## 2 Materials and Methods
### Testing  
The density was determined according to EN 323:1996 [39], the moisture content according to EN 322:1996 [40], and the thickness swelling (TS) and water absorption (WA) according to EN 317:1996 [41], with specimens with dimensions of 50 mm \(\times\) 50 mm (length \(\times\) width). The samples were immersed in distilled water at a temperature of 20 \({}^{\circ}\)C for 24 h. The thickness and the mass of the samples were recorded before and after immersion in water at 2, 24, 48, 72, 96, 120, 144, 168 192 and 216 h. An electronic calliper with an accuracy of 0.01 mm and the electronic scale with the accuracy of 0.01 g were used to measure the thickness and to weigh the samples. The purpose of this test was to assess the water resistance and the dimensional stability of the hybrid composites. The flexural strength (MOR) and modulus of elasticity (MOE) were evaluated according to EN 310:1996 [42], the tests being carried out with the help of a Zwick/Roell Z010 universal testing machine (Ulm, Germany). Six samples were used for each type of hybrid composites and each test.  
The hybrid structures were designed to resist the humid environment. Thus, in addition to the water immersion test, they were also subjected to cyclic moisture resistance tests according to EN 321:2003 [43]. The experimental panels of 300 mm \(\times\) 300 mm were subjected to three exposure cycles consisting of immersion in water followed by freezing and then drying at a high temperature. After cyclic testing, their thickness, mass, MOR and MOE were measured.  
Figure 3: The manufacturing process of the hybrid composites.  
A water bath at a temperature of 20 \(\pm\) 1 \({}^{\circ}\)C was used for the water immersion of panels in the first cycle for 24 h. The other two cycles were carried out in a test chamber of HL type [DongGuan City, China] set to the 24 h freezing cycle at temperatures between \(-\)12 \({}^{\circ}\)C and \(-\)25 \({}^{\circ}\)C and drying for 72 h at 70 \(\pm\) 2 \({}^{\circ}\)C. Three cycles were performed per edge of each composite. After completing these cycles, the panels were conditioned at 23 \({}^{\circ}\)C and 65% relative humidity of air until constant mass. The changes in thickness, mass and flexural properties were tested and compared to those obtained before cyclic testing. The increase in thickness (dT) and the mass decrease (dM) of panels after cyclic testing were calculated with the following equations:  
\[\text{dT}=\frac{tf-ti}{ti}\times 100 \tag{1}\]  
where  
_tf_--the final thickness after cyclic testing, in mm;  
_ti_--the initial thickness before cyclic testing, in mm.  
\[\text{dM}=\frac{Mf-Mi}{Mi}\times 100 \tag{2}\]  
where  
_Mf_--the final mass after cyclic testing, in g;  
_Mi_--the initial mass before cyclic testing, in g.  
A microscope investigation of the samples before and after cyclic testing was carried out using a NIKON SMZ 18-LOT2 (Nikon Corporation, Tokyo, Japan) stereo-microscope. This allowed for a better visualization of the structures and of the adhesion between the component layers. The microscopic analysis was made on the cross sections of the experimental hybrid composites with 30\(\times\) magnification. Six samples from each experimental composite, were prepared to perform the test.