Effect of molecular structure on extrudate swell behavior for different thermoplastic melts in an electro‐magnetized die

The extrudate swell ratio of five different thermoplastic melts flowing in a constant shear rate rheometer having a capillary die with and without application of magnetic field was studied. The effects of the magnetic flux direction and density, die temperature, and wall shear rate on the extrudate swell and flow properties were investigated. The experimental results suggested that an increasing wall shear rate increased the swelling ratio for the polystyrene (PS), LLDPE, and PVC melts, but the opposite effect was observed for the ABS and PC melts. The extrudate swell ratio for the PS, ABS, PC, and LLDPE melts decreased with increasing die temperature, the effect being reversed for the PVC melt. Thermoplastic melts having high benzene content in the side‐chain and exhibiting anisotropic character were apparently affected by the magnetic field, the extrudate swell ratio increasing with magnetic flux density. The effect of the magnetic field on the extrudate swell ratio decreased in the order of PS → ABS → PC. The extrudate swell ratio for the co‐parallel magnetic field system was slightly higher than that for the counter‐parallel magnetic field system at a high magnetic flux density. POLYM. ENG. SCI., 47:270–280, 2007. © 2007 Society of Plastics Engineers.     

Melt elasticity behavior and extrudate characteristics of rigid poly(vinyl chloride)/epoxidized natural rubber miscible blends

A study on the melt elasticity behavior and extrudate characteristics of melts of rigid poly(vinyl chloride), PVC, and rigid poly(vinyl chloride)/epoxidized natural rubber (ENR) miscible blends were conducted. Extrusion studies were carried out in a capillary rheometer and examinations of the surface characteristics of the extrudate were made by taking photomicrographs in a scanning electron microscope. The anomalous behavior in the die swell ratio of rigid PVC arising from the particle agglomerates continued in its blends up to 50 wt% composition of ENR. Temperature independence for high ENR blends was noted for the principal normal stress difference and elastic shear modulus, when shear stress was held constant. Recoverable shear strain and die swell ratio behaved identically in terms of blend composition and processing temperature. Factors which control the extrudate distortion and melt fracture of the melts of rigid PVC/ENR systems were fusion of particle agglomerates and strength of melts. Diamond cavitations were typical of the extrudate surface of PVC melts as those of the fracture surface of the tensile failure of PVC. Conditions to obtain a smooth extrudate surface of rigid PVC melts in blends with ENR have been found to be the low ENR content, low shear rate, or stress and high processing temperature.     

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.     

Experimental studies on extrudate swell behavior of PS and LLDPE melts in single and dual capillary dies

Extrudate swell behavior of polystyrene (PS) and linear low‐density polyethylene (LLDPE) melts was investigated using a constant shear rate capillary rheometer. Two capillary dies with different design configurations were used, one being a single flow channel and the other being a dual flow channel. A number of extrudate swell related parameters were examined, and used to explain the discrepancies in the extrudate swell results obtained from the single and dual flow channel dies, the parameters including output rate and output rate ratio, power law index, wall shear rate, wall shear stress, melt residence time, pressure drop induced temperature rise, flow channel position relative to the barrel centerline, and the flow patterns. It was found in this work that the power law index (n value) was the main parameter to determine the output rate ratio and the extrudate swell between the large and small holes for the dual flow channel die: the greater the n value the lower the output rate ratio and thus decreased extrudate swell ratio. The differences in the extrudate swell ratio and flow properties for PS and LLDPE melts resulted from the output rate ratio and the molecular chain structure, respectively. The extrudate swell was observed to increase with wall shear rate. The discrepancies in the extrudate swell results from single and dual dies for a given shear rate were caused by differences in the flow patterns in the barrel and die, and the change in the melt velocities flowing from the barrel and in the die to the die exit. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1713–1722, 2003     

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     

Ultrasonic improvement of rheological behavior of polystyrene

The effects of ultrasonic oscillation on die pressure, productivity of extrusion, melt viscosity, and melt oscillating flow of polystyrene (PS) as well as their mechanism were studied in a special ultrasonic oscillations extrusion system developed in our lab. The experimental results show that in the presence of ultrasonic oscillations, the PS melt oscillating flow or surface distortion of PS extrudate is inhibited or disappears. The surface appearance of PS extrudate gets greatly improved. The die pressure, melt viscosity, flow activation energy, and consistency efficiency of PS decreased and the productivity of PS extrudate increased in the presence of ultrasonic oscillation. The shear sensitivity of PS melt viscosity is decreased because of the increase of its power law index in the presence of ultrasonic oscillation. Introduction of ultrasonic oscillation into PS melt can greatly improve the processibility of PS. Their possible mechanism is also proposed in this article. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 2451–2460, 2002     

Die design for rigid PVC—the effect of die land length on extrudate swell

PVC profile extrusion compounds have a unique morphology. While other polymers gradually decrease in extrusion die swell with increasing length/thickness (L/D) ratio, PVC profile extrusion compounds have a low die swell, quite independent of the die's L/D ratio in the range of 5 to 20. The fact that the die land length can be changed without changing the extrudate swell is an important consideration, which makes die design and balancing dies simpler and easier for PVC profile extrusion compounds. While other polymers substantially increase extrudate swell with increased shear rate, the swell of the PVC profile compounds is not much affected by shear or extrusion rate. This unique behavior allows wider processing latitude in profile extrusion and faster extrusion rates than with other polymers. Another unique factor in the rheology of PVC profile extrusion compounds is that extrusion die swell increases with increasing melt temperature, while other polymers have decreasing die swell with increasing melt temperature. The unusual rheology of PVC profile extrusion compounds is attributed to its unique melt morphology, where the melt flow units are 1 um bundles and molecules that have low surface to surface interaction and entanglement at low processing temperatures but increased melting and increased entanglement at higher processing temperatures. Other polymers, unlike PVC, have melt flow at the molecular level.     

A parallel coextrusion technique for simultaneous measurements of radial die swell and velocity profiles of a polymer melt in a capillary rheometer

This article proposes a new experimental technique to simultaneously measure radial die swell and velocity profiles of polystyrene melt flowing in the capillary die of a constant shear rate rheometer. The proposed technique was based on parallel coextrusion of colored melt‐layers into uncolored melt‐stream from the barrel into and out of the capillary die. The size (thickness) ratio of the generated melt layers flowing in and out of the die was monitored to produce the extrudate swell ratio for any given radial position across the die diameter. The radial velocity profiles of the melt were measured by introducing relatively light and small particles into the melt layers, and the times taken for the particles to travel for a given distance were measured. The proposed experimental technique was found to be both very simple and useful for the simultaneous and accurate measurement of radial die swell and velocity profiles of highly viscous fluids in an extrusion process. The variations in radial die swell profiles were explained in terms of changes in melt velocity, shear rate, and residence time at radial positions across the die. The radial die swell and velocity profiles for PS melt determined experimentally in this work were accurate to 92.2% and 90.8%, respectively. The overall die swell ratio of the melt ranged from 1.25 to 1.38. The overall die swell ratio was found to increase with increasing piston speed (shear rate). The radial extrudate swell profiles could not be reasoned by the shear rate change, but were closely linked with the development of the velocity profiles of the melt in the die. The die swell ratio was high at the center (～1.9) and low (～0.9) near the die wall. The die swell ratio at the center of the die reduced slightly as the piston speed was increased. Polym. Eng. Sci. 44:1960–1969, 2004. © 2004 Society of Plastics Engineers.     

Melt extrudate swell behavior of multi‐walled carbon nanotubes filled‐polypropylene composites

The extrudate swell behavior of polypropylene (PP) composite melts filled with multi‐walled carbon nanotubes (MWCNTs) was studied using a capillary rheometer in a temperature range from 190 to 230°C and at various apparent shear rates varying from 50 to 800 s⁻¹. It was found that the values of the extrudate swell ratio of the composites increased nonlinearly with increasing apparent shear rates, while the values of the extrudate swell ratio decreased almost linearly with increasing temperature. The values of the melt extrudate swell ratio increased approximately linearly with increasing shear stress, while decreased approximately nonlinearly with an increase of the MWCNT weight fraction. In addition, the extrudate swell mechanisms were discussed with observation of the fracture surface of the extrudate using a scanning electronic microscopy. This study provides a basis for further development of MWCNTs reinforced polymer composites with desirable mechanical and thermal properties. POLYM. COMPOS., 38:2433–2439, 2017. © 2015 Society of Plastics Engineers     

High density polyethylene/polystyrene blends: Phase distribution morphology,rheological measurements,extrusion, and melt spinning behavior

An experimental study of the development of phase morphology, rheological properties, and processing behavior of mechanical blends of a polystyrene (PS) and a high density polyethylene (PE) is presented. Phase morphologies were determined by scanning electron microscopy for (i) products prepared in a screw extruder/static mixer system, (ii) samples removed from a cone-plate viscometer, (iii) extrudates, and (iv) melt spun fibers. Disperse phase dimensions were measured. The values varied from 1–5 μm in the products from static mixers. The dimensions of the dispersed phase in the blend products from the cone plate and capillary die were of the same order. The melt-spun fibers exhibited disperse phase dimensions as low as 0.35 μm. Polystyrene was extracted from the blend fibers producing small diameter, PE fibrils, or minifibers. Both the initial melts and the blends were rheologically characterized. The shear viscosity and principal normal stress difference N₁ exhibit maxima and minima when plotted as a function of composition. The characteristics of extrudates and melt spinning behavior of the blends were investigated. The shrinkage of extrudates of PE is much greater than PS. Additional small amounts of PE to PS greatly increase its shrinkage. Addition of PE to PS initially increases extrudate swell, though the swell shows maxima and minima when considered as a function of composition. The positions of the maxima and minima correspond to those of N₁. The onset of draw resonance has been investigated in isothermal melt spinning. Wide angle X-ray diffraction studies have been carried out on blend fibers and the orientation of the crystalline polyethylene regions has been determined as a function of process conditions. This orientation decreases rapidly with the addition of polystyrene when the melt-spun filaments are compared at the same spinline stress or drawdown ratio.     

Rheological properties of poly(vinyl chloride) / epoxidized natural rubber blends. Part II: The effect of processing variables

The effect of processing variables on the rheological properties of PVC/ENR blends was investigated. The role of crosslinking in determining the flow behavior of blends was also examined by means of dynamically cured blends. It was found that PVC/ENR blends yield melts that are power law fluids. The flow of the melts improves with an increase in temperature and shear rate. However, the introduction of crosslinks reverses this trend, although under more rigorous conditions, the influence of crosslinks is superseded, and subsequently, flow becomes shear rate and temperature dependent. PVC/ENR systems also manifested elastic phenomena. The dependence of the elastic phenomena such as die swell and melt fracture on L/D ratio of the die was demonstrated.     

Measurement of melt viscoelastic properties of polyethylenes and their blends—a comparison of experimental techniques

Two polyethylene resins (LDPE and HDPE) and their blends were characterized for dynamic shear rheology, extrudate swell in a capillary rheometer, and recoverable strain as measured by the Melt Elasticity Indexer in attempts to compare parameters related to the so-called “melt elasticity” as obtained by different experimental techniques. Such parameters may be useful in screening materials for their melt processability. Data were obtained at equivalent shear rates/frequencies and different temperatures. With respect to the individual blend components, the LDPE resin with the lower Melt Index (MI) had higher storage modulus and Weissenberg number than the HDPE resin. However, by using criteria based on “recoil” and strain recovery, ranking was different with the LDPE resin shown to exhibit lower “melt elasticity.” In this case, extrudate swell data were found to correlate reasonably well with equilibrium recoverable strain data. With respect to blends, complex viscosity and storage modulus versus composition curves showed positive deviations from linearity, similar to those observed in melt heterogeneous blends. Similarities between the short time recoverable strain vs. composition and the storage modulus vs. composition curves suggest that similar morphological states may exist in the melt over the experimental times and conditions applicable to these different experimental techniques.     

Effects of blend ratio and vulcanizate powders on the processability of silicone rubber/fluororubber blends

While silicone rubber has a much lower shear viscosity than that of fluororubber, the viscosity of silicone rubber/fluororubber blends is closer to that of silicone rubber. All rubber compositions show pseudoplastic behavior. While the viscosity of a 50/50 silicone rubber/fluororubber blend increases on replacement of the silicone rubber by silicone rubber vulcanizate powder (SVP), the effect of fluororubber replacement by the corresponding fluororubber vulcanizate powder (FVP) on the shear viscosity is less pronounced. The difference in viscosity between SVP‐replaced silicone rubber and FVP‐replaced fluororubber becomes less prominent at higher shear rates. Fluororubber exhibits a higher extrudate die swell than that of silicone rubber. The die swell of the silicone rubber/fluororubber blends is higher than are the figures obtained by the additivity rule. Replacement of constituent rubbers in the blend by the corresponding vulcanizate powders causes an increase in the die swell. While the silicone rubber extrudate surface is smooth, the fluororubber extrudate shows melt fracture. The extrudate surfaces of the silicone rubber/fluororubber blends are similar to that of the silicone rubber extrudate. Replacement of constituent rubbers by the corresponding vulcanizate powders increases the roughness of the extrudate surface, which is more prominent in the case of silicone rubber replacement by SVP. SVP can replace 50% of silicone rubber and FVP can replace 75% of fluororubber in the 50/50 silicone rubber/fluororubber blend. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 2377–2387, 2003     

Blends of low density polyethylene/plasticized poly(vinyl chloride): The rheological,morphological, and mechanical characterization

An experimental study was conducted to investigate the rheological, morphological, and mechanical properties of a heterogeneous polymer blend system consisting of low density polyethylene (LDPE) and plasticized poly(vinyl chloride) (PVC). The components were mixed using a single-screw extruder, which was equipped with a special measuring head for the determination of rheological quantities. The morphology of blends was examined by scanning electron microscopy. Die swell was determined by photography. The velocity of ultrasound through the polymer melt was also measured. The dependencies of viscosity, die swell, and ultrasonic velocity on blend composition were qualitatively similar, exhibiting a minimum at about 70 wt % PVC. The morphology of the blend system at this blending ratio was different from morphologies of the other blends. Tensile properties of blends, except elongation at break, were not significantly inferior to those of the LDPE component.     

Melt processing,mechanical, and fatigue crack propagation properties of reactively compatibilized blends of polyamide 6 and acrylonitrile–butadiene–styrene copolymer

Within a IUPAC study, melt processing, mechanical, and fatigue crack growth properties of blends of polyamide 6 (PA 6) and poly(acrylonitrile–butadiene–styrene) (ABS) were investigated. We focused on the influence of reactive compatibilization on blend properties using a styrene–acrylonitrile–maleic anhydride random terpolymer (SANMA). Two series of PA 6/ABS blends with 30 wt % PA 6 and 70 wt % PA 6, respectively, were prepared with varying amounts of SANMA. Our experiments revealed that the morphology of the matrix (PA 6 or ABS) strongly affects the blend properties. The viscosity of PA 6/ABS blends monotonically increases with SANMA concentration because of the formation of high‐molecular weight graft copolymers. The extrudate swell of the blends was much larger than that of neat PA 6 and ABS and decreased with increasing SANMA concentrations at a constant extrusion pressure. This observation can be explained by the effect of the capillary number. The fracture resistance of these blends, including specific work to break and impact strength, is lower than that of PA 6 or ABS alone, but increases with SANMA concentration. This effect is most strongly pronounced for blends with 70 wt % PA 6. Fatigue crack growth experiments showed that the addition of 1–2 wt % SANMA enhances the resistance against crack propagation for ABS‐based blends. The correlation between blend composition, morphology and processing/end‐use properties of reactively compatibilized PA 6/ABS blends is discussed. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Melt fracture,melt viscosities,and die swell of polypropylene resin in capillary flow

The melt fracture, shear viscosity, extensional viscosity, and die swell of a polypropylene resin were studied using a capillary rheometer and dies with a 0.05‐cm diameter and length/radius ratios of 10, 40, and 60. A temperature of 190°C and shear rates between 1 and 5000 s^?1 were used. A modified Bagley plot was used with consideration of pressure effects on both the melt viscosity and end effect. The shear viscosity was calculated from the true wall shear stress. When the true wall shear stress increased, the end effect increased and showed critical stresses at around 0.1 and 0.17 MPa. The extensional viscosity was calculated from the end effect and it showed a decreasing trend when the strain rate increased. Both the shear and extensional viscosities correlated well with another polypropylene reported previously. The die swell was higher for shorter dies and increased when shear stress increased. When the shear rates increased, the extrudate changed from smooth to gross melt fracture with regular patterns (spurt) and then turned into an irregular shape. In the regular stage the wavelength of the extrudates increased when the shear rate increased. The frequency of melt fracture was almost independent of the shear rate, but it decreased slightly when the die length increased. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1587–1594, 2003     

Studies on binary and ternary blends of polypropylene with ABS and LDPE. I. Melt rheological behavior

Studies are presented on melt rheological properties of binary blend of polypropylene (PP) and acrylonitrile–butadiene–styrene terpolymer (ABS), and ternary blend of PP, ABS, and low-den-sity polyethylene (LDPE). Data obtained in capillary rheometer are presented to describe the effect of blending ratio, shear stress, and shear rate on flow properties, melt viscosity, and melt elasticity. At a blend composition corresponding to 10 wt % ABS content, both binary and ternary blends show maximum in melt viscosity accompanied by minimum in melt elasticity. Pseudoplasticity of the melt decreases with increasing ABS content. In ternary blends, LDPE facilitates the flow at low LDPE contents and obstructs the flow at high LDPE contents. Scanning electron microscopic studies are also presented to illustrate the state of dispersion and its variation with blend composition.     

Blends of poly(vinyl chloride) with α‐methylstyrene‐acrylonitrile‐butadiene‐styrene copolymer: Thermal properties,mechanical properties,and morphology

Binary blends of poly(vinyl chloride) (PVC) with α‐methylstyrene‐acrylonitrile‐butadiene‐styrene copolymer (AMS‐ABS) were prepared via melt blending. A single glass transition temperature (T_g) was observed by differential scanning calorimetry, thus indicating that PVC is miscible with the α‐methylstyrene‐acrylonitrile‐styrene in AMS‐ABS. The results from attenuated total reflection Fourier transform infrared spectra indicated that specific strong interactions were not available in the blends. With increasing amounts of AMS‐ABS, both heat distortion temperature and thermal stability were increased considerably. With regard to mechanical properties, flexural and tensile properties decreased with increasing AMS‐ABS content. A synergism was observed in impact strength. The morphology of both impact‐fractured and tensile‐fractured surfaces, observed by scanning electron microscopy, correlated well with the mechanical properties. It is suggested that there was a transition of fracture mechanisms with the changing composition of the binary blends—from shear yielding for blends rich in PVC to cavitation for blends rich in AMS‐ABS. J. VINYL ADDIT. TECHNOL., 19:1–10, 2013. © 2013 Society of Plastics Engineers     

Measurement and simulation of low‐density polyethylene extrudate swell through a circular die

BACKGROUND: Extrudate swell is a common phenomenon in polymer processing. The investigation of its mechanism is of both scientific and industrial interest. RESULTS: The rheological parameters of a material described by the viscoelastic PTT (Phan‐Thien–Tanner) constitutive model are obtained by fitting the distributions of material functions detected with a strain‐controlled rheometer. The swelling ratios of low‐density polyethylene (LDPE) under different volume flow rates are indirectly obtained using a photographic technique. A mathematical model of extrudate swell is established and its finite element model is derived. A penalty method is employed to solve the extrudate swell problem with a decoupled algorithm. Computation stability is improved by using the discrete elastic‐viscous split stress algorithm incorporating the inconsistent streamline‐upwind scheme. CONCLUSION: The swell phenomenon of LDPE through a circular die is investigated using both experimental measurement and numerical simulation. The swelling ratios obtained from the simulation are compared with those measured: they agree well with each other. The essential flow characteristics of polymer melts are predicted and the mechanism of the swell phenomenon is further discussed. Copyright © 2009 Society of Chemical Industry     

LLDPE／LDPE共混物熔体挤出胀大行为的研究

应用毛细管流变仪，考察了实验条件下线性低密度聚乙烯与低密度聚乙烯共混物（ＬＬＤＰＥ／ＬＤＰＥ）熔体挤出胀大行为及其影响因素。结果表明，挤出胀大比Ｂ与剪切应力近似呈幂律关系，而随口型长径比的增加呈指数衰减，当共混比为５０／５０时，Ｂ产生局部极涉值现象。