# Toughened Vinyl Ester Resin Reinforced with Natural Flax Fabrics
## 2 Materials and Methods
### Resin Characterization Methods  
Tensile tests were carried out according to ISO 527-1A using an MTS Criterion model 43 universal tensile (MTS Systems Corporation, Eden Prairie, MN, USA) testing machine equipped with an extensimeter. The testing machine was equipped with a 10 kN cell load and interfaced with a computer through MTS Elite Software (version 4.0). Five samples were tested for each thermal treatment of both resins. Two velocities were settled, the first one at 2 mm/min and the second one at 10 mm/min, after removing the extensimeter at an extent of 0.5 mm and with a grip distance of 80 mm. Tests were carried out after 48 h, and during that period, samples were stored in a dry keeper with a controlled atmosphere (room temperature and 50% relative humidity). Flexural tests were carried out with an MTS Criterion model 43 equipped with a 10 kN cell load. The tests were carried out using a constant load punch lowering speed of 2 mm/min for the entire duration of the test. A support span-to-depth ratio of 20:1 was used. Specimens with a thickness greater than 3.2 mm should have a width of 12.7 mm. Each specimen was deflected until rupture occurred at the outer surface. A minimum of five samples per formulation were tested, and the average flexural modulus was reported. Flexural stress and modulus were derived from the data. For a homogeneous elastic material under flexural testing, the maximum stress at the outer surface occurs at the midpoint. This stress can be calculated at any point on the load-deflection curve using Equation (3):  
\[\sigma=\frac{3 PL}{2 bd^{2}} \tag{3}\]  
where the stress in the outer fibers at midpoint is \(\sigma\) (MPa), the load at a given point on the load-deflection curve is \(P\) (N), the support span is \(L\) (mm), the width of the specimen is (mm), and the depth of the specimen is \(d\) (mm). The flexural modulus was calculated from stress-strain curves using Equation (4):  
\[E=\frac{L^{3}m}{4 bd^{3}} \tag{4}\]  
where the modulus of elasticity in bending is \(E\) (MPa), the support span is \(L\) (mm), the width of the specimen is \(b\) (mm), the depth of the specimen is \(d\) (mm), and the angular coefficient of the linear elastic part of the load-deflection curve is \(m\) \(\left(\frac{N}{mm}\right)\). The impact properties of the composites were evaluated according to ISO 179 using an Instron CEAST 9050 machine (INSTRON, Canton, MA, USA) with a 15 J Charpy pendulum. Specimens were V-notched in the center with a 2 mm, 45\({}^{\circ}\) notch using a manual cutter. At least five specimens were tested, and the average value was reported. The Charpy impact test measures the energy absorbed by a notched specimen during fracture under an impact load. This involves striking the specimen with a hammer on a pendulum arm, opposite the notch, and measuring the decrease in the pendulum's movement to determine the absorbed energy [33]. Fracture toughness tests were carried out with an MTS Criterion model 43 equipped with a 10 kN cell load using a test velocity of 10 mm/min. First, it was crucial to prepare the resin sample in the form of single-edge noticed bend (SENB, as it is possible to see in Figure 2). The specimen should have a sharp crack, 5 mm length, on one side. The specimen was placed in the universal testing machine, and a test using a tensile load applied perpendicular to the crack plane was started. It is recommended to use at least five replicates. The load vs. load-point displacement curve was obtained once the test was completed. The software recorded the load at which the crack started to propagate. This is the critical load (Pc). Finally, it was possible to calculate the fracture toughness (KIC) using Equation (5):  
\[K_{\text{Q}}=f\frac{P_{Q}}{BW^{1/2}} \tag{5}\]  
where \(W\) is the width and \(B\) the thickness of the specimen. All the experiments in this study were performed using a Differential Scanning Calorimeter (Q200 TA-DSC). A 10 mg sample of the cured resin was put in an aluminum pan for the DSC analysis.