Epoxy bests polyester in boat repair trial

West System shows that polyester boat hulls can be best repaired with epoxy-based systems.

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Recent tests conducted at West System Inc. (Bay City, Mich.), a supplier of resins, hardeners, fillers, additives, reinforcements, application tools and instruction for boat repair, supported the company’s contention that today’s glass/polyester composite boat bulls can be repaired most effectively with epoxy-based repair systems.

Because the chopped strand glass mat used in early boats (30+ years ago) isn’t as strong as woven and stitched fabrics, their hulls were thick and stiff, so a repair technician could grind away the damage, sand a taper around the perimeter and apply an effective glass/polyester patch. But West System technical staffer Tom Pawlak points out that improved polyester resin chemistries make it possible to fabricate today’s hulls thinner, lighter and more flexible (impact-resistant) but also makes them more difficult to repair. Modern polyesters crosslink more completely during cure, he contends, resulting in fewer available sites for primary bonds with a polyester patch. Also, polyester is typically “air-inhibited” (it remains uncured or tacky when a thin film is applied on a surface exposed to air), making it less than desirable as an adhesive. In contrast, Pawlak says epoxies are able to form superior secondary bonds and readily adhere to properly prepared, cured polyester, without volatile emissions. Epoxy resists microcracking better than polyester and has excellent thin film characteristics, is more flexible with higher tensile elongation. Further, polyesters must be cured at temperatures above 60°F/16°C, but hardeners are available that make it possible to cure epoxy repairs at temperatures as low as 40°F/4°C. Pawlak also notes that polyester resin shrinkage, while fairly insignificant in a small repair, can become a factor in large repair areas, because the shrinkage subjects the bond line to stress before the repair has a chance to experience working loads.

Pawlak and his group backed these contentions with the results of testing that pitted West System epoxy against dicyclopentadiene (DCPD) polyester resin. Trials were conducted on large, well-cured fiberglass/DCPD laminate coupons made with triaxial fiberglass and glass mat, with G-10 fiberglass laminate tabs added to each end to provide a grip area for the test machine. Coupons were ground to a 12:1 bevel along one edge, sanded with 80-grit sandpaper, then repaired with multiple plies of fiberglass and various repair resins. The tensile breaking strength of the repaired laminates using the same DCPD polyester resin and fiberglass material averaged 18,460 psi or 70.5 percent of original strength (determined by testing unrepaired coupons). The breaking strength of the repaired laminates using West System epoxy and the same fiberglass material averaged 21,404 psi or 81.7 percent of the original laminate strength. Pawlak points out that all repairs were done using the minimum 12:1 bevel, but longer bevel angles at 15:1 or 20:1 would yield higher repair strengths.