Long-chain Branched Polypropylene Research Articles

Influence of various pretreatments on molecular and rheological properties of a linear and a long-chain branched polypropylene

The influence of a mechanical or a thermal pretreatment of a linear (L-PP) and a long-chain branched polypropylene (LCB-PP) in the molten state was studied. The molar mass distributions and the branching structure were determined by high-temperature gel permeation chromatography HT-GPC coupled with laser-light scattering. The samples were extruded through long or short capillaries of various geometries corresponding to a predominant shear or elongational deformation. As a rheological probe, the extrudate swell at low stresses was measured for the differently pretreated samples. For the L-PP, neither molecular nor rheological changes were observed. However, the extrudate swell of the LCB-PP was found to decrease with increasing volume throughput. It was more strongly affected by shear in the capillary than by molecule stretching in the entry region. The smaller extrudate swell was accompanied by a decrease of the high molar mass tail of the LCB-PP, which could be the reason for the decay of swell, in principle. However, a comparable degradation of the high molar mass tail was obtained by a pure thermal treatment that was shown to leave the extrudate swell unchanged. This result and the unaffected branching structures found by high-temperature gel permeation chromatography (HT-GPC) support the hypothesis of a change of the branching topography by the mechanical pretreatment.

Synergistic effect of thermoplastic polyester elastomer and polytetrafluoroethylene in situ fibrillation on the resilience and compression strength of long‐chain branched polypropylene foam

AbstractExcellent resilience and compression resistance are the key characteristics of cushioning and water–oil separation materials. Although polypropylene (PP) offers excellent performance and affordability of PP, it exhibits poor resilience. To address this shortcoming, polymer blends with long‐chain branched PP (LCBPP), thermoplastic polyester elastomer (TPEE), and polytetrafluoroethylene (PTFE) were prepared via melt blending, and their mechanical properties were tested. The LCBPP/TPEE/PTFE composites were subjected to physical foaming using supercritical CO2 (scCO2) as the blowing agent. The synergistic toughening mechanism of the elastomer/nanofiber and the effect of the structure and crystallization of the dispersed phase on the foam resilience and foam compression strength were investigated. The results revealed that fibrous PTFE and spherical TPEE formed a composite dispersed phase in LCBPP. The impact and tensile strengths of the unfoamed LCBPP were found to have been simultaneously optimized owing to the synergistic effect of the elastomeric toughening of TPEE and the reinforcement of the PTFE fiber–induced fibrous crystal structure. Compared with pure LCBPP, the impact strength of the LCBPP/TPEE/PTFE composite increased by 216%, whereas its tensile strength decreased by only 6%. In particular, the synergistic effect of TPEE/PTFE improved the resilience and compression strength of the LCBPP foam. Compared with the pure LCBPP foam, the permanent strain of the LCBPP/TPEE/PTFE foam was reduced by 13.8% after five cycles of compression and its compression strength increased by 27.3%. The experimental results of our study could be used to prepare LCBPP foams with exceptional resilience, high compression strength, and good application prospects.

Molecular and thermo-mechanical assessment of long-chain branched polypropylene: Effect of irradiation dose, multifunctional monomer content and molecular weight

The influence of irradiation dose, trimetylolpropane trimethacrylate (TMPTMA) amount, as a three-functional monomer, and polypropylene (PP) molecular weight on the performance of irradiation technique to create long chain branched PP (LCB-PP) was investigated through molecular, rheological and thermo-mechanical characterizations. With increasing doses or amount of TMPTMA, a reduction in molar mass and an increment in long chain branches (LCBs) number as well as a change in the topology of LCBs from star to tree-like were found. PP grade with low molar mass had more branching ability and its LCBs topology was different. Irradiation declined the glass transition and melting temperature of PP, while it did not affect its crystallization temperature and its degree. Two opposing effects of molar mass reduction and LCBs formation controlled the tensile strength and toughness of irradiated PP samples.

Crystallisation regulation of long‐chain branched polypropylene on dielectric performance and energy density for metallised film capacitors

AbstractThe degradation of dielectric properties of polypropylene (PP) at high temperature affects the operational stability of metallised film capacitors. This study presents a method to improve the high‐temperature dielectric performance of PP through long‐chain branching modification and regulating the crystalline properties. Different amounts of nucleating agents were added to long‐chain branched polypropylene (LCBPP) to study the effect of crystallisation on electrical properties. The results show that the long‐chain branches improve thermal stability by enhancing the chain entanglement, and also increase the number of crystals due to heterogeneous nucleation. The addition of nucleating agents promotes the crystallisation of LCBPP, which increases the nucleation density and improves the crystallinity. The regulation of the microstructure of PP suppresses the relaxation of the chains and limits the charge transport. The dielectric constant of the modified films with long‐chain branching is slightly increased, and the doping of nucleating agents has little effect on the dielectric constant and loss of LCBPP. At 125°C, the conductivity of the modified LCBPP films with the addition of 0.05 wt% nucleating agent decreased by 2 orders of magnitude and the breakdown strength increased by 26.3%. This study has implications for improving the dielectric properties of PP films.

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.

Trouble-free combination of ω-alkenylmethyldichlorosilane copolymerization-hydrolysis chemistry and metallocene catalyst system for highly effective and efficient direct synthesis of long-chain-branched polypropylene

This paper reports a highly effective and efficient synthesis of long-chain-branched polypropylene (LCB-PP) rendered by combination of ω -alkenylmethyldichlorosilane copolymerization-hydrolysis (ACH) chemistry and metallocene catalyst system. The potential side reaction, the dichlorosilane-methylaluminoxane (MAO) double replacement reaction, is revealed to be slow-occurring and largely suppressible by slightly increasing the steric hindrance of the alkyl substituent(s) on the reactants. Thus, with 5-hexenylmethyldichlorosilane and 7-octenylmethyldichlorosilane of some increased alkenyl chain lengths, the main/side reaction competition dynamics in ω -alkenylmethyldichlorosilane/propylene copolymerization with metallocene ( rac -Me 2 Si-[2-Me-4-Ph(Ind)] 2 ZrCl 2 )-modified MAO complex involving the dichlorosilane-MAO double replacement reaction could shift disproportionally to the main copolymerization to the extent that the effect of the side reaction is not detectable. The synthesized LCB-PPs are characterized by narrow molecular weight distribution, with the LCB structure being H-shape and the interlinkage well defined as 1,2-dihydroxylsiloxane. The successful combination of ACH chemistry and metallocene catalyst system based on conventional metallocene and MAO assures a facile, well-defined, and, most importantly, highly effective and efficient synthesis of H-shape LCB-PP, as compared to two existing outstanding chemistries rendering the same type of LCB-PP, including ACH chemistry combined with heterogeneous Ziegler-Natta catalyst system and nonconjugated α,ω -diolefin copolymerization (NDC) chemistry exercised with 1,9-decadiene and metallocene catalyst system. • ω -Alkenylmethyldichlorosilane copolymerization-hydrolysis chemistry is successfully combined with metallocene catalyst system. • The dichlorosilane-MAO double replacement reaction is weak with ω -alkenylmethyldichlorosilanes of increased alkenyl lengths. • The chemistry results in highly effective and efficient synthesis of H-shape LCB-PP.

Fabrication strategy for long‐chain branched polypropylene foams with high resilience and compressive strength

AbstractA flexible and controllable is proposed for fabricating long‐chain branched polypropylene (LCBPP) foams with high resilience and strength. LCBPP was blended with thermoplastic polyester elastomer (TPEE) using polyethylene octene elastomer grafted glycidyl methacrylate (POE‐g‐GMA) as a compatibilizer at different mass ratios, and the blends were then used to prepare foams at different temperatures by the supercritical CO2 batch foaming method. The foams were then tested for their compressive permanent strain and compressive strength. The results showed that introducing TPEE and POE‐g‐GMA increased the resilience. In particular, the permanent strain of the LCBPP/TPEE/POE‐g‐GMA foam at a ratio of 90/10/7 decreased from an initial 14.5% for the original LCBPP foam to 5% after five compression cycles. However, the compressive strength decreased from 65.59 to 39.82 kPa. Therefore, the foam density (ρ) and cell size (d) were adjusted to balance the resilience and strength. Increasing ρ/d from 0.32 to 3.10 increased the compressive strength significantly from 39.82 to 91.67 kPa while the permanent strain only increased 3.5%. Thus, the proposed strategy realized LCBPP foams with high resilience and compressive strength.

Dielectric Property and Breakdown Strength Performance of Long-Chain Branched Polypropylene for Metallized Film Capacitors.

At high temperatures, the insulation performance of polypropylene (PP) decreases, making it challenging to meet the application requirements of metallized film capacitors. In this paper, the dielectric performance of PP is improved by long-chain branching modification and adding different kinds of nucleating agents. The added nucleating agents are organic phosphate nucleating agent (NA-21), sorbitol nucleating agent (DMDBS), rare earth nucleating agent (WBG-Ⅱ) and acylamino nucleating agent (TMB-5). The results show that the long-chain branches promote heterogeneous nucleation and inhibit the motion of molecular chains, thereby enhancing the dielectric properties at high temperatures. Nucleating agents modulate the crystalline morphology of long-chain branched polypropylene (LCBPP), which leads to a decrease in the mean free path of carriers and an increase in trap energy level and trap density. Therefore, the conductivity is reduced and the breakdown strength is improved. Among the added nucleating agents, NA-21 showed a significant improvement in the electrical properties of LCBPP films. At 125 °C, compared with PP, the breakdown strength of the modified film is increased by 26.3%, and the energy density is increased by 66.1%. This method provides a reference for improving the dielectric properties of PP.

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.

Effect of viscoelasticity on the foaming behaviour of long-chain branched polypropylene with different branching degrees analysed by using bubble-growth modelling

Long-chain branched polypropylene (LCB-PP) was generated by a melt grafting reaction with trimethylolpropane triacrylate (TMPTA) as the grafting monomer and zinc dimethyldithiocarbamate (ZDMC) or styrene (St) as the co-monomers. High-temperature size exclusion chromatography (HT-SEC) was used to characterize the molecular mass, molecular mass distribution and branching degree of the modified PP. Subsequently, linear and nonlinear viscoelasticity data were obtained by a small amplitude oscillatory shear (SAOS) test and uniaxial extensional viscosity measurement. The pressure balanced bubble-growth (PBB) model was used to calculate the critical coalescence radius (CCR) curves for PP to quantitatively correlate the viscoelasticity and foamability and evaluate the foamability of PP combined with batch foaming. It is shown that with increasing branching degree, the foam shows a better resistance to bubble coalescence, resulting in a more regular bubble structure and a higher expansion ratio. Moreover, viscosity should be controlled over an appropriate range for realising good foamability; otherwise, excessive viscosity can suppress bubble growth and limit the expansion ratio. MPPZ4 modified with 4 wt% TMPTA and 0.4 wt% ZDMC showed the best foamability. A combination of the PBB model and batch foaming experiment is used to quantitatively explain the contribution of LCB to foaming.

Effect of Introducing Long Chain Branching on Fiber Diameter and Fiber Diameter Distribution in Melt Blowing Process of Polypropylene

Abstract In the melt blowing process, the molten polymers extruded from nozzles are elongated by high-velocity and high-temperature air flow. In this study, with the aim of stabilizing the melt blowing process for producing nonwoven webs with fine diameter fibers, the effect of the control of polymer rheology by the introduction of either low melt flow rate (MFR) polypropylene (PP) or long chain branched PP (LCB-PP) to regular high MFR PP was investigated. Introduction of low MFR PP into regular PP increased shear viscosity and fibers of larger diameter were produced in the melt blowing process, while introduction of low MFR LCB-PP suppressed the elongational viscosity reduction with the increase of strain rate, and eventually spinning was stabilized. It was found that the blending of an optimum amount of LCB-PP to regular PP caused the stabilization of the melt blowing process. As a result, the formation of nonwoven webs consisting of fine fibers of rather uniform diameter distribution could be achieved.

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.

Industrial Adaptability of the Ziegler–Natta Catalyst-Friendly Synthesis of Long-Chain-Branched Polypropylene Based on ω-Alkenylmethyldichlorosilane-Assisted Propylene Polymerization

ω-Alkenylmethyldichlorosilane-assisted propylene polymerization for the synthesis of long-chain-branched polypropylene (LCB-PP) was applied in a liquid bulk polymerization process to verify its ind...

Upcycling of polypropylene with various concentrations of peroxydicarbonate and dilauroyl peroxide and two processing steps

AbstractThe influence of two peroxides (peroxydicarbonate/dilauroyl peroxide) with various concentrations (10–200 mmol/kg PP) and their effective opportunity to introduce long chain branched (LCB) were investigated. The dependence of a single and double extrusion step and the changes of the properties were studied. Experiments were carried out in a single screw extruder at 180°C for the first extrusion step (modification) and at 240°C for the second extrusion step (processing simulation). Melt flow rate and dynamic rheological properties were studied at a measuring temperature of 230°C. For the definitive determination of long chain branched polypropylene (LCB‐PP) served the extensional rheology measurements. The mechanical properties were examined via tensile test and impact tensile test. Summarized, LCB (melt strength) could be observed via extensional rheology for all modified specimens and the mechanical properties were maintained or even improved for the modified samples. Particularly, samples containing dilauroyl peroxide display excellent mechanical properties in this study.

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.

ω-Alkenylmethyldichlorosilane-assisted propylene polymerization with Ziegler-Natta catalyst to long chain-branched polypropylene

Long chain-branched polypropylene (LCB-PP) with high melt strength characteristics has long been a class of high-end PP materials modified from their linear prototypes. Recent advances in catalyst technology has enabled direct synthesis of LCB-PP using metallocene/non-metallocene catalysts via, among others, non-conjugated α,ω-diolefin/propylene copolymerization. However, for Ziegler-Natta catalysts (MgCl2/TiCl4 catalysts) which dominate commercial PP production, chemistries for LCB-PP synthesis are still rare. To resolve the dilemma, this paper reports a new LCB-PP synthetic chemistry based on Ziegler-Natta catalyst technology. The new chemistry uses ω-alkenylmethyldichlorosilane as an LCB reagent to help with propylene polymerization, where it leverages its two distinct functionalities of α-olefin and alkylmethyldichlorosilane in main polymerization (with Ziegler-Natta catalyst) and during polymer workup (with water) to construct dialkylsiloxane oligomer interlinkages between PP chains, thus synthesizing H-shape LCB-PP. By completely ditching macromonomer copolymerization as being essential for LCB structure formation, the new chemistry is greatly reduced of steric barrier and suits Ziegler-Natta catalysts. Gelation can be prevented by containing the addition dosage of ω-alkenylmethyldichlorosilane. The synthesized LCB-PP samples exhibit strong LCB characteristics in rheology test. LCB densities are controlled by ω-alkenylmethyldichlorosilane incorporation rates in main propylene polymerization. Of different ω-alkenylmethyldichlorosilane derivatives, one that is more reactive for copolymerization with propylene is more efficient for LCB-PP synthesis. For the three synthesized ω-alkenylmethyldichlorosilanes their efficiencies for LCB-PP synthesis follow the order of 5-hexenylmethyldichlorosilane > 7-octenylmethyldichlorosilane > 3-butenylmethyldichlorosilane.

Long-chain branched polypropylene: crystallization under high pressure and polymorphic composition

High-pressure crystallization and resulting polymorphic composition of long-chain branched polypropylene (LCB-PP) were studied and compared with common linear isotactic polypropylene (PP). Commercially available LCB-PP and PP with similar melt flow indexes were crystallized under several high pressures (20, 40, 80, 120 and 160 MPa) at constant cooling rate 5 °C min−1. Structure of crystallized samples was evaluated via wide-angle X-ray scattering, differential scanning calorimetry and scanning electron microscopy. It was shown that under low pressure LCB-PP crystallizes at higher crystallization temperature than PP due to its higher nucleating density. The opposite situation is observed at high pressures (120 and 160 MPa): crystallization temperature of PP exceeds that of LCB-PP as a negative effect of branching is pronounced. Polymorphic analysis proved that LCB-PP tends to crystallize into orthorhombic γ-form. This crystalline form becomes to be dominant at 40 MPa, and LCB-PP samples crystallized at 120 and 160 MPa contain solely γ-form. On the other hand, no pure γ-form sample was prepared from PP in this study, although positive effect of pressure on its formation is observed. Thermodynamic stability of LCB-PP crystalline structure is systematically lower compared to PP. With pronounced crystallization pressure, the melting peak broadens and finally splits, indicating the presence of dominant amount of γ-form in LCB-PP. In comparison with PP, crystallites in LCB-PP structure are considerably smaller due to lower crystal growth rate and higher nucleating density.

Foams with Enhanced Ductility and Impact Behavior Based on Polypropylene Composites.

In this work, formulations based on composites of a linear polypropylene (L-PP), a long-chain branched polypropylene (LCB-PP), a polypropylene–graft–maleic anhydride (PP-MA), a styrene-ethylene-butylene-styrene copolymer (SEBS), glass fibers (GF), and halloysite nanotubes (HNT-QM) have been foamed by using the improved compression molding route (ICM), obtaining relative densities of about 0.62. The combination of the inclusion of elastomer and rigid phases with the use of the LCB-PP led to foams with a better cellular structure, an improved ductility, and considerable values of the elastic modulus. Consequently, the produced foams presented simultaneously an excellent impact performance and a high stiffness with respect to their corresponding solid counterparts.

Effect of β‐nucleating agent and zeolite molecular sieves on supercritical CO2‐assisted cell and crystal nucleation during the long‐chain‐branched polypropylene foaming

ABSTRACTIt is of great interest and importance to design and fabricate the double crystal melting peak (DCMP) structure for the expanded polypropylene (EPP) bead foams during sintering. Two kinds of methods with different mechanisms were proposed to prepare long‐chain‐branched polypropylene (LCBPP) samples with a DCMP structure: One was that the DCMP structure was prepared by generating crystals of different perfection degrees and the other one was that the DCMP structure was prepared by adding β‐NA to produce various crystal forms. In addition, zeolite molecular sieve (ZMS) as the synergist was introduced into LCBPP/β‐NA system to increase the efficiency of β‐NA. The crystallization behaviors of various PP and their foams were tested by differential scanning calorimetry and wide‐angle X‐ray diffraction, as well as the cellular morphologies of various LCBPP foams were observed by scanning electron microscope. The results showed that a DCMP structure was formed in pure LCBPP in the presence of CO2 at 168–172 °C. The content of β‐crystal in LCBPP/β‐NA system was the highest when the content of β‐NA optimized as 1.5 phr. The percentage of β‐crystal in LCBPP/β‐NA system was further increased when a small content (0.045 phr) of ZMS was added. Compared with that in unfoamed LCBPP, the percentage of β‐crystal in LCBPP foams was obviously enhanced, due to the coupling effect of crystal nucleation and cell nucleation. In the LCBPP foams, the cell size became smaller and the cell density became larger, owing to the addition of ZMS. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 48183.