: While that's true in theory, it's somewhat different in practice.
: Carbon fiber tends to lay much thinner than Kevlar, probably
: because the stiff fibers stay in relatively tight contact as
: they weave around each other. Kevlar fibers are softer and more
: bulky, which creates more air spaces in the weave that will
: ultimately be filled with resin when laid up. Kevlar is also
: buoyant and will readily float on top of the resin. The upshot
: of this is that in order to keep the fiber-to-resin ratio near
: optimum, you need to compress Kevlar more than you do carbon
: fiber. Vacuum bagging, resin infusion, packing tape or some
: other compression method will compact the fibers and reduce the
: space in the weave for resin, reducing the weight and improving
: the strength of the layup.

Brian,

I agree that some form of compaction is useful for maintaining a near optimum fiber/resin ratio. However, I think that Mike's original statements are misleading – maybe that could be could be cleared up by using more specific language. More than once on this board someone has proposed saving weight by using similar areal weight carbon cloth in place of glass cloth. Mike's comments could be read as support for that thinking. But the density difference between carbon and glass means that for the same fiber/volume ratio, the carbon lay-up will weigh significantly more than the glass lay-up regardless of any savings based on the weave style.

I don’t see any reason why stiff fibers would stay in tighter contact than softer fibers “as they weave around each other”. I can imagine stiffer fibers creating more open space at yarn intersections, compared to softer fibers.

I don’t know what you mean by Kevlar is more “bulky”. The typical PAN carbon (T-300) fiber diameters are around 8-microns. Typical Kevlar fiber diameters are around 12-microns. Not much difference there, particularly since they both have the same shape. Looking at the dry thickness of 5-oz HS carbon compared to 5-oz HS Kevlar, the difference is easily accounted for by density, not excess space in the weave. The same can be said for the composite weight of hand laid 5-oz Kevlar/epoxy compared to 5-oz carbon epoxy. Therefore, some unspecified "bulkiness" argument is unnecessary. Besides, with the same fiber shape, the packing factor (fiber/resin volume ratio) can be the same regardless of the fiber diameters (bulkiness?). Of course, for equal areal weight fabric and fiber/resin volume ratio, Kevlar will require more resin than carbon because of the density difference (fiber/resin weight ratio).

Typical specific gravity of PAN carbon (T-300) is 1.8 and that of Kevlar is 1.45. Typical specific gravity of cured epoxy resins is 1.2 to 1.3. Unless one is using an epoxy with lots of VOC’s that cause an uncured density much different than the cured density, I think it’s unlikely to get Kevlar to 'float' in the resin. Besides, floating or not floating, there is no reason to assume that the spaces between fibers would be affected by density differences between the fibers and the matrix. Because of the large fiber surface area to volume ratio, fiber/resin interface affects should be much more important than density and it's affect on buoyancy.

Sam