Bernard Monasse

Shear-induced crystallization of polypropylene: Influence of molecular weight

Three series of isotactic polypropylene characterized by different molecular weights and the same isotactic index have been studied during crystallization under static and shearing conditions. The shear is induced by the displacement of a glass fiber in the molten polymer. The monoclinic α-phase is here formed under shear with a columnar organization at the surface of the glass fiber, and does not appear under static condition. The growth-rate, constant during the shear-induced crystallization experiment, is compared with the result obtained from static crystallization. An important increase of the growth-rate due to the shear flow is observed. This increase depends on the molecular structure. The average molecular weights ―Mw and ―Mz seem to be the most important molecular parameters, for which an excellent correlation is obtained. The increase of these parameters ―Mw and ―Mz leads to a significant enhancement of the growth-rate, which can be multiplied by a factor of 10 in the present conditions.

Rheology of polypropylene in the solid state

The tensile behaviour of a commercial grade of isotactic polypropylene was tested in a temperature range between 20 and 150 °C with a video-controlled testing system which is capable of imposing a constant true strain-rate within the neck automatically. The results are displayed in the form of effective stress-strain curves and modelled by a constitutive equation in a multiplicative form. It is thus shown that, for each temperature, the plastic response can be described up to very large strains (ɛ ≃ 2.0) by a set of four parameters. The assumptions introduced in this modelling are critically discussed in order to check the validity of the simplifying hypotheses (strain homogeneity, isochoric deformation, etc.). The constitutive equation thus obtained was utilized in a finite difference code in order to predict the development of stretching instabilities of polypropylene. The simulation gives access to the engineering stress-strain response of the stretched test piece and to the detailed kinetics of the incipient neck. It is found that the severity of the instabilities is less at room temperature than near the melting point because of the decrease of the strain-hardening and of the strain-rate sensitivity with temperature.

Shear-induced Crystallization of polypropylene. Growth enhancement and rheology in the Crystallization range

Crystallization under shear of many different polypropylenes has been studied using a fiber pull-out device. It appears that growth can be considerably enhanced by flow. The best correlation is obtained with weight average molecular weight. Modeling the flow pattern gives access to the mechanical parameters at the growth front (shear rate and shear stress) as well as to the total strain applied to the polymer. The residual strain can be calculated taking into account relaxation processes.

Numerical and Physical Modeling of Polymer Crystallization: Part II: Isothermal and Non-isothermal Crystallization Kinetics of a Polypropylene in 2 Dimensions – Experiments and Simulation

Abstract Crystallization of thin polypropylene films was performed in isothermal, constant cooling-rate and mixed conditions. The experiments were first analyzed using the classical procedures based on simplified forms (Avrami, Ozawa) of the general Kolmogoroff-Avrami-Evans (KAE) theory. These analyses, which can be applied over an unusually wide transformation range, show that the crystallizations are actually 2 D. Then, a procedure has been established for the determination of the nucleation and growth parameters involved in the theoretical model presented in the first paper of this series. These parameters have been introduced into the model in order to predict the crystallization behavior in isothermal, constant-cooling-rate and mixed-conditions: transformed fraction, number of activated nuclei, final size distribution of semi-crystalline entities. A very good agreement is generally found between predictions and experimental results.

Structure Development in Injection Molding: A 3D Simulation with a Differential Formulation of the Kinetic Equations

Abstract The purpose of the present work is to introduce a crystallization law into Rem3D, a 3D code written in C++ and dedicated to the injection molding of polymers. We kept the basic hypotheses of Avramis model and cast the kinetic equations into a differential system that is solved numerically. The variation of the density of potential nuclei with temperature is taken into account. Furthermore, the distribution of mean spherulite sizes can be deduced from the calculations. The second part of the paper is an experimental study of crystallization in well-controlled conditions (2D, isothermal or constant cooling-rate). It establishes a procedure for the determination of the nucleation and growth parameters used in the theoretical model, and gives a first validation of this model. Finally, the crystallization equations are introduced into Rem3D, in order to assess the feasibility of our new approach. Some typical results concerning the evolution of the transformed volume fraction in injection-molded parts are presented.

Influence of Stretching and Cooling Conditions in Cast Film Extrusion of PP Films

Abstract In cast film extrusion, the polymer melt is extruded through a slit die, slightly stretched in air and then cooled on a chill-roll. An important part of the present work was to study the effect of the roll temperature on the polymer crystallization in the thickness of extruded films. A thermomechanical model of the whole process taking into account the polymer crystallization was used to determine the mean stretching stress in air, local temperature, local cooling rate and local crystallization development. The morphologies in the thickness of the films were observed by optical microscopy on microtomed slices and the crystalline structures were investigated by wide-angle X-ray diffraction and DSC measurements. From DSC melting curves of thin sections it is possible to know the local crystallization temperature and to check the thermomechanical model. The model is validated and explains the influence of the roll temperature on the formation of structures and morphologies. Nevertheless neither the existence of three different zones in the thickness nor the high p phase concentration are predicted by the model, which needs new developments.

Influence of Molecular Weight and Crystallinity of HDPE upon the Initial Friction and Transfer Behavior

The influence of molecular weight, crystalline structure, and surface energy of high-density polyethylene (HDPE) upon initial friction and transfer behavior was studied on a pin-and-disk tribometer, at low speed (1.6 mm/s) 20°C and 12 percent relative humidity, using: (a) Composite PE pins made from thin film (< 400 μm) of defined molecular weight, Mw (5.104, 4.105, 4.106 g/Mole) and crystalline structure, controlled by the temperature of crystallization (120°C < Tc < 132°C), stuck onto a PE support (6 × 6 × 20 mm) (b) Thoroughly polished, plasma-cleaned steel disks (Ra < 0.02 μm) It was shown, that any increase in the PE surface energy (by plasma or chemical treatment) greatly influences the initial friction and transfer behavior. Depending on the superficial cross-linking density, which is dependent upon the surface treatment, either transfer of polymer onto the metal, or the formation of metal-containing transfer particles on the polymer is promoted during the first few rotations. The influence of the ...