J.M. Gloaguen

Elaboration of EVA–nanoclay systems—characterization, thermal behaviour and fire performance

In this study, the effect of several parameters (nature of clay and clay loading) on the fire retardancy of the nanocomposite is investigated. It is observed that the nature of the cations, which compensates the negative charge of the silicate layers, affects the fire performance even though they are improved for both investigated montmorillonite-type fillers. The clay loading also affects the fire properties. In a second part, the materials are characterized using thermogravimetric analysis and small-angle X-ray diffraction in order to better understand the obtained fire retardant performance. It is observed that the degree of dispersion determined by X-ray diffraction can be correlated with the fire performance of the polymer clay composite.

Plastic deformation behaviour of thermoplastic/clay nanocomposites

Abstract Polymer/clay nanocomposites are materials that display rather unique properties, even at low clay content, by comparison to more conventional mineral-filled polymers. Two systems are considered in the present study: the first one consists of nylon 6/clay hybrids in which in situ polymerisation is aimed at obtaining a nylon matrix strongly bonded to the delaminated clay platelets. The second one is prepared by melt dispersion of organophilic clay in polypropylene, which should in principle result in a reduced degree of polymer–clay interaction. Dynamic viscoelastic analysis is indeed indicative of a noticeable difference when referring to the molecular dynamics of the glass transition. Plasticity results, in which volume strain is recorded by video-extensometry, show extensive cavitational behaviour while retaining a fairly large strain at break, as long as deformation is performed above the glass transition temperature of the matrix. In the particular case of PA6, it is clear that the usual shear banding plastic deformation mode is altered at least in its initiation step. Localised interfacial damage promotes extensive polymer matrix fibrillation and fracture occurs predominantly in areas where delamination of the clay platelets was not fully achieved.

Elasto-viscoplastic constitutive equations for the description of glassy polymers behavior at constant strain rate

In this study, a modelization of the viscoplastic behavior of amorphous polymers is proposed, from an approach originally developed for metal behavior at high temperature, in which state variable constitutive equations have been modified. A procedure for the identification of model parameters is developed through the use of experimental data from both uniaxial compressive tests extracted from the literature and uniaxial tensile tests performed in this study across a variety of strain rates. The numerical algorithm shows that the predictions of this model well describe qualitatively and quantitatively the intrinsic softening immediately after yielding and the subsequent progressive orientational hardening corresponding to the response of two polymers, amorphous polyethylene terephthalate and rubber toughened polymethyl methacrylate.

Plasticity of rubber-toughened poly(methyl methacrylate): effect of rubber particle size

The plastic deformation of rubber-toughened poly(methyl methacrylate) has been investigated in compression at constant strain rate as a function of particle volume fraction and particle size. The behaviour at yield appears insensitive to particle size for particle diameters d in the range 80 nm < d < 300 nm. In contrast, work-hardening rate measurements in the pre-yield region reveal an abrupt change in the materials ability to develop plasticity. This transition from difficult to easy shear-band formation as the rubber volume fraction is varied is shown to occur for the same surface-to-surface interparticle distance, whatever the particle size.

Constitutive modelling of the large inelastic deformation behaviour of rubber-toughened poly(methyl methacrylate): effects of strain rate, temperature and rubber-phase volume fraction

A combined approach including experimental investigation and constitutive modelling was followed in this work to study the stress–strain behaviour of rubber-toughened glassy polymers. The large inelastic deformation response of rubber-toughened poly(methyl methacrylate) (RT-PMMA) was experimentally studied under uniaxial compression tests at different strain rates and temperatures. The studied composite system consists of spherical core–shell (PMMA hard shell and soft rubber core) particles embedded in a PMMA matrix. The influence of particle concentration (ranging from 0% to 45%) on the macroscopic behaviour was also investigated from small to large strain. The physically based hyperelastic–viscoplastic constitutive model of Boyce–Socrate–Llana was extended to describe the stress–strain behaviour of rubber-toughened glassy polymers. The model accounts for the effective contribution of the two polymeric phases to the overall composite macroscopic behaviour, by including in the original model the hyperelastic deformation of rubber particles. The capabilities of the model to describe the rate-dependent yield and post-yield behaviour of PMMA over a wide range of temperatures and strain rates are pointed out. The model is able to successfully capture the significant features of the stress–strain behaviour including the initial linear elasticity, the gradual rollover to yield, the strain softening after yield (when it exists) followed by the strain hardening. Its predictive capabilities are further tested by comparison with compression data on RT-PMMA for different rubber contents.

Finite Element Analysis of Plastic Strain Distribution in Multipass ECAE Process of High Density Polyethylene

Equal channel angular extrusion (ECAE) is a relatively novel forming process to modify microstructure via severe plastic deformation without modification of the sample cross section. In this study, an optimized design of die geometry is presented, which improves homogeneity of the plastic deformation and decreases the pressing force required for extrusion. Then, a typical semicrystalline polymer (high density polyethylene) was subjected to multipass ECAE using two different processing routes: route A where the sample orientation is kept constant between passes and route C where the sample is rotated by 180 deg. Compression tests at room temperature and under different strain rates were used to identify the material parameters of a phenomenological elastic-viscoplastic model. Two-dimensional finite element analysis of ECAE process was carried out, thus allowing to check out the homogeneity of the plastic strain distribution. The effects of die geometry, number of passes, processing route, and friction coefficient on the plastic strain distribution were studied. The simulations were performed for three channel angles (i.e., 90 deg, 120 deg, and 135 deg), considering different corner angles. According to simulation results, recommendations on the angular extrusion of the polymer are provided for improving die and process performance.

Analysis of Polypropylene Deformation in a 135° ECAE Die: Experiments and Three-Dimensional Finite Element Simulations

Plastic deformation of polypropylene (PP) resulting from equal channel angular extrusion (ECAE) process was investigated in a 135° die. A phenomenological elastic-viscoplastic constitutive model was identified and coupled with the three-dimensional finite element (FE) method in order to predict the different processing parameters governing the deformation behaviour of PP during the extrusion. An optimal agreement between FE results and experimental data was obtained for a friction coefficient of 0.2. A detailed three-dimensional FE analysis of stress-strain field distribution was then carried out. The effects of both the number of extrusion passes and the processing routes were experimentally highlighted. The results show that the pressing force decreases with the increase of the number of extrusion passes and reaches its saturation state rapidly for routes A and C while, for routes BA and BC, it requires a high number of passes.

Mechanistic insights on nanosilica self-networking inducing ultra-toughness of rubber-modified polylactide-based materials

Abstract Developing novel strategies to improve the impact strength of PLA-based materials is gaining a significant importance in order to enlarge the range of applications for this renewable polymer. Recently, the authors have designed ultra-tough polylactide (PLA)-based materials through co-addition of rubber-like poly(ϵ-caprolactone-co-d,l-lactide) (P[CL-co-LA]) impact modifier and silica nanoparticles (SiO2) using extrusion techniques. The addition of silica nanoparticles into these immiscible PLA/P[CL-co-LA] blends altered their final morphology, changing it from rubbery spherical inclusions to almost oblong structures. A synergistic toughening effect of the combination of P[CL-co-LA] copolymer and silica nanoparticles on the resulting PLA-based materials therefore occurred. To explain this particular behavior, the present work hence aims at establishing the mechanistic features about the nanoparticle-induced impact enhancement in these immiscible PLA/impact modifier blends. Incorporation of silica nanoparticles of different surface treatments and sizes was thereby investigated by means of rheological, mechanical and morphological methods in order to highlight the key parameters responsible for the final impact performances of the as-produced PLA-based materials. Relying on video-controlled tensile testing experiments, a toughening mechanism was finally proposed to account for the impact behavior of resulting nanocomposites.

Micromechanical Modeling of the Effective Viscoelastic Response of Polyamide-6-Based Nanocomposites Reinforced with Modified and Unmodified Montmorillonite Clay

A micromechanics-based approach using a self-consistent scheme based on the double-inclusion model is adopted to develop a pertinent model for describing the viscoelastic response of polymer/clay nanocomposites. The relationship between the intercalated nanostructure and the effective nanocomposite stiffness is constructed using an equivalent stiffness method in which the clay stacks are replaced by homogeneous nanoparticles with predetermined equivalent anisotropic stiffness. The capabilities of the proposed micromechanics-based model are checked by comparing with the experimental viscoelastic (glassy to rubbery) response of two polyamide-6-based nanocomposite systems reinforced with a modified montmorillonite clay (Cloisite 30B) and an unmodified sodium montmorillonite clay (Cloisite Na+), favoring, respectively, exfoliation and intercalation states.

Critical energy for crack initiation in rubber-toughened poly(methyl methacrylate)

Abstract A single specimen J -integral method has been used to evaluate the critical energy for crack initiation in rubber-toughened poly(methyl methacrylate). An abrupt transition is observed in the evolution of J Ic as a function of particle volume fraction and the sudden increase in toughness appears to be directly related to a comparable change in the materialss ability to nucleate plasticity.