Several commercial grades of homo-polymer and its blends were selected to prepare microporous membranes through melt extrusion-annealing-uniaxial stretching technique (MEAUS). Branched or very fluid polypropylene was employed to modify the polymeric composition. In some blends, micro-sized calcium carbonate was added. We analysed the influence of sample composition, extrusion draw ratio, and we performed a deep study concerning the uniaxial strain rate, using in some cases extreme strain rates and strain extents. The crystalline features were studied by Differential Scanning Calorimetry (DSC), and the morphology of porous structure was analyzed through Scanning Electron Microscopy (SEM). Thermal stability and thermomechanical performance was measured by thermogravimetric analysis (TGA) and dynamic-mechanical-thermal (DTMA) study. A close relationship was found between crystalline characteristics, porous morphology and the trends registered for permeability.
Abstract:In this study, block and random copolymers of polypropylene-ethylene are selected to prepare porous membranes through the melt extrusion-annealing-uniaxial stretching technique (MEAUS), at a constant draw ratio. In some cases, these copolymers were blended with a homopolymer grade. The variation of temperature in the stages of extrusion, annealing and uniaxial strain was analysed. Several characterisation techniques were employed to study this influence. The crystalline orientation was analysed by polarised infrared spectroscopy (FT-IR), and crystalline features were studied by differential scanning calorimetry (DSC). The thermal stability of the membranes was checked by thermogravimetric analysis (TGA). Tensile tests were performed to ascertain the stiffness and ductility of the produced samples. The results were correlated with the porous morphology, global porosity, and permeability to air. A close relationship was found between crystalline characteristics, porous morphology and the trends registered. An improved pore distribution along the membrane surface was found when copolymers were employed.
This work is made to ascertain the effects of mineral fillers, namely calcium carbonate and talc, on the morphology and properties of multilayer polypropylene (PP)/high-density polyethylene (HDPE) porous membranes. Multilayer membranes were prepared using the three-stage Melt-Extrusion, Annealing and Uniaxial Stretching (MEAUS) process. The orientation of PP’s crystalline phase was affected by both the flow-induced crystallization and the heterogeneous nucleation promoted by the fillers. A synergistic effect was observed in the filled samples due to the generation of pores after the stretching-induced lamellae separation and the debonding of mineral fillers from the polymeric matrix. The fillers increased the porous surface, leading to an increase of permeance to air, being this effect more marked at higher filler contents. Talc showed a higher efficiency to create porous surfaces when compared to calcium carbonate. The thermal stability of the membranes increased with filler addition, as well as their stiffness and strength.
Several commercial grades of talc were selected to develop polypropylene based microporous membranes through MEAUS process (melt extrusion-annealinguniaxial strain). Talc commercial grades differed in particle size, aspect ratio, and crystalline morphology. Different filler percentages were added to polypropylene (1, 5, 10 wt.%) Parameters such as draw ratio during extrusion, annealing temperature strain rate, and strain extension were kept as constant to analyze the effect of the talc characteristics and content of the obtained membranes. Small particle size and high aspect ratio tend to provide membranes with small pore size, high porous area, and high Gurley permeability values.
Polymer films are used in a wide variety of applications. Their toughness is often a basic requisite to meet some industry needs.The Linear Elastic Fracture Mechanics (LEFM) approach is used to study fractures occurring at nominal stresses well below the material yield stress. The dissipated energy is confined in a small area near the crack tip, and the fracture is brittle. The LEFM approach is not applicable when the plasticity around the crack tip becomes large; in those cases the Elastic Plastic Fracture Mechanics (EPFM) is applied. When the crack propagation occurs through a highly deformed and yielded material then Post-Yield Fracture Mechanics can also be applied and the Essential Work of Fracture (EWF) is a suitable methodology.The EWF approach has become very popular to characterise fracture of polymer films and is increasingly used due to its apparent simple preparation and easy testing. The EWF characterises the plane stress toughness in mode I, generally using the double edge notched tension (DENT) configuration for the specimens.In spite of the apparent simplicity of the EWF test, some aspects of the validity of this technique remain controversial; there are intricate details that seem to play an important role in the repeatability and reproducibility of the test. This problem has been and still is a topic of much debate, and these questions indicate that the EWF procedure is not yet sufficiently well-defined to be standardised. Some of the aspects of the test validity are related to the specimen manufacture, particularly the quality of the notches. Two sets of specimens have been prepared, the first one sharpened by the femtosecond laser ablation technique, and the second one sharpened by the classical razor blade sliding technique. These two sets of specimens have Test Method
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