Why Epoxy Resin, not Polyester?
Shown above: the 'resin master', the Gougeon/Pro-Set epoxy gear pump, which meters precise ratios of resin and hardener at will.
The Gougeon brothers pioneered the use of epoxy resin as the adhesive/laminating resin in performance sailboats and powerboats, over 40 years ago. I am old enough that I can remember the revolution that these three Michiganders caused in the boatbuilding industry, it was a tsunami of epic proportions.
We use only Gougeon epoxy in the construction of the I-20, not vinyl ester resin, or polyester resin. Here is a link to the Gougeon website; the West System link is the 'consumer' epoxy line, and the Pro-Set link is the 'professional' line. We use the Pro-Set line:
Why epoxy? Two reasons. One, epoxy is much stronger and more resilient than polyester resin. Two, well, polyester resin just sucks. Polyester resin has one thing going for it: it is cheap. The offgassing of VOCs (Volatile Organic Compounds), styrene, and the like with polyester resin is hazardous to your health. Here is the Wikipedia take on epoxy, in a comparison with polyester resin:
While it is common to associate polyester resins and epoxy resins, their properties are sufficiently different that they are properly treated as distinct materials. Polyester resins are typically low strength unless used with a reinforcing material like glass fibre, are relatively brittle unless reinforced, and have low adhesion. Epoxies, by contrast, are inherently strong, somewhat flexible and have excellent adhesion. However, polyester resins are much cheaper.
Epoxy resins typically require a precise mix of two components which form a third chemical. Depending on the properties required, the ratio may be anything from 1:1 or over 10:1, but in every case they must be mixed exactly. The final product is then a precise thermo-setting plastic. Until they are mixed the two elements are relatively inert, although the 'hardeners' tend to be more chemically active and should be protected from the atmosphere and moisture. The rate of the reaction can be changed by using different hardeners, which may change the nature of the final product, or by controlling the temperature.
By contrast, polyester resins are usually made available in a 'promoted' form, such that the progress of previously-mixed resins from liquid to solid is already underway, albeit very slowly. The only variable available to the user is to change the rate of this process using a catalyst, often Methyl-Ethyl-Ketone-Peroxide (MEKP), which is very toxic. The presence of the catalyst in the final product actually detracts from the desirable properties, so that small amounts of catalyst are preferable, so long as the hardening proceeds at an acceptable pace. The rate of cure of polyesters can therefore be controlled both by the amount of catalyst and by the temperature.
As adhesives, epoxies bond in three ways: a) Mechanically, because the bonding surfaces are roughened; b) By proximity, because the cured resins are physically so close to the bonding surfaces that they are hard to separate; c) Ionically, because the epoxy resins form ionic bonds at an atomic level with the bonding surfaces. This last is substantially the strongest of the three. By contrast, polyester resins can only bond using the first two of these, which greatly reduces their utility as adhesives and in marine repair.
More required reading:
The jury is still out as to whether or not styrene, a principal component of polyester and vinylester resins, is carcinogenic. The unpleasantness of the offgassing of styrene in polyester layup is enough to dictate use of epoxy in any laminate.
There are solid engineering reasons why the new Boeing 787 Dreamliner is made of a reinforced epoxy laminate. Your new boat should be, as well.
If you need more convincing than that, give us a call.