Abderrahim Maazouz

Fracture behavior of epoxy polymers modified with core-shell rubber particles

Physical and thermomechanical properties of BDMA-catalyzed DGEBA/Dicy epoxy toughened with core-shell particles were studied. Relationships between these properties and the level of toughening are reported. The blends have been made in well-defined processing conditions. In fact, the resulting properties depend on the state of dispersion of the particles in the prepolymer matrix before crosslinking and on the cure schedule. The considered core-shell particles were of two types: i) poly(butadiene-co-styrene) core/carboxy-functionalized poly(methyl methacrylate-co-styrene) shell. Such core-shell particles have been dispersed in the epoxy matrix at different volume fractions (from 9.5 to 24%); ii) poly(butyl acrylate) core/carboxy functionalized poly(methyl methacrylate-co-styrene) shell. These particles have been dispersed at a volume fraction of 24%. Static mechanical tests were performed in tension and compression modes on these core-shell polyepoxy blends. A slight decrease of Youngs modulus and an increase of the ability to plastic deformation were noticed as the volume fraction of the core-shell particles increased. Using linear fracture mechanics (LEFM), an improvement of the fracture properties (KIC) was measured. Fatigue crack-growth studied for blends demonstrates that the Pariss law can be used to describe the behavior of the materials. Increasing the volume fraction of core-shell particles leads to an improvement of the resistance to fatigue crack-propagation. The same trend is noted on the impact behavior studied by means of high-speed tests performed in a large range of temperatures.

Toughening of epoxy networks using pre-formed core-shell particles or reactive rubbers

The influence of several additives (liquid reactive rubbers or core-shell particles-CSR) is studied on thermal and mechanical properties of an epoxy network based on diglycidyl ether of bisphenol-A (DGEBA) and dicy (DDA). Using high speed stirring, without any solvent, a good dispersion of the CSR is achieved. It is shown that the glass transition temperature (Tg)of the networks decreases when liquid rubbers are added but remains constant with the addition of CSR.It is demonstrated that every kind of particles favors the shear yielding of the networks, and well fit a modified Von Mises criterion.The fracture energies increase with the addition of rubbers, especially for the rubber with the higher acrylonitrile content. The same toughening effect is obtained with CSR if compared to the lower acrylonitrile content of liquid reactive rubbers but without any decrease in Tg of the epoxy network.

Curing of dicyandiamide epoxy resins accelerated with substituted ureas

An epoxy resin based on bisphenol-A-diglycidylether (DGEBA) was cured with dicyandiamide (DDA) and different substituted ureas (Urones) as accelerators. Differential scanning calorimetry, size exclusion chromatography, and Fourier transform infrared (IR) analysis indicated complex reaction mechanisms. The DDA granulometry seemed not to be a predominant parameter, but its content clearly influences Tg of the network. The maximum Tg value was found for an amine-to-epoxy molar ratio of 0.6. IR spectra revealed the appearance of oxazolidone rings, which are the first product of epoxy reactions. Urone accelerators also acted as homopolymerization initiators; they modified the epoxy conversion at gelation and changed of the structure of the obtained network. Influence of uron nature and content on reaction rates were also studied.

Shear and compression behaviour of sheet moulding compounds

At the present time, the rheology of sheet moulding compounds (SMC) during forming is not well known. In order to provide better experimental data, an experimental program was carried out as a result of the development of a new rheometer especially dedicated to this kind of material. Homogeneous simple compression and simple shear tests are presented. They allow the evaluation of the influence of the main parameters on the SMC behaviour: strain rate, temperature and fibre fraction. It is shown that the SMC can be considered as a strongly anisotropic non-linear viscous medium.

A Method of Estimating Kinetic Parameters of Thermoset Cures: Application to a Dicyanate Ester Resin

In this work, temperature modulated DSC is used to measure the heat flow evolved during the cure reaction of a dicyanate ester resin. This technique enables the heat capacity contribution to be separated from the heat source term (from the kinetic behavior) in nonisothermal conditions. At any given moment, therefore, the true reaction rate can be calculated without any assumptions about the shape of the baseline of the thermogram. Resulting accurate data are used for the characterization of cure kinetics. The isoconversional analysis shows that the apparent activation energy varies with conversion degree, so that a Kamal & Sourour kinetic model is proposed. Its kinetic parameters are obtained by an inverse method, and such a model is successfully used to simulate nonisothermal, isothermal and multistep cure experiments.

Fatigue behaviour of glass bead filled epoxy

There is a relatively abundant literature on the mechanical properties of particle filled thermosets. Detailed experimental data are available on the effect of variables, such as the filler volume fraction, its surface treatment or shape factor, on the usual properties. In the case of epoxy matrix composites, data have been published on elastic properties. Kinetic studies on thermoplastics, as well as microscopic investigations clearly show that each particle acts as a crack initiation site. The present study deals with thermoset epoxy-glass bead composites. A noticeable advantage of the sphericity of the glass beads over the previously studied mineral fillers is that theoretical calculations, for instance of interparticle average distance, are easier. Some results are reported concerning the eventual role of the geometrical characteristics, including particle diameter, number of particles per volume unit, particle-matrix contact area, interparticle distance, on the fatigue characteristics of the composite as assessed from Paris or Wöhler plots. In addition, quasi-static crack propagation characteristics will be compared with dynamic ones. It is clearly shown that glass beads improved the fatigue crack propagation. Despite this fact, it is also shown that even a small amount of mineral filler, acting as crack initiator, can considerably reduce the fatigue life of epoxy composites.

Synthesis of hairy acrylic core-shell particles as toughening agents for epoxy networks

Hairy poly(butyl actylate) (PBuAs) core-shell particles with a crosslinked shell were prepared by soapfree emulsion polymerization. The incorporation of the haity structure was obtained either by surface polymerization of a methacrylate (MMA) terminated poly(ethylene oxide) (PEO), or by physical adsorption of poly(ethylene oxide)-poly(propylene oxide) (PPO)-poly(entylene oxide) triblock copolymers. The particle shell was crosslinked during the synthesis to as to keep the integrity and morphology of the particle upen curing the epoxy network. Particle sizes and size distributions were determined both by quasi-elastic light scattering and transmission electron microcopy. Particle morphology was investigated by electron and atomic force microscopies. The presence of the poly(ethylene oxide) layer was evidenced by direct analysis of the latexes by means of 1 H NMR spectroscopy. Mixing of the core shell rubber particles with the reactive epoxy and processing of the toughened-epoxy networks are described. The influence of the hairy layer (with regarts to density and PEO chain length) on particle dispersion within the epoxy network, the resulting rheological behaviour of the core-shell (CS)/prepolyomer epoxy blends, and machanical properties of the modified epoxy were examined and discussed.

Rheological, Morphological and Mechanical Studies of Sustainably Sourced Polymer Blends Based on Poly(Lactic Acid) and Polyamide 11

The objective of this study was to gain a deep understanding of composition and compatibilization effects on the properties of entirely sustainably sourced polymer blends based on polylactide (PLA) and polyamide 11 (PA11). Generally, PLA cannot challenge regular commodity polymers due to its weak thermo-mechanical properties and its poor elongation properties. With this work, however, we present a promising route to overcome these drawbacks in order to enhance the processability of PLA: blending the polymer with various compositions of other ductile biopolymers such as PA11, as well as mixing PLA/PA11 blends with various amounts of a chain extender, Joncryl ADR®-4368, containing reactive epoxy functions, in a laboratory-scale twin-screw extruder. The effects on the rheological, morphological and mechanical properties were investigated. Results showed that a “self compatibilization” between PLA and PA11 chains can occur but it was found to be insufficient, contrary to recent work reported in the literature. The role of Joncryl as a compatibilizer for the PLA/PA11 system has been demonstrated by the significant decrease of particle size and interfacial tension as well as the improvement of ductile properties. Moreover, a new relaxation peak appeared in the relaxation spectrum, indicating the generation of a copolymer at the polymer-polymer interface.

A nonlinear shear and elongation rheological study of interfacial failure in compatible bilayer systems

This work aims to examine whether or not interfacial failure can occur in a compatible polymer bilayer system under large shear and elongation deformations, as well as to probe the sensitivity of nonlinear transient rheology to the presence of interface/interphase at neighboring layers. For this, stress relaxation after a step strain and fast startup in simple shear and uniaxial extension experiments have been performed on healed and coextruded poly(methyl methacrylate)/poly(vinylidene fluoride) bilayers with the presence of a robust diffuse interphase as evaluated by energy dispersive X ray. For unhealed bilayers, interfacial failure occurred in shear flows at intermediate deformations, while for healed bilayers the interphase greatly delayed the onset of interfacial failure to larger deformation steps and to a higher deformation rate in the startup shear. Extensional rheology demonstrated that the presence of an interphase in the bilayers greatly enhanced the transient extensional viscosity ηE+(t) as we...

Chemo-rheological study of a dicyanate ester for the simulation of the resin-transfer molding process

Abstract The kinetic and rheological behavior of a liquid dicyanate ester catalysed by a copper salt has been studied with a view to establishing behavioral laws and investigating the resin-transfer molding (RTM) process. The time/temperature transformation diagram of the system has been studied in order to predict gelation and vitrification, and the rheological study has enabled us to determine filling times during injection. An empirical model relating viscosity and conversion has been established in order to carry out a simulation of the RTM process.