Maëlenn Aufray

Residual stresses and practical adhesion: effect of organo-metallic complex formation and crystallization

Epoxy-amine liquid pre-polymers are often applied onto metallic substrates and cured to obtain painted materials or bonded joint structures. The overall performance of such systems depends on the interphase created between the epoxy-amine polymer and the metallic substrate. When epoxy-amine liquid mixtures are applied onto a metallic oxide layer, concomitant amine chemisorption and oxide dissolution occur leading to organo-metallic complex formation. Depending on the amine nature, as soon as the organo-metallic complex concentration is higher than the solubility product (e.g., isophoronediamine (IPDA)), these organo-metallic complexes crystallize as sharp needles. At the same time, the uncrystallized organo-metallic complexes react with the epoxy monomer to form, after curing cycle, a new network. Moreover, the crystal size increases with the solid/liquid contact time leading to an increase of intrinsic residual stresses and Youngs modulus. When aliphatic diethylenetriamine (DETA) was used, no crystallization occurred, but the interphase formation was observed. The aim of this study was to understand and to establish the role of crystallization of organo-metallic complexes formed within the interphase on the practical adhesion performance. As the crystallization of the organo-metallic complex depends on the nature of the amine, two amine hardeners were used (IPDA inducing the formation of crystals and DETA without formation of crystals). For DGEBA-IPDA systems, the ultimate load decreases while residual stresses increase when the liquid/solid contact time increases. When no crystal formation was observed (e.g., DGEBA-DETA system), residual stresses, coating Youngs modulus and ultimate load values all remained nearly constant irrespective of the liquid/solid contact time.

The role of the residual stresses of the epoxy-aluminum interphase on the interfacial fracture toughness

When an epoxy-diamine system (DGEBA-IPDA) is applied onto aluminum alloy (5754) and cured, an interphase having chemical, physical, and mechanical properties quite different from those of the bulk polymer is created between the substrate and the part of the polymer having bulk properties. To get a better understanding of the role of the interphase on the interfacial fracture toughness either a tri-layer (bulk coating/interphase/substrate) or a bi-layer model (bulk coating/substrate) were used for quantitative determination of the critical strain energy release rate (noted Gc). Indeed, as the interphase formation results from both dissolution and diffusion phenomena, we were able to control the interphase formation within coated systems by controlling the liquid-solid contact time and then to make tri- or bi-layered systems. The particularity of models used is to consider residual stress profiles developed within the entire system leading to an intrinsic parameter representing the work of adhesion between the polymer and the metallic substrate. The aim of this publication is to clearly establish the role of the interphase mechanical properties, such as Youngs modulus and residual stress on the interfacial fracture toughness. Results are presented and discussed for three different aluminum surface treatments (chemical etching, degreasing and anodizing).

Effects of maleic anhydride grafted ethylene/vinyl acetate copolymer (EVA) on the properties of EVA/silica nanocomposites

AbstractTernary nanocomposites based on ethylene/vinyl acetate copolymer (EVA), maleic anhydride-grafted EVA (EVAgMA), and nanosilica were prepared in a Haake Rheomixer. The structure of the EVA/EVAgMA/silica nanocomposites was characterized by Fourier transform infrared spectroscopy and field emission scanning electron microscopy. The blending sequence was found to have a significant effect on the microstructure of EVA/EVAgMA/silica nanocomposites and the dispersion behavior of the nanosilica in the EVA matrix. The tensile properties (tensile strength and elongation at break), thermal behavior, crystalline structure and weatherability of the nanocomposites were also studied. The results showed that the above properties of the nanocomposites were enhanced remarkably using 1 wt% EVAgMA.

Mechanical and Surface Properties of Chemical Vapor Deposited Protective Aluminium Oxide Films on TA6V Alloy

Mechanical, barrier and surface properties of aluminium oxide films were investigated by nanoindentation, microscratch and micro tensile tests, by isothermal oxidation and voltammetry, and by contact angle measurement. The films were grown on TA6V substrates by a low pressure MOCVD process from aluminium tri-isopropoxide. Modelling of local gas flow, gas concentration and deposition rate profiles was performed using the CFD code Fluent on the basis of an apparent kinetic law. Films grown at 350 °C are amorphous AlO(OH), the one at 480 °C is amorphous Al2O3 and the one at 700 °C is nanocrystalline -Al2O3. Scratch tests and micro tensile tests resulted in adhesive failure on the two films grown at low temperature whereas cohesive failure was observed for the high temperature growth. Sample processed at 350 °C presents significantly lower oxidation kinetics in dry air than the bare substrate. Contact angle changes approximately from 100 to 50 degrees for films processed at 350-480 °C and 700 °C, respectively. Concerning the electrochemical behavior in NaCl environment, polarization curves revealed that amorphous alumina coatings improved the corrosion resistance by comparison with the others oxide films. These consolidated results reveal promising combination of properties for the films grown at different temperatures with regard to the targeted applications.

Influence of moisture on the electrical properties of XLPE insulation

During their operating service, insulated power cables can be exposed to wet environment. The presence of moisture in cables surroundings may affect the properties of the used insulation material for instance, XLPE widely employed in MV and HV power cables insulation and therefore the reliability of the insulated cables. In order to examine the influence of wet aging conditions on the performances of XLPE insulated cables, samples (plates moulded from granules HFDE 4201-EC) of the same insulation material were exposed during 5600 hours to wet environment inside a cell simulating moisture. The XLPE material used in the present investigation is employed as insulation for medium voltage (MV) cables (18/30 kV). This work presents the results of the effect of aging under wet conditions on the electrical properties of XLPE. The goal of this paper was to investigate the eventual degradation of XLPE insulation under humidity effect by characterization techniques. For this purpose, measurements of dielectric losses factor, relative permittivity, volume resistivity and dielectric strength were performed.

Dielectric Spectra Analysis: Reliable Parameter Estimation Using Interval Analysis

Dielectric spectra of materials are often difficult to analyze since the common software algorithms and line shape functions do not always provide unambiguous data for the fitted parameters. In this paper, a new algorithm (called S.A.D.E. as SIVIA (Set inversion Via Interval Analysis) Applied to DiElectric spectroscopy), based on a global optimization algorithm which uses interval analysis, is presented. Taking into account the experimental error of each data point in the measured dielectric spectrum, our algorithm provides a confidence interval for every parameter of the dielectric function implemented in the software. This is demonstrated for an epoxy monomer with a sum of Debye relaxators as dielectric line shape function. S.A.D.E. is also able to deliver and guarantee the number of relaxation processes even if they are in part masked by other phenomena like conductivity or electrode polarization.

Study of SU-8 reliability in wet thermal ambient for application to polymer micro-optics on VCSELs

We present experimental data on the reliability of SU-8 polymer when used as a core material for the integration of microlenses on vertical-cavity surface-emitting lasers (VCSELs). The respective effects of a hot and humid environment on structural, mechanical and optical properties of this epoxy resist are investigated. High aspect-ratio SU-8 micropillars are found to keep a good surface morphology and a stable optical transmission, as well as a good adherence on the wafer. Thermal cycling is also studied to check material stability under electro-thermal actuation in SU-8 micro-opto-electro-mechanical system (MOEMS). These results are of great importance for the collective integration of low-cost SU-8-based passive or active microlens arrays onto VCSELs wafers for optical interconnects and optical sensing applications.

Adherence Measurements and Corrosion Resistance in Primer/Hot-Dip Galvanized Steel Systems

This paper focuses on the adherence during ageing of a primer (made of polyester resins crosslinked with melamine) applied onto hot-dip galvanized (HDG) steel for coil coating application and its influence on corrosion protection. A chromium-free surface treatment, composed of fluorotitanic acid, phosphoric acid, manganese phosphate, and vinylphenol was applied on the HDG steel to obtain high corrosion resistance and high adherence of a polyester and melamine primer. The influence of the manganese phosphate on the corrosion and adherence was investigated. To measure the adherence between the metal and the primer, a three-point flexure test was set up. The adherence was then linked with corrosion resistance during ageing, using electrochemical impedance spectroscopy.

Interval analysis applied to dielectric spectroscopy: a guaranteed parameter estimation

Abstract Dielectric spectra of materials are often difficult to analyze since the common software algorithms and line shape functions do not always provide unambiguous data for the fitted parameters. In particular, this article deals with epoxy/ceramics nano-prepolymers studied by dielectric spectroscopy. In this situation, both system (the prepolymer with nanofillers) and method (the dielectric spectroscopy) are complex. Taking into account the experimental error of each data point in the measured dielectric spectrum, the sofware based on a global optimization algorithm which uses interval analysis, provides a confidence interval for every parameter of the dielectric function implemented in the software. Then, this software is able to deliver and guarantee the number of relaxation processes even if they are in part masked by other phenomena like conductivity or electrode polarization.

Effect of moisture on breakdown voltage of polyesterimide used in electrical machines

This paper deals with the influence of humidity on breakdown voltage of polyesterimide. The specimens were submitted to an hydrolytic aging, under 90% humidity and 30°C temperature, until 3000 h. The breakdown tests were executed under AC and DC voltage. A statistical analysis of the breakdown voltage data was carried out using Weibull model. The 90% confidence bounds were calculated and plotted. The study shows that breakdown voltage varies versus the aging time. Its increase is allotted to the crosslinking of the material structure. Whereas its decrease is ascribed to the plastificization of the polymer. The breakdown voltage under DC ramp is higher than that obtained under AC ramp. The DC breakdown voltage depends on the polarity.