Epoxy versus vinyl ester

My understanding is it's the shape and attachment of molecular segments during the realignment (poly) or molecular reassembly (epoxy) in the cure. On a molecular level the polyester bonds appear 2 dimensional, basically layers of spaghetti laying on top of each other. Some of the strands that touch other strands form bonds, but that's it. Vinylester is similar, but introduces a broken segment interlock that engages more strands and across more layers, making for a stronger bond. Epoxy on the other hand is a different animal, the molecule reassembles into a hairy string, but with several ends. These ends and the hairs themselves all can connect to other epoxy molecules, which produces a very dense crosslink in all three dimensions. So epoxy's attributes come from it's physical arrangement on a molecular level (3 dimensional crosslink), as does the weaknesses in the linear polys (mostly 2 dimensional bonding).

 

Paul

Epoxy forms a much stronger matrix and it's brilliant with some fibres like carbon or wood. It's just not so good with glass.

You want the matrix to hold the fibers in place, provide some compressive strength and fill the voids. You want it to form a very strong adhesive bond with the fiber surface. Carbon Epoxy is an excellent material. But epoxy glass unfortunately is nothing like it even when dry.

If the fibre matrix bond fails then the fibre becomes a loose cable in a conduit so the matrix carries all the local (micro) stress and cracks further. Micro cracks then grow into major cracks and not simply running to a fiber and stopping which is what should ideally occur.
Then you have the matrix forming larger crack planes and stress concentration mechanisms which lead to fibre failures and more local stresses more matrix failure etc.

So many of our composites, cores, adhesives are not tested in the state they will inevitably be found in in a marine environment. I know a lot of engineers who have found their nice FOS disappeared when they looked into the long term immersed properties of some composites.

Vinylesters don't blister as easily as Polyesters but they still have limitations as a structural material which a lot of designers don't seem to be aware of.



I just looked up the figure for Vinyl Ester- E glass. Close to saturation it loses 30% of it's tensile strength. That's just a straight one cycle tensile test, the S-N curves are a worse scenario.

 

Although this is the standard test, it is very limited. It restricts the manufactures of plywood to using glues that aren't softened at 100 degrees C. Of all places, Zimbabwe made superb plywood with epoxy resins that were used to build a charter fleet on Lake Kariba. Unfortunately no export market could be found for these sheets as they couldn't pass BS1088.

 

...no, it is not incompatible, we use it all the time as the first few layers.

 

The other item that gets mentioned, then forgotten from time to time is the 'sizing' found on most cloth. Some say it makes a big difference , and that Epoxy shouldnt be used with 'sized' cloth, others say it doesnt matter.

Still researching that.

 

..yes that is correct, the binding of the cloth is dissolved by the styrene in poly type csm, there are specific bindings for differing materials.

 

No, sizing doesn't make a difference. Okay it does, but not much. The styrene soluble sizing just ends up in suspension in an epoxy matrix, so it's no big deal. Think of it as filler.

 

After the vynil ester laminate it can be iso resin or epoxy. If you use epoxy, you cannot switch back to poly or vynilester. That is where the compatibility issue comes in. Epoxy over poly yes but not poly over epoxy.

 

Resin does not like to stick to unfinished fiber so "sizing" or finish is added. There are two types, the mechanical finish where the strand is abraded or pitted or the chemical type. Chemical finish promotes the adhesion of resin to the fiber. Some are compatible with poly, others with epoxy or vynil. Most finish is compatible with at least two resin type and is usually indicated on the data sheet.

 

Looking at comparative data of cast resins, the tensile strength of vynil ester is much higher than iso or epoxy and the elongation is greater than iso or epoxy.

In my opinion, the high tensile strength and the "stretch" or "give" gives it a much higher impact strength. This material property would be less prone to cracking by its ability to resist deformation.

It is not however a cure all for impact strength as there are four categories of impact response of a laminate. I am just saying that it it one of the factor that helps prevent initial micro cracking.

 

Thanks for that extra bit of info - you can see what I am up against though, advice ranges from "sizing doesnt matter" through to "specific bindings for differing materials".

In practice, no fiber retailer I have spoken to even knows what "sizing" is ? (southern australia is a composite expert free zone)

Now I hear there are data sheets for fibre! - that will give some local retailers a headache today.

 

reference site

great site about resins here

http://www.mdacomposites.org/mda/psgbridge_CB_Materials2_Resins.html

 

RW
Here's the coating types availbable. From the Glassfibre handbook, RH Warring.

 

Sizing is very important. I was about to type the finishes but Mikejohns beat me to it.

Once, we received fiberglass from a supplier. It was cheap but there was no indicated finish. True enough, when we laid it, we could not wet it thouroughly. There were dry patches showing up and we could only remove it by continously rolling it. Doubled the effort. It turned out it was untreated fiberglass intended for commercial application, not for boatbuilding.

There was also an instance when we were bidding for a military type boat. The specs called for a "Garan" type of finish for the fiber but specified "high quality isopthalic resin" for the matrix. Garan is for epoxy.