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Crystallization Kinetics of Ipp: Influence of Operating Conditions CORE Metadata, citation and similar papers at core.ac.uk Provided by Archivio istituzionale della ricerca - Università di Palermo Crystallization Kinetics of iPP: Influence of Operating Conditions and Molecular Parameters V. La Carrubba, S. Piccarolo, V. Brucato Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, Universita` di Palermo, Viale delle Scienze, 90128 Palermo, Italy Received 5 September 2006; accepted 21 November 2006 DOI 10.1002/app.25871 Published online in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: An analysis of the crystallization kinetics of dif- the crystalline phases. The whole body of results (including cal- ferent grades of isotactic polypropylene (iPP) is here presented. orimetric ones) provides a wide basis for the identification To describe the crystallization kinetics as a function of molecu- of a crystallization model suitable to describe solidification in lar and operating parameters, the methodological path fol- polymer-processing operations, based on the Kolmogoroff– lowed was the preparation of quenched samples of known Avrami–Evans nonisothermal approach. The kinetic parame- cooling histories, calorimetric crystallization isotherms tests, dif- ters, determined for all the materials, are discussed, highlight- ferential scanning calorimetry cooling ramps, wide angle X-ray ing the effect of molecular parameters on the crystallization diffraction (WAXD) measurements, and density determination. kinetics: molecular mass and distribution, tacticity, nucleating The WAXD analysis performed on the quenched iPP samples agents, and ethylene units content. Ó 2007 Wiley Periodicals, Inc. confirmed that during the fast cooling at least a crystalline J Appl Polym Sci 104: 1358–1367, 2007 structure and a mesomorphic one form. The diffractograms were analyzed by a deconvolution procedure, to identify the Key words: WAXS; density; crystallization; polyolefins; relationship between the cooling history and the distribution of processing INTRODUCTION nonisothermal conditions and analysis of crystalliza- tion kinetics is not straightforward. The crystalline structure of the polymer is extremely With reference to isotactic polypropylene (iPP), it is relevant for practical purposes as it significantly influ- well known that crystallization toward monoclinic ences the properties of final products. Generally crystalline structure is preferred at high temperature, speaking, polymer crystallization under processing whereas a mesomorphic highly disordered phase is conditions cannot be strictly considered an ‘‘equilib- mainly obtained at lower temperatures.7,8 rium’’ phenomenon, since it is not possible to separate Several attempts have been made to describe noniso- thermodynamics from kinetic effects on processes. thermal crystallization kinetics with simplifying assump- Furthermore, polymeric materials crystallization is tions9–13 and procedures have also been developed to always limited by molecular mobility, and very often determine the relevant rate parameters with no concern leads to metastable phases.1 Evidences of the forma- on the experimental conditions encountered during pro- tion of metastable phases under drastic conditions cessing where drastic solidification conditions are deter- (high cooling rates and/or high deformation rates) mined by large pressures, stresses, and temperature gra- have been widely reported for several semicrystalline dients.9,10 As a matter of fact, the data obtained from tra- polymers.2–6 ditional techniques, such as calorimetric cooling ramps, From the aforementioned arguments, it comes out are restricted to few degrees Celsius per second. Such that the structure of semicrystalline polymers, being cooling rates are orders of magnitude lower than those kinetically controlled, is always strongly affected by experienced by the material during polymer processing. processing conditions. In actual industrial polymer In recent years, our group undertook quantitative studies processing, crystallization generally occurs under of crystallization under high cooling rates (continuous cooling transformation, CCT14) with reference to differ- ent materials, such as poly(ethylene terephthalate) Correspondence to: V. La Carrubba (lacarrubba@dicpm. 15 16,17 unipa.it). (PET) and polyamide6 (PA6). Contract grant sponsor: EU Brite; contract grant number: Furthermore, the crystalline structure of iPP quen- BRPR.CT96.0147. ched from the melt is affected not only by cooling rate, Contract grant sponsor: The Italian Government MURST or generally by processing conditions, but also by mo- Funds (40% Quota). lecular parameters like molecular mass (Mw) and mo- Journal of Applied Polymer Science, Vol. 104, 1358–1367 (2007) lecular mass distribution (MWD). Different configura- VC 2007 Wiley Periodicals, Inc. tions (isotacticity and head-to-tail sequences) or addi- CRYSTALLIZATION KINETICS OF ISOTACTIC POLYPROPYLENE 1359 tion of small monomeric units and nucleating agents samples to outline, when possible, the influence on can also influence the final structure.18–25 the crystallization kinetics of average molecular mass, Influence of molecular weight on polymer crystalliza- molecular-mass distribution, isotacticity, copolymer- tion is controversial. Stem length indeed interferes with ization with small amount of ethylene units, and the entanglement density, thus determining a rate-con- addition of nucleating agents. trolled segregation regime of topological constraints in noncrystalline regions. Very low molecular weight tails THEORY: CRYSTALLIZATION of the distribution are shown to positively affect crystal- KINETICS MODEL lization kinetics, although their thermodynamic action should not favor perfection of crystallites.1 When dealing with crystallization of iPP, the numer- It is known from the literature that crystallization ous crystalline modifications of this material must be kinetics of semicrystalline polymers is influenced by accounted for, since a, b,org crystals may form upon the presence of contaminants. The main effect of the solidification from the melt. The resulting complex addition of a nucleating agent is an increase of the frame can be simplified based on some experimental 19,22–24 final crystallinity level together with a higher final evidences, supported by several references. As density and a finer and homogeneous crystal size dis- for the b phase, it basically shows up only if specific b tribution. This typical effect of enhancement of the nucleants are added; therefore, for commercial non-b- 22–24 overall crystallization kinetics allows one to infer that nucleated iPPs, it does not form ; traces of g form crystallization kinetics is nucleation-controlled, being crystals are often present, but always in minor amount the nucleation step the rate-determining one, while and in a narrow window of operating conditions (i.e., the growth rate remains almost unaffected.22,23 cooling rates), hence its presence is neglected without 19 On the other hand, the incorporation of a small con- affecting the reliability of the results. tent of ethylene units in the polypropylene chains has Under the aforementioned hypotheses, as two dif- an influence on the regularity of the molecular struc- ferent crystalline phases are formed (a and mesomor- ture. In fact, a change in tacticity induced by the short- phic), at least two kinetic processes take place simulta- ening of isotactic sequences was observed.26 Although neously. The simplest model is a parallel of two ki- this has a negative influence on crystallization kinetics, netic processes noninteracting and competing for the an opposite effect should come from the enhanced mo- available molten material. The kinetic equation adop- bility due to the presence of the ethylene sequences. As ted here for both processes is the nonisothermal for- 9,10 a result of these counteracting effects, a relatively nar- mulation by Nakamura et al. of the Kolmogoroff– 27–30 row window of cooling rates exists in which an Avrami–Evans model. enhancement of crystallization kinetics sets in.19 The model is based on the following equation: A better understanding of the relation between pro- XðtÞ cessing and properties can be achieved if the absolute ¼ 1 À exp½EðtÞ (1) crystallinity during transformation can be predicted as a X1 function of processing conditions. This prediction has to be supported by a crystallization kinetics model; in this where X(t) and X1 are the crystallized volume frac- article, a modified two-phase nonisothermal form of the tion at time t and in equilibrium conditions, respec- Kolmogoroff–Avrami–Evans model was used to describe tively. For simplicity and for the sake of generaliza- the crystallization kinetics.27–32 tion, X1 is here assumed to be a material constant, The main purpose of this study is to underline the although it has been reported its dependence upon relevance of thermal history resulting from various the crystallization history (crystal size distribution 33 cooling conditions on the crystallization kinetics of dif- and degree of perfection). ferent grades of iPP containing various additives such E(t) is the expectancy of crystallized volume fraction as nucleating agents and small content of ethylene. if no impingement would occur. A different formula- More specifically, the article attempts to identify rel- tion of the model can be easily obtained by differentia- evant material parameters determining quiescent non- tion of eq. (1), thus getting isothermal crystallization kinetics simulating polymer dx _ solidification under processing conditions.
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