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

• Reversible/irreversible modifications of biocomposites due to hydrothermal aging.
• The changes in mechanical properties were mostly reversible.
• Irreversible changes increased with immersion temperature and water content.
• Irreversible changes were mostly characterized by a drop of mechanical properties.
• Despite composites degradation, presence of fibers drastically improved lifetime.

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.