Effect of Immersion in Water or Alkali Solution on the Structures and Properties of Epoxy Resin

The durability of fiber-reinforced polymer (FRP) composites is significantly dependent on the structures and properties of the resin matrix. In the present paper, the effects of physical or chemical interactions between the molecular chain of the epoxy resin matrix and water molecules or alkaline groups on the water absorption, mechanical structures, and microstructures of epoxy resin samples were studied experimentally. The results showed that the water uptake curves of the epoxy resin immersed in water and an alkali solution over time presented a three-stage variation. At different immersion stages, the water uptake behavior of the resin showed unique characteristics owing to the coupling effects of the solution concentration gradient diffusion, molecular hydrolysis reaction, and molecular segment movement. In comparison with the water immersion, the alkali solution environment promoted the hydrolysis reaction of the epoxy resin molecular chain. After the immersion in water or the alkali solution for one month, the water uptake of the resin was close to saturate, and the viscoelasticity was observed to decrease significantly. The micropore and free volume space on the surface and in the interior of the resin gradually increased, while the original large-scale free volume space decreased. The tensile strength decreased to the lowest point after the immersion in water and the alkali solution for one month, and the decrease percentages at 20 degrees C and 60 degrees C water or 60 degrees C alkali solution were 24%, 28%, and 22%, respectively. Afterward, the tensile strength recovered with the further extension of immersion time. In addition, it can be found that the effect of the alkali solution and water on the tensile strength of the epoxy resin was basically the same.

Automated summary

The durability of fiber-reinforced polymer composites is highly dependent on the resin matrix, and the interaction between the epoxy resin molecular chain and water or alkaline groups can significantly affect the water absorption, mechanical structures, and microstructures of the resin. Immersion in water or alkaline solutions leads to unique characteristics in water uptake behavior due to diffusion gradients, hydrolysis reactions, and molecular movement. Both water and alkaline solutions decrease the tensile strength of the epoxy resin, with alkali solutions promoting hydrolysis reactions and water leading to near saturation of water uptake after one month of immersion.