Epoxy–amine systems are ubiquitous in the field of industrial thermosetting polymers, often used in a moist atmosphere. In addition, previous studies showed amine–metal interactions through the ...formation of an interphase, with the formation of surface complexes that may involve the formation of water molecules. However, to date, the impact of water on an epoxy/amine–metal interphase has not been specifically addressed. In this work, we examined for the first time the role of this potential fourth component by way of a dual experimental/computational approach. The effect of water on the glass-transition temperature of the obtained polymers was quantified. The in situ formation of a DETA–Al–water interphase was followed by mixing calorimetry. The DETA–water interaction was highly exothermic, and the underlying mechanism was discussed on the basis of DETA hydration, which was confirmed by density functional theory (DFT) and Monte Carlo simulations. Taking into account the pre-existing interaction between diethylenetriamine (DETA) molecules allowed us to model all experimental data. Comparison of experimental and calculated IR spectra contributed to validate the simulation parameters used. Our findings indicate that the presence of water may noticeably affect epoxy–amine-based systems. Mixing calorimetry and computational modeling appear as particularly adapted tools for the comprehension of such complex systems.
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•Exploring the formation of the amine-metal interphase by in situ mixing calorimetry•The amine-metal is significantly exothermic and the phenomenon was quantified•Al and Cu surface ...reactivity with DETA amine were compared•Copper exhibit a significantly more exothermic reaction with DETA than aluminum
Epoxy resins are ubiquitously encountered in industrial applications as in adhesives and composites. The properties of epoxy-amine networks are directly impacted by the presence of metal (hydr-oxidized) surfaces, leading to a modification of their glass transition temperature Tg. We propose here an innovative experimental approach, investigating the interaction of DETA amine and DGEBA epoxy with Al and Cu powder substrates (partially (hydr)oxidized). We explored for the first time the formation of the amine-metal interphase by in situ mixing calorimetry to evaluate the energetics of interaction. While DGEBA interacted only slightly with Al-based surface, the reaction with DETA was associated with a high exothermic enthalpy of reaction. The enhancing role of surface hydroxylation was also evidenced by comparing boehmited Al to a simply oxidized counterpart. An even larger exothermic effect was measured with copper, which was related to the high chelating power of Cu compared to Al. The possible underlying mechanism of amine-metal interphase formation was discussed with a generalized schematic.
Flax fibers are particularly relevant in composite fabrication due to natural availability and mechanical properties close to glass fibers. We explore flax fiber-containing epoxy-amine (DGEBA/DETA) ...polymers with wide industrial applicability. Flax fibers impact the glass transition temperature (DSC), with a Tg drop of 67 °C at 30 wt% loading. For deeper insight, we develop here an original mixing calorimetry approach to follow in situ DGEBA/fibers and DETA/fibers interphases. DGEBA does not interact quantitatively with flax fibers while DETA/fibers interaction is significantly exothermic and directly related to fibers content. DETA/water interaction only accounts for 25% of total heat effect. Experiments run by contacting DETA with each of the main components of flax fibers (cellulose, hemicelluloses, lignin) point out systematic exothermic effects. Considering their mean proportion in flax fibers, the sum of enthalpy contributions agrees well with the overall effect measured with entire fibers. The main contribution (58%) arises from cellulose. XRD analyses and literature considerations allow concluding on the direct implication of DETA molecules with cellulose chains, resulting in Tg decrease. This work clarifies the chemical role of flax fibers in DGEBA/DETA thermosets, down to the molecular-scale, and highlights that mixing calorimetry is a powerful tool to follow interphase formation in situ.
We develop a mixing calorimetry approach to follow in situ interphase formation between flax fibers and DGEBA or DETA. DETA/fibers interaction is particularly exothermic. Each component of flax fibers leads to exothermic effects, cellulose giving the most prominent enthalpy contribution. DETA interacts with cellulosic chains, which limits the amount of “reactive” amino functions for polymerization and decreases the Tg value. Display omitted
•Addition of flax fibers in epoxy-amine mixture leads to Tg decrease (67 characters) .•Mixing microcalorimetry does not show interaction between DGEBA and flax fibers (84 characters) .•DETA/fibers interaction is significantly exothermic (63 characters).•Cellulose, hemicellulose, lignin and water reacted exothermically with DETA (76 characters) .•Cellulose is responsible for 58% of the heat effect despite its crystallized state (83 characters).
•Infrared radiation provides a substantial decrease in the cure time.•Under Infrared, a non-thermal effect can be highlighted at low temperature.•It arises from the absorption of infrared radiation ...by epoxy groups.•It promotes only reaction between epoxy groups and primary amines.
In the industry, the cure time of two-component adhesives is very important for a cost-effective manufacturing. Too fast, it does not favor the application of the product and the control of bonded joints. Too slow, it leads to long process times and too high process costs. The best compromises are two-component adhesives that cure slowly at room temperature and can reach full polymerization in minutes, on demand. In this paper, the curing behavior of a model poly-epoxide adhesive (a stoichiometric mixture of a pure epoxy and amine) polymerized with infrared radiation will be studied. The kinetic follow-up of this polymerization will be carried out by thermal analysis (determination of the residual heat peak by Differential Scanning Calorimetry-DSC). This study paves the way to a cold and universal cure-on-demand process, which means achieved in few minutes at low temperature without any initiators, catalysts or accelerators. Basically, infrared curing can be possible thanks to an increase in temperature (called thermal effect). But it has been shown that a “non-thermal effect” could also be involved in accelerating kinetics with infrared. This increase due to a non-thermal effect, suggested as a function of the infrared radiative flux, has been shown to be possible thanks to the absorption of infrared radiation, leading to a reduction in the energy barrier of the primary epoxy/amine reaction.
In the framework of the adhesive bonding, the assessment of interfacial properties has an essential role in determining the adhesive joints' global responses. Various surface preparations are ...available for each type of metallic substrate. There are plenty of tests widely used for mechanical characterisation to determine the adhesive properties and few tests to assess interfacial properties. For such a case, a specific three-point bending test can be applied to examine the interactions between adhesive and substrate. On the other hand, a direct comparison of the critical force is not always possible because of geometrical incompatibility. A practical solution for the last issue is applying a coupled stress-energy criterion (CC) since the interface properties are independent of the substrate thickness. Hence, CC and the macro-element technique were applied to determine the interfacial properties using an aluminium alloy 2024-T3 as substrate and the adhesive DGEBA/DETA™ under many preparation conditions.
As a result, the three-point bending test's overall behaviour was established in terms of interface strength (adherence), including incremental energy release rate and critical stress. Thus, this paper can be read as the first work towards the ability to predict the interface failure in the frame of three-point bending test using different geometries. In conclusion, the occurrence of an adhesive or cohesive failure and the unstable or a stable failure propagation of the bonded joints were sorted and classified as a function of interface properties.
In the framework of lightweight structures, the bonded joints appear as a suitable solution for increasing the mass-to-strength ratio. However, the full understanding of the interface debonding is a ...key obstacle to be overcome in critical systems. For characterizing adhesive-to-adherend interface crack initiation, the three-point bending test has been shown as a useful test that provides an identifiable small round zone. The samples were manufactured using an aluminum alloy 2024-T3 etched with nitric acid. The adhesive consisted in an epoxy pre-polymer DGEBA, DETA amine and an organosilane GLYMO directly introduced in the mixture. A coupled stress and energy criterion was used for the assessment of the properties of the interfacial debonding, since a stress concentration appears near to the corner of the specimen. For a quick computation, the application of the coupled criterion (CC) used the macro-element technique. On the other hand, most finite–element-based software has already implemented the cohesive zone modeling (CZM). In this sense, the fracture parameters computed via CC were used for the assessment of cohesive zone modeling of adhesive-to-adherend interface crack initiation. The results of fracture initiation using the CZM showed a good agreement in a macroscale response with the experimental campaign, thus providing a useful tool for a rapid estimation of cracking initiation.
In the present work, a specific three-point bending test is applied to evaluate how the roughness can impact the bond strength (adherence) and the mechanism of interfacial failure initiation. The ...study is conducted using an aluminum alloy 2024-T3 as substrate and the DGEBA (polyepoxide bisphenol A diglycidyl ether) /DETA (diethylenetriamine) as adhesive, considering different abrasive surface treatments. An optimal roughness is reached to maximize the critical force during failure initiation; besides, the roughness impacted the failure propagation mechanism and the failure initiation area for each abrasive treatment. A power-law regression is considered to correlate the critical force and the failure initiation area, considering different average roughness. Local assessment using a mechanical profilometer and Scanning Electron Microscope (SEM) with Focused Ion Beam (FIB) are applied to measure the residual adhesive thickness at failure initiation and failure propagation zones as well as the initiation-to-propagation transitions. It is constated that the residual adhesive thickness is dependent on the average roughness of the substrate. Finally, Weibull's analysis is undertaken to demonstrate that the mechanism of failure initiation for all substrate groups is essentially the same, although the failure propagation mechanism can be different.
Recently, the aerospace industry has been facing many challenges, including an increase in the production rates to meet the market needs. In the context of adhesives and liquid shim applications, ...this means the possibility of on-demand curing. In other words, adhesives must cure slowly at room temperature and this process must be accelerated at any time to allow for the fastest polymerization possible. However, while on-demand curing is possible in several ways (ultraviolet radiation, induction, or microwave), the route chosen in this study is infrared (IR) radiation. This is because this method allows curing at low temperatures (i.e., around 50°C) and is universal, hence requiring no modification in the adhesive formulation.
Given that the acceleration of polymerization using thermal (temperature) and nonthermal (radiation–matter interaction) effects has been demonstrated in another study, it is now important to study the properties of such an adhesive after curing under IR radiation.
In this study, we measured the following properties: adherence on aluminum 2024-T3 via three-point bending, tensile strength and modulus, and flexural strength and modulus. We also studied the parameters of the IR lamp, including the lamp–adhesive distance and the rate and temperature of polymerization. For this purpose, a composite design of experiments was used, which generally has two main advantages: screening and response surface methodology. On the one hand, screening allows determining the factors, among those selected, that have a significant influence on the studied responses. At the same time, it allows determining the interactions (synergistic effects) between the influencing parameters. On the other hand, response surface methodology allows quantifying the influence of the parameters and determining the optimal ones.
Bi-functional water-soluble polymers have been investigated as bonding layers (BL) on aluminum surfaces to promote the adhesion of adhesives for structural bonding applications in the automotive ...sector. The BLs are located at the interface between two parts: the aluminum substrate and the adhesive (an epoxy-dicyandiamide). The formation and the morphology are studied mainly by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). Many process parameters have been investigated such as: i) long or short immersion time in the treatment baths containing the polymers, ii) formulation using three different concentrations. The performances of the BLs under these different operating conditions are evaluated via several mechanical tests: micro-scratch, three-point bending, single lap shear. Results were compared with results obtained with already existing products, launched for the same application, surface treatment prior to structural adhesive bonding of aluminum in transportation.
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