Characterization of melt flow instabilities in polyethylene/carbon nanotube composites

The effect of single (SWCNT) and multi- (MWCNT) walled carbon nanotubes on the melt flow instabilities of polyethylenes with different topologies was characterized by means of a novel capillary rheometer allowing in-situ measurements of the pressure fluctuations inside the die and by scanning electron microscopy (SEM) analysis. Our results show that carbon nanotubes modify the main characteristics of the spurt instability developed by the linear polyethylene. Furthermore, the sharkskin instability, developed in short chain branched polyethylene, is reduced at low amounts of MWCNT and SWCNT. Noteworthy, the critical shear rate for the on-set of the spurt and the sharkskin instabilities decreases in the nanocomposites due to the physical interactions between the polymer and the nanofiller.At high shear rates, the gross melt fracture instability is completely erased in the nanocomposites based on the linear polymer whereas in short chain branched polyethylene the amplitude of this bulk distortion is rather moderated. These changes were confirmed by on-line pressure measurements using the novel capillary rheometer set-up. Based on our results, it is concluded that carbon nanotubes drastically affect the non-linear molecular dynamic leading to polyethylene melt flow instabilities.     

Preparation and characterization of linear low‐density polyethylene/dickite nanocomposites prepared by the direct melt blending of linear low‐density polyethylene with exfoliated dickite

Dickite particles were exfoliated by the thermal decomposition of molecular urea in the interlayer of dickite. The exfoliated dickite (ED) was composed with linear low‐density polyethylene (LLDPE) to prepare a novel LLDPE/dickite nanocomposite (LDN‐5). X‐ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were used to evaluate the exfoliation effect. FTIR spectra showed that the inner‐surface hydroxyls of dickite decreased because of the sufficient exfoliation of the dickite layers. The 001 diffraction of dickite in the XRD pattern almost disappeared after exfoliation; this indicated the random orientation of dickite platelets. SEM and TEM micrographs confirmed the effective thermal decomposition of the interlamellar molecular urea ED layers, which resulted in smaller particle sizes and better dispersions of dickite in the resulting LLDPE/dickite composite. The microstructure of LDN‐5 showed that most of the dickite platelets were exfoliated and homogeneously dispersed in the LLDPE; this led to increases in the anticorrosion properties and thermal stabilities of LDN‐5. The results of salt‐spray tests illustrated that the corrosion rate of the iron coupon decreased from 23% (LLDPE packing) to 0.4% (LDN‐5 packing). Moreover, the thermal degradation temperature corresponding to a mass loss of 10% increased from 330°C (pure LLDPE) to 379°C (LDN‐5). © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

The influence of drawing temperature on mechanical properties and organization of melt spun polyethylene solid-state drawn in the pseudo-affine regime

Mechanical properties of high density polyethylene (HDPE) solid-state drawn with fixed draw ratio at different temperatures in a fiber/tape spin line were investigated. All drawing experiments were performed in the pseudo-affine regime, i.e. no effective relaxation of the molecules occurs during drawing. For such conditions, the Young's modulus is uniquely determined by the applied draw ratio. The general appearance of the stress-strain behavior of drawn HDPE, and in particular its yield strength, however, is strongly influenced by the stretching temperature applied. For a fixed draw ratio, a significant drop in yield stress can be observed with decreasing drawing temperature. Characterization of structure and organization of the solid-state drawn HDPE was performed using various analytical techniques, such as wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC). It is proposed that solid-state drawing at temperatures above the α-relaxation temperature results in relative large crystals so that corresponding tapes show a high yield point. Drawing at low temperatures below the α-relaxation temperature of PE, however, causes formation of small or imperfect crystals that can be destructed at low stress (low yield point), which is a preferable start situation for a second solid-state drawing step in a multiple drawing process.     

Elongational flow properties of low-density polyethylene and linear low-density polyethylene from nonisothermal melt spinning experiments

Low-density polyethylene (LDPE) and also linear low-density polyethylene (LLDPE) resins can be characterized by the degree of strain hardening and down-gaging during elongation. A new method for the determination of the apparent elongational flow characteristics is presented. In a small scale apparatus, a molten monofilament is stretched under nonisothermal conditions similar to those found in tubular film extrusion. Measurement of resistance to elongational flow and apparent elongational strain rates permit the comparison of the process-ability of different resins under specified conditions. The effect of melt temperature and extension ratio are examined. The importance of the molecular structure of both LDPE and LLDPE resins on these properties is also outlined.     

Large scale formation of various highly oriented structures in polyethylene/polycarbonate microfibril blends subjected to secondary melt flow

A strong shear flow was imposed on the melt of polycarbonate (PC) microfibril reinforced high density Polyethylene (HDPE) during a secondary melt flow process, i.e. gas assisted injection molding (GAIM). Classic shish-kebabs and typical transcrystallinity were simultaneously observed in the entire thickness of the GAIM HDPE/PC microfibril composites, which were closely related to the strong shear flow that was further amplified and distributed by incorporated PC microfibrils. Interestingly, some nano-sized ultrafine PC microfibril inclined to absorb extended chain bundles to form shish nuclei on its surface first, which subsequently evolved into hybrid shish-kebab superstructures. It was deemed that the induced formation of hybrid shish-kebab superstructures on nano-sized ultrafine PC microfibril was due to the absorbing of extended chain bundles for hybrid shish nuclei with the strong shear flow serving as the driving force. Importantly, large scale formation of these highly oriented crystalline superstructures can bring significant mechanical reinforcement in GAIM HDPE/PC microfibril composite. For GAIM HDPE/PC microfibril composites, its yield strength is increased by 68% and 66%, compared to the GAIM HDPE parts and the common injection molded (CIM) HDPE/PC composites, respectively; meanwhile, the Young's modulus is enhanced by 253% and 17%, compared to the GAIM HDPE parts and the CIM HDPE/PC composites, respectively.     

Structure of blown film from blends of polyethylene and high melt strength polypropylene

The structure of blown film processed from linear low density polyethylene blended with up to 30 wt[percnt] of a high melt strength polypropylene (hmsPP) was examined using primarily atomic force microscopy and wide angle X-ray scattering. The study focused on two polyethylene resins with the same density: a conventional Ziegler-Natta catalyzed linear low density polyethylene (znPE) and a blend of a Ziegler-Natta catalyzed and a metallocene catalyzed linear low density polyethylene (zn/mPE). Parallel characterization was performed on blown film of the hmsPP and blown film of each of the polyethylene resins. In films of the blends, the hmsPP was well-dispersed in the polyethylene matrix as elongated domains. In the domains, the hmsPP crystallized as planar row-nucleated structures with the long axis of the lamellae perpendicular to the extrusion direction. Row-nucleated hmsPP lamellae provided a template for epitaxial crystallization of polyethylene lamellae. The 42° angle of the lattice match imparted a characteristic herringbone texture to the polyethylene. Blending with hmsPP increased the tensile modulus and strength of polyethylene film without significantly affecting the ultimate elongation.     

A molar mass induced transition in the yielding properties of linear polyethylene

The mechanism of tensile deformation in a semi-crystalline polymer like PE changes at four critical points which can be associated with (A) the onset of single slip-processes, (B) a turnover into a collective activity of slips, (C) the beginning of a disaggregation of the crystal blocks, followed by fibril formation, and (D) the onset of chain disentangling. Studies of the deformation behavior of series of LPEs with increasing molar mass show an abrupt rise of the critical strains ε_H(C) and ε_H(D) at a molar mass of M_t≈(1 5-2)×10⁵ g mol⁻¹. As indicated by dynamic mechanical tests the rise originates from a change in the mobility and compliance of the amorphous regions, which are melt-like for M_w>M_t, but reduced for M_w<M_t. As a consequence, both, crystal block disaggregation and chain disentangling, set in at lower strains.     

Molecular weight dependence of melt crystallization behavior and crystal morphology of low molecular weight linear polyethylene fractions

Melt crystallization behavior and corresponding crystal morphology of five low molecular weight (3,900 ≤ MW ≤ 20,800) linear polyethylene (PE) fractions have been investigated. The overall crystallization data indicate that the lower molecular weight (MW) fraction possesses a higher crystallization rate at the same undercooling (ΔT). On the contrary, at the same crystallization temperature (T_c) the rate increases with MW. The Avrami exponent (n) varies from ca. 3 to 4 with decreasing ΔT for the fractions studied, which implies the nucleation process changes from athermal type to thermal type as T_c increases. For the low MW PE’s, the different crystal growth regimes (regime I and II) have been first time identified via linear crystal growth rate (G) measurements. The regime I/II transition temperatures are close to previously reported data, which were obtained through a different method. As reported for intermediate MW PE’s, the transitions occur at an almost constant ΔT of 17.5±1 °C for each fraction studied. Morphological study shows that single crystals could be formed isothermally at low ΔT’s. Typical banded spherulites and axialites, which are MW and ΔT dependent, are also observed. Orthorhombic structure is ascertained to be the dominant crystal structure that exists irrespective of MW and crystal growth regime.     

Anomalous rheological behavior of polyethylene melts in the gross melt fracture regime in the capillary extrusion

During the capillary extrusion with several different polyethylenes, we observe an abnormal rheological behavior. The nominal viscosity of polyethylene melt in the gross melt fracture regime does not change with a temperature. All polyethylenes tested show same behaviors. More interestingly, the nominal viscosity in the gross melt fracture regime shows even no molecular weight dependency when PEs have similar molecular structures (degree of branching and co-monomer content). From various experiments, we conclude that this abnormal phenomenon is relevant to the structural change with the melt temperature.     

Three-dimensional shape of polyethylene single crystals grown from dilute solutions and from the melt

Three-dimensional morphology of polyethylene single crystals grown from dilute solutions and from the melt has been examined by atomic force microscopy. The observation of single crystals clarifies the morphology of chair-type as well as hollow pyramidal type for solution crystallization. From the melt, only chair-type was obtained. It has been confirmed that the screw dislocations in the chair-type follow a selection rule of the handedness in a manner to relieve the distortion in the chair-type. The meaning of the selection is discussed in connection with the twisting correlation in the banded spherulites grown from the melt of non-chiral polymers, such as polyethylene.     

Electrical conductivity and self-temperature-control heating properties of carbon nanotubes filled polyethylene films

Electrical properties of polyethylene and carbon nanotube composite films were investigated, when the composite films were set in heating box or under electric field at constant voltage. The composite films were prepared by gelation/crystallization from dilute solution. The mixture of ultra-high molecular weight polyethylene (UHMWPE) and branched low molecular weight polyethylene (LMWPE) was used as matrix, and multi-walled carbon nanotubes (MWNTs) were used as fillers. The filler content was chosen to be 10 wt% (ca. 5.25 vol%) which is a relatively higher loading than the percolation threshold to ensure to act as heating element in plane heater of composite film. The focus was concentrated on the temperature dependences of electric conductivity by external heating and by exothermic effect concerning self-temperature-control heating properties which were measured for the three kinds of UHMWPE-LMWPE composites with the same content of MWNTs in the composites. When a certain voltage was applied to the composite, the surface temperature of film reaches the equilibrium value within less than 100 s. The maximum surface temperature as the equilibrium state of the resultant composite film can be easily controlled by adjusting the composite ratio represented as UHMWPE/LMWPE. The high efficiency of heating and wide adjustability of stable temperature suggested its good application in high efficient plane heater.     

Prediction and quality control of the melt index during production of high-density polyethylene

In polyolefin processes the melt index (MI) is the most important control variable indicating product quality. Because of the difficulty in the on-line measurement of MI, a lot of MI estimation and correlation methods have been proposed. In this work a new dynamic MI estimation scheme is developed based on system identification techniques. The empirical MI estimation equation proposed in the present study is derived from the 1 ^st -order dynamic models. Effectiveness of the present estimation scheme was illustrated by numerical simulations based on plant operation data including grade change operations in high density polyethylene (HDPE) processes. From the comparisons with other estimation methods it was found that the proposed estimation scheme showed better performance in MI predictions. The virtual sensor model developed based on the estimation scheme was combined with the virtual on-line analyzer (VOA) to give a quality control system to be implemented in the actual HDPE plant. From the application of the present control system, significant reduction of transition time and the amount of off-spec during grade changes was achieved     

Evidence for isothermal lamellar thickening at and behind the growth front as polyethylene crystallizes from the melt

The poorly characterized and little understood phenomenon of isothermal lamellar thickening, central to melt crystallization, has been studied morphologically in polyethylene rows, grown around high-melting fibres as linear nuclei revealing that thickening is a function of position within the morphology as well as of elapsed time. In contrast to polyethylene spherulites whose central lamellae are the thickest, in rows the first lamellae to form remain the thinnest because, being close-packed, they have no space into which to thicken. The thickness of lamellae at the growth front increases linearly with the logarithm of elapsed time but, as the thickest lamellae are found at finite radius, thickening must also occur behind the growth front. The data are consistent with a uniform rate of thickening throughout spherulitic polyethylene but melt crystallization must now be envisaged as occurring not at an interface in steady-state condition but at one whose thickness increases asymptotically and where interference will reduce thickening.     

On the time for fold surfaces to order during the crystallization of polyethylene from the melt and its dependence on molecular parameters

New experiments underpin the interpretation of the basic division in crystallization behaviour of polyethylene in terms of whether or not there is time for the fold surface to order before the next molecular layer is added at the growth front. For typical growth rates, in Régime II, polyethylene lamellae form with disordered {001} fold surfaces then transform, with lamellar thickening and twisting, towards the more-ordered condition found for slower crystallization in Régime I, in which lamellae form with and retain {201} fold surfaces. Several linear and linear-low-density polyethylenes have been used to show that, for the same polymer crystallized alone or in a blend, the growth rate at which the change in initial lamellar condition occurs is reasonably constant thereby supporting the concept of a specific time for surfaces to attain the ordered {201} state. This specific time, in the range from milliseconds to seconds, increases with molecular length, and in linear-low-density polymer, for higher branch contents.     

Influence of the die configuration on the gross melt fracture of linear polyethylene

In this study, capillary extrusion experiments and rupture visualization experiments were carried out with a combined die configuration where two different dies were attached in a capillary rheometer. We observed that the gross melt fracture (GMF) and rupture of the melt occurred simultaneously when the diameter of the die located at the upstream position was larger than that of the die located downstream. However, when the location of the dies were interchanged, that is, the diameter of the upstream die was smaller than that of the downstream die, the upstream rupture did not accompany GMF up to a certain extent of shear rate. From these observations, we present a new theory on the origin of GMF. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Effect of mixing on melt flow properties of high-density polyethylene

An investigation has been made into the effect of mixing by a screw extruder upon the molecular characteristics and the melt flow properties of high-density polyethylene. The crosslinking between molecules predominates over the scission of a molecule at an early stage of mixing, but with further increase in the degree of mixing, this situation reverses itself to bring about the formation of branched polymers. Static flow and dynamic viscoelastic properties of molten high-density polyethylene change considerably with increase in the degree of mixing. The apparent viscosity drops rather sharply after mixing. The dynamic properties for the original resin show smaller frequency dependence of the viscosity and larger dependence of the rigidity than those for extrudates. The relaxation spectra become broader with increase in mixing. These may mainly be due to the change of the molecular characteristics of high-density polyethylene with mixing by an extruder.     

Transient flow-induced crystallization of a polyethylene melt

Extensional, flow-induced crystallization (FIC) of a high-density polyethylene (HDPE) melt has been studied using a four-roll mill flow cell. Simultaneous measurement of the birefringence and scattering dichroism are used to quantify the crystallization process during and following transient flow deformation in planar extensional flow. Suspension of the HDPE phase as a droplet in a linear low-density polyethylene carrier phase prevents die blockage on crystallization and allows measurement of the flow kinematics. Initial crystallization rates following a transient flow deformation show a stress-strain dependence. Crystallization induction times during flow correlate with the extension rate during the transient flow deformation. Measurement of the HDPE melt steady and oscillatory flow rheological properties, along with measurements of time constants following step-shear and extensional strains, allow determination of the viscoelastic properties which enhance FIC. Parameters obtained from these experiments are used in a phenomenological model for FIC which allows qualitative and semiquantitative analysis of the data trends, particularly the relaxation behavior of the birefringence during flow cessation/crystallization. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 2165–2176, 1997     

On the influence of molecular weight and crystallisation condition on the development of defect in highly drawn polyethylene

Longitudinal morphology of highly drawn high density polyethylenes has been investigated. Three grades of HDPEs with different molecular weights were used. Two different initial morphologies were obtained by quenching and slow cooling from the melt. The samples were drawn at 75 °C close to their breaking points and their longitudinal morphologies examined under SEM after etching. Two types of defects were observed. These are the structure containing a large number of parallel longitudinal etched pockets laid in the regions between almost parallel continuous transverse bands or ‘Pisa’ structure recently reported and transverse cracks. The appearance of these defects was found to correlate with molecular weight and thermal history of the samples. For all quenched samples, both Pisa structure and transverse cracks were observed. The number of transverse cracks was found to increase with increasing molecular weight. Apparently, the transverse cracks, which thought to limit the drawing to high draw ratios, can be retarded by drawing at higher temperature (100 °C). This suggests that the formation of transverse cracks relates to chain mobility and drawability of the materials. The Pisa structure was found to disappear from slowly cooled low molecular weight samples. For slowly cooled high molecular weight sample, the Pisa structure became less salient. It is suggested that the formation of Pisa structure is determined by molecular entanglement. Analysis of the band separation of the Pisa structure shows that there seems to be a characteristic value which depends on drawing temperature.     

The relation between melt flow properties and molecular weight of polyethylene

The non-Newtonian behavior of commercial linear polyethylene samples and their fractions were studied at 190°C. The viscosity η versus shear rate \documentclass{article}\pagestyle{empty}\begin{document}$ \dot \gamma $\end{document} curves of whole polymers could be superimposed onto a single master curve despite the variations of their molecular weights and molecular weight distributions. For fractions, however, the same master curve was inapplicable, and the sensitivity of the viscosity to shear rate was found to be greater than those of the whole polymers. The zero-shear viscosities η₀ of fractions were related to the 3.42 power of the weight-average molecular weight M_u as follows: For whole polymers, the zero-shear viscosities were found to be considerably higher at the same M_w and markedly lower at the same z-average molecular weight M_z than those of the fractions. Thus, it was concluded that η₀ corresponds to an average of molecular weight between M_w and M_z. It was found that the molecular relaxation time τ is proportional to M_z^5.3 for whole polymers and to η₀M_w for fractions. Using these relations it was possible to relate the flow ratio, the ratio of flow rates at two different shear stresses, with the molecular weight distribution.     

Characteristics of slug flow in a fluidized bed of polyethylene particles

The slug flow behavior of polyethylene particles was examined in a fluidized bed of 7 cm ID and 50 cm in height. The employed polymer particles were high density polyethylene (HDPE) with the average particle size of 603 μm. The slugging flow of polyethylene particles was analyzed from the measured pressure drop signals by classical statistical methods such as absolute average deviation, probability density function, power spectrum, auto-cor-relation, and cross-correlation. The results show that in spite of high dielectric constant of polymer particles, the slugging phenomena such as incipient slugging velocity, slugging frequency and slugging rise velocity were very similar to the Geldart B type non-polymeric particles. It was observed that slug frequencies decreased with gas velocity and the limiting slug frequency was observed for the gas velocities in this study.