Chilling epoxy to slow setup

Discussion in 'Fiberglass and Composite Boat Building' started by flightofone, Dec 6, 2008.

  1. flightofone
    Joined: Jun 2007
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    flightofone New Member

    Hi, I'm trying to figure out if I can use Nomex honeycomb on a catamaran, without commercial prepreg and adhesive film, and without infusion which doesn't work well with honeycomb. I'm going to test it first on some small panels. The thought is:
    1. Prepreging the fiberglass with epoxy using a homemade impregnator, controlling the resin ratio, at room temperature, onto a roll with a plastic sheet rolled in between layers.
    2. Hand layup all the layers, including the honeycomb at near-freezing or sub-freezing temperatures. This could take a full day.
    3. Vacuum-bagging the structure.
    4. Heating it all up to achieve the cure.
    The reason for the honeycomb versus Core-cell / infusion is that I can get it very cheaply ($40 per 4'x8'x1" sheet). I'll be testing the process for good honeycomb bonding fillets. The questions I cannot find any information about are:
    A) How stiff / easy-to-layout will the prepreg be at near-freezing temps?
    B) How much does the working time increase for normal epoxy systems if they are kept near freezing?
    C) Are there any adverse strength effects due to the low temperature?
    Thanks in advance for your input.
     
  2. steele m.a.
    Joined: Dec 2007
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    steele m.a. Designer/Engineer

    As far as I've experienced , a frozen sheet of pre-preg is not workable.
    It's frozen to prevent it from curing.You can't shape it as you would a
    hand layup.
     
  3. marshmat
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    marshmat Senior Member

    Aerospace prepregs are generally not workable in their frozen state. They have to be thawed out to room temperature, or a little lower, to lay up. How your home-made prepreg would behave is hard to predict; it depends on the fibre you choose and the properties of that particular resin.
    Hard to predict without more information. You might get an extra half-hour, you might get something that never cures. My advice would be to talk to your suppliers directly, see if they have something suitable, and buy a gallon or so to run some tests before you go and order a whole pallet of resin drums that may or may not do what you want.
    Maybe. I know there has been some research done on this by Gougeon Brothers, with regards to how much strength is lost if the shop cools down during the cure. I think they did find a measurable, but not huge, loss in strength.

    I'm really not convinced that freezing regular resin will work. It'll thaw awfully quickly once it's out in the air, unless you're working in a frozen room. If you do that, you're inviting human error, because your crew will be frozen too.

    I think the way to do what you're thinking of, flightofone, is to get a fairly high-viscosity, slow-setting resin, lay up the first skin, and vac-bag the Nomex onto that. Then, once it's cured, start the second skin, using a very tightly woven, presaturated layer against the core so that the resin doesn't drip into the cells too much. Bag that layer too (even crappy honeycomb can take a few atmospheres of autoclave pressure, you can't break it with a vac bag alone). You may find it valuable to do a few flat test panels at various angles, then cut them open to see if any resin filled the cells.

    Best of luck.
     
  4. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Instead of farting around with a cold room, where viscosity issues will be a constant battle, why don't you just get a super slow resin formulated for you. I have a extra supper slow made up for me and it's not that costly. Now I have to admit the formulator is local and I've worked closely with the chemist, but it's doable and you don't have to freeze your butt off.
     
  5. Ilan Voyager
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    Ilan Voyager Senior Member

    I have worked several tons of prepeg; it's made with rather thick resins which never fully cure at room temperature. Prepegs are kept frozen for storage lasting weeks, but are used unfrozen at room temp and always cured at high temp in a oven. Good ovens of the size of a hull, with equal temperature everywhere are expensive...

    Par is right, you can get very slow hardeners, any formulator can get you one if you buy enough. A slow hardener with a post-cure moderate temperature of 60 celsius degrees (so you do not need a oven, but a simple heated tent) will work for you. A lot of commercial resins (look at the resins sold for experimental home built planes) comply with these requisites.

    Besides, it seems to me very difficult to make my own prepeg with a regular resin and a big tooling is needed. That explains the price of prepegs.
     
  6. Ilan Voyager
    Joined: May 2004
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    Ilan Voyager Senior Member

    I have worked several tons of prepeg; it's made with rather thick resins which never fully cure at room temperature. Prepegs are kept frozen for storage lasting weeks, but are used unfrozen at room temp and always cured at high temp in a oven. Good ovens of the size of a hull, with equal temperature everywhere are expensive...

    Par is right, you can get very slow hardeners, any formulator can get you one if you buy enough. A slow hardener with a post-cure moderate temperature of 60 celsius degrees (so you do not need a oven, but a simple heated tent) will work for you. A lot of commercial resins (look at the resins sold for experimental home built planes) comply with these requisites.

    Besides, it seems to me very difficult to make my own prepeg with a regular resin. That explains the price of prepegs.

    My own experience shows that most honeycombs are difficult to work in boat building, and often with disappointing results.

    Honeycombs are not equal: aluminum are heavy and soon destroyed by corrosion, common phenolic paper can go from the worst to the rather good because of sensibility to humidity and poor bonding, some are good in static loads but are destroyed by alternate dynamic loads, a lot have low shear modulus.

    The true Nomex brand (phenolic kevlar paper with hexagonal cells) costs an arm and leg by square inch.

    I have gotten good results in :
    -flat panels as it's not hard to make it correctly.
    -hulls and decks with the made in France Nidaplast (sold in the States as Nidacore), made with polypropylene, impervious to water and other stuff, very resilient, and easy to work. It's not as rigid as a Nomex but it's very strong. It cost about 100 US$ the 8*4 sheet in 1 inch at retail price. Take a look at the Nidacore internet site to see the features.

    You should examine closely the honeycomb you can get at 40 US$ sheet because if it is paper honeycomb for making panels for trucks it won't last in a boat. It must be waterproof guaranteed, not simply weatherproof.
     
  7. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    ProCore is an alternative to NidaCore and is more cost effective (cheaper). It's also a polypropylene with a scrim. Obviously you want to go light, which is understandable, but you do want to insure stiffness, durability, control costs and workability. Engineering sandwich composite panels requires a fair high amount of understanding, with the physical properties of the separate materials, plus how they'll interact bonded to each other, before you can successfully expect to generate new laminate schedules that will meet or exceed your desires/requirements.
     
  8. Ilan Voyager
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    Ilan Voyager Senior Member

    Sandwich engineering can be tricky, and honeycomb totally tricky. Requires a experienced engineer. Often the same engineer, for validating his calculations, which needed kilos of aspirin and wild guessing, discreetly go to a research facility and destroys a good number of samples at his own expense. Liability demands are expensive...

    The good data, the truly good one and the better methods of calculation are no public. Nope, you won't find it in internet, or books because or it's military classified and/or pertaining to a company who spent millions to develop the methods and softs, so you'll have to pay at high price.

    You'll only find approaches, rather empiric, for foams and low tech honeycombs in internet, books, scholar and providers manuals and these infos must be corrected or you take a 400% safety coefficient. None describes for example the destruction of the walls of a honeycomb in alternating loads with cyclic vibrations of the skins (a situation found in big multihulls, fast boats, and planes), the maths involved are of PhD level. If it was mastered, all the fighters would me be made entirely all carbon/nomex, and not in steel-aluminium like since 70 years.

    The builder must be also good, skilled and experienced. Sometimes that remembers me of a beginner buying a concert Pleyel piano and starting his piano training with Lizt or Rachmaninoff, reading the "The concert piano made easy in ten lessons".

    Worst, after spending a lot of time in experiments, brain-juice engineering, space age materials, failures and so money, often the final result in small boats is so disappointing...all this money to save a few pounds, more expensive that the same weight in 2 carats diamonds. Simply the method is not valuable in a boat with small surfaces where the skins thickness are more dictated by the local loads like a big skipper jumping on the deck or the trailering stresses which are superior to the efforts induced by sailing, so the weight savings become marginal. It's a different story on a race boat, where durability is not a concern.

    I have seen a Class C hulls prepeg carbon/Nomex heavier than those made by the Yellow Pages team in 3mm Okoume plywood in a garage...

    A lot of people underestimates totally the money expended and the technological background needed to make successfully a high tech boat. Some have been millionaire failures. Impossible to remake in a common shipyard, and it's reflected by the low opinion that have surveyors of the majority of the sandwich boats.

    But you can get excellent results with simpler technics than prepeg, and honeycombs may be interesting to stiffen flat or almost flat planels like bulkheads, topsides in multihulls with little compound curvature, decks and cockpits, that are a big part of the total surface. It can induce savings as it asks less labor than a meticulous structure of stringers, deck beams and ribs.
     
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  9. CanQua
    Joined: Jul 2008
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    CanQua Junior Member

    With that said, best of luck and I hope you can get it figured out in a method that works for you and the equipment you have. I'm not saying it can't be done. Just offering a bit of advice on my limited knowledge. If you have the money to play with and could do some small trials to figure it out, it would be valuable information indeed.
     
  10. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Small trials are fine, but of limited usefulness. The bottom line is the calculations, based on physical attributes for the anticipated loads. This is way more calculation and material testing (accurate) then the run of the mill backyard builder is capable of comprehending, let alone performing.

    Considering "Flightofone" hasn't replied to this thread and the only other thread he has posted was to copy another person's hull design, I see it as very unlikely this person will attempt or complete the project. This is in light of the questions on the other post, where he was unsure of performance degradation if the copy was off by 4 mm or so in spots. If he's this unaware of the ramifications in his "modifications" they no amount of prepeg, honeycomb or other "fancy" engineering/construction attempts will help this fellow or his yacht.
     
  11. flightofone
    Joined: Jun 2007
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    flightofone New Member

    Thanks for suggestions

    The gist of this project is that I teach a CAD/Construction/Robotics class at a high school in Massachusetts, and all of the materials have now been donated to us. Our first application of the materials is making panels for our FRC robot shipping crate. We just vacuum bagged the first side of a test panel using:
    0.090 FRP panel from Lowes
    1208H Fiberglass, 2 layers
    BMS8-124P CL4 TY5 GR3 0.75" Aramid honeycomb
    I'll post some pictures after we add two more 1208H layers and do some strength testing.

    I'd like to then build a small dingy using the same sandwich, around an internal frame. The FRP should allow us to make it "mold-less", using the internal frame, FRP, and supplemental temporary internal framing to withstand the vacuum bagging pressure.

    We use Solidworks to design all our structures (and the robot - see www.colonelrobotics.com ). Assuming we can make this all work, I'd like to then take on a bigger catamaran design - which is where we'll need a very slow resin setup time to get it all put together and vacuum bagged.

    Thanks
     

  12. steele m.a.
    Joined: Dec 2007
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    Location: Victoria B.C. Canada

    steele m.a. Designer/Engineer

    Noble robotics

    Noble mission.I'm an aeroengineer , it's nice to see profs giving hands-on
    experience to their students.FRP is it's own industry , with very
    unique problems because of the nature of industrial epoxies/vinylesters ,
    polyesters , etc.
    Home builders and experimenters often discover that , instead of fast
    production times and inexhaustable construction shapes , what
    happens is unexpected curing times , leading to unexpected results.
    Like concrete molding , FRP has a time window when all of your
    preparation either leads to glory or failure.
    Cure times for resins and vinylesters are not designed around working
    times.A slow curing epoxy isn't designed to have a larger window of
    opportunity to work it.If you have a volume to fill , like with a medium
    sized project , what your after is viscosity , not longer curing times.
    When a shop lays up a FRP hull , from resin mix to set-up , as far as
    'working time' is 15 to 20 minutes.
    Your resin has to have been introduced to where it's supposed to be
    during that time - there isn't a choice.
    Although pre-pregs are cold , they're cold to hold off the chemical
    reaction that makes them set.
    To fill a vacuum bagged mold , you need an unusually high viscosity
    resin , and these set differently than the regular stuff.
    They also generate heat and gasses when they set , you must prepare for that.
    As your resin hardens , it may eat through your structures , it also
    might eat through the vacuum bags.
     
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