Pierre G. Lafleur

RHEOLOGY OF SHORT GLASS-FIBER REINFORCED POLYPROPYLENE

An anisotropic rheological constitutive equation for short fiber reinforced polypropylene is presented. The bulk stress is the sum of a contribution of the Newtonian suspending fluid and of the fiber contribution. This contribution depends on the orientation which is described by an orientation tensor. The rate of change in orientation is calculated using semidilute suspension theories. After simplifications the model contains only one adjustable parameter and the transient relative viscosity depends only on the strain and not on the shear rate or time separately. Transient and steady‐shear viscosities were determined for two reinforced polypropylenes using a constant stress and a rotational parallel plate rheometer. An overshoot in the viscosity versus time curve was observed. It is attributed to changes in fiber orientation, and it depends on the fiber content but not on the shear rate. The model predictions for 20 and 30 wt. % glass fiber reinforced polypropylene are fairly good.

Investigation of Bubble Instabilities in Film Blowing Process

Abstract This paper describes an original on-line video device developed in order to study bubble instabilities occurring in the film blowing process, taking into account their three-dimensional behavior. For a linear low-density polyethylene, two forms of instabilities and combination have been observed: draw resonance and helical instability. These instabilities could be quantitatively described and differences in behavior could be assessed using real objective measurements and criteria. The influence of key processing conditions was investigated and the results showed that the instabilities are enhanced by increasing the draw ratio, blow up ratio and frost line height. These first results are in agreement with the majority of the results reported in the literature, but allow for a more accurate analysis of the phenomena.

On-Line Birefringence Measurement in Film Blowing of a Linear Low Density Polyethylene

Abstract Blown film properties depend on the thermo-mechanical history experienced by molten polymer during biaxial deformation. In this study on-line birefringence measurements along the length of the bubble in film blowing of a linear low density polyethylene (LLDPE) were carried out in order to assess the stress level in the melt zone and total orientation in the solid zone. Bubble temperature measurements were carried out to find out the onset and the end of crystallization. Strain rates were also determined from bubble diameter and axial velocity measurements. We have focused on the effects of key processing parameters on the thermo-mechanical history of polymers. The relations between the birefringence and temperature profiles are described. The birefringence value is shown to be very small in the molten zone and increases rapidly as crystallization proceeds. The birefringence of the solidified film is strongly dominated by the crystalline phase contribution. Stresses in the molten blown film were calculated using the data of birefringence and pressure inside the bubble. The birefringence technique appears to be a promising but limited tool to determine stresses occurring in film blowing.

Polymer-Grafted Metal Nanoparticles for Fuel Applications

Ultrafine metal powders have been identified as very promising fuels for future energetic material formulations. However, the large specific surface area that gives these powders a high reactivity also makes them particularly difficult to remain in a nonoxidized state. They also agglomerate easily during compounding processes due to strong particle-to-particle interactions. The coating of the particles with a polymer may offer a solution to these problems. We investigated two in situ polymerization processes using thermoplastic and thermoset coatings. Polyolefins such as polyethylene and polypropylene were obtained using a modified Ziegler-Natta reaction scheme. This process was found to be flexible enough to control the amount of polyethylene grafted onto the powders. The second type of coating was based on polyurethane chemistry. Nanometric-sized aluminum and boron powders were treated and characterized by means of thermogravimetric analysis, electronic microscopy, and x-ray photoelectron microscopy. The barrier properties of the polymer layer grafted onto the particles were evaluated using a chemical digestion method and thermoanalytical techniques. Polyethylene-coated particles showed a better resistance to early aging under stringent conditions of humidity and temperature and therefore would be expected to demonstrate a longer shelf life in a propellant formulation.

Dynamically Vulcanized Nanocomposite Thermoplastic Elastomers Based on EPDM/PP (Rheology & Morphology)

Abstract This study examines the rheological and morphological properties of dynamically vulcanized nanocomposite thermoplastic elastomers (TPV nanocomposites) based on PP/EPDM. Rubber contents of 20, 40, and 60% were used with polypropylene of different viscosities at 2 wt.% of nanoclay. We performed rheological and morphological characterizations on the nanocomposites using X-ray diffraction, transmission electron microscopy, scanning electron microscopy and rheometry in small amplitude oscillatory shear. The effects of polypropylene viscosity, maleic anhydride grafted polypropylene (PPMA), and composition were also investigated. The storage modulus (G) of the TPV nanocomposites (without PPMA) containing 20, 40, and 60% rubber significantly increased in comparison with similar but unfilled samples and also a further increase in the G from the incorporation of the PPMA in the samples. The agglomeration of the clay considerably decreased when the rubber content was increased in the TPV nanocomposites. The yield stress of the prepared TPV nanocomposite, based on a low-viscosity PP, increased more than that of the sample from high viscosity PP. The TPV nanocomposites containing 20, 40 and 60% EPDM exhibited a strong elastic modulus that tended to level off (plateau) at low shear rates. These results were attributed to strong interfacial interactions between the nanoclay and TPV matrix and, also, the existence of the physical three-dimensional network structure formed between the cured rubber particles, as evidenced by the morphological features of the samples. A Carreau-Yasuda law with yield stress and a linear viscoelastic model, taking into account the maximum packing volume () were used to describe the melt linear viscoelastic properties of the TPV nanocomposites.

Comparison of rheological properties of fiber suspensions with model predictions

The rheological behavior of fiber suspensions in a Newtonian fluid is compared with the simulations results of two families of models. The well-known Folgar-Tucker model for fiber motion, combined with the Lipscomb constitutive equation, was used and compared to an extended Jeffery model developed by Grmela et al. (2004). The high shear-thinning behavior and the constant normal stress differences predicted by this model, for the fiber suspensions in Newtonian matrix, required modifications and the stress tensor equation was replaced by the Lipscomb constitutive equation. The steady-state and stress growth behavior of fibers suspensions in a Newtonian polybutene are fairly well predicted by both models. The viscosity and normal stress overshoots during stress growth in forward and reverse flows are better described by both models when using a slip parameter and λ larger than I in the Folgar-Tucker-Lipscomb model. The extended Jeffery model has more potential to describe properly the behavior of concentrated fiber suspensions, but this is achieved by using a large number of parameters.

Influence of particle-matrix interface, temperature, and agglomeration on heat conduction in dispersions

A combination of the effective medium and the phonon approaches is used to investigate heat conduction in heterogeneous media composed of a homogeneous matrix in which spherical particles of micro and nanosizes are dispersed. In particular, we explore the effect of different types of scattering on the particle-matrix interface, temperature dependence of the effective heat conduction coefficient, and the effect of various degrees of agglomeration of the particles. Predictions calculated explicitly for Si nanoparticles dispersed in Ge matrix agree with available Monte Carlo simulations. Our predictions show that the higher is the temperature the lower is the heat conductivity and the smaller is the influence of the details of the particle-matrix interactions. As for the influence of the agglomeration, we predict both decrease and increase of the heat conduction depending on the degree of the agglomeration.

Effect of chemical and physical branching on rheological behavior of polylactide

This study was aimed at improving the process rheology of polylactide (PLA) melts by means of two strategies. First, PLAs of different branched structures, i.e., star shaped, comblike, and hyper branched, were synthesized and blended with a linear grade analog. Shear and extensional flow rheometry tests were performed on pure materials and their blends to evaluate their rheological properties. It was shown that the presence of branched poly(L-lactide) (PLLA) increased the shear thinning, shear and extensional viscosity, and elastic modulus of linear PLLA at the same time; the star shaped PLLA providing the most significant change. Second, poly(D-lactides) (PDLA) with similar molecular architectures were synthesized to have a double branching effect. In addition to the presence of branched architecture, physical cross-links due to the stereocomplex formation exist between PLLA and PDLA chains. Based on the rheological characterizations in shear and extensional mode, a greater improvement in PLA melt rheolo...

Modeling Filler Dispersion along a Twin-Screw Extruder

Abstract Particle size distribution strongly affects physical and mechanical properties of filled polymers. A new model has been developed to predict agglomerate size distribution in a twin-screw extruder (TSE). The model considers the break-up and erosion processes and it uses agglomerate size population balance in its mathematical formulation. The model parameters were evaluated in simple field flow. This paper shows the validation of the model along the extruder using different screw configurations of a short twin screw extruder. Flow parameters along of the TSE necessaries to apply the new dispersion model have been calculated with ©Ludovic software. Calcium carbonate filled polypropylene system was used as model compound. The agglomerate size distribution was evaluated from micrographs of polished samples at different locations along the extruder obtained by reflected light microscopy in conjunction with-semiautomatic image analysis.

Effective heat conduction in dispersion of wires

We derive a formula for the heat conductivity coefficient of dispersions of wires in a homogeneous matrix. Such formula is particularly useful for thermoelectric applications. The method used to derive this type of formula in Behrang et al. [J. Appl. Phys. 114, 014305 (2013)] for spherical particles is adapted to generally oriented wires of a finite length. Both diffuse and specular scatterings on the wire-matrix interface are considered. The results obtained previously from numerical solutions of the phonon kinetic equation under the assumption of diffuse scattering agree with predictions based on the formula.