The hygroscopic behavior of plant fibers: a review

Abstract

Environmental concern has resulted in a renewed interest in bio-based materials. Among them, plant fibers are perceived as an environmentally friendly substitute to glass fibers for the reinforcement of composites, particularly in automotive engineering. Due to their wide availability, low cost, low density, high-specific mechanical properties, and eco-friendly image, they are increasingly being employed as reinforcements in polymer matrix composites. Indeed, their complex microstructure as a composite material makes plant fiber a really interesting and challenging subject to study. Research subjects about such fibers are abundant because there are always some issues to prevent their use at large scale (poor adhesion, variability, low thermal resistance, hydrophilic behavior). The choice of natural fibers rather than glass fibers as filler yields a change of the final properties of the composite. One of the most relevant differences between the two kinds of fiber is their response to humidity. Actually, glass fibers are considered as hydrophobic whereas plant fibers have a pronounced hydrophilic behavior. Composite materials are often submitted to variable climatic conditions during their lifetime, including unsteady hygroscopic conditions. However, in humid conditions, strong hydrophilic behavior of such reinforcing fibers leads to high level of moisture absorption in wet environments. This results in the structural modification of the fibers and an evolution of their mechanical properties together with the composites in which they are fitted in. Thereby, the understanding of these moisture absorption mechanisms as well as the influence of water on the final properties of these fibers and their composites is of great interest to get a better control of such new biomaterials. This is the topic of this review paper.

Keywords: ageing effects; composite materials; durability; hydrophilic behaviors; natural fibers.

Figure 1

Classification of natural fibers [inspired by Baley (2004)].

Figure 2

Multi-scale structure of the flax fiber [Célino et al. (2013, 2014a,b) inspired by Baley (2002) and Morvan et al. (2003)]. (A) Stem of a flax plant, (B) bundle of flax fibres, (C) represntation of an elementary fibre, (D) the S2 layer of elementary fibres.

Figure 3

Young's modulus evolution of flax fibers vs. number of mechanical cycles (Baley, 2002).

Figure 4

(A) Matrix cracking, (B) Fracture running along the interface, (C) Fiber/matrix debonding due to attack by water molecules (Dhakal et al., 2007).

Figure 5

Infrared spectra bands impacted by increasing relative humidity for sisal fiber. p-values scores (used with a threshold of 0.05), indicating significant impact of the water uptake on the FTIR bands, were marked using red dots (Célino et al., 2013, 2014a,b).

Figure 6

Equilibrium water vapor sorption isotherm for modified agave fibers at 25°C [inspired by Bessadok et al. (2009)].