Heterophasic Ethylene-propylene Copolymers Research Articles

Role of polymorphism in solid-state production of double-melting expanded polypropylene beads

The production of expanded polypropylene (EPP) beads is of great significance for its wide range of industrial applications. However, it requires special grades of polypropylene with high melt strength (HMS-PP) and the capability of forming double-melting expanded beads to enable sintering during post-processing of EPP. In this study, conventional Ziegler-Natta PP grades of heterophasic ethylene-propylene copolymer, random propylene-ethylene-butene-1 terpolymer, and their 50 wt% blends were studied in a wide range of foaming conditions from solid to semi-liquid state. While, all the polypropylenes were capable of producing EPP beads with double-melting behavior when foamed in a semi-liquid state, in solid-state foaming, the double-melting behavior appears only in polymorphic polypropylenes with low temperature and high temperature melting peaks associated with γ and α phase crystals, respectively. The EPP samples foamed in the solid-state show a closed-cell morphology with high cellular density (>1010 number of cells/cm3) and low cell size (0.3–5 μ m) resulting from the high extensional strength of the polymeric matrix when foamed at a temperature prior to the initial melting. Using polymorphism to induce double-melting behavior and performing the expansion in solid-state is promising for producing nano cellular EPP beads.

NMR Characterization of Vis‐Broken Heterophasic Ethylene−Propylene Copolymers

AbstractSolution 13C nuclear magnetic resonance (NMR) is used in conjunction with in situ solid‐state NMR to determine the effect of peroxide treatment on the chemical structure and morphology of ethylene−propylene copolymers. The copolymers contain increasing quantities of ethylene with the lowest ethylene content corresponding to pure isotactic polypropylene. The vis‐breaking of heterophasic ethylene‐propylene copolymers (HEPCs) is found to be influenced by the homogeneity of the chain sequences. The in situ vis‐breaking in solid‐state NMR reveals significant changes in mobility of the crystalline domains and the formation of a disordered crystal phase within the α structure is observed. Both solution and solid‐state NMR experiments confirm the greater sensitivity of copolymers containing short ethylene sequences to peroxide degradation. It is proposed that longer chain ethylene sequences resist chemical changes and provide some degree of protection from degradation as a result of the reactivity of the ethylene units.

Heterophasic ethylene-propylene copolymers: New insights on complex microstructure by combined molar mass fractionation and high temperature liquid chromatography

The present study investigates the changes in the microstructure of heterophasic ethylene propylene copolymers (HEPCs) with increasing ethylene content, before and after visbreaking. Three samples obtained from commercial gas process reactors at Sasol Polymers at varying times after the addition of the ethylene comonomer are further visbroken to produce three more samples. Bulk sample analyses indicate that the ethylene-rich rubber phase which is essential in aiding peroxide mobility during visbreaking as observed in narrower dispersities and lower peak molar masses with increasing ethylene content. Further investigations of the microstructure via offline coupling of preparative molar mass fractionation (pMMF) to advanced analytical techniques such as solvent gradient interaction chromatography (SGIC) reveal that ethylene-rich copolymer chains are not significantly affected by the peroxide as the polypropylene homopolymer. Consequently, the polypropylene (PP) homopolymer and polypropylene-rich fractions with high molar mass diminish after visbreaking. Furthermore, the increase in ethylene content was observed to reduce the impact of the peroxide on the fraction quantities before and after visbreaking implying that visbreaking affects more the polyolefin chains with more PP segments and less those with more ethylene comonomer.

Unrevealing the effect of different dispersion agents on the properties of ethylene–propylene copolymer/halloysite nanocomposites

The properties of polyolefin nanocomposites strongly depend on the dispersion level of the nanoparticles and the addition of dispersion agents contributes to improving their performance. In this study, the morphology and the mechanical properties of heterophasic ethylene-propylene copolymer/halloysite nanocomposites were tailored by using two hydrogenated hydrocarbon resins: 90% and 100% hydrogenated and two compatibilizing agents: poly(propylene-g-maleic anhydride) and poly(ethylene-octene-g-maleic anhydride). The transmission electron microscopy indicated that the best dispersion of the halloysite nanotubes was achieved when hydrogenated hydrocarbon resins and poly(propylene-g-maleic anhydride) were used simultaneously. All nanocomposites showed an increase in mechanical stiffness and the most pronounced increase of 46% in the Young modulus was achieved with the system containing halloysite, poly(propylene-g-maleic anhydride) and the hydrocarbon resin with the higher degree of hydrogenation. Poly(ethylene-octene-g-maleic anhydride) caused the halloysite nanoparticles to concentrate preferentially in the rubber domains, wherein these hindered the crystallization of polypropylene and polyethylene chains, as showed by atomic force microscopy. In this case, the composite exhibited both high stiffness and improved toughness. These results highlight the key role of dispersion agents in promoting a good balance in the mechanical properties of resulting nanocomposites based on halloysite particles and heterophasic ethylene-propylene copolymers.

Structure-property relations of heterophasic ethylene-propylene copolymers based on a single-site catalyst

Structure-property relations of heterophasic ethylene-propylene copolymers based on a single-site catalyst

Ionic liquid tailored interfaces in halloysite nanotube/heterophasic ethylene–propylene copolymer nanocomposites with enhanced mechanical properties

In this study, imidazolium ionic liquids (IL) were investigated as novel compatibilizers to improve the dispersion of halloysite nanotubes (HNT) in heterophasic ethylene-propylene copolymer (EPP) matrix to enhance the mechanical properties of the EPP/HNT nanocomposites prepared by melt-blending approach. The results highlight that the dispersion of HNT nanotubes and consequently the mechanical properties of nanocomposites depends on the surface modification of HNT with IL, which in turn strongly depends on the chemical structure of IL, i.e. anion and cation N-alkyl chain length, as well as on the adsorption conditions, e.g. solvent polarity and IL concentration adopted for the modification of HNT surface. In details the nanocomposites with HNT modified by IL (m-HNT) exhibited a more homogeneous HNT dispersion above all when the IL surface modification was conducted in less polar solvents as dichloromethane. Furthermore, the IL allowed to tuning the stiffness and the toughness with a right compromise. The EPP/m-HNT nanocomposites with bis(trifluoromethylsulfonyl)imide [NTf2] anion and 1-methyl-3-n-octadecylimidazolium cation-based IL, exhibited an improvement of elastic modulus of about 34% with respect to the pristine polymer, without loss of the impact properties at 23°C. The morphological and mechanical results confirm the IL acted effectively as compatibilizer, improving both the interfacial interaction and HNT dispersion into the EPP matrix.

The Use of Solid‐State NMR to Investigate the Development of Segmental Mobility in Commercial Heterophasic Ethylene Propylene Copolymers (HEPCs)

The effect increasing ethylene incorporation on the development of commercial heterophasic ethylene propylene copolymers (HEPCs) was previously evaluated in terms of particle morphology, chemical composition, crystallinity, and microstructure. These polymers were obtained from a commercial gas‐phase process and enabled the visualization of the early development of the copolymer phase while including the effects of large‐scale production processes on the resulting polymer properties. Based on the ethylene‐dependent changes observed for the HEPCs as well as some semi‐crystalline TREF fractions, further variable temperature solid‐state 13C NMR experiments were done to observe temperature‐dependent shifts in localised mobility within the bulk samples during a heating/melting profile. From these experiments, the development of amorphous polypropylene signals was observed with increasing temperature, which could indirectly be related to the introduction of ethylene defects. It was also observed that polypropylene components with high rigidity remained at high temperatures for the HEPCs but not for the homopolymer. T1ρ experiments were used to differentiate between changes in crystalline and non‐crystalline phases based on ethylene incorporation as well as during melting. In this publication, the observations from the perspective of the solid‐state analyses are outlined and placed in the context of the evolution of microscopic, chemical, microstructural, and mechanical properties.

The effect of controlled degradation on the molecular characteristics of heterophasic ethylene–propylene copolymers

ABSTRACTA commercial heterophasic ethylene–propylene copolymer (HEPC) produced by Sasol Polymers using a Ziegler‐Natta catalysed gas‐phase process was vis‐broken (controlled degradation) to various degrees by making use of an organic peroxide. The effects of the amount of vis‐breaking on the molecular characteristics and physical properties were subsequently studied by making use of preparative Temperature Rising Elution Fractionation (p‐TREF), Nuclear Magnetic Resonance (NMR), Differential Scanning Calorimetry (DSC), High Temperature Size Exclusion Chromatography (HT‐SEC), and deposition of the SEC fractions via the LC Transform Interface (SEC‐FTIR). It was found that by increasing the amount of organic peroxide, the molecular characteristics of the heterophasic copolymer are severely affected and hence influence the physical characteristics of the polymer dramatically. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41783.

Defining the distribution of ethylene-propylene copolymer phases in heterophasic ethylene-propylene copolymers by a sequential xylene extraction method: Chemical and morphological analysis

Sequential xylene extraction (SXE) in combination with scanning electron microscopy (SEM) and high temperature solvent gradient interaction chromatography (HT-SGIC) were employed for the detailed visualisation and understanding of the evolution of phase morphology in heterophasic ethylene-propylene copolymer (HEPC) particles. The study focused on sequentially extracting the soluble EP rubber phase and EP block or segmented copolymers by SXE at various temperatures. Major changes in the particle morphology (significant increase in both size and number of voids in the particles) after SXE were revealed using SEM imaging. These void structures are believed to result from the amorphous phase (EPR) being removed during the xylene extraction which was further confirmed by differential scanning calorimetry (DSC), high temperature 13C NMR spectroscopy and HT-SGIC. The extractables obtained at 100 °C were found to be a mixture of EP random copolymers, semi-crystalline EP (block or segmented) copolymers, as well as iPP and PE homopolymers. Upon complete dissolution of the particles by SXE at 100 °C, significant amounts of the EP rubber fractions were obtained. Our results show a very heterogeneous distribution of EPR components with varying chemical compositions in HEPC particles produced by dual-reactor processes. For the layered structure observed from this study, truly amorphous EP rubber was found in the outer most regions followed by continuous regions of EP random copolymers having increasing ethylene contents. Propylene-rich EPR, extracted at temperatures of 100 and 130 °C, was observed as the inner EPR phase. Semi-crystalline EP (segmented or block copolymer) and PE homopolymer were detected as intermediate structures between these two EPR regions, inside and outside the pores of the iPP particles. Based on these findings a modified multi-layered core–shell structure was proposed. The results obtained by the proposed SXE fractionation method and its combination with various analytical approaches are found to be very effective for the investigation of the phase composition present in HEPC particles. The present approach is general and can also be used for other multiphase semi-crystalline polyolefins.

Chemical Composition Separation of EP Copolymers by CEF and HT‐SGIC: Crystallization versus Adsorption

Preparative temperature rising elution fractionation (prep TREF) suffers from co‐crystallization effects and, therefore, cannot provide reliable chemical composition distribution (CCD) information. This limitation can be overcome when prep TREF is combined with further fractionation methods such as crystallization elution fractionation (CEF) or high‐temperature solvent‐gradient‐interaction chromatography (HT‐SGIC) as a new approach. By CEF, significant amounts of (co‐crystallizing) amorphous ethylene‐propylene (EP) copolymer are identified in semicrystalline TREF fractions of a heterophasic ethylene/propylene copolymer (HEPC). Complete compositional fractionation with no influence of crystallization effects is accomplished by HT‐SGIC. Prep TREF–HT‐SGIC is found to be the most selective and suitable method for the fast and complete CCD analysis of such complex EP copolymers with CEF providing complementary information. image

The effect of in-process ethylene incorporation on the evolution of particle morphology and molecular characteristics of commercial heterophasic ethylene propylene copolymers (HEPCs)

The development of particle morphology, composition, crystallinity and microstructure were investigated for commercial heterophasic (high impact) propylene–ethylene copolymers (HEPCs). Samples from gas phase polymerization processes were collected at set intervals after the introduction of ethylene into the second reactor. TREF profiles show pronounced differences between these samples where the amount of the lower crystallinity fractions (30–80°C), increased with increasing ethylene content, with a corresponding decrease in the relative amount of SEM analysis of the external surfaces of all the samples has indicated very little porosity, whereas the internal porosity has been maintained (as indicated by FE-SEM). FE-SEM images have also shown the presence of thin threads of copolymer between polypropylene globules for the lowest ethylene content sample and this detail seemingly disappears with higher ethylene contents as larger parts of the surface are covered through copolymer pooling effects. Solution 13C NMR has shown a disproportionate microstructural development where continuous ethylene (E) sequences seem to increase at a higher rate than random-like mixed ethylene (E) and propylene (P) sequences. The overall length of continuous E sequences is however limited by the total ethylene content. Solid state NMR has shown that the sample with the highest ethylene content has at least some ethylene monomers incorporated in rigid amorphous areas within the polymer. From GPC analysis, it was observed that molecular weight and -distribution increased with ethylene content. SEC–FTIR results for bulk samples have shown that both ethylene content and crystallinity is prevalent in the high molecular weight area of the curves, whereas the same properties for propylene were found in the lower molecular weight portion.

Crystal structure and melting behavior of homo-polypropylene and heterophasic ethylene–propylene copolymer after long time heat treatment

The homo-polypropylenes and heterophasic ethylene–propylene copolymers (EP-HECO) were prepared by the non-isothermal and isothermal crystallization methods. The crystal structure and crystal phase were detected by the wide-angle X-ray diffraction (WAXD). The morphology of the spherulite growth process was observed under a polarized optical microscope (POM) with a hot stage. The melting behavior was performed by the differential scanning calorimetry (DSC). It was found that the restricted ethylene segment in the long propylene sequences could act as a heterogeneous nucleating agent of β phase only when the crystallization temperature was very high. PP with long molecular chain had more polymer sequences entering crystal lattice to form large lamellar thickness. Nucleation and spherulitic growth occurred at the same time. Both the final spherulite size and the crystallinity of iPP were related to the melt flow rate. When samples crystallized isothermally at a very high temperature, shorter molecular chain would crystallize into some small-size defect crystals, and this resulted in a multiple melting behavior in the DSC endotherm.

Phase interactions and structure evolution of heterophasic ethylene–propylene copolymers as a function of system composition

AbstractHeterophasic copolymers comprised of polypropylene (PP) matrix and ethylene–propylene copolymer (EPC) dispersed phase were investigated with respect to the dispersed phase composition, i.e., ethylene/propylene ratio. The rheological properties, morphology, as well as thermal and mechanical relaxation behavior were studied to describe the structure evolution and phase interactions between the components of the PP copolymers. Decrease of the ethylene content of the EPC leads to a higher matrix‐dispersed phase compatibility, as evaluated by the shift of the glass transition temperatures of EPC and PP towards each other. At ethylene content of EPC of 17 wt %, the glass transition temperatures of the both phases merged into a joint relaxation. The effect of the EPC composition on the internal structure of the dispersed domains and on the morphology development of the heterophasic copolymers was demonstrated. Decreasing ethylene content was found to induce a refinement of the dispersed phase with several orders of magnitude down to 0.18 μm for propylene‐rich EPC. Optical microscopy observations showed that the dispersed propylene‐rich phase is preferably rejected at the interlamellar regions of the spherulites and/or at the interspherulitic regions, while the ethylene‐rich domains are engulfed within the PP spherulites. Both of these processes impose an additional energetic barrier and influence the spherulite growth rate of the heterophasic materials. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 2825–2837, 2006

Fracture characteristics and deformation behavior of heterophasic ethylene–propylene copolymers as a function of the dispersed phase composition

The deformation and fracture behavior of in reactor produced heterophasic copolymers, comprising a polypropylene (PP) matrix and an ethylene propylene copolymer (EPC) dispersed phase, have been studied as a function of the dispersed phase composition (ethylene/propylene ratio). Conventional and instrumented Charpy as well as instrumented drop weight tests were employed to quantify the response of the materials to impact loading. Scanning and high-voltage electron microscopy was used for characterization of the deformation mechanisms. Decreasing ethylene content of the EPC led to an enhancement of the matrix/dispersed phase compatibility, reduction of the dispersed phase particle size and therewith to a systematic increase of the impact strength at room temperature and a decrease of the brittle-to-tough transition temperature ( T BTT) of the materials. The low temperature impact strength was predominantly dependent upon the glass transition temperature of the EPC phase. The results are discussed from the viewpoint of interfacial interactions, size and spatial packing of the dispersed phase domains and the observed deformation mechanisms.

PARTIAL CROSSLINKING OF THE HETEROPHASIC ETHYLENE-PROPYLENE COPOLYMER IN THE SOLID PHASE

The influence of radical modification with peroxides under solid phase conditions on mechanical properties and the morphology of an heterophasic ethylene-propylene copolymer (HECO) were investigated. The extent of crosslinking and degradation was characterized by the melt flow index and extraction data. Influence of modification on dynamic viscoelasticity of HECO was also investigated. The morphology was studied by the transmission electron microscopy (TEM). Tensil and impact tests were performed on samples of the modified HECO. Crosslinking occurred in the dispersed EPR phase of HECO in the investigated peroxide concentration range. The extent of crosslinking and degradation was dependent on the type of peroxide as a result of the modification in the melt. The modification with higher peroxide concentrations led to the morphology close to the one of the unmodified HECO powder before and also after the extrusion step. The elongation at break of HECO increased at low peroxide concentrations. Improved impact strength at 23°C was attained. Loss of toughness at −20°C was found probably due to slight degradation of PP matrix and due to an inhomogeneous crosslinking of EPR in HECO.

Morphology of photodegraded heterophasic ethylene-propylene copolymers

The morphology of photooxidative degraded films of heterophasic ethylene-propylene copolymer (EPQ-30R) was investigated and compared with isotactic polypropylene and linear low-density polyethylene by scanning electron microscopy. Surface damage caused by polychromatic ultraviolet irradiation (λ ≥ 290 nm) at 55°C in air is presented in different micrographs. Changes occurring due to the formation of polar groups during photooxidative degradation are discussed. Morphological study of these photodegraded polymer samples show very good correlation with the photodegradation results. The rate of photooxidation is very fast in case of isotactic polypropylene, compared with polyethylene and ethylene-propylene copolymers.

The oxidative degradation of heterophasic ethylene-propylene copolymers: a comparison of photoproduct formation under natural and accelerated conditions

The natural weathering behaviour of various heterophasic ethylene–propylene (E–P) copolymers and their fractions was studied. The results are compared with those for polypropylene homopolymer (PP). The extent of natural weathering was monitored by Fourier transform infrared spectroscopy. Also, the photo-oxidation behaviour of isotactic PP, heterophasic E–P copolymers and their fractions under natural exposure were compared with corresponding photoirradiated samples. The photoproducts are produced in the same relative proportions under accelerated and natural exposure, which shows that the mechanisms by which they are formed are not related to the temperature and the photon flux. ©1997 SCI

Effect of UV Irradiation on Gas Permeability in Heterophasic Ethylene-Propylene Copolymer Films

Abstract The permeability variations of carbon dioxide, oxygen, and nitrogen through isotactic polypropylene, heterophasic ethylene-propylene (E-P) copolymers, and polyethylene films were measured during photo-oxidation using polychromatic light at 60°C in air. For polypropylenebased samples the permeability decreases with increasing photooxidation, whereas for polyethylene and an elastomeric fraction from a heterophasic E-P copolymer an increase in permeability with photooxidation is observed.

Effect of UV Irradiation on The Structure of Heterophasic Ethylene-Propylene Copolymers

Changes in [η] and crystallinity of commercial samples of isotactic polypropylene (i-PP), heterophasic ethylene–propylene copolymers [E–P copolymer] containing <16 mol% of ethylene and their fractions (having ethylene content 40.9 and 55.7 mol%) upon polychromatic irradiation in air at 55°C were studied by comparing unirradiated samples. Crystallinity was evaluated by DSC and X-ray diffraction. Viscosity results showed a decrease of molecular weight upon irradiation. DSC and X-ray studies revealed that the transition of α-modification to β-phase of polypropylene and also that crystallinity of the copolymer decreased initially upon irradiation. However, upon longer irradiation, the crystallinity increased again.

Kinetics of Thermal Decomposition of Heterophasic Ethylene-Propylene Copolymers

Abstract Kinetics of thermal decomposition of various heterophasic ethylene-propylene copolymers in the temperature range 215-432°C by thermogravimetry (TG) and 150-180°C by differential scanning calorimetry (DSC), where volatile formation was negligible, was investigated and compared with isotactic polypropylene. Under nitrogen flux the activation energy for decomposition was in the range of 71-89 kcal-mol−1 in TG and 42-64 kcal-mol−1 in DSC. The thermal stability depends on the copolymer composition. The effect of heating rate on DSC thermograms has been studied.