Zfennell said…
"Let me ask the question this way.
If the limiting factor in composite construction (of surfboards) is compressive strength of the laminate, what are the properties of epoxy which maximise the compressive strength of the laminate?"
Bill responed…
Interesting observation zfennell!
I have a couple of thoughts on this whole subject.
Compression strength -
Many seem to feel that the deck skins are failing on a Polyester board due to some inferior mechanical properties of the Polyester Resin. (I don’t think that is true.) Hence their desire to replace the Polyester with Epoxy.
In my view the problem is not really the lack of compression strength in Polyester. That said, in this case and thread then, the mechanical properties comparing poly to epoxy, don’t really matter.
The problem with boards buckling is all about the skin not staying in column under compression. That being so, there are really only two things that take place that allows boards to buckle.
1 - The skin buckles up.
2 - The skin buckles down.
The two factors effecting this are
1 - The foam’s strength.
2 - The foam’s bond to the skin.
If the board’s skin buckles down it is because the foam isn’t strong enough to support the skin and keep it in column. Therefore, the foam crushes in and allows the skin to shift downward and out of column. .
If the skin buckles up, it is because the foam isn’t strong enough and tears apart allowing the skin to move upwards and out of column. I should note here that it isn’t the resin’s adhesion to the foam that is insuffecient. It is the foam that rips to shreds. Some stays attached to the (resin) skin.
Once either of these happen, it depends on how much further the forces continue to bend the board. But that is another issue which we will likely touch on later.
If what I am saying above is accurate, then to a great extent it hardly matters what the resin material is that is used in the single skin, if the backing material it is attached to, is not as strong as or properly matched to the skins ability to stay in column. What is needed, in this case, is a different foam that can better resist these forces. Crispier or more rubbery, the foam needs properities that will allow it to resist or flow with these forces.
I haven’t read or been involved in all the discussions on Swaylock’s regarding this area of surfboard design. But what I have seen is, a huge emphasis on the resin used rather than a thorough discussion of the real problem. Hence Sfennell’s earlier question, “ultimately it does fail, but what’s the mechanism?”
The solution I have seen most commonly discussed here is to turn the single skin that is standard, contemporary PuPe constuction, into a sandwich skin using Epoxy resins and crispier foams. I am not challenging this skin construction. That can be discussed in another thread at another time. But it seems only fair to note that the STRUCTURAL FORM of this kind of skin is vastly different than the commonly used single skin’s form, found on contemporary boards. It is this FORM STABILITY that changes the way the boards resist breaking, not as it often sounds, the Epoxy Resin. If one were to create a similar skin of Polyester, Fiberglass and Divinycell and overlay in on any foam core it would perform quite well.
Having lived through the evolution of these processes, it is my experience and belief that Epoxy leapt into this mix because of 3 primarily reasons.
1 - The Poly would go off faster than you could get the thing layed up, bagged and a vacuum pulled. (Greg’s comments in an earlier thread alluded to this. That not only are Epoxies easier to use then Polyesters, in vacuum bagged composites but his are designed to be both easer and safer).
2 - The desire for stronger and lighter boards (particularily sailboards) led people into using Styrofoam based cores which would desolve if used under Polyester resins. Thus epoxy use was near manditory.
3 - The use of heavier, sandwiched skins, using heavier foams, required and also allowed the use of lighter Styrofoam cores, requiring Epoxy resins.
In my experience, it wasn’t the strength of Epoxy resins that drove their use. It was issues of practicality of use, that required it.
Now I am not saying this wasn’t a good idea. Or that Epoxy’s other advantages aren’t beneficial to the package. Or that we shouldn’t continue to use it. Or that we should replace it with Polyester. So don’t all you members of the Epoxy Clan jump all over me. I AM NOT PUTTING EPOXY DOWN, AND YOU DON’T NEED TO DEFEND IT.
We are all just board builders of varying knowledge and experience. And the more we understand about the materials, techniques, and historical uses of them, the better off we will all be in using them to create all the varied boards that we and the marketplace desires. I should note here that “we” are also the marketplace. Home built surfboards by individuals for themselves or friends, is a huge part of the economic pie that is consuming surfboard materials.
The compressive failure starts at the rail and works its way in.