Adhesive properties of low-cross-linking density cured epoxy resin
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Off-stoichiometry effect on the physical properties of epoxy resins
Amines generally tend to segregate at the interface of the epoxy resins cured with amines. To gain better insight into the aggregation states and physical properties at the adhesive interface, we examined the cross-linking structure and physical properties of the cured epoxy resins with an off-stoichiometric ratio of epoxy and amine. As the excess amine increased, the amine not only remained as unreacted monomers or low-molecular-weight isolated chains within the cross-linking structure but also formed dangling chain ends, increasing heterogeneity in the cured epoxy resin. As a result, the cross-linking density and mass density decreased, along with a reduction in the glass transition temperature. On the other hand, Young’s modulus increased with the excess amount of amine. Wide-angle X-ray scattering experiments, in conjunction with molecular dynamics simulations, revealed that the excess amine suppressed the widening of the distance between phenyl groups during the curing reaction, suggesting that this could act as steric resistance during deformation.
An innovative methodology for testing and selecting greener solvents for varnishing paintings
Organic solvents remain needed for certain treatments in cultural heritage conservation and restoration, including the application of traditional and synthetic resin varnishes on paintings. This research describes a collaborative approach for selecting and testing alternative greener sustainable solvents for this purpose. With commonly used solvent xylene targeted for substitution, initial solvent suggestions were made employing the SUSSOL (sustainable solvent selection and substitution) software tool considering key physical properties of the solvents and specified safety, health and environment hazard boundaries. In parallel HSPiP (Hansen Solubility Parameters in Practice) software was employed after the experimental determination of Hansen solubility parameters and the solubility of selected resins. Fourteen alternative solutions were selected and prepared from commonly used resins for application testing on reference substrates (e.g., Leneta cards and prepared canvas test boards). This testing included (1) the assessment of essential working properties, paint film solvent sensitivity, drying and film formation, and (2) standard coating tests such as water resistance, gloss, and color measurements. Comparable reference resin solutions used as varnishes, prepared with xylene and other solvents, were simultaneously evaluated. First standardized solvent tests provided a practical assessment of the sensitivity of the paint films to the substitute and reference solvents. Varnishing tests were subsequently performed on two historical paintings in order to evaluate the working properties and esthetic qualities of the resin solutions. The novel varnish Regalrez 1094 in isoamyl acetate showed very good working properties and gave a good visual result on both paintings. The resin solution Paraloid B72 in anisole showed good visual results on both paintings, while demonstrating acceptable working properties. This study illustrated that good working properties, protective and visual film qualities could be achieved with varnishes based on typical resins in alternative solvents. Various alternative greener solvents and their mixtures can thereby be suggested for dissolving and applying these resins. The approach within this study acknowledges the individuality of an artwork, which guides the selection of the most appropriate resin solution.
High-performance, multi-component epoxy resin simulation for predicting thermo-mechanical property evolution during curing
High-performance epoxy systems are extensively used in structural polymer‒matrix composites for aerospace vehicles. The evolution of the thermomechanical properties of these epoxies significantly impacts the evolution of process-induced residual stresses. The corresponding process parameters need to be optimized via multiscale process modeling to minimize the residual stresses and maximize the composite strength and durability. In this study, the thermomechanical properties of a multicomponent epoxy system are predicted via molecular dynamics (MD) simulation as a function of the degree of cure to provide critical property evolution data for process modeling. In addition, the experimentally validated results of this study provide critical insight into MD modeling protocols. Among these insights, harmonic- and Morse-bond-based force fields predict similar mechanical properties. However, simulations with the Morse-bond potential fail at intermediate strain values because of cross-term energy dominance. Additionally, crosslinking simulations should be conducted at the corresponding processing temperature, because the simulation temperature impacts shrinkage evolution significantly. Multiple analysis methods are utilized to process MD heating/cooling data for glass transition temperature prediction, and the results indicate that neither method has a significant advantage. These results are important for effective and comprehensive process modeling within the ICME (Integrated Computational Materials Engineering) and Materials Genome Initiative frameworks.
Fracture analysis under modes I and II of adhesive joints on CFRP in saline environment
This study analyzes the delamination behavior of adhesive joints after exposure to a saline environment for zero, one, and twelve weeks. Delamination was assessed under static and fatigue loading conditions in fracture Modes I and II, with a detailed analysis of fracture surfaces using Scanning Electron Microscopy (SEM) and Backscattered Electron (BSE) detection. The 3D images reveal significant morphological differences in fracture surfaces, showing variations in fatigue lines and the presence of impurities depending on the fracture mode. A probabilistic fatigue life analysis was performed using a Weibull regression model, showing notable changes, especially in Mode I at a high number of cycles. A chemical analysis using EDX and FTIR-ATR complemented the mechanical study, revealing an increase in sodium and chlorine concentrations with prolonged saline exposure. Oxidative degradation was also observed, with carbonyl groups increasing significantly over time, particularly in areas most exposed to the saline mist.
Immediate and delayed micro shear bond strength evaluation of two glass ionomer cements to composite resin by using different bonding techniques—an in vitro study
Evaluating immediate and delayed micro shear bond strength (µSBS) between composite resin and glass ionomer cements using different adhesive systems and mechanical surface treatment.
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