## 3 Results and Discussion
### Mechanical Properties  
In Figures 9 and 10 are presented the flexural strength (MOR) and modulus of elasticity (MOE) values before and after cyclic testing. The DP/K hybrid composite exhibited the highest values of flexural strength and modulus of elasticity: 82.24 N/mm\({}^{2}\) and 6248 N/mm\({}^{2}\), respectively. Similar results were found by [30], who also confirmed that hybrid composites with Kevlar show higher mechanical properties. Figure 10: Comparative values of MOE for the 3 structures before and after exposure to cyclic testing (CT) (variables with the same letter mean that the difference is not statistically significant and those with different letters are statistically significant). Figure 9: Comparative values of MOR for the 3 structures before and after exposure to cyclic testing (CT) (variables with the same letter mean that the difference is not statistically significant and those with different letters are statistically significant). Lower values of MOR and MOE were achieved for DP/C and DP composites, the values ranging between 70.78 N/mm\({}^{2}\) and 75.4 N/mm\({}^{2}\) and between 5774 N/mm\({}^{2}\) and 5028 N/mm\({}^{2}\), respectively. The natural fibre composites exhibit inferior mechanical properties compared to synthetic fibres, and in the hybrid composites, these properties mainly depend on the binding ability between the matrix and the fibres [57]. DP/C had the lowest density and the adhesive amount applied to each layer was about 40% lower than that applied to DP; consequently, the flexural strength decreased by 14% compared to DP. The cotton fibres did not improve the strength performance of the hybrid composite, due to weaker fibre-matrix interfacial bonding. After cyclic testing, the mechanical properties were analysed and no substantial changing was observed in the case of DP/K. The resin was much easier to apply to the Kevlar sheets and the resin amount was greater than in the case of DP/C. The resin penetrated better in all layers and no voids were observed in the structure (Figure 8f). In the case of the other two composites (DP and DP/C), when subjected to water immersion (first cycle), the softening of fibres occurred, as hydroxyl groups contained in date palm and cotton absorbed water. Then, the expansion of water molecules and contraction of the polymer matrix happened in the freeze cycle, leading to the degradation of the bonding between the fibres and the matrix, further influencing the mechanical properties. DP/C registered a decrease in MOR and MOE by 24% and 30%, respectively, and DP registered decreases by 12% and 23%, respectively, compared to the DP/K composite. The flexural strength (MOR) and modulus of elasticity (MOE) before cyclic testing were not influenced by cotton and Kevlar fibres in the structures, as a statistical analysis displays no significant effect (\(p<\) (0.2-0.6)). However, after the cyclic test, the thickness swelling, MOR and MOE were statistically significantly affected by Kevlar layers (\(p<\) (0.01-0.004)), whereas cotton layers had no significant effect on MOR and MOE after cyclic testing (\(p<\) (0.1-0.4)). This demonstrates a better interface between polyester and Kevlar fibres compared to cotton fibre, the behaviour of which is influenced by its hydrophilic properties. Due to their low density and good performance in the freeze-dry test, the experimental hybrid composites with Kevlar (DP/K) could be alternative materials for metal, which in the marine condition is susceptible to corrosion and cracks in time. On the other hand, due to their high flexural strength (75-82 N/mm\({}^{2}\)) and minimal changes in thickness after water immersion for 216 h and cyclic testing (between 2.3% and 4%), structures DP/K is more reliable for application in construction for floors or roof tiles where moisture resistance is required.