Laminating 50%Glass/50%Aramid Cloth with Polyester Resin on a Foam Core

Discussion in 'Fiberglass and Composite Boat Building' started by Paulo, Apr 17, 2007.

  1. Paulo
    Joined: Apr 2003
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    Paulo Junior Member

    Does any one here has practical experience laminating 50% glass - 50% aramid cloth with polyester resin?

    The objective is to build a one-off fast fishing boat using a MDF frame to give the foam core the shape of the hull, laminating outside first, turning the boat upside up, removing the whole MDF and laminating inside, so that the shell can then be stiffened and the boat completed. I believe so far there are no surprises.

    For economical reasons, I would like to use polyester resin on the whole boat. I understand it is possible to laminate on foam core, as long as some precautions are taken: impregnate the surface of the foam core with resin and allow it to cure before starting the actual laminating of the fiberglass skin. So far so good.

    Next, I thought about using fiberglass bi-axial cloth with the usual fiberglass matt for ensured adhesion between layers. The great advantage I am after here is weight reduction of the finished hull.

    One step further on this line of thought would be to substitute some of the glass fibers in the bi-axial cloth by some aramid fibers. In my view, the bi-axial 50/50 cloth is very interesting and I am able to buy it locally made, which is a even bigger 'plus'.

    I understand that Aramid fibers are meant to be used with epoxy resin. My hope is that the 50% glass mixed with the 50% aramid will aid solving the adhesion problems the aramid-polyester combination presents.

    I also understand that Aramid is much better at tension then compression. Therefore, my intention is to use the 50/50 cloth only on the outside skin of my sandwich laminate, next to the foam core and with one or maybe two layers of 100% glass cloth on the outside.

    Finally we arrive at the question posed at the beginning of this post:
    "Does any one here has practical experience laminating 50% glass - 50% aramid cloths with polyester resin?"

    I would be very greatfull, if some one could shine some light into this problem.

    Thank you (for the pasions of reading my post :) ),
    Paulo
     
  2. frosh
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    frosh Senior Member

    Hi Paulo, I would be surprised to see any responders to your post say that they have used fibreglass-aramid hybrid cloth with polyester resin for boat construction.
    It seems strange that you might outlay considerable money for the reinforcement, then save money by using polyester resin which has many inferior properties to epoxy. I believe also that you are right that adhesion and wetting out for kevlar using polyester is suspect. What about using vinylester resin with biaxial or triaxial glass, to get a good result for cheaper than an epoxy laminate. I don't think that you need kevlar unless the hull is going to be contacting rocks or other hazards. Using foam sandwich construction even with woven fibreglass only is already going to give a huge weight saving over an equivalent strength single skin glass boat, as found in most commercial hull constructions for fishing boats. If you really want to use kevlar hybrid cloth then only vinylester or epoxy is recommended. The tensile strength of kevlar is much more valuable on the inside skin however to resist deformation inwards. But I say once again, it is not necessary.
    What is the size of the boat? Which foam, type, density, thickness do you want to use?
    I would think again about the molding method. If a simple MDF frame would suffice, then some professionals would already be doing it, as it it would be much cheaper than more conventional molds. I think it will not work, but I suggest you ask a professional boat builder before you go ahead.
     
  3. Paulo
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    Paulo Junior Member

    The Project

    Hi, Frosh!
    Thank you for your post. And don't worry, you are not off the topic. Not yet, anyway. :)

    The reason for my question is that in Brasil we have some difficulty to get some good boatbuilding epoxy resin and when we can it is imported with so much tax added. That makes it as good as impossible, economically speaking.

    I fully agree with every thing you sad in your post: It is very strange indeed. However I am trying to make my project work and should any one out there have tried it before I would have liked to know how it went and if I should considere it as an option. As it turns out, if anybody has ever tried it, he or she is never going to earn up to it. Not to worry.

    The boat I will be beginning to build next month is the one in the images.
    It is an open fishing boat, LOA = 8.85m and BOA = 3.15m, in new money.

    The method I use is to make a frame out of MDF sheets, cut with CNC and assembled, so that the hull is upside down. It is then a matter of covering this frame with 20mm DiviniCell, sand any inperfections out of the foam, apply a thin layer of modeling past and sand down again to a fine finish. The surface is then cleaned and is ready to receive the laminate.
    Once the laminate is finished, we can very carefully turn the boat, remove all MDF and laminate the inside of the foam. In so doing, we complete the sandwich laminate. The next step would be to lay the internal structure of the boat and go on to the decks and so on.
    Very much simplifide, this is how we professionals do it here in Brazil.
    Why other people don't do it this way, could be a question for another thread perhaps. What do you think?

    I am sorry, if I gave the impression that I am an amateur. I didn't mean it. Maybe my question wasn't all that clever, but who knows, maybe some cleverer person has worked out a way to protect the hull and build the boat as cost effective as possible and just maybe this could include aramid and polyester resin. I am some what isolated in Brazil. The only contact I have with the outside world is the internet and some magazines I receive down here.

    Thank you once again,
    Paulo
     

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  4. marshmat
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    marshmat Senior Member

    I've only used an aramid/glass hybrid cloth once. It was a lower grade prepreg and was absolutely disgusting to work with.
    Kevlar's not as stiff as glass, and so the engineering is a bit different. Your method seems interesting, I suppose it's at least worth a try.
    Polyester and Kevlar don't get along too well. I'd go with at least a vinylester if you really need the Kevlar. (Its main benefit is impact resistance, if you're not hitting rocks you might be better off with just glass.) If you're hooked on poly, I'd suggest going with a higher quality glass, rather than trying to coax Kevlar to mate with a resin that it probably won't like very much.
     
  5. Roly
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    Roly Senior Member

    Hi paulo & Matt, sorry for butting in here. It is kinda on topic......
    Could you elaborate on that please Matt.
    I have this kevlar knit that I was going to combine with e-glass (triax) athwardship bothsides of balsa thru the mast area of the deck, and sheer to sheer in the chain plate area and fore and aft to the turn of the bilge to create a large "washer" to spread the load from the keel bolts.
    The chart on my wall :) sayes the tensile modulus of kevlar is 18 and E glass
    only 10.5;I thought the higher the modulus the greater the force to "stretch".
    I understand that with a change of modulus, one is going to load up before the other;which aint ideal, so how do you use them in conjunction with each other?
    Thanks for your time.
     
  6. marshmat
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    marshmat Senior Member

    E-glass has an elastic modulus of 72 GPa, S-glass is 87 GPa. Kevlar's elastic modulus is approximately 100 GPa (give or take a bit depending on grade) for a tensile load axial to the polymer chains. So the fibre itself is somewhat stiffer. However, the individual fibres of a Kevlar cloth are incredibly thin compared to glass fibres, and the polymer chains do not crosslink within a single fibre nearly as much as in glass (which is more of an amorphous network crystal). Kevlar's also not nearly as stiff in compression as it is in tension. So although Kevlar is somewhat stiffer than glass when the loads are purely axial to the polymer chains, it usually ends up being a lot less stiff when woven into a cloth and set in resin. It is, though, generally a fair bit stronger for the same weight.
     
  7. frosh
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    frosh Senior Member

    Hi Paulo, So are you a professional boatbuilder? If so what does this MDF framework look like? Is it just a series of bulkhead hull shapes representing cross sections at intervals from bow to stern? Are there longitudinal timbers also? Doesn't the hull collapse outwards once it is turned and the MDF removed, before you have fitted the internal framing?
    Another point regarding resin choice is the need for strong adhesion between laminate/resin and foam core. The surface of the foam is not really flat but a series of tiny hollow spaces with narrow ridges. Epoxy is easily the best for adhesion, but filling the surface of the foam with a slurry of Q-cells in epoxy resin is worth while before laminating even if you are using epoxy in the laminating process. Save money on the reinforcement by using fibreglass and spend more on the resin. Vinylester=good Epoxy=best
     
  8. Paulo
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    Paulo Junior Member

    With regard to resin choice

    Hi Frosh and Marchmat,

    From what I can gather, the best I should do is stick to what I am used to and have had quite a bit of success with thus far: bi-axial cloths of s-glass and polyester resin.

    I like the suggestion of using the vinylester resin. It has the added bonus of avoiding the print-through of the woven.

    The aramid hybrid cloths (and any aramid or carbon cloth for that matter) really is a big investment and, by what I have seen here, should be used with epoxy resin only. I am still not sure in which case should someone choose to use 50-50 aramid-glass cloth, given the differences between the two reinforcements and the difficulty of engineering the laminate as pointed out in this thread.

    I would like to leave you with a little note that should explain my reason for chosing polyester resin and for opening this thread: I can buy polyester resin at US$3.00/kg, while the epoxy resin would set me back US$37.00/kg. In other words, I can buy 12kg of polyester resin for the price of 1kg of epoxy resin and still get some change for my money!
    I am always looking for improvements through technology adapted to my reality and cost efficiency.

    Thanks for the attention,
    Paulo
     
  9. Paulo
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    Paulo Junior Member

    About the MDF frame construction method

    Hi Frosh,

    Yes, I am a professional boatbuilder based in Brazil, graduate from Southampton-UK. I work with boatdesign and project development, as well as boatyard consultancy and yacht brokerage.

    I thought the best way to answer your questions about the Foam over MDF frame one-off construction method would be to explain the entire process with the aid of the 4 images attached to this post.

    We begin with a 3D model of the boat that we wish to build. I use the MaxSurf design program to design my hull shape. Then I set the sections 500mm apart for the bow area and 700-750mm for the rest of the length of the boat. In so doing, I can generate a set of cut files to send to the CNC milling machine. I use 15mm thick MDF sheets. The milling machine works with a precision of + or - 2mm! Pretty good cutting!

    Image 'paulo01.jpg':
    First we build a bed out of solid timber (the only part of the frame that is re-usable) and set it leveled. Then we can assemble the cut sections in their predefined spacing. Stringers, cut out of MDF sheets, fasten the sections and maintain the spacing throughout the length of the structure. Once the structure is finished, all the edges facing outside are covered with wrapping tape to keep the epoxy adhesive from sticking to the MDF.

    Image 'paulo02.jpg':
    Depending on the hull curvature, we can use entire sheets cut to shape to fit the hull, on the areas with less curvature, and strips cut out of the foam sheets, using a circular saw, on the areas with greater curvature. Normally, the bow requires the strips, for example.

    Image 'paulo03.jpg':
    The strips are used with the strip-planking construction method. The strips are glued together and held in place on the frame with some nails and some 25x25mm pieces of board.

    Image 'paulo04.jpg':
    Once the epoxy, in the case of this example, has cured, we can just pull the nails out, plug the wholes and begin the finishing process with some sanding down. At this point it is very easy to work out any kinks and irregularities in the hull shape. I like to use twice the scatling thickness for the foam core exactly for this step of the fairing process. It allows me to feel free to sand quite a bit down without any scares of sanding a hole through the core.

    To be continued . . .
     

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  10. Paulo
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    Paulo Junior Member

    About the MDF frame construction method - continued

    . . .
    The hull is now ready to receive its outside laminate. I like to apply a first layer of resin without any reinforcement. This has the role of filling the bubbles in the foam and thus sealing the foam core. Once the resin has cured, it is business as usual with the laminating.

    The surface finish is done as usual with some resin mixed with micro balloons, a lot of sanding and a fair dose of passions. For surface coating, there are a great deal of options available. I like to use a gelcoat variation, designed to be applied in the case of restorations without the need of additives. It looks nice and is good for the finances.

    The next step would be to turn the boat upside up. First we make a cradle for the boat. Then, once the boat is turned and secured on its cradle, we can remove all MDF sections out of the hull's skin. Finally, we use 3 or 4 cross beams set at pre-defined sections and width to make sure the hull maintains its intended shape. Now we are ready to prepare the inside surface, sanding it down and making sure all holes have been properly filled, to begin laminating the inside of our sandwich hull laminate. Once we have some of the internal structure in place, it is possible to remove the cross beams and carry on with the work without any obstruction in the way.

    I personally like this method because it is possible to play around with the weights of the whole boat and achieve a prototype with the same characteristics of a production boat. That is possible, because the weight of the laminate we save by using a sandwich laminate. This weight is then used on the modeling and fairing process. The weight of the prototype beeing the same as the one of the production boat, we can test the boat on the water before making the molds for production without any reservations as to the veracity of the results.

    I hope this explanation has been clear and that you have been able to understand everything. I would like to say that this method has been used and approved time and time again not only by me, but also by some of the best boatbuilders in Brazil. Should anybody have any further questions, I would be more then happy to clarify them.

    Thanks for reading this post,
    This is Paulo signing off! :)
     
  11. frosh
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    frosh Senior Member

    Thanks Paulo, for your clear answers and photos showing the method with the MDF mold. Based on the cost of polyester vs. epoxy and the fact that this is a commercial enterprise, then if you have had no delamination problems in the past, it makes no sense to spend hugely more money.
    Obviously with your training the mold is far better than I imagined from your initial post, and the receiving cradle is also a necessity. Is this economic for the production of one boat? You do mention also that you can use the prototype to make a commercial mold, but would you always have orders for more than one? Biax glass is a really good product for the price having used it recently, as well as in the past woven glass, dynel, kevlar, and carbon fibre woven and uni-axial.
    My interest is sailing craft which are much more weight sensitive than powerboats.
     

  12. Paulo
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    Paulo Junior Member

    The same for sailing boats!

    Hi Frosh,

    I am glad that you liked my post.
    In your case, as a sailboat man, goes exactly the same process, but then we can turn on the skill nob full on and turn the name of the game into "as light as possible"! ;)
    I am also attaching some pictures of a sail boat. In this case a mini 650 to be used in the minitransat. I guess you are familiar with these boats, hey?

    The economy of this building process for one-offs is mostly time and mold fabrication. Most of the time I have build with this method, the client ended up making the molds for production, because there were other that liked the boat so much, they too wished to have one.
    This process is an option for production boats, when the customer wishes to run some tests before proceeding with the building and marketing.

    Paulo

    Hope you like these images . . .
     

    Attached Files:

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