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LARRY
ROBINSON ON EPOXY
In
an email with Larry, I said I had heard that epoxy can 'melt'. This
started a discussion, which follows, where Larry said:
Yes, some epoxies will soften with heat. It becomes less of a problem over
time as the material ages. It does not melt, or should not, if the proper
resin ratio is used. If the ratio specified in the instructions is not
followed, un-reacted resin or hardener will remain after cure, and the
product will remain soft, or even gooey. Different epoxy resins use
different ratios. Nonetheless, conventional epoxies are generally
softer than polyester resins.
For this reason, special boat building epoxies have been developed. But
why use epoxy in the first place? Advantages: it has greater impact
resistance and it does not smell up the house - - very important if you
are married. :-)
I should mention that it's difficult to apply an epoxy gel coat to a boat
that is made from a mold. Spraying epoxy is difficult to do safely. With
time many people become highly allergic to the stuff, and cannot tolerate
even the slightest exposure.
Epoxy choices: The common WEST System epoxies are fairly soft. System
Three (Seattle, Washington) makes conventional epoxies, but the best
choice for boat building is their 'Phase Two' epoxy. In the finished
state, it actually consists of two phases at the microscopic level that
gives it the desirable properties.
After conventional cure overnight the material is very stiff, but
extremely brittle. While still in the mold, it is 'post cured' it at about
140 deg. F for several hours. It's extra work, but the resulting product
is very stiff but with a very high resistance to impact. Impact resistance
is probably not that much of an issue for the IOM, but it certainly is for
the EC-12. Many EC-12s made of polyester have been holed by the bow of
another boat. Eighteen pounds of lead has a lot of energy when moving! Tom
Jordin at one time made EC-12 hulls that were considered the gold standard
of the fleet, using System Three's Phase Two epoxy. It's been quite a
number of years now, but to my knowledge, none of his hulls ever suffered
a fracture, even after severe abuse. Frankly, this is probably overkill
for an IOM, but it would solve the problem of heat softening down in
Florida if an epoxy hull is desired and conventional epoxy has not proven
satisfactory.
Another choice is vinyl ester. It's like polyester, but a little less
brittle to my understanding.
Well, this covered epoxy, but I had also been
warned about polyester being soft and also liable to 'melt'. Larry had
this to say:
I have not heard of this problem, but polyester is a little soft when
green, and becomes increasingly brittle with age. That's why older boats
are more prone to punctures. The IOM bow bumper largely eliminates this
risk I would think. Nonetheless, I would not want to store a brand new
polyester hull (or conventional epoxy hull for that matter) in a car in
the hot Florida sun. This is a good way to wreck sails too, as many have
sadly found out.
Although less sensitive to improper ratios than epoxy, it is possible that
inappropriate catalyst use could have lead to reports of cured polyester
'melting'. If the polyester resin is grossly under-catalyzed in relation
to the ambient temperature and lay-up thickness, and it is not well mixed
with the catalyst, it may never harden properly. However, if the resin is
not outdated, even if it is significantly under-catalyzed, given enough
time (perhaps months) it will harden, and the product will be useable.
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