Efectos de la incorporación de polietilentereftalato sobre la estructura y propiedades mecánicas de materiales compuesto polipropileno-vidrio

Esta investigación pretende contribuir al conocimiento de las relaciones entre la microestructura, las propiedades y las condiciones del procesado de los materiales compuestos polipropileno-vidrio. El propósito fue el de actuar sobre la interfase, al ser ésta una región del material de relevante importancia por su influencia sobre las propiedades mecánicas del material compuesto.Se prepararon y caracterizaron una amplia gama de compuestos con un porcentaje en volumen (26%) de microesferas de vidrio. Para modificar el grado de adhesión entre las fases se actuó por dos vías: incorporación de un copolímero polipropileno-co-anhídrido maleico (MAPP) y/o polietilentereftalato (PET), y aplicación de tratamientos superficiales basados en silanos organofuncionales sobre las microesferas de vidrio como promotores de la adhesión.La composición se realizó mediante un equipo de extrusión de doble husillo corrotatorio, que garantiza una óptima dispersión de las fases constituyentes del compuesto. Para la caracterización se moldearon probetas por inyección con varías geometrías. Los materiales preparados se caracterizaron por su densidad, concentración de vidrio e índice de fluidez..Primeramente, se estudiaron las mezclas sin vidrio PP/PET y PP/MAPP/PET. Su morfología de fases fue analizada mediante microscopía electrónica de transmisión (TEM) y de barrido (SEM). Su estructura se estudia mediante espectroscopía infrarroja por transformada de Fourier (FT-IR), espectroscopía Raman y difracción de rayos X (WAXD). La acción compatibilizante del MAPP en esta mezcla se puso de manifiesto por la reducción de tamaños de la fase dispersa de PET, diferencias en su estructura cristalina, así como en el espectro Raman del polipropileno. El comportamiento de cristalización se estudió mediante calorimetría diferencial de barrido (DSC), observándose una marcada acción nucleante debida a la presencia de PET, así como ligeros cambios en la cristalinidad. En las muestras no cargadas pudo medirse el tamaño medio esferulítico del polipropileno mediante microscopía óptica de luz polarizada.Por otro lado, se analizaron las orientaciones de la fase cristalina (α) del PP en los distintos compuestos inyectados, a través de medidas realizadas mediante WAXD. Esta técnica proporcionó además valores de porcentaje de cristalinidad, así como de la fracción de fase β. El análisis térmico mecánico-dinámico (DMTA) proporcionó información acerca de la temperatura de transición vítrea (Tg) del polipropileno así como de una relajación denominada α'. Mientras que no se observaron diferencias significativas en la Tg, la distinta composición de los materiales hizo variar de manera importante la relajación α', lo cual se relacionó con variaciones en las interfases cristalinas. Las variaciones observadas, igualmente, en los valores del factor de pérdidas viscoelásticas (tan δ) se asociaron a diferencias en rigidez y grado de adhesión interfacial. La caracterización mecánica, llevada a cabo mediante ensayos de tracción, permitió obtener valores del módulo de Young, resistencia máxima y alargamiento a rotura. Las diferencias encontradas evidenciaron el diferente grado de adhesión interfacial desarrollado. Estos resultados se complementaron con observaciones por SEM, que permitieron apreciar, además de la adhesión, una tendencia destacable del PET a encapsular a las microesferas de vidrio.Para caracterizar el comportamiento a fractura se tuvieron que aplicar varios conceptos. Los compuestos con una fuerte adhesión matriz-carga presentaron una rotura frágil, por lo que fueron analizados mediante ensayos de la Mecánica de la Fractura Elástico-Lineal. Por su parte, la fractura de los materiales con baja adhesión se caracterizó a través de la integral J.Por último, se pretendió verificar la aplicabilidad del concepto del Trabajo Esencial de Fractura (EWF) a este tipo de materiales. Para ello, se realizaron ensayos específicos del EWF sobre aquellos compuestos más dúctiles.In this work we have intended to deep into the knowledge of the correlation between the microstructure, properties and processing conditions of polypropylene-glass composite materials. The original aim was to act onto the interface, as it is a material region with a significant importance, due to its influence on the mechanical properties of the composite.This leaded us to design, compound and characterize a wide number of composites with a constant glass (sphere-shaped) percentage, choosing 26% by volume in order to maximize the effect of the interface. The adhesion degree between phases was modified following two routes: by one hand, maleic anhydride grafted polypropylene (MAPP) and/or poly (ethylene terephthalate) (PET) were added and by other hand as adhesion promoters surface treatments with organofunctional silanes were applied onto glass beads.The composition was carried out by a co-rotating twin screw extruder, which guarantees an optimal phase dispersion. Different samples were injection-moulded to characterize the composites. The composites were initially characterized by its density, glass bead content and melt flow index. Firstly, unfilled PP/PET and PP/MAPP/PET blends were studied. Their phase morphology was analysed through transmission (TEM) and scanning (SEM) electron microscopy. Their microstructure was analysed through Fourier transformed infrared spectroscopy (FT-IR), Raman spectroscopy and wide angle X-ray diffraction (WAXD). The compatibilizing action of MAPP in this blend was manifested by a reduction of average PET size domains and differences in the crystalline PET microstructure, as well as changes in the polypropylene Raman spectrum.The crystallization behaviour was analysed through differential scanning calorimetry (DSC), noticing a marked nucleating activity of PET, as well as slight differences in the degree of crystallinity. In the unfilled samples, the medium spherulitic size of polypropylene could not be measured through polarised light microscopy.By other hand, the orientations of the α-form of PP crystalline phase were estimated through WAXD. This procedure also allowed to obtain values of the degree of crystallinity, as well as values of β-phase fraction.Dynamic-mechanical thermal analysis (DMTA) provided information about the PP glass transition temperature (Tg), as well as about the relaxation α'. No significant differences were observed in the Tg values, but the different sample composition resulted in notable variations of relaxation α', which was related with differences in the crystalline interfaces. The differences observed in the values of the loss tangent (tan δ) were associated with variations in both stiffness and interfacial adhesion.The mechanical characterization, carried out through tensile tests, allowed to obtain values of Young's modulus, maximum tensile strength and elongation at break. Significant differences were found, related with the modification of the interfacial adhesion degree. These results were complemented with SEM observations, which showed besides the adhesion, the tendency of PET to encapsulate the glass beads.Different concepts had to be applied to characterize the fracture behaviour. Composites with a high filler-matrix adhesion showed a brittle failure, being analysed by specific tests of the Linear-Elastic Fracture Mechanics (LEFM). By other hand, the fracture of composites with low interfacial adhesion were analysed through J-integral.Finally, the applicability of the Essential Work of Fracture (EWF) concept to these materials was checked. For this reason, specific tests of the EWF were carried out on the more ductile composites. Moreover, the effect of the thickness sample was analysed

The post-yield fracture of a ductile polymer film: Notch quality, essential work of fracture, crack tip opening displacement, and J-integral

Double edge notched tension (DENT) specimens of a polyethylene terephthalate (PET) film were tested in an universal testing machine, measuring the displacements and the ligament lengths with a digital image correlation (DIC) system. With these data the essential work of fracture (EWF), crack tip opening displacement (CTOD), and the J-integral fracture methods were compared. The specimens were tested in mode I under plane stress conditions, verifying that the crack always propagated through a fully yielded ligament. It has been proved that we, the specific essential work of fracture was the specific energy just up to crack initiation and has the same value that J-integral at crack initiation, Jo. The relationship of these parameters with the CTOD was also shown. The influence of the notch quality on the fracture behaviour when the specimens were sharpened by two different methods, femtosecond laser ablation or by razor blade sliding, has also been analysed in detaiPeer ReviewedPostprint (author's final draft

Effect of filler content, size, aspect ratio and morphology on thermal, morphological and permeability properties of porous talc filled—Polypropylene obtained through MEAUS process

Several commercial grades of talc were selected to develop polypropylene based microporous membranes through MEAUS process (melt extrusion—annealing—uniaxial 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 valuesPostprint (published version

Notch effect on the fracture of a polymeric film

The fracture behavior of a bio-based thermoplastic copolyester film has been studied by combining the essential work of fracture, the J-integral, and the crack tip opening displacement characterization methods on double edge notched tension specimens, in an attempt to provide a better understanding of the details that play an important role in the repeatability and the reproducibility of the essential work of fracture test, with particular attention to the effect of the quality of the notches generated by two different notch sharpening techniques. Specifically, the femtosecond pulsed laser ablation and the classic razor blade sliding techniques have been applied. The equivalence, reported and discussed in the literature, between the specific essential work of fracture and the J value at crack initiation, as well as their relationship with the crack tip opening displacement, have been confirmed. It is also identified the parabolic shape of the stress-displacement curves, which modeled relates the specific non-essential work of fracture with the stress at initiation and the extension ratio during the crack growth.Peer ReviewedPostprint (author's final draft

Essential work of fracture, crack tip opening displacement, and J-integral relationship for a ductile polymer film

A unique set of double-edge notched tension specimens of a Polyethylene Terephthalate Glycol-modi¿ed ¿lm was tested in mode I, plane stress. The load was registered on a universal testing machine. The displacements, ligament lengths, and video frames were recorded by a Digital Image Correlation system. With these registered data, the essential work of fracture, J-integral, and crack tip opening displacement (CTOD) fracture concepts have been applied. The onset of crack initiation was through a complete yielded ligament. The analysis showed that the intrinsic speci¿c work of fracture, we, is the speci¿c energy just up to crack initiation, which is an initiation value. we has both a coincident value and the same con-ceptual meaning as Jo, the J-integral at the onset of crack initiation. The relationship between Jo and CTOD is also determined. The in¿uence on the notch quality when the specimens were sharpened by two different procedures, femtosecond laser ablation and razor blade sliding, was analysed in detail.Peer ReviewedPostprint (author's final draft

Estudio de la presión interna en el molde de inyección en la producción de materiales poliméricos microcelulares de etileno-propileno con espumación física

Se efectuó el estudio de la presión en la cavidad de un molde de inyección en dos posiciones diferentes respecto al punto de inyección para evaluar los efectos de los parámetros del proceso y la distancia de dicho punto en la morfología celular del material resultante. Con los datos obtenidos es factible definir criterios para la aplicación de este método de espumación microcelular en la fabricación de productos viables con materiales poliméricos aligerados.Peer ReviewedPostprint (published version

The fracture testing of ductile polymer films: Effect of the specimen notching

The fracture of a ductile polymer film, a heterophase ethylene-propylene block copolymer, has been studied, combining a range of characterisation methods in an attempt to provide a better understanding of the intricate details that play an important role in the repeatability and reproducibility of the essential work of fracture test. The experimental factors that have a strong influence on the resulting parameters are clearly explained, with particular attention to the effect of the quality of the notches, the non-collinearity of the two edge notches in double edge notched tension specimens, and the lack of alignment of the specimen with the load axis once it is mounted on the load train. Furthermore, the influence of these experimental factors on the registered stress-displacement curves is also studied, and a criterion and the method for separating non-valid specimens are established.Peer ReviewedPostprint (author's final draft

Multifunctional foams made of polymer nanocomposites

Peer Reviewe

Microstructure anisotropy in polyolefin flexible foams

The use of polyolefin flexible foams with typical thicknesses between 1 and 3 mm produced by a physical foaming extrusion process is nowadays quite widespread in the packaging sector. Their high flexibility and closed-cell structure allows them to show good energy absorption properties under low loading conditions. Although the compressive response of these materials is well known, the inner microstructure developed during processing induce a high anisotropy that is responsible for their direction-dependent tensile and fracture behaviours. In this work, two different polyolefin-based foams, with densities ranging from 20 to 45 kg/m3, were studied. The induced microstructure anisotropy was characterized by micro- Raman. With this technique, the relative orientations of both crystalline and amorphous phases in the foam’s base polymer could be determined and thus related to their mechanical properties measured in the different directions.Peer ReviewedPostprint (author’s final draft

Effects of a phosphorus flame retardant system on the mechanical and fire behavior of microcellular ABS

The presentwork dealswith the study of phosphorus flame retardantmicrocellular acrylonitrile– butadiene–styrene (ABS) parts and the effects of weight reduction on the fire and mechanical performance. Phosphorus-based flame retardant additives (PFR), aluminum diethylphosphinate and ammonium polyphosphate, were used as a more environmentally friendly alternative to halogenated flame retardants. A 25 wt % of such PFR system was added to the polymer using a co-rotating twin-screw extruder. Subsequently, microcellular parts with 10, 15, and 20% of nominal weight reduction were prepared using a MuCell® injection-molding process. The results indicate that the presence of PFR particles increased the storage modulus and decreased the impact energy determined by means of dynamic-mechanical-thermal analysis and falling weight impact tests respectively. Nevertheless, the reduction of impact energy was found to be lower in ABS/PFR samples than in neat ABS with increasing weight reduction. This effect was attributed to the lower cell sizes and higher cell densities of the microcellular core of ABS/PFR parts. All ABS/PFR foams showed a self-extinguishing behavior under UL-94 burning vertical tests, independently of the weight reduction. Gradual decreases of the second peak of heat release rate and time of combustion with similar intumescent effect were observed with increasing weight reduction under cone calorimeter testsPostprint (published version