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Understanding how it controls print-through in
epoxy laminates can help builders who work
E
poxy resins
have many
wonderfu l
characteristics,
including good
elongation, excellent
toughness,
and resistance to
delamination. But,
although epoxies
are different from
styrenated resins
in many respects,
they share two important
"weaknesses":
they must be
fully cured to function
at their rated
performance levels,
and, if an epoxy
laminate is exposed
to elevated temperatures
before
the resin is completely
cured, the
reinforcing fabric will print-through
like mad.
One reason is that
the hardener-toresin
ratios of epoxies
are fixed, not
adjustable as with
polyesters. As a
result, the nonstyrenated
resins
are much more sensitive
to temperature—especially
chilly
shop air,
which can really
retard the cure
cycle (even when
special hardeners
are used to compensate).
Post-curing,
however, can
solve this problem.
But there''s anothInside
the post-cure oven at
Gougeon Brothers, radiant electric heaters are
aimed at the inner walls—not at the laminate being cured—in order to create
a heat sink. An electric fan circulates the heated air through ductwork. The oven is
lined with about 12" of fiberglass insulation, as well as 1½" of drywall for fire safety.
Fortunately, controlled post-curing—
subjecting "green" laminates to a regulated
heating cycle for an extended
period—will push an epoxy''s cure state
further toward completion, making it
both stronger and more resistant to
print-through.
Even though print-through affects
both polyester and epoxy boats, postcuring
is more critical for epoxy parts
because they soften and print at lower
temperatures. While cured polyester
and vinyl ester resins get soft from
170°F to 225°F, cured epoxies soften
at between 130°F and 160°F, depending
on the formulation. Dark-colored
hulls and decks, especially when cored,
can easily heat up to more than 150°F,
meaning that most laminates, particularly
those involving epoxies, are close
to or beyond the thermal edge.
The softening and expansion of boatbuilding
resins (see accompanying
story on page 47) are directly connected
to print-through, and most
epoxy users I''ve talked to post-cure
primarily to improve part finish. (They
cite increases in resin strength and
toughness as a secondary benefit.) The
lessons they have learned about controlling
print-through are relevant to
all boatbuilders regardless of the type
of resin being used, because the same
mechanism causes the problem.
Who Is Post-Curing, and Why
We''ve established that a full cure is
critical to the quality of any laminate,
but why do many epoxy users achieve
it through post-curing, while most users
of polyesters and vinyl esters do not?
er reason why postcuring
is enjoying
an increasingly
strong following,
and, again, it involves temperature.
Many modern epoxy boats incorporate
high-performance reinforcements like
carbon and Kevlar instead of E-glass.
That means thinner skins and, usually,
vacuum bagging, which reduces
resin content. The end result is a less
bulky, "drier" laminate with a lower
exotherm cycle. In combination, these
factors can aggravate undercure.
Notes Mark Lindsay at Lindsay
Boatworks (Gloucester, Massachusetts),
"We began post-curing roomtemperature-cure
[RTC] laminates in the
late 70s, using primitive ''tents'' and a
variety of heat sources. Many of our
laminates were very thin, and fully curing
them at room temperature was
simply too slow and inefficient. A
vacuum-bagged .075" skin that was
laid up without much resin would
remain gummy for days unless it was
post-cured."
Like Lindsay, many of the builders
DECEMBER/JANUARY 1992 45
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