Valérie Nassiet

A Study of Mechanisms of Poly (PhenyleneSulfide) Thermal Degradation in Air

Because of its special properties and commercial significance, Poly(phenylene sulfide) (PPS) has been the subject of many research efforts since its commercial introduction in 1967. Intensive work has been done on its crystalline structure and morphology and its thermal behaviour. But fewer investigations have been carried out to understand long term behaviour in high temperature environments. Always anticipating industrial needs linked to power integration, we have launched an extensive study on thermal aging in air of PPS at 250°C. This study has shown that PPS thermal degradation in air happens by intermolecular branching reaction, similar to crosslinking. This phenomenon was already known for temperature above 300°C. This crosslinking is evidenced by rheometry where the relative position of G’ and G’’ above melting temperature changes with aging. IR spectroscopy confirms that para substituted benzene in PPS molecule is transformed in 1,2,4 trisubstituted benzene. DSC measurements evidence both an elevation of melting temperature and a change in melting endotherm showing significant changes in crystalline morphology along aging, which tends to indicate that crosslinking occurs in crystalline phase. Then degradation implies drastic loss of mechanical properties leading to destruction of the sample.

High Temperature Epoxy Foam: Optimization of Process Parameters

For many years, reduction of fuel consumption has been a major aim in terms of both costs and environmental concerns. One option is to reduce the weight of fuel consumers. For this purpose, the use of a lightweight material based on rigid foams is a relevant choice. This paper deals with a new high temperature epoxy expanded material as substitution of phenolic resin, classified as potentially mutagenic by European directive Reach. The optimization of thermoset foam depends on two major parameters, the reticulation process and the expansion of the foaming agent. Controlling these two phenomena can lead to a fully expanded and cured material. The rheological behavior of epoxy resin is studied and gel time is determined at various temperatures. The expansion of foaming agent is investigated by thermomechanical analysis. Results are correlated and compared with samples foamed in the same temperature conditions. The ideal foaming/gelation temperature is then determined. The second part of this research concerns the optimization of curing cycle of a high temperature trifunctional epoxy resin. A two-step curing cycle was defined by considering the influence of different curing schedules on the glass transition temperature of the material. The final foamed material has a glass transition temperature of 270 °C.

Chemical Structure Influence of Silicone Adhesives on Curing Process

The density increase of components used in power converters involves cooling problems and results to the use of new materials and innovative assembly processes (such as adhesive bonding). Searching for an adhesive family leads to the silicones. This paper focuses on the silicone adhesive formulation as regards the use requirements (temperature range, chemical agents). This first article is dealing with the influence of chemical structure on vulcanization process parameters and physical properties before ageing. The first adhesive family is the oxime terminated polydimethylsiloxane (PDMS) adhesive’s one which cures as soon as it enters in contact with air moisture. This cure process seems too sensitive to humidity and temperature and involves a too long time process for industrial applications. On the contrary the process for silicone cured by polyaddition is easy. Besides, the introduction of a few phenyl groups in dimethylsiloxane backbone makes the PDMS free from crystallisation ensuring the silicone flexibility in a higher use temperature range.

STUDY OF BONDED PLASMA-TREATED POLYETHERIMIDE COMPONENTS FOR POWER INTEGRATION: DURABILITY IN A HOT/WET ENVIRONMENT

This work deals with the study of the durability, in a hot/wet environment, of structural adhesively bonded polyetherimide (PEI) assemblies used in power electronics packaging technology. An overall approach is proposed, for which the epoxy joint-PEI substrates assembly on the one hand, and the adhesive system components (substrate surface and bulk adhesive) on the other hand, are studied separately with different analytical techniques. The first part of this work was devoted to the substrate surface state and to its modification using a cold plasma treatment of the PEI surface. Then for chosen parameters (power, duration) contact angle measurements indicated an increased surface tension resulting from surface decontamination (removal of release agent and carbon contaminants) and from the creation of polar species, such as esters or carboxylic acid groups, on the PEI surface (XPS analyses). The second part of this study concerned the bulk adhesive ageing in an ethylene glycol-water solution at 70°C. Mass uptake measurements versus time showed the liquid diffusion in the bulk adhesive associated with a microscopic damage of the epoxy system. An overall plasticizing of the adhesive with a considerable decay of the α-transition temperature of one of the two adhesive epoxy-amine networks (TGDDM-BAPP) was also highlighted using rheometry. However, in these ageing conditions, the adhesive glassy modulus decreases slighty because of the thermomechanical stability of the other epoxy network. In the third part, the asymmetric wedge test showed the beneficial effect of the cold plasma treatment on the epoxy/PEI interface durability in the aggressive medium.

Development of an Adhesion Test for Characterizing the Interface Fiber/Polymer Matrix

There are several models on the relationship structures and properties of the composite fiber / matrix interface [1]. Including literature proposes the development of micromechanical tests suitable for assessing the shear strength of the interface fiber / polymer matrix. The first test which allowed to characterize the fiber / matrix interface is the pull-out test developed by Broutman [2]. It consists in extracting the fiber from the matrix that can be in block form, gout or disk of resin. To reduce the variation in results due to the geometries used, it was agreed to use a drop of resin with small dimensions. The test is to characterize the fiber / matrix interface of natural thermosetting or thermoplastic by determining the shear stress.

Graft Interpenetrating Continuous Epoxy-Polysiloxane Polymeric Network

The objective is to design a joint, suitable for use from low to high temperature by combination of two adhesives along the overlap length in single lap joint. This mixed modulus concept is called Multi-module Bond line (MMBL). At high temperatures, a brittle adhesive (high modulus) in the middle of the joint retains the strength and transfers the entire load. At low temperatures, a ductile adhesive at the ends of the joint is the load-bearing adhesive. The first part of this work deals with the formulation of adhesives with differend stiffnesses to be used in the MMBL concept. Starting from a DGEBA resin/DETDA hardener system, different contents of amine terminated polysiloxane modifiers are added to the original mixture. A phase-separated structure is observed via scanning electron microscopy. The thermal, mechanical and dynamic viscoelastic properties of polysiloxane modified epoxy networks are studied. The second part of this paper will present the infinite element study of the assembly with two formulated adhesives in order to verify if they respect the MMBL concept.

In Situ Determination of Glass Transition Temperatures of Epoxy Adhesives in Structural Ceramic Assemblies

A method is described for measuring the glass transition temperature (Tg) of epoxy joints bonding ceramic (SiC) substrates. This method is based on the strain measure of a single-lap joint subjected to a temperature variation. The resulting displacement (d) is observed as a function of the temperature (T) by means of a contact strain gauge extensometer. Thus Tg value can be determined using (d-T) curves recorded. The influence of joint parameters (joint thickness and overlap length) and of other parameters such as the applied load and the surface state of substrates were studied for a structural epoxy adhesive showing different thermomechanical behaviours. The results show that it is possible, with appropriate experimental conditions, to measure Tg with this method and to find Tg values similar to those found by classical techniques such as the static thermomechanical analysis (TMA) and the differential scanning calorimetry (DSC). This method shows similarities with TMA, but it has the advantage to allow the thermomechanical analysis of adhesives without destructive sampling of joints.

Study of DGEBA and Novolac Adhesives Hydrothermal Ageing for a Ceramic/ Steel Substrates Bonding Assembly

The industrial application (not described for confidential reasons) needs the assembly of a ceramic piece and a steel one. This PhD research work aims to provide a reliable industrial bonding between ceramic and steel substrate using structural adhesive. This industrial joint shall withstand a wide range of temperature with pretty brutal thermal changes and also severe humidity conditions. As prerequisite to this bonding assembly qualification, the purpose of this work is the study of the durability, in a hot-wet environment. Depending on the formulations, we observe different behaviours in term of weight uptake, morphological, thermomechanical and chemical modifications of the epoxy network.

Study of DGEBA and Novolac Adhesive Solutions between Ceramic and Steel Substrates

The industrial application (under a confidential clause) concerns a ceramic/steel assembly. This PhD research is concerned with providing a reliable industrial bonding between ceramic and steel using structural adhesive. This industrial joint shall withstand a wide range of temperatures with brutal thermal changes. This paper focuses on the adhesive formulation. Indeed, we needed to compromise the adhesive flexibility: soft enough to resist the thermal stresses and compensate the coefficient of thermal expansion mismatch between the two substrates. We find this good compromise with DGEBA and Novolac epoxy based adhesive separated or blended. The influence of the different formulation parameters on the initial properties was studied by chemical, rheological and thermal tests on adhesive samples. Depending on those formulations, we observed different behaviours in term of glass transition temperature, cross-linking time and flexibility.