Absorption of water and corrosion performance of a clear and pigmented epoxy coating on Al-2024 alloy

The corrosion performance of Al-2024 substrates coated with an epoxy clear coating and a pigmented epoxy coating was investigated. The absorption of water in the pigmented coating was high compared to the clear coating. The clear coating provided a good barrier which is stable beyond 120 days of immersion in a 0.5 M NaCl solution. The shape of the Nyquist plot of the pigmented coating changed with immersion time and reveals the existence of physico-chemical processes in the coating and/or at the interface. Its impedance magnitude at low frequencies remained very high (~ 109 .cm2) after 72 days of immersion in sodium chloride solution. Both coatings exhibited good dry adhesion on Al-2024. The wet adhesion of the clear coating was poor while the stress (~ 9 MPa) required in a pull off adhesion test of the wet pigmented coating remained high

UV-cured self-replenishing hydrophobic polymer films

Self-healing functional polymer surfaces, designed with an intrinsic and spontaneous mechanism which replenishes the damaged surfaces with the original chemical functionalities, are of great interest to maintain a high performance of the functionality and extend the life-time of materials. We report self-replenishing UV-cured hydrophobic polymer films prepared through the incorporation of methacrylate-terminated perfluorinated-dangling chains into poly(ethylene glycol diacrylate) (PEGDA)-based networks. The films are able to spontaneously and fully self-replenish the surface hydrophobicity, multiple times, upon consecutive intentional damages. The rate of recovery was found to be dependent on the glass transition temperature (Tg) of the networks, which directly correlates to the PEG block length in the PEGDA oligomer used. This study demonstrates that an intrinsic self-replenishing mechanism can be implemented in new network architectures, created rapidly and efficiently by free radical UV-polymerization, which allows preparing self-healing functional polymer films in a faster and eco-friendlier way

Natural versus accelerated weathering : understanding water kinetics in bilayer coatings

Exposure to water is a key issue in the performance of multilayer coatings. It may take place in different forms, i.e. as rainfall, dew and humidity variation. Consequently, coatings will experience time-dependent water activity fluctuations. In industrial practice, coatings are subjected to artificial water activity fluctuations in weathering tests. Little is known about the connection between these tests and the reality experience by a coating. This article presents a theoretical investigation of the response of multilayer coatings to water activity fluctuations. This investigation is performed on the basis of a validated model for water transport in hydrophilic base coat/hydrophobic top coat systems. The study aims to understand how permeability and sorption properties determine the overall coating response to fluctuations. It is concluded that present accelerated weathering tests do not mimic natural weathering due to the response time of the considered systems, which are insensitive to rapid fluctuations

Natural versus accelerated weathering : understanding water kinetics in bilayer coatings

Exposure to water is a key issue in the performance of multilayer coatings. It may take place in different forms, i.e. as rainfall, dew and humidity variation. Consequently, coatings will experience time-dependent water activity fluctuations. In industrial practice, coatings are subjected to artificial water activity fluctuations in weathering tests. Little is known about the connection between these tests and the reality experience by a coating. This article presents a theoretical investigation of the response of multilayer coatings to water activity fluctuations. This investigation is performed on the basis of a validated model for water transport in hydrophilic base coat/hydrophobic top coat systems. The study aims to understand how permeability and sorption properties determine the overall coating response to fluctuations. It is concluded that present accelerated weathering tests do not mimic natural weathering due to the response time of the considered systems, which are insensitive to rapid fluctuations

Quantitative conductive atomic force microscopy on single-walled carbon nanotube-based polymer composites

Conductive atomic force microscopy (C-AFM) is a valuable technique for correlating the electrical properties of a material with its topographic features and for identifying and characterizing conductive pathways in polymer composites. However, aspects such as compatibility between tip material and sample, contact force and area between the tip and the sample, tip degradation and environmental conditions render quantifying the results quite challenging. This study aims at finding the suitable conditions for C-AFM to generate reliable, reproducible, and quantitative current maps that can be used to calculate the resistance in each point of a single-walled carbon nanotube (SWCNT) network, nonimpregnated as well as impregnated with a polymer. The results obtained emphasize the technique's limitation at the macroscale as the resistance of these highly conductive samples cannot be distinguished from the tip-sample contact resistance. Quantitative C-AFM measurements on thin composite sections of 150-350 nm enable the separation of sample and tip-sample contact resistance, but also indicate that these sections are not representative for the overall SWCNT network. Nevertheless, the technique was successfully used to characterize the local electrical properties of the composite material, such as sample homogeneity and resistance range of individual SWCNT clusters, at the nano- and microscale

Single-walled carbon nanotube networks in conductive composite materials

Electrically conductive composite materials can be used for a wide range of applications because they combine the advantages of a specific polymeric material (e.g., thermal and mechanical properties) with the electrical properties of conductive filler particles. However, the overall electrical behaviour of these composite materials is usually much below the potential of the conductive fillers, mainly because by mixing two different components, new interfaces and interphases are created, changing the properties and behaviours of both. Our goal is to characterize and understand the nature and influence of these interfaces on the electrical properties of composite materials. We have improved a technique based on the use of sodium carboxymethyl cellulose (CMC) to disperse single-walled carbon nanotubes (SWCNTs) in water, followed by coating glass substrates, and drying and removing the CMC with a nitric acid treatment. We used electron microscopy and atomic force microscopy techniques to characterize the SWCNT films, and developed an in situ resistance measurement technique to analyse the influence of both the individual components and the mixture of an epoxy/amine system on the electrical behaviour of the SWCNTs. The results showed that impregnating a SWCNT network with a polymer is not the only factor that affects the film resistance; air exposure, temperature, physical and chemical properties of the individual polymer components, and also the formation of a polymeric network, can all have an influence on the macroscopic electrical properties of the initial SWCNT network. These results emphasize the importance of understanding the effects that each of the components can have on each other before trying to prepare an efficient polymer composite material

Extreme wet adhesion of a novel epoxy-amine coating on aluminium alloy 2024-T3

Amine-epoxy polymer systems are widely used, for example as matrix materials for structural composites employed in aerospace industry and in industrial coatings on metal substrates for corrosion protection. This work focuses on the investigation of different epoxy-amine coatings on the adhesion performance on aluminum AA-2024 substrates. Two different epoxies (Epikote 828 (aromatic) and Eponex 1510 (aliphatic)) and four different amines (1,8-diaminooctane, Dytek A, Jeffamine EDR148 and Jeffamine D230) as curing agent were used in different stoichiometric ratios. These different epoxy-amine coatings were characterized using differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), tensile tests (pull-off) and water-uptake measurements. Pull-off tests in dry conditions showed comparable adhesion of the coatings. Surprisingly, pull-off results showed after water soaking a higher wet adhesion of the coatings prepared with Eponex 1510 as compared to coatings prepared with Epikote 828. Moreover, the combination of Eponex 1510-Jeffamine EDR148 coatings resulted in high adhesion values (~7 MPa) with pull-off tests and these values did not change after immersion for two weeks in water. This combination shows extreme good wet adhesion performance as compared to any other epoxy-amine coating. Complete recovery was demonstrated of the adhesion of Eponex 1510-Jeffamine D230 coating after being immersed for two weeks in water and dried for two weeks. Furthermore, in contrast with Epikote 828 water uptake measurements showed almost nil water uptake for all coatings prepared with Eponex 1510. Optical microscopy investigations on the residues of the coatings after pull-off tests revealed adhesive failure in wet condition for Epikote 828, while coatings prepared with Eponex 1510 showed cohesive failure

Single-walled carbon nanotube networks in conductive composite materials

Electrically conductive composite materials can be used for a wide range of applications because they combine the advantages of a specific polymeric material (e.g., thermal and mechanical properties) with the electrical properties of conductive filler particles. However, the overall electrical behaviour of these composite materials is usually much below the potential of the conductive fillers, mainly because by mixing two different components, new interfaces and interphases are created, changing the properties and behaviours of both. Our goal is to characterize and understand the nature and influence of these interfaces on the electrical properties of composite materials. We have improved a technique based on the use of sodium carboxymethyl cellulose (CMC) to disperse single-walled carbon nanotubes (SWCNTs) in water, followed by coating glass substrates, and drying and removing the CMC with a nitric acid treatment. We used electron microscopy and atomic force microscopy techniques to characterize the SWCNT films, and developed an in situ resistance measurement technique to analyse the influence of both the individual components and the mixture of an epoxy/amine system on the electrical behaviour of the SWCNTs. The results showed that impregnating a SWCNT network with a polymer is not the only factor that affects the film resistance; air exposure, temperature, physical and chemical properties of the individual polymer components, and also the formation of a polymeric network, can all have an influence on the macroscopic electrical properties of the initial SWCNT network. These results emphasize the importance of understanding the effects that each of the components can have on each other before trying to prepare an efficient polymer composite material

Tip-enhanced Raman mapping of single-walled carbon nanotube networks in conductive composite materials

Identifying and characterizing the structural integrity of single-walled carbon nanotubes (SWCNTs) that are fully embedded in a polymer matrix without causing any damage to them is a difficult task to achieve for bulk samples. Using tip-enhanced Raman spectroscopy, the surface of a polymer-embedded conductive network of SWCNTs was mapped underneath a thin layer of pure polymer. The technique was also used to detect tube-breaking within the composite sample caused by mechanical stress, beyond the 'visual' capabilities of scanning electron microscopy techniques. Results show that tip-enhanced Raman mapping can be used to successfully identify and characterize SWCNTs even underneath a layer of polymer

Hierarchical multi-scale simulations of adhesion at polymer-metal interfaces: dry and wet conditions

We performed hierarchical multi-scale simulations to study the adhesion properties of various epoxy–aluminium interfaces in the absence and presence of water. The epoxies studied differ from each other in their hexagonal ring structures where one contains aromatic and the other aliphatic rings. As aluminium is unavoidably covered with alumina, a cross-linked epoxy structure near an alumina substrate is created and relaxed by performing coarse-grained simulations. To that purpose, we employ a recently developed parameterization method for variable bead sizes. For polymer–metal interactions, a multi-scale parameterization scheme is applied where the relative adsorption of each bead type is quantified. At the mesoscopic scale, the adhesion properties of different epoxy systems are discussed in terms of their interfacial structure and adsorption behavior. To further perform all-atom simulations, the mesoscopic structures are transformed into atomistic coordinates by applying a reverse-mapping procedure. Interface internal energies are quantified and the simulation results observed at different scales are compared with each other as well as with the available experimental data. The good agreement between observations from simulations and experiments shows the usefulness of such an approach to better understand polymer–metal oxide adhesion