Abstract
Fumed silica (FS) is widely used in numerous fields of application, the plastics industry being one of the most significance, where FS has proved to be successful as an efficient thickening, thixotropic, and anti-settling agent, as well as reinforcing filler. Chemical modification of silica surface enlarges its functional capabilities. In particular, silica with grafted silicon hydride groups was found to be active in the processes of hydrosilylation of alkene and alkyne bonds in monomers during their polymerization, resulting in the formation of reinforced polymeric composites. Recently, specific epoxy resins have gained significance, and FS was found to be useful, particularly as rheological additive. The aim of this study was to evaluate the efficiency of hydride-silylated FS (HFS) as a potentially active reinforcing component for epoxy-based polymers. The activation energy for hydrosilylation of olefins is higher than that for ring-opening polymerization of epoxides, therefore, one may expect the latter process with participation of ≡SiH groups to proceed more readily. HFS was obtained via FS treatment with triethoxysilane. The presence of grafted silicon hydride groups was confirmed by means of IR spectroscopy, and their concentration measured by titrimetric and spectrophotometric analysis was found to be about 0.4 mmol/g. FS-epoxy and HFS-epoxy composites were prepared by the corresponding filler introduction (2 wt. % loading) into the mixture of epoxy monomer and amine hardener. The resulted materials after curing were subject to compression, bending, and adhesion tests. Compression tests revealed that filling with FS and HFS reduced the compressive strength by 10%, however, HFS-epoxy composite was found to possess an increased by 20 % Young’s modulus for compression as compared to that for the unfilled epoxy polymer. Upon this, 2 wt. % loading with silicas keeps the ductility of the polymer. Also, silica-containing epoxy polymers showed an improved bending strength and bending modulus, the former being two times higher for HFS-epoxy composite than that for the unfilled polymer. The adhesion to steel was found to increase by more than 2 times upon filling with silicas, HFS-epoxy composite being also superior as compared to the FS-epoxy one. Thus, preliminary results indicate that fumed silica with grafted silicon hydride groups shows promise as active reinforcing filler for epoxy polymers.
Highlights
Fumed silica (FS) is widely used in numerous fields of application [1, 2], the plastics industry being one of the most significance, where FS has proved to be successful as an efficient thickening, thixotropic, and anti-settling agent, as well as reinforcing filler [3]
Modification with triethoxysilane under quasi fluidised bed conditions applied only slightly decreased the specific surface of fumed silica – from 319 m2/g (A-300) to 310 m2/g (SiH-2)
As for the role of the specific surface groups, it appears that ≡SiOH groups are important for the bending strength improvement as well as for the increase in Young’s modulus and adhesive strength, while surface ≡SiH groups are responsible for an increase in Young’s modulus and adhesive strength. The latter parameter increases in higher extend when hydrophobic hydride-silylated silica is used instead of the hydrophilic-hydrophobic one. All of these findings suggest that, beside the stiffing function of rigid silica nanoparticles [11], the structuring role of surface ≡SiOH and ≡SiH groups may be an important factor for epoxy polymer reinforcement
Summary
Fumed silica (FS) is widely used in numerous fields of application [1, 2], the plastics industry being one of the most significance, where FS has proved to be successful as an efficient thickening, thixotropic, and anti-settling agent, as well as reinforcing filler [3]. Among the variety of the polymers, epoxides represent an important class of thermosetting resins. The abundance of their applications is vast, including, for example, aircraft, train and car manufacturing, electronic assembling, jewelry, and so on [7]. The activation energy for hydrosilylation of olefins (140–160 kJ/mol) is higher than that for ringopening polymerization of epoxides (~110 kJ/mol) [9], one may expect the latter process to proceed more readily with participation of ≡SiH groups at elevated temperature during the epoxy curing, and likely with no need to use the costly catalysts
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