Hygrothermal aging behavior and mechanism of multi-filler reinforced epoxy composites for steel structure coatings

The long-term hygrothermal resistance of epoxy-based composites plays a key role as an anti-corrosion and anti-wear coating for steel structures when exposed to complex service environments. The hygrothermal aging will cause an irreversible damage to the service performances of composites. In the present paper, the multi-fillers reinforced epoxy composites (MFREC) had been successfully prepared and have been given higher expectations for steel coatings. Its hygrothermal aging behavior was experimentally investigated through the immersion in distilled water (DW) and saline water (SW) at 20, 40, and 60 degrees C as long as 120 days. The moisture uptake, thermal properties, mechanical properties and microstructure analysis were tested to evaluate the long-term hygrothermal evolution. The research results showed that the quasi-equilibrium moisture uptake content (3.72%) of MFREC in SW was lower than that (4.14%) in DW owing to the preferential permeation and occupation effects of small ions (Na+, Cl-). Hygrothermal exposure led to a maximum degradation of tensile strength, elongation at break and glass transition temperature of MFREC, up to 38.2%, 46.6% and 20.8% in DW, and 34.5%, 40.8% and 18.4% in SW, respectively. This was because the epoxy matrix had generated relaxation and reinforced-fillers/epoxy interfaces had been destroyed, which can be verified by hydrogen bond increasing (maximum 75.2%) from infrared test, fiber pulling out from epoxy matrix (morphology analysis), elastic modulus decreasing (maximum 37.8%) from micro-hardness measurement and pore volume increasing (maximum 53.9%) from N2 adsorption test compared to unaged samples. Based on the Arrhenius theory, the long-term life prediction of MFREC tensile strength under three typical bridge service environments was conducted to evaluate the service time as coatings. It can be found that the stable strength retentions were 63.19% and 66.03% in the simulated pure water and marine environments, respectively. Furthermore, an additional strength degradation percentage up to 11.5% can be found for MFREC exposed to steel bridge coatings in southern as long as 3 years compared to the northern environment.