Hygrothermal Aging History of Amine-Epoxy Resins: Effects on Thermo-Mechanical Properties

Abstract

Epoxy systems are widely used as matrix resins for fiber reinforced polymers (FRP) and, therefore, often have to withstand harsh environmental conditions. Especially in marine and offshore environments, moisture or direct water contact leads to water absorption into the epoxy resin. As a result, the mechanical properties change during application. Since diffusion at room or colder temperatures is slow, industry and academia typically use accelerated aging methods at elevated temperatures for durability prediction. However, as the water-polymer interaction is a complex combination of plasticization, physical aging, and molecular interaction, all of these mechanisms are expected to be affected by the ambient temperature. To reveal the impact of aging time and temperature on the thermo-mechanical properties of an amine-epoxy system, this publication includes various hygrothermal aging conditions, like water bath and relative humidity aging at temperatures ranging from 8°C to 70°C and relative humidity from 20% to 90%. Thus, it is demonstrated via long-term aging, DMTA and FTIR investigations that, e.g., strength, stiffness, strain to failure, and the glass transition temperature (Tg) can differ significantly depending on aging time and temperature. For example, it can be shown that water absorption at cold temperatures leads to the strongest and longest-lasting reduction in strength, although the maximum water absorption amount is lower than at higher temperatures. For the application, this means that strength differences of up to 26% can be obtained, depending on the aging method selected. Furthermore, it can be shown that conventional prediction models, such as Eyring correlation, which consider the mobility of the molecular structure for the prediction of thermo-mechanical properties, can only be used to a limited extent for prediction in hygrothermal aging. The reasons for this are seen to be, in particular, the different characteristics of the water-polymer interactions depending on the aging temperature. While plasticization dominates in cold conditions, relaxation and strong water-molecule bonds predominate in warm conditions.

Improving epoxy adhesion with steel adherends using a tannic acid‐based additive: Impact on resin properties and interfacial bonding

“…It can be inferred that the addition of 1 wt% TA‐AGE into the resin, has increased by over 1%. Moisture uptake in epoxies can be explained by the amount of free volume present and polymer–water interactions that arise due to the presence of polar groups in the matrix 42 . Availability of free volume depends on the polymer structure and crosslink density.…”

“…Solving Equation 2gives D values of Neat and TA-AGE1 aged at 60 C as 7.5 Â 10 À7 mm 2 /s and 1.1 Â 10 À6 mm 2 /s respectively. Some studies have noted that with an increase in aging temperature, the coefficient of diffusion (D) increases, but it does not have a significant influence on M ∞ : 38,40,41 While others have reported lower M ∞ for specimens aged at low temperatures compared to those aged above 50 C. 42,43 After 3 M aging at RT, water uptake in Neat and TA-AGE1 did not reach an equilibrium M ∞ . By assuming M ∞ as equilibrium water content under strict Fickian response, value of D can be obtained for Neat and TA-AGE1 at RT using Equation 2.…”

“…Moisture uptake in epoxies can be explained by the amount of free volume present and polymer-water interactions that arise due to the presence of polar groups in the matrix. 42 Availability of free volume depends on the polymer structure and crosslink density. A general trend in epoxies is that moisture diffusivity increases with decreasing crosslink densities.…”

Improving interfacial adhesive bond durability of a structural epoxy with steel using a tannic acid‐based additive for underwater applications

“…An increase in water uptake is often attributed to decreased crosslinking density and increased water-polymer interactions. 34 Zhou and Lucas 35 have classified the absorbed water into a polymer as Type-I and Type-II. Type-I bound water forms single hydrogen bonds at temperatures lower than the T g of the material.…”

Clarifying the chemical reactions of the weakening of adhesion between epoxy resin and aluminum by molecular dynamic simulation and experiment

“…Karuth simulated the hydrothermal degradation of cross-linked epoxy resin, and successfully simulated the protonation of water molecules and the nucleophilic attack on the CO bond of ether bond in epoxy amine network (Karuth et al, 2022). At present, many experiment have been carried out on the aging of epoxy resin under different conditions, such as physical aging (Kong et al, 1981;Moosburger-Will et al, 2014), thermal aging (Gao et al, 2011;Chen, 2013), hygrothermal aging (Stocchi et al, 2006;Niu et al, 2021;Gibhardt et al, 2022), photooxidation aging (Zhang et al, 1994;Pan, 2016). In terms of aging experiment, Yang studied the high temperature thermal oxidative aging behavior of bisphenol A epoxy resin fiber reinforced composites cured with anhydride.…”