## 1 Introduction  
The use of composite structures in a wide range of fields is still increasing, thanks to their high specific strength and stiffness over metals. The significant weight reduction of these structures translates into a benefit for the industry in terms of cost savings and environmental benefits, especially in terms of fuel savings [1], with the recycling phase of the composites remaining the key environmental issue. However, it is well known that the mechanical properties of fibre-reinforced composites can be very sensitive to environmental parameters. Temperature, humidity, light, or environmental impact phenomena such as rain, lightning, or nail affect the durability of composite materials, becoming a crucial issue when they are used as structural materials [2]. These severe environmental conditions can limit the use of polymeric composites, especially for applications in which high performances are required. In an aircraft, some primary structures are always subjected to varying temperature and moisture exposures throughout their entire service life. Continuous exposure to high temperature and moisture levels may lead to a change in the mechanical behaviour of the aircraft's structural components [3, 4]. Furthermore, cyclic thermal, moisture and mechanical loadings could even further deteriorate the performance of the structures leading to premature failure. These environmental conditions,especially temperature and moisture exposure, vary widely during a flight from the take-off to the landing. Specifically, Kevlar-reinforced composites are prone to moisture uptake, due to the strong hygroscopic nature of both the resin and the fibre. Epoxy resins are strongly sensitive to water absorption because of large amounts of polar OH groups in the networks which facilitate attractive interactions with polar molecules of water [5; 6; 7]. It was found that the maximum moisture absorption for epoxy is 2 wt% at 70 \({}^{\circ}\)C and 100% RH [8] and 6 wt% for Kevlar 49 aramid fibres when they are exposed to a high-humidity environment (96% RH) [9]. The absorbed moisture leads to the plasticization phenomenon in the matrix resin, decreasing the glass transition temperature and also generating changes in the state of stress in favour of cracking through swelling [10; 11; 12], degrading the mechanical properties of the material [2; 3; 4]. As a consequence, the stress transfer at the fibre-matrix interphase is compromised and the overall behaviour of the composite becomes more brittle: the impact strength and fracture toughness can even drop by 50% [13]. In aerospace applications, the use of Kevlar honeycomb sandwiches is widespread, thanks to their excellent mechanical performances, i.e., their high bending-stiffness-to-weight ratio [14]. Nevertheless, the honeycomb core absorbs water via the capillary phenomenon [15; 16]. The moisture locally accumulates inside the honeycomb cells, thereby jeopardising the original purpose of adopting sandwich structures for weight reduction and seriously affecting the integrity of composite structures and aircraft safety [17; 18]. The industrial solution currently used is based on aluminium paints applied through a flame-spray technique, thanks to the combined barrier effect against water adsorption and anistatic properties [19; 20; 21]. Although these barrier films have a water vapor transmission rate (WVTR) equal to \(\sim\)10\({}^{-1}\) g/m\({}^{2}\)h [22; 23; 24], they also have some drawbacks, i.e., their high weight and fragility [25]. Alternatively, polymer-based films, such as polyvinyl fluoride (PVF) films, are extensively used in aerospace applications thanks to their low permeability of 24.5 g/m\({}^{2}\)day for vapours and low ignition time [26; 27]. These hydrophobic films are usually co-bonded to surfaces of the glass, carbon, and Kevlar composite structures, providing a moisture absorption \(<\)0.5% [28]. In the last decade, the possibility of using graphite nanoplatelets (GNPs) to improve the barrier properties of polymers has been investigated [29].