| In this study, a series of novel long-chain silane coupling agent (SCA), containing a terminal glycidol ether group and polyether segment, was synthesized and employed as a modifier for silica particles in the aqueous sol. The modified silica particles were then added into the epoxy resin, the influences of different SCA and their amount on the surface modification of silica particles was investigated, including the rheological behavior of the silica/isopropanol sol, the dispersion of silica particles in epoxy matrix, and the strengthening and toughening effect of the modified silica particles on epoxy adhesives.The synthesis of the novel SCA involved two steps: the epoxidation of the allyl polyether (APP) with epichlorohydrin and sodium hydroxide, and hydrosilation of epoxidated APP with triethyoxyl silane. The structure of the product and the influences of catalyst, reaction time, temperature and molecular weight of APP on the reaction conversion were analyzed by infrared spectrometry, ~1H NMR and chemical titration (titration of epoxide value and iodine value), and the optimized condition were chosen in following research.Silica particles in silica sol were transferred into isopropanol by azeotropic distillation, and then treated by SCA(both KH560 and GPPTES), thus hydrophilic silica particles were converted into hydrophobic ones. After evaporating the solvent of the silica/isopropanol sol, and extracting with toluene several times, silica particles obtained were analyzed by Infrared spectrometry (IR) and Thermal Gravity Analysis (TGA). The results confirmed that a considerable amount of SCA molecules were grafted onto the surface of silica particles. Furthermore, the dispersion of silicaparticles in isopropanol sol was investigated by Transmission Electron Microscopy (TEM) and Dynamic Lightening Scattering (DLS), showing that a homogenous dispersion was obtained by SCA modification.The influences of different SCA and their amount on the surface active silanol value of silica were studied by silanol titration with sodium hydroxide. The results suggested that active silanol value was decreased as the increasing of the molecular weight of the SCA and their amount, which suggested that, unlike KH560, SCA with longer molecular chain covered the silanol groups physically besides reacted with them chemically.The rheological behavior of silica/isopropanol sol was studied and a strong shear thinning behavior was observed. On one hand, shear thinning behavior of the sol became apparent with increasing of the chain length and amount of the SCA, on the other hand, it maintained stable when saturation grafting was achieved.The modified silica particles were employed as a modifier for the epoxy adhesives by first dispersing silica/isopropanol sol in the solution of epoxy in acetone by ultrasonic and then evaporating the solvents under vacuum at 50°C.Transmission Electron Microscopy (TEM) was used to observe the dispersion of silica in epoxy adhesives. KH560 modified silica showed slight better dispersion than unmodified silica in an aggregate range of about 100nm, but GPPTES modified silica helped silica particles reached an even more homogenous dispersion in epoxy resin both before curing and after curing with no aggregation.Mechanical properties of silica filled epoxy adhesives were studied. Shear strength and uneven tear strength were both increased by addition of treated silica, but deceased by addition of untreated one. The longer molecular chain of the SCA, the better performance of epoxy adhesive showed in mechanical properties, whichresulted from better dispersion of silica and softer interface. In the range of 1%～5%, when silica particles disperse homogenously in epoxy matrix, shear strength reaches a peak at 2%, while uneven tear strength grows first dramatically(before 2%) and then gently(after2%).
|
PDF Full Text Request