Impact of fillers on physical and mechanical properties of an epoxy matrix, and the effect of surface modification using a silane coupling agent.

Abstract

Fillers are used in coating and composite materials to enhance or introduce new properties. Deterioration of certain properties have been explained by a lack of interactions between the polymer and the filler. The interactions between a diepoxy-diamine matrix and four different fillers have been investigated in this work. The physical and mechanical properties of the epoxy matrix/filler materials have also been tested, mainly as free films, using a filler loading of 10 % of the CPVC of the fillers. Barium sulfate (BaSO4), mica (KAl2(AlSi3O10)(OH)2), aramid (Kevlar fibers) and wollastonite (CaSiO3) were the four fillers tested. XPS and FT-IR analysis were used to detect the possible interactions between the epoxy matrix and the filler particles. New peaks or chemical shifts were not observed, indicating lack of interactions. The surface of the filler particles was modified by a silane coupling agent, 3- aminopropyltriethoxysilane, to obtain a chemical bond between the epoxy matrix and the filler particles. The surface modified filler particles were examined with XPS and a successful surface modification was observed by presence of carbon, silicon and nitrogen (amine groups and protonated amine groups) on the surface of the fillers. Neither XPS nor FT-IR analysis revealed indications of interactions after surface modification. By using SEM for analysis of fractured surfaces of the epoxy/filler films it was observed that the surface modified filler particles were slightly more attached to the epoxy matrix than the unmodified filler particles. The stiffness and the Tg of the samples were analyzed using DMA and showed that all fillers increased the stiffness of the epoxy matrix, however, the addition of filler particles did not change the Tg. The strength and toughness of the epoxy/filler samples were tested with tensile and flexural tests using an UTM. The strength and toughness of the epoxy matrix decreased by the addition of all the four various fillers. This circumstance was explained by the filler particles acting as mechanical hinders for the elongation of the epoxy matrix. The permeability of water vapor was tested with Payne permeability cups and were found to decrease slightly by addition of fillers. The strength, toughness and permeability properties were improved by surface modification of the filler particles (stated with its exceptions). The differences observed in the physical and mechanical properties between the epoxy/filler samples (with unmodified and modified particles) were explained to be influenced the most by the size, shape and rigidity of the filler particles since differences in the interactions or main chemistry of the epoxy/filler samples (epoxy/BaSO4, epoxy/mica, epoxy/aramid and epoxy/wollastonite) were not observed.