## 3 Results
### Moisture Absorption  
Figure 4 shows the moisture absorption curves obtained for neat epoxy resin and composites reinforced with the different types of graphene nanoparticles. Firstly, it is worth pointing out that the pristine epoxy resin studied in this work is aircraft quality and, therefore, its hydrothermal resistance is initially very high, absorbing only 1.75% \(w/w\) of water. For this reason, the enhancement reached by graphene addition is less pronounced than other published works with epoxy resins of different nature, with a curing process at room temperature [5; 6]. The crosslinking degree of these resins is lower and thus, their water uptake was higher, close to 6% \(w/w\). At the first stage, the water uptake curves present a tendency that can be approximated to linear growth. After this first region, the tendency changes and the curves asymptotically approach the equilibrium. From these results, the diffusion coefficient can be calculated by knowing the water absortion rate, according to the [PERSON]'s model from the initial slope of the absorption curves [16]. The other characteristic parameter is the maximum absorbed water content when the saturation is reached. These parameters are summarized in Table 2 for all the studied samples. It is worthy to note that the incorporation of graphene nanoplatelets into the epoxy matrix causes different effects on the water uptake of the resin as a function of the GNPs geometry. In fact, it is striking that the addition of GNPs induces a reduction of the maximum absorbed moisture content in most cases except for GNP1/epoxy composites. However, this is the nanofiller with the highest specific area and aspect ratio, therefore, it would be expected that this composite presents the highest barrier properties. These controversial results can be explained by the morphology of the composites. As it is shown in Figure 3d, GNP1 suffers an important phenomenon of weaving. This implies two different effects: A decrease of its effective area and a weak interphase with the matrix, \begin{table}
\begin{tabular}{c c c} \hline \hline
**Sample** & **D \(\times\) 10\({}^{9}\) (cm\({}^{2}\cdot\)s\({}^{-1}\))** & **Woo (\%)** \\ \hline
**Epoxy resin** & 10.8 & 1.75 \\
**GNP1/epoxy** & 7.99 & 1.79 \\
**GNP2/epoxy** & 7.97 & 1.78 \\
**GNP3/epoxy** & 6.56 & 1.46 \\
**GNP4/epoxy** & 7.53 & 1.45 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Water diffusion coefficients (D) calculated by Fick model and weight fraction of water at the saturation equilibrium (Woo). Figure 4: Moisture absorption of neat epoxy resin and epoxy composites reinforced with different types of graphene, commercially named GNP1, GNP2, GNP3 and GNP4, were \(t\) is time, and \(a\) is area. that results in areas with low wettability. Despite GNPs difficult water absorption, the final amount of absorbed water is higher due to this weaker interface, which allows water to go in between. In contrast, the addition of only a 0.5 wt % of GNP2, GNP3 and GNP4 causes an important reduction of water absorption. It confirms that, when the graphite nanosheets are stretched, they act as effective barriers. This affirmation is corroborated by the diffusion coefficient. The water absorption rate decreases by the GNPs addition. This parameter is lower for epoxy/graphene composites with regards to neat epoxy resin, but this remains practically constant for all the studied composites. This decrease of diffusion coefficient by graphitic nanofiller addition has been already observed by other authors. As it is indicated by [PERSON] et al. [5; 6], water absorption is a complex phenomenon, influenced by many factors, including free volume, crosslinking degree of the matrix, morphology, hydrophobicity, etc. [17]. They explain the increase of water resistance in epoxy composites, reinforced with nanofillers, by two effects. In the first place, nanofillers act as efficient barriers against water absorption due to the increased tortuosity for water molecules diffusing through the epoxy matrix.