How to increase adhesion?

Discussion in 'Materials' started by Blueknarr, May 22, 2018.

  1. Blueknarr
    Joined: Aug 2017
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    Blueknarr Senior Member

    The motorcycle shop across the way occasionally sends me projects.

    1527031954516511766324.jpg

    This is from an 05 something or other.
    The original repair used epoxy fairing. I don't know the type of plastic which forms the base, it is deaply scratched for adhesion.

    I plan to epoxy (west) FG cloth (bi-ax)
    cross joint then fair.

    How can I get my repair to adhere better than the previous one?
     
  2. fallguy
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    fallguy Senior Member

    Wire brush.

    Use 150psi air and see if you can part any of what is still there first.

    Then wire brush on an electric drill on high.

    It won't get much better on plastic.

    Personally, it amazes me noone has found a glazing/compound that would soften the plastic, then allow a chemical bond to epoxy when the plastic hardens back up, but afaik there isn't.
     
  3. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    I can't understand what's going on in this photo.
    Is the photo sideways?
    Is this the result of the failed repair?
    What is the scale in the photo?
    Is it a m/c cowling?
     
  4. BlueBell
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    BlueBell Ahhhhh...

    If it's plastic then forget it.
    In my experience, if you don't know what kind of "plastic" it is, it'll never stick.
     
  5. upchurchmr
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    upchurchmr Senior Member

    One thing to attempt is to run a propane torch over the "plastic".
    Not until it melts, just to heat it up.
    There is supposed to be a chemical change that promotes adhesion.
    Probably depends upon the plastic.

    I have had real success with using JB weld on plastics I couldn't get to stick.
    You might try a very thin layer - sort of like a primer coat.

    No gurantees, sorry.
     
  6. Blueknarr
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    Blueknarr Senior Member


    I'm not mc type, part may be out of proper orentation.
    Someone added speakerbox to cowling. Main cowling harh extension soft. Fairing up to 3\4 inch thick
     
  7. BlueBell
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    BlueBell Ahhhhh...

    Still not clear.
    "Main cowling harh extension soft" ???
    Scale?


    Remove modification would be all I can suggest with the limited info given.
    Try a m/c forum.
     
  8. PAR
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    PAR Yacht Designer/Builder

    The first thing you need to do is ease off those dramatic direction changes, by feathering all the edges, at least 8:1, with 12:1 being ideal. This will provide an area for the old work to blend and bond to the new work.

    The resin type isn't critical, though West System is the most costly and many formulators offer goo for 1/2 or less their price. Something to consider if you use enough epoxy.

    To get a good stick to the underlying plastic, you'll need to "caramelize" the surface, which as previously described, is waving a torch over the area. A quick waving is all that's necessary. This will break down the partly oxidized polymer chains and make the surface much better to bond to. Also previously mentioned a good tooth is necessary for epoxy to bond well to some plastics. 40 grit is what I use.
     
  9. Blueknarr
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    Blueknarr Senior Member

    Thanks for the responses.

    Motorcycle shop wanted some apparent rock chips filled and painted. When I tried to flick off loose material around one of the chips, all hell broke out.

    Appears that a soft (possibly vinyl) speaker box inserted into hard (ab) cowling. Plastics were abundantly scuffed by wire wheel, as that the 36 grit I used to feather edges actually smoothed the soft plastic. There was a void between soft plastic and epoxy filler. Filler did not chip off of hard. Filler thickness over hard was less than 1/8 inch. Filler up to 3/4 inch on soft. Fail point at hard - soft intersection 1/8 thickness.
     
  10. fallguy
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    fallguy Senior Member

    Another thing is to avoid going too heavy on additives and lean for a wetter mix vs drier at the plastic. I realize this is not easy, but by running a fast batch low, you can add another thicker batch above it and the wetter epoxy will work better to bond while the thicker stuff can be used to get fairer and bond well to the hot primary.

    Running a drier coating oftentimes reduces the adhesion; though I may be patronizing.
     
  11. fallguy
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    fallguy Senior Member

    Not fully following the soft hard thing, but you might consider some glass if the soft area is failing. You'd go hard-soft-hard with the glass if I understand at all here.

    That way in order for it to fail; the glass would need to flex.
     
  12. PAR
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    PAR Yacht Designer/Builder

    Bond area and transition slope are the keys to good bonds, assuming appropriate materials and prep. There has to be a significant transition from old to new, with most goo formulators suggesting a 12:1 slope in transition, though I've used 8:1 on low load and vibration repairs. Just scuffing it up with a disk on a DA or a wire wheel isn't going to get this done. If you have areas that need a deep filler coat, do so in stages or add material so the filler isn't 3/4" thick.
     
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  13. Blueknarr
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    Blueknarr Senior Member

    You're not patronizing. I'm hoping against odds that someone will mention something I either forgot or didn't know. The perverbial magical bullet. So bring on all ideas.

    My current plan is to:
    Scratch and flashfry
    Epoxy bi- axial bridge stapled to soft plastic.
    Epoxy fairing fortified with fibers

    Just surprised by the epoxy adhesion fail over appearantly well prepared substrate.
     
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  14. rxcomposite
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    rxcomposite Senior Member

    The problem is the substrate itself. Most of these car/motorcycle parts are made of ABS plastic. It is a terpolymer plastic + rubber. It is tough and flexible. It is very hard for epoxy to bond into rubber so the bond area must be increased as suggested by PAR. Second would be the flexibility of the patched part. Try CSM and high stretch epoxy like Silvertip which has an elongation ratio of 8%.
     
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  15. PAR
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    PAR Yacht Designer/Builder

    A place for G-Flex resin should be added, as it's elongation is likely what you'd need.
     
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