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donee · 03-21-2008, 06:51 PM

Hi All,

Epoxy is used because Polyester is incompatible with Styrofoam. This started with surf boards. Fiberglass boats tend to use PVC foam, so either skin material can be used. Epoxy resists blistering better than polyester skin. A gallon of epoxy resin for laminating was like $75 the last time I was buying any. I imagine its twice that now. I prefer Epoxy because of the better cure one can get in the home shop situation. There are two main brands for this West Systems, and System III. System III is available at Wood Craft stores. West Systems is available at many boat maintenance shops.

The PVC foam is not cheap either. The main trade name of the PVC foam is Divinycell (spelling?), and a few others I forget. In Europe they may use Rohacell (its polyamidimid I believe), but its very expensive here, and probably more so now. DOW makes rigid Styrofoam brand board (its blue) which is good for composite construction. The pink insulation board stuff has been "ruggedized" so it wont break as easy itself, but this greatly decreases the compressive strength (so the board can bend), which is critical for stressed skin construction. Boats core materials can also be aluminum or polypropylene honeycomb, or edge grain balsa wood. Or just simply marine grade plywood. Remeber, boats can be allot heavier than cars.

The PP Core has been used in land transportation applications. There is a company in Germany that makes a whole travel trailer, the kind that can be towed by cars, out of PP stressed skin construction, with about a 1 inch thick core. The brand name of this material is NIDA Core, and they are in Florida. This PP core has Polyester film stretched tight and bonded to the PP honeycomb, and the polyester has been scrimmed (made fibrous) on the outside for the glue to stick to. I have some (much too thick for car stuff) here and use it for vibration isolating platforms. PP being a polyolefin does not bond by gluing readily. NIDA Core has figured out how to do the bonding to the polyester film (probably thermally).

Another good core material, I forgot the name, for this stuff would be the PC (Polycarbonate) tetrahedral dimpled stuff. Its amazing too, and allot cheaper, but does not have the compressive strength of honey comb. But , I would think it would be very servicable for aero feature parts. PC can be glued directly. So, there is no need for the Polyester film and scrim, like with PP. It may not be available in thin enough core for this tho. Where the NIDA core can be made to order in its thickness.

Remember most of these materials are plastics, which can have tremendously variable modulus' with temperature. I made a sample with PVC foam, and Polyester fiberglass skins once, that was about 1 inch thick, 2 inches wide, and 9 inches long. At room temperature one could flex this about 1/8 inch over its length. At 10 F (I took it for a walk for an hour) the sample became very very rigid, and I could not feel any deflection at all hard as I tried to bend it.

03-21-2008, 06:51 PM	#5 (permalink)
donee EcoModding Apprentice Join Date: Jan 2008 Location: Chicagoland Posts: 204 - - '10 Toyota Prius III w/Navi Thanks: 4 Thanked 12 Times in 12 Posts	Hi All, Epoxy is used because Polyester is incompatible with Styrofoam. This started with surf boards. Fiberglass boats tend to use PVC foam, so either skin material can be used. Epoxy resists blistering better than polyester skin. A gallon of epoxy resin for laminating was like $75 the last time I was buying any. I imagine its twice that now. I prefer Epoxy because of the better cure one can get in the home shop situation. There are two main brands for this West Systems, and System III. System III is available at Wood Craft stores. West Systems is available at many boat maintenance shops. The PVC foam is not cheap either. The main trade name of the PVC foam is Divinycell (spelling?), and a few others I forget. In Europe they may use Rohacell (its polyamidimid I believe), but its very expensive here, and probably more so now. DOW makes rigid Styrofoam brand board (its blue) which is good for composite construction. The pink insulation board stuff has been "ruggedized" so it wont break as easy itself, but this greatly decreases the compressive strength (so the board can bend), which is critical for stressed skin construction. Boats core materials can also be aluminum or polypropylene honeycomb, or edge grain balsa wood. Or just simply marine grade plywood. Remeber, boats can be allot heavier than cars. The PP Core has been used in land transportation applications. There is a company in Germany that makes a whole travel trailer, the kind that can be towed by cars, out of PP stressed skin construction, with about a 1 inch thick core. The brand name of this material is NIDA Core, and they are in Florida. This PP core has Polyester film stretched tight and bonded to the PP honeycomb, and the polyester has been scrimmed (made fibrous) on the outside for the glue to stick to. I have some (much too thick for car stuff) here and use it for vibration isolating platforms. PP being a polyolefin does not bond by gluing readily. NIDA Core has figured out how to do the bonding to the polyester film (probably thermally). Another good core material, I forgot the name, for this stuff would be the PC (Polycarbonate) tetrahedral dimpled stuff. Its amazing too, and allot cheaper, but does not have the compressive strength of honey comb. But , I would think it would be very servicable for aero feature parts. PC can be glued directly. So, there is no need for the Polyester film and scrim, like with PP. It may not be available in thin enough core for this tho. Where the NIDA core can be made to order in its thickness. Remember most of these materials are plastics, which can have tremendously variable modulus' with temperature. I made a sample with PVC foam, and Polyester fiberglass skins once, that was about 1 inch thick, 2 inches wide, and 9 inches long. At room temperature one could flex this about 1/8 inch over its length. At 10 F (I took it for a walk for an hour) the sample became very very rigid, and I could not feel any deflection at all hard as I tried to bend it.