Abdellatif Ait-Kadi

Morphology, thermomechanical and barrier properties of polypropylene-ethylene vinyl alcohol blends

Processing as well as physical properties of polypropylene (PP)/ethylene vinyl alcohol (EVOH) and PP/EVOH/high density polyethylene (HDPE) blends have been investigated. Blends were prepared both on a batch mixer and a twin-screw extruder. Extrusion was followed by drawing between two roll mills. Minor phase deformation during stretching was analysed using shape relaxation theory. When PP/EVOH blends were prepared on the internal mixer, low aspect ratio EVOH particles were obtained. This did not lead to appreciable barrier properties improvement. In contrast, an EVOH fibrillar and lamellar morphology obtained by extrusion and drawing was found to induce a significant decrease (85%) in O2 permeability for only 20 vol.% EVOH content. The barrier properties of the PP/EVOH extruded blends were predicted using a hybrid diffusion model that takes into account the morphology of the blends. When PP.g.Maleic-Anhydride is added to PP/EVOH blends and for high draw ratios, elongation at break remains important, even at 20 vol.% EVOH. This result, together with the large gain in barrier properties, makes the PP/PP.g.MA-EVOH blends a proper material for applications such as food packaging. The use of HDPE/EVOH blend with PP intended to keep the EVOH well dispersed in the system did not give the expected barrier properties, due to the poor adhesion at the PP-HDPE interface.

NUMERICAL SIMULATION OF VISCOELASTIC FLOWS THROUGH A PLANAR CONTRACTION

Abstract In this study, three nonlinear rheological models consisting of the Giesekus, the FENE-P, and the Phan-Thien-Tanner model are used to simulate the flow of a viscoelastic fluid through a planar 4:1 contraction. Both stress and velocity fields are examined at different sections of the flow and the predictions of the numerical simulations are compared with the experimental results of L.M. Quinzani, R.C. Armstrong adn R.A. Brown, J. Non-Newtonian Fluid Mech., 52 (1994) 1–36. Overall, the numerical simulations allow a description of the essential features of the flow, and reproduce much of the experimental results with good accuracy. Excellent qualitative agreement between the numerical results and the experimental observations is reported. However, the agreement remains semi-quantitative especially for the first normal stress difference around the entry section of the flow. This is to be expected in view that the simulations were limited to only one-mode models.

Effect of used-tire-derived pyrolytic oil residue on the properties of polymer-modified asphalts

Abstract Polymer-modified asphalts are prepared by incorporating recycled polyethylene (RPE) and a used-tire-derived pyrolytic oil residue in asphalt. A thorough characterisation showed superior properties for the modified asphalts at high temperatures. Low temperature properties of the base asphalt were found to deteriorate after addition of polymer. The asphalts modified with 10% pyrolytic oil (H18) obtained by vacuum pyrolysis of used tires showed improved low temperature properties. Based on optical micrographs it was concluded that contribution of the Ostwald ripening mechanism to phase separation of the dispersed polymer phase is negligible. It was found that the RPE structure changes in the mixture do not depend on the type of asphalt. The SHRPs high temperature ( T SHRP ) criterion was found to correlate with the Ring and Ball temperature ( T RB ) with a systematic 20° difference between the two temperatures (T SHRP >T R&B ). The asphalt mixtures containing 10% H18 and 1% polymer were found to have the best low and high temperature performance.

Mixing Hydrodynamics in a Double Planetary Mixer

The mixing hydrodynamics in a double planetary mixer is investigated numerically and experimentally over the course of a cross-linking reaction. Using various visualization techniques, it is shown that this mixer provides good radial dispersion capabilities but poor axial (top-to-bottom) pumping, irrespective of the viscosity level. The power drawn by the mixer evolves dramatically from about 180 W m −3 at 40% conversion up to approximately 4kWm −3 at 85% conversion. Overall, the numerical predictions and the experimental results exhibit good agreement although at 85% conversion, the numerical model is not accurate enough to predict adequately the power consumption due to physical phenomena not considered in the computations.

Investigation of the abrupt contraction flow of fiber suspensions in polymeric fluids

Abstract Numerical simulations of the flow of rigid fibres through a 4:1 planar contraction, and the predicted flow pattern and fiber orientation are presented. Entirely new is the examination of the nature of the suspending matrix which may consist of either a Newtonian fluid or a polymer melt. In the case of a polymer matrix three rheological models, the Phan-Thien–Tanner, FENE-CR, and Carreau models have been used to investigate the effects of shear-thinning and elasticity on the flow and the orientation of the fibers. The effects of inertia are neglected, and the governing equations for the flow field, polymer stress, and fiber orientation are coupled and simultaneously solved. A parametric study is used to explore the effects of different dimensionless parameters on the velocity field, the fiber orientation, the pressure drop, as well as the vortex size measured by the dimensionless reattachment length. We particularly focus on the role of the fibers aspect ratio, volume fraction, and interaction coefficient which measures the intensity of fiber interaction in the suspension. Furthermore, we evaluate and compare the results of four different closure approximations: the quadratic, linear, hybrid A and T, and natural closures.

Rheological properties of bitumen modified with pyrolytic carbon black

Pyrolytic carbon black (CBp), a by-product of scrap tyre pyrolysis, was blended with a 150200 penetration grade bitumen at concentrations ranging from 5 to 30 wt%. The fundamental rheological properties of standard and modified bitumens as well as their classical characteristics were determined. The interactions between CBp and bitumen components using ESCA, i.r. and u.v. spectroscopy were investigated. The results indicate that pyrolytic carbon black can be used as a filler to reduce the influence of temperature on physical response characteristics of the mixtures. The addition of CBp in amounts of 5–30 wt% of the binder changed the rheological properties of the blends positively.

Weld lines in injection-moulded immiscible blends : model predictions and experimental results

Abstract In this work, a theoretical approach based on diffusion and the Flory—Huggins theory of the free energy of mixing is considered to predict the weld-line strength in injection-moulded polycarbonate (PC) and its blend with high-density polyethylene (PE) as a function of melt temperature and contact time. A comparison between the model predictions and experimental results was also performed for pure PC as well as for its blend. Different melt temperatures ranging from 220 to 300°C and a constant total cooling time of 15 s were used. Two types of specimen were produced, one with a dual-gate mould cavity and another with a single gate. In the case of PC, the model predicts satisfactorily the weld-line strength as a function of injection temperature. High-density PE was also moulded in the same conditions but in a lower temperature range (145 to 220°C). The results showed that the tensile strength of PE remains unchanged in the presence of weld lines. For the PC/PE blend, the results showed a good agreement between the model predictions and the experimental results. It was also possible to predict the weld-line strength of the blend as a function of its composition and temperature.

Tensile properties of polymerization-filled Kevlar pulp/polyethylene composites

Polymerization-filled composites (PFC) and melt-blended composites (MBC) of Kevlar pulp/high density polyethylene composites were prepared to compare their mechanical properties. It was found that break strains of PFC composites were by far higher than MBC composites for a similar fiber concentration. Tensile data were then used to compare several models of short fiber polymer composites. Of all the models tested, it was found that Berlins approach in combination with Rosens model for critical aspect ratio give reasonable prediction. It was also found that fibers aspect ratio in composites is strongly related to the processing technique used.

Polyethylene-Ultra High Modulus Polyethylene Short Fibers Composites

Composites based on chopped ultra high modulus polyethylene fibers (UHMPE) and low density polyethylene (LDPE) were prepared in the melt state. It is shown that the UHMPE fibers do not melt during the preparation operations, and an im portant enhancement of the mechanical properties is obtained as a function of fiber content and length. The efficiency of these fibers was found to be comparable to those of Kevlar® fibers, and higher than those of glass fibers. An important transcrystalline region has been observed for all the fibers, however, its effect on their efficiency as a reinforcing agent is questionable. It is also shown that UHMPE fibers can advantageously replace Kevlar® fibers in some particular applications.

Torsion Properties of High Density Polyethylene Foams

High density closed-cell HDPE foams (450–950 kg/m3) were prepared by compression molding, and torsion rectangular tests were performed to measure their shear modulus in order to study: (1) the relationship between the modulus as a function of the density, and (2) to determine the effect of thin skins. Based on the assumption that the twist stiffness (product of shear modulus and moment of inertia) of the foams is the sum of the twist stiffness of the skin layers and the core part, several structural foam models are proposed. We found that structural foam models give better results than uniform foam models, indicating that thin skins have an important effect of the shear modulus of polymer foams.