Theonis Riccò

Toughening of polypropylene by different elastomeric systems

Abstract Rubber toughening of a series of blends constituted by a polypropylene (PP) matrix added with talc, and modified by ethylene–propylene (EPR) and/or ethylene–butene rubber (EBR), with different molecular weights, was investigated. The fracture toughness was measured by an elasto-plastic fracture mechanics approach, applying the methodology of the essential work of fracture, and by conventional testing. It was found that: (i) EBR has a higher toughening efficiency than EPR; and (ii) the fracture toughness increases by increasing the molecular weight of the dispersed elastomeric phase. Measurements of volume change and temperature increase in specimens stretched during tensile tests indicated that the presence of EBR in the blends reduces cavitation and crazing, and induces a large amount of shear yielding. Some difference in the microcavitation mechanisms induced by each of the two elastomers was shown by electron microscopy analysis. An explanation of this behaviour on the basis of structural changes within the matrix or at the rubber–matrix interface was sought. Dynamic thermomechanical analysis indicated that EBR has a stronger interaction than EPR with the PP matrix.

Crystallization and morphology of reactor-made blends of isotactic polypropylene and ethylene-propylene rubber

The crystallization and morphology of reactor-made blends of isotactic polypropylene (PP) with a large content of ethylene-propylene rubber (EPR) (i.e., > 50%) were investigated. In the blends, PP was found to form spherulites during the crystallization process, with the growth rate constant under isothermal conditions. For crystallization temperatures in the range of 118–152°C, the birefringence of the spherulites varied from negative to positive by decreasing crystallization temperature, while homopolypropylene (homo-PP), the same as used in the blends as a matrix, showed negative spherulites in the whole temperature range investigated (118–152°C). Both the spherulite growth rate and the overall crystallization rate were slower for the blends than for homo-PP. The density of the crystallization nuclei was lower in the blends than in the homo-PP. It was concluded that a large amount of EPR content in the reactor-made blends of PP retards and hinders the crystallization of the matrix.

Fatigue crack propagation in polypropylene reinforced with short glass fibres

Fatigue crack propagation (FCP) behaviour of polypropylene composites reinforced with short glass fibres has been investigated as a function of fibre content and frequency of the sinusoidal applied load. The FCP resistance of the composites was found to improve as the fibre weight fraction increased. Results for all composites showed a dramatic decrease in the crack growth rate per cycle as a result of increasing frequency, at any given crack length. A further analysis of the data indicated that crack propagation was governed by viscoelastic creep which produced, at the lower frequencies, a crack speed approximately independent of frequency. However, it was recognized that at the highest frequency hysteretic heating at the crack tip induced a higher crack speed, associated with non-isothermal creep processes.

Developments in dynamic testing of rubber compounds: assessment of non-linear effects

Abstract In the present work, a test method to characterize the dynamic behaviour of rubber compounds by electrodynamic shaker (ES) in the frequency range of 10–1000 Hz was developed. Data of dynamic moduli of two different rubber compounds were determined through the analysis of the transmissibility of a suitably designed test system. The results were compared with those of dynamic moduli master curves obtained through frequency–temperature reduction of data measured by a commercial dynamic mechanical thermal analyser (DMTA), by scanning temperature at various frequencies in the range 0.3–30 Hz. Very good agreement of the data obtained by the two different aproaches were found, in spite of the different range of frequency explored by the two instruments, ES and DMTA, respectively. For one of the material examined, non-linear effects at low strain amplitudes were investigated by the two experimental methods considered.

High-rate fracture toughness of polypropylene-based, hybrid, particulate composites

The high-rate fracture behaviour of two hybrid, particulate, composite systems consisting of a polypropylene matrix, containing both inclusions of ethylene-propylene rubber and particles of talc or calcium carbonate, was investigated by varying the formulation of the dispersed phase. Depending on the behaviour of the different materials, linear elastic or elasto-plastic fracture mechanics was applied to determine their fracture resistance. It was found that, for certain compositions, the hybridization of the secondary phase produces synergistic effects with respect to the corresponding binary systems.

Development of toughness in ABS resins

Abstract Different ABS systems, produced by means of different polymerisation techniques (emulsion and mass polymerisations) and with different microstructural features were characterised and studied. Elastoplastic fracture mechanics tests were performed in order to compare the effects of the different morphologies of the dispersed phase on the mechanical responses. The plastic deformation mechanisms that give rise to energy dissipation were investigated in situ by means of transmission electron microscopy, and correlated to the observed fracture properties. Information about the stress field around and inside the dispersed particles was also obtained by dynamic mechanical analysis. The microstructural characteristics of the materials account well for the performance differences: particles from emulsion polymerisation, in particular, can cavitate and then promote plastic deformation through shear yielding.

Recovery of post-yielding deformations in semicrystalline poly(ethylene-terephthalate)

In this paper, aspects of the non-elastic deformation of semicrystalline poly(ethylene-terephthalate) (PET) films were studied from strain recovery and differential scanning calorimetric tests. The results show the existence of two components of non-elastic deformation, i.e. a fast-relaxing component (called anelastic) and a slow-relaxing component (usually called plastic). These strain components are both reversible and distinguished only on the basis of their different recovery times at temperatures far below the glass transition. A strain recovery master curve was built from the results of recovery tests at increasing recovery temperature. The shift-factor for the strain recovery master curve was then compared with the shift-factor for the construction of the dynamic storage modulus master curve obtained in linear regime (small strain). The aim of this comparison was to investigate the viscoelastic nature of yielding and post-yielding behavior in a semicrystalline polymer.

Fatigue Fracture of Neat and Short Glass Fiber Reinforced Polypropylene: Effect of Frequency and Material Orientation

The influence of glass fibers content and testing conditions (frequency and mean load) on the fatigue crack propagation rate in injection molded neat and fiber reinforced polypropylene has been investigated. The resistance to fatigue crack propagation increases as the fiber volume fraction increases. The fatigue resistance of either neat and fiber reinforced polypropylene is tremendously dependent on the crack propagation direction, with higher values for crack propagating transversely to the melt flow direction in the molded plaques. A dramatic increase of the crack growth rate per cycle by decreasing frequency, at any given crack length was found for all the materials investigated. Analysis of data obtained at various frequencies and applied mean loads suggests that crack propagation is determined mostly by viscoelastic creep processes at the crack tip, the role of pure fatigue appearing quite secondary.

Crack growth in discontinuous glass fibre reinforced polypropylene under dynamic and static loading conditions

Crack propagation in single edge notched tensile specimens of isotactic polypropylene reinforced with short E-glass fibres has been investigated under both fatigue and creep loading conditions. Fatigue crack propagation (FCP) experiments have been performed at three different frequencies (0.1, 1, 10 Hz) and at a mean applied tensile load of 1200 N. Isothermal creep crack propagation (CCP) tests have been conducted under a constant tensile applied load of 1200 N at various temperatures in the range from 32 to 60 °C. Analysis of FCP data allowed an estimation of the pure fatigue and pure creep components of the crack velocity under the adopted cyclic loading conditions. Crack growth at low frequencies (0.1 and 1 Hz) is mainly associated with a non-isothermal creep process. At higher frequency (10 Hz), the pure fatigue contribution appeared more pronounced. Finally, the comparison of FCP and CCP as a function of the mean applied stress intensity factor confirmed the major contribution of creep crack growth during FCP process at low frequencies.

Creep crack growth in a short glass fibres reinforced polypropylene composite

In this paper the creep crack propagation in a short glass fibre reinforced polypropylene composite has been investigated at various temperatures in the range from 32 to 60°C. Creep crack speed (da/dt) resulted initially decreasing till a minimum value, and then gradually increasing up to instability and fracture. Both initial and minimum crack speed values were found to strongly increase as test temperature increased. Moreover, isothermal curves of the applied stress intensity factor Kappl as a function of the crack speed (da/dt) were obtained at various temperatures. Portions of these curves in the stable crack acceleration region were hence shifted along the da/dt, axis according to a time-temperature reduction scheme, thus allowing the construction of a creep crack propagation master curve. The shift factor values, aT for the creep crack propagation master curve appeared to be higher than those obtained, in the same temperature range, from dynamic mechanical measurements in a linear viscoelastic regime.