Fracture toughness of controlled-rheology polypropylenes

Abstract

Abstract:The rheological, thermal, tensile and fracture properties of controlledrheology polypropylene processed via organic peroxides have been evaluated. Different concentrations of di-tert-butylperoxide (DTBP) were used in this work. Both melt flow index and the crystallinity index increased slightly with the peroxide content while the melting temperature tend to decrease. This was attributed to the decrease in the molecular weight which mainly affects the amorphous region, especially the entanglement density of this semycrystalline polymer. As a consequence, the tensile stress and strain at break reduced as the peroxide content increased. The elastic-plastic response of the different polypropylenes at room temperature promotes the use of J-integral to determine the fracture toughness. A multiple specimens method was utilized. The crack initiation resistance and the crack propagation resistance were influenced by the molecular weight, decreasing as the peroxide content increased. The micromechanisms of failure were analyzed via scanning electron microscopy.

Relating fracture behavior to spherulite size in conrolled‐rheology polypropylenes

“…At room temperature and at quasi‐static conditions (low loading rates), the mechanical response for either the PP homopolymer or the controlled‐rheology PPs was clearly nonlinear with stable fracture [35]. Hence, the appropriate fracture mechanics procedure to characterize the PP homopolymer and the controlled‐rheology PPs was the multiple specimen method.…”

“…Table 1 lists the molecular properties, such as the MWD, obtained by gel permeation chromatography (GPC), the rheological properties, determined from melt flow rate measurements (MFR), and the morphological properties as the thermal parameters, the melting temperature, T m , the crystallinity temperature, T c , and the crystallinity index, α, measured via differential scanning calorimetry (DSC) for the PPs with 0, 154, 402, and 546 ppm of peroxide content termed as PP0, CRPP154, CRPP402, and CRPP546, respectively [35]. The X‐ray diffraction pattern analyzed by Salazar et al [35] reported the presence of mainly monoclinic α‐form of iPP. The dependence of the rheological and thermal properties with the addition of peroxide content has been extensively described by Salazar et al [35].…”

“…The X‐ray diffraction pattern analyzed by Salazar et al [35] reported the presence of mainly monoclinic α‐form of iPP. The dependence of the rheological and thermal properties with the addition of peroxide content has been extensively described by Salazar et al [35].…”

“…The Young's modulus, E , was determined via flexure tests carried out under stroke control at a cross‐head speed of 2 mm/min as ISO178:2003 standard recommends. The influence of the peroxide content on the mechanical properties of the materials under study has been analyzed elsewhere [35].…”

“…Although extensive studies have been focused on the reactive mechanism, the rheological and crystallization behavior as well as the mechanical properties of controlled rheology PPs [3–8, 12–14, 16–19, 34], few reports are concerned with the influence of the spherulite size and distribution on the fracture mechanics parameters [35–37]. Salazar et al [35, 36] analyzed the evolution of the fracture toughness under Elastic–Plastic Fracture Mechanics (EPFM) of different grades of PP homopolymer [35] and ethylene‐propylene block copolymers [36] prepared controlling the addition of di‐ tert ‐butylperoxide (DTBP) in reactive extrusion. In turn, Sheng et al [37] analyzed the effect on the fracture behavior under Post‐Yield Fracture Mechanics approach through the evaluation of Essential Work of Fracture (EWF) of rheology controlled ethylene‐propylene block copolymers films.…”