D. Arencón

Fracture Toughness of Polypropylene-Based Particulate Composites

The fracture behaviour of polymers is strongly affected by the addition of rigid particles. Several features of the particles have a decisive influence on the values of the fracture toughness: shape and size, chemical nature, surface nature, concentration by volume, and orientation. Among those of thermoplastic matrix, polypropylene (PP) composites are the most industrially employed for many different application fields. Here, a review on the fracture behaviour of PP-based particulate composites is carried out, considering the basic topics and experimental techniques of Fracture Mechanics, the mechanisms of deformation and fracture, and values of fracture toughness for different PP composites prepared with different particle scale size, either micrometric or nanometric.

Mechanical characterization of closed-cell polyolefin foams

Three different experimental techniques [compression experiments at low strain rates, instrumented falling-weight impact tests, and dynamic mechanical analysis (DMA)] have been used for the mechanical characterization of a collection of crosslinked closed-cell polyolefin foams of different chemical compositions, densities, and type of cellular structure. The experimental results that it is possible to obtain from each technique are shown, and related to the different applications of these materials. The relationships between the structure and the mechanical properties are also presented.

Influence of the injection moulding parameters on the microstructure and thermal properties of microcellular polyethylene terephthalate glycol foams

Microcellular injection moulding is capable of producing lightweight polymeric products. The present study analyses the influence of several representative injection moulding parameters on the foam’s morphology, apparent density and thermo-mechanical properties of PETG, poly(ethylene terephthalate-co-1,4-cyclohexylene-dimethylene terephthalate) specimens. A strong variation of the cell morphology along the melt flow direction has been found, as well as a dependence on the shot volume. The most homogeneous microcellular structure is achieved when low shot volume, intermediate injection speed and low mould temperature are employed. The skin–core structure of the injected parts, determined the thermo-mechanical features of the specimen, which are ruled by the skin layer.

Application of instrumented falling dart impact to the mechanical characterization of thermoplastic foams

The applicability of instrumented falling weight impact techniques in characterizing mechanically thermoplastic foams at relatively high strain rates is presented in this paper. In order to try simulating impact loading of foams against sharp elements, an instrumented dart having a hemispherical headstock was employed in the tests. Failure strength and toughness values were obtained from high-energy impact experiments, and the elastic modulus could be measured from both flexed plate and indentation low-energy impact tests. The results indicate a dependence of the failure strength, toughness, and the elastic modulus on the foam density, the foaming process, and the chemical composition. This influence was found to be similar to that of pure nonfoamed materials and also to that observed from low-rate compression tests. The results also indicate that the indentation low-energy impact tests were more accurate in obtaining right values of the elastic modulus than the flexed plate low-energy impact tests usually used to characterize rigid plastics. The foam indentation observed with this test configuration contributes to obtaining erroneous values of the elastic modulus if only a simple flexural analysis of plates is applied.

Polypropylene filled with flame retardant fillers : Mechanical and fracture properties

Two grades of isotactic polypropylene (homopolymer and block copolymer) were filled with magnesium and aluminium hydroxides, and studied focusing the mechanical and fracture characteristics of the composites. As expected, dispersion of such fillers in PP resulted in improved stiffness and reduced tensile yield strength. By one hand, the composites fracture resistance was characterised at low strain rate applying the J-integral concept; the resistance to crack growth initiation (J IC ) was found decreasing as the Mg(OH) 2 concentration was raised in the copolymer PP matrix. By the other hand, the linear-elastic fracture mechanics (LEFM) parameters were determined by means of instrumented impact tests at 1 m/s on the homopolymer PP filled with uncoated Al(OH) 3 particles. The higher the Al(OH) 3 mean particle size, the lower the composite fracture energy (G IC ). In the opposite, with commercial surface-coated filler grades it was not possible to achieve LEFM conditions to characterise the fracture toughness of filled PP at 1 m/s, because the Mg(OH) 2 surface coating, which is applied in practice to improve the melt processing, acts increasing the composite plasticity and reducing the tensile yield strength.

Polypropylene-Based Porous Membranes: Influence of Polymer Composition, Extrusion Draw Ratio and Uniaxial Strain

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.

Low Energy Dart Test for Mechanical Evaluation of Ophthalmic Materials

Purpose. Many impact tests fail to rigorously analyze the polymer behavior at impact, because they are performed in an energy range too different from real-life incidents, use specimens with other geometries than those of their final application, or they do not take in account polymer viscoelastic nature. A novel low energy impact method that overcomes current method limitations is presented for ophthalmic polymers and advances our understanding of the behavior of these materials under impact conditions. Method. Plate-shaped specimens of two known materials, CR-39 and Superfin, were tested in an energy range around their failure limit. A non-conservative model was proposed to predict the dynamic response of the specimens that did not fail. Both the deflection and indentation mechanisms were introduced in the model, which was solved using a fourth order Runge-Kutta numerical method. Damper coefficients that were introduced to model the energy dissipation and elastic modulus were obtained after the fitting process. Rupture stress and absorbed energy at failure were obtained from the specimens that failed. Results. Very good agreement between experimental and calculated data was observed. Under non-failure conditions, Superfin and CR-39 showed similar elastic modulus, although slightly larger energy dissipation was observed for CR-39. However, Superfin clearly outperformed CR-39 when measuring rupture stress and absorbed energy at failure with values 54% and 170% larger, respectively. Conclusions. Low energy impact methods are a very powerful tool to study and compare ophthalmic materials. The model satisfactorily predicted the behavior of materials in low energy impact conditions and can be used to obtain critical material characteristics. In this particular case, the method was used to quantify mechanical differences among CR-39 and Superfin. Of these two, the latter is the best performing material.

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 foams base polymer could be determined and thus related to their mechanical properties measured in the different directions.

Diffusion of CO2 in Polymer Nanocomposites Containing Different Types of Carbon Nanoparticles for Solid-State Microcellular Foaming Applications

This work considers the study of the diffusion of carbon dioxide in polypropylene and amorphous polymers containing carbon nanoparticles, particularly carbon nanofibres and graphene, as well as nanoclays, to be used in microcellular foaming. The diffusion of CO2 out and into the nanocomposites was studied during high pressure CO2 dissolution, as the amount of CO2 dissolved into the nanocomposite and CO2 desorption rate are crucial in order to have a proper control of foaming. Comparatively, platelet-like nanoparticles slowed down the desorption of CO2 out of the nanocomposites by means of a physical barrier effect, enabling a higher concentration of CO2 to remain in the polymer and be used in foaming. As a consequence of the higher amount of CO2 retained in the polymer and the cell nucleation effect promoted by the nanoparticles, polymer nanocomposite foams presented finer microcellular structures, in the case of PMMA even sub-microcellular, and higher specific moduli and electrical conductivities when compared to their pure counterparts.

Porous membranes based on polypropylene-ethylene copolymers. Influence of temperature on extrusion, annealing and uniaxial strain stages

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.