OSMOTIC BLISTER REPAIR
THE PROBLEM
If boat hulls spent most of their lives in air
rather than water, the Water Soluble Materials (WSM), which composes
them, would be saved from the blight of blisters. However, the majority
of vessels are simply not adaptable to high and dry storage facilities.
So, understanding how water permeates a hull is crucial to grasping the
osmotic process.
Hydrolysis is the ability of water to break down and alter the
chemical bonds of polyester resins. It is through permeation that water
comes into contact with WSMs suspended in the laminate layers.
Professional Boat Builders Magazine in its February 1992
issue lists several known WSMs: MEKP, Silica compounds, PVA, styrene,
and the binders in chopped strand matt, just to name a few. It was
further noted that a reaction of the water and WSMs left behind a
corrosive material, which created ?fiber whiting?. Fiber whiting occurs
when resin surrounding the fiber has been hydrolyzed or has totally
dissipated. In many older vessels you may only see crazing or cracking
of the gelcote. You may never see a blister. In this scenario, the gases
created by water reacting with WSMs simply escape through the lesions in
the gelcote.
A blister forms when the permeation of water
into the laminate exceeds the flow of hydrolyzed fluid trying to escape.
The solution, now increased in molecular size, cannot readily pass
through the laminate structure. The WSMs are hydroscopic and this
absorbing capability attracts more fluid into the same void. It is the
continuous attraction of the additional WSMs and their chemical reaction
in a limited space, which creates the blister. This usually occurs more
rapidly in gelcote with a tight molecular structure. The accumulation of
a large number of blisters will, in turn, create a serious hull
delamination. The degree of structural damage will increase as the
blisters fracture and penetrate deeper into the layers of the hull.
THE SOLUTION
Osprey Marine Composites, Inc. always begins
the blister repair process with a laminate profile. The profile consists
of four steps. First, high-speed grinders remove a sample of material at
selected sites. Second, depth calipers record the thickness of each
layer that is removed. Third, a moisture meter determines the amount of
moisture in each layer. Fourth, a Barcol Impressor determines the amount
of resin deterioration. This information determines the repair
procedure, i.e. how much material has been damaged and must be removed
and replaced with new material.
The next step is to remove the damaged material. The hydraulic peeler
is set to the appropriate depth, and the damaged material is removed
evenly over the entire hull. A hard sanding of the hull follows this
procedure.
The next phase of the repair is to dry the hull. It is our opinion
that the vessel must be dried to a reading of less than 10 on an Electro
physics moisture meter. With a low reading, only a minimal amount of
moisture is trapped in the hull and a higher degree of secondary bonding
between the original resin and the new material will occur. Some hulls
dry very quickly after the peeling process. Others may be force dried
with propane heat alternated with moving air.
If more than 80 mils of material had to be removed, the vessel will
usually require a new laminate. OMC uses Knytex 1808dbm structural
fabric for all re-laminations. This is a state of the art boatbuilding
fabric which adds strength to the hull. Unlike chop strand matt, this
fabric contains no WSMs. If less than 80 mils of material is removed,
usually a barrier coating is all that is required. In either case, the
hull is faired to original shape before the barrier coating is
applied.
FAIRING AFTER
LAMINATE
OMC uses a Dow Chemical vinyl ester resin for
both re-laminations and barrier coatings. This is a top of the line
vinyl ester resin which is considerably more expensive than commercial
v.e. resin, and has excellent moisture barrier properties.
All OMC warranty barriers are applied to a thickness of 50 mils of
vinyl ester resin. OMC gives a three year warranty to boats with barrier
coat only, and a five year warranty with re-laminations. One coat of
anti-fouling paint is included with both repair
packages.
