flax composite stitch & glue

Discussion in 'Materials' started by magwas, Jul 6, 2010.

  1. magwas
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    magwas Senior Member

    I like stitch&glue except the scarfing part, and interested in flax composites. So the idea came in mixing the two together.

    Has anyone did something similar to this? What are your experiences? What do you think?

    I plan to use some ordinary flax textile. I will make flat panels by simply wetting the textile with epoxy. If I feel it is still too flexible, use another layer. When I reached the flexibility similar to 4mm plywood (actually I guess something less would be enough), I use stitch&glue to build a kayak. I might do some experiment here to strategically cut the material in order to build non-developable shape.
    After I reached the needed shape, I continue laminating until I reach "production strength".

    My bets:

    - I can get away without scarfing.
    - Hopefully I will get panels which can be more easily bent&cut than plywood, so more shapes are possible.
    - It will be weaker than glass composite, maybe weaker than plywood, so I will end up with greater wall thickness.
    - Vacuum bagging may be necessary, especially when I laminate more layers in one run. At the first experiment I will try to go away without it.
    - With carefully selected and prepared textile I could end up with half as strong material as glass, but I doubt I can easily obtain ideal textile (Alkali treated european flax, 50 twist/meter, densely woven. Wet retted is even out of question.), so I will just select the one looking strongest of the easily obtainable ones.
    - I might or might not end up with a cheaper boat than a plywood stitch&glue. It is not a consideration for me.
    - I would not do it with polyesther.
    - I guess bigger boats would be possible with this technique (provided it actually works), just the base panels should be made with more layers.
     
  2. Herman
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    Herman Senior Member

    All natural fibers I have seen so far are not like a woven glass fabric, but more like a needled mat, or a unifilo, and are more suitable for press moulding of some sort, then laminating by hand.
     
  3. magwas
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    magwas Senior Member

    What I have seen so far, is that natural fiber in composites is mostly used as a replacement for wood filler, using from some agricultural byproduct like short cell american flax. These are mostly not for structural elements (your BMW's door panel for example), and often use a thermoplastic matrix.

    But I do encounter natural fibers not unlike woven glass fabric. For example the clothes I am wearing right now are made of such material:)
    I am interested in using old school textile as a structural material.
     
  4. lewisboats
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    lewisboats Obsessed Member

    You are going to use a fortune in epoxy to do this and it will be heavier than plywood. Shipping wt on my last order of epoxy was over 20 lbs...so around 19 lbs for 1.5 gal of epoxy. That would probably be enough to do one half to 3/4 of a sheet worth of 4mm laminate. A full sheet of 4 mm occume runs what? 16 lbs. Scarfing is not hard...and you are in for a butt load more work trying to get your panels consistently even and voidless. Not even going into the natural fiber thing...the best natural fiber you in can use in hull building is obtained from a tree...in the form of wood.
     
  5. magwas
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    magwas Senior Member

    You are most probably right. I will try it anyway.
    If it would be easy to get hand on textile specifically made for the application, that would mean approx. twice the wall thickness of glass. Which means twice as much epoxy as with glass. Epoxy being four times as expensive as polyesther, it means eight times the price for the matrix. I have no idea about price comparison for the textile vs woven glass, as I do not know source for the textile.

    On the other hand for my current project (4.2m kayak) I could obtain plywood for the price of 10kg epoxy and I guess I will use some 3 kg epoxy until I finish. Neglecting the price of textile, but calculating with 40% textile content I end up with a 20kg textile composite boat for approx the same price ((10+3)/0.6). I guess a 20kg kayak at 4.2m would be a rather stiff one if made from glass, and it might be good enough if made from textile.

    Well, my scarfs are scary, and one does not do DIY boatbuilding to minimise price, so I will try this textile composite thing using off-the shelf textile anyway. I will report back.
    Remember, failed experiments enhance knowledge, too:)
     
  6. gonzo
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    gonzo Senior Member

    You will end up with a heavy and weak structure. The glass fibers are what give fiberglass its strength. The resin mainly bonds them together. Wood panels are much stiffer by weight than fiberglass. On small boats, stiffness is the one of the main design parameters. Scarfs are easy to make. Also, to make a flat panel is not a easy as it seems. The difference in temperature when it cures will warp it, unless you cool down the base you are laminating over.
     
  7. Herman
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    Herman Senior Member

    Gonzo, you will be surprised how strong natural fibers are. Here in Holland they are now experimenting with hemp (no pun, please...) which is exceeding the strength of glass.

    Also, in the polyester industry, thickness is what brings stiffness, and for many products a natural fiber, with an open structure, suitable for RTM(light), giving plenty of thickness, could be ideal. I am not claiming it is a perfect material for boats (I am not denying that either) but to just put it in the bin because it is natural fiber, is not the right way to go either.
     
  8. gonzo
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    gonzo Senior Member

    How do you compare strenght? By weight, size...
     
  9. magwas
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    magwas Senior Member

    Strength have well established definitions, see http://en.wikipedia.org/wiki/Strength_of_materials All kind of strengths are measured in (giga)Pascals (at least in this side of the pond). I guess we most frequently use yielding tensile strength, which describes how much pulling force a material with a given cross section area will withstand without irreversible deformation.
     
  10. magwas
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    magwas Senior Member

    I have done some preliminary tests and calculations. Nothing serious yet, just laminated 7 various flax textile specimens at 5 layers, and measured their width and weight. I am yet to make dogbones out of them and find someone to test them.

    I did the lamination by hand, no vacuum bagging. They contain somewhere around 30% of flax (24-36). The width is 1.8 mm for 125 g/m2, and 3.2 mm for 320 g/m2. The divident of width and textile weight seem to be linear.

    The 320g/m2 one seems to be a reasoneable material for boat building. It is less stiff than 4mm okoume plywood, but seems to be stiff enough. According to literature its tensile strength should be greater than that of 4mm okoume.

    I am just finishing a 4.2m kayak. It is a big one (nearly 600 kg displacement at flood draft), and I did some calculations with respect to weight and price using that design as a baseline.
    Using the 320g/m2 textile, the kayak would weight some 6 kg. (Baseline weights 16 kg.)
    The price of the textile would be slightly higher than the price of the plywood. I did calculate with all the plywood I used (nearly twice of the area of the strakes), but with textile I used the area of the strakes. So your mileage might be as much as twice the plywood price, however I think that it is easier to be economic with textile five times the area than with plywood once the area.
    I did use nearly 5 kgs of epoxy for the plywood kayak, the calculation for the flax one is a bit less than 4kg. Again, I am comparing actual usage with calculation. Well, vacuum bagging would mean even less epoxy (I won't do that I guess), maybe with a less stiff hull. However the bulk of epoxy used for the plywood one is went to coating. I guess coating would be a good idea for the flax one as well, but not 3 thick layers, only one thin one.

    Bottom line: using flax composite seems to be a reasoneable choice from the mechanical point of view, gives slightly or considerably higher price than plywood stitch&glue.

    I expect to be able to use some combination of origami and lamination (first one or two layers laminated flat, "sewn" to shape much like steel origami, and the following layers are laminated on top of that "mold". I hope this would be much faster than plywood stitch&glue, so I will try it.

    I do not know much about glass fiber to make a reasoneable price comparison. How much layers would one use of what kind of glass to make this kind of kayak?

    I have found a manufacturer who is willing to create a textile specifically designed for composite application. They have some difficulties because it is hard to wove low-spun threads tightly, but they have promised a specimen for the next week. I am hardly awaiting it.
     
  11. uncookedlentil
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    uncookedlentil Junior Member

    could you post some pictures of your work?
     
  12. magwas
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    magwas Senior Member

    I don't know exactly do you mean the specimens or the boat?

    I will make pictures of the specimen and post them here, and make a separate thread for the boat with plans and pictures.

    In the meantime here is my spreadsheet with my calculations in pdf (because the forum disables upload of spreadsheet in standard odf format).
     

    Attached Files:

  13. Steve W
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    Steve W Senior Member

    Magwas,if your main concern with using plywood is scarfing the panels together,dont worry,you dont need to,there is a simple method of achieving the same result,ie,a joint equally as strong as the rest of the panel that will bend in a fair curve.All you need to do is butt the 2 panels to be joined together with a layer of glass tape/epoxy over the join on each side.This method works well with the thin 3-4mm ply typically used in stitch and tape construction of small boats.
    Steve.
     
  14. gonzo
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    gonzo Senior Member

    Tensile strength is not the main concern in a small boat but stiffnes is. Wikipedia is not a really reliable source of engineering data.
     

  15. uncookedlentil
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    uncookedlentil Junior Member

    magwas, I was mainly interested in pictures of your design, just pm me when you start your build thread, please.

    Native people here in north america have been using bark or animal skins over frame work for thousands of years. the same style of construction was used by the Basques and the island Celts. Dyson did an updated kayak with aluminum poles and hi-tech fabric that worked very well, but i'd love to see a more planet friendly approach tried.

    the main problem with these boats is that they're not useful for transporting social/environmental plunder:)
     
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