BY Tom Pawlak Download PDF version of this article
By Tom Pawlak
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.
Plastic’s bonding drawbacks
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.
A repair that works
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.
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.
Flame treating a plastic surface for bonding
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.
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