Polypropylene Melt Research Articles

Effects of high molecular weight species on shear-induced orientation and crystallization of isotactic polypropylene

In situ rheo-SAXS (small-angle X-ray scattering) and—rheo-WAXD (wide-angle X-ray diffraction) techniques were used to investigate the role of high molecular weight species on the evolution of oriented microstructure in isotactic polypropylene (iPP) melt under shear flow. The two iPP samples, designated as PP-A and PP-B, respectively, had the same number-average (Mn) but different weight-average (Mw) and Z-average (Mz) molecular weights. Molecular weight distribution (MWD) of PP-A and PP-B was such that for MW<105 the MWD curves overlapped; whereas in the high MW tail region, the amount of high molecular weight species was higher in PP-B than PP-A. Both samples were subjected to an identical shear condition (rate=60s−1, duration=5s, T=155°C). In situ 2D SAXS and WAXD images allowed the tracking of shear-induced oriented structures in the melt. It was found that the shish structures evolved much earlier, and the degree of crystal orientation and oriented crystal fractions were higher in PP-B than PP-A. Moreover, PP-B exhibited faster crystallization kinetics than PP-A. These results, along with the predictions of double reptation models of chain motion and experimental studies of chain conformation dynamics in dilute solutions under flow, suggest the following: When a polymer melt that consists of entangled chains of different lengths is deformed, the chain segments aligned with the flow eigenvector can undergo the abrupt coil-stretch-like transition, while other segments would remain in the coiled state. Since, flow-induced orientation decays much more slowly for long chains than for short chains, oriented high molecular weight species play a prominent role in formation of the stretched sections, where shish originates. Our experimental results are strong evidence of the hypothesis that even a small increase in the concentration of high molecular weight species causes a significant increase in the the formation, stability and concentration of the flow-induced oriented microstructure.

Comparison of different -nucleators for isotactic polypropylene, characterisation by DSC and temperature-modulated DSC (TMDSC) measurements

The nucleating efficiency and selectivity of different β-nucleating agents was characterised and compared by differential scanning calorimetry, (DSC) and temperature-modulated DSC (TMDSC). The nucleating agents were the calcium salts of pimelic and suberic acid (Ca-pim and Ca-sub), linear trans-γ-quinacridone (LTQ), a commercial nucleator NJ Star (NJS) and an experimental product (CGX-220). The efficiency and the selectivity of Ca-sub and Ca-pim are extremely high. NJS is efficient above a critical concentration, which is connected with its partial dissolution in polypropylene melt. LTQ and CGX-220 possess strong overall nucleating ability and moderate selectivity. Using TMDSC, we found that three consecutive processes take place during the heating of β-nucleated samples cooled down to room temperature: reversible partial melting of the β-form, irreversible βα-recrystallisation, and the melting of the α-modification formed during βα-recrystallisation or being present in samples prepared with non-selective β-nucleators. Melting of the α-phase contains both reversible and irreversible components.

Influence of process Conditions on Melt Blown Web Structure. Part IV - Fiber Diameter

We are continuing an effort to quantitatively measure the influence of processing variables on the structure of polypropylene melt blown webs. In this paper, we report experimental measurements of the influence of die-to-collector distance, primary airflow rate, die temperature, collector speed and resin throughput rate on the diameter of fibers in fully-formed webs. This enabled us to quantitatively compare the influence of these processing variables on fiber diameter as well as to achieve greater understanding of the melt blowing process.

Shear‐Induced Orientation and Structure Development in Isotactic Polypropylene Melt Containing Modified Carbon Nanofibers

The shear‐induced crystallization behavior of isotactic polypropylene (iPP) nanocomposite melt containing modified carbon nanofibers (MCNFs) was investigated by rheo‐SAXS (small‐angle X‐ray scattering) and rheo‐WAXD (wide‐angle X‐ray diffraction) techniques using synchrotron radiation. Under quiescent conditions, the nucleating effect of MCNFs on crystallization of iPP was pronounced and the system exhibited a remarkably low saturation point (ca. 0.05 wt% of MCNF). In‐situ SAXS and WAXD results showed the development of shear‐induced crystalline structures and lamellar morphology in nanocomposite melts. Under the same shear conditions, the filled system exhibited notably faster kinetics compared with the unfilled system. The oriented crystalline fraction was found to decrease with the MCNF loading, indicating the competition between oriented crystals (induced by shear) and unoriented crystals (due to the nucleating effect of MCNF). At the early stages of crystallization, the amount of the oriented crystals increased with the MCNF concentration, suggesting that the nanofiller hindered the motion of polymer chains after the cessation of flow resulting in the delayed relaxation of stretched polymer segments. Dedicated to Prof. Phillip H. Geil's seventy‐fifth birthday.

Particulate multi-phase polymeric nanocomposites

The present research focuses on a model system of a blend of two immiscible polymers containing an inorganic nanofiller, exploring the effect of various melt processing procedures on structuring and on the resulting properties. Binary polypropylene/alumina and ternary polypropylene/nylon/alumina composites were produced by batch compounding under various processing conditions and compositions. Several types of polypropylene and alumina grades were examined. The alumina nanoparticles vary primarily in their mean particle size (13, 50, 500 nm). The morphology of the nanocomposites produced and their thermal, and rheological properties were studied. It was found that under certain processing conditions the nanoparticles significantly affect the morphology of the polypropylene/nylon polymer blend causing compatibilization. Also, the alumina nanoparticles affect the melt viscosity of the polypropylene/nylon blends, whereas their effect on either polypropylene or nylon melts was found insignificant. POLYM. COMPOS., 27:425–430, 2006. © 2006 Society of Plastics Engineers

Shear‐Enhanced Nucleation of Isotactic Polypropylene in Limited Space

The nucleation rate was measured by directly counting the number of nuclei, which were developed while an isotactic polypropylene melt was flowing under shear in a thin film. The nucleation rate was enhanced with an increased rate of shear, e.g., by a factor of 10 larger at the rate of shear of 14 s−1 compared with the quiescent state, at 134°C. The ratio of the shear‐enhanced nucleation rate to the nucleation rate in the quiescent state was larger at a higher temperature of crystallization, i.e., about 10 times at 134°C to 590 times at 140°C. The increase of the nucleation rate under shear flow was explained by a reduction of the lateral and end (fold) surface free energies; the product σ s 2 σ e decreased to 3.2×10−7 for the sheared melt, from 6.0×10−7 (J m−2)3 for the isotropic state. The free energy reduction was caused by transition of the nucleus formation mode from three‐dimensional folded chain nuclei to two‐dimensional bundle nuclei, in which chains lie down on the glass substrate, aligning parallel to the flow direction.

Influence of Processing Conditions on Melt Blown Web Structure. Part III — Water Quench

The overall goal of our recent research has been to quantitatively measure the influence of commercial processing variables on the structure of polypropylene melt blown webs. In this paper, we applied a cool water quench spray to fibers to increase their thermal quenching rate. Then, we collected webs and measured single fiber diameter, shot content, fiber bundle size, basis weight uniformity and pore structure. The structure of webs produced with and without quenching was compared to access the influence of quenching on webs.

Phase morphology evolution and compatibility improvement of PP/EPDM by ultrasound irradiation

The effects of ultrasonic irradiation on morphology and compatibility of polypropylene (PP)/ethylene–propylene–diene terpolymer (EPDM) blends were examined. Results show that ultrasound irradiation lead to stable morphology with reduced disperse phase size. Dynamic rheological analysis indicates the more homogeneous internal structure of PP/EPDM blend is obtained with ultrasonic irradiation. Increase the residence time of the blends in the ultrasound irradiating zone, the structure of the blends get even more homogeneous. The interfacial tension between PP and EPDM melts obtained from the Palierne emulsion model decreases with applying of ultrasound irradiation and increasing of residence time. The glass transition temperatures of PP and EPDM phase become closer to each other with ultrasonic irradiation. FTIR analysis indicates in situ copolymers formation between PP and EPDM happens with ultrasound irradiation and the copolymer consequently acts as compatiblizers for the blend.

Development of highly oriented polymer crystals from row assemblies

The development of organized structures from sheared isotactic polypropylene melts with different amounts of dibenzylidene sorbitol (DBS) as a nucleating agent has been followed in real time using small angle X-ray scattering in conjunction with synchrotron radiation. The results show that, above a certain DBS concentration threshold, the lamellar crystals are highly oriented perpendicular to the direction of the prior applied shear field. The role of the directing agent in nucleating the crystallization and templating the crystal anisotropy is discussed.

Influence of Processing Conditions on Melt Blown Web Structure: Part 2 -Primary Airflow Rate

We are continuing an effort to quantitatively measure the influence of processing variables on the detailed structure of commercial polypropylene melt blown (MB) webs. In this paper, we report the influence of primary airflow rate on fiber entanglement, global fiber orientation and pore structure in webs. This enabled us to quantify the influence of primary airflow on web structural features as well as achieve greater understanding of the commercial MB process.

A comparison of some models for describing polymer crystallization at low deformation rates

There is now in the literature a number of mathematical models of the effects of flow on crystallization of polymers, and here we compare the performance of various models in slow simple shearing and sinusoidal strain (oscillatory) flows. We compare the model predictions to the experiments of Wassner and Maier [E. Wassner, R.-D. Maier, Shear-induced crystallization of polypropylene melts. in: D.M. Binding, et al. (Ed.), Proceedings of the XIII International Congress on Rheology, Cambridge, 2000, pp. 1–183] on isotactic polypropylene. Both the Kolmogorov [A.N. Kolmogorov, On the statistics of the crystallization process on metals. Bull. Akad. Sci. USSR, Class Sci., Math. Nat. 1 (1937) 355–359] and Nakamura et al. [K. Nakamura, T. Watanabe, K. Katayama, T. Amano, Some aspects of non-isothermal crystallization of polymers I, J. Appl. Polym. Sci. 16 (1972) 1077–1091] crystallinity formulations are discussed, and the advantages of the former are emphasized. For the cases considered, we see that strain-based crystallinity enhancement formulations appear to give results close to the experiments at the low flow rates which were studied, although they have some drawbacks. Several other ideas also lead to acceptable results; a new formulation based on both strain and strain-rate appears to give the best overall fit.

The effect of temperature on the viscoelastic response of polymer melts

Observations are reported in isothermal torsional oscillation tests on melts of isotactic polypropylene (iPP) and low-density polyethylene (LDPE) in the intervals of temperature between 190 and 250 °C (iPP) and between 120 and 190 °C (LDPE). With reference to the concept of transient networks, constitutive equations are developed for the viscoelastic response of polymer melts at three-dimensional deformations with small strains. A melt is treated as an equivalent network of strands bridged by temporary junctions (entanglements and physical cross-links whose life-times exceed the characteristic time of deformation). The time-dependent behavior of the network is modelled as detachment of active strands from their junctions and merging of dangling strands with the network. The network is assumed to be strongly heterogeneous in the sense that different junctions have different activation energies for separation of strands. The stress–strain relations involve three adjustable parameters (the plateau modulus, the average activation energy for rearrangement of strands and the standard deviation of activation energies) that are determined by matching the dependencies of storage and loss moduli on frequency of oscillations. The difference in the effects of temperature on the material constants of iPP and LDPE is associated with the difference in their molecular architecture.

Shear-Induced Molecular Orientation and Crystallization in Isotactic Polypropylene: Effects of the Deformation Rate and Strain

Studies of dilute polymer solutions in shear flow suggest that the mean fractional extension of molecules increases gradually with the Weissenberg number (Wi = shear rate × longest relaxation time) and approaches an asymptotic value of 0.4−0.5, while in elongational flow it approaches full contour length above a certain critical strain rate. In an entangled polymer melt, this behavior is more complex due to inter- and intramolecular interactions. In situ rheo-SAXS (small-angle X-ray scattering) and -WAXD (wide-angle X-ray diffraction) experiments were performed to investigate the effects of shear rate, shear duration, and Wi on the extent of molecular orientation/extension and crystal orientation in an isotactic polypropylene (iPP) melt. Two series of experiments were designed: (1) variation of shear rate (30, 45, and 60 s-1) at a constant shear duration (5 s) and (2) variation of shear duration (1.3, 3, and 5 s) at a constant rate (60 s-1). The degree of crystal orientation (Herman's orientation function, f) observed at 165 °C and fraction of oriented crystals (Xo) observed in a fully crystallized sample at room temperature increased with both shear rate and shear duration. Interestingly, at a constant strain (rate × duration), short-duration shear at a high rate was found to be more effective (i.e., higher f and Xo) than long-duration shear at a low rate. The longest relaxation time for the iPP sample and Wi were estimated from the dynamic moduli data. Both f and Xo were found to gradually increase with Wi and approached plateau values at high values of Wi. Results indicated that, even under a very intense shear field (or high Wi values), molecules do not extend to full contour length, and there is a limiting value for mean orientation/extension and subsequent crystal orientation in a polymer matrix. Characteristic dimensions of the shish-kebab entity formed in a sheared iPP melt at 165 °C were determined from the rheo-SAXS data. It was found that the average shish length was 700−750 nm and the average spacing between adjacent kebabs was 60−70 nm.

The effect of slip in the flow of a branched PP melt: experiments and simulations

In this paper visualisation and direct velocity profile measurement experiments for a branched polypropylene melt in a 10:1 axisymmetric contraction demonstrate the onset of wall slip. Video processing of the flow shows the formation of vortices and their diminution with increasing flow rate. Numerical simulations using a multimode K-BKZ viscoelastic and a purely viscous (Cross) model—both of them incorporating a nonlinear slip law—were used to predict the flow kinematics and dynamics as well as to deduce the slip velocity function by performing fitting to the velocity profiles. It was found that the numerical predictions agree well with the experimental results for the velocity profiles, and vortex formation, growth and reduction. It is suggested that such experiments (visualisation of entrance flow and direct velocity profile measurement) can be useful in evaluating the validity of constitutive equations and slip laws in the flow of polymer melts through processing equipment.

Effect of maleated polypropylene on the performance of polypropylene/cellulose composite

Composites consisting of a polypropylene (PP) and highly crystalline cellulosic microfibers were prepared by melting mixing with the maleic anhydride grafted polypropylene (MAPP) as a compatibilizer. The results show that even with addition of a small amount of MAPP, the mechanical properties of the composites improved dramatically. The improvement is attributed to stronger interfacial adhesion caused by esterification between anhydride groups of MAPP and hydroxyl groups of cellulose, although the number of the ester bonds is too few to be detected by FT-IR spectroscopy. It was also found that tensile strength and Young's modulus increased with the increasing MAPP contents in the composites, and the optimum MAPP content is about 10 wt% for the composite with cellulose content of 30 wt%. SEM indicated that the interfacial adhesion between cellulose fibers and PP improved in MAPP-containing composites. The DSC results showed that MAPP has little effect on melting and crystallization temperatures of PP in the composites. POLYM. COMPOS., 26:448–453, 2005. © 2005 Society of Plastics Engineers

Viscoelastic Simulation of Deformation-Induced Bubble Coalescence in Foaming Process

Viscoelastic simulations of deformation-induced bubble coalescence in forming process under isothermal condition were carried out. Multi mode PTT model was used in a simplified model for deformation of Polypropylene(PP) melt between bubbles in plane strain. 2-Dimentional Lagrangian FEM was adopted in the unsteady creeping flow simulation. The numerical results demonstrates that the thinnest part between neighboring gas bubbles is easily deformable and consequently coalescence occurs. Still more, creep recovery by elastic residual stress promotes the coalescence. The strain-hardening property is very effective to the coalescence prevention. Furthermore, the relaxation time distribution of polymer melts has significant influence on the behavior. Composition of the relaxation time that is a little longer than the elongational time makes a deformed bubble unstable and causes bubbles to coalesce. The numerical simulation could explain the deference in appearances of foamed sheets made of various PP.

On-line Visualization of PS/PP Melting Mechanisms in a Co-rotating Twin Screw Extruder

Abstract The melting and deformation mechanisms of polystyrene (PS) and polypropylene (PP) blends were investigated through on-line visualization of the co-rotating twin-screw extrusion process. Two compositions, PP/PS (80:20) and PS/PP (80: 20) were chosen as the model systems for this study. A sliding barrel technique was used to realize the on-line visualization using a glass window in the barrel. The axial temperature and pressure profiles along the screw channel were measured using the same sliding technique. It was found that in the PP/PS (80:20) blend, in which PP was the major phase, there was a combined melting of PS and PP, whereas in the PS/PP (80:20) blend, in which PS was the major phase, there was initial melting of PS alone and then combined melting of PS and PP. In the partially filled region, heat conduction from the hot barrel was the major source for heating polymer pellets under the conditions studied here; while in the fully filled region, viscous energy dissipation (VED) generated most of the heat for melting of polymer pellets. The pressure profiles along the extrusion channel gave us insight into the melting process for the two blends. It was found that if there was some molten polymer in the fully filled region, the overall melting process was accelerated due to heating from viscous dissipation.

Pressure−Volume−Temperature Dependence of Polypropylene/Organoclay Nanocomposites

The pressure−volume−temperature (PVT) dependencies of commercial polypropylene melt (PP) and its nanocomposites containing X wt % of organoclay (Cloisite-15A, or C15) and 2X wt % of a compatibilizer were determined at T = 450−530 K and P = 0.1−190 MPa. C15 was used at concentrations: X = 0, 2, and 4 wt %. Three functionalized PP's were used as compatibilizers: two maleated and one grafted with glycidyl methacrylate. Incorporation of X = 2 wt % C15 into PP resulted in reduction of specific volume by ΔV ≈ 1%, but that of free volume (hole) fraction by Δh ≈ 5%. The latter quantity was computed from the Simha−Somcynsky lattice−hole equation of state. Furthermore, at constant T and P the hole fraction was found to be linearly related to the bulk-average energetic interaction parameter and to be a sensitive indicator of structural changes. In binary (polymer + organoclay) systems Δh is linearly related to the interlayer spacing, d001. So, where statements from the previous paper are repeated or there are similarities, this is done for purposes of comparison. In three-component systems (with a compatibilizer) the proportionality has been preserved, but large changes of Δh result in relatively small changes of d001. Mechanical properties hardly correlate with either Δh or d001, as polymer/compatibilizer morphology and crystallinity complicate the behavior.

Modeling the Flow in the Metering Zone of Single Screw Extruders Using a Helical Geometry Model

Abstract This research work is devoted to the development of a finite element model for the simulation of the flow of polymer melts through the metering zone of single screw extruders. The sets of the governing equations (flow and energy) were solved using the continuous penalty scheme and a streamline upwinding Petrov-Galerkin method, respectively. The non-Newtonian rheological behavior of the fluid was also taken into account by the use of the power-law model. The novelty of this work is the application of this model on the flow of a polypropylene melt through the helical geometry of a single screw extruder. To validate this model, the results of the computer simulations were compared with experimentally measured mass flow rate and also results from a previous model, which was based on the simulation of the flow in a flat flow domain. Comparisons showed that not only there are very good agreements between the model predictions and actual data but also inclusion of the curvature of the screw flights has an important influence on the accuracy of the results.

Isothermal crystallization behavior of polypropylene catalloys

AbstractThe isothermal crystallization behavior of polypropylene (PP) catalloys and neat PP were studied with differential scanning calorimetry and polarized optical microscopy (POM). The crystallization kinetics of the samples were described with the well‐known Avrami equation. The crystallization rate depended remarkably on the content of the ethylene component in the PP catalloys. The crystallization half‐time increased obviously with the increase of the ethylene component in the PP catalloys. We also observed by POM that in isothermal crystallization, there were many more nuclei in the PP catalloys than that in neat PP and with an increase of the ethylene component, the average size of the spherulites decreased obviously. Even when ethylene content was as high as 27%, the crystallization rate still increased apparently, and this was quite different from common PP melting blends, in which the crystallization rate decreased when the ethylene content was relatively high because of the obstruction effect of dispersed droplets to the spherulite growth of the PP matrix. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 877–882, 2004