Marky V wrote:…The great thing about polystyrene is that you can hotwire any shape you like into it once set up
Please tell me how to do this! I’d love to be able to carve the bowl on the nose of many of my boards, spoon out the deck, shape the thumb grips, and carve out the kneewells with a hot wire and EPS foam.
Marky V wrote:…with so many exciting and flexible new materials around us, and so much experience, I think it is time to take it to a new dimension…
I certainly wouldn’t describe the use of epoxy (and/and or carbon fiber and balsa wood) as a “new dimension”. My first resin/glass project (model airplane fuselage)--in 1958--used epoxy resin (Epon 828 resin, curing agent U). I also built “Pelamis” in 1973 or 1974 using an “aircraft grade” epoxy resin and fiberglass cloth, and participated in the construction of a homebuilt aircraft using epoxy resin, polystyrene foam, and hot wiring in the mid-to-late 70’s.
Unless the specific characteristics of epoxy are called for by the materials or the application, I favor the use of polyester resin as long as it is adequate for the task (however, your mileage may vary). As with the usage of epoxy resin, you need to tailor the materials for the task. While it may be true that Resin Research epoxy resin is more flexible than the polyester resins typically used for surfboard construction, that doesn’t have to be the case. For example, the percent elongation of the latter to failure is about 1.5 percent. But for the boards I built during the 70’s, I used Dion (Diamond Shamrock) DRA-602 polyester resin that had an elongation to failure of 10.5 percent.
It is true that epoxy resins bond better to many (but not all) materials than does polyester resin. But the practical question is: “Does polyester resin bond well enough to accomplish the desired objectives?” If some other component of the structure (e.g. the fiber reinforcements) fail before the bond between the resin and the fibers (or, more correctly, between the resin and the sizing), it doesn’t matter if the bond to the sizing with polyester resin would have failed if the load could have been increased by another 50 percent, or increased by another 100 percent with epoxy resin.
It appears to me that the primary reason for using epoxy resin with fiberglass reinforcement is the desire to use expanded poly-styrene (EPS) foam since the styrene in polyester resin would dissolve the surface of the blank. Examination of broken boards of polyurethane (foam) and polyester (resin) (i.e. PU/PE) construction indicates to me that when the glass peels from the foam, it is not a consequence of the resin parting from the foam, but rather the foam failing in a layer interior to the bonding of the glass to the foam. Hence the foam/glass bond in PU/PE construction appears to be more than adequate to the task.
The flex-rails in my boards have substantial flex—in some cases, more than is desirable as the wave size increases. If the bond between the resin and sizing were breaking down, one would expect to see some visual evidence (e.g. appearance of small areas with the cloth weave visible as small white-colored segments corresponding to the fiberglass strands). With few exceptions, that has not been the case. The few instances that I have seen such a pattern have been associated with slipping on rocks while entering the water, with the board landing on a rock (and sometimes me following by landing on the board).
It is generally true, however, that the use of epoxy is required to achieve the full benefit of advanced (but not necessarily new) fibers. The most common of the latter in the surfboard industry is carbon fiber (sometimes erroneously referred to as “graphite” fiber, but the two cross-linking structures are substantially different). Carbon fiber has a number of attractive qualities. For example, it is about 30 percent lighter than glass fibers, and the stiffness is about 5-1/2 times that of E-glass or S-glass. It is also much more resistant to fatigue than glass fibers. On the hand, its tensile strength is only 4 percent greater than E-glass, and 20 to 24 percent less than S-glass. This set of parameters has some significant consequences when building a board with flex.
The following table summarizes the properties of several types of fibers, including some relatively recent types. The values are all normalized to that of E-glass (the glass commonly used in surfboard construction). For example, “density” is equal to the density of the fiber indicated divided by the density of E-glass. Hence all the values for E-glass will be 1.00.
Fiber…………....…….Den……Stiff…Strength…Twang…Max Flex
------------------ ------ ----- ----- ----- ------
E-Glass…………......1.00……1.00……1.00……1.00……1.00
S-Glass……………...0.98……1.18……1.30……1.05……1.16
Carbon(T-100)……0.69……3.19……1.09……1.60……0.61
Boron……….....…….1.01…….5.52……1.04……1.53……0.44
Aramid(Kevlar49)..0.57……1.81……1.12……1.54……0.83
PBO(ZylonAS)…....0.61……2.48……1.70……1.61……1.08
LCP(DeefranHS)...0.92……0.96……0.88……1.33……0.90
Den = density
Stiff = Stiffness (resistance to bending or stretching)
Strength = Maximum stress at failure
Twang = A measure of the frequency of oscillation if flexed and then released (for a laminate thickness that produces the same deflection for the same loading on a glass fiber board)
Max Flex = Degree of bending before failure occurs (for a laminate thickness that produces the same deflection for the same loading as on a fiberglass board)
Now let’s see how these fibers compare with regard to a flex spoon by comparing various properties of the spoon, depending on the fiber used. In these calculations, we assume that the lay-ups for both boards have the same thickness, the fiber/resin ratios are identical, and the same resin (epoxy) is used for both boards, and essentially all the loading is carried by the reinforcing fibers.
Case I (both boards are built with the lay-ups built to the same thickness and have equal weights of foam):
1. A board built with carbon fiber will weigh somewhere between 0 percent (fiber weight is negligible relative to the weights of the other components) and 31 percent less (if the board could be built out of fiber alone—i.e. no resin, foam, etc.). For a 2:1 resin/glass ratio, the weight saving over a 10 lb fiberglass board would be roughly about 1 lb.
2. A board built with carbon fiber would require about 5-6 times the force applied to it to produce the same degree of flex (i.e. it would be MUCH stiffer).
3. A board built with carbon fiber would be about 9 percent stronger than the board built out of E-glass, but about 21 percent weaker than a board built out of s-glass (assuming that buckling is not the failure mode).
4. The “twang” of any flex developed for a carbon fiber board would be a little more than twice that of a board built using glass fibers.
5. The maximum degree of flex that a carbon fiber board could undergo before tensile failure of the fibers begins to occur would be about one third that of a board with glass fibers.
Case II (both boards are built to the same weight, assuming that both use equal weights of foam):
1. Both boards weigh about the same, but the laminate thicknesses of the carbon fiber board would be about 10 percent greater than for the glass fiber board (again assuming a 2:1 resin/glass ratio).
2. A board built with carbon fiber would require about 6-7.5 times the force applied to it to produce the same degree of flex as a glass fiber board.
3. A board built with carbon fiber would be about 20 percent stronger than a board built out of E-glass, but about 10 percent weaker than a board built out of S-glass.
4. The “twang” of a carbon fiber board would be increased by about another 20 percent (to about 2-1/2 times that of a fiberglass board).
5. The maximum degree of flex that a carbon fiber board could undergo before tensile failure of the fibers begins to occur would be increased by about 20 percent (to about 40 percent that of the fiber glass board).
Case III - The Carbon Fiber tail laminate thickness is chosen so as to produce the same degree of flex as the glass fiber laminate when both are subject to the same loading.
1. The "twang" of the carbon fiber board is about 60 percent greater than for the glass fiber board.
2. The amount of flex the tail/rails can undergo before the laminate starts to break down is about 40 percent less than the glass fiber board.
Are these the qualities you really want? I don’t think so for my designs. In particular, the substantially reduced degree of flex that can occur with the carbon fiber board before it begins to fail is worrysome in all cases. And in Cases I and II, the greatly increased stiffness and the reduced allowable flex (without breaking) has essentially converted a flex spoon into a non-flexing spoon.
The substantially increased “twang” associated with the carbon fibers is nice…but it remains to be seen if that consequences of twang contributes significantly to more forward thrust, or is actually more of a difference in the “feel” of the board (the latter certainly occurs, the former is more uncertain).
The potential for a reduction in weight is also nice, but the greater the weight savings, the more the carbon fiber board falls behind the glass fiber board with regard resistance to failure by tensile loadings.
My conclusions (yours may vary):
For my type of boards, changing from fiberglass to carbon fiber would degrade many of the qualities I like in the board, and which were a specific component of the design. In particular, it would be difficult to achieve the same degree of flex without greatly reducing the strength of the board (and the board would have to be substantially re-engineered).
If I don’t need carbon fiber, why would I need to use epoxy resin? Polyester resin costs about one-half as much, and seems to have adequate bonding strength, flex, and fatigue resistance to survive at least 23 years of usage (as demonstrated by “Cetor”). It is also much more convenient when making up small batches (since the full strength of epoxy depends on getting the proper mixture of resin and hardner—e.g. requiring a lab beam balance, or something equivalent), and has a more effective uv-blocker than epoxy resin, or the uv-blocking coatings applied over epoxy. In addition, there is the added risk of developing a allergic sensitivy to epoxy. And finally, I like to tint my hulls red—and I don’t know how to do that with epoxy.
mtb
Experience gained is in proportion to equipment ruined.
