José Ignacio Velasco

CRYSTALLIZATION BEHAVIOR OF POLYPROPYLENE FILLED WITH SURFACE-MODIFIED TALC

Talc-filled polypropylene (PP) composites were prepared by extrusion in a wide composition range (0–40 wt %). To improve the affinity relation between talc and the PP matrix, we modified the talc surface with silane coupling agents. Differential scanning calorimetry investigations on test samples, prepared by injection moulding, revealed that the talc content and its surface modification had a pronounced effect on the crystallization behavior of the filled PP composites. The experimental results indicate that a talc concentration of 2 wt % strongly affects the nonisothermal crystallization process of the PP, especially when talc is silane treated Isothermal crystallization experiments on samples with minimum amounts of talc (2 wt %) revealed an improved nucleation activity with silane-treated talc.

CONSTRAINED CRYSTALLIZATION AND ACTIVITY OF FILLER IN SURFACE MODIFIED TALC POLYPROPYLENE COMPOSITES

Abstract Two different series of talc-polypropylene composites ranged between 0 and 40 wt % were prepared by extrusion. In one of the series, the talc surface was modified with silane coupling agents in order to improve the affinity relation between the filler and the PP matrix. By means of X-ray diffraction (WAXD) and differential scanning calorimetry (DSC) techniques, the configuration of the PP crystalline phase and the nucleating activity of the mineral have been stated in both series.

Dynamic mechanical analysis of injection-moulded discs of polypropylene and untreated and silane-treated talc-filled polypropylene composites

Abstract Dynamic mechanical analysis (DMA) is used to study the dynamic mechanical properties of injection-moulded discs of polypropylene, talc-filled polypropylene composites and silane-treated talc-filled polypropylene composites. Silane addition in a very low proportion improves the adhesion between the filler and the polymer and enables composites to be used where otherwise they could not. Therefore, on the one hand, the composites have different properties due to the filler and the coupling agent, and on the other hand the injection processing induces a structure, resulting in inhomogeneity and anisotropy for the moulded discs. Scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD) techniques are used to help state the microscopic structure and justify the macroscopic properties measured with DMA.

Essential work of fracture on PET films: influence of the thickness and the orientation

Abstract This paper presents the fracture behaviour of films of a bioriented poly(ethylene terephthalate) (BOPET), that was studied by the Essential Work of Fracture (EWF) method. The influence of specimen thickness and molecular orientation was investigated. The results show that this method is a useful alternative for studying the plane-stress fracture of this material, finding that the specific essential work of fracture is strongly affected by the orientation [ w e was smaller in the direction of extrusion (MD) than in the perpendicular one (TD)], but independent of the specimen thickness in a range from 50 to 250 μm. On the other hand, the plastic work item is sensitive to variations of thickness but does not depend on orientation.

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.

The effect of filler type, morphology and coating on the anisotropy and microstructure heterogeneity of injection-moulded discs of polypropylene filled with aluminium and magnesium hydroxides. Part 2. Thermal and dynamic mechanical properties

Abstract Thermal and dynamic mechanical characteristics of injection-moulded discs of polypropylene (PP) filled with 40% by weight of magnesium and aluminium hydroxides have been studied by means of differential scanning calorimetry and dynamic mechanical thermal analysis, and have been related to the anisotropy and microstructure heterogeneity of the discs. The effect of filler type, particle morphology and surface coating has been analysed. The nucleation activity of filler particles on PP has been quantified and found to be reduced in coated grades of magnesium hydroxide. The employed coatings worked isolating and preserving particles surface from direct interaction with polymer chains. The different orientations of both filler particles and PP α crystals were found to be the main cause of the differences observed in mechanical properties.

Determination of J-R curve of polypropylene copolymers using the normalization method

In this paper the applicability of the load normalization method to determine J-R curves of polypropylene copolymers (PP) is analyzed. This method allows the determination of resistance curves ideally from a single fracture test, and it is based on the load separation principle, which assumes that load can be separated in two multiplicative functions, the geometry function, G(a/W), and the deformation function, H(νpl/W), which depend of the crack depth and the plastic displacement, respectively. The load separation validity has been checked for two different PP copolymers (block and random copolymers) and the load normalization method has been applied in order to determine and analyze the resistance curves, which have been compared, as a reference, with those obtained by the multiple specimen method. The applicability of the load normalization method to PP copolymers is analyzed by introducing some variations in the general procedure: Firstly, the deformation function is determined using either a power law fit or the so-called LMN function. With the power law, two different fitting methods have been tested: the usual “6 + 1” method and a “6 + 6” method proposed here for giving more weight to the final point of the curve. Secondly, the influence of the material crack tip blunting has been analyzed quantifying it through different values of the constriction factor (m) in the general expression of the blunting line. Finally, the effect of the separable blunting region extension on the J-R curve has been also analyzed by establishing different separable blunting zones.

Vegetable fibres from agricultural residues as thermo-mechanical reinforcement in recycled polypropylene-based green foams

Novel lightweight composite foams based on recycled polypropylene reinforced with cellulosic fibres obtained from agricultural residues were prepared and characterized. These composites, initially prepared by melt-mixing recycled polypropylene with variable fibre concentrations (10-25 wt.%), were foamed by high-pressure CO(2) dissolution, a clean process which avoids the use of chemical blowing agents. With the aim of studying the influence of the fibre characteristics on the resultant foams, two chemical treatments were applied to the barley straw in order to increase the α-cellulose content of the fibres. The chemical composition, morphology and thermal stability of the fibres and composites were analyzed. Results indicate that fibre chemical treatment and later foaming of the composites resulted in foams with characteristic closed-cell microcellular structures, their specific storage modulus significantly increasing due to the higher stiffness of the fibres. The addition of the fibres also resulted in an increase in the glass transition temperature of PP in both the solid composites and more significantly in the foams.

Moulded polypropylene foams produced using chemical or physical blowing agents: structure–properties relationship

Polypropylene (PP) foams have become essential items due to their excellent properties. Nevertheless, obtaining net-shaped PP foams with medium relative densities is a complicated issue. In this article, two processes able to produce moulded PP foams in this density range are presented. One of them is based on a modification of the pressure quench foaming method and therefore uses a physical blowing agent (CO2). The second one is the improved compression moulding technique which uses a chemical blowing agent (azodicarbonamide). PP foams with relative densities in the range between 0.25 and 0.6 and cylindrical shape were prepared using these foaming techniques. A common PP grade (instead a highly branched one) was used to obtain the samples, showing, that by combining the appropriate foaming technique, the adequate moulds, suitable blowing agent and proper foaming parameters, net-shaped PP foams with excellent properties can be produced starting from a conventional PP grade. Samples were characterized by analyzing their cellular structure and their mechanical properties. Results have showed that depending on the chosen foaming route isotropic or anisotropic structures with cell sizes ranging from 40 to 350 μm and open cell content in the range between 0 and 65% can be obtained. Moreover, mechanical properties are highly influenced by the production route and chemical composition of the foams. For instance, the stiffer materials at relative densities higher than 0.4 are the ones produced using the chemical blowing agent while at relative densities lower than 0.4 are the ones produced using the physical blowing agent.