Re: Question about using epoxy resin with 1708 cloth?
Not one of those "corrections" are accurate. They are however, good examples of why people should do their homework with product mfgs documented specs and frp engineering specs to get accuracy.
1.Standard mat is not epoxy compatible.
"Standard" is a relative term. If you consider Bondo mat from the local home store or "Sea Glass" mat from West Marine standard, both are epoxy compatible. You can still find mat that's not, sure, but it's a shrinking minority of the market, because it's cheaper for manufacturers to make mat with just one binder that works with epoxy, poly, and VE resins, rather than to make one for poly and one for epoxy. This has been the case for about 10 or so years.
By the way, I'm not going to pick up the bait you're trolling with the comment on "why people should do their homework"... I encourage the various readers of this board to look around this board, read up, and do their own research, then decide who to believe
2.Epoxy mat has always been more difficult to find.
It was when it first came out. It's not now.
3.1708 has mat for engineering (stiffness) and adhesion reasons...for poly or epoxy resins.
Stiffness is dictated by the thickness of the layup, and thickness depends on how much glass, mat, and resin you use, so I suppose you could say that you add mat to add stiffness, because it adds thickness. But it's always weaker than adding a similar weight of woven fabric.
Epoxy needs no mat or anything else for adhesion assistance. Think about it, when have you ever seen carbon fiber mat or kevlar mat? You haven't, because those fabrics are used for high end boats (with epoxy) and mat isn't needed. Mat is used in low end boats because they're made with poly, which is cheap, and mat is a nice cheap way to add bulk to a layup.
Mat stitched to biaxial fabric is very easy to handle, and coincidentally forms most of a "ply" or standard fractional thickness of a hull. Most hand laid up hulls use an alternating series of 2-3 (or more) "ply" layers, so the layup schedule goes something like this, outside of hull to inside:
gelcoat->surfacing veil->mat->fabric->mat->fabric->mat->roving
I won't speculate on stitched fabric/mat uses outside the boat building industry, but I'd suspect it's a lot easier for anyone working with glass to use a stitched combo like 1708 than multiple separate pieces of mat or fabric, it's stronger than chopper gun layup (which is easier to build), and cheaper than using fabric only. Plus as mentioned, if you use poly, you need the mat to make it all adhere.
4.Stiffness is not dependent on thickness unless you want high weight.
Stiffness is dependent upon two things: Total composite thickness and the properties of the skin layers of said composite (tensile strength and elasticity). It's a requirement that the core of the composite have sufficient shear strength and crush resistance to hold the skins (inner and outer) together and force them to bend in unison, but that's not a hard goal to meet. You can make a solid panel of glass, essentially using the same glass/resin mix for the "core" and "skin", but the core doesn't take any more stress than a lighter material used in its place. It's just easier to build, and heavier.
A given thickness of hull made of solid poly with mat, woven, roving, or a chopper gun will have about the same stiffness as the same hull section made with plywood core covered with a couple layers of woven cloth. Probably the composite sandwich will be stronger, and it will certainly be lighter. There's a pretty good discussion of hull cores here, with some interesting info if you want to read:
http://www.powerandmotoryacht.com/boat/hard-core-facts/
5.There are not many, if any at all, high or low end boat mfgs that don't use mat.
Bill P.
Here's links to a couple:
www.cigaretteracing.com
www.fountainpowerboats.com
Once again, mat is used because it's a cheap way to make a hull thicker. The only cheaper way is with a chopper gun, which is even weaker. If you need strength and/or don't care about price, you use epoxy and glass, aramid, or carbon fabrics, not mat.
Erik
