural filler to do a good enough job.

    Getting back to the original question, may I suggest mixing up some of your candidate epoxy and cotton flox into a a small cube and seeing how many sandbags you can pile on it. Crude by effective. SO the same with epoxy and another structural filler. You can also mix some up, use it to to join two blocks of say maple together with a carefully measured joint thickness, then pull them apart with your sand bags rigged up to some pulleys. This exercise will give you some rough numbers that you can derate to be conservative.

    Generally though, if you have enough overlap on the joints, the glue is not the weak link. And you can make up some test joints and pull them apart to prove that you have enough bonding surface and a good enoiugh bond that the surrounding structure fails first.

    So what are you building?

    Best of luck,

    Wes
    N78PS

  7. #7

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    Typical shear strength of a paste adhesive is 3500 psi with around 40 lbs/in width (piw) peel to provide a combination of required properties. You can take a laminating resin and force it to be an adhesive with fillers but won't achieve the same beneficial values. In the non-met business, you would make lap shear coupons and pull them to obtain a shear strength and do a climbing drum peel test to establish that value. If you do such tests, you will find that a formulated structural paste adhesive has higher properties.

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