Establishing polymers crystallization temperature by the self nudeation test (original) (raw)
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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.
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.
Chemical Engineering Science, 2002
A new experimental route for investigating polymer crystallization under very high cooling rates (up to 2000 • C=s) is described. A complete and exhaustive description of the apparatus employed for preparing thin quenched samples (100 -200 m thick) is reported, the cooling mechanism and the temperature distribution across sample thickness is also analysed, showing that the ÿnal structure is determined only by the thermal history imposed by the fast quench apparatus.
Polymer, 2012
The structure of semi-crystalline polymers is strongly influenced by the conditions applied during processing and is of major importance for the final properties of the product. A method is presented to quantify the effect of thermal and pressure history on the isotropic and quiescent crystallization kinetics of four important structures of polypropylene, i.e. the a-, b-, gand mesomorphic phase. The approach is based on nucleation and growth of spherulites during non-isothermal solidification, described by the Schneider rate equations combined with the Komogoroff-Avrami expression for space filling. Using an optimization routine the time-resolved multi-phase structure development is accurately described using crystal phase dependent growth rates and an overall nucleation density, all as function of temperature and pressure. It is shown that the maximum growth rate of the a-, and g-phase increases with applied pressure, while it decreases for the mesomorphic phase. Addition of b-nucleation agent is interpreted as a secondary nucleation density with a coupled b-phase growth. This complete crystallization kinetics characterization of isotactic polypropylene allows prediction of the multi-phase structure development for a wide range of quiescent processing conditions.
Crystallization of isotactic polypropylene containing beta-phase nucleating agent at rapid cooling
European Polymer Journal, 2013
ABSTRACT The crystallization behavior of isotactic polypropylene containing a β-phase nucleating agent has been investigated, focusing on evaluation of the effects of cooling rate and/or supercooling of the melt on the generation of different polymorphs. It has been found that β-crystals only form on cooling the melt at rates lower than about 50 K s−1 while cooling at rates between 50 and 300 K s−1 leads to formation of α-crystals; even faster cooling is connected with mesophase formation or vitrification of the entire melt. Fast scanning chip calorimetry revealed different mechanisms of nucleation at low and high supercooling. In comparison to non-nucleated iPP the presence of the β-phase nucleating agent only affects the crystallization kinetics at low supercooling, supporting the idea that ordering at high supercooling is governed by homogeneous nucleation. β-crystals, formed initially on slow cooling, melt below about 420 K on slow heating, followed by formation of few α-crystals on continuation of heating. The mesophase initially formed on fast cooling and aging at ambient temperature, in contrast, re-crystallizes directly into α-structure. The results of the present work provide comprehensive information about the condition of formation and the stability of different polymorphs in isotactic polypropylene containing a β-phase nucleating agent.
Journal of Polymer Science Part B-polymer Physics, 1999
Crystallization of polyethylene terephthalate and i-polypropylene in nonisothermal conditions is studied by means of differential scanning calorimetry. Measurements, carried out at several constant cooling rates, are interpreted in terms of a new theory 1,2 that takes into account effects related to a transient, nonsteady-state course of the process as well as athermal nucleation, which may occur under such circumstances. This article gives preliminary results based on analysis of final crystallinity reached at the end of cooling. Results indicate that the classical isokinetic approach is not adequate to describe crystallization kinetics at high cooling rates. A parameter quantizing the magnitude of deviations from isokinetic law is evaluated.