## 3 Results and Discussion
### Thickness Swelling (TS) and Water Absorption (WA)  
The values obtained for TS and WA during the period of 216 h (9 days) of immersion in water are presented in Figures 5 and 6. By analysing the thickness swelling values, it can be seen that after 24 h and 216 h, the lowest values were obtained for the composites with Kevlar (1.22% and 2.3%, respectively) followed by those with cotton (1.01% and 3.0%, respectively) and date palm fibres (2.74% 5.42%, respectively) (Figure 5).  
Figure 4: Density and moisture content (average values) (A—variables with the same letter mean that the difference is not statistically significant).  
Figure 5: The dynamics of swelling in thickness for the 3 structures (DP, DP/C and DP/K).  
In the first 24 h, the thickness increased at about 0.02%/hour for the composites DP/C and DP/K and at about 0.09%/hour for DP. These results can be explained by the sensitivity of date palm fibres to water absorption due to their hydrophilic character, as was also observed by [46]. Another cause may be related to the presence of voids and pores in the structure, due to uneven application of the adhesive on the surfaces of the layers.  
For all composites, the water uptake increased more rapidly in the first 72 h; then, until the end of the period, the dynamic of the water uptake decreased. This is in agreement with [47,48] (Figure 6).  
The rate of swelling and absorption was more pronounced for the DP composite compared to the other two. The presence of high voids in DP specimens increased the water absorption, as water was trapped in the voids. After 24 h and 216 h, the water absorptions were 15% and 24.3%, respectively, for DP; 5.6% and 10.8%, respectively, for DP/C; and only 4.2% and 8.6%, respectively, for DP/K. Reinforcement with Kevlar fibres improved the performance in terms of water uptake by 20% compared to DP/C and by about 60% compared to DP.  
However, the addition of the cotton and Kevlar fibres in the structure of the hybrid composite significantly (\(p<0.05\)) affected both TA and WA after 24 h (\(p<0.01\)) and 216 h, for which this influence was more evident, since \(p<0.009\). This explains the better interface between polyester resin with cotton and Kevlar fibres.  
In Table 2, the equations that describe the dynamic of thickness swelling and water absorption during the period of testing are presented. The variation in TS and WA over time is expressed by a logarithmic curve with a high correlation factor (R\({}^{2}>0.9\)).  
The natural fibres' cell walls contain free hydroxyl (OH) groups, which are a favourable medium for water absorption, leading to the swelling of fibres within the matrix. As a consequence, the interfacial adhesion between the matrix and fibres can be destroyed and voids  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Code** & **TS** & **WA** \\ \hline DP & y = 1.8459 ln(x) + 1.1636 & y = 4.5477 ln(x) + 12.594 \\  & R\({}^{2}\) = 0.9226 & R\({}^{2}\) = 0.911 \\ DP/C & y = 1.067 ln(x) + 0.4358 & y = 2.5452 ln(x)  
or cracks can occur, negatively influencing the mechanical properties and performance of the composites [49; 50].  
These disadvantages of natural fibres can be reduced or removed by hybridization with synthetic fibres, as in this case with Kevlar, which led to higher resistance to moisture and better aging properties.