Hygroscopic multi-scale behavior of polypropylene matrix reinforced with flax fibers

Abstract

Due to the hydrophilic behavior of natural fibers, bio-based composites made of vegetal fibers are still facing some obstacles that impair their large-scale development in the modern and promising industrial applications. Therefore, the aim of this study is to evaluate the multi-scale moisture absorption phenomena within a unidirectional bio-based composite material reinforced with flax fibers. The effect of fiber fraction, coupling agent and relative humidity on diffusion kinetics have been studied. In this work, composites are made of polypropylene matrix with various flax fiber contents, with the addition of maleic anhydride as a coupling agent. Samples were submitted to wet aging at 75% and 95% relative humidities (RH) under room temperature of 23 °C. Results have shown significant increase of moisture absorption with the increase of fiber content and relative humidity. In fact, the samples reinforced with 40% flax fiber content absorb as much as 9.15% moisture at saturation at 95% RH, whereas the samples reinforced with 20% flax fiber content only absorb 4.91% moisture at the same RH condition. However, with the addition of a coupling agent, composites undergo a decrease in moisture absorption due to an improved adhesion at the fiber/matrix interface. The moisture content of the samples reinforced with 40% flax fiber has indeed been reduced to 7.52% at 95% RH owing to the addition of the coupling agent. In this work, for the first time, the prediction of the maximum moisture absorption capacity of a bio-based composite has been studied, using a multiscale homogenization method. The obtained results highlight that once the fibers are incorporated within the matrix, the overall macroscopic property of the composite does not always satisfy the classical law of mixture, even for the maximum moisture absorption capacity. In fact, it has been concluded that the difference between the experimental results and those of the homogenization method is primarily due to a hygro-mechanical coupling, induced by the containment effect the matrix exerts on the fibers. This prediction strongly depends on the fiber content, the environmental relative humidity and the flax fiber sorption behavior which can vary from a fiber to another.