The Montauk Catamaran Company Chronicles 12/02/14: Epoxy Glue

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David Ryan

David Ryan is a boat builder and USCG licensed master captain. He is the owner of Sailing Montauk and skipper of Montauk''s charter sailing catamaran MON TIKI You can follow him on Twitter @CaptDavidRyan

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13 Responses

  1. When the surfaces are mated with glue, the glue and primed surfaces will bond at a chemical level.

    Is there a timing issue here? My understanding is that once the first layer is fully cured, a second layer of epoxy will provide only a mechanical bond. Of course, some epoxies can be handled pretty roughly before they’re fully cured.Report

    • This is a really good question, and one that I should have addressed in the post.

      The answer is yes, once the epoxy is fully cured you can only get mechanical bonding, which is plenty, but to get that you need to rough up the surface with sand paper.

      But, and this is a big but…

      Our formulator tells us that epoxy remains “chemically open” long after it’s dry to the touch, which means you can prime one day and glue the next and still get chemical bonding, which is even better than mechanical bonding. (Epoxy has super tenacious mechanical bonding, which is what makes it such a good boat-building material.)

      We’re usually working wet on wet on wet, but sometimes things don’t’ go as quickly as you thought they would. Fortunately (and especially at the temperatures we’re working at) you can come back to it the next day and still get chemical bonding.Report

  2. Avatar Brian says:

    David,

    Just found your blog…Add to your song list: One Particular Harbor – Buffet. Very happy to see the sort of tips you plan to post as they apply to all sorts of Wharrams.Report

  3. Avatar Jim Heffman says:

    “Diatomaceous”, I think you mean. Because it’s got diatoms in it, is what they mean.

    ****

    One other reason to add fillers is to ensure a minimum bondline–as you point out, glue doesn’t glue if there’s no glue. Beads of a calibrated size are often added (the idea being that if you fill the glue with beads that are .020″ diameter, and then you clamp the hell out of the joint so that it squeezes down as far as possible, then you’ll have exactly .020″ bondline.)

    It’s interesting that you find that thicker bondlines are better; in general, having a thick bondline is a problem because it allows the adhesive material to deform under load, and you get rotation of the load action line which causes the bondline to be loaded in peel rather than shear. Think of it as though the bondline were a piece of rubber–as you start to slide one piece along the other, the rubber will turn from a rectangle into a parallelogram, and the ends will start “pulling” away from the piece rather than “sliding” along it. Adding the fiber fill helps with this because you’re now making millions of tiny little bond surfaces all through the adhesive material, rather than just one big ol’ chunk of glue.

    Bonds are a lot stronger (and, often, stiffer) when loaded in shear rather than peel. This is why your Velcro-strap shoes stay tight even though you can just rip the strap off with your fingers. It’s also why carpenters use biscuits, dadoes, or dovetail joints to put things together instead of just butting an edge against a face; more effective length of “shear” joint rather than “peel” joint.Report

    • I’ve heard of using calibrated beads when using epoxy to fasten machined parts, but .020 is a way way finer tolerance than even master cabinet-maker is going to be able to achieve when working with wood at a boat-scale. And yeah, “diatomaceous”. What did I write? 🙂Report

      • Avatar Jim Heffman in reply to David Ryan says:

        One of the issues I often find with coworkers is trying to convince them that accepting wider tolerances and planning to mitigate them is cheaper than putting “profile +/-.005” on the drawing because it’s “standard machined part tolerance” and then having to write up a defect report for the inevitable out-of-range conditions.Report

      • Avatar Mad Rocket Scientist in reply to David Ryan says:

        That is the curse of engineers & designers who haven’t spent enough time in a machine shop.Report

    • Avatar Mad Rocket Scientist in reply to Jim Heffman says:

      Oh good, the other engineer mentioned this.

      Epoxies are really cool polymers and in my opinion, one of the coolest things we clever monkeys have ever come up with that 99% of the population just shrugs about (right up there with teflon & carbon fiber).

      When I get around to building my WIG, I suspect I’ll be needing to talk to you about suppliers, I don’t think Home Depot or Lowe’s is gonna cut it.Report