# Experimental Study on the Surface Properties of Nanoalumina-Filled Epoxy Resin Nanocomposites  
[PERSON]  
1 CQM--Centro de Quimica da Madeira, University of Madeira, Campus da Penteada, 9020-105 Funchal, Portugal; [EMAIL_ADDRESS]  
[PERSON]  
2 CEMMPRRE, Department of Mechanical Engineering, University of Coimbra, 3030-788 Coimbra, Portugal  
nano hardeners (i.e., nanoparticles (NPs)) at very low contents (usually described in terms of the weight fraction (wt. (%)) or volume fraction (vol. (%)) [2; 3; 7; 8; 9; 10; 11]. In recent years, special attention has been paid to EPs filled with NPs. These nanofillers (e.g., silicon dioxide (SiO\({}_{2}\)) [12; 13; 14], titanium dioxide (TiO\({}_{2}\)) [15; 16], aluminum oxide (Al\({}_{2}\)O\({}_{3}\)) [17; 18; 19]) are characterized by their specific shape and size (nanoscale), high surface areas, surface pretreatment, and the degree of dispersion into the EP matrix [1; 2; 4; 20; 21]. In recent years, many studies have demonstrated that the inclusion of inorganic NPs into EPs has the capability to improve the stiffness, toughness, flexural modulus, fracture toughness, strength, hardness, and many other properties of the final PCs [1; 2; 3; 7; 8], which are necessary in many engineering applications. These improvements are achieved without sacrificing the basic properties of EPs and with low percentages of NP loading [3; 8; 17; 22; 23; 24]. Among the available NPs that have been studied and that can be used to reinforce EPs, alumina NPs constitute a good solution. The good results published in recent years, combined with the low cost of alumina NPs when compared to other NPs (for instance, alumina NPs are cheaper than titanium NPs), its high modulus (about 300 GPa), and high thermal resistance in comparison with others metal oxides NPs, show that functionalized alumina NPs are a viable and very promising solution to be used as nanoenforce for PCs [1; 2; 4; 8; 22; 24]. Among the curing agents used as hardeners for EPs, some studies show that bis (4-aminophen) methane (DDM) [1; 2; 25; 26; 27; 28] or 3-dodec-2-enyloxolane-2,5-dione (DDSA) + 8-methyl-3a,4,7,7a-tetrahydro-4,7-methano-2-benzofuran-1,3-dione (MNA) [28] are suitable for EP systems. Both the physicochemical and topographical properties of surface materials are important for adhesion and coatings applications [29; 30]. EP adhesives filled with NPs have also been recently investigated [20; 21; 31; 32]. It was found that the inclusion of NPs can improve the wetting behavior, surface roughness, wear resistance, and shear strength, which are important properties for the resulting epoxy resin nanocomposites (EPNCs) to be used as adhesives or coatings. The wettability of solid surfaces, including PCs, can be studied through the contact angle (CA) or the spreading area of a liquid over the surface. The contact angle, \(\theta\), is the angle conventionally measured through the liquid, where a liquid-vapor interface meets a solid surface (see Figure 1). Smaller CAs or larger spreading areas imply higher wettability [20]. The CA on a rough surface of the same material, also called the apparent (or measured) contact angle \(\theta^{*}\) (see Figure 1), is smaller. Therefore, the material is more wettable. This is because the surface area is increased due to roughness (the real surface area is higher than the geometrical surface area). Therefore, wettability and roughness are strongly related. The wetting characteristics of surface materials using functionalized NPs depend strongly on the NP distribution within the surface layers [21]. The surface roughness of PCs can be analyzed by atomic force microscopy (AFM), which is a relatively new technique used for the surface characterization of polymers. By using AFM, it is possible to obtain images of a non-conducting polymer surface and study its mechanical properties. Compared with other microscopy techniques, AFM does not involve any chemical etching, staining,  
Figure 1: Definition of contact angle. or electron beam radiation, which can damage the polymer surface [6].