Environmental durability of moisture-cured urethane adhesive joints

Hot-dipped galvanized steel was bonded to fiber-reinforced unsaturated polyester (FRP) with a moisture cure urethane. Single lap shear joints were made with bondline thicknesses of 0.76 mm and bond areas of 25.4 mm×25.4 mm. Single lap shear joints were aged by cataplasma, cyclic moisture, QUV®, Weather-Ometer® and an oven at 60°C. Cataplasma aging and cyclic moisture aging resulted in a 23 and a 28% drop, respectively, in lap shear strength of joints made with acetone-wiped surfaces. QUV® and oven aged joints resulted in increased lap shear strength, and oven aged joints changed in failure mode from adhesive at the galvanized steel interface to cohesive failure. Weather-Ometer aged joints failed adhesively at the FRP interface, but after closer inspection by environmental scanning electron microscope (ESEM) adhesive could be seen on the surface of the FRP. QUV® light may not always penetrate the adherend materials even though natural sunlight penetrates the same materials. When conducting accelerated aging of adhesive joints made with FRP adherends that allow light to penetrate to the joint, workers must carefully consider the light source and adherend material used in the design of accelerated aging experiments.