Metallocene Polyethylene Research Articles

An Evaluation of Dilution Rheology for the Characterization of Long Chain Branching of Polyethylenes

Abstract The purpose of this paper was to re-evaluate the novel rheological method of Crosby et al. [J. Rheol. 46 (2002) 401] to characterise long chain branching (LCB) in polyethylenes (PE) using the rheology of concentrated solutions. The feasibility of this dilution method centred on knowing the class of branched material and the molecular tube theory-based technique for the determination of two topological parameters (n, bU), where n is the number of entanglements between branch points while bU is the probability of meeting a branch point when tracing along the molecule from a random monomer against the direction of polymerisation. This paper proposes new possible approaches to calculate the two topological parameters (n, bU) set for metallocene polyethylenes (mPE), and their ambiguity discussed, as the results are dependent on the approach taken, though the previous authors mentioned only one. In addition, their approach requires an input value of LCB/1000C obtained from the standard analytical solution (SEC-V or SEC-LALLS) methods, hence, until now, without proper demonstration of the potential advantage of the dilution rheology method for LCB characterisation, as the main premise of their published article was to characterise the degree of LCB via rheological measurements without recourse to other methods of LCB characterisation.

Properties and preparation of compatibilized nylon 6 nanocomposites/ABS blends using functionalized metallocene polyolefin elastomer. I. Impact properties

AbstractThe impact behaviors of nanoclay‐filled nylon 6 (nano‐nylon 6) blended with poly(acrylonitrile–butadiene–styrene) terpolymers (ABS) prepared through a twin screw mixing process were investigated here using metallocene polyethylene grafted maleic anhydride (POE‐g‐MA) as a compatibilizer to enhance the interface interaction. No clear effect of compatibilizer on the dispersion of clay and crystalline structure of nano‐nylon 6 has been observed. In view of morphology and rheological behaviors, the effect of compatibilizer on the mechanical properties could be elucidated. It is found that impact strength increases with the addition of compatibilizer at various ABS compositions. Similar effects are also observed with decreasing test temperature at the nano‐nylon 6/ABS blend composition of 80/20. As for thermal properties, the heat distortion temperature shows a marginal decrease in the nano‐nylon 6/ABS blends. Rheological behavior indicates that increased viscosity is found for the investigated compatibilized systems. Through morphology observations, the etched ABS particle sizes tend to decrease with the addition of compatibilizer for the blends, but are larger with higher contents of ABS concentrations. Those observations account for impact behaviors of the investigated blends. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 1364–1371, 2006

Addition of filled or unfilled fluoropolymers and their effects on the electrification of a metallocene polyethylene during capillary flow

During the extrusion of polymers, the extrudates exhibit distortions at high shear rates such as sharkskin, spurt or gross melt fracture that limit industrial production rates. One solution to reduce or eliminate these defects is to induce slip at the die wall or in the polymer bulk for example by adding some polymer processing additives (PPA). In this study, we used the addition of different fluoropolymers with and without inorganic fillers to promote wall and interlayer slippage in a metallocene polyethylene (mPE). As expected, defects onset was delayed and shear stress was decreased in any case due to the increase of slip velocity. In a previous work, we showed that the electrical charging of the extrudate during the extrusion could be correlated with slip velocity. Present results confirmed the relationship between electrical charging and wall or interlayer slippage. Indeed, the higher the amount of PPA without filler in the melt, the higher the wall-slippage and the higher the electrification. The filled fluoropolymer exhibited a more efficient postponing of extrusion defects than unfilled ones concomitant with a strong increase of the extrudate electrical charging. Finally, electrical charging versus wall-slippage curves witnessed the influence of additives concentration and nature on electrification as expected from the double-layer theory.

Experimental set-up and characterization of a mLLDPE-PP co-extruded bilayer system

The rheological behavior of the bilayer system mLLDPE (metallocene linear low density polyethylene)-PP (polypropylene) obtained by a coextrusion process has been investigated in this work. Metallocene polyethylenes are characterized by a superior toughness, heat sealing, and optical properties when compared with conventional polyethylenes. The purpose of the addition of PP in a two layer structure is to confer a better process facility and an enhanced rigidity to the structure itself. A rheological characterization of the two polymers was performed in order to study the flow behavior in each resin, and to verify their compatibility in a coextrusion process. Films, containing different percentages of metallocene polyethylene, were produced by a cast coextrusion process. Moreover, a mathematical modeling of the process was done in order to predict the die pressure gradient, the relative thicknesses, and the shear stress distributions as function of working conditions and rheological properties in the individual polymers. A preliminary investigation on the effect of film composition on the performance of the coextruded bilayers was realized by performing some mechanical tests. POLYM. ENG. SCI. 46:874–881, 2006. © 2006 Society of Plastics Engineers

Phase separation in polypropylene and metallocene polyethylene blends

Blends of a metallocene linear low density polyethylene (m-LLDPE) and polypropylene random copolymer (PP) have been prepared using a twin screw extruder and characterized by thermal analysis, mechanical properties, and wide angle X-ray scattering to determine their degree of compatibility. The blends were either directly quenched in water from the melt- or slow-cooled to room temperature. In both cases, the two components formed separate phases and crystallized independently. The slow-cooled specimens had higher yield stress, tensile modulus, and lower elongation at break consistent with higher degree of crystallinity. The elongation to break also varied with composition reaching a minimum at 50% consistent with the incompatible nature of the blends. Crystallization kinetics and melting studies confirm that the two components formed separate phases and crystallized independently. POLYM. ENG. SCI., 46:889–895, 2006. © 2006 Society of Plastics Engineers

Morphology and gas barrier properties of polyethylene‐based nanocomposites

AbstractNanocomposites were processed by melt blending two reference matrices, a metallocene polyethylene and a low density maleic anhydride‐grafted polyethylene with an organo‐modified montmorillonite. It was shown that the introduction of a maleated polyethylene compatibilizer was required to improve the clay nanoplatelet dispersion in the metallocene polyethylene‐based nanocomposites. Increasing the montmorillonite content led to a significant increase of the barrier properties. Interfacial agents such as oxidized paraffins were shown to be more effective to reduce the gas permeability than maleated polyethylene and the dependence of the gas transport properties was discussed not only as a function of the clay dispersion but also as a function of the clay/compatibilizer and compatibilizer/matrix interactions. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 431–440, 2006

The thermo-oxidative degradation of metallocene polyethylenes. Part 1: Long-term thermal oxidation in the solid state

The long-term thermo-oxidative degradation of different metallocene polyethylenes (mPEs) was investigated in the solid state (oven ageing in air at 90 °C). The mPEs differed essentially according to their initial melt index, molar mass distribution, density and ash content. Two Phillips-type HDPEs were also studied for comparison. The rate of oxidation was assessed by carbonyl index measurements. Initial vinyl unsaturation and short chain branch contents were correlated with induction times. The hydroperoxide concentrations of the aged materials and the levels of metal catalyst residues were also evaluated. Outstanding oxidative stability was exhibited for all the mPEs in the solid state relative to the Phillips HDPEs. This was believed to be due to the low concentration of both adventitious metal catalyst residues and initial vinyl unsaturation. Degree of short chain branching also appeared to influence the oxidative stability of the mPEs. Besides inherent properties, the density/crystallinity appeared to be the main factor influencing their oxidative stability. In the case of the Phillips HDPEs, the high level of vinyl unsaturation together with chromium catalyst residues may contribute to their poor oxidative stability.

The thermo-oxidative degradation of metallocene polyethylenes: Part 2: Thermal oxidation in the melt state

The thermo-oxidative melt degradation of different metallocene polyethylenes (mPEs) was investigated in a torque rheometer open to air at 225 °C and 10 rpm. The mPEs differed essentially according to their initial melt index, molar mass distribution, density and ash content, but one characteristic was changed at a time in order to assess the influence of each specific property in the thermo-oxidative degradation of the PEs investigated. Crosslinking was found to dominate at the early stages of degradation during mastication for most polymers where reactions of alkyl radicals to vinyl groups were considered to be the dominant reaction. Furthermore, discolouration was attributed to both excessive levels of catalyst residues and extensive formation of conjugated systems. Finally, it was concluded that the polymer melt viscosity, i.e., molar mass and shape of molar mass distribution, appeared to govern the processing stability of the mPE. These results confirm the importance of shear as the major source for initiation of free radicals formed by homolytic fission caused via mechanical cleavage of polymer chains.

Processability of a metallocene‐catalyzed linear PE improved by blending with a small amount of UHMWPE

AbstractThe processability of a metallocene‐catalyzed polyethylene blended in the melt with low amounts of ultra‐high molecular weight polyethylene was evaluated. Our morphological and thermal studies revealed that the blends were structurally heterogeneous, formed by a matrix of metallocene polyethylene and homogeneously distributed particles of ultra‐high molecular weight material. The visible particles were smaller than those of the reactor powder. Also observed was some degree of interaction between both phases. Rheological data indicated an intense effect of morphology on viscoelastic functions, beyond that expected for a system composed of noninteractive phases. Collectively, our findings suggest the existence, to some extent, of a homogeneous phase at the molecular level composed of a fraction of ultra‐high molecular weight species and metallocene polyethylene. This gives rise to a striking behavior during processing in that the blends show improved extrusion performance the higher their ultra‐high molecular weight polyethylene content. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2963–2971, 2005

Pressure effects on viscosity and flow stability of polyethylene melts during extrusion

In the present work, the effects of pressure on the viscosity and flow stability of four commercial grade polyethylenes (PEs) have been studied: linear-low-density polyethylene copolymer, high-density polyethylene, metallocene polyethylenes with short-chain branches (mPE-SCB), and metallocene polyethylenes with long chain branching (mPE-LCB). The range of shear rates considered covers both stable and unstable flow regimes. “Enhanced exit-pressure” experiments have been performed attaining pressures of the order of 500×105 Pa at the die exit. The necessary experimental conditions have been clearly defined so that dissipative heating can be neglected and pressure effects isolated. The results obtained show an exponential increase in both shear and entrance-flow pressure drop with mean pressure when shear rate is fixed and as long as flow is stable. These pressure effects are described by two pressure coefficients, βS under shear and, βE under elongation, that are calculated using time–pressure superposition and that are independent of mean pressure and flow rate. For three out of four PE, pressure coefficient values can be considered equal under shear and under elongation. However, for the mPE-LCB, the pressure coefficient under elongation is found to be about 30% lower than under shear. Flow instabilities in the form of oscillating flows or of upstream instabilities appear at lower shear rates as mean pressure increases. Nevertheless, the critical shear stress at which they are triggered remains independent of mean pressure. Moreover, it is found that the βS values obtained for stable flows do not differ much from the values obtained during upstream instability regimes, and differ really from pressure effects observed under oscillating flow and slip conditions.

Fracture behaviors of PP/mPE thermoplastic vulcanizate via peroxide crosslinking

Peroxide cured PP/metallocene polyethylene (mPE) to form a novel thermoplastic vulcanizate has been prepared. Metallocene polyethylenes with two different levels of comonomer contents were chosen for the investigation. The mixing of mPE (40 wt%) and PP (60 wt%) containing various dosages of peroxide was performed to activate dynamic cure using an internal mixer. Thermal behaviors remain largely unchanged for PP, indicating the dominant effect of cured mPE domains on mechanical properties for the vulcanizates. Tensile strength decreases at first with increasing the levels of peroxide concentrations yet increases again at higher level of peroxide concentrations at all test temperatures. Tear strength generally decreases with reduced energy dissipation and tends to level off at higher degree of cure. The values of cutting strength are comparable with the results for different types of materials and are generally two orders of magnitude smaller than the results from tear measurements. Strength of materials was clearly dominated by the crystalline yielding effect than viscoelastic effect, even in this nano-fracture zone of deformation in the cutting measurement. Morphological observations were also elucidated as well.

Fracture behaviors of silane‐cured metallocene‐catalyzed polyethylene thermoplastic vulcanizate

AbstractSilane moisture‐cured metallocene‐catalyzed polyethylene (mPE) blend to form a novel thermoplastic vulcanizate (TPV) has been prepared. Metallocene polyethylenes with two different levels of comonomer contents were grafted with various amounts of vinyltriethoxy silane. As a result, tensile strength varies slightly with increasing the levels of silane concentrations, at all test temperatures. Tear strength generally decreases with reduced energy dissipation, at higher degrees of cure. “Threshold” fracture energy is roughly proportional to the reciprocal square root of Young's modulus. By relating tensile strength to tear strength, it was found that the corrected average depth of flaw is in the range of 29.3 ± 7.2 μm, which successfully confirms the extension of Rivlin and Thomas's theory for conventional elastomers to TPVs under an elaborate treatment, due to the limitation of the theory. Cutting strength of mPE TPVs gives an intermediate value when compared with that of crystalline plastics and conventional elastomers, which further signifies the importance of crystalline yielding even in the nanofracture zone of deformation. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2207–2218, 2005

The effect of nanoclays on the processibility of polyolefins

AbstractThe influence of a new processing additive (particles of organically modified nanoclays) on the processibility of polyolefins in extrusion is studied. The equipment used includes an Instron capillary rheometer with two types of dies, namely capillary dies and special annular dies (Nokia Maillefer wire coating crosshead) attached to the rheometer. Ziegler Natta and metallocene polyethylenes and one polypropylene were tested using these two pieces of equipment. It was found that the nanoclay additive had a significant effect on the extrudate appearance of polyethylene. It eliminates surface melt fracture and postpones the critical shear rate for the onset of gross melt fracture to significantly higher values depending on resin type, temperature, and additive concentration (typically 0.05 to 0.5 wt%). To explain the possible mechanism for the effect of the additive on the processibility of the resins, shear and extensional rheological measurements were carried out for the pure resins as well those loaded with the nanoclay additives. It seems that the presence of nanoclays suppresses the development of extensional stresses to such high levels that can cause melt fracture phenomena. Finally, it was found that the combination of nanoclays with traditional processing aids such as fluoropolymers produce an enhanced processing aid that can increase the critical shear rates for the onset of melt fracture to levels much higher than the individual constituents when they are used independently. POLYM. ENG. SCI., 45:1098–1107, 2005. © 2005 Society of Plastics Engineers

Understanding heat transfer mechanisms during the cooling phase of blow molding using infrared thermography

The cooling phase of the extrusion blow molding process has a large influence on the cycle time of the process as well as on the properties and quality of the molded products. A better understanding of the heat transfer mechanisms occurring during the cooling phase will help in the optimization of both mold cooling channels and operating conditions. A continuous extrusion blow molding machine and a rectangular bottle (motor oil type) mold were used to produce bottles. A high density polyethylene (HDPE) and a metallocene polyethylene (mPE) having different rheological properties were tested. Melt and mold temperatures, cooling time, inflating pressure and die gap were varied systematically. An infrared camera was used to measure the temperature distribution of the plastic part just after mold opening as well as after part ejection. The wall thickness and dimensions of the bottles of the finished parts were measured in order to determine the shrinkage and warpage. Finally, the infrared temperature fingerprints were used to explain what happens during the cooling phase and correlated with the final part characteristics.

A New High-Performance mVLDPE

Metallocene polyethylenes are well known for providing superior toughness, heat sealing, and clarity when compared to the conventional polyethylenes. Their unique attributes have been employed for over a decade in the manufacture of many different types of blown and cast films for numerous end-use applications. A new family of very low density metallocene polyethylenes, defined here as mVLDPE, exhibits outstanding toughness and surprising heat seal and hot tack performance. Both monolayer and coex blown film studies have demonstrated that the unique properties of these mVLDPEs can be used to produce high-performance films for a wide variety of packaging applications. Selected data from these studies are used to demonstrate that the properties of these mVLDPEs are superior to those of conventional very low density polyethylenes (VLDPEs), linear low density polyethylenes (LLDPEs), metallocene linear low density polyethylenes (mLLDPEs), and ethylene vinyl acetate copolymers (EVAs).

An investigation of the relationship between thermal relaxations and the impact performance of rotationally moulded linear low density polyethylenes

The objective of this paper is to provide a better understanding of how thermal relaxations in linear low density polyethylenes are related to the crack initiation energy of rotomoulded parts. Trials were carried out on two Ziegler-Natta catalysed linear low density polyethylenes (LIDPE) and two metallocene catalysed linear low-density polyethylenes (mLLDPE). Instrumented impact tests and dynamic mechanical thermal analysis (DMTA) were carried out on each material at a wide range of temperatures. The frequency of impact at each test temperature was determined, and the DMTA results were shifted to a corresponding frequency. A correlation can be seen between changes in loss modulus and crack initiation energy with temperature. When comparing the crack initiation energy of the samples and the tan Δ values, it can be seen that the β transition affects the way in which the polymer behaves under impact. A method for predicting impact performance over a wide temperature range is proposed.

Electrical charging during the sharkskin instability of a metallocene melt

Flow instabilities are widely studied because of their economical and theoretical interest; however, few results have been published about the polymer electrification during the extrusion. Nevertheless, the generation of the electrical charges is characteristic of the interaction between the polymer melt and the die wall. In our study, the capillary extrusion of a metallocene polyethylene (mPE) through a tungsten carbide die is characterized through accurate electrical measurements thanks a Faraday pail. No significant charges are observed since the extrudate surface remains smooth. However, as soon as the sharkskin distortion appears, measurable charges are collected (around 5 × 10 −8 C/m 2). Higher levels of charges are measured during the spurt or the gross-melt fracture (gmf) defects. This work is focused on the electrical charging during the sharkskin instability. The variation of the electrical charges versus the apparent wall shear stress is investigated for different die geometries. This curve exhibits a linear increase, followed by a sudden growth just before the onset of the spurt instability. This abrupt charging corresponds also to the end of the sharkskin instability. It is also well known that wall slip appears just at the same time, with smaller velocity values than during spurt flow. Our results indicate that electrification could be a signature of the wall slip. We show also that the electrification curves can be shifted according to the time–temperature superposition principle, leading to the conclusion that molecular features of the polymer are also involved in this process.

Rheological evaluation of metallocene polyethylenes with processing aids by multi‐wave oscillations

AbstractDuring the die flow of metallocene polyethylenes, flow instabilities may occur. Namely, wall slip, “sharkskin,” and stick‐slip (pressure oscillations) and gross fracture may be obtained depending on the volume flow rate and die geometry. It was reported that fluoroelastomers and boron nitride powders with hexagonal crystal structure can be used as suitable processing aids in melt extrusion processes. Fluoroelastomers at low concentrations act as die lubricants and may eliminate flow instabilities such as surface and stick‐slip melt fracture. On the other hand, specific boron nitride powders may not only eliminate surface and stick‐slip melt fracture, but also postpone gross melt fracture to higher volume flow rates. In this paper, a way for quantitative differentiation of the influence of polymer processing additives on rheological behavior is shown. Standard material functions show no clear‐cut differences. However, using multi‐wave oscillations with higher strain amplitudes make a quantitative assessment possible. Polym. Eng. Sci. 44:2047–2051, 2004. © 2004 Society of Plastics Engineers.

Organic–inorganic polymeric nanocomposites involving novel titanium tetraisopropylate in polyethylene–octene elastomer

AbstractThe new nanocomposites, by means of an in situ sol–gel process consisting of metallocene polyethylene–octene elastomer (POE) and titanium tetraisopropylate (TTIP), were investigated. In addition, the acrylic acid grafted POE (POE‐g‐AA) was studied as an alternative to POE. Fourier transform infrared (FTIR) spectroscopy, a dynamic mechanical analyzer (DMA) spectrometer, an X‐ray diffractometer (XRD), differential scanning calorimetry (DSC), a thermogravimetric analyzer (TGA), an Instron mechanical tester, and a scanning electron microscope (SEM) were used to characterize and examine the samples. The results indicate that the POE‐g‐AA/TiO2 hybrid could have a positive effect on the properties of the POE/TiO2 hybrid because the carboxylic acid groups of acrylic acid should act as coordination sites for the titania phase to form a TiOC chemical bond. The strength of interfacial bonding between the polymer chains and the ceramic phase depended on the amount of TiO2, as shown by the change in glass‐transition temperature (Tg) with TiO2 content. The result of mechanical and thermal tests showed that both the tensile strength and the Tg increased to a maximum value and then decreased with an increasing of TiO2 because excess particles (e.g., greater than 10 wt % TiO2) might cause separation or segregation between the organic and inorganic phases. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 4272–4280, 2004

Effects of Sparse Long Chain Branching on the Spinning Stability of LLDPEs

Abstract The influence of sparse long chain branching on the onset and propagation of isothermal draw resonance in fiber spinning of polyethylene melts was investigated. Six polyethylene melts were used in this study: three sparsely branched metallocene polyethylenes, a linear low-density metallocene polyethylene, a conventional linear low-density polyethylene, and a conventional low-density polyethylene (LDPE). The sparsely branched metallocene polyethylenes have almost identical shear rheology and molecular weight distributions, but strain harden to different extents under extensional deformation because of slight differences in the amount of sparse long chain branching. Critical draw ratios and the ratios of minimum to maximum diameter were found to be different for each of these polyethylenes. The two linear low-density polyethylenes, which have no long chain branching, had critical draw ratios similar to those of the sparsely branched polyethylenes, but failed (necked to the point of filament breakage) during monofilament extrusion at draw ratios significantly lower than those measured for the sparsely branched polyethylenes. In contrast, the LDPE, which has the highest degree of branching and largest molecular weight distribution, had a much higher critical draw ratio than that obtained for the other five polyethylenes. These results suggest that the degree of extensional strain hardening, arising from differences in long chain branching, has a significant effect on the onset and propagation of draw resonance in isothermal fiber spinning. In the case of LLDPE, broadening the MWD seemed to affect the drawability of LLDPE, but had no effect on the critical draw ratio.