Polyester resin laminates achieve their highest strength when the bonds between layers of fiberglass are chemical or primary bonds as opposed to mechanical or secondary bonds. The manufacturing process for polyester boats has been developed so that a chemical bond exists between the gelcoat and the laminate. When a polyester boat is built, polyester gelcoat is first sprayed onto the mold surface. The first layer of the laminate is then applied to gelcoat, which has not completely cured. The two layers eventually cure together with a chemical bond between them.
The test program we developed was designed to determine the effect of moisture on the adhesion of polyester gelcoat applied over fully cured epoxy. The tests were done on a gelcoat finish over a simulated repair made with epoxy. The test included a variety of finishing processes. In addition, we varied the length of time the epoxy was allowed to cure before being subjected to high moisture conditions. For comparison, we also tested gelcoat adhesion to a polyester laminate using West Marine Boaters Resin™.
The test panels were laminated from three layers of 738 Biaxial Fabric (15 oz biaxial with .8 oz mat) wet out with 105 Resin and 205 Hardener. Panels were covered with 879 Release Fabric and allowed to cure. This laminate provided a stiff panel that simulated a patch used to repair the bottom of a fiberglass boat. After the Release Fabric was removed, a variety of finishing processes were applied to the panels. These included using 742 Glass Fabric (6 oz), 407 Low-Density Filler, and 410 Microlight™. All of the test panels had a sealer coat of 105 Resin/206 Slow Hardener to fill the weave and seal the scratches in the sanded fairing compounds. Slow Hardener was chosen because its slower reaction time would be more likely than 205 Fast Hardener to reveal any adhesion problems.
The overall effect of moisture on the gelcoat's bond to both the polyester and epoxy panels was negligible. The graphs show very little change in the adhesive strength, and much of the variance is within the tolerance of the test method. When the studs were pulled, the failures occurred in the fairing compound, cohesive failure of the gelcoat, or in the bond between the gelcoat and laminate. Since these three modes of failure occurred under similar loads, it indicates that the strength of the bond is close to the cohesive strength of both the gelcoat and the fairing compound. The nine-week exposure may not simulate what could happen after many years, but it does indicate that the gelcoat bond to epoxy laminates is a good bond and should perform well.