Molded
plastic canoes and kayaks are incredibly tough and durable. Occasionally though
people damage them and call us for repair recommendations. Considering that
plastic film is often used as a mold release for epoxy and cured epoxy easily
pops out of our reusable plastic mixing cups, you can see what we're up against
when we try to bond to it.
The
most common plastics used for molded boats and other toys are low surface energy
plastics such as polyethylene and polypropylene. Characteristics that make them
desirable for the manufacture of small boats also make them difficult to bond
to.
Low
surface energy plastics are not able to overcome the surface tension of
adhesives and therefore inhibit the spreading and wetting of the adhesive on the
surface. These plastics are not porous and provide little opportunity for a
mechanical bond. They are unreactive, or chemical resistant, inhibiting a
chemical bond. Also, low molecular weight molecules tend to migrate to the
surface of these plastics. Molecules with low molecular weight have low tensile
strength, resulting in a weak boundary layer at the surface.
The
repair method I’m about to describe overcomes some of these drawbacks and
produces a functional repair. However, it will be fairly obvious that the boat
has been repaired. In other words, it may not be pretty. Surface preparation,
bonding area, and fiberglass preparation and placement are key to the success of
this repair.
1.
Begin the repair by forcing the damaged section of hull back to its
original shape
2.
Clean the area with solvent and paper towels well beyond the break.
Solvents will remove oil, wax or other contaminants that would inhibit bonding.
3.
Sand an area several inches beyond the break in all directions with 60 or
80 grit sand paper. Scratch the surface aggressively, making certain the entire
repair surface has been abraded with no shiny places left. This step helps to
overcome the plastic’s low porosity.
4.
Flame treat the surface. (See Flame
treating a plastic surface for bonding below.) Flame treating will burn off
contaminants and weak boundary layers and produce surface oxidization. Oxidation
increases the surface energy allowing better wetout and makes the surface more
reactive for potential chemical bonding.
5.
Apply a coat of clear epoxy to the surface within an hour or two of the
flame treatment. The best bonds are formed soon after flame treatment.
6.
Cut the fiberglass patches, making certain each layer is staggered in
size from any other layer on either side of the repair. Cut the largest
fiberglass patch to extend several inches beyond the actual break in the hull
and at least two inches beyond the next smaller fiberglass layer. (See the comments on fiberglass preparation below.) It usually
isn’t necessary to bevel the repair area.
7.
Position the largest fiberglass patch into the wet epoxy and apply more
epoxy to thoroughly wet out each layer prior to positioning each additional
layer. Allow this repair to cure before repeating the process on the other side.
8.
Repeat steps 2–7 on the backside of the hull. Stagger the ends of
fiberglass patches on each side of the repair so they don’t end in the same
place as layers on the opposite side. This allows the repair laminate to bend
more uniformly when it is stressed.
9.
Sand the fiberglass to smooth out the edges of the fiberglass steps.
10.
Fair the stepped layers of glass with an application of epoxy thickened
with a fairing filler if you wish to dress up the repair prior to painting. Sand
the cured fairing compound and apply 2 sealer coats of epoxy to the area and
allow to cure.
11.
Wet sand with 220-grit paper to prepare the surface for paint. Apply an
automotive or other matching paint to finish the repair.
The
center of repaired sections will be stiffer than the rest of the hull, but the
edges of the repair will flex and stay attached under considerable abuse. How
much abuse it will handle will depend on the surface preparation, how far the
first layer of fiberglass is applied beyond the break, what weight fiberglass
was used and how uniformly the repair layers have been staggered.
To
flame treat a plastic surface, hold a propane torch flame about 4" to
6" from the plastic (with the tip of the flame just above the surface) and
move it across the surface at a rate of 2 or 3 inches per second overlapping the
previous pass slightly. Keep the torch moving and only allow the exhaust gases
to hit the surface. If done correctly, the surface will not discolor or burn in
any obvious way. This technique oxidizes the surface and improves adhesion. For
best adhesion, bond to the surface within 30 minutes of treatment.
Fiberglass
preparation
Use
lightweight fiberglass fabric for the repair. One reason this repair works is
because a single layer of 2 to 4 oz fiberglass and epoxy is flexible enough to
bend to a fairly tight radius when cured. With the fiberglass stepped down to a
single layer at the perimeter, the outer edges of the repair should be as
flexible and bendable as the hull itself. This flexibility is helpful if someone
severely dents the hull alongside the repair. It will reduce the likelihood the
repair will peel or debond from the hull.
If
the break in your hull is only a split or slight tear, three layers of 4 oz
fiberglass or six layers of 2 oz fiberglass are usually adequate for this type
of repair. Multiple layers of light fiberglass fabric are best. Two layers of 4
oz fiberglass (or four layers of 2 oz) are applied to the inside of the hull and
one layer of 4 oz (or two layers of 2 oz) are applied to the outside of the
break to complete a repair for canoes and kayaks. More layers are used on the
inside because higher tensile loads are expected on the inside if there is an
impact on the outside.
A good mail order source for
light fiberglass fabrics ranging from .5 oz to 10 oz and other composite
materials is: Composite Structures Technology, PO Box 622, 16330 Harris Rd.,
Mountain Valley Airport #2, Tehachapi, CA 93581-0622, Fax: 661-822-4121 www.cstsales.com.
Epoxyworks
16 / Fall 2000
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reserved. This page is maintained by Gougeon Brothers, Inc. Last Modified on Tuesday, May 08, 2001. WEST SYSTEM and EPOXYWORKS are registered trademarks of Gougeon Brothers, Inc. Bay City, Michigan, USA. |