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

Dennis Gibhardt,Bodo Fiedler,Christina Buggisch,Devin Meyer

doi:10.3389/fmats.2022.826076

Dennis Gibhardt, Bodo Fiedler + Show 2 more

Open Access

https://doi.org/10.3389/fmats.2022.826076

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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.

Highlights

To achieve efficient and sustainable use of raw materials and energy, structures made of lightweight composites and thermosetting polymers or adhesives are in wide use today (Gagani et al, 2019; Budhe et al, 2018; Rocha et al, 2017), while the demand is still increasing (Amano, 2017); but to make a decisive contribution to the energy and mobility transition, their lightweight and durability potential has to be exploited even further
The results presented show a strong dependence of the mechanical properties on the time-temperature history of hygrothermal aging
Considering water absorption and thermomechanical analyses, it can be assumed that not all properties will converge to the same values for different operating temperatures, even after extremely long periods

Summary

Introduction

To achieve efficient and sustainable use of raw materials and energy, structures made of lightweight composites and thermosetting polymers or adhesives are in wide use today (Gagani et al, 2019; Budhe et al, 2018; Rocha et al, 2017), while the demand is still increasing (Amano, 2017); but to make a decisive contribution to the energy and mobility transition, their lightweight and durability potential has to be exploited even further. Most of them have in common that, due to slow diffusion rates at moderate ambient temperatures, they rely on accelerated aging processes at elevated temperatures for prediction of property development (Ilioni et al, 2019; Bordes et al, 2009; Jaksic et al, 2018). Thanks to these studies, it is known that absorbed water can cause significant changes in thermomechanical properties due to simultaneously occurring plasticization and relaxation (physical aging) (Le Guen-Geffroy et al, 2019; Zheng et al, 2004). It has been shown that the tensile strength and the Tg are the most affected properties by water absorption as they can be reduced by a factor of 2 or even more (Zhou and Lucas, 1999b; Startsev et al, 2018)

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