Why Epoxy is Preferable to Vinylester and both are Preferable to Polyester.

There are many good reasons to use epoxy instead of polyester.

First, stretch to failure. Glass fabric has about 6% stretch to failure. So does most room-temperature epoxy, and many vinylesters. Polyester stretch to failure is, everyone? About 1%. So in tension, the glass is only loaded to about 17% of its strength before the resin matrix starts to break apart and become a necklace.

Polyester will be bonding with water throughout its life, and it will gain surprising weight from that water, while losing strength properties. If I may quote from a D570 water weight gain test, "The orthophathlic casting had more than a 2.5% weight gain after 4 days, then showed a weight loss on the 7th and 14th days. That means the polymer is being broken down, solubilized, leached out of the composite, and replaced with water."

The biggest benefit of epoxy is that epoxy is more forgiving during construction. Get the mix ratio right and you are there. With polyester, or even vinylester, one has to vary the MEKP level and vary the N,N-DMA level, oppositely, as a function of temperature change. Or, also, vary the BPO level and again vary the N,N-DMA level oppositely, as a function of temperature. The catylist/promotors have to be done exactly right to get a good degree of cure. The catylist/promoters will have amounts of down to fractions of a percent. Those amounts must be very precise if good results are intended.

Also, if the part is stored at less than 10C it may never cure fully, even with a later post cure. Using epoxy, you can improve the laminate properties with a post cure, almost always.

Interplastic Corp. has some great papers on this, which I am quoting from. Specifically, Proper Cure of Vinyl Ester Resins and A 15-Year Study of the Effective Use of Permeation Barriers in Marine Composites to Prevent Corrosion and Blistering.

I've never been able to afford a temperature controlled shop, so this

matters a great deal to me.

Until someone is using a Barcol hardness tool or even better, a D790

flexural strength test to prove it, I don't want to hear how a laminate is acceptable with any or all the above conditions off, just because the layup looks good.

The point is if the temperature was too cold when you did an

epoxy laminate, you could bring it up to close to 100% cure later.

If you, for example, do a polyester laminate at 15C with 1% BPO and 0.3% N,N-DMA, you will have only an 80% cure and it probably cannot ever be improved.

That same formulation however will give 96% cure at 25C.

Most room temperature cure epoxies post-cure at around 65C to 70C. Static properties increase, but so does toughness against impacts.

One of the best reasons to post-cure is that you get to keep your paint job smooth. Ideally every bit of composite structure and fairing bog should be post-cured. Post-curing parts before the final fairing can be a problem later on.

Heat will treat laminates, and fairing, kind of like a muffin in the oven. Post cure gets the muffin to rise fully, as it were. Then, only after post cure, you prime and paint, so it won't rise later if it gets heated. I recall that the F40 trimaran Scissors was white most of its career. When it was painted red for the advertisement filming in the tropics, the surface then solar post cured and it looked like a big, red waffle. They apparently used contour core and lots of bog in between the squares. It was literally a red waffle.

Blisters are also a huge issue. With polyester, especially with orthophthalic resin, as Terry McCabe of Interplastic said, "Its not if it will form blisters, but when." The useful paper is "A 15 Year Study of the Effective Use of Permeation Barriers in Marine Composites to Prevent Corrosion and Blistering". Again, Interplastic.

Shelf life is another issue. The useful shelf life of epoxy resin is years. Hardner has a shorter shelf life, but it is still good for years. Both vinylester and polyester components have a shelf live of just months.

I have always wondered why Kelsall with his KSS flat panel construction prefers polyester to epoxy, in the face of all the evidence against polyester. Especially when the KSS panels are being bent around one way or the other. With only about 1% give in the resin, the polyester panels will microfissure quickly compared to epoxy ones. Then I remembered that as far as I know, there still isn’t a good epoxy gelcoat. I wonder if that’s the reason. Gelcoat. In my opinion, a painted part will be a far lighter part and worth the time spent smoothing the part.

In many parts of the US, hand lay-up polyester is illegal due to the VOCs (fumes). In those areas, KSS could only be done with infusion and trapping equipment. Epoxy does not emit VOCs.

In conclusion, both epoxy and vinylester are much preferred to polyester. Structurally, epoxy and vinylester are close in properties. Epoxy however is much easier to work with, and is much more forgiving.