Dennis,
I think the whole weight "advantage" of polyester resin is lost in the noise. And like Aussie Greg suggests, you need to be pretty specific about what you are trying to achieve/improve, or it may not be better, just different. Which is ok, it's a hobby!
For manufacturers, the compelling reason to use polyester resin is lower cost. For a hobby project it may matter more if you have a quart of polyester resin that you need to use up and that provides a (legitimate?) reason to go that route.
I was hard-pressed to find data sheets that support your argument that polyester resin is lighter than epoxy laminating resin. There is a range for each of course, but both are in the 1.1 to 1.45 gm/cm3 zone. The best direct comparison I could make was that an available laminating resin was ~5% heavier than polyester. So if you used even 3 oz. of material for your reinforcement, which would lay down a very large amount of 0.5 oz/yd^2 cloth, your weight penalty when holding thickness constant would be 4-5 grams. That is less than likely variations coming from wet-out techniques.
You need enough resin to get the laminate down and attached and all the filaments anchored to each other. If you are only using one layer of glass, then it is lightest to get it stuck down but don't try to fill the outer weave with matrix (resin); instead, use a lightweight filler. This will have a greater impact on weight than the difference in cured densities of polyester vs. epoxy.
If engineering is driving your materials choice, then consider the Specific Stiffness and Specific Strength of the cured laminate. The epoxy is stronger, so you need less of it in general--except there has to be enough to stick down one layer, if that is your idea here. The bending stiffness and strength should all be coming from the fiber. But the resin has everything to do with damage tolerance. (The military had impact requirements for dropped tools, which amounts to the same thing.)
Personally, I am more comfortable with what I believe are reduced safety risks of working with epoxy than the polyester. The trick with epoxy is to limit your exposure so that you do not become sensitized to it. I don't like having MEKP around at all. The last plane I worked on that was glassed with polyester resin (an Other People's Plane that they never finished) was a brittle disaster. I still have it and fly it, but the wing did snap in half...so I repaired it with epoxy.
The Divot McSlow
PS--I tried to stay out of this topic and see what the other guys had to say---but as a "sometimes mass props guy" in aerospace I was curious, and that lead to....
Topic: Fiberglass polyester resin over dope? (Read 1455 times)