High Melt Strength Polypropylene Research Articles

Enhancing the sound and thermal insulation properties of polypropylene foam by preparing high melt strength polypropylene.

High-performance polypropylene (PP) foam is a vital polymer product in industrial areas. However, the poor melt strength of ordinary PP homopolymer limits its foaming molding. In this work, high melt strength polypropylene (HMSPP) was prepared by using styrene (St) and tripropylene glycol diacrylate (TPGDA) as comonomers, and then PP foams were prepared by mold foaming method. The results showed that adding St in the grafting process of TPGDA will obviously improve the melt strength of the PP matrix, and its melt strength (28184Pa.s) is 7.4 times higher than that of pure PP. HMSPP foam has more regular and uniform cells and higher cell density, which significantly improves the sound and thermal insulation properties of PP foam. Compared with pure PP foam, the average sound transmission loss (52.9dB) of HMSPP foam with a low foaming ratio increased by 64%, and the thermal conductivity (0.0867W/mK) decreased by 46%. Therefore, the obtained HMSPP foam can be used in sound insulation or thermal insulation area. This work provides an available route for the high-performance utilization of PP foam. This article is protected by copyright. All rights reserved.

In Situ Dispersion of Lignin in Polypropylene via Supercritical CO2 Extrusion Foaming: Effects of Lignin on Cell Nucleation and Foam Compression Properties.

Supercritical CO2 (scCO2) extrusion foamed high-melt-strength (HMS) polypropylene (PP) often suffers from low cell density, large cell sizes, and poor cell structure uniformity due to the poor nucleation rates of CO2 in the PP. To remedy this, various inorganic fillers have been used as heterogeneous nucleation agents. Although their efficient nucleation effects have been demonstrated, the preparation of these fillers causes some adverse effects on the environment/human health or involves relatively expensive processes or non-eco-friendly chemicals. In this work, biomass-based lignin is studied as a sustainable, lightweight, and cost-effective nucleating agent. It is found that scCO2 could assist in situ dispersion of lignin in the PP in the foaming process, leading to significantly increased cell density, smaller cells, and improved cell uniformity. The Expansion Ratio is also simultaneously improved due to reduced diffusive gas loss. The PP/lignin foams with low lignin loadings exhibit higher compression moduli and plateau strengths than the PP foams with the same densities owing to the improved cell uniformity and probably also the reinforcing effect of the small lignin particles in cell walls. Moreover, the energy absorption capability of the PP/lignin foam with 1 wt% lignin could match the PP foam with similar compression plateau strengths; even the density of the former is 28% lower than the latter. Therefore, this work provides a promising approach to a cleaner and more sustainable production of HMS PP foams.

Design of thermal conductive polymer composites with precisely controlling graphene nanoplatelets at the interface of polypropylene and high melt strength polypropylene via elongation flow

AbstractThe migration and selective localization of graphene nanoplatelets (GNPs) in a polypropylene/high melt strength polypropylene (PP/HMSPP) blend under elongational and shear forces are studied. The effects of elongational flow upon the elongational rheology, X‐ray diffraction patterns, thermal properties, and mechanical properties are investigated. The results show that the elongational viscosity and melt strength of HMSPP are higher than those of PP under elongational flow, whereby HMSPP shows elongational strain hardening behavior, and the migrating of GNPs from the PP to the HMSPP phase is hindered. As a result, the GNPs are constrained to remain at the interface between the two phases. In addition, the elongational force can exfoliate GNPs more effectively, and the orientation of GNPs along the phase interface will reduce the migration rate of GNPs at the interface, while precisely controlling the location of the GNPs at the phase interface. Thus, more thermal conductivity pathways are constructed and, hence, the thermal conductivity of the PP/HMSPP/GNPs composite prepared under elongational flow is higher than that prepared under shear flow. The present work provides a new method for the preparation of highly thermally conductive polymer composites.

Fundamental influences of propylene‐based elastomer on the foaming properties of high melt strength polypropylene based on extrusion foaming

AbstractHigh melt strength polypropylene (HMS‐PP)/propylene‐based elastomer (PBE) microcellular foams were prepared by the extrusion foam method. The fundamental mechanism of PBE improving the foaming properties of HMS‐PP was investigated. PBE 6102, with a low melt flow rate and high ethylene content, enables HMS‐PP/PBE blends to form a microphase‐separated structure and maintain good melt strength and pronounced strain hardening, which would promote cell nucleation and hold cell structure at relatively high temperatures. Furthermore, PBE 6102 postpones HMS‐PP crystallization and decreases the optimum foaming temperature, which helps balance melt elasticity and viscosity near the foaming window to avoid cell rupturing and merging. The introduction of PBE 6102 does not reduce the crystallization rate of HMS‐PP nor the crystallinity of the foamed samples, fixing the cell structure to prevent collapse during the cell solidification stage. Microcellular HMS‐PP/PBE 6102 foams with a high expansion ratio, small cell size, and large cell count were successfully fabricated.

Supercritical CO 2 foaming of open‐cell polypropylene/ethylene propylene diene monomer composite foams with oriented cellular structures for water treatment

Efficient treatment of tiny media in wastewater is a global challenge. Herein, an ingenious design to fabricate the highly open-cell polypropylene (PP)/ethylene propylene diene monomer (EPDM) composite foams for water treatment by using supercritical CO2 (scCO2) extrusion foaming coupling with post-stretching is presented. The tensile strain of high melt strength PP is fully utilized in an extensional flow field during foaming. Meanwhile, EPDM is added into PP matrix for obtaining the highly open-cell and oriented porous structures. Interestingly, a variety of structural transformations (like uniform, hollow and oriented cellular structures) can be achieved during extrusion foaming. Furthermore, the effects of different cellular structures on their selective filtration properties are studied. More interestingly, the as-prepared composite foam strips, with the highly open-cell content (~94.4%) and orientation degree (~89.6%), exhibit excellent selective filtration performance, which can successfully remove waste with particle size larger than 1 μm (its filtration efficiency:~ 99.52%). Water droplets can easily pass through the lengthy foam strips in seconds, but tiny particles (like oil droplets or glass microbeads) will be blocked. This work provides a new pathway to prepare highly open-cell composite foams with anisotropic porous structures for water treatment.

Extruded polypropylene foams with radially gradient porous structures and selective filtration property via supercritical CO2 foaming

Original polypropylene (PP) foams with radially gradient porous structures and selective filtration were developed through the supercritical CO2 assisted extrusion foaming couple with the stretching auxiliary. Firstly, the relationship among the physical characteristics, processing technology and microstructure of the polymer foams were studied. Specially, the effect of the tensile strain hardening and its spatial confinement of the high melt strength PP (HMSPP) in the extensional flow field on the condensed matter physics and foaming behavior of the polymer foams were revealed. Remarkably, the orientation degree of the modified PP foams could be ~47.2 higher than the traditional ones in different zones. Secondly, the compression experiments and theoretical calculated analysis showed the mechanical properties of the as-prepared PP foams exhibited obvious directionality, which is beneficial to recycle and squeeze wastewater. Besides, the corresponding characterization methods and self-made filter device were used to demonstrate the selective filtration performance of the as-prepared PP foams. Interestingly, the water droplet could pass the 3D lengthy PP foam strips, while the ultrafine wasted materials were blocked due to the unique micro-structures. Gradient porous polymer foams without any modification are firstly demonstrated to be interesting materials with the potential applications in the selective filtration properties.

Expanded Beads of High Melt Strength Polypropylene Moldable at Low Steam Pressure by Foam Extrusion.

We explore the foam extrusion of expanded polypropylene with a long chain branched random co-polypropylene to make its production process simpler and cheaper. The results show that the presence of long chain branches infer high melt strength and, hence, a wide foamability window. We explored the entire window of foaming conditions (namely, temperature and pressure) by means of an ad-hoc extrusion pilot line design. It is shown that the density of the beads can be varied from 20 to 100 kg/m3 using CO2 and isobutane as a blowing agent. The foamed beads were molded by steam-chest molding using moderate steam pressures of 0.3 to 0.35 MPa independently of the closed cell content. A characterization of the mechanical properties was performed on the molded parts. The steam molding pressure for sintering expanded polypropylene beads with a long chain branched random co-polypropylene is lower than the one usually needed for standard polypropylene beads by extrusion. The energy saving for the sintering makes the entire manufacturing processes cost efficient and can trigger new applications.

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.

Preparation of polypropylene/high‐melt‐strength PP open‐cell foam for oil absorption

AbstractOil spills not only cause significant economic losses; they also harm the environment. In this study, high‐melt‐strength polypropylene (HMSPP) and linear polypropylene (PP) were compounded at different ratios and foamed by supercritical CO2at various temperatures and pressures to develop open‐cell foams for oil absorption applications. The experimental results show that adding HMSPP increased the foam expansion ratio and the open‐cell content and that the foam processing window widened with increasing HMSPP content. The oil absorption experiment results show that the amount of oil absorption increased with the foam expansion ratio and that low oil viscosity resulted in high oil absorption. In addition, the amount of oil absorption remained unchanged after 10 cycles of oil absorption experiments. The oil absorption kinetics study shows that the oil absorption process complied with the pseudo‐second‐order law.

Effect of MWCNT carboxyl functionalization on the shear rheological and electrical properties of HMS-PP/MWCNT foams

Different concentrations of multiwall carbon nanotubes (MWCNT) and carboxyl functionalized MWCNT (MWCNT-COOH) were added to a high melt strength polypropylene (HMS-PP) to produce foams with high dielectric constants, using azodicarbonamide (AZO) as blowing agent. The AZO foaming behavior and the crystallization, thermal properties, steady state and oscillatory shear rheological properties of the nanocomposites were analyzed by polarized light optical microscopy (PLOM), differential scanning calorimetry (DSC), thermogravimetry analyses (TGA) and parallel plate rheometry. The morphology, the dielectric and dynamic mechanical properties (DMTA) of the foams were also studied by scanning electron microscopy (SEM), impedance spectroscopy and bending method, respectively. A decrease in crystallite size and an increase in the HMS-PP overall crystallinity promoted by the presence of both types of MWCNTs was observed, as well as an increase in the crystallization temperatures. From these results and from the analyses of the rheological properties, it was possible to predict that the 5 wt.% MWCNT foam would have the lowest bubble growth rate, the 1.5 wt.% MWCNT-COOH the highest, while the 3 wt.% MWCNT-COOH composition would have the slowest bubble stability (and consequently the highest cellular density) and the 1.5 wt.% MWCNT-COOH the fastest. Also, it was possible to predict that only the 5 wt.% MWCNT-COOH foam would have a percolated and electrically conductive structure. All these predictions were confirmed by the resultant morphology and impedance spectroscopy results. The highest mechanical damping was displayed by the 3 wt.% MWCNT-COOH foam, while the lowest by the 5 wt.% MWCNT-COOH foam. Regarding the dielectric properties, the 1.5 wt% MWCNT-COOH foam was found to be the most suitable to be used as a capacitor material; this foam was also the less dense of all the samples.

Preparation of high melt strength polypropylene by addition of an ionically modified polypropylene

Polypropylene-graft-maleic anhydride (PP-g-MAH) ionically modified with zinc(II) (PP-g-MAHZn) in varying amounts of ionic associations were prepared via melt reaction. Absorption bands in FTIR-ATR and shifted binding energy in X-ray photoelectron spectroscopy showed the formation of zinc carboxylates in PP-g-MAHZn. Decreased crystallinity, gradually increased viscosities, and storage modulus with the increment of zinc carboxylates in the PP-g-MAHZn were investigated by thermal and rheological analysis, which were attributed to the restriction of chain mobility via ionic interactions between polymer chains and aggregations of ionic domains. Furthermore, PP-g-MAHZn was introduced to polypropylene as an additive to promote foamability, and the PP/PP-g-MAHZn blends displayed higher viscosities and storage modulus compared with pristine PP in rheological measurements. In addition, the PP/PP-g-MAHZn blends were foamed by supercritical CO2 and cryogenically fractured surfaces were observed by scanning electron microscope. PP/PP-g-MAHZn foams presented high closed-cell content, uniformity of cells, and increasing cell density owing to the combined effects of rheological properties, while maintaining the cell structures and good dispersion of the PP-g-MAHZn as nucleating sites. These results reveal that PP-g-MAHZn is a suitable additive to produce high melt strength PP for foam production.

A facile methodology to effectively improve the melt strength and microcellular foamability of isotactic polypropylene

Melt strength is crucial in foam-processing of polymer materials, especially for linear molecular structured isotactic polypropylene (iPP), which has comparatively weak melt strength to hinder its foaming. In this study, a commercial high melt strength polypropylene (HMSPP) was added into the iPP matrix, cooperated with a thermal treatment, aiming to enhance the melt strength and microcellular foamability of iPP under supercritical CO2. Melt strength enhancement effect was proved by rheological and microcellular foaming results. The crystallization and batch foaming behaviors of iPP and its HMSPP added blends at different foaming conditions were investigated. It was observed that the addition of HMSPP could refine the crystals of the blends, and the crystal nucleation sites could be increased apparently. Cooperated with the suitable thermal control schedule during the batch foaming process, the introduced crystals could correspondingly improve the melt strength and cell nucleating ability of HMSPP added iPP. At a given HMSPP content of 20 wt.%, quite fine microcellular foams with the average cell size of 0.8 μm, and cell density up to 1012 cells/cm3 were fabricated.

De-compositing honeycombs: Rheology-architecture relationships in melt-stretched sandwich panels

Single-material melt-stretched thermoplastic sandwich panels are a way to create environmentally-sustainable alternatives to conventional honeycomb sandwich structures. In order to efficiently develop them, it is necessary to understand the relationship between molecular structure, rheology, and panel architecture. In this study, seven polymer grades were tested: four high melt strength polypropylenes, two polycarbonates, and a linear polypropylene. The resulting honeycombs were characterized by direct measurement of web thicknesses and out-of-plane compression testing. Honeycombs manufactured from high melt strength polypropylene were better able to draw material from the melt pool into the honeycomb webs, and thus exhibited a larger fraction of the total material volume contributing to the webs than seen in honeycombs manufactured from the other polymers. Strain-hardening – or lack thereof – in the polymer melt was implicated as the primary cause of the observed differences in architecture and mechanical performance between the different grades.

Improved cell nucleating effect of partially melted crystal structure to enhance the microcellular foaming and impact properties of isotactic polypropylene

The effects of a partially melted crystal structure (PMCS) on enhancing the cell nucleating behavior, as well as on the related foaming behavior of isotactic polypropylene (iPP), were investigated. The incorporation of PMCS and the corresponding effects were characterized by rheological, thermal and X-ray testing methods. The microcellular foaming results revealed that the addition of PMCS could dramatically increase the melt strength and the cell nucleating sites of the matrix. The synergistic effects of the PMCS and high melt strength polypropylene (HMSPP) could decrease the average cell size to approximately 2 μm and increase the cell density to 1.49×1011 cells/cm3. Additionally, the gradual melting of the PMCS could also prove its enhancing effect on the melt strength and cell nucleating ability. The uniform microcellular cells obtained could positively affect the notched impact strength of the foamed iPP-HMSPP blends.

Rheological optimization of reactively modified polypropylene to enhance the foam extrusion performance

A high melt strength polypropylene (PP) suitable for foam extrusion was prepared from a commercial homo-polypropylene by long chain branching using reactive extrusion. Effects of four variables consist of monomer, inhibitor, compatibilizer and initiator concentrations on the rheological properties of modified PP were investigated applying Taguchi method of experimental design. The results showed that each factors had different effect on the melt strength of modified polypropylene. The optimum samples were selected for foam extrusion. DSC analysis was used to study the thermal behavior of obtained optimal samples. The SEM images of the foam extruded optimized sample revealed the uniform cell dispersion with closed fine cell morphology.

In-situ cooling of adsorbed water to control cellular structure of polypropylene composite foam during CO2 batch foaming process

The fixation stage of cellular structure during polymer foaming process is difficult to control due to difficult removing of internal heat, especially for crystalline polymers such as polypropylene (PP). In order to control cellular structure in the fixation stage, we established an universal method by introducing water as an in-situ internal cooling agent during the foaming process of PP. Particularly, hydrophilic fillers were added into the high melt flow index polypropylene (HMI-PP) with poor foaming ability to study the effect of adsorbed water on the foaming process of PP/filler composites during sc-CO2 batch foaming process. Compared to pure HMI-PP, HMI-PP/hydrophilic filler composites showed increased adsorption amount for water and melt viscosity. The water within the composites played a key role in the foaming process, which not only served as a physical foaming agent to increase the volume expansion ratio of PP composite foam, but also showed the function of rapid cooling foam proved by thermal imaging technology. As a result, the presence of some hydrophilic fillers within HMI-PP matrix significantly improved the foaming properties of HMI-PP, including preventing internal bubble collapse and significantly increasing the volume expansion ratio of the foam. The applicability of this method was validated using other PP materials besides HMI-PP, including high molecular weight PP (HMW-PP), high melt strength PP (HMS-PP).

Rheological, crystallization and foaming behaviors of high melt strength polypropylene in the presence of polyvinyl acetate

Polyvinyl acetate (PVAc) is a kind of CO2-philic materials with high solubility of CO2. For improving the supercritical carbon dioxide (Sc-CO2) foaming behavior of isotactic polypropylene (iPP), a high melt strength polypropylene (HMSPP) was prepared using styrene (St) as grafting monomer. The effect of PVAc on the preparation, rheological, crystallization and foaming behaviors of HMSPP was investigated. The high temperature gel permeation chromatography (HT-GPC) results showed that the PVAc had a promotive effect on melt grafting reaction. With the addition of PVAc, the weight-average molecular weight (Mw) of HMSPP increased from 217,158 to 240,733 g/mol. Thus, the HMSPP presented higher complex viscosity and storage modular, and lower loss angle, which indicated that the melt viscosity and melt strength of HMSPP was increased by adding PVAc. The crystallization behavior of HMSPP was investigated using differential scanning calorimetry (DSC). Double crystallization peaks were observed on the DSC cooling curves of HMSPP in the presence of PVAc, which was ascribed to incomplete molten of iPP with long chain branching (LCB) structure at low end melting temperature. Moreover, the prepared HMSPP exhibited better foaming behavior in the presence of PVAc. With the addition of PVAc, the average cell diameter of HMSPP decreased from 93 to 59 μm, and the cell density increased from 2.83 × 107 to 9.79 × 107 cell/cm3.

Continuous ultrasound‐assisted fabrication of lightweight polypropylene foam with low thermal conductivity and excellent sound insulation properties

Ultrasonic oscillation as a new polymer processing technology, was introduced into the polymer melt to improve the high melt strength Polypropylene (HMSPP) foaming ability, the effect of the ultrasonic oscillation power, Azodicarbonamide (AC) and talc content on the cell morphology, cell size, apparent density, cell density and nucleation density of the HMSPP foam was systematically studied. When the ultrasound and talc were introduced into the HMSPP melt, the HMSPP foam had a larger cell density and smaller the size as well as more uniform distribution. Interestingly, when the talc was introduced into the HMSPP, the cell morphologies changed from sphere to oval. Compared to the HMSPP foam without ultrasonic oscillation, the minimum thermal conductivity of the HMSPP foam with ultrasonic power 100 W reached 0.055 W/mK. When the talc was introduced into HMSPP melt, the average sound transmission loss value of the HMSPP foam were enhanced, and reached 30.37 dB, which was 13.7% higher than that of the HMSPP foam without talc and ultrasonic oscillation. The ultrasonic‐assisted extrusion foaming had a promising to construct a new method to continuously and rapidly fabricate polymer foam with low thermal conductivity and excellent sound insulation properties. POLYM. ENG. SCI., 58:1678–1684, 2018. © 2017 Society of Plastics Engineers

Rheology of high melt strength polypropylene for additive manufacturing

Rheology of high melt strength polypropylene for additive manufacturing

Extensional rheology, cellular structure, mechanical behavior relationships in HMS PP/montmorillonite foams with similar densities

The main goal of this work is to analyze the relationships between the extensional rheological behavior of solid nanocomposites based on high melt strength polypropylene (HMS PP) and montmorillonites (MMT) and the cellular structure and mechanical properties of foams produced from these materials. For this purpose two systems have been analyzed. The first one incorporates organomodified MMT and a compatibilizer and the second system contains natural clays and is produced without the compatibilizer. Results indicate that the extensional rheological behavior of both materials is completely different. The strain hardening of the polymer containing organomodified clays decreases as the clay content increases. As a consequence, the open cell content of this material increases with the clay content and hence, the mechanical properties get worse. However, in the materials produced with natural clays this relationship is not so clear. While no changes are detected in the extensional rheological behavior by adding these particles, the nano-filled materials show an open cell structure, opposite to the closed cell structure of the pure polymer, which is caused by the fact of having particle agglomerates with a size larger than the thickness of the cell walls and a poor compatibility between the clays and the polymer.