The Gougeon Brothers on Boat Construction - WEST SYSTEM Epoxy

22 Fundamentals of Wood/Epoxy Composite Boatbuilding
contact with air, with WEST SYSTEM epoxy and use
it as a bonding adhesive in all joints and laminates.
When applied in correct quantities, the epoxy forms
a thin continuous film, which serves as a moisture
barrier on exterior surfaces, in joints, and between
layers of veneer.
While this barrier is not perfect, it is far more realistic
than wrapping a boat in plastic. Moisture passage into
wood is limited to such an extent that any change in
moisture content within the wood itself is minimal. If
dry wood encapsulated in our epoxy is put in a high
humidity chamber and left for months, the wood’s
moisture content will eventually rise. <strong>The</strong> rate of
moisture change in wood/epoxy composites is so slow
under normal circumstances, however, that wood
remains at virtually constant levels in exact equilibrium
with average annual humidity. In most areas, this
equilibrium is between 8% and 12% moisture content.
A minimum of two unsanded coats of WEST SYSTEM
epoxy on all surfaces will form an adequate moisture
barrier. More coatings are desirable, especially if the
boat will be subjected to extremes between dry and
moist environments. Coating effectiveness reaches a
point of diminishing returns at five or six coats, where
improved moisture resistance does not balance the
weight of the extra epoxy. Any more than six coats of
epoxy may contribute to abrasion resistance but will
add little to the moisture barrier.
Large amounts of epoxy are used in coating and bonding
boats. Although ratios vary, depending primarily on
hull size, it’s not uncommon to find that 25% of the
weight of the basic hull structure is epoxy. About 20%
is more typical. Wood performance dominates, but
because so much of a boat is epoxy, the physical properties
of the epoxy have a large effect on the physical
properties of the composite. For this reason, it’s important
that the epoxy portion of the composite contribute
more than weight.
A large range of physical properties can be developed
within present epoxy technology. Very flexible resins,
with good resistance to a single impact, can be formulated,
but they otherwise might contribute little to a
hull’s overall structure in their ability to pay for their
own weight in added stiffness and strength. Flexible
resins also have high strain rates, which can limit longterm
fatigue performance at higher load levels.
WEST SYSTEM epoxy is specifically formulated to
develop maximum physical properties while retaining
just enough toughness and flexibility to live with the
slight deformations which inevitably occur in any boat.
<strong>The</strong> most significant driving force in developing our
WEST SYSTEM epoxy formulation has been long-term
fatigue resistance. Wood is one of the most fatigue
resistant materials on earth. Any bonding adhesive used
to make joints or laminates must be of equally high
capability or the wood’s long-term performance will
be compromised. We have conducted a great deal
of fatigue testing on WEST SYSTEM epoxy and have
achieved performance at 10 million cycles, which is
approximately 40% of ultimate in torsional shear. (See
Appendix C.)
Determining Moisture Content
When using wood in composite with WEST SYSTEM
epoxy, it’s important to use only wood which has a
moisture content of 12% or less; 8% to 12% moisture
content is ideal. Boats can be built with wood with
moisture levels up to 18%, but in long-term equilibrium,
they may lose some moisture, which could affect
structural integrity because of internal stressing from
shrinkage.
As we explained earlier, the moisture content of wood
varies according to the relative humidity and temperature
of the atmosphere which surrounds it. For every
combination of temperature and humidity, there is a
moisture level that wood will seek, absorbing and
dispelling moisture until it reaches this equilibrium.
We keep our shop at 65°F (18°C), with relative
humidity averaging around 50%. In these conditions,
the moisture level equilibrium is 8%, and most of the
wood we work with has a moisture content near that
percentage.
Atmospheric conditions vary according to geography
and type of shelter. In most situations, the prevailing
conditions will be such that the equilibrium percentage
is below 12%. Figure 3-11 shows the equilibrium