# Mechanical Properties of Sandwich Composites Reinforced by Nanoclas: An Overview
## 2 Sandwich Composites Incorporating Nanoclays into the Core
In terms of mechanical properties, the benefits observed were explained by the brittleness of the nanophased material, reducing the strain percentage, while the lowest moisture absorption is due to the nanoclays that acted as a moisture barrier and, therefore, did not allow the water to penetrate the material [21]. Similar tests were carried out with materials exposed to moisture, and the best results were obtained with samples enhanced by nanoclays. This evidence was explained by the lower water absorption of the nano-enhanced material. In 2011, [PERSON] et al. [22] studied the mechanical performance of nanoclay-filled syntactic foams after immersion into salt water or de-ionized water. The weight fraction of nanoclays used in this study ranged from 0% to 5%; the volume fraction of glass microballoons was around 60%. After hygothermal conditioning, the samples were subjected to high strain rate, compression and dynamic mechanical analysis tests. It was possible to conclude that the addition of nanoclays had a negligible effect on the uniaxial compressive properties, although some improvements were observed for samples with 2% nanoclays. Regarding the viscoelastic properties--and when compared to neat syntactic foam--the storage modulus and T\({}_{\text{g}}\) increased up to 2 wt% of nanoclays, regardless of whether the samples were dry or wet. However, exposures to salt water promoted the worst results due to the weakening of chemical bonds between polymer chains. In 2013, [PERSON] et al. [23] evaluated the low velocity impact response of a sandwich composite produced by carbon fiber skins and polyurethane (PU) foam core. Three organically modified nanoclays (Cloisite(r) 10A, Cloisite(r) 30B and Nanomer(r) 128E) and maple wood flour were used as fillers for the PU foam core. Sandwich composites contained 0.5 wt% and 2.5 wt% of nanoclays and wood flour, respectively. The impact energies used were 10, 20 and 30 J; the pneumatic rebound brake system prevented multiple impacts on the specimen. It was possible to conclude that, for energies up to 20 J, the nanophasic core sandwic had higher peak loads, however, when wood flour was incorporated instead of nanoclays, the peak load decreased in comparison to the neat cores. On the other hand, the peak load values obtained for 30 J did not show as much improvement as those observed for 10 J and 20 J. The failure analysis evidenced extensive damage in all sandwic for the energy of 30 J, while for 10 J only penetration in the top facesheet and partial penetration of the core was observed. Regarding the sandwic tested at 20 J, authors observed delaminations at the interfaces between the core and facesheets, except for those involving organically modified nanoclays from Cloisite(r). Later, in 2014, [PERSON] et al. [24] performed impact testes to evaluate the mechanical properties of sandwich panels with nanoclay-filled polyurethane foams and glass fiber-reinforced polyamide and polypropylene facesheets. Nano-enhanced foam cores with organophilic montmorillonite loadings between 0 wt%-10 wt% were produced for this study. All sandwich panels were subjected to low energy impact tests, in a drop weight testing machine with an energy of 15 J, as well as compression tests to determine their quasi-static behavior. These authors achieved improvements in terms of absorption energy between 66% and 92% for sandwic with polypropylene facesheets and 23% and 43% for sandwic with polyamide facesheets. In terms of quasi-static tests, the usage of different facesheets showed a negligible effect on the compressive properties. However, for both materials, the compressive strength decreased with increasing filler loading. However, an increase in the compression modulus of 20%-37% was observed for sandwic with polyamide facesheets. the authors explained this behavior by the reaction of the clay with the foam core matrix, which has weaker characteristics and leads to the creation of large voids.