Fillet putty increasing viscosity after applying.

Discussion in 'Fiberglass and Composite Boat Building' started by Jetboy, Aug 1, 2012.

  1. Jetboy
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    Jetboy Senior Member

    I'm using epoxy with fumed silica to fillet joints in the boat I'm building. I'm struggling with it because I can mix it as thick as I can stir it, then after I apply it to the fillet joint, it kicks and heats up a lot and ends up losing it's structure and slumps down or leaks out of the joints. Any great ideas on how to avoid this? Should I add another thickening agent along with the fumed silica? It's a PITA because I then end up having to sand off the runs and also do every joint twice to fill in where the fillet has slumped in.
     
  2. Herman
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    Herman Senior Member

    Add even more. My experience with silica is that it is a filler which does ot decrease viscosity as much as for instance microballoons.
    You really need to go very thick, for the fillet to stand up. Another factor can be the mixing cup. Deep cups have a tendency to not let the silica go down. So the top part of the epoxy is thick enough, but the bottom part is still too liquid. Different mixing technique solves this. (get resin up, and work the putty down.)

    As a last resort you can add some fibers (cotton or even glass) to the mixture. But the surface will be slightly courser.
     
  3. Doug Lord
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    Doug Lord Flight Ready

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    In my experience fumed silica is a ***** to use with epoxy because of the heat it generates. You might consider this stuff-I've used it as a core on molds and it works very well. It warms up but doesn't get hot or melt.
    http://www.freemansupply.com/SpecialtyToolingPa.htm
     
  4. PAR
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    PAR Yacht Designer/Builder

    It's easier if you approach your mixtures with a few things in mind: do you need a bulking agent - do you also need to control viscosity - mechanical properties adjustments? Each of these will require different materials.

    Balloons and other materials are bulking agents, designed to take up space in the mixture, but not dramatically affect viscosity. They will make things thicker, but it'll still sag. Other materials like wood flour will do a little of both, while silica is really used just to control viscosity.

    Most mixtures need a little silica and a bunch of something else. Silica is used to control viscosity, to prevent sagging. Coupled with a bulking (or other) agent, you can get the physical attributes you want, for the task at hand.

    Lastly, when making highly thickened mixtures, you can't do this in a cup and expect to have good success, the exotherm will kick your butt. As soon as you have the mixture modestly thick, get it out of the cup and spread it into a thin, flat sheet on a mixing board or big mixing tray. This well prevent the epoxy from kicking off on you so quickly, from the exothermic reaction. It's also a lot easier to mix epoxy as a flat sheet of goo, then attempting to stir a thickened mixture in a cup.
     
  5. gonzo
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    gonzo Senior Member

    How long do you take to mix and use? I would say that the putty should be on the fillet no more than 15 minutes after you started mixing.
     
  6. Jetboy
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    Jetboy Senior Member

    It's on the fillet within 1-2 minutes max. As soon as it's mixed, it's being applied. I mix very small portions or it hardens before I can get it on the boat. THere's a period shortly before it solidifies that it heats rapidly and becomes runny. It sounds like I need to add a bulking agent rather than just pure fumed silica to thicken. I will also try chilling the epoxy liquid. Right now I'm doing it in roughly 95* F temperature.
     
  7. PAR
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    PAR Yacht Designer/Builder

    In 95 degree heat, you'll need an extra slow hardener, and even these will not give you a lot of time, unless it's a special formulation. West 207 will give you a 20 - 30 minute window in these temperatures, with other slow hardeners being half this, generally.

    Again, you can't mix epoxy in a thick mass and not expect to kick off rapidly. It has to be spread out, so it can vent the heat through the increased surface area.
     
  8. Tungsten
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    Tungsten Senior Member

    i've been mixing mine with cellulose fiber till it won't drip any more then some cabosil to firm it up.the cabosil really helps it from sagging.i let my fillet kick for 2 hours and no sagging,i apply FG tape over the fillet after the 2 hours.
     
  9. missinginaction
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    missinginaction Senior Member

    Sure has been a hot summer. Besides what has been mentioned here you might try this.

    If it is a hot day make sure that you keep your resin and hardener cool before you start mixing. You can stick it in a refrigerator or keep it on ice in an old cooler. Have all of your materials ready to go before you start mixing things up. Spreaders, small brushes, acetone and paper towels, gloves, everything you need should be ready to go. I also have had good luck masking off the joints that need fillets. This saves me a lot of time when making the fillets as I don't waste time cleaning resin from adjacent areas.

    Work in the early morning if you can. I was actually gluing up a flybridge panel at 7:00 this morning. Although we reached almost 90 degrees in the afternoon it was a cool 60 degrees when I was working and even fast hardener gave me plenty of time. I was much more comfortable too.

    You're probably doing a lot of what I mentioned already. Hope some of this helps you out and good building to you!

    MIA
     
  10. Herman
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    Herman Senior Member

    Hot summer, huh? Over here we had a nice April, since then we had a lousy summer. Just like last year. We are already joking that April is the summer, and may-november is autumn.

    Anyhow, it looks PAR is right on the resin: preferably get something slower. This will also lower the heat generated, and a bit of working time would be nice as well.

    To add to MIA above: Take an aluminium pipe, diameter the same as your desired fillet. Run this pipe through the corner. It will leave 2 black stripes. Now tape along the stripes, apply bog, and use the pipe again to smooth the bog. Any residue will end up on the tape, and can be cleaned up with a spreader or putty knife. Remove the tape directly afterwards. Presto, a clean fillet!
     
  11. PAR
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    PAR Yacht Designer/Builder

    Instead of pipe use a ball of the appropriate diameter. A ball will always make the same size fillet, regardless of it's orientation, while a pipe can change diameter, just because the angle you've dragged it at has changed.
     
  12. Herman
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    Herman Senior Member

    Stainless balls will not make the nice black lines to tape along. For the rest they work OK.
     

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  13. missinginaction
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    missinginaction Senior Member

    I'm going to try that pipe technique Herman. PAR. I see your point as well, but where do I get those balls of steel, or should I go for brass? :p

    I was thinking that with Herman's idea perhaps I could attach (glue) a wedge of plywood onto the pipe as a guide to keep the angle constant as I dragged the pipe down the joint. For instance, just cut a 45 degree right triangle.

    Thanks for the tip guys,
     
  14. Herman
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    Herman Senior Member

    Do not make it too difficult for yourself. Just run a pipe trhough the putty, and you are done. Just eyeball the angle, it is really hard to do it wrong.

    For these balls look for "filleting balls". They are mainly used with wax fillets. (prototyping)
     

  15. Jetboy
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    Jetboy Senior Member

    Thanks for all the tips. 90* is pretty common here. Actually 100* is kinda the normal summer high temps. The slow cure hardner gives me a good working time until I add the fumed silica. It must act as a catalyst because it speeds up the reaction a lot.
     
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