Correlation between entry pressure losses and elongation viscosity of polyethylene melts

The correlation between the entry pressure drop and elongation viscosity during entry converging flow of polymer melts was discussed in this article. The entry pressure drop during extrusion of a low density polyethylene (LDPE) melt and a linear low density polyethylene (LLDPE) melt was measured by means of a capillary rheometer under test conditions with temperature of 170 °C and shear rate varying from 10 to 300 s⁻¹. The results showed that the entry pressure drop increased nonlinearly with an increase of the shear stain rate, and the variation of entry pressure drop of the two melts was close to each other. The melt elongation viscosity of the two resins was estimated using Cogswell equation from the measured entry pressure drop data, and the predictions were compared with the melt extension viscosity measured by using a melt spinning technique published in literature. It was found that the melt extension viscosity from entry converging flow was slightly lower than that from melt spinning technique under the same temperature and extension strain rate.     

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     

Enhanced flow behaviors of metallocene‐catalyzed linear low‐density polyethylene during ultrasound‐assisted extrusion

The effect of ultrasound on flow behaviors of metallocene‐catalyzed linear low‐density polyethylene (mLLDPE) melt in capillary‐like die during the extrusion is investigated in this article. The rise in die temperature is found with increasing ultrasound power, especially at lower initial die temperature. At the same die temperature, the presence of ultrasound can decrease the apparent viscosity and the viscous flow activation energy of mLLDPE melt then increase its slip velocity at the capillary wall in the die. The flow behavior of mLLDPE melt is enhanced during ultrasound‐assisted extrusion as the presence of ultrasound can enhance the mobility and the orientation of entangled segments. It is also found that ultrasound can break the dispersed phase of mLLDPE/polyolefin elastomer (POE) blend into small pieces thus improve the homogeneous dispersion of POE phase in mLLDPE matrix. A possible mechanism for enhanced flow behaviors of mLLDPE melt because of the presence of ultrasound is also proposed. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers     

Synthesis and characterization of melt‐processable polyimides derived from 1,4‐bis(4‐amino‐2‐trifluoromethylphenoxy)benzene

A series of molecular‐weight‐controlled imide resins end‐capped with phenylethynyl groups were prepared through the polycondensation of a mixture of 1,4‐bis(4‐amino‐2‐trifluoromethylphenoxy)benzene and 1,3‐bis(4‐aminophenoxy)benzene with 4,4′‐oxydiphthalic anhydride in the presence of 4‐phenylethynylphthalic anhydride as an end‐capping agent. The effects of the resin chemical structures and molecular weights on their melt processability and thermal properties were systematically investigated. The experimental results demonstrated that the molecular‐weight‐controlled imide resins exhibited not only meltability and melt stability but also low melt viscosity and high fluidability at temperatures lower than 280°C. The molecular‐weight‐controlled imide resins could be thermally cured at 371°C to yield thermoset polyimides by polymer chain extension and crosslinking. The neat thermoset polyimides showed excellent thermal stability, with an initial thermal decomposition temperature of more than 500°C and high glass‐transition temperatures greater than 290°C, and good mechanical properties, with flexural strengths in the range of 140.1–163.6 MPa, flexural moduli of 3.0–3.6 GPa, tensile strengths of 60.7–93.8 MPa, and elongations at break as high as 14.7%. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

Experimental investigation of the rheological behaviors of polypropylene in a capillary flow

The rheological characterization of polymer melts is strongly related to their material properties. In this study, we focused on the rheological behaviors of a polypropylene (PP) melt through a capillary die. With an advanced twin‐bore capillary rheometer with dies measuring 1.0, 0.5, and 0.25 mm in diameter, experiments were performed over a shear‐rate range of 3 × 10² to 5 × 10³ s^?1 at three temperatures, 210, 220, and 230 °C. The results demonstrate that the geometry dependence of the PP viscosity relied on the die diameter and the temperature of the PP melt. The viscosity values of the PP melt in the 0.25‐mm diameter die were higher than were those in the 0.5‐ and 1.0‐mm dies at 220 and 230 °C. However, the viscosity values in all of the tested dies were similar at 210 °C. The tendency for the viscosity to decrease as the temperature of the polymer melt increased weakened in the 0.25‐mm diameter die. As a result, the pressure applied to the PP melt in the 0.25‐mm diameter die increased; this caused a decrease in the free volume between molecules. On the basis of the Barus equation, the contribution of pressure to the changed viscosity in each die at each of the tested temperatures was calculated and was found to be as high as 32.86% in the 0.25‐mm die at 230 °C. Additionally, the effect of the wall slip on the geometry dependence of the PP viscosity in the tested dies was investigated with a modified Mooney method. The values of the slip velocity revealed that wall slip occurred only in the 0.25‐mm die at 210 °C. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43459.     

Relationships between processing conditions and rheological behavior of polyethylenes

The rheological properties of high‐density polyethylene melts were found to change drastically after treatment with oxygen or peroxide. Unusual features of the treated melts in shear flow (190°C) included (a) increase in length of time to reach steady state values of shear stress in start‐up experiments; (b) a non‐reproducibility of the low‐shear rate sections of the flow curves measured at increasing and decreasing shear rate; (c) an increase of viscosity at low shear rates compared to the neat sample. Under non‐stationary extensional flow (a regime of constant force) the treatment leads to a change in shape of the strain development with time, an increase of the apparent elongational viscosity, and an increase in time to break. At 150–170°C, the rheological behavior of the treated polyethylenes is completely identical to the corresponding behavior of the untreated. These results, together with data from IR‐spectroscopy and GPC suggest the following mechanism: The oxidation or peroxidation leads to reactive sites in the polymer chain that incorporate a few long branches during the initial contact with oxygen or peroxide. These reactive sites remain in the polymer after cooling/solidification and can become activated again upon heating to 190°C causing additional changes in molecular structure. Formation of the long‐chain branches results in an increased resistance of the melt to extensional deformation, and an improvement in processing behavior, as well as the quality of bottles produced by the blow‐molding process. Polym. Eng. Sci. 44:615–624, 2004. © 2004 Society of Plastics Engineers.     

Surface Energy of Modified Nanoclays and Its Effect on Polymer/Clay Nanocomposites

Surface properties of thermally stable phosphonium-modified montmorillonite were investigated at both room temperature and 220°C. These properties were compared with those of pristine and ammonium-modified montmorillonite. Surface properties at room temperature were calculated from contact angles measured using sessile drops. Several liquids with known polar and dispersion components of surface tension were used. Surface energy of nanofillers at 220°C was calculated from contact angles, using sessile drops of polymer melts. Two commercial polystyrene (PS) resins, with different melt flow characteristics, and high-density polyethylene (HDPE) were used. Isothermal TGA experiments were used to determine the thermal stability of the resins and nanofillers. The dispersion behavior and mechanical properties of the nanocomposites are correlated with the values of the Hamaker constant and thermodynamic work of adhesion for these polymer–nanoclay systems.     

Measurement and prediction of LDPE/CO2 solution viscosity

When CO₂ is dissolved into a polymer, the viscosity of the polymer is drastically reduced. In this paper, the melt viscosities of low‐density polyethylene (LDPE)/supercritical CO₂ solutions were measured with a capillary rheometer equipped at a foaming extruder, where CO₂ was injected into a middle of its barrel and dissolved into the molten LDPE. The viscosity measurements were performed by varying the content of CO₂ in the range of 0 to 5.0 wt% and temperature in the range of 150°C to 175°C, while monitoring the dissolved CO₂ concentration on‐line by Near Infrared spectroscopy. Pressures in the capillary tube were maintained higher than an equilibrium saturation pressure so as to prevent foaming in the tube and to realize single‐phase polymer/CO₂ solutions. By measuring the pressure drop and flow rate of polymer running through the tube, the melt viscosities were calculated. The experimental results indicated that the viscosity of LDPE/CO₂ solution was reduced to 30% of the neat polymer by dissolving CO₂ up to 5.0 wt% at temperature 150°C. A mathematical model was proposed to predict viscosity reduction owing to CO₂ dissolution. The model was developed by combining the Cross‐Carreau model with Doolittle's equation in terms of the free volume concept. With the Sanchez‐Lacombe equation of state and the solubility data measured by a magnetic suspension balance, the free volume fractions of LDPE/CO₂ solutions were calculated to accommodate the effects of temperature, pressure and CO₂ content. The developed model can successfully predict the viscosity of LDPE/CO₂ solutions from PVT data of the neat polymer and CO₂ solubility data.     

Rheological hybrid effect in dually filled polycarbonate melt containing liquid crystalline polymer

Polycarbonate (PC)/liquid crystalline polymer (LCP) blends dually filled with glass fiber and nano‐SiO₂ were prepared by melt blending, with the use of a commercial Vectra A130 as the source of LCP and glass fiber. In these dually filled PC/LCP melts, rheological hybrid effect occurred, confirmed by the melt viscosity of the quadruple polymer blends decreased with increasing nano‐silica loading, influenced by the minor LCP phase in the blend. The drastic viscosity reduction closely correlates with the deformation and fibrillation of LCP droplets in the system. The LCP fibrillation was controlled jointly by the thermodynamic and hydrodynamic driving forces. Finally, the dually filled PC/LCP melt had decreased viscosity lower than those of pure PC, silica‐filled PC, and PC/Vectra A130 blends, and furthermore had decreased glass fiber breakage, shown by larger average aspect ratio than that in PC/Vectra A130 blends. POLYM. ENG. SCI., 2012. © 2011 Society of Plastics Engineers     

Melt flow behavior in capillary extrusion of nanosized calcium carbonate‐filled poly(L‐lactic acid) biocomposites

Nanosized calcium carbonate (nano‐CaCO₃)‐filled poly‐L ‐lactide (PLLA) biocomposites were compounded by using a twin‐screw extruder. The melt flow behavior of the composites, including their entry pressure drop, melt shear flow curves, and melt shear viscosity were measured through a capillary rheometer operated at a temperature range of 170–200°C and shear rates of 50–10³ s^?1. The entry pressure drop showed a nonlinear increase with increasing shear stress and reached a minimum for the filler weight fraction of 2% owing to the “bearing effect” of the nanometer particles in the polymer matrix melt. The melt shear flow roughly followed the power law, while the effect of temperature on the melt shear viscosity was estimated by using the Arrhenius equation. Hence, adding a small amount of nano‐CaCO₃ into the PLLA could improve the melt flow behavior of the composite. POLYM. ENG. SCI., 52:1839–1844, 2012. © 2012 Society of Plastics Engineers     

Ultrasonic oscillations effect on rheological and processing properties of metallocene‐catalyzed linear low density polyethylene

The effects of ultrasonic oscillations and die materials on die pressure, productivity of extrusion, melt viscosity of metallocene‐catalyzed linear low density polyethylene (mLLDPE), as well as their mechanism were studied in a special ultrasonic oscillations extrusion system developed in our lab. Die materials used in our experiment included steel, brass, and polytetrafluoroethylene (PTFE)_. The experimental results showed that ultrasonic oscillations as well as die materials have great influence on the rheological and processing behavior of mLLDPE. Ultrasonic oscillations can greatly increase the productivity of mLLDPE melt extruded through different dies, and can decrease the die pressure and the melt viscosity of mLLDPE. Compared with steel or brass die, mLLDPE melt extruded through PTFE die is more sensitive to ultrasonic oscillations. A possible mechanism for the improved processability of mLLDPE is proposed in this article. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 1873–1878, 2003     

Melt flow indexer evidence of high‐temperature transitions in molten high‐density polyethylenes

A melt flow indexer (MFI) was used to investigate high‐temperature transitions in melts of high‐density polyethylene (HDPE). The MFI data were obtained in the range 190–230°C. These transitions were found in the MFI at about 210 and 225°C and reproduced in a Haake melt blender. Polystyrene was used in the blender experiment to demonstrate typical amorphous behavior. For HDPE melts, the MFI–temperature behavior and the torque–temperature data of the blender were found to be alternative images of the same anomalous temperature dependency in the range 210–225°C. Also, the Haake melt blender was able to reproduce the 150°C transition observed by Kolnaar and Keller in the extrusion of HDPE. Regardless of the simplicity of the MFI device, results are in agreement with our previous DSC findings. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 1309–1313, 2004     

Comparisons of the rheological behaviors of polypropylene and polymethyl methacrylate in a capillary die

In this study, the rheological characteristics of polypropylene (PP) melt at 210, 220, and 230 °C and polymethyl methacrylate (PMMA) melt at 230, 240, and 250 °C in a micro die were investigated. The experiments were performed over a shear rate range of 3 × 10² to 5 × 10³ s^?1 using an advanced twin‐bore capillary rheometer. Dies with diameters of 1.0, 0.5, and 0.25 mm were used. The results indicated that the geometric dependences of the PP and PMMA viscosities were not identical at different shear rates and temperatures and that the micro size effect had a profound influence on the PP viscosity. The analysis demonstrated that the variations in the shear viscosity of the PP and PMMA melts in the micro die were partially attributed to the contribution of the pressure applied to the polymer melts. Additionally, the effect of wall slip on the PP and PMMA viscosities in the tested dies was investigated based on the modified Mooney method. The results implied that wall slip easily occurred in the PP melt flowing through the 0.25 mm die at 210 °C due to the distinct size effect. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134 , 44617.     

Influence of rheological properties on the sagging of polypropylene and abs sheet for thermoforming applications

The isothermal sagging resistance of different grades of conventional and a high melt strength (HMS) PP has been correlated with the rheological characteristics of the polymers, such as dynamic shear properties, melt strength, and zero shear viscosity. A thermoforming grade of acrylonitrile‐butadiene‐styrene (ABS) was used as a reference material. At 190°C, ABS had the highest viscosity and elastic modulus in the frequency range measured, showing that this polymer is highly elastic. HMS PP had a greater shear thinning behavior than conventional PP because of its broader molecular weight distribution. The tan δ of the polymers showed that conventional PP had a higher tendency to flow than HMS PP and ABS when heated above 172°C. This was confirmed with sagging experiments performed in an air circulating oven, where the rate of sagging decreased as the melt strength and the zero shear viscosity of the polymer increased.     

Poly(trimethylene terephthalate)/magnesium hydroxide composites with on‐demand thickening at high temperatures

Magnesium hydroxide, when melt extruded with poly(trimethylene terephthalate) (PTT) at low concentrations (1–3 wt.%), can be melt processed during extrusion and injection molding. When the polymer is heated in the melt to 295°C, the viscosity of the composite behaves similarly to control samples and the viscosity decreases as a function of temperature. When the temperature of the composite is raised above 295°C (above the normal processing conditions for PTT), a significant change in the melt rheology of the composite is observed relative to unfilled compositions. This change in melt rheology coincides with the decomposition temperature of magnesium hydroxide and formation of magnesium oxide, a common thickening agent employed in unsaturated polyester resins. Lower processing temperature polyesters, including PTT, enable processing of the polymer in the presence of magnesium hydroxide under normal conditions. The viscosity modifier ‘turns on’ when the composite sees elevated temperature where magnesium oxide is known to form. The magnesium oxide is hypothesized to interact with PTT carboxyl end groups, providing the observed increase in viscosity. The rheological response observed in the composites is dependent on the concentration of magnesium hydroxide. During burning, the viscosity modifier results in a non‐dripping formulation of PTT. Copyright © 2012 John Wiley & Sons, Ltd.     

Geometrical dependence of viscosity of polymethylmethacrylate melt in capillary flow

The shear viscosity of polymethylmethacrylate (PMMA) melt is particularly investigated by using a twin‐bore capillary rheometer at four temperatures of 210, 225, 240, and 255°C with different capillary dies. Experimental results show that the geometrical dependence of shear viscosity is significantly dependent on melt pressure as well as melt temperature. The measured shear viscosity increases with the decrease of die diameter at lower temperatures (210 and 225°C) but decreases with the decrease of die diameter at higher temperatures (240 and 255°C). Based on the deviation of shear viscosity curves and Mooney method, negative slip velocity is obtained at low temperatures and positive slip velocity is obtained at high temperatures, respectively. Geometrical dependence and pressure sensitivity of shear viscosity as well as temperature effect are emphasized for this viscosity deviation. Moreover, shear viscosity curve at 210°C deviates from the power law model above a critical pressure and then becomes less thinning. Mechanisms of the negative slip velocity at low temperatures are explored through Doolittle viscosity model and Barus equation, in which the pressure drop is used to obtain the pressure coefficient by curve fitting. Dependence of pressure coefficient on melt temperature suggests that the pressure sensitivity of shear viscosity is significantly affected by temperature. Geometrical dependence of shear viscosity can be somewhat weakened by increasing melt temperature. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 3384–3394, 2013     

Effect of diatomite/polyethylene glycol binary processing aids on the rheology of a metallocene linear low‐density polyethylene

Flow performance of metallocene linear low‐density polyethylene (mLLDPE) containing small amounts of polyethylene glycol (PEG) diatomite and diatomite/PEG binary processing aids respectively was investigated. The mLLDPE melt viscosity is increased by the addition of diatomite, but is decreased by addition of PEG or the diatomite/PEG binary processing aids. It was also found that the viscosity reduction of mLLDPE with the addition of diatomite/PEG binary processing aid was significantly greater than that obtained with the addition of only PEG. The flow curves of mLLDPE containing diatomite/PEG binary processing aid show extremely lower value and stronger dependence on shear rate than the others. It is suggested that the rheological improvement of mLLDPE with diatomite/PEG binary processing aids resulted not entirely from the wall slip promoted by PEG; the intrinsic structure may have changed under the application of shear flow. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 1546–1552, 2004     

Kinetic modeling of esterification of cardanol‐based epoxy resin in the presence of triphenylphosphine for producing vinyl ester resin: Mechanistic rate equation

In this study, cardanol‐based epoxidized novolac resins and methacrylic acid were used to produce cardanol‐based epoxidised novolac vinyl ester resins. The reactions were conducted under nonstoichiometric condition using triphenylphosphine as catalyst in the temperature range of 80–100°C with an interval of 5°C. The first‐order rate equation and mechanism based rate equation were examined. Parameters were evaluated by least square method. A comparison of mechnism based rate equation and experimental data showed an excellent agreement. Finally, Arrhenius equation and activation energy were presented. The specific rate constants, based on linear regression analysis, were found to obey Arrhenius equation. The values of activation energy, frequency factor, enthalpy, entropy, and free energy of the reaction revealed that the reaction was spontaneous and irreversible and produced a highly activated complex. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Synthesis of novolac‐type phenolic resins using glucose as the substitute for formaldehyde

Novel Novolac type phenolic resins were prepared using glucose as the substitute for toxic formaldehyde (a carcinogenic chemical). The resins were synthesized with varying molar ratios of phenol to glucose, catalyzed by strong acid (such as sulfuric acid) at 120–150°C. Analysis of the resins using gel permeation chromatography (GPC) and proton nuclear magnetic resonance (¹H‐NMR) showed that they were broadly distributed oligomers derived from the Fridel‐Crafts condensation of phenol and glucose. Using hexamethylenetetramine (HMTA) as the curing agent, the phenol‐glucose resins could be thermally cured and exhibited exothermic peaks at 130–180°C, typical of thermosetting phenolic resins. The cured resins showed satisfactory thermal stability, e.g., they started to decompose at >280°C with residual carbon yields of above 58% at 600°C. Based on the thermal properties, phenol‐glucose resin with a molar ratio of 1 : 0.5 is promising as it could be cured at a lower temperature (147°C) and exhibited a satisfactorily good thermal stability: it started to decompose at >300°C with a residual carbon yield of >64% at 600°C. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Effect of dynamic cross‐linking on melt rheological properties of polypropylene/ethylene‐propylene‐diene rubber blends

Study of melts rheological properties of unvulcanized and dynamically vulcanized polypropylene (PP)/ethylene‐propylene‐diene rubber (EPDM) blends, at blending ratios 10–40 wt %, EPDM, are reported. Blends were prepared by melt mixing in an internal mixer at 190°C and rheological parameters have been evaluated at 220°C by single screw capillary rheometer. Vulcanization was performed with dimethylol phenolic resin. The effects of (i) blend composition; (ii) shear rate or shear stress on melt viscosity; (iii) shear sensitivity and flow characteristics at processing shear; (iv) melt elasticity of the extrudate; and (v) dynamic cross‐linking effect on the processing characteristics of the blends were studied. The melt viscosity increases with increasing EPDM concentration and decreased with increasing intensity of the shear mixing for all compositions. In comparison to the unvulcanized blends, dynamically vulcanized blends display highly pseudoplastic behavior provides unique processing characteristics that enable to perform well in both injection molding and extusion. The high viscosity at low shear rate provides the integrity of the extrudate during extrusion, and the low viscosity at high shear rate enables low injection pressure and less injection time. The low die‐swell characteristics of vulcanizate blends also give high precision for dimensional control during extrusion. The property differences for vulcanizate blends have also been explained in the light of differences in the morphology developed. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 1488–1505, 2000