Metallocene-catalyzed Linear Low-density Polyethylene Research Articles

Polylactic acid/polycaprolactone nanocomposite

This article investigates the performance of polylactic acid (PLA)/polycaprolactone (PCL)/montmorillonite (MMT) nanocomposites toughened with metallocene-catalyzed linear low-density polyethylene (mLLDPE), in terms of mechanical, thermal, and morphological properties. All the results were compared and the influence of MMT and mLLDPE on the final properties was observed and reported. mLLDPE decreased the modulus and the strength of PLA/PCL and its nanocomposites due to its inherent lower rigidity compared to PLA/PCL nanocomposites. Moreover, incorporation of clay significantly increased mechanical and thermal stability of the nanocomposites. Scanning electron microscopic images confirmed that MMT acted as a compatibilizer, whereas it reduced the size of the droplets. It has been suggested that improvements in properties are related to good dispersion of clays within the matrix. However, further addition of MMT beyond 2 parts per hundred (phr) level decreases all the properties of PLA/PCL/mLLDPE/MMT nanocomposites. X-Ray diffraction patterns were used to discover the reason of reduction in the properties of PLA/PCL/mLLDPE/MMT at 4 phr. It was revealed that mLLDPE-toughened blends have intercalated structure below 4 phr MMT content as the interlayer spacing decreased at 4 phr MMT.

Study of Metallocene-Catalyzed Linear Low-Density Polyethylene Solid-State Extrusion: Processing and Mechanical Properties

The effect of extrusion temperature and extrusion drawing ratio (EDR) on the die swell ratio (DSR) and mechanical properties of metallocene-catalyzed linear low-density polyethylene (m-LLDPE) was examined with the application of solid-state extrusion (SSE). Scanning electron microscopy (SEM) was employed to characterize the microstructure and morphology of the extrudates. Extruded from a convergence-divergence die, compared with samples obtained by melt-state extrusion (MSE), the DSR decreases for SSE samples prepared at low extrusion temperature and high EDR. Mechanically strong SSE samples were also obtained at low extrusion temperatures and high EDR. Mechanically strong SSE samples were also obtained at low extrusion temperatures and high EDR. SEM indicates that the microstructures of the MSE samples consist primarily of ring-banded spherulites; the microstructure of the SSE samples was microfibers oriented along the direction of extrusion. The highly oriented microfibers contribute to the improved mechanical strength of the SSE samples.

Sparse Long-chain Branching's Effect on the Film-casting Behavior of PE

Abstract The degree of film-width reduction or necking during film-casting is analyzed for three metallocene-catalyzed linear low density polyethylenes, LLDPE, with varying degrees of sparse long-chain branching, LCB. The resins included three sparsely LCB polyethylene, PE, materials with varying LCB content ranging from linear to 0.57 LCB/10000 CH2 along with a Ziegler-Natta polymerized LLDPE and a tubular free-radical polymerized low density polyethylene, LDPE. The last two resins are used as reference materials. The primary objective of this analysis is to evaluate whether uniaxial extensional rheological characteristics, in particular strain-hardening, that are a result of LCB influence the film-necking properties. At the lowest drawdown ratio, necking is observed to be reduced with increasing LCB, and thus strain-hardening characteristics. At the higher drawdown ratios it is observed that LCB no longer reduces necking and the curves merge to the results found for linear PE, except in the case of LDPE, which shows reduced necking at all drawdown ratios. Furthermore, comparisons of film necking are also made to separate the effects of molecular weight distribution, MWD, and LCB. The results indicate that both broadening the MWD and the addition of sparse LCB reduce the degree of necking, but to a lesser degree in the case of broadening the MWD. Analysis of the uniaxial extensional and dynamic shear rheology with the pom-pom constitutive model reveals that a distribution of branches along shorter relaxation time modes is important in reducing necking at higher drawdown ratios. Factors such as shear viscosity effects, extrudate swell, and non-isothermal behavior were eliminated as contributing factors.

Effect of Metallocene-Catalyzed Polyethylene on the Physicomechanical Properties of Blends with High-Density Polyethylene or Low-Density Polyethylene

The physicomechanical properties of polymer blend formulations comprising different grades of metallocene-catalyzed linear low-density polyethylenes (mLLDPEs) with high-density polyethylenes (HDPEs) or a low-density polyethylene (LDPE) were investigated. For blends with HDPE, the addition of mLLDPE improves the Izod impact strength and some tensile properties. For blends with LDPE, adding mLLDPE increases the ductility and the percent elongation at break.

Melting behavior and isothermal crystallization kinetics of PP/mLLDPE blends

The melting/crystallization behavior and isothermal crystallization kinetics of polypropylene (PP)/metallocene-catalyzed linear low density polyethylene (mLLDPE) blends were studied with differential scanning calorimetry (DSC). The results showed that PP and mLLDPE are partially miscible and interactions mainly exist between the mLLDPE chains and the PE segments in PP molecules. The isothermal crystallization kinetics of the blends was described with the Avrami equation. Values of the Avrami exponent indicated that crystallization nucleation of the blends is heterogeneous, the growth of spherulites is almost three-dimensional, and the crystallization mechanism of PP is not affected much by mLLDPE. The Avrami exponents of the blends are higher than that of pure PP, showing that the mLLDPE helps PP to form perfect spherulites. The crystallization rates of PP are decreased by mLLDPE because the crystallization temperature of PP was decreased by addition of mLLDPE and consequently the supercooling of the PP was correspondingly lower. The crystallization activation energy was estimated by the Friedman equation, and the result showed that the activation energy increased by a small degree by addition of mLLDPE, but changed little with increasing content of mLLDPE in the blends. The nucleation constant (Kg) was determined by the Hoffman–Lauritzen theory.

The Rotational Molding Characteristics of Metallocene Polyethylene Skin/Foam Structures

This article reports on the results of ongoing work in which the foaming characteristics of metallocene-catalyzed linear low density polyethylenes for rotational molding are investigated. Earlier publications related rheological and thermal parameters to the polymer structure and mechanical properties and found that metallocene polyethylene can be used in rotational foam molding to produce a foam that will perform as well as a Ziegler-Natta catalyzed foam. Through adjustments to molding conditions, the significant processing and physical material parameters, which optimize metallocene catalyzed linear low-density polyethylene foam structure, have been identified. This article details the optimum processing route for the production of two layer skin/foam parts using the drop box method.

The Effect of Metallocene-Catalyzed Polyethylene on the Physicomechanical Properties of Film Blends with Conventional Linear Low-Density Polyethylene

Film blends of metallocene-catalyzed linear low-density polyethylenes (mLLDPEs) with conventional linear low-density polyethylene (LLDPE) – with both LLDPEs being made using hexene comonomer – were prepared and investigated. The addition of mLLDPE to conventional LLDPE significantly improves the dart impact strength but has little effect on the overall clarity of the resulting films. Conversely, the addition of a small amount of LLDPE to mLLDPE improves the optical properties of the latter.

The Effect of Downgauging on the Physicomechanical Properties of Film Blends of Linear Low-density Polyethylene with Low-density Polyethylene

Two series of film formulations are investigated comprising blends of a metallocene-catalyzed linear low-density polyethylene (mLLDPE) or a conventional linear low-density polyethylene (LLDPE) with a low-density polyethylene (LDPE). The physicomechanical and optical properties are studied at various gauge lengths to investigate the effects of downgauging the films, particularly those containing mLLDPE. The potential for downgauging is found to be optimum at higher levels of mLLDPE with the best results achieved for a film containing 40 wt.% mLLDPE at 60 mm. This formulation has significantly better properties than an analogous blend containing 40 wt.% LLDPE at 100 mm. The potential materials and cost savings associated with downgauging films is also briefly investigated.

The effect of metallocene-catalyzed linear low-density polyethylene on the physicomechanical properties of its film blends with low-density polyethylene

Films comprising a metallocene-catalyzed linear low-density polyethylene (mLLDPE) blended with either of two different low-density polyethylene (LDPE) materials were prepared. The physicomechanical, optical and melt flow properties of the films were measured. A novel adaptation of conventional radar plots was used to process the acquired data to identify the level at which mLLDPE should be incorporated in either of the LDPEs to produce optimal overall properties. In general, the addition of mLLDPE to LDPE improved most of the properties considered and the LDPE material having the higher polydispersity produced blends having superior properties. A level of mLLDPE of between 20–30% (w/w) was required in order to achieve optimization.

Effect of the Material of Construction of Die on Processing Behavior of Metallocene-Catalyzed Linear Low Density Polyethylene

The effects of die materials on die pressure, productivity of extrusion, melt viscosity, and melt fracture of metallocene-catalyzed linear low density polyethylene (mLLDPE) as well as their mechanism were studied in a single screw extrusion system. Die materials used in our experiment include steel, brass, and polytetrafluoroethylene (PTFE). The experimental results show that die materials have great influence on the processing behavior of mLLDPE. The PTFE die can greatly inhibit the occurrence of melt fracture or surface distortion of mLLDPE extrudate. The surface appearance of mLLDPE extrudate is greatly improved. The PTFE die can greatly increase the productivity of mLLDPE extrudate and decrease the die pressure and the melt viscosity of mLLDPE. A possible mechanism for the improved processability of mLLDPE is proposed.

Quantitative analysis of polyethylene blends by Fourier transform infrared spectroscopy.

The quantitative analysis of binary polyethylene (PE) blends by Fourier transform infrared (FT-IR) spectroscopy has been achieved based on the ratio of two absorbance peaks in an FT-IR spectrum. The frequencies for the absorbance ratio are selected based on structural entities of the PE components in the blend. A linear relationship between the absorbance ratio and the blend composition was found to exist if one of the absorbance peaks is distinct to one of the components and the other peak is common to both components. It was also found that any peak resulting from short-chain branching in copolymers (such as linear low-density polyethylene (LLDPE) or metallocene-catalyzed LLDPE (mLLDPE)), is suitable for use as the peak that is designated as being distinct to that component. In order to optimize the linearity of the equation, however, the selection of the second common peak is the most important and depends on the blend system studied. Indeed, under certain circumstances peaks that are not spectrally distinct can be used successfully to apply the method. The method exhibits potential for the routine analysis of PE blends that have been calibrated prior to its application.

Novel characterization of the crystalline segment distribution and its effect on the crystallization of branched polyethylene by differential scanning calorimetry

Based on a thermal segregation treatment, a novel semiquantitative method for the characterization of the crystalline segment distribution in branched polyethylene copolymers was established by the results of differential scanning calorimetry being treated with the Gibbs–Thomson equation. The method was used to describe the segment distribution of Ziegler–Natta-catalyzed linear low-density polyethylene (Z–N LLDPE), metallocene-catalyzed linear low-density polyethylene (m-LLDPE), and a commercial linear low-density polyethylene with a wide molecular weight distribution. The isothermal crystallization kinetics of Z–N LLDPE and m-LLDPE were studied to assess the effect of different segment distributions. According to their molecular characteristics, the crystallization behaviors were analyzed. They indicated that the different segment distributions of the two polymers resulted in different crystallization processes, including the nucleation and growth of crystals under various crystallization conditions. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2107–2118, 2002

Comparison of structure and properties of conventional and “high crystallinity” isotactic polypropylenes and their blends with metallocene‐catalyzed linear low density polyethylene. II. Morphological studies

The morphology of “high crystallinity” polypropylene (h.cr.PP) and metallocene-catalyzed linear low density polyethylene blends has been investigated using optical microscopy, scanning electron microscopy, and wide-angle X-ray scattering. The data obtained were analyzed and compared with results from physical testing. It was concluded that the improved physical performance of pure h.cr.PP, as well as its blends, compared with conventional PP and its blends was due to the alteration (suppression) of spherulitic structure of the former caused by the presence of nucleating agents. These additives (nucleating agents) appear also to cause the formation of fine lamellae in h.cr.PP, which could be an additional factor contributing to the improvement of the properties. A co-continuous structure of the blends comprised from two phases is shown to be superior to a matrix-dispersed one for improving the balance of mechanical properties. Co-continuity can exist and contribute to the improvement of the properties in systems either with or without spherulites. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 831–840, 2001

Comparison of structure and properties of conventional and ?high crystallinity? isotactic polypropylenes and their blends with metallocene-catalyzed linear low density polyethylene. II. Morphological studies

The morphology of “high crystallinity” polypropylene (h.cr.PP) and metallocene-catalyzed linear low density polyethylene blends has been investigated using optical microscopy, scanning electron microscopy, and wide-angle X-ray scattering. The data obtained were analyzed and compared with results from physical testing. It was concluded that the improved physical performance of pure h.cr.PP, as well as its blends, compared with conventional PP and its blends was due to the alteration (suppression) of spherulitic structure of the former caused by the presence of nucleating agents. These additives (nucleating agents) appear also to cause the formation of fine lamellae in h.cr.PP, which could be an additional factor contributing to the improvement of the properties. A co-continuous structure of the blends comprised from two phases is shown to be superior to a matrix-dispersed one for improving the balance of mechanical properties. Co-continuity can exist and contribute to the improvement of the properties in systems either with or without spherulites. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 831–840, 2001

Blown Film Characterization of Binary Blends of Metallocene-Catalyzed LLDPES

We have explored the effects of blending metallocene-catalyzed linear low-density polyethylenes (LLDPE) of reasonably differing molecular weights on the performance and orientation characteristics of blown films. The presence of relatively longer molecules in a metallocene-catalyzed LLDPE resulted in blown films with relatively lower machine direction (MD) tear resistance, lower impact strength and higher transverse direction (TD) tear resistance. While all of the blown films investigated displayed the Keller-Machin “row” structure, the degree of orientation was observed to increase with increasing concentration of long molecules (or high molecular weight blend component). The observed influence of orientation in the crystalline and non-crystalline phases on the Elmendorf tear resistance of the blown films was observed to be consistent with previous investigations on a wide spectrum of commercial LLDPE blown films [15]. Morphological investigations of the blend films indicated the potential significance of microstructural anisotropy on blown film impact performance. This calls into question the wide-spread notion that co-monomer distribution profile is largely responsible for distinguishing the impact performance of various classes of LLDPE blown films.

Solid-State 13C NMR Analyses for the Structure and Molecular Motion in the α Relaxation Temperature Region for Metallocene-Catalyzed Linear Low-Density Polyethylene

The structure and molecular motion of metallocene-catalyzed linear low-density polyethylenes (MLLDPE) have been investigated in the α relaxation temperature region by solid-state 13C NMR spectroscopy. Fully relaxed dipolar decoupling (DD)/MAS 13C NMR spectra are well resolved into the crystalline, crystalline−amorphous interfacial, and rubbery amorphous components at different temperatures above room temperature. Prominent broadening of the crystalline resonance line in the DD/MAS spectra is observed above 60 °C, while the chemical shift anisotropy (CSA) spectra stay almost unchanged from the line shape in the rigid state. This fact indicates, in good accord with the previous results, that the 180° flip motion around the molecular chain axis occurs above 60 °C in the crystalline region with the rate of about 105 Hz. Moreover, the segmental exchange between the crystalline and noncrystalline regions, which will be closely associated with the multistep forward and backward 180° flip motions, is clearly conf...

Influence of phase structure on impact toughening of isotactic polypropylene by metallocene-catalyzed linear low-density polyethylene

In the present study isotactic polypropylene (PP) and metallocene-catalyzed linear low-density polyethylene (mLLDPE) were blended together to obtain thermoplastic materials (compositions) with improved toughness. Structure–property relationships were determined for these compositions with the help of scanning electron microscopy (SEM). Special emphasis was made on tracing the morphological features that led to the optimum mechanical performance. A co-continuous type of structure was found to have much superior toughness as compared to a dispersed-matrix structural type, for blends comprised of the same components (PP and mLLDPE). The study showed the fascinating possibility of creating toughened PP blends by inducing a co-continuous structure. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 1011–1018, 2000

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

Comparison of structure and properties of conventional and ?high-crystallinity? isotactic polypropylenes and their blends with metallocene-catalyzed linear low-density polyethylene. I. Relationships between rheological behavior and thermal and physical properties

The present study was conducted to compare the structure and properties of conventional and so-called “high-crystallinity” (hcr) polypropylene (PP) and to establish characteristic features of the latter that are responsible for its superior thermal and mechanical performance. Moreover, structure–properties relationships of hcr PP blends with metallocene-catalyzed, linear low-density polyethylene (mLLDPE) were compared with those of conventional PP/mLLDPE blends. In Part 1, relationships between rheological behavior (viscosity and melt density) and thermal (transition temperatures and level of crystallinity) and mechanical properties (impact strength and Young's modulus) were analyzed with reference to composition. The rheological and MDSC tests showed that both types of the blends were miscible at the processing temperatures, whereas immiscible in the solid state and in vicinity of the PP melting point. It was found that the improved mechanical properties and the extraordinary high crystallization temperature of hcr PP (and, correspondingly, hcr PP/mLLDPE blends) are not due to the assumed high level of crystallinity but due to alteration of internal structure of this polypropylene. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 1591–1599, 2000

Molecular character of sharkskin phenomenon in metallocene linear low density polyethylenes

The effect of molecular weight on sharkskin dynamics is systematically studied using three metallocene-catalyzed linear low density polyethylenes (mLLDPE) of different molecular weights. A new experimental method is developed to enable in-situ characterization of the time scale r on which sharkskin forms on the extrudate. This laser scattering technique directly measures the periodic variation of the refracted light of the rough sharkskin-like extrudate surface. It is found that r increases with molecular weight as strongly as the overall molecular chain relaxation time, τ * , determined from oscillatory shear measurements. The observed molecular weight dependence provides additional support for the recently proposed interfacial stress growth/relaxation mechanism for sharkskin formation.