# Mechanical Properties of Sandwich Composites Reinforced by Nanoclas: An Overview
## 3 Sandwich Composites Incorporating Nanoclays into the Skin
The maximum percentage of moisture absorption after 744 h of soaking decreased by 1.5 times the value obtained for neat composite laminates. This was explained due to impermeable fillers, like clay and glass fibers, which act as barriers and prevent the direct contact between jute and water [34]. In 2014, [PERSON] et al. [35] analyzed the benefits obtained with nanoclays in a sandwich structure composed of bidirectional E-glass fiber, epoxy resin and high density thermocol. For this purpose, experimental tests were carried out in tensile and flexural mode and the amounts of nanoclays dispersed into the epoxy resin were 1, 3 and 5 wt%. It was observed that the mechanical properties obtained increased with increasing the nanoclays content, but while the maximum tensile strength was achieved with 3 wt% of nanoclays (33% higher compared to the control samples), the maximum bending strength was achieved with 5 wt% of nanoclays (47.8% higher compared to the control samples). The bending strength was also obtained after immersion into water for 15 days and, while the decrease compared with the dry samples was 13.8% for control samples (neat resin), this value was only 4.2% for composites reinforced with 5 wt% of nanoclays. In fact, the weight gain was about 3.2% for neat samples, while for composites enhanced with 5 wt% of nanoclays was only 1.6%. Later, in 2015, [PERSON] et al. [36] gave continuity to previous studies and, using similar sandwich laminates (described in ref. [32]), evaluated the influence of nanoclays content on flexural, impact and water absorption properties. Initially, they studied only the fiber reinforced polymer (FRP) used in the skins and, for that purpose, flexural tests were carried out. They noticed that the flexural properties started to decrease after 4 wt% of nanoclays, therefore, only 0 wt% and 4 wt% of nanoclays  
Figure 2: Residual flexural strength versus impact energy [12]. were used to analyze the sandwich laminates. In this case, when 4 wt% of nanoclays were added to the FRP laminates, improvements of 40% and 57% were achieved in terms of bending strength and bending modulus, respectively. Regarding the sandwich laminates, the benefits obtained were about 60.4% and 21.8 times higher than the values obtained with the neat FRP laminate (fiber glass/polyester resin), respectively. From the Charpy impact tests carried out with a velocity of 0.75 m/s, it was possible to conclude that the impact strength of sandwich laminate was 43% higher over the FRP laminate. Finally, the maximum percentage of moisture absorption after 744 h was about 5% for neat sandwich laminates and 4% for sandwich laminates with 4 wt% of nanoclays [36]. In 2016, [PERSON] et al. [37] compared the mechanical properties of two sandwich composites. One of them was composed of glass fiber skin and jute core and the other one by glass fiber skin and PS-foam core. The matrix used was a polyester resin, in which 2, 4 and 6 wt% of nanoclays were added. In terms of flexural properties, it was noticed an increase with the percentage of nano-reinforcements, reaching the maximum values with 4 wt% of nanoclays. Compared to the neat samples, improvements of 16% and 19.5% were observed in terms of flexural strength for jute sandwic and PS-foam sandwic, respectively, while these values for the flexural modulus were around 45% and 42%, respectively. Regardless of the same trend observed for the Charpy impact strength and water absorption, in this case, the most significant improvements were achieved for 6 wt% of nanoclays. Therefore, comparatively to the neat samples, the Charpy impact strength increases around 22.4% and 49% for the jute sandwic and PS-foam sandwic, respectively, whereas the reduction in water absorption was more than 50% and 72% of the values observed for jute sandwic and PS-foam sandwic, correspondingly. In this case, the higher water permeability of the PS-foam core in comparison with the jute core must be emphasized.