Reversible and irreversible changes in physical and mechanical properties of biocomposites during hydrothermal aging

Abstract

The use of biocomposites on a daily basis for industrial products brings to light the influence of environmental factors on the evolution of mechanical properties. This aging has a major influence in the lifetime of any product based on such materials. Among biobased composites, poly(lactic acid) reinforced with plant fibers are known to be sensitive to hydrothermal aging due to the intrinsic nature of their components. Although some papers studied the influence of temperature and water absorption on such materials, so far the difference between reversible and irreversible effects of aging has been hardly studied. This distinction is the purpose of this study. Poly(lactic acid) samples reinforced with various content of flax fibers (0%, 10%, and 30%) were immersed in water at different temperature (20, 35, and 50°C) up to 51 days. The physical and chemical phenomena responsible for the changes in the mechanical properties of biocomposites were evaluated. It was observed that these changes were mainly reversible. However, irreversible effects of aging turned out to increase drastically with the amount of fiber and the aging temperature. While hydrolysis drastically deteriorated PLA for longer aging times, the lifetime of biocomposites was significantly extended (+ 230% at 50°C from 0% to 10% in fiber content) by the presence of fibers which postponed the failure of PLA.2

Coupled hydro-mechanical aging of short flax fiber reinforced composites

“…Then, this phenomenon usually affects amorphous polymers. Indeed PLA was mostly amorphous at this stage of aging [30]. Finally, the highly probable orientation of polymer chains in the direction of the stress due to the sample forming process was consistent with the mechanisms of environmental stress cracking [47].…”

“…The morphology of PLA was assessed during aging by evaluating its number average molar mass by size exclusion chromatography [30]. Figure 3 shows the number average molar mass of PLA for each material and at each aging temperature before and after 24 h and 144 h of aging.…”

Modeling of hydrothermal aging of short flax fiber reinforced composites

“…The different temperature/humidity combinations applied and the subsequent water content at saturation are summarized in Table 1. Similarly to immersion tests, the dynamic elastic modulus was evaluated at room temperature from free vibrations tests [11]. The dynamic elastic moduli of samples subjected either to immersion or wet air were plotted as a function of their water content in Fig.…”

“…However, for PLA-F30, the simulated moduli were underestimated but only in the transient region. These inaccuracies of the simulated values were explained by the model approximations and more specifically by the absence of consideration of irreversible mechanisms [11]. Nevertheless, underestimated values were more likely due to an inaccurate evaluation of the water gradient or an inappropriate estimation of the flexural equivalent homogeneous modulus.…”

“…5). Secondly, the occurrence of damage at the fiber/matrix interface [11] would result in a overestimation of the fibers modulus for high water contents. Nevertheless these results are globally in accordance to data found in the literature [26][27][28].…”