Time Dependent Rheological Properties of Polypropylene-Polyethylene-EPR Blends and Ethylene-Propylene Copolymers

JORGE ARIEL RODRÍGUEZ FRIS; LUIS PALMA; MARCELO DANIEL FAILLA; LIDIA MARÍA QUINZANI

Rio de Janeiro

Congreso; 2nd. Brazilian Conference on Rheology; 2004

The low capacity of polypropylene (PP) to absorb impact energy at low temperature can be improved in essentially two different ways [1-3]: by blending PP with elastomeric polyolefins, such as polyethylene (PE) or ethylene-propylene rubber (EPR), or by copolymerizing propylene with other a-olefins, mainly ethylene.  The propylene-ethylene copolymers (PEC) are generally obtained in a sequential reactor process which produces blends of PP, PE and a variety of poly(propylene-co-ethylene) molecules with different monomer ratios. Since PE and PP are essentially immiscible and incompatible polymers [2,3], all the above mentioned processes generate, in general, multiphasic systems with domains of different shapes and sizes.  The properties of immiscible polymer blends can be optimized by controlling the blend morphology, which in turn is strongly dependent on the rheological behavior of the blend.  The rheological properties of immiscible blends depend on the processing conditions, composition, rheological properties of the components, shape and size of the phases, and the interactions between the phases [3].  In the case of blends based in PP and PE, the rheological behavior and phase stability have no received much attention [4-6]. In the present work, the phase stability of blends of PP/PE and PP/PE/EPR are analyzed and compared that of a PEC.  The combined effects that the composition and annealing have on the phase structure organization and melt rheological behavior of these systems are considered.The materials used are: an isotactic PP from Petroquímica Cuyo S.A.I.C. (Mw = 400,000 Da, Mw/Mn = 5.8), a high density PE from Petropol S.A. (Mw = 55,000 Da, Mw/Mn = 4.4), an EPR from EniChem Corp. (Dutral CO043, 45 wt% of propylene, Mw » 50,000 Da), and a PEC from Petroquímica Cuyo S.A.I.C. (16.4 mol% of ethylene, Mw » 298,000 Da, Mw/Mn » 4.3 [5,7]).  The poly(propylene-co-ethylene) in PEC constitutes approximately 12% of the total mass and it has an ethylene content of ~50 mol%. The blends analyzed are: 70/30, 50/50 and 30/70 of PP/PE, and 70/28/2, 70/25/5, 70/20/10, 75/20/5 and 78/20/2 of PP/PE/EPR.  The blends were prepared in a Brabender mixer (Plastograph) at 190°C, 30 rpm of blade speed, and 14 min of mixing time.  Nitrogen atmosphere was used to avoid oxidation of the polymers.  The individual polymers were also processed in the mixer using the conditions mentioned above to ensure that all the materials used in the rheo­logical study have the same initial thermal and deformation histories.  The melted polymers were removed from the mixer bowl and rapidly molded at room temperature to obtain plates of about 1.5 mm thick.  Discs of 25 mm diameter were cut from these plates to be used in the rheological study. All the materials were characterized from the molecular, morphological and rheological point of view.  The molecular characterization was made using infrared spectroscopy and size exclusion chromatography.  The morphological analysis of the blends was performed observing by scanning electron microscopy (SEM) fracture surfaces that were etched using a method specially developed to attain phase contrast.  The rheological behavior was analyzed measuring the dynamic moduli of the materials in small amplitude oscillatory shear flow as a function of frequency and temperature.  Nitrogen atmosphere was used to avoid the degradation of the polymers.  To analyze the effect of annealing on the blends, some samples were subjected to a sequence of rheological tests that included an initial dynamic frequency sweep (DFS), followed by a period of annealing at constant temperature (that lasted up to 90 min), and a final dynamic frequency sweep.  During the annealing, some samples were kept at rest and some were subjected to constant frequency oscillatory flow(at 0.1 or 1 s-1). All the analyzed blends form immiscible systems with unstable initial morpholo­gies.  In all the cases, when the materials are kept in the molten state, the phase structure of the blends generated during the mixing evolves towards final, thermodynamically stable, morphologies.  The 50/50 and 30/70 blends of PP and PE reach phase cocontinuity after the annealing, displaying occlusions of domains of one component inside the phase formed by the other.  This cocontinuity may be associated to the proximity to the inversion phase concentration.  In the case of immiscible blends formed by materials of equal viscosity, the inversion phase concentration is 50%.  When one of the components has a larger viscosity, the inversion phase concentration shifts towards lower concentrations of that component.  Since the PP has a much larger viscosity than the PE, the phase inversion concentration of their blends shifts to smaller concentrations of PP, explaining the cocontinuity observed in the 30/70 blend. The 70/30 blend of PP and PE and all the blends with EPR showed phase structures with domains of ethylene-rich materials dispersed in the PP matrix.  The dispersed domains grow during the annealing reaching their final size after approximately 8 min.  The size of the final domains decreases as the concentration of EPR increases (for 70 wt% of PP) and it is practically independent of the relative concentration of PP and EPR (for 20 wt% of PE).  The freshly mixed PEC also presents small quasi-spherical domains homogeneously dispersed in the PP matrix.  This morphology also changes with time while the material is in the molten state.  However, the behavior of PEC is different to the one observed in the case of the blends of PP.  During the annealing, the ethylene-rich domains in PEC coalesce forming bundle of particles that keep their individual aspect.  The coalescence and growth of the dispersed phase continues while the materials are kept in the molten state.  The PEC did not reach an stable morphology during the length of the annealing processes used in this study (up to 90 min). The rheological behavior of all the blends corresponds to that typically observed in immiscible systems [3].  The presence of the interphase produces an increment in the dynamic properties at low frequencies that is more noticeable in the elastic modulus.  This increment was determined by comparison of the experimental data with predictions of a logarithmic mixing rule.  The dynamic moduli measured for the PP blends, before and after the annealing, were practically equivalent.  This coincidence agrees with the fact that most of the changes in the morphology of these blends occurs in approximately 8 min.  This means that, by the time the samples reach the test temperature in the rheometer chamber and a rheological test begins, the PP blends have practically reached their final phase structures.  On the other hand, in the case of PEC, which needs a much longer time to reach the equilibrium morphology, noticeable differences were observed between the dynamic moduli before and after the annealing.  Furthermore, the obtained moduli depended on the deformation history applied during the annealing.  The data from the final DFS of samples kept at rest in the molten state are larger than those of the initial DFS.  This increase is larger when the elapsed time is longer.  An increase, although smaller, it is also observed when the material is subjected to a continuous oscillatory flow of small frequency (0.1 s-1).  On the other hand, when a constant frequency of 1 s-1 is used, the final moduli are smaller than the initial ones.  In all the cases the larger differences are obtained in the range of smaller frequencies.