Preparation and characterization of epoxy composites filled with functionalized nanosilica particles obtained via sol–gel process
Introduction
Organic materials cannot be used alone for high performance applications because they have limited properties. Therefore, organic/inorganic composites are frequently employed in order to overcome the limitation. One of the widely used organic/inorganic composites is an epoxy/silica system. Since epoxy resins as organic matrix have excellent heat, moisture, and chemical resistance and good adhesion to many substrates, they are mostly applied in the field of coatings, adhesives, casting, potting, composites, laminates and encapsulation of semiconductor devices [1], [2]. However, epoxy resins, due to their low-mechanical properties and high coefficients of thermal expansion (CTE) value compared with inorganic materials, cannot meet the requirements especially for the applications of electrical and structural such as epoxy molding compounds (EMC). Thus, silica particles are commonly used for the reinforcement of epoxy matrix to lower shrinkage on curing, to decrease thermal expansion coefficients, to improve thermal conductivity, and to meet mechanical requirements.
The intrinsic properties of each component, the shape of fillers, the nature of the interface, and so forth largely affect the properties of composite [3], [4]. It is well known that the load applying on the composites is mainly transferred to be fillers via the interface. Therefore, for excellent properties, strong interfaces between components are needed. Another important factors of fillers for affecting composite properties are their contents and size. To enhance the properties, smaller size and larger amount of fillers are required. It has been already reported that the increase of specific surface area and contents of fillers enhance the mechanical and impact properties of composite [5]. However, when the size of fillers becomes smaller and the content of fillers becomes higher, the viscosity of composite resin will be too high to process. In that case, the interfacial strength will be more important factor due to their increasing surface area of fillers. One of the most promising solutions for enhancing processibility at high filler-loading system is suggested to be the surface modification of filler [6].
In general, sol–gel method is widely applied to prepare oxide particles with various properties such as large porous gel spheres and small particles with high density. The preparation procedures of silica particles from silicon alkoxides in alcoholic solution were developed by Stober et al. and their resultant particles are excellent in monodispersity [7]. The advantages of the sol–gel process over the traditional ceramic synthesis process are the abilities to form pure and homogeneous products at low temperature [8], [9].
In this study, to investigate the effect of surface modification of fillers on composite properties, especially dynamic mechanical behavior and thermal properties, silica particles are synthesized by sol–gel process to get uniform size and surface properties. And then their surfaces are modified by substituting silanol groups into other functional groups by changing the degree of substitution (DS). It is also investigated how the surface modification of silica affects their dispersity and interfacial properties in composites.
Section snippets
Materials
Tetraethoxysilane (TEOS, Fluka) as alkoxide, epichlorohydrin (Acros), Tolylene-2,4-diisocyanate (TDI, Aldrich) and aminopropyltriethoxy silane (Aldrich) as surface modifiers were used as received without further purification. Ammonium hydroxide (Mallinckrodt AR, 26 degree) for sol–gel process of alkoxide and Pottasium iodide (Aldrich) and crown ether (Aldrich) for epoxidization were used as catalyst, respectively. Toluene (Daejung) as reaction medium were dried by distillation with sodium for 3
Silica preparation
In sol–gel process, many researchers have already reported that the size and uniformity of resultant silica particles are greatly affected by the concentrations of TEOS, water and catalysts, the types of catalysts, reaction temperature, etc. [14], [15], [16]. Especially, the types of catalysts, acidic or basic, have been considered as the most important factors in determining which mechanisms in silica growth are dominant. These mechanisms are generally classified into two types such as
Conclusions
The properties of epoxy/silica composites are highly influenced by the interfacial strength between matrix and fillers. Compared with S–NCO with bulky substituents, S–epoxide and S–NH2 with smaller and more reactive substituents show the better distribution of silica particles in epoxy matrix, because of better resin-wetability. Furthermore, strong interfaces are observed in fracture surfaces of composites filled with silica particles reactable with matrix below the temperatures of mixing and
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