Epoxy versus vinyl ester

There's a 100+ year old German built steel ketch on a mooring in the bay here and most of the steel is pristine and original thickness, only the plating and framing in the wet bilge areas has been replaced. There's no forseable reason why that vessel won't make another one or two hundred years.


Some of the class societies have no extra thickness for corrosion allowance for sailboats. Others only add .3 of a mm.



No Ones mentioned normal aging effects of polyester and vinylesters GRP yet. There's a significant loss of strength of GRP through time, a big part of that property change is that the matrix creeps.

 

Are you sure that's steel Mike and not iron.

 

Yes Paul, I tested it. The owner had some of the hull that had been removed when the bilge was patched. There are very few true black iron boats around. Iron was a bit soft and after around 1900 shipbuilding was nearly all carbon steel.

 

And FRP, like concrete, gets better with age. the tensile strength goes up. Not much something like 5,990 psi to 6,140 in 20 years.

And I forgot to add, after the 30% loss in 25 years, it levels off. No more degradation.

 

Unfortunately thats only the positive side of the coin, as you should know!

The negative has more weight.

 

Quite right Richard. FRP is worst in terms of fatigue resistance. However, graphite/epoxy outperforms steel and aluminum.

With poly resin, ortho is the worst. Iso outperforms ortho and vynilester is tops among the three.

 

I know, you know!

The graphite / ep will have to stand the test of time first, before I buy that. My steel and wooden Museums ships did that already.

 

You may not buy it. I did not carry out the test. I am just stating published data by Hercules and Marine composites so that I can share.

 

I know what you are saying !! I had a suspicion vinylester was pretty good but this sort of confirms that .but would like to know more !!

 

The composites were talking about here have an increase in shear compressive and bending failure stress levels after an increase in time due specifically to increased curing. But tensile? And be careful tensile strength can be a very misleading figure.

I'd be interested in how that could occur. Since if there are inline fibers (which you'd expect for tensile stress testing) the tension is taken by the fibers not by the matrix and the modulus of elasticity of the two is quite dissimilar.
But with all materials it's fatigue strength and with composites aging effects (including visco elastic effects) that have to be considered in the Factor of safety.

As I mentioned before another important issue is stress relaxation or creep, this property sees a loss in tensile strength over time if the composite is permanently stressed. And the loss is significant.

For example for CSM Polyester FRP the loss in flexural strength over only 1000 hours with constant load is abysmal, at saturation in water and at 25 degrees C it loses nearly 3/4 of its strength. That's how important creep is. Even in air at 25 degrees it loses around 40% of its strength. CSM is the best example of matrix creep, and it's mitigated of course by fibre orientation, but you cant orient fibres very well for shear stresses.

For comparison creep of 'dry laminate'' glass epoxy shows around a 30% drop in flexural strength over 5 years and carbon epoxy around 15%.


I've quite a lot of material on this but it's a heavy mathematical engineering text so there's not many pretty graphs to pull out and post because there are so many variables. Which is why I distrust simple suggestions of increase in tensile over 20 years. I'd like to see the study parameters.

Carbon fibre epoxy in good lab conditions with a professional layup has a fatigue limit around 20 % better than A36 mild steel. But some alloy steels beat it for strength lightness and fatigue resistance ! They are just even more expensive and when thin they are prone to buckling.

 

When the vinylesters first appearred, they all required elevated temperature curing (ETC). When the relative merits of ep, ve, poly were compared on a graph, the vinylester was only slightly better than the iso poly when the vinylester was NOT ETC'ed. When it was ETC'ed, the vinylester was almost as good as the epoxy in many respects (testing and graphs by Systems Three). However, none of the builders I've spoken with recently do any sort of temp control. Does anyone know - has the formulation changed so that ETC is no longer required? Or are the quoted properties just salesman BS, as usual?

 

Being used with temp. control on our Wood epoxy boats, we DO in fact post cure our VE hulls and structures, to achieve the ultimate ratios.
But we are moving to prepregs in the near future. For us, VE was not the winner.

Regards
Richard

 

Thanks Richard. I will assume, then, that vinylester still does not reach its published specs without the post cure. In this neighborhood nobody post cures and they all think they're acheiving something great by using the ve.
I'm with you - not the winner. Here in the subtropics the short shelf life was a problem. Switched to exclusively epoxy 25 years ago and never looked back.

 

Mike,
The tension-tension fatigue graph was presented in SAMPE 2004 and reprinted in Marine Composites by Eric Green. It is accredited to a study made by Hercules. I don't have study parameters. Maybe other member of the forum can provide the link.

The physical property data for the 20 year test was done by USCG and Owens-Corning Fiberglass.

Rx

 

Wise decision,

especially in the light of marketing value against resin cost!

VE does reach the published spec.s, of course.
Two, amongst others, are bad weathering, and bad creeping values.
Lets face it, it is a ....esther member, with all the well known disadvantages.

I, personally, got it too late, that a well executed and designed, carbon prepreg structure is cheaper to execute than even a chopped strand rubbish.

Given the product has the same structural strength. (leave the weight out here)

Well, we pay for our skills, no doubt.

If it is not wood epoxy (still one of the best solutions, up to 30 meter imho), it should be straight high tech*. Carbon prepreg, ep film, bought in stringer and engine bedding, and cooked in one go.

There was a nice series of articles in Pro Boat, a while ago. I learned a lot! Especially about my own bias.
Result was, either dirt cheap and crappy glass ..esther, or high tech carbon ep. I vote for the latter.

Regards
Richard
* Wood Epoxy layups can be a rather high, "high tec. " job, when done as I do! That is not well known in the US unfortunately.

But