pine sawdust at the same loading rate and the diffusivity was about two times higher (**Table 4**).

Furthermore, as the ambient temperature increases, the activity of the polymer molecules increases, which accelerates the diffusion of water molecules within the bio-composites. In the same line, Beg and Pickering [98] observed that increasing temperature increases the water uptake kinetics of bio-composites (**Figure 8**).

**Table 4** summarizes the diffusion coefficient values of different bio-composites reported in the literature.

*Hydro/Hygrothermal Behavior of Plant Fibers and Its Influence on Bio-Composite Properties DOI: http://dx.doi.org/10.5772/intechopen.102580*

#### **4.3 Hydro/hygrothermal aging**

Hydro/hygrothermal aging can be qualified as a physicochemical degradation. Indeed, in the short term, the composite undergoes plasticization due to the infiltration of water molecules between the polymer chains. This type of aging is mostly reversible when the material returns to its initial state. In addition, plant fibers are known to have an affinity for water, unlike most matrices. This causes differential swelling which can also lead to micro-cracks at the fiber/matrix interphase. These cracks can propagate further through a succession of adsorption/desorption cycles (**Figure 9**). This damage is irreversible and affects the performance of bio-composites. In addition, chemical degradation of the matrix and even of the fibers can occur; this is called hydrolysis. The presence of temperature can aggravate these phenomena [103].

Law and Ishak [99] found a plasticization effect on Kenaf/PP composites after saturation, with a decrease of between 14 and 35% in all tensile and flexural mechanical properties for the composites at 40% loading. After drying the samples again, a partial recovery of their initial mechanical properties (in flexion and in traction) was noted.

Beg and Pickering [98] observed degradation in the tensile mechanical properties of kraft fiber (40%)/PP composites, with and without 4% MAPP, aged for 238 days at different temperatures of 30, 50, and 70°C. The property retention capacity was lower with increasing temperature. The authors associate these results with the degradation of the fibers and/or the fiber/matrix interface (**Figure 10**).

Freund [104] carried out a study of cyclic hygrothermal aging at 80°C on Lin/Elium composites during his thesis. Each cycle contains two phases: a saturation phase at 80%

**Figure 9.**

*Degradation mechanism during hygrothermal aging [13]: (a) fibre swells and matrix microcrack at the interface (b) water infiltration into microcracks and the interface (c) dissolution of some fibre component (d) fibres-matrix debonding.*

#### **Figure 10.**

*Effects of hygrothermal aging on the fracture surface SEMs of composites (kraft fiber (40%)/PP) of: (a) unaged composites and (b) composites aged at 70C for 238 days [98].*

**Figure 11.** *Evolution of Young's modulus and maximum stress of composites flax/Elium as a function of humidity [104].*

relative humidity and a drying phase at 10% relative humidity. It was found that after each cycle the composites lost mass and mechanical properties with a decrease in stiffness of about 50% after five cycles and a decrease in tensile stress from 110 to 30 MPa (**Figure 11**). These degradations also had an effect on the adsorption kinetics: the more the material undergoes aging cycles, the faster its adsorption becomes.

The author summarized this change in behavior into two reasons: swelling of the composite, especially the fibers, and degradation of the fibers. These findings were justified by a change in the failure mechanism of the composite due to a degradation of the fiber/matrix interface (**Figure 12**), and the potential degradation of the flax fibers were justified by a decrease in their crystallinity index from 77.2 to 38.4%.

On the other hand, Berges [3] studied the cyclic aging of composites at 70°C with two humidity conditions 90% RH and 15% RH for adsorption and desorption, respectively. For each cycle, a total duration of 4 days was chosen, that is, 2 days for each half cycle. Tensile tests carried out after four and nine cycles revealed a reproducible behavior for tensile modulus with a decrease in elongation at break and stress at the break with the number of cycles undergone. This reduction was related to fiber/ matrix decohesion and resin damage.

Wang and Petru [105] compared the results of the mechanical properties of unidirectional (30% by volume)/epoxy flax fiber composites undergoing natural aging outdoors and artificial aging by immersion in water at 60°C. It was concluded that 60, 120, and 180 days of natural aging correspond to 1.1, 37.2, and 167.9 h of artificial aging, respectively. Comparisons were made regarding flexural strength and flexural modulus.

*Hydro/Hygrothermal Behavior of Plant Fibers and Its Influence on Bio-Composite Properties DOI: http://dx.doi.org/10.5772/intechopen.102580*

**Figure 12.** *Fracture facies of flax/Elium composite before and after aging [104].*
