Cold-Cured Epoxy-Based Organic–Inorganic Hybrid Resins Containing Deep Eutectic Solvents

Abstract

The development of improved cold-cured resins, to be used as either adhesives or matrices for FRP (fiber reinforced polymer) composites employed in the construction industry, has become the focus of several academic and industrial research projects. It is expected that the use of nano-structured organic–inorganic hybrid materials could represent a realistic alternative to commercial epoxy-based resins due to their superior properties, especially in terms of higher durability against: moisture, temperatures, harsh environments, and fire. In this context, organic–inorganic epoxy hybrids were synthesized by a modified sol–gel method without the addition of water. The experimental formulations were prepared starting from a mixture of a silane-functionalized epoxy resin, alkoxysilane components and a deep eutectic solvent (DES) based on a blend of choline chloride and urea. The latter was added in two different loads in order to analyze in depth its effect as a promoter for an effective dispersion of silica nano-phases, formed through hydrolysis and condensation reactions, into the cross-linked epoxy network. The produced formulations were cold-cured for different time spans in the presence of two hardeners, both suitable for a curing process at ambient temperature. In this first part of a wider experimental program, several analyses were carried out on the liquid (rheological and calorimetric) and cold-cured (calorimetric, thermogravimetric, dynamic-mechanical, flexural mechanical, and morphological) systems to evaluate and quantify the improvement in properties brought about by the presence of two different phases (organic and inorganic) in the same epoxy-based hybrid system.

Recent Advances and Trends of Nanofilled/Nanostructured Epoxies

“…These systems gained a large success due to their outstanding synergistic properties derived from the beneficial combination of nanosized phases into a bulk epoxy, differing in chemical composition, physical properties, and morphology of the final phases. They can find an advantageous application, for instance, in the field of “cold-cured” adhesives, i.e., those able to set and harden at ambient temperature, due to a marked improvement in properties, in particular the glass transition temperature and durability [ 67 , 68 ].…”

“…The extraordinary chemical and physical properties of a wide variety of experimental nano-epoxies enable a wide range of applications, varying from coatings to structural adhesives, from FRP matrix to electrical/electronic components. Epoxy nanocomposites have found advantageous application in many industrial fields [ 59 , 120 ] such as aerospace [ 114 ], aircraft [ 119 ], automotive [ 33 , 110 ], buildings [ 18 , 67 , 145 , 146 ], electronic devices [ 17 , 86 , 99 , 128 ], sensors [ 90 , 110 , 115 , 127 , 144 ] (even for biomedical applications), anti-static materials [ 9 ], abrasive tools [ 103 ], anti-corrosive coatings (for example in marine environment [ 147 ] or pipes [ 10 ]), and structural applications [ 29 , 68 , 109 ] with self-sensing properties [ 127 ]. Often, the improved properties allow an efficient usage for multifunctional purposes [ 27 , 69 , 148 ].…”

Experimental Cold-Cured Nanostructured Epoxy-Based Hybrid Formulations: Properties and Durability Performance

“…It cannot be excluded, however, that the systems cured with TETA hardener, irrespective of whether it is used in the hybrid or its control neat epoxy, did not reach the stable T g state (plateau) even after one year-curing, suggesting that it could continue to rise further at longer curing/aging times. The high T g obtained in the Hyb-L-B system, however, can be partly attributed to rigidity of the aliphatic ring in the PACM molecule [23]. Once again, evaporation was not observed in DSC curves of Hyb-L-A system ( Figure 2c); furthermore, it was possible to observe and measure a residual heat of reaction (Figure 3b), that was comparable with that measured for Hybrid DGEBA and Hyb-L-B systems.…”

“…Compared to the neat epoxy resin, the hybrid systems in Figure 7 exhibit a significant weight loss between 100 and 300 • C, due to evaporation of volatiles (water and mostly alkanols) arising from the condensation reactions within the siloxane domains. In the case of Hyb-L-B system, the observed weight loss is also due to the thermal decomposition of ammonium cations associated with DES [23].…”

“…DES was obtained by mixing choline chloride (ChCl) and urea (U) in a 2:1 molar ratio; the two chemicals were supplied by Iolitec GmbH (Heilbronn, Germany) with a purity greater than 97%. The content of ChCl-U was set at 2.5 parts per hundred resins (phr), corresponding to 1.5% by weight of total mixture [23].…”