Thermal and rheological studies on the molecular composition and structure of metallocene‐ and Ziegler–Natta‐catalyzed ethylene–α‐olefin copolymers

The relationship between the molecular structure and the thermal and rheological behaviors of metallocene‐ and Ziegler–Natta (ZN)‐catalyzed ethylene copolymers and high‐density polyethylenes was studied. Of special interest in this work were the differences and similarities of the metallocene‐catalyzed (homogeneous) polymers with conventional coordination‐catalyzed (heterogeneous) polyethylenes and low‐density polyethylenes. The short‐chain branching distribution was analyzed with stepwise crystallization by differential scanning calorimetry and by dynamic mechanical analysis. The metallocene copolymers exhibited much more effective comonomer incorporation in the chain than the ZN copolymers; they also exhibited narrower lamellar thickness distributions. Homogeneous, vanadium‐catalyzed ZN copolymers displayed a very similar comonomer incorporation to metallocene copolymers at the same density level. The small amplitude rheological measurements revealed the expected trend of increasing viscosity with weight‐average molecular weight and shear‐thinning tendency with polydispersity for the heterogeneous linear low‐density polyethylene and very‐low‐density polyethylene resins. The high activation energy values (34–53 kJ/mol) and elevated elasticity found for some of our experimental metallocene polymers suggest the presence of long‐chain branching in these polymers. This was also supported by the comparison of the relationship between low shear rate viscosity and molecular weight. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 1140–1156, 2002     

An investigation into the relationship between the impact performance of rotationally molded polyethylene products and their dynamic mechanical properties

In this paper, a study of the relationship between the impact performance of rotationally molded polyethylenes and their dynamic mechanical properties is carried out. A wide range of conventional linear low density polyethylene powders and met‐allocene catalyzed linear low density polyethylene powders were rotationally molded and tested. Instrumented falling weight impact tests were carried out over a temperature range of ?60°C to 20°C. Dynamic Mechanical Thermal Analysis (DMTA) tests were also carried out between ?100°C and 90°C, at a frequency of 100 Hz. Comparisons between the impact performance of metallocene catalyzed LLDPEs and Ziegler‐Natta LLDPEs are made. The transitions evident in the DMTA results are related to changes in impact performance with temperature. The beta transition is found to fall in the transition region between high impact performance at low temperatures and lower impact performance at ambient temperatures.     

Ultraviolet‐induced degradation of Ziegler‐Natta and metallocene catalyzed polyethylenes

The metallocene revolution has aroused a storm of interest and associated questions regarding the performance and durability of polyolefins. This new technology has impacted the additives used to stabilize and process polymers. In this work, Ziegler‐Natta and metallocene polyethylene (PE) samples were exposed to natural weather conditions under high doses of ultraviolet radiation, high temperature, and increased humidity. Weather‐induced degradation of the two sets of PEs was studied using gel permeation chromatography, mechanical properties testing, differential scanning calorimetry, and Fourier transform infrared spectroscopy. The gel permeation chromatography analysis indicated the change in molecular weight distribution and molecular weights of metallocene PE to be more stable than conventional Ziegler‐Natta PE. The tensile properties of metallocene PE are known to have higher values than Ziegler‐Natta PE. The former exhibited a lower drop rate in mechanical properties when exposed to natural weather. Formations of nonvolatile carbonyl oxidation products, which absorb in the infrared region with a maximum absorbance level at 1742 cm⁻¹ were determined. This indicated a higher rate of photo‐oxidative and thermal degradation of Ziegler‐Natta PE as compared with metallocene PE. The ultraviolet stabilization of metallocene PE may require different doses and a new kind of stabilizer system that can impart a longer useful lifetime and are cost effective for PE used for outdoor purposes. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 1591–1596, 2000     

Effects of type of polymerization catalyst system on the degradation of polyethylenes in the melt state. Part 1: Unstabilized polyethylenes (including metallocene types)

Several polyethylene resins namely, high‐density polyethylene (HDPE) (Phillips metal oxide catalyst) and linear low‐density polyethylenes (LLDPE) (formed by using Ziegler‐Natta and metallocene catalyst technologies), were used in order to acquire insight into the effect of different polymerization catalyst systems on the production of degradation products during melt processing. Infrared spectroscopy, color measurement, hydroperoxide determination, and melt flow rate measurement were used to monitor the degradation as a function of the number of passes through a twin‐screw extruder. The metallocene PEs were shown to exhibit superior melt stability relative to Phillips HDPE. The latter showed high levels of hydroperoxide formation. The superior thermo‐oxidative stability of the metallocene PEs was attributed to low levels of metallic catalyst residues, together with low vinyl unsaturation content. In all of the PEs examined, the rate of crosslinking was greater than that of chain scission. IR spectroscopy indicated that crosslinking (most prevalent in the Phillips HDPE) proceeded via the addition of macroradicals to vinyl unsaturation. The Ziegler‐Natta LLDPE showed an intermediate tendency for crosslinking but notable formation of trans‐vinylidene and the most noticeable color development. J. VINYL ADDIT. TECHNOL., 2011. © 2011 Society of Plastics Engineers     

Distributions of Crystal Size from DSC Melting Traces for Polyethylenes

Melting curves, obtained by differential scanning calorimetry, are used to estimate crystal size distributions. The proposed theoretical analysis is applied to different types of polyethylene, including high‐density polyethylene (HDPE), metallocene catalyzed linear low‐density polyethylenes (m‐LLDPE), blends of m‐LLDPEs, and Ziegler‐Natta catalyzed LLDPEs (ZN‐LLDPE). Theoretical predictions are in agreement with experimental results. A generalized melting temperature equation successfully predicts the melting temperatures of all the LLDPEs, although it was initially proposed for homogeneous copolymers with excluded comonomers. A new definition of the heat of fusion for pure crystals is proposed. This heat of fusion can be calculated from the average crystal size or the crystal size number distribution.     

Effects of ultrasonic oscillations on processing behavior and mechanical properties of metallocene‐catalyzed linear low‐density polyethylene/low‐density polyethylene blends

The influences of ultrasonic oscillations on rheological behavior and mechanical properties of metallocene‐catalyzed linear low‐density polyethylene (mLLDPE)/low‐density polyethylene (LDPE) blends were investigated. The experimental results showed that the presence of ultrasonic oscillations can increase the extrusion productivity of mLLDPE/LDPE blends and decrease their die pressure and melt viscosity during extrusion. Incorporation of LDPE increases the critical shear rate for sharkskin formation of extrudate, crystallinity, and mechanical properties of mLLDPE. The processing behavior and mechanical properties of mLLDPE/LDPE blends were further improved in the presence of ultrasonic oscillations during extrusion. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 2522–2527, 2004     

Morphology development for single‐site ethylene copolymers in rotational molding

The morphology development of ethylene copolymers was modeled with the modified phase‐field theory. The metastability of polymer crystallization was also considered in the modeling. Modeling and experimental work were simultaneously undertaken to compare the crystallization kinetics of single‐site‐catalyzed and Ziegler‐catalyzed resins and their influence on morphology development in the rotational‐molding process. With a more uniform short‐chain branch distribution, the single‐site copolymers developed well‐defined spherulitic structures. The Ziegler–Natta catalyst resins were characterized by a higher nuclei density and a faster crystallization rate and produced finer structures in the molded parts. The modeling approach proposed in our work allowed an evaluation of the processing and the material effects on the development of morphological features during melt solidification. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Optical properties of blown and cast polyethylene films: Surface versus bulk structural considerations

In this article we report on some surprising, and we believe new, findings regarding the factors affecting the optical properties (haze) of polyethylene blown and cast films. A comprehensive investigation of blown and cast films made from conventional Ziegler‐Natta catalyzed linear low density polyethylene (LLDPE) as well as metallocene‐catalyzed LLDPE (mLLDPE) resins was conducted. The large majority of the contribution to the total haze in the blown and cast films was observed to come from the surface roughness of the films, with the bulk (internal) contribution being relatively minor. Using a variety of analysis and characterization methods, including atomic force microscopy, small angle light scattering, and wide angle X‐ray scattering, we determined that the surface roughness in these films was a result of the development of distinct spherulitic‐like superstructures formed during the blown or cast film processing. Furthermore, these superstructures were observed only in the mLLDPE blown films, and not in the LLDPE blown films processed at similar conditions. Analysis of the rheological and molecular characteristics of these various mLLDPE and LLDPE resins revealed that the mLLDPE resins exhibited considerably lower molecular weight, narrower molecular weight distribution, lower zero shear viscosity, and lower melt elasticity compared with the LLDPE resins of similar melt index. These observations support our general finding and primary conclusion from this work that in polyethylene blown and cast films made using typical processing conditions, the optical haze properties are adversely affected because of enhanced surface roughness caused by the formation of spherulitic‐like superstructures in polymer melts that possess fast relaxing and low melt elasticity rheological characteristics. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 2845–2864, 2000     

Haze of polyethylene films—effects of material parameters and clarifying agents

Effects of material parameters on the haze of blown films were analyzed. Four linearlow‐density polyethylenes (two metallocene grades and two Ziegler‐Natta grades) were studied in combination with three additives (two sorbitol‐based clarifying agents and a low‐molecular‐weight long‐chain branched polyethylene). One of the sorbitol‐based additives reduced the haze of both the metallocene materials in this study, but did not have any positive effect on the two Ziegler‐Natta materials. The variation in haze among the four base materials was directly related to the root‐mean‐square surface roughness (σ). When considering all 16 material/additive combinations, the link between haze and surface topography was not a simple σ‐haze relationship, but the haze was correlated with the average distance between adjacent surface profile peaks, the average slope, and the power spectral density at high lateral frequencies. Both of the mechanisms referred to in the literature, extrusion‐induced haze and crystallization‐induced haze, were probably active for the films in this study.     

Insulation performance and microstructure in modified polyethylene by MPE

Electrical measurements have shown to be able to provide useful information on physical, chemical, and microstructural properties of dielectric material. In this article, the depolarization characteristics of low‐density polyethylene blended with a small amount of metallocene catalyzed polyethylene were measured by pulsed electro‐acoustic method under various stresses. According to space charge limited current theory, the derivation of quantities such as mean volume density of space charge, apparent trap‐controlled mobility, trap depth distribution, and threshold stress were discussed. The test results showed that low‐density polyethylene blended with 1 wt % metallocene catalyzed polyethylene could effectively decrease deep trap density, increase shallow trap density, and then improve the mobility of charges. We also measured the breakdown voltage and tensile strength of the blends. It was found that low‐density polyethylene blended with a small amount of metallocene catalyzed polyethylene could effectively improve its breakdown voltage and tensile strength but reduce the material tenacity. Finally, mechanical relax and crystalline morphology of blends were studied by dynamic mechanical measurement, wide‐angle X‐ray diffraction, and small‐angle X‐ray scattering experiments. The results showed that the improvement of electrical properties and mechanical strength in the blends were relevant to the crystalline morphology. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Implications of melt compatibility/incompatibility on thermal and mechanical properties of metallocene and Ziegler–Natta linear low density polyethylene (LLDPE) blends with high density polyethylene (HDPE): influence of composition distribution and branch content of LLDPE

In this paper, the implications of melt compatibility on thermal and solid‐state properties of linear low density polyethylene/high density polyethylene (LLDPE/HDPE) blends were assessed with respect to the effect of composition distribution (CD) and branch content (BC). The effect of CD was studied by melt blending a metallocene (m‐LLDPE) and a Ziegler‐Natta (ZN) LLDPE with the same HDPE at 190 °C. Similarly, the effect of BC was examined. In both cases, resins were paired to study one molecular variable at a time. Thermal and solid‐state properties were measured in a differential scanning calorimeter and in an Instron mechanical testing instrument, respectively. The low‐BC m‐LLDPE (BC = 14.5 CH₃/1000 C) blends with HDPE were compatible at all compositions: rheological, thermal and some mechanical properties followed additivity rules. For incompatible high‐BC (42.0 CH₃/1000 C) m‐LLDPE‐rich blends, elongation at break and work of rupture showed synergistic effects, while modulus was lower than predictions of linear additivity. The CD of LLDPE showed no significant effect on thermal properties, elongation at break or work of rupture; however, it resulted in low moduli for ZN‐LLDPE blends with HDPE. For miscible blends, no effect for BC or CD of LLDPE was observed. The BC of LLDPE has, in general, a stronger influence on melt and solid‐state properties of blends than the CD. Copyright © 2004 Society of Chemical Industry     

Characterization of commercial linear low‐density polyethylene by TREF‐DSC and TREF‐SEC cross‐fractionation

A new technique for characterization of linear low‐density polyethylene (LLDPE) is presented in this report. The molecular structure of two commercial LLDPEs, produced by copolymerization of ethylene with 1‐butene over a Ziegler‐Natta and a metallocene catalyst, was investigated. The LLDPE resins were fractionated by temperature rising elution fractionation (TREF), and the TREF fractions were further analyzed by size exclusion chromatography and differential scanning calorimetry (DSC) coupled with successive nucleation/annealing (SNA). The cross‐fractionation techniques provided detailed information about the molecular structure of different types of LLDPEs; of particular interest is the TREF‐SNA‐DSC cross‐fractionation which allowed a direct observation of methylene sequence distribution and thus short chain branch (SCB) distribution. TREF‐size exclusion chromatography cross‐fractionation showed that the molar mass of the Ziegler‐Natta LLDPE increased monotonically with decreasing SCB, whereas the plot of M_w vs SCB for the metallocene LLDPE showed a maximum. TREF‐SNA‐DSC cross‐fractionation clearly showed that the metallocene LLDPE only had intramolecular heterogeneity in SCB distribution, whereas the Ziegler‐Natta LLDPEs exhibited both intermolecular and intramolecular heterogeneity. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 960–967, 2000     

The role of the interface in melt linear viscoelastic properties of LLDPE/LDPE blends: Effect of the molecular architecture of the matrix

The rheological properties of blends consisting of a long chain branched low‐density polyethylene (LDPE) and two linear low‐density polyethylenes (LLDPE) are studied in detail. The weight fractions of the LDPE used in the blends are 5 and 15%. The linear viscoelastic characterization is performed at different temperatures for all the blends to check thermorheological behavior and miscibility in the melt state. Blends containing metallocene LLDPE as the matrix display thermorheologically complex behavior and show evidences of immiscibility in the melt state. The linear viscoelastic response exhibits the typical additional relaxation ascribed to the form deformation mechanism of dispersed phase droplets (LDPE). The Palierne model satisfactorily describes the behavior of these blends in the whole frequency range explored. However, those blends with Ziegler‐Natta LLDPE as the matrix fulfill the time‐temperature superposition, but exhibit a broad linear viscoelastic response, further than the expected for an immiscible system with a sharp interface. The rheological analysis reveals that, in addition to the droplets form relaxation, another mechanism at lower frequencies exists. The broad linear response of the blends with the Ziegler‐Natta LLDPE can be explained by hypothesizing a strong interaction between the high molecular weight linear fraction of the LLDPE and the low molecular weight (almost linear) chains of the LDPE phase, forming a thick interface with its own viscoelastic properties. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Metallocene polypropylene crystallization kinetic during cooling in rotational molding thermal condition

This article is part of an ambitious project. The aim is to simulate mechanical properties of rotomolded part from microstructure consideration. Main objective here is to consider metallocene polypropylene crystallization kinetic (PP) during cooling stage in rotational molding. Crystallization kinetic of metallocene PP is so rapid that microscopy cannot help to observe nucleation and growth. Crystallization rate can be estimated by a global kinetic. Given that cooling in rotational molding is dynamic with a constant rate, Ozawa law appears more appropriate. Ozawa parameters have been estimated by differential scanning calorimetry. In rotational molding thermal condition, Avrami index identifies a complex nucleation intermediate between spontaneous and sporadic. Ozawa rate constant is 68 times higher than this obtained for Ziegler–Natta PP. By coupling transformation rate from Ozawa model and a thermal model developed earlier, the difference between theory and experimental is less than 1%. To optimize rotational molding, study has been completed by sensitivity to adjustable parameters. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

LLDPE‐based mono‐ and multilayer blown films: Effect of processing parameters on properties

In this study we investigated the effect of processing parameters on the end‐use properties of mono‐ and five‐layer coextruded polyethylene (PE) blown films using three different linear low‐density PE (LLDPE) resins. The three investigated LLDPEs were a conventional Ziegler‐Natta gas phase ethylene‐butene copolymer, and two solution ethylene‐octene resins produced with Ziegler‐Natta and single‐site catalysts. The octene copolymers were produced using NOVA Chemicals Advanced SCLAIRTECH? process and catalyst technologies. It was found that within the investigated range of processing conditions, tear strength increased in the direction perpendicular to the highest orientation, impact and puncture strength increased with the overall orientation, and the effect of orientation due to shear stresses in the die was negligible because of rapid macromolecular relaxation before crystallization. Finally, it was also shown that due to changes in the size of the crystallites, haze increased with die gap and frost line height (FLH), and decreased as take‐up speed (TUS) increased. POLYM. ENG. SCI., 45:1214–1221, 2005. © 2005 Society of Plastics Engineers     

Polypropylene/linear low‐density polyethylene blends: Morphology,crystal structure,optical, and mechanical properties

The morphology, crystal structure, crystallization behavior, optical, and mechanical properties of isotactic polypropylene (iPP) blended with metallocene linear low‐density polyethylene (mLLDPE) and Ziegler–Natta linear low‐density polyethylene (zLLDPE), with and without nucleating agents, were investigated. The correlation between the structures and optical properties was investigated. The addition of linear low‐density polyethylenes (LLDPEs), nucleating agents, and poly(ethylene‐co‐octene) (POE) had little influence on the crystal form of the iPP. The growth along the b axis was favorable in the presence of nucleating agents and LLDPEs. The LLDPEs led to much finer crystal morphologies, and the nucleating agents further prohibited spherulite formation; consequently, light scattering from the bulk crystalline structure was reduced. In all blends, biphase morphology was observed, and POE could improve the adhesion between the iPP and mLLDPE. After blending with LLDPEs, the haze and stiffness decreased, and the gloss increased. mLLDPE enhanced the toughness whereas zLLDPE had a slight influence on it. The nucleating agents decreased the haze, increased the gloss more, and ameliorated the stiffness; however, they changed the toughness little. POE increased the toughness of the blend significantly, accompanied by a much lower haze, higher gloss, and almost the same stiffness. When the concentration of 1,3 : 2,4‐bis(3,4‐dimethyl‐benzylidene sorbitol) exceeded 0.25 wt %, the optical properties and mechanical properties leveled off. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Characterization of an amorphous surface layer on blown polyolefin film

This study concerned an amorphous surface layer on blown polyethylene film with a composition different from that of the bulk. The surface layer was characterized by gentle probing with an atomic force microscope. The demonstration of an amorphous layer uniformly covering the surface of a blown Ziegler–Natta‐catalyzed polyethylene (znPE) film reproduced previous reports. Removing the surface layer by solvent washing confirmed the hypothesis that the layer consisted of lower molecular weight, higher branch content fractions. A blown film of znPE blended with up to 30 wt % impact‐modified high‐melt‐strength polypropylene (hmsPP) also exhibited an amorphous surface layer. In thin films, it was advantageous for the mobile, amorphous fractions of ethylene–propylene rubber (EPR) to locate at the surface rather than at the phase interface. The amorphous EPR tended to segregate into pools on the film surface, and this pointed to a substantial difference between the amorphous surface layers on the znPE and hmsPP/znPE blend films. Surface enrichment best described the compositional gradient that resulted from the concentration of lower molecular weight, higher branch content chains at the surface of the znPE film. Surface segregation was more appropriate for the emergence of EPR fractions as a separate phase on the surface of the hmsPP blend film. Films blown from a blend of a Ziegler–Natta‐catalyzed polyethylene and a metallocene‐catalyzed polyethylene (zn/mPE) and its blend with hmsPP reproduced the primary features of surface enrichment and surface segregation. Some differences between the znPE and zn/mPE films were attributed to the metallocene constituent of zn/mPE. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 3625–3635, 2002     

LLDPE‐based mono‐ and multilayer blown films: Property enhancement through coextrusion

In this study we investigated the performance of multilayer coextruded linear low‐density polyethylene (LLDPE) blown films. Five‐layer films were compared with monolayer dry‐blended films, and the effects of layer composition and layout on the end‐use properties of the coextruded films were highlighted. Three different LLDPEs were used: a conventional Ziegler‐Natta LLDPE gas phase butene copolymer, an advanced Ziegler‐Natta LLDPE solution octene copolymer, and a single‐site LLDPE solution octene copolymer. Numerous five‐layer coextruded structures comprising the single‐site resin and the other two Ziegler‐Natta resins were produced. The coextruded structures composed of the LLDPE butene and the single‐site resin yielded improved end‐use properties relative to the monolayer‐blended films. This result was ascribed to the presence of interfacial transcrystalline layers. Also, blends of the single‐site LLDPE and the advanced Ziegler‐Natta LLDPE octene resins within selected layers of coextruded films showed slightly enhanced tear resistance. Finally, it was found that haze was significantly reduced when the outside layers were composed of the single‐site resin. POLYM. ENG. SCI., 45:1222–1230, 2005. © 2005 Society of Plastics Engineers     

Analysis of the effects of the polymerization route of ethylene‐propylene‐diene rubbers (EPDM) on the properties of polypropylene‐EPDM blends

A comparative study of two ethylene‐propylene‐diene rubbers (EPDM) polymerized by both conventional (Ziegler–Natta catalysts) and new techniques (metallocene catalysts) is presented. For this purpose, thermoplastic elastomers based on isotactic polypropylene (iPP) and EPDM blends at different percentages were prepared and their properties examined. In particular, the processing behavior and mechanical properties are reported. So, the flow properties analyzed by torque value, melt index, and rheological study reveal that the blends containing EPDM synthesized by metallocene catalyst present a smaller viscosity, thus offering better processing behavior. On the other hand, the mechanical properties show that metallocene EPDM rubbers give rise to more elastic materials with a higher deformation at break and resilience as well as a lower compression set. Moreover, the effectiveness of these innovative EPDM rubbers as impact modifiers for PP is demonstrated. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 25–37, 2002     

Morphology and coalescence of ethylene copolymers: Influence of thermal treatments and sorbitol nucleating agents

Coalescence of polymer particles is a key phenomenon in many powder processing technologies. Both the extent and the rate of coalescence between particles govern the production cycle and the performance of the end‐product. It is well accepted that both processing conditions and material formulation affect the morphology of molded parts and consequently their mechanical properties. The interest of this study is to evaluate the impact that changes in morphological features caused by the imposition of different thermal treatments as well as by the addition of a nucleating agent (bis 3,4 dimethylbenzylidene sorbitol) have on the coalescing behavior of ethylene copolymers produced from Ziegler–Natta and metallocene catalyst technologies. Results showed that samples produced using slower cooling rates exhibited higher crystallinity and increased thermal stability. Variations in thermal treatments, however, only resulted in minor changes in the coalescing behavior of the resins considered in this work. However, the addition of the nucleating agent to Ziegler–Natta ethylene copolymers led to the formation of crystalline structures with increased material thermal stability, but reduced chain mobility, and consequently resulted in a slower coalescing rate. These effects, however, are dependent on the molecular structure of the copolymers. The addition of the sorbitol nucleating agent influenced the morphological structure of the metallocene copolymer used in this work but did not result in any significant changes in the coalescence behavior of the resin. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 5443–5455, 2006