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

Abstract

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 a-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 ß-phase fraction. Dynamic-mechanical thermal analysis (DMTA) provided information about the PP glass transition temperature (Tg), as well as about the relaxation a. No significant differences were observed in the Tg values, but the different sample composition resulted in notable variations of relaxation a, which was related with differences in the crystalline interfaces. The differences observed in the values of the loss tangent (tan d) were associated with variations in both stiffness and interfacial adhesion. The mechanical characterization, carried out through tensile tests, allowed to obtain values of Youngs 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.