# Toughened Vinyl Ester Resin Reinforced with Natural Flax Fabrics
## 3 Results and Discussion
### Composite Characterizations  
Composites were produced using the vacuum bagging infusion process, starting with an initial pressure of 700 mbar, which was gradually decreased to optimize the infusion process and achieve a homogeneous piece. Based on the mechanical properties of the resin, a post-curing treatment of 1 h at 90 \({}^{\circ}\)C was selected as it appeared to be the most promising thermal treatment. All samples underwent three-point bending, tensile, KIC, and impact testing to determine their mechanical resistance. Firstly, the number of piles to insert into the composites was calculated with Equation (2); the result was four piles of flax fabrics. Figure 7 illustrates the differences between the Dion 9500 1% matrix and the composites, both subjected to the aforementioned thermal treatment. Regarding flexural properties, there was an increment of 1.25% in flexural strength and 3.13% in the flexural modulus, which is slightly lower than expected. However, significant improvements were observed in tensile properties, with the Young's modulus reaching a very positive value of 6.52 GPa, nearly tripling the matrix modulus. The strong interfacial forces between the flax fibers and the matrix are primarily responsible for the improvement in mechanical properties  
Figure 6: Viscosity evaluation of Dion 9500 with both 1% and 2% of Butanox. when flax fibers are incorporated into the matrix. These forces contribute to improved composite performance [44] and are significantly higher than those observed with other plant fibers [45]. This result is highly encouraging for developing a sustainable alternative to glass fiber panel composites. Additionally, the tensile stress value of 77.38 MPa is very promising for vinyl ester/flax coupling. Furthermore, notable enhancements were found in elongation at break and impact strength, suggesting that flax fibers could improve the toughening mechanism. The Interlaminar Shear Stress (ILSS) was evaluated using a short beam shear test, as described in more detail in the section on methods for characterizing composite materials. Shear stress is inherent in bending tests, especially when the distance between supports is small relative to the specimen thickness, resulting in significant shear stresses compared to normal stresses. This allows for the measurement of shear strength in brittle matrix materials. Either the shear strength of the matrix or the quality of the fiber-matrix bond can be indicated by the type of failure observed. Due to the thickness range of 2.20-2.70 mm for glass composite samples, the ILSS could not be calculated as the pin distance could not be adjusted with the available gauge system. Conversely, for the flax fiber composites, the greater thickness achieved (3.20 mm) allowed the ILSS to be calculated, giving a value of 15.56 MPa (\(\pm\)1.25). Images of the delaminated composites are shown in Figure 8. The relatively low ILSS value suggests poor adhesion between the flax fibers and the matrix. The delamination started from the middle of the thickness (interlayer shear) as shown in Figure 8a. The interlaminar shear cracking was discontinuous and did not lead to continuous cracking. Upon further loading, typical flexural failure occurred in the lower part of the composite (tension mode), as indicated by the yellow arrow in Figure 8b. Figure 7: Mechanical proprieties comparison between flax fiber composites and Dion 9500-1-25-90-1. From Figure 9, it is possible to observe the overall composite morphology at low magnification. This micrograph verifies that the infusion process allowed us to obtain composites without any voids. The implication would be that water absorption, which is critical for automotive interior applications, should be low. In order to verify this, a water adsorption analysis was carried out. When the reinforcement is not fully impregnated with the polymer matrix during production and there is inadequate wettability, capillary transport in the voids and imperfections at the fiber-matrix interface occur [46].