# Mechanical Properties of Sandwich Composites Reinforced by Nanoclas: An Overview
## 2 Sandwich Composites Incorporating Nanoclays into the Core
While in neat samples the energy was absorbed by the collapse of the foam cells and by shear fracture, the samples with nanoclays failed due to the brittle fracture of the cells [15]. In 2008, [PERSON] et al. [16] studied the addition of nanoclays (between 0 wt% and 2 wt%) to a polyurethane foam for application in sandwich composites. The benefits were analyzed in terms of compressive, flexural and dynamic properties. They observed that the addition of nanoclays improved the storage modulus, as well as the glass transition of the material up to 0.5 wt%. For compression and flexural tests, the best results were achieved for 0.5 wt% and 1 wt% of nanoclays, respectively [16]. In 2009, [PERSON] et al. [17] compared the bending and thermal behavior of syntactic foams with and without nanoclays. The content of nanoclays used ranged from 0 wt% to 3 wt%. Regarding the flexural properties, they increased with the addition of nanoclays up to 2 wt% wt%, with values around 42%, 38% and 18% for flexural strength, flexural strain and flexural modulus, respectively. Higher contents promoted agglomerations and, consequently, lower bending properties were observed [17]. In terms of dynamic properties, similar behavior was observed, where the maximum modulus and loss modulus were obtained for 2 wt% of nanoclays. Compared with the values obtained for neat samples, 2 wt% of nanoclays promoted increases of 30.3% and 27.8%, respectively, for modulus and loss modulus. In 2009, [PERSON] et al. [18] published a study where the flexural properties of sandwich structures produced with syntactic foam enhanced by nanoclays were evaluated. Two different densities of microballoons (220 and 460 kg/m\({}^{3}\)) were mixed with a constant content of 1 wt% of nanoclays. Three and four-point bending tests were performed to compare the bending properties of the materials. These authors concluded that the incorporation of nanoclays in the foam core increased the shear stress and facing bending stress of the material, because more energy was needed to separate the interfaces. In this case, multiple cracks were observed with the presence of nanoclays, evidencing the brittleness of the material, but also due to the difficulty of crack propagation with the presence of nano-reinforcements [18]. In 2010, [PERSON] et al. [19] studied the mechanical properties of cyanate ester synthetic foams containing 2 wt% and 4 wt% of nanoclays. Tensile, flexural and compressive properties were evaluated--as well as the dynamic and thermal properties of the materials. An increase in tensile, flexural and compressive strength was observed with the content of nanoclays in the foams. In terms of compressive strength, increases of around 73% and 150% were observed when 2% and 4% of nanoclays were added to the syntactic foams, respectively, while the tensile and bending strengths increased about by 63% and 94%, and 55% and 97%, respectively. This increase was justified by the presence of nanoclays, because they provided higher interaction areas with the matrix, and restricted the mobility of polymer chains and, consequently, the crack propagation through the material [19]. In 2011, [PERSON] et al. [20] characterized a sandwich material composed of a polyurethane/layered silicate foam core and aluminum skins. Nanoclays were added to the foam in contents ranging from 0% to 7.5 wt%, and impact tests were performed using a drop weight testing machine with an energy of 45 J. The best impact performance was obtained for specimens with 7.5 wt% of nanoclays. In 2011, [PERSON] et al. [21] produced a green sandwich composite material composed of a jute skin and a biopol core enhanced by nanoclays (between 0 wt% and 4 wt%). They analyzed the dynamic and bending properties, as well as the moisture absorption of the materials. The best results achieved in all tests were obtained with the highest content of nanoclays.