Why does Dacron not stick to epoxy when fiberglass does?

We call that bleeder cloth in the industry. It's there to do what you observe. It gives the excess resin someplace to go. They make expensive release cloth which is glass coated with PTFE or some other slick, inert material, too. But, as you've discovered, other types of cloth will work.

Polyester's low surface energy and slick finish allow it to release from the cured resin.

The surface left behind after removal of the bleeder cloth takes on the texture of of the fabric used. This may or may not be an ideal bonding surface, but if it seems to work OK for you, then go with it.

Glass cloth is specially treated to enhance the wetting properties of the fibers so the resin will adhere readily to it. I think it's a siloxane coating, but, now I'm in over my head.

I'm sure some of the brighter bulbs in the pack can explain it in more detail and with more intelligence.

Thanks LynLynch for the prompt response.

No, this is not bleeder material. I use that too when bagging for lightweight layups for the reasons you cited.

Peel ply is used whether the part is bagged or not, room temp cured or heated in an AC.

I'm exploring the use of common organic fibers for some very light low stress parts made with foam cores and was concerned about the use of cotton blends using polyester.

Many years ago, Burt Rutan showed me tests he conducted for the purpose of comparing conventional "S" glass and ordinary bed sheets. Absolutely amazing what you can do with bed sheets!

Thanks

L.J.

Dear,

strength of adhesives depend upon the surface tension, molecular arrangements, temperature conditions of two contacting surfaces.

from the first part the molecular arrangement and its free volume allows to dissolve 1,3-Dioxalane, it is compatible with both glycol and with formaldehide.

epoxys are compatible with toluene(1:1 to palacate the alergic nature of epoxy and volatile nature of toluene).

cross linkage and molecular arrangement of epoxy's surface never detroirate the FRP or GRP surface, but create vaccum to hold.

polymer engineer.

Though no expert I think you summed it up ahsum haleem,

when you said no cross linkage. The surface wets well but has no chemical bond. The wetting leaves no place for voids to be on the surface.

Brad

This article may (or may not) shed some light.

Something interesting brought up in the article is that the use of siloxane-coated peel ply fabric resulted in poor subsequent bond strength. (Some siloxane is apparently left behind.) I will remember this if tempted to use anything other than plain polyester.

Also interesting is their contention that there must be a chemical bond for good bond strength. I am not sure that I agree. I've always thought of epoxy secondary bonding as being singularly un-chemical -- thus the requirement for good surface preparation (particularly roughness). Also, I think the difference between glass fibers and Dacron (as it relates to epoxy bonding to one and not the other) is a mechanical (surface tension, static attraction, Van der Waal attraction,etc.) rather than chemical difference. Gecko feet adhere well to all sorts of materials, apparently smooth and apparently rough, by Van der Waal attraction, it seems, and i suspect a similar thing happens with glass and fails to happen with Dacron. This relates to the chemical nature of each fabric, but does not, to me, mean that the glass chemically bonds with the glass, any more than Gecko feet chemically bond to a window. I think for a chemical bond to occur, there would need to an interface in which one found glass molecules interspersed in epoxy "molecules" (or even more strictly, silicon atoms bonded to carbon chains).

But I really don't know -- I'm making this up as I go.

An article I read in Scientific American many years ago said that Scotch tape acted like thousands of suction cups. That explains its ability to be removed fromn glass without leaving a trace, but does not explain its ability to stick to porous substances, such as paper and cloth.

I always rest assured in the knowledge that epoxy will stick permanently and agressively to a valuable mold treated with mold realease, but that secondary bonds will separate easily if someone sneezes too close to one.

I think dacron doesn't stick because:

1. The epoxy doesn't wet the fiber. Dacron appears to repel both water and oily materials. (Try using some in the workshop to wipe up oil etc, it doesn't absorb).

2. Being a monofilament, the surface is smooth and there are not the spaces between fibers for the epoxy to key into.

Epoxies do form secondary chemical bonds with many materials but I doubt if bonding to fiberglass is anything other than mechanical.

I think that your #1 is correct, although I am not completely sure why. I think it gets back to Van der Waals attraction and the nature of the molecules involved. Maybe the Dacron and epoxy, both being organic, and both having somewhat similar molecular structures, operate according to the "likes repel" theory. On the other hand, if both molecules were liquid, you'd expect them to mix quite well, at least according to the polar vs non-polar theory.

Also interesting, (to me at least) is the fact that lots of carbon fiber comes from a polyester fiber carbon fiber precursor. The pyrolysis, (to which the polyester is subjected to make carbon fiber) I guess, gets rid of the components of poyester that prevent bonding.

It's interesting that the FAA article starts off by saying the the bonding mechanism is not understood at an atomic level. Keep that in mind next time you ride in a airplane with composite pieces.