Environmental aging of cold‐cured epoxy‐silica hybrids prepared by sol−gel process

Abstract

The article investigates the effects of long term environmental aging on thermal and mechanical properties of epoxy-silica hybrids. These nanostructured materials, prepared by non-aqueous sol-gel process and in situ generation of nanosilica during epoxy curing at room temperature, present the potential to be used as cold-cured adhesives for civil engineering and Cultural Heritage applications. A specifically developed conditioning procedure for these cold-cured nanostructured materials was applied before moisture/water absorption tests. The work evidenced the superior durability of the studied epoxy-silica hybrid, which kept its performances in severe, but realistic, environmental conditions with respect to traditional epoxy adhesives. The reduction in the glass transition temperature and mechanical properties of the studied epoxy-silica hybrid, observed in the first weeks of environmental aging, was followed by a significant recovery. This was attributed to two concomitant phenomena: the reactivation of the incomplete curing reactions in the epoxy domains and the continuation of the condensation reactions in the siloxane domains activated by the absorbed water. Finally, the Fickian behavior, presented by the studied epoxy-silica hybrid, was used as an indirect indication of the homogeneity of achieved microstructure, with well dispersed silica nanostructures in the epoxy network. V C 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40093.

Environmental effects on the adhesion properties of nanostructured epoxy‐silica hybrids

“…The O‐I epoxy‐silica hybrid system, in fact, exhibits a comparable stiffness but a noticeable improvement in flexural strength (almost doubled) compared to the control one, which presents mechanical properties comparable with those found in commercial epoxy adhesives tested in previous works . This distinct advantage of the O‐I hybrid cured at ambient temperature over the parent epoxy resin is due to the unique morphology of the nanostructured adhesive, resulting from the chemical interaction of the silica nanodomains with the thermosetting matrix at a molecular level and the restriction of molecular dynamics at the interface between the organic and inorganic regions of the adhesive, as previously found by the authors …”

“…Moreover, the strength after water immersion of the O‐I hybrid resin (40.2 MPa) is about 53% higher than the strength of the un‐immersed control resin (26.2 MPa). This remarkable result is due to the fact that the siloxane domains of the epoxy‐silica hybrid cured at ambient temperature undergo further sol–gel reactions during aging under high humidity conditions, which counteracts the plasticization phenomena . This constitutes a distinct advantage of epoxy‐silica hybrid systems over corresponding conventional epoxy resins cured at ambient temperature for outdoor applications.…”

“…Before application, part A and part B were mixed at laboratory temperature for 10 min. After curing, the hybrid adhesive had a nominal silica content of about 15 wt %, which, from previous work of the same authors, has been found be the optimal silica content able to provide the adhesive in isolation with the highest mechanical strength and the maximum retention of properties when the adhesive is exposed to mild temperatures or immersed in water …”

“…Unlike other O‐I hybrid epoxies reported in literature, the hybrid epoxy‐silica systems proposed by the authors are the only ones able to polymerize at ambient temperature, thus developing superior performances compared to unmodified epoxy resins cured at ambient temperature at comparable production costs. From previous studies has been, in fact, verified that the presence of inorganic co‐continuous domains in epoxy‐silica hybrids enhances the load‐bearing properties of the adhesive cured at ambient temperature …”

Curing kinetics of epoxy-deep eutectic solvent mixtures

“…ILs can overcome the major drawbacks of commercial latent curing agents, i.e., a possible inhomogeneous cure due to a difficult dispersion of these solid compounds in the resin, especially in the case of a fiber composite matrix. The control of the composition of epoxy systems with new curing agents or catalytic components is a topic of considerable interest due to the great versatility of epoxy resins for different applications, including coatings, electric and electronic devices, fiber-reinforced composite materials [24], structural adhesives [25,26], polymer nanocomposites [27], and many others [28,29].…”