Thermal/mechanical properties of incompletely cured carbon fiber/vinyl ester composites after hydrothermal aging

Carbon fiber/vinyl ester resin composite (CF/VE) was gradually applied to large-thickness and large-scale ship structural parts due to its light weight, high strength and corrosion resistance. Due to the limitation of size and manufacturing equipment, the structural parts manufactured in the actual environment were not post-cured, and the resin matrix in the composite structure was often incompletely cured. The durability of incompletely cured vinyl ester resin (VER) and its carbon fiber composites in hydrothermal environment were studied. The surface morphology change and hydrolysis mechanism of the resin were analyzed. The dynamic thermodynamic properties of VER and CF/VE and the mechanical properties at high temperature were studied.

Liquid chromatography-mass spectrometry (LC-MS) was used to qualitatively analyze the resin leaching solution. Fourier transform infrared spectroscopy (FTIR) was used to analyze the structure of the resin surface before and after hydrothermal aging. The mass changes of VER and CF/VE were recorded by weighing method. The storage modulus and glass transition temperature () of the samples after different aging time were measured by dynamic thermomechanical analyzer (DMA). The in-plane thermal expansion coefficient of the samples was measured by electronic thermal dilatometer. The mechanical properties of the samples at different aging time and temperature were tested by universal testing machine. The surface and fracture surface of the samples were analyzed by field emission scanning electron microscopy (SEM).

①The results of LC-MS and FTIR show that the ester bonds in the resin molecular chain are strongly hydrolyzed; dense micropores, pores and microcracks are formed on the surface of the resin. ②The resin reaches the maximum mass growth rate of 0.56% at 120 h of aging, and the CF/VE reaches the maximum mass growth rate of 0.26% at 240 h of aging, then both show a downward trend, the mass of CF/VE is lower than the initial value after aging for 4000 h. ③The results of DMA show that after hydrothermal aging for 1176 h, the of resin and CF/VE increases by 21.94°C and 25.88°C, respectively; and the storage modulus of both increases significantly. The difference of thermal expansion coefficient between the two is 74.66×10/k before aging, and the difference is further expanded to 77.61×10/k after aging for 1176 h. ④After hydrothermal aging for 1176 h, the compressive and interlaminar shear strength of incompletely cured CF/VE at room temperature decreases by 23.0% and 46.6%, respectively, the tensile strength decreases by only 0.8%, and the in-plane shear strength increases by 3.1%. After hydrothermal aging, the tensile and compressive strength of the resin at 70°C increase compared with that before aging; and the compressive and in-plane shear strength of CF/VE at 70℃ increase compared with that before aging. ⑤The tensile strength and compressive strength of the resin at 70°C are 47.9% and 57.7% lower than those at room temperature, respectively. Compared with room temperature, the compressive and in-plane/interlaminar shear strength of CF/VE at 70°C decreases by 26.4%, 36.7% and 27.7%, respectively.Conclusions: The thermal/mechanical properties of incompletely cured VER and CF/VE before and after hydrothermal aging are studied. The results show that during the hydrothermal aging process, due to the strong hydrolysis reaction and osmotic cracking of the resin from the surface to the inside, a large number of organic solutes migrate to the medium, resulting in a decrease in the mass growth rate of the material after reaching the maximum value. Due to the post-curing reaction of the resin matrix, the thermal properties of the two are significantly improved: the storage modulus and increase in the early stage of hydrothermal aging, and the thermal expansion coefficient decreases, and the relative rigidity of the material increases; in the later stage of aging, recovers due to the plasticization of water. Hydrolysis leads to microcracks, holes and other defects in the resin; the coating of resin to fiber is reduced, and the intra-layer fiber/resin bonding properties and inter-layer bonding properties of the composites are weakened, resulting in a significant decrease in the compression and interlaminar shear strength of CF/VE in the short term of hydrothermal aging. Due to the increase of thermal stability and relative rigidity of the resin matrix, some mechanical properties of VER and CF/VE at 70°C increase.