Improved processing behaviors and mechanical properties of polyolefin elastomer blends prepared by ultrasound‐assisted extrusion

To improve the processability of ethylene‐α‐olefin copolymers (POE), POE and POE/polystyrene (PS) blends were extruded in the presence of ultrasound. On the one hand, the effect of ultrasound on the die pressure drop, extrudate productivity, melt viscosity of POE, and the processing behaviors of POE and POE/PS (80/20) blend were studied. The results showed that with increasing ultrasound power, the die pressure and melt‐apparent viscosity of POE decreased whereas the productivity of POE extrudates increased, then the processability of POE was greatly improved. On the other hand, the effects of ultrasound on the morphology, rheological, and mechanical properties of POE/PS (80/20) blend were studied. Capillary rheological results showed that the merger of ultrasound and the addition of PS showed a synergistic improvement of processability of POE. From morphological observation and rheological analysis, the compatibility of the blend was also improved in the presence of 200W ultrasound. As a result, the stress at break of compatibilized POE/PS (80/20) blend increased from 9.2 to 11.0 MPa, and the dynamic storage modulus increased at experimental temperature range, indicating that the mechanical properties of POE/PS blends can be improved by ultrasound‐assisted extrusion. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

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

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

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     

An in‐process ultrasonic approach to investigating the relaxation of orientation and disorientation of polymer melts

The orientation and relaxation behaviors of a low‐density polyethylene melt and polypropylene melts with different melt indices undergoing a shear flow in a restricted channel were investigated by using ultrasound. A capillary rheometer was used to force the polymer melt through a slit die equipped with pressure, temperature, and ultrasound sensors, and the variation of ultrasound velocity traversing the melt was measured. Experimental results revealed that due to different mechanisms involved, the relaxations of orientation and disorientation processes show different dependences of ultrasound velocity on shear rate, temperature, and melt index. POLYM. ENG. SCI., 2008. Published 2008 Society of Plastics Engineers     

Prediction of elongation viscosity of polymer melts

Melt extension flow is a common flow pattern during polymer processing, such as entrance converging flow in die extrusion or runner injection of polymer melts from an extruder barrel, blow molding, blowing film and melt spinning. Extensional viscosity is one of the important characterizations of the flow characteristics for polymer fluids. A new extension viscosity equation was established based on White‐Metzner model, Vinogradov‐Malkin viscosity equation and a new relaxation time equation in the present paper. The melt elongation viscosities of metallocene linear low‐density polyethylene (mLLDPE) and polyvinyl butyral (PVB) resins at 130°C were estimated applying this viscosity equation, and the predictions were compared with the measured data of mLLDPE and PVB resins at 130°C reported from reference. The results showed that calculations were close to the experimental data. The parameters in this equation were easy to be determined and the equation was convenient to use for estimating the extension viscosity of polymer melts. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers     

Effects of ultrasonic oscillations on rheological behavior and mechanical properties of novel propylene‐based plastomers

The effects of ultrasonic oscillations on the die pressure, productivity of extrusion, melt apparent viscosity, melt surface appearance, and die swell of novel propylene‐based plastomers were studied in a specially designed ultrasonic oscillations extrusion system developed in our laboratory. The effects of ultrasonic oscillations on molecular weights, tensile strength, and dynamic mechanical properties of extrudates were also studied. The experimental results showed that the presence of ultrasonic oscillations during extrusion could significantly increase the productivity of plastomers at the same die pressure, and reduce die swell and melt fracture such as sharkskin at a given screw rotation speed. The die pressure and apparent viscosity of plastomers remarkably decreased with increasing ultrasonic intensity. Introduction of ultrasonic oscillations into plastomer melts can improve their processibility. The possible mechanism for ultrasonic improvement of rheological behavior was also proposed in this article. Under certain conditions, ultrasound‐assisted extrusion could slightly decrease the glass transition temperature (T_g) and storage modulus of plastomers due to the minor reduction in molecular weights, but showed no significant impact on yield strength and strength at break. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Melt fracture of two broad molecular weight distribution high‐density polyethylenes

The melt fracture instabilities of two broad molecular weight distribution (MWD) high‐density polyethylenes (one Ziegler–Natta and one metallocene HDPEs) are studied as functions of the temperature and geometrical details and type of die (cylindrical, slit, and annular). It is found that sharkskin and other melt fracture phenomena are distinctly different for these resins, despite their almost identical rheology. It is also found that the critical conditions for the onset of various melt fracture phenomena depend significantly on the type of die used for their study. For example, sharkskin melt fracture in slit and capillary extrusion was obtained at much small critical shear stress values compared with those found in annular extrusion. Moreover, the metallocene HDPE shows significant slip at the die wall in the sharkskin flow regime. On the other hand, the Ziegler–Natta HDPE has shown no sign of slip. These differences are discussed on the basis of differences in their MWDs that influence their melt elasticity. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers     

In‐line versus off‐line rheological measurements on S‐PVC formulations releant for flow in extrusion dies

The supermolecular structures present in S‐PVC during processing are dependent on the thermomechanical history and affect the rheological behavior. Motivated by the need to have reliable rheological data relevant for numerical modeling of 3D flows in extrusion dies, we compared standard off‐line capillary measurements with‐inline measurements obtained from an instrumented die mounted on an extruder. Different PVC formulations showed different slip behavior in‐line, and also differences in comparison with the off‐line results. Cogswell analysis on entry pressure drops was used in order to obtain elongational information. For one formulation, with plug flow both in off‐line and in‐line measurements, an analysis based on the resistance to flow in contracting zones was the only way to get flow data that could be used in 3D profile simulations. The temperature sensitivity of measurements was used in order to discuss our results in terms of structural changes within the melt due to differences in thermomechanical history and formulation. Finally, an example of a 3D simulation of the flow of this formulation in an industrial extrusion die is given and compared with observations.     

Experimental studies on radial extrudate swell and velocity profiles of flowing PS melt in an electro‐magnetized die of an extrusion rheometer

This article investigates the radial extrudate swell and velocity profiles of polystyrene melt in a capillary die of a constant shear‐rate extrusion rheometer, using a parallel coextrusion technique. An electro‐magnetized capillary die was used to monitor the changes in the radial extrudate swell profiles of the melt, which is relatively novel in polymer processing. The magnetic flux density applied to the capillary die was varied in a parallel direction to the melt flow, and all tests were performed under the critical condition at which sharkskin and melt fracture did not occur in the normal die. The experimental results suggest that the overall extrudate swell for all shear rates increased with increasing magnetic flux density to a maximum value and then decreased at higher densities. The maximum swelling peak of the melt appeared to shift to higher magnetic flux density, and the value of the maximum swell decreased with increasing wall shear rate and die temperature. The effect of magnetic torque on the extrudate swell ratio of PS melt was more pronounced when extruding the melt at low shear rates and low die temperatures. For radial extrudate swell and velocity profiles, the radial swell ratio for a given shear rate decreased with increasing r/R position. There were two regions where the changes in the extrudate swell ratio across the die diameter were obvious with changing magnetic torque and shear rate, one around the duct center and the other around r/R of 0.65–0.85. The changes in the extrudate swell profiles across the die diameter were associated with, and can be explained using, the melt velocity profiles generated during the flow. In summary, the changes in the overall extrudate swell ratio of PS melt in a capillary die were influenced more by the swelling of the melt around the center of the die. Polym. Eng. Sci. 44:2298–2307, 2004. © 2004 Society of Plastics Engineers.     

Factorial optimization of the effects of extrusion temperature profile and polymer grade on as‐spun aliphatic–aromatic copolyester fibers. II. Crystallographic order

By using factorial experimental design, a range of crystallographic orders for as‐spun linear aliphatic–aromatic copolyester fibers have been characterized with the aid of wide angle X‐ray diffraction measurements. Full‐Width Half‐Maximum of an X‐ray scattering profile (FWHM) has been quantitatively assessed as responses to polymer grades denoted by melt flow index (MFI) and to extrusion temperature zones in the extrusion equipment used to produce the as‐spun fibers. With the advantages of the factorial experimental design in the development of fiber process technology, the enhanced statistical approach specifies the direction of change of the polymer's melt flow index and extrusion temperature profile for increasing or reducing crystallographic order. The produced as‐spun aliphatic aromatic copolyester fiber is an environmentally‐friendly attractive, alternative to conventional chemical fibers for different applications. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

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

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

Maleic anhydride/styrene melt grafting and crosslinking onto ethylene‐octene copolymer

Melt grafting of the multimonomer system of maleic anhydride (MAH)/styrene (St) onto ethylene‐octene copolymer (POE) was performed by a twin‐screw extruder. The effects of St and initiator contents as well as MAH/St on the grafting reaction were investigated. The structure and properties of the grafted POE were characterized by the Fourier transform infrared spectroscopy, melt flow index, dynamic rheological behaviors, and thermogravimetric analysis. It is shown that the addition of St can significantly enhance MAH grafting degree onto POE. MFI values of grafted POE are affected not only by MAH/St copolymer concentration, but also by initiator concentration. These data indicate that the interaction and reaction between MAH and St monomers plays an important role in the grafting reaction. St improves the grafting reactivity of MAH and reacts with MAH before the two monomers graft onto POE. And high grafting degree can be obtained while the gel content is still low. Compared with neat POE, grafted POE shows different dynamic rheological behaviors and high thermal stability. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Anomalous rheological behavior of polyethylene melts in the gross melt fracture regime in the capillary extrusion: Effect of long‐chain branching

During a capillary extrusion with several different polyethylenes, we observe an abnormal rheological behavior. The nominal viscosity of some polyethylene melt in the gross melt fracture regime does not change with the temperature. Several metallocene‐catalyzed linear low density polyethylene are investigated. Among them, polyethylenes, which have long‐chain branches in their main chain, show this abnormal rheological behavior. By capillary extrusion experiments with various dies of different L/D ratios, it is inferred that the abnormal rheological behavior is originated in the die land, not die entrance nor die exit. From various experiments, we notice that this abnormal phenomenon may be used to detect long‐chain branch of PE. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

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     

The influence of carbon-based nanofillers on the melt flow singularity of linear polyethylene

A quantitative power law relationship between molecular weight and flow criticalities is uncovered that links stick-slip theory with the melt flow singularity observed during capillary flow. The singularity arises from decrease in pressure during melt flow of linear polyethylene through capillary die in a narrow temperature window. The molecular origin of the window effect is attributed to slip flow arising from disengagement of chains adsorbed to melt-wall interface from free chains in melt. Considering the similar molecular configuration of the linear polyethylene and carbon nanotubes (CNTs), the stretched chain conformation of the adsorbed chains in the presence of CNTs is influenced. The potential to broaden the window temperature interval in the presence of CNTs is explored. For the study, multi-walled carbon nanotubes (MWCNTs) are added in the linear polyethylene. The influence of the filler aspect ratio on the window effect is further investigated in the presence of carbon black (CBs). The presence of MWCNTs broadens the window temperature interval and increases the decrease in pressure. Contrary to MWCNTs in the presence of CBs the extrusion window narrows and reduction in the pressure-decrease occurs. It is also found that the two carbon nanofillers have a significant impact on flow-induced solidification. Both, the crystallisation rate and the onset temperature of crystallisation, increase with the loading of the two nanofillers. A closer comparison between the two carbon nanofillers at the same loading suggests that the crystallisation is more influenced in MWCNT-PE composites.     

Ultrasound initiated maleic anhydride grafted onto a novel polypropylene copolymer

The reaction of maleic anhydride (MAH) grafted onto propylene‐based copolymer (DP) without adding any initiator was conducted through ultrasound assisted extrusion in this article. The effects of ultrasound power, die temperature, and MAH content on the grafting degree and efficiency were studied. With increasing ultrasound power, the grafting degree and efficiency of DP‐g‐MAH increase. The presence of ultrasound with higher power and lower die temperature is beneficial to increase the grafting degree and efficiency. The increase of MAH content can increase the grafting degree but reduce the grafting efficiency. Based on the results of melt flow index, dynamical rheological, gel permeation chromatograph (GPC), and Fourier transform infrared spectroscopy (FTIR) tests, the mechanisms of the grafting reaction were proposed. POLYM. ENG. SCI., 2012. © 2011 Society of Plastics Engineers     

Effect of short‐chain branching on melt fracture behavior of metallocene and conventional poly(ethylene/α‐olefin) copolymers

A phenomenon that can represent a great problem in melt processing is extrudate distortion. This effect can range in intensity from a loss of gloss to gross distortion and is the factor that limits the production rate in certain processes such as the blown film extrusion of linear low‐density polyethylene (LLDPE). The aim of this work was to investigate the effects that molecular weight distribution and short‐chain branch length have on the observed melt fracture phenomena for poly(ethylene/α‐olefin) resins with similar weight comonomer content and molecular weight. The flow stability analysis conducted in this study has shown that, even increasing of few carbon atoms the short‐chain branch length of the resins, the surface melt fracture phenomena are reduced and/or eliminated. Moreover, the comparison between the metallocene (mLLDPE) and conventional LLDPE samples, with the same comonomer (hexene), showed that the metallocene‐catalyzed resin exhibits early onset and more severe melt fracture, due to its narrower molecular weight distribution. POLYM. ENG. SCI., 52:1968–1977, 2012. © 2012 Society of Plastics Engineers     

Modeling of coat‐hanger die under vibrational extrusion

The distributions of the pulsatile pressure field, the pulsatile velocity field, and the pulsatile resident time of the polymeric melt in the coat‐hanger die are derived by using the pulsation of volumetric flow rate and pressure. Subsequently, formulae of the manifold radius and the slope of the manifold are deduced via volumetric flow rate pulsation. Polypropylene (PP) was employed for the experiments of the vibrational extrusion. The results indicate that the average extrusion pressure declines with frequency or amplitude decreasing; the distribution of residence time along the width of the coat‐hanger die performs uniformly during the vibrational extrusion process; the theoretical extrusion pressure well agrees with the experimental pressure; the experiments of tensile test, impact test implicate that vibration improves the mechanical properties of products; differential scanning calorimetry testing demonstrates that the melting point of PP is moved to a higher temperature value, and the endothermic enthalpy and the crystallinity are improved as well when superimposing the vibrational force field. Accordingly, the model of the coat‐hanger die under vibrational extrusion is well consistent with the experiments. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Structure and melt rheology of long‐chain branching polypropylene/clay nanocomposites

Long‐chain branching polypropylene (LCB‐PP)/clay nanocomposites were prepared by melt blending in a twin‐screw extruder. The microstructure and melt rheology of these nanocomposites were investigated using x‐ray diffraction, transmission electron microscopy, oscillatory shear rheology, and melt elongation testing. The results show that, the clay layers are intercalated by polymer molecular chains and exfoliate well in LCB‐PP matrix in the presence of maleic anhydride grafted PP. Rheological characteristics, such as higher storage modulus at low‐frequency and solid‐like plateau in tan‐ω curve, indicate that a compact and stable filler network structure is formed when clay is loaded at 4 phr (parts per hundred parts of) or higher. The response of the nanocomposite under melt extension reveals an initial decrease in the melt strength and elongational viscosity with increasing clay concentration up to 6 phr. Later, the melt strength and elongational viscosity show slight increases with further increasing clay concentration. These results might be caused by a reduction in the molecular weight of the LCB‐PP matrix and by the intercalation of LCB‐PP molecular chains into the clay layers. Increases in the melt strength and elongational viscosity for the nanocomposites with decreasing extrusion temperature are also observed, which is due to flow‐induced crystallization under lower extrusion temperature. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Modeling of three‐dimensional flow and heat transfer in polystyrene foam extrusion dies

A three‐dimensional mathematical model was developed to investigate the nonisothermal, non‐Newtonian polymer flow through the dies used in the polystyrene foam extrusion process. The model, based on the computational fluid dynamics (CFD) code, Polyflow, allowed for the shear rate and temperature dependence of the shear viscosity of the blowing agent laden polystyrene melt. The model also accounted for viscous heating. The shear viscosity of the polystyrene‐blowing agent mixture was measured experimentally at several temperatures. The model was used to calculate pressure, flow, and temperature distributions in two different dies used for industrial‐scale extrusion of polystyrene foams. The article presents a selection of computed results to illustrate the effect of die design on uniformity of flow at the die exit, the overall pressure drop in the die, relative magnitudes of pressure drop in the land section versus the rest of the die, and temperature distribution in the die. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers.