Quantification of non-isothermal, multi-phase crystallization of isotactic polypropylene: The influence of cooling rate and pressure (original) (raw)
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Modeling of non-isothermal crystallization kinetics of isotactic polypropylene
Polymer, 2001
Isothermal and non-isothermal melt crystallization kinetics of isotactic polypropylene (iPP) were investigated via differential scanning calorimetry (DSC). Isothermal melt crystallization kinetics were analyzed using the Avrami equation. An Avrami exponent close to three was obtained for iPP, which implies growth of three-dimensional spherulitic superstructures following heterogeneous nucleation. Nonisothermal crystallization kinetics data obtained from DSC in conjunction with a non-linear regression method were employed to estimate the kinetic parameters of mathematical models describing the non-isothermal crystallization of iPP. The results suggest that the available mathematical models are not successful in describing the non-isothermal crystallization of iPP over a wide range of cooling rates. It was found that the non-isothermal crystallization kinetics of iPP, over a wide range of cooling rates, could best be described by modifying the Ozawa model to include induction times.
European Polymer Journal
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Isotactic polypropylene crystallization: Analysis and modeling
European Polymer Journal, 2011
The relevance of crystallization phenomena during the processing of semi-crystalline polymers is well known. Despite the huge investigation work done in the field, a full description of the crystallization kinetics under non-isothermal conditions still lacks. In this work, a commercial semi-crystalline polymer, an isotactic polypropylene, was studied in term of isothermal crystallization and of crystallization during cooling in a large range of cooling rate. The entire experimental behavior – solidification at high cooling rates, alpha-phase crystallization kinetics at intermediate cooling rates, and secondary crystallization at low cooling rates – was carefully quantified. Finally, a model able to reproduce all the observed features was proposed and tuned, to build a tool useful for the transformation processes simulation codes.The iPP samples final phase content is function of the cooling rate. A robust model able to capture all the complex behavior shown by a crystallizing iPP was proposed and tuned.
Primary Crystal Nucleation and Growth Regime Transition in Isotactic Polypropylene
Journal of Macromolecular Science, Part B, 2003
We studied nucleation and crystal growth in two polypropylene samples by analyzing the number of spherulites, their size, and shape for various treatments in a hot-stage microscope. The consistency of nucleation and growth data were tested by using overall crystallization experiments and the Jonhson -Mehl-Avrami -Kolmogorov Q1 equation. In isothermal conditions, for temperatures, T C , varying between 1238 and 1388C, the number of crystallites per unit area does not depend on crystallization time and temperature. Instead, at small undercoolings (for T C . 1388C), the total number of nuclei per unit area remains independent of crystallization time, but decreases with increasing temperature. We discuss the implications of this finding for the final microstructure of the crystallized samples. It is noteworthy that the passage between these two nucleation behaviors coincides with the transition from regime III to regime II, observed in the crystal growth kinetic analysis. This remarkable correlation between primary nucleation and crystal growth regimes is discussed.
Crystallization of polypropylene at high cooling rates
International Journal of Material Forming, 2009
In the context of polymer crystallization under high and constant cooling rates, a new survey is presented. The growth kinetics of spherulites from the molten state and the corresponding temperature of crystallization under similar cooling rates are considered. An industrial grade of isotactic polypropylene (iPP) is investigated in the range of slow (from 1 to 10 ° C/min), of relatively moderate (from 30 to 500 ° C/min) and of high (from 500 to 1600 ° C/min) constant cooling rates. The growth kinetics for the α-modification of iPP versus the temperature of crystallization is in agreement with the data published by Janeschitz-Kriegl [Macromolecules, 2006]. To understand the growth mechanism, the Hoffman equation is used and then extrapolated to lower temperatures of crystallization. Finally, to describe the non-isothermal crystallization, the equation of Ozawa is used. The benefits to use the spherulitic growth kinetics to extrapolate data from alternative methods, i.e., differential scanning calorimetry (DSC), are briefly discussed.
Journal of Polymer Science Part B-polymer Physics, 2002
Reliable experimental data for semicrystalline polymers crystallized under pressure are supplied on the basis of a model experiment in which drastic solidification conditions are applied. The influence of the pressure and cooling rate on some properties, such as the density and microhardness, and on the product morphology, as investigated with wide-angle X-ray scattering (WAXS), is stressed. Results for isotactic polypropylene (iPP) samples display a lower density and a lower microhardness with increasing pressure over a wide range of cooling rates (from 0.01 to 20 °C/s). Polyamide-6 (PA6) samples exhibit the opposite behavior, with the density and microhardness increasing at higher pressures over the entire range of cooling rates investigated (from 1 to 200 °C/s). A deconvolution technique applied to iPP and PA6 WAXS patterns has allowed us to evaluate the final phase content and to assess the crystallization kinetics. A negative influence of pressure on the α-crystalline phase crystallization kinetics can be observed for iPP, whereas a slightly positive influence of pressure on the crystallization kinetics of PA6 can be noted. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 40: 153–175, 2002
Crystallization kinetics of iPP: Influence of operating conditions and molecular parameters
Journal of Applied Polymer Science, 2007
An 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
Crystallization of polypropylene at various cooling rates
Materials Science and Engineering: A, 2005
Crystallization of polypropylene (PP) can result in formation of different crystal modifications depending on external conditions. The mechanisms of formation of various crystal modifications in polypropylene are still under discussion. We have investigated non-isothermal melt crystallization of isotactic polypropylene at cooling rates ranging from 1 up to 180,000 K/min using two types of differential scanning calorimeters-standard device Perkin-Elmer DSC Pyris-1 and ultra-fast calorimeter. Additional results were obtained by means of wide angle X-ray scattering and optical microscopy. At cooling rates below 6000 K/min there is only one exothermic peak corresponding to simultaneous crystallization of ␣and modifications. At cooling rates higher than 6000 K/min there is additional low temperature DSC peak corresponding to formation of mesomorphic phase. At the rates higher than 36,000 K/min there is no trace of formation of any ordered phase. In our opinion this complex behavior observed during crystallization of polypropylene can be explained using the concept of metastable phases. An increase of  content in samples with quinacridone pigment has been observed only at very low cooling rates, corresponding to high temperatures of crystallization and low homogeneous nucleation rate.