Crystallization Kinetics Of iPP Research Articles

Effects of Pressure and Cooling Rates on Crystallization Behavior and Morphology of Isotactic Polypropylene

Isotactic Polypropylene (iPP) is a widely used polymer due to its excellent mechanical and thermal properties, as well as its chemical resistance. The crystallization behavior of polypropylene is influenced by several factors, such as temperature, cooling rate, and pressure. The effect of pressure is significant for both scientific and technological points of view, since in important industrial processing techniques the polymer solidifies under high pressures. In this paper, the study of the effect of pressure on the crystallization kinetics of iPP was conducted using a dilatometer in the pressure range from 100 to 600 bar and under two cooling rates: 0.1 and 1 °C/s. The morphology of the samples was characterized using DSC, optical microscopy, and X-ray diffraction. The results showed that pressure had a larger effect on specific volume changes at higher temperatures (in the melt state) than at lower temperatures (in the solid state). The polymer crystallization, which determined the transition between the melt and solid state, occurred at higher temperatures with increasing pressure. The cooling rate affected the crystallization process, with higher cooling rates leading to crystallization at lower temperatures. The size of the spherulites decreased with increasing cooling rates. The crystallinity evolution curves showed a linear relationship between the crystallization temperature and pressure. The study used a Kolmogoroff–Avrami–Evans model to describe the evolution into isotropic structures, and the predictions of the model accurately described the effect of pressure and cooling rates on the final spherulite radii.

New Insights into Crystallization of Heterophasic Isotactic Polypropylene by Fast Scanning Chip Calorimetry.

The crystallization kinetics of metallocene-catalyzed heterophasic isotactic polypropylene composed of a matrix of isotactic polypropylene (iPP) and rubbery particles made of random ethylene–propylene copolymers (EPC), often denoted as heterophasic iPP copolymers, was analyzed as a function of the cooling rate and supercooling in nonisothermal and isothermal crystallization experiments, respectively. Fast scanning chip calorimetry (FSC) allowed assessing crystallization at processing-relevant conditions, and variation of the content (0–39 wt %) and composition (0–35 wt % propylene counits) of the EPC particles revealed qualitatively new insight about mechanisms of heterogeneous crystal nucleation. For neat iPP homopolymer, the characteristic bimodal temperature dependence of the crystallization rate due to predominance of heterogeneous and homogeneous crystal nucleation at high and low temperatures, respectively, is reconfirmed. At high temperatures, in heterophasic iPP, the here studied ethylene-(C2)-rich EPC particles accelerate crystallization of the iPP-matrix, with the acceleration or nucleation efficacy correlating with the EPC-particle content. The crystallization time reduces by more than half in presence of 39 wt % EPC particles. An additional nucleating effect of the EPC particles on iPP-matrix crystallization is detected after their crystallization, suggesting that liquid/rubbery particles are less effective than solid/semicrystalline particles in affecting crystallization of the surrounding iPP-matrix. At low temperature, homogeneous crystal nucleation in the iPP-matrix outpaces all heterogeneous nucleation effects, and the matrix-crystallization rate is independent of the sample composition. The obtained results lead to the conclusion that the crystallization kinetics of iPP can be affected significantly by the content and composition of EPC particles, even towards superfast crystallizing iPP grades.

Isothermal and non-isothermal crystallization of isotactic polypropylene in the presence of an α nucleating agent and zeolite 13X

The isothermal and non-isothermal crystallization kinetics of isotactic polypropylene (iPP) and iPP nucleated with the nucleating agent sodium 2,2′-methylene-bis(4,6-di-tertbutylphenyl)-phosphate (NA11), zeolite 13X (Z13X), and both NA11 and Z13X were investigated. First, isothermal crystallization analyses show that the iPP/NA11/Z13X system provided the smallest half-crystallization time compared with the other three iPP systems even though a smaller amount of NA11 was added in the iPP/NA11/Z13X system (0.08 wt%) relative to that of the iPP/NA11 system (0.2 wt%). Moreover, the fold surface energy of the iPP/NA11/Z13X system was also the least among all samples considered. Then, during non-isothermal crystallization, the iPP/NA11/Z13X system demonstrated the smallest value of F(T), indicating that the required cooling rate to attain a specified relative crystallinity for the system was the lowest among all systems considered. Furthermore, a comparison of the crystallization activation energy shows that the NA11 and NA11/Z13X additives primarily improved the crystallization of iPP through an acceleration of the nucleation process. Finally, polarized optical microscopy results show that the NA11 and Z13X additives affected only the crystallization kinetics of iPP, and did not affect the crystallization morphology.

Non-isothermal crystallization kinetics of montmorillonite filled β-isotactic polypropylene nanocomposites

Montmorillonite filled isotactic polypropylene nanocomposites (iPP/MMT) generally form α-modification due to the heterogeneous α-nucleation of MMT for iPP crystallization. To obtain MMT filled β-iPP nanocomposites, MMT with β-nucleating surface (β-MMT) was prepared by supporting calcium pimelate (CaPA) as β-nucleator on the surface of MMT particles. The crystallization behavior, melting characteristics, and non-isothermal crystallization kinetics and crystallization activation energies of MMT and β-MMT filled iPP nanocomposites were studied by differential scanning calorimetry. It is found that the crystallization peak temperatures of β-MMT filled iPP nanocomposites were higher than those of β-iPP and MMT filled iPP nanocomposites, which indicated that the heterogeneous nucleation of β-MMT is stronger than that of MMT. Jeziorny and Mo methods were applicable to study the non-isothermal crystallization kinetics of iPP and its nanocomposites. Addition of CaPA, MMT and β-MMT can increase the crystallization rate. The crystallization activation energies were calculated by Friedman methods, and nucleating activities were calculated by Dobreva methods. It is indicated that β-nucleation decreased the crystallization activation energy of iPP, and the nucleating activity of β-MMT is higher than that of CaPA and MMT.

Influence of a particulate nucleating agent on the quiescent and flow-induced crystallization of isotactic polypropylene

Flow induced crystallization of commercial isotactic polypropylene (iPP) and its blends with sodium 2,2′-methylene bis-(4,6-di-tert-butylphenyl) phosphate (also known as NA11) is studied by means of in-situ time resolved small-angle X-ray scattering (SAXS). The isothermal crystallization at 145 °C (i.e. well below melting temperature of polymer) is performed after the application of steady shear to probe the anisotropic structure formation. In order to separate the influence of shear rate and shear time on polymer crystallization, four different shear conditions (60 s−1 for 1 s, 30 s−1 for 2 s, 15 s−1 for 4 s and 6 s−1 for 10 s) are applied while maintaining the same imposed strain in the polymer melt. Further the effect of different concentration of nucleating agent on the crystallization kinetics of iPP is examined both under quiescent and shear flow conditions. For instance, under quiescent condition, the crystallization half-time (τ1/2) decreases with the increasing concentration of nucleating agent in the polymer. Under shear flow conditions, our observations are as follows: In the case of neat iPP, τ1/2 decreases significantly at higher shear rates (≥30 s−1). Compared to the neat iPP, for the same concentration of NA11 in the NA11/iPP blends differences in τ1/2 with the increase in applied shear rates are significantly smaller. In other words, the crystallization kinetics is dominated by the amount of nucleating agent in the NA11/iPP blends as opposed to shear rates in the neat iPP. The present study shows that the critical value of shear rate required for chain orientation in the molten polymer is lower in the presence of the nucleating agent compared to neat iPP. The self-nucleation process investigated with the aid of differential scanning calorimetry (DSC), indicates that the nucleating efficiency of NA11 on iPP is around 60%.

Effect of chain disentanglement on melt crystallization behavior of isotactic polypropylene

Isotactic polypropylene (iPP) with “disentangled” chains was generated through crystallization of iPP from its mineral oil solution. TGA test assured complete removal of mineral oil from iPP precipitates. Time sweep rheological measurements showed the modulus build-up with time indicating the formation of “disentangled” chains in iPP after the sample disentanglement treatment. The “disentangled” chains could preserve for a certain time before completely re-entangled during melting. Crystallization kinetics of iPP with “disentangled” chains was studied by using polarized optical microscope. The growth rate of spherulites in “disentangled” iPP was faster than that in the entangled one.

Crystallization kinetics of isotactic polypropylene nucleated with organic dicarboxylic acid salts

AbstractBicyclo[2.2.1]hept‐5‐ene‐2,3‐dicarboxylate salts (BCHED) were synthesized by hydrothermal method. The crystallization kinetics of isotactic polypropylene (iPP) and nucleated iPP with BCHED were investigated by differential scanning calorimeter (DSC) under isothermal and nonisothermal conditions. Isothermal crystallization kinetics was described by Avrami equation and the fold surface free energy (σe) of iPP was calculated by Hoffman theory. The results showed Avrami exponents of nucleated iPP become smaller, the values of t1/2 dramatically decrease, the crystallization rate constants Z(T) greatly increase and the fold surface free energy (σe) of iPP decrease with the addition of the nucleating agents. Under nonisothermal condition the Caze method was applied to deal with the crystallization kinetics of iPP and the crystallization active energy of iPP was determined by Kissinger method. The results showed the crystallization peak temperatures (Tcp) of nucleated iPP greatly increase, but Avrami exponents of iPP were slightly influenced and the crystallization active energy of iPP increases by the addition of BCHED. It can be concluded that BCHED act as nucleating agents and BCHE11 shows the best nucleating effect. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Non‐isothermal Crystallization Kinetics and Morphology of Isotactic Polypropylene (iPP) Nucleated with Rosin‐based Nucleating Agents

The non‐isothermal crystallization kinetics of iPP with 0.3 wt.% of nucleating agent potassium dehydroabietate (DHAA‐K) or sodium dehydroabietate (DHAA‐Na) at different cooling rates was investigated using a differential scanning calorimetry technique together with Jeziorny's, Mo's, and Cazé‐Chuah's methods. The results indicated that all three models gave good fitting to experimental data. The Avrami exponents of crystallization for iPP, PP‐K, and PP‐Na were about 3, 2, and 2, respectively, suggesting a change of crystal growth mechanism for the nucleated iPP. The activation energies of the crystallization, estimated with Kissinger's method, increased in magnitude for the nucleated iPP compared with iPP. Moreover, the results of PLM and SEM showed that detectable spherulites were suppressed by the nucleating agents.

The influence of thermoelastomers on the crystallization behavior of isotactic polypropylene under shear

AbstractThe crystallization behaviors of isotactic polypropylene (iPP) and its blends with thermoelastomers have been investigated with in situ X‐ray scattering and optic microscopy. At quiescent condition, the crystallization kinetics of iPP is not affected by the presence of elastomers; while determined by the viscosity, the differences are observed on sheared samples. With a fixed shear strain, the crystallization rate increases with increasing the shear rate. The fraction of oriented lamellar crystals in blends is higher than that in pure iPP sample, while the percentage of β phase is reduced by the presence of the elastomers. On the basis of experimental results, no direct correlation among the fraction of oriented lamellae, the percentage of β phase, and growth rate can be deduced. The evolution of the fraction of oriented lamellae supports that shear field promotes nucleation rather than growth process. Shear flow induces the formation of nuclei not only with preferring orientation but also with random orientation. The total density of nuclei, which determines the crystallization kinetics, does not control the ratio between nuclei with and without preferring orientation, which determines the fraction of oriented lamellae. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1188–1198, 2006

Nature of Shear-Induced Primary Nuclei in iPP Melt

Although observations of molecular processes in the formation of primary nuclei prior to actual crystallization are beyond the detection limits of current instrumentation, we attempted to probe the nature of primary nuclei in sheared isotactic polypropylene (iPP) polymer melt. In situ rheo-SAXS (small-angle X-ray scattering) and -WAXD (wide angle X-ray diffraction) experiments using synchrotron radiation were carried out to evaluate the effects of an addition of a high molecular weight atactic polypropylene (aPP) (5 wt%), which is compatible with the iPP matrix but does not crystallize, on the evolution of oriented structures in the sheared iPP melt and its crystallization kinetics. It is unlikely that the aPP chain segments can be incorporated into iPP nuclei or crystal; hence, its addition effects, if any, would be seen only in the amorphous melt prior to crystallization. The results showed stonger orientation and improved crystallization kinetics in the iPP/aPP blend compared to pure iPP. Observations that the presence of long chains of an amorphous polymer aid in nucleation and crystallization kinetics of iPP, combined with our previous synchrotron results of sheared iPP melts at high temperature (165°C), lead us to conclude that primary nuclei in iPP most likely consist of liquid-crystalline or mesomorphic bundles of aligned chain segments prior to the formation of crystals.

Crystallization kinetics of iPP. Model and experiments

The model proposed by Ziabicki [1] [2] for non-isothermal crystallization kinetics was adopted in this work, to describe the crystallization kinetics of a commercial iPP under a very wide range of conditions (i.e. isothermal, slow-cooling rate and high-cooling rate (up to 200 °C/s) from the melt). A modification of the model was required in order to achieve a good agreement between model predictions and the whole set of experimental data.

Characterization of a maleic anhydride-modified polypropylene as an adhesion promoter for glass fiber composites

Isothermal and nonisothermal crystallizations of isotactic polypropylene (iPP), maleic anhydride (MAH)-grafted PP, and MAH-modified iPP were studied by differential scanning calorimetry (DSC), to evaluate the influence of a small amount of MAH-grafted PP in iPP on its crystallization behavior. Isothermal crystallization was followed in the temperature range from 391 K to 403 K, and the rate constant and Avrami exponents were determined. Nonisothermal crystallization was carried out at different cooling rates (1-20 K/min). It was found that the crystallization kinetics of iPP was significantly altered by modification with the MAH-grafted polymer. A decreased equilibrium melting temperature, as well as decreased surface energy of folding and critical dimensions of a growing nucleus, was determined for the MAH-modified iPP, indicating faster growth of lamellae and a higher rate of crystallization. The improved nucleation ability of the modified polymer was shown to cause a shift in the crystallization peak temperature towards higher values (from 393.7 K to 399.6 K, at a cooling rate of 1 K/min), resulting in crystal structures less disposed to recrystallization. Model composites of iPP and MAH-modified iPP with glass fibers were also analysed. The apparent shear strength of single-fiber model composites with MAH-modified iPP was drastically increased compared with homo-iPP.