Grades Of Isotactic Polypropylene Research Articles

Nodular structure of isotactic polypropylene crystallizes from the melt

AbstractThis article highlights the melt crystallization behavior of different grades of isotactic polypropylene (iPP) using a hot‐stage polarizing optical microscopy. iPP samples were heated up at a heating rate of 10°C/min passing the melting temperature and then kept for 3 min at a temperature range of 175–200°C before they cooled rapidly at 40°C/min to crystallize isothermally at a range of 130–145°C. It has been found that the temperature at which the samples were kept has a strong effect on the crystallization mode; for samples heated up and kept at temperatures below 190°C, the crystallization started with thin and long rods or nodules, which grew in the circumferential direction only while their lengths remain unchanged as the time passed. The shape of the nodules can be straight, circular, branched, or entangled, and they can grow parallel to each other or they can be crossed or in a random way. This phenomenon disappeared completely for samples melted and kept at temperatures above 195°C. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Cyclic viscoplasticity of semicrystalline polymers

Abstract Observations are reported on three grades of isotactic polypropylene in uniaxial cyclic tensile tests with various maximum strains. Constitutive equations are derived for the viscoplastic behavior of a semicrystalline polymer. A characteristic feature of the model is that the rate-of-strain tensor for plastic flow is split into the sum of two components, one of which is proportional to the deviator of the stress tensor, while the other is proportional to the rate-of-strain tensor. The stress–strain relations involve four adjustable parameters that are found by fitting the experimental data. Numerical simulation demonstrates that the model correctly predicts the mechanical response of semicrystalline polymers in cyclic tests.

Crystallization kinetics of iPP: Influence of operating conditions and molecular parameters

AbstractAn analysis of the crystallization kinetics of different grades of isotactic polypropylene (iPP) is here presented. To describe the crystallization kinetics as a function of molecular and operating parameters, the methodological path followed was the preparation of quenched samples of known cooling histories, calorimetric crystallization isotherms tests, differential scanning calorimetry cooling ramps, wide angle X‐ray diffraction (WAXD) measurements, and density determination. The WAXD analysis performed on the quenched iPP samples confirmed that during the fast cooling at least a crystalline structure and a mesomorphic one form. The diffractograms were analyzed by a deconvolution procedure, to identify the relationship between the cooling history and the distribution of the crystalline phases. The whole body of results (including calorimetric ones) provides a wide basis for the identification of a crystallization model suitable to describe solidification in polymer‐processing operations, based on the Kolmogoroff–Avrami–Evans nonisothermal approach. The kinetic parameters, determined for all the materials, are discussed, highlighting the effect of molecular parameters on the crystallization kinetics: molecular mass and distribution, tacticity, nucleating agents, and ethylene units content. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 1358–1367, 2007

Mechanical Properties of Impact i-PP/CSM Rubber Blends

ABSTRACT The impact strength and the tensile behavior of an impact grade of isotactic polypropylene (impact i-pp)/chlorosulfonated polyethylene (CSM) rubber blends are studied at the CSM rubber concentrations 0 to 23.4 vol%. The impact strength, which increased with CSM rubber concentration, has been analyzed on the basis of the interphase adhesion and crazing mechanisms. Tensile modulus and strength decreased whereas breaking elongation increased with increase in CSM rubber content. Predictive models have been used to explain the tensile modulus and strength properties. Scanning electron microscopy has been employed to study the phase structure.

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.

Highly active thermally stable ?-nucleating agents for isotactic polypropylene

Calcium salts of suberic (Ca-Sub) and pimelic (Ca-Pim) acids were synthesized and implemented as in different grades of isotactic polypropylene (iPP). Propylene homopolymer, as well as random and block copolymers containing these additives, crystallized iPP into pure or nearly pure β modification in the isothermal and nonisothermal crystallization experiments. Recently, Ca-Sub proved to be the most effective β-nucleating agent of iPP. The Ca-Sub nucleating agent widens the upper crystallization temperature range of pure β-iPP formation up to 140°C. In this study the effect of the these additives on the crystallization, melting characteristics, and structure of the PP were studied. The degree of crystallinity of β-iPP was markedly higher than that of α-iPP. A widening in the melting peak of the samples crystallized in a high temperature range was first observed and discussed in regard to literature results of the same phenomenon for α-iPP. The morphology of the β-iPP samples was revealed by scanning electron microscopy. Independent of the type of polymer or nucleating agent, hedritic structures were found in the early stages of growth of the β-spherulites. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2357–2368, 1999

Crystallization and Melting of β-Nucleated Isotactic Polypropylene

Ca salts of suberic (Ca-Sub) and pimelic acid (Ca-Pim) were synthesized and used as β-nucleating agents in different grades of isotactic polypropylene (IPP). Propylene homo-, random- and block-copolymers containing these additives crystallize principally in pure β-modification as demonstrated in isothermal and non-isothermal crystallization experiments. Ca-Sub proved the most effective β-nucleating agent known, so far. It broadens the upper crystallization temperature range of pure β-IPP formation up to 140°C. The effect of the additives on the crystallization and melting characteristics of the polymers was studied. The degree of crystallinity of the β-modification was found to be markedly higher than that of α-IPP. High temperature melting peak broadening was first observed and discussed in literary results regarding the same phenomenon for α-IPP.

Wide-Angle X-Ray, Densitometric and Microscopical Studies on Injection Molded Polypropylene Disks

Abstract Circular disks (280 mm diameter, 2 mm thickness) were injection molded from two grades of isotactic polypropylene under systematic variation of processing parameters (melt temperature, mold temperature, flow rate, packing pressure, and packing time) according to a two-level fractional factorial design. The disks showed ring-shaped zones (“halos”) of enhanced turbidity. The wide-angle X-ray scattering of the disks was measured as a function of the distance from the gate at 13 positions along the radius, using a diffractometer and reflection geometry. These measurements yielded radial profiles of the orientation of α-PP and of the distribution of β-PP crystallites. Both kinds of profiles revealed clear correlations with the polymer grade and with the melt and/or mold temperature. Radial profiles of optical turbidity of the disks were obtained by means of a microdensitometer. Pronounced maxima of turbidity were found to coincide with the maxima observed in the radial distribution of β-PP. An analysis of variance (ANOVA) of the experiments showed that the mass temperature has the greatest influence on the radial distances of the maxima of turbidity. Increasing mass temperature led to decreasing radial distances of the turbidity maxima, whereas the mold temperature as well as the flow rate only had a small influence on the radial positions of the turbidity maxima. The investigation of cross-sections, which were taken from some disks at selected distances from gate, by means of optical microscopy using polarized light revealed significant changes in the morphology with increasing distance from gate, e.g., changes in the size and distribution of β-PP spherulites.

Processing and physical property relationships in injection-molded isotactic polypropylene. 2. morphology and crystallinity

A preceding article referred to the processing of various grades of isotactic polypropylene (iPP) and propylene/ethylene copolymers by conventional and shear-controlled orientation injection molding (SCORIM), and considered the mechanical properties of the molded polymers in relation to the processing conditions applied. This article is principally concerned with the morphology and phase relationships that apply in injection-molded iPP. The γ-phase of isotactic polypropylene is reported to occur in moldings with pronounced molecular orientation. The presence of γ-phase in injection moldings is indicative of enhanced Young's modulus, and tensile strength provided that the melt has not been subjected to excessive deformation during processing. The common view that isotactic polypropylene exhibits only α- and β-phases in commercially produced moldings is challenged. SCORIM moldings contain less β-phase than conventional moldings, and exhibit greater overall crystallinity than moldings produced by conventional methods. The difference in the relative proportions of α-, β-, and γ-phases is marked and depends on processing conditions and the molding method. Shear-controlled orientation injection molding (SCORIM) results in more pronounced molecular orientation than conventional injection molding, which is consistent with the substantial increase in Young's modulus of moldings produced by SCORIM. The improved mechanical properties of iPP moldings is attributed to shish-kebab morphology developed by the action of shear to the solidifying melt. A new model is proposed for the oriented region morphology in iPP injection moldings based on the results presented in this article. © 1997 John Wiley & Sons, Inc.

Effect of orientation on melting of isotactic polypropylene

AbstractThe effect of uniaxial orientation on the melting of a commercial grade of isotactic polypropylene has been investigated by thermal analytical techniques. Orientation progressively shifts the onset of and the observed melting point to higher temperatures and increases the peak heights of differential scanning calorimetric (DSC) endotherms. The increase in melting point with orientation is attributed to the increase in configurational entropy of the extended melt into which the polymer crystallites melt.