Linear Polypropylene Research Articles

Conductivity Insights Into the Carbon Nanotubes Mobility Restricted by the Long‐Branched Chains During the Thermal Annealing, Fast Shear Flow and Melt to Solid Cooling Process

AbstractThe present work provides a unique insight to reveal the long chain branching (LCB) influence to the nano‐fillers, that is via analyzing the conductivity evolution during the thermal annealing, fast shear flow and melt to solid cooling process. Meanwhile the crystallization behavior is also discussed. A linear polypropylene (PPC) and a long chain branched polypropylene (PPH) are chosen as the two polymer matrices with carbon nanotubes (CNTs) as the nanofillers. During the thermal annealing process, a more obvious growth of electrical conductivity of linear PPC nanocomposites are observed compared with LCB PPH nanocomposites. The harder movement of CNTs inside LCB PPH matrix may be the main reason. This also can be reflected by the conductivity evolution during slow cooling process where a decrease‐then‐increase (DTI) phenomenon is found for PPC/CNTs systems due to the rebuilding of CNTs conductivity network after crystallization. By contrast, this does not happen for PPH/CNTs systems.

Unemployment hysteresis in Central and Eastern European countries during EU-membership

AbstractThis study aims to analyse the time series properties of the unemployment rates in 10 Central and Eastern European countries after joining the European Union. Three types of unit root tests were conducted: (1) linear unit root tests, namely ADF, PP, LM, and RALS-LM tests; (2) LM and RALS-LM unit root tests with two structural breaks; and (3) LM and RALS-LM unit root tests with Fourier function. The results reveal that the hysteresis hypothesis is valid for Bulgaria, Czechia, Hungary, Latvia, Lithuania, Romania and Slovenia, whereas the structuralist hypothesis is valid for Estonia and Poland. However, the natural rate hypothesis holds only for Slovakia.

Ultra high density carbon fiber derived from isotactic polypropylene fiber without skin‐core structure

AbstractPolypropylene (PP)‐based carbon fibers were prepared by sulfonation process of isotactic PP fibers with concentrated sulfuric acid, followed by stress‐less carbonization under nitrogen atmosphere. The stabilization behaviors of PP fiber under different sulfonation temperatures and time were discussed. The carbonization behavior of the stabilized PP fibers under different carbonization temperatures, as well as the mechanical performance of the obtained carbon fibers were investigated. The results indicated that linear PP molecule were effectively converted into thermally stable structure at higher temperature (≥130°C) in short time (2 h) through sulfonation‐desulfonation reaction, among which ordered graphite structure has been formed prior to the carbonization process. Meanwhile, the carbon fibers were considerably densified by increasing the sulfonation temperature and carbonization temperature, and a bulk density of 1.96 g/cm3 was achieved. Moreover, the temperature and time of the sulfonation process as well as the temperature of the carbonization process were regulated, and carbon fibers with tensile strength of 262.3 MPa was obtained, which was superior to that of 208.1 MPa for the linear low density polyethylene‐based carbon fibers reported previously. Isotactic PP was proved to be a promising candidate to develop carbon fibers with tunable graphite structure and mechanical performance.

Helical polyamines.

Polymer microstructures rely on tacticity, yet exploration in polyamines has focused predominantly on atactic polymers. We introduce a method to synthesize a diverse library of ortho and para-cyanobenzenesulfonyl-activated-methyl aziridines using R, S, and racemic alaninol. Living anionic ring-opening polymerization of racemic sulfonyl aziridines yields soluble polymers, while enantiomerically-pure sulfonyl aziridines follow a dispersion polymerization with complete monomer conversion giving access to stereoblock copolymers. Removal of activation groups is achieved using dodecanethiol and tert-butylimino-tri(pyrrolidino)phosphorane to obtain isotactic or atactic linear polypropylene imines (LPPIs). High-purity L-PPIs are obtained in salt and neutral forms with high yields. Stereoblock copolymers of poly-R-block-S-polysulfonamides and respective polypropylene imine stereoblocks are synthesized, revealing helical structures in water influenced by the monomer type and sequence in CD spectroscopy. Molecular dynamics simulations confirm the helical nature of isotactic LPPIs in water. Bulk characterization demonstrates the first crystalline isotactic polyamines via spherulite growth in polarized light, atomic force microscopy and XRD analyses. In cell-transfection studies, the synthesized isotactic LPPIs exhibit lower toxicity and transfection efficiency than commercial hyperbranched polyethylene imine, with longer chains showing increased transfection efficiency. These isotactic polymers open avenues for complex macromolecular architectures with optically active polyamines akin to poly(amino acid)s but lacking hydrolytically cleavable amide links.

Polypropylene blends: Impact of long chain-branched polypropylene on crystallization of linear polypropylene

Polypropylene blends: Impact of long chain-branched polypropylene on crystallization of linear polypropylene

Unveiling the restricted mobility of carbon nanotubes inside a long chain branched polymer matrix via probing the shear flow effects on the rheological and electrical properties of the filled systems.

The present work has aimed at gaining a deeper understanding of the effects of shear flow on the behaviors of nano filler evolution inside linear and long chain branched polymer matrices. Accordingly, measurements consisting of transient start-up shear rheology coupled with small amplitude oscillatory sweep (SAOS) and dielectric tests were designed. Linear polypropylene (PPC) and polypropylene (PPH) with long chain branching (LCB) were chosen as the polymer matrices and carbon nanotubes (CNTs) as the nanofillers. The percolation threshold of the LCB PPH nanocomposites was found to be higher than for linear PPC, due to the high viscosity and elasticity of LCB PPH. A transient shear with different shear rates was imposed on the composites after which SAOS and electrical conductivity measurements were conducted. The liquid-solid transitions of the nanocomposites were found to be different and to depend on the shear flow conditions (shear rate). For the linear PPC, higher shear rates caused the filler network to break down while lower shear rates helped the nanofillers to agglomerate. Interestingly, for LCB PPH, both higher and lower pre-shear rates resulted in the breakup of the filler networks, which was due to the restricted mobility of the CNTs by the LCB. The confinement of the polymer chains to the CNTs and their aggregates made it difficult for the fillers to move thus causing the formed network to be easily destroyed even under slow and slight shears. Similarly, the trend was also found after shear flows as reflected by the increase and decrease of electrical conductivities.

Influence of molecular weight on high- and low-expansion foam injection molding using linear polypropylene

Long-chain branched (LCB) polypropylene (PP) exhibits strain hardening and increased melt strength not seen in typical linear PP. These benefits have seen LCB PP used broadly in foaming applications to produce high cell density foams due to decreased cell coalescence. However, these benefits may come at a significantly increased material cost. This work evaluates linear PP resins of varying molecular weight and polydispersity to determine their influence on foam expansion and cell nucleation behavior in a foam injection molding application. Resins with higher molecular weight and polydispersity reached higher expansion ratios owing to increased crystallization temperatures and improved extensional rheology. Low molecular weight was correlated to improved cell nucleation capability and higher cell density foams. This trend was attributed to decreased interfacial tension which decreases the energy barrier for cell nucleation. The results can inform future application-based resin design and selection by providing insights into the properties governing expansion and cell nucleation.

Non‐isothermal Crystallization Kinetics, Thermal, and Rheological Behavior of Linear/Branched Polypropylene Blends

AbstractThis work aims at evaluating the rheological and thermal properties and non‐isothermal crystallization kinetics of blends of linear homopolymer polypropylene (HPP) and branched polypropylene (BPP). Two linear polypropylene's of different melt flow index (MFI) are used: H301 (10 g per 10 min) and H604 (1.5 g per 10 min); and one BPP (2 g pr 10 min). The rheological result shows that the H301/BPP blends have an increase in complex viscosity proportional to the addition of BPP amount, while for the H604/BPP blends, it is observed a higher influence of H604 on rheological properties. With the addition of 25 wt% of BPP, the strain hardening behavior is observed in the extensional rheology tests of the polymer blend. The DSC results show that the melt temperature and the crystallinity content in blends with BPP are affected by the molecular weight (MW) of the linear polymer. Results suggest that Pseudo‐Avrami/Jeziorny and Mo models can be used to predict the experimental results of crystallization kinetics of the blends with sufficient precision for all systems studied, regardless of MW. Therefore, it is possible to use the blends of HPP/BPP in processes that demand a combination of rheological properties, such as high strain hardening, and fast crystallization.

Extrusion foaming of linear and branched polypropylenes – input of the thermomechanical analysis of pressure drop in the die

Abstract This paper aims at a better understanding of the polypropylene (PP) physical extrusion foaming process with the objective of obtaining the lowest possible foam density. Two branched PPs were compared to the corresponding linear ones. Their shear and elongation viscosities were measured as well as their crystalline properties. Trials were conducted in a single screw extruder equipped with a gear pump and a static mixer cooler to adjust the melt temperature at the final die. The effect of decreasing this temperature on the PP foamability and on the pressure drop in the die was analyzed. The foam density of branched PPs varies from high to low values while decreasing the foaming temperature. In the same processing conditions, the foam density of linear PPs does not decrease so much, as already evidenced in the literature. The foamability transition coincides with an increase of the pressure drop in the die. The originality of the work lies in the thermomechanical analysis of the polymer flow in the die which allows the identification of the relevant physical phenomena for a good foamability. The comparison of the experimental pressure drops in the die and the computed ones with the identified purely viscous behavior points out the influence of the foaming temperature and of the PP structure. At high foaming temperature the discrepancy between experimental measurements and the computed pressure drops remains limited. It increases when decreasing the foaming temperature, but the mismatch is much more important for branched PPs than for linear ones. This difference is analyzed as a combination of the activation energy of the viscosity, the elongational viscosity in the convergent geometry of the die which is much more important for branched PPs than for linear ones, and the onset of crystallization which occurs at higher temperature for branched PPs than for linear PPs.

Foaming Performance of Linear Polypropylene Ionomers

Linear polypropylene (PP) exhibits poor foamability because of its inherently low melt strength. Incorporation of long-chain branches is a common approach to increase melt strength; however, it is a costly process. To bridge this gap, we set out to study the effect of ionic content on the foaming behavior of linear PP ionomers using a PP homopolymer as a control and a commercial high melt strength long-chain branched PP as a benchmark. Small-angle X-ray scattering confirms the presence of ion clusters in the ionomers, which act as physical cross-linking points as was recently shown, providing enhancement to the dynamic moduli, complex viscosity, and strain hardening behavior, even at ion contents below 0.1 mol %. We found that, upon foaming, the PP ionomer with a higher ionic content exhibited higher foam uniformity, higher cell density (∼108 cells/cm3), and smaller cell size (as small as 37 μm) compared to those of its linear counterpart. Furthermore, the linear ionomer resin and the commercial branched PP showed similar foam morphologies and mechanical responses under cyclic indentations over the temperature range 155–175 °C. These findings reveal a new, and potentially cost-effective, route to produce high performance foams using linear PPs that are compliant to industrial standards.

Breakdown strength improvement of polypropylene by introducing long-chain branched structures for film capacitors

The breakdown strength of polypropylene (PP) for film capacitors drops sharply in a high-temperature environment, which would lead to capacitor insulation failure frequently. This paper proposes a method to improve the breakdown strength of PP at high temperatures by introducing long-chain branches (LCBs). Different from traditional linear PP, the results show that LCBs can increase the number of spherulites and refine the grains due to heterogeneous nucleation, resulting in a reduction in weak areas in the films and an increase in trap densities and energy levels in long-chain branched polypropylene (LCBPP). In addition, the higher melt strength of LCBPP indicates that LCBs promote entanglement between molecular chains and hinder the sliding of chains, thereby reducing the influence of temperature on the free volume. Therefore, compared with PP, the conductivity of LCBPP is lower at high temperatures, and the breakdown strength increases by 16.2% at 105 °C. This method provides an idea for improving the breakdown strength of PP films at elevated temperatures from the perspective of regulating the microstructure.

Breakdown Performance of Polypropylene and Its Long-Chain Branched Blending for Metalized Film Capacitors

A modification method of polypropylene (PP) films is proposed in this article for capacitors, based on the physical blending of the linear PP and long-chain branched PP (LCBPP). In order to evaluate the comprehensive properties of the samples with different mixing ratios, the micro morphologies, thermal, and electrical characteristics are studied. LCBPP is proven to possess the ability of heterogeneous nucleation. On the one hand, the addition of long-chain branched structures strengthens the intermolecular interaction and promotes the overlap of microcrystalline, which helps to reduce the amorphous region and improve the dimensional stability of the film. On the other hand, impurities and interfaces are introduced, resulting in the increased polarization. Therefore, the conductivity of the PP/LCBPP film reduces, the loss decreases, and the permittivity is basically unchanged. At 20 °C, a low proportion of the LCBPP is beneficial to the increase of the breakdown strength. While at 70 °C or 120 °C, the dense microstructures, promoted by a high proportion of the LCBPP, play a decisive role in suppressing the local field distortion and improving the breakdown properties. This method provides a feasible idea to improve the high-temperature resistance of the film from the perspective of molecular and aggregate structure design.

Development of high melt strength polypropylene and its application in thermoplastic elastomeric composition

High melt strength polypropylene (HMS-PP) with a long-chain branched structure is a modified form of polypropylene (PP) which has basic properties of regular PP but with superior melt drawability. This paper reports on the development of gel-free HMS-PP from a linear isotactic PP through the introduction of long-chain branching on its backbone via a reactive extrusion process, using dicetyl-peroxydicarbonate (PODIC) alone or in combination with a coagent. The melt strength and the mechanical properties such as impact and flexural strength of PP showed improvements with the modification with PODIC. 5000 ppm by weight of PODIC was found to provide the best balance of properties. The efficacies of zinc diethyldithiocarbamate (ZDC) and tetramethyl thiuram disulphide (TMTD) as coagents in combination with PODIC to augment properties of HMS-PP further were explored. TMTD offered slightly enhanced performance benefits as compared to ZDC at an optimized concentration of 100 ppm by weight. The application potential of HMS-PP in thermoplastic elastomeric blends of HMS-PP with ethylene-propylene-diene monomer (EPDM) rubber at a fixed ratio of 35/65 by weight was also investigated. Structure-property correlations were established between the extent of long-chain branching in the modified PP and the properties of the resultant thermoplastic elastomeric composition.

Improved Cell Morphology and Surface Roughness in High-Temperature Foam Injection Molding Using a Long-Chain Branched Polypropylene.

Long-chain branched polypropylene (LCB PP) has been used extensively to improve cell morphologies in foaming applications. However, most research focuses on low melt flow rate (MFR) resins, whereas foam production methods such as mold-opening foam injection molding (MO-FIM) require high-MFR resins to improve processability. A systematic study was conducted comparing a conventional linear PP, a broad molecular weight distribution (BMWD) linear PP, and a newly developed BMWD LCB PP for use in MO-FIM. The effects of foaming temperature and molecular architecture on cell morphology, surface roughness, and mechanical properties were studied by utilizing two chemical blowing agents (CBAs) with different activation temperatures and varying packing times. At the highest foaming temperatures, BMWD LCB PP foams exhibited 887% higher cell density, 46% smaller cell sizes, and more uniform cell structures than BWMD linear PP. Linear PP was found to have a surface roughness 23% higher on average than other resins. The BMWD LCB PP was found to have increased flexural modulus (44%) at the cost of decreased toughness (−88%) compared to linear PP. The branched architecture and high molecular weight of the BMWD LCB PP contributed to improved foam morphologies and surface quality in high-temperature MO-FIM conditions.

Mechanical pretreatment of polymer melts: Critical aspects and new rheological investigations on a linear and a long-chain branched polypropylene

It has been known for some time that mechanical pretreatments of melts of long-chain branched polymers can increase the melt index and reduce the extrudate swell. This effect is not due to changes of the molar mass, its distribution, or the degree of branching because it may be reversed by long-term annealing or dissolution of the polymeric material and subsequent precipitation. Furthermore, molecular analysis shows a molecular structure unaltered by the treatment. The effect has not been completely understood up to now, and it is still an open question as to whether it is restricted to long-chain branched polymers, which have been preferably studied. This paper gives an overview of results from the literature and presents comparable measurements on a well characterized long-chain branched and a linear polypropylene. A wide range of linear and nonlinear rheological properties were studied, and it became obvious that long-chain branching is a strong structural precondition for the pronounced pretreatment effect occurring in processing. Based on this result, models from the literature are discussed, and it is found that chain alignment may qualitatively explain the rheological measurements.

Effect of silica‐coated TiO2 nanorods on the foamability of polypropylene and photostability of foamed polypropylene

Researches about the heterogeneous nucleation ability of nanorods and the weather resistance of polypropylene (PP) foams were very limited. In this study, the well‐defined rutile TiO2 nanorods with UV‐absorbing function were coated with silica and then calcined. The obtained nanorods (denoted as “TS”) could be uniformly dispersed in PP matrix by melt blending. The foaming behavior of linear PP and its composites was investigated by using supercritical carbon dioxide (scCO2) as a physical foaming agent. The results demonstrated that the addition of 5 wt% TS made PP/TS‐5% foam have higher cell density, regular cell structure and uniform cell distribution owing to the large numbers of nucleation sites provided by well‐dispersed TS nanorods. At the same time, the aging performance of PP foam samples was evaluated by the changes of macro or micro morphology, melting and crystallization behavior before and after UVB irradiation. It was intuitively found that the damage to the surface of PP foams caused by UV light, involving the erosion of surface, the collapse of cells and the degradation of PP chains could be significantly suppressed by the incorporation of TS nanorods. These findings indicated that TS nanorods could be used as multifunctional nanofillers to achieve the PP foams with improved foamability and photostability.

Improving Rheological and Mechanical Properties of Various Virgin and Recycled Polypropylenes by Blending with Long-Chain Branched Polypropylene.

Blends of two long-chain branched polypropylenes (LCB-PP) and five linear polypropylenes (L-PP) were prepared in a single screw extruder at 240 °C. The two LCB-PPs were self-created via reactive extrusion at 180 °C by using dimyristyl peroxydicarbonate (PODIC C126) and dilauroyl peroxide (LP) as peroxides. For blending two virgin and three recycled PPs like coffee caps, yoghurt cups and buckets with different melt flow rate (MFR) values were used. The influence of using blends was assessed by investigating the rheological (dynamic and extensional rheology) and mechanical properties (tensile test and impact tensile test). The dynamic rheology indicated that the molecular weight as well as the molecular weight distribution could be increased or broadened. Also the melt strength behavior could be improved by using the two peroxide modified LCB-PP blends on the basis of PODIC C126 or PEROXAN LP (dilauroyl peroxide). In addition, the mechanical properties were consistently enhanced or at least kept constant compared to the original material. In particular, the impact tensile strength but also the elongation at break could be increased considerably. This study showed that the blending of LCB-PP can increase the investigated properties and represents a promising option, especially when using recycled PP, which demonstrates a real “up-cycling” process.

Polypropylene/thermoplastic polyester elastomer blend: Crystallization properties, rheological behavior, and foaming performance

AbstractThe inherently linear polypropylene suffers unsatisfying foaming behavior due to its low melt strength. To overcome this drawback, polypropylene‐/polypropylene‐grafted glycidyl methacrylate/thermoplastic polyester elastomer (PP/PP‐g‐GMA/TPEE) blending foam is prepared by the chemical foaming method in this study. The foaming mechanism of blending was studied from the aspects of the rheological behavior and the crystallization property. The results show that TPEE disperses in the PP matrix with fine particles and forms ideal interfaces, which provide a large number of nucleation sites for the foaming process of blending, inducing heterogeneous nucleation. Both of the 15 wt% and 20 wt% TPEE‐modified PP composites show higher shear viscosity and obvious strain hardening behavior. It has been proved that the cross‐linking network structure formed by TPEE and PP‐g‐GMA reaction improves the melt strength. The cell size decreases from 37.6 to 24.8 μm, and the cell density increases from 2.9 × 106 cells/cm3to 2.5 × 107 cells/cm3. Compared with PP composites, the foaming window of the PP/PP‐g‐GMA/TPEE composites was widened.

Effect on the Impact Properties of Adding Long‐Chain Branched Polypropylene in Polypropylene‐Polyolefin Elastomer Cellular Polymers Produced by Core‐Back Injection Molding

AbstractDifferent blends of a linear polypropylene (L‐PP), a long‐chain branched polypropylene (LCB‐PP), and a polyolefin elastomer (POE) are prepared and foamed using core‐back injection molding. The objective is to analyze the use of POE and LCB‐PP as complementary strategies to improve cellular polymers’ impact response. Different parameters are then studied in blends and injection molded plaques to understand the samples’ mechanical behavior. Parameters such as the interfacial tension, elastomer morphology, extensional rheology, cellular structure, and crystallization behavior are characterized in detail. The addition of POE allows improving the impact response of both the solids and cellular polymers, but the stiffness is reduced. On the other hand, the substitution of L‐PP by LCB‐PP, in blends containing POE, results in solids and cellular materials with both better stiffness and better impact properties, due to the different crystalline morphology comprising the samples, a different cellular orientation, and thicker solid skin.

Synchronously promoted discharged energy density and discharged efficiency in PVDF films through a linear dielectric polymer PP blending approach

Owing to the large dielectric constant, ferroelectric poly(vinylidene fluoride) (PVDF) has emerged as an ideal potential component for dielectric capacitors with high energy density (Ue). However, the intrinsic polarity of PVDF induces a large hysteresis loss, which dramatically reduces the discharged efficiency (η) and thus hinders its practical application. Herein, we demonstrate a simple modified polymer blending approach to enhance the η and meanwhile Ue of PVDF through the employment of nonpolar polypropylene (PP). Despite the poor miscible nature of PVDF and PP, we realized the homogenous dispersion of PP within PVDF and their good interaction using a nonequilibrium manufacturing process. A minor addition of PP (< 15 wt%) demonstrated to be effective in depressing the hysteresis loss without degrading the polarization of PVDF, inducing superior η and Ue. By further constructing a sandwich-structured for PP/PVDF blend composites, η and Ue can be maximized to be 8.1 J/cm3 and 75%, respectively.