线型与支化聚烯烃熔体高速挤出时的不稳定扰动源

采用恒速型双毛细管流变仪对比研究线型与支化聚烯烃熔体在高速流场中的流动曲线、挤出畸变、挤出压力变化及粘弹性的特征,分析讨论了引发熔体不稳定流动的扰动源位置及扰动性质.结果表明,高速流场中的扰动源有:口模入口区的扰动、口模壁处的扰动、口模出口区的扰动.支化聚合物易受入口区的扰动干扰,造成挤出物无规破裂;线型聚合物易受口模壁处的扰动干扰,造成挤出压力振荡和挤出物有规畸变;鲨鱼皮畸变主要由于口模出口区的振荡扰动造成.     

经不同程度降解聚丙烯的熔体流变性质

<正> 聚丙烯(PP)的热降解和化学控制降解法七十年代开始应用于聚合物工业加工过程。聚丙烯在高温下热降解,一般认为服从无规断链机理。随降解过程进行,分子量变小,分子量分布变窄,由定向聚合反应产生的特征性高分子量尾端降解最为显著。该过程往往可显著改善PP熔体加工性,因此研究降解过程对PP熔体流变性质的影响殊属     

The melt elastic behavior of polypropylene/glass bead composites in capillary flow

The melt extrudate swell and entry pressure losses are important characteristics of elastic properties during die extrusion of polymeric fluids. They are usually expressed with die-swell ratio (B) and entry pressure drop (ΔP_o). In the present paper, the die-swell behavior and entrance pressure drop of a polypropylene (PP) filled with A-glass beads were investigated by using a Rosand capillary rheometer to identify the effects of the filler contents and extrusion rate on the elastic behavior of the sample melts. The experiments were carried out under the conditions with an apparent shear rate range of 50–10⁴ s⁻¹ and a temperature of 190 °C. The results showed that B increased nonlinearly with increasing shear rate at the wall (γ_w), and increased linearly with the increase of shear stress at the wall (τ_w). With the increase of the volume fraction of the fillers B decreased nonlinearly. Similarly, the entry pressure drop increased linearly with the increase of τ_w, whereas the influence of the filler concentration on ΔP_o was insignificant in this case. Furthermore, B increased as a linear function of ΔP_o, and extension stress (σ_e) increased nonlinearly with increasing γ_w.     

Solid-state extrusion of chain-extended polyethylene

Billets of chain-extended polyethylene were prepared from Alathon 7050 (M_w 59,000, M_n 19,000) in an Instron capillary rheometer by crystallization at a constant pressure of 460 MPa, at a series of teimperatures from 198 to 221°C corresponding to varying degrees of undercooling. This gives chain-extended morphologies with a range of crystallinites and lamellar thicknesses. The billets were then solid-state extruded at 100°C through a conical die with 20° entrance angle up to an extrusion draw ration 23.4. Thermal behavior was studied with differential scanning calorimetry. The orientation function measured by wide-angle x-ray diffraction showed higher orientation function measured by wide-angle x-ray diffraction showed higher orientation at equivalent draw ratio when the initial billets were crystallized at lower temperatures. Drawing efficiency, defined as the ratio of molecular draw ratio (from shrinkage) to extrusion draw ratio correspondingly increases, reaching a maximum of 0.71 in our solid-state extrusion. These studies show that highly chain-extended polyethylene, i.e., with few chain entanglements, draws poorly. Drawability was improved by increasing chain entanglements by lowering the crystallization temperature. Electron micrographs of fracture surface replicas of extrudates revealed the coexistence of undeformed, tilted, partially drawn lamellae and fibrillar structure consistent with the cahange of morphologies in Peterlin's model of plastic deformation.     

Extrudate swell and flow analysis of polystyrene melt flowing in an electro‐magnetized die in a single screw extruder

An electro‐magnetized capillary die via a parallel co‐extrusion technique was used to study the changes in the overall and radial extrudate swell ratio of polystyrene (PS) melt flowing in a single screw extruder. The effects of magnetic flux density, wall shear rate (screw rotating speed) and die temperature were studied. The results suggested that, in the case of non‐magnetic die the average overall swell ratio of the melt ranged from 1.25 to 1.55. The swelling ratio increased with increasing wall shear rate up to 8.5 sec^?1 and then decreased at 17.1 sec^?1. Increasing die temperature caused a reduction of extrudate swell ratio. The changes in extrudate swell ratio can be explained using the simultaneously measured velocity profiles during the flow in the die, and the swell ratio decreased with increasing radial position. Melt contraction of the melt layer near the die wall was observed. The die temperature was found to have no effect on the change of the radial extrudate swell profiles. When an electro‐magnetized die was used, the average overall swell ratio was found to increase with increasing magnetic flux density to a maximum value and then decreased at higher flux densities. The magnetic flux density of the maximum swell was changed by the wall shear rate. It was associated with a balance of elastic and magnetic energies during the flow. The magnetic energy was thought to have a pronounced effect on the swell ratio at low shear rate and low die temperature. Considering the radial position, the highest swell ratio occurred at the duct center, in the range 2.4–3.3. There was no extrudate contraction of the melt layer near the die wall. Copyright © 2005 John Wiley & Sons, Ltd.     

Rheological characterization of the die swell phenomenon of rubber compounds

The die swell phenomenon of rubber compounds in capillary experiments with various ratios of length to diameter of capillaries is investigated. This knowledge is important for the design of injection heads for the extrusion of rubber profiles. The die swell of viscoelastic rubber compounds depends on the geometry of the capillary dies, on the melt temperature and on the shear strain rate. One empirical relationship will consider all these dependencies. Usage of this equation and identification of only one new material parameter enables the comparison and assessment of the die swell of different materials, independently of the corresponding geometry of the capillary die used. Furthermore, the influence of melt temperature, molecular structure and extrusion process on the die swell can be identified. The investigation was performed with various rubber compounds as well as rubber blends used in industry, mainly EPDM and carbon black in different compositions.     

Characteristics and design procedure of hyperbolic dies

Nozzle profiles capable of generating constant extensional strain rates are termed hyperbolic dies. When used in polymer extrusion, they exhibit greater potential in inducing and retaining polymer molecular orientation than conventional capillary dies. Most mathematical expressions found in the literature involve several processing variables in describing and designing such nozzle profiles. This report reveals that a hyperbolic die profile, although rather complicated, can be expressed with equations in terms of two ordinary geometrical parameters—the exit diameter and the hyperbolic length. This finding greatly simplifies the design procedure of hyperbolic dies. The extensional strain rate of a hyperbolic die can be related to the length-to-diameter ratio for any given exit diameter. Examples of various types of die profiles are presented and their constant extensional strain-rate characteristics are discussed.     

Die swell ratio of polystyrene melt from an electro‐magnetized capillary die in an extrusion rheometer: effects of barrel diameter,shear rate and die temperature

A constant shear‐rate extrusion rheometer with an electro‐magnetized capillary die was utilized to investigate die swell behavior and flow properties of a polystyrene melt as the application of an electro‐magnetic field to the capillary die was relatively novel in polymer processing. The test conditions such as magnetic flux density, barrel diameter, extrusion rate and die temperature were studied. The results suggest that the maximum swelling of the polystyrene melt with application of the electro‐magnetic field could be enhanced up to 2.6 times (260%) whereas that without the electro‐magnetic field was 1.9 times (190%). The barrel diameter of 30 mm was found to be a critical value in the case of the die swell ratio and flow properties of the polystyrene melt were significantly affected by the magnetic flux density. This involved the number and angle of magnetic flux lines around the barrel part. Under the electro‐magnetic field, there were two mechanical forces influencing the die swell ratio and the flow properties; magnetic torque and shearing force. The die swell at wall shear rates less than 11.2 sec^?1 was caused by the magnetic torque, whereas at higher wall shear rates it was dependent on the shearing force. For a given magnetic flux density, the maximum increase in the die swell ratio as a result of the magnetic torque was calculated to be approximately 20%. Increasing the die temperature from 180 to 200°C reduced the overall die swell ratio and suppressed the effect of the magnetic flux density. Copyright © 2004 John Wiley & Sons, Ltd.     

Effect of thermal degradation on rheological properties for poly(3-hydroxybutyrate)

Thermal degradation at processing temperature and the effect on the rheological properties for poly(3-hydroxybutyrate) have been studied by means of oscillatory shear modulus and capillary extrusion properties, with the aid of molecular weight measurements. Thermal history at processing temperature depresses the viscosity because of random chain scission. As a result, gross melt fracture hardly takes place with increasing the residence time in a capillary rheometer. Moreover, it was also found that the molecular weight distribution is independent of the residence time, whereas the inverse of the average molecular weight is proportional to the residence time. Prediction of average molecular weight with a constant molecular weight distribution makes it possible to calculate the flow curve following generalized Newtonian fluid equation proposed by Carreau as a function of temperature as well as the residence time.     

Hydrostatic extrusion of polyoxymethylene

The hydrostatic extrusion behavior of polyoxymethylene (POM) is described. Extrusions were performed at 164°C for a range of different molecular weight grades. Excellent unflawed lengths of extrudate were obtained with axial Young's moduli, measured at room temperature, reaching values as high as 24 GPa. The extrusion characteristics are discussed in terms of the very strong dependence of the flow stress of POM on strain and strain rate and pressure. In addition to measurements of Young's modulus over a wide temperature range, data for shear modulus and transvers modulus are also presented. A limited amount of other structural measurements is presented.     

Effect of pressure in capillary flow of polystyrene

Several corrections possibly required for capillary flow are based on the existence of a linear relationship between the pressure drop along the capillary and the length-to-diameter ratio at a given temperature and shear rate. Recently, the appearance of nonlinearities in this relationship has created some concern as to the cause of this behavior. The occurrence and an explanation of the nonlinearities for polystyrene form the basis of this study. A narrow-distribution, low molecular weight (20,400) polystyrene was tested in eight capillaries at temperatures of 140 and 160°C to initiate the discussion of the nonlinearity in a ΔP (pressure) versus L/D (length/diameter of capillary) plot. The sample exhibits negligible extrudate swelling at all pressures which reinforces the idea that pressure is influencing the flow. The pressure dependence of viscosity is determined using the equivalent expression of the WLF equation derived from free volume theory. Justification for its use is presented. A pressure correction, representing the increased shear stress necessary for flow of the higher viscosity material, is found to linearize the ΔP versus L/D data. A narrow-distribution, high molecular weight polystyrene (670,000) is subjected to a similar analysis at 165°C by using nine capillaries. The situation is quite different, as the high molecular weight sample is not nearly as ideal as the low molecular weight polystyrene.     

Melt elongation flow behaviour of LDPE/LLDPE blends

The extensional rheological properties of low density polyethylene (LDPE)/linear low density polyethylene (LLDPE) blend melts were measured using a melt spinning technique under temperatures ranging from 160 to 200 °C and die extrusion velocities varying from 9 to 36 mm/s. The results showed that the melt elongation stress decreased with a rise of temperature while it increased with increasing extensional strain rate and the LDPE weight fraction. The dependence of the melt elongation viscosity on temperature roughly obeyed the Arrhenius equation, it increased with increasing extensional strain rate and the LDPE weight fraction when the extensional strain rate was lower than 0.5 s⁻¹, and it reached a maximum when the extensional strain rate was about 0.5 s⁻¹, which can be attributed to the stress hardening effect.     

Rheology of polydimethylsiloxane swollen with supercritical carbon dioxide

Viscosity curves were measured for polydimethyl siloxane (PDMS) melts swollen with dissolved carbon dioxide at 50 and 80°C for shear rates ranging from 40 to 2300 s⁻¹, and for carbon dioxide contents ranging from 0 to 21 wt %. The measurements were performed with a capillary extrusion rheometer modified for sealed, high-pressure operation to prevent degassing of the melt during extrusion. The concentration-dependent viscosity curves for these systems are self-similar in shape, exhibiting low-shear rate Newtonian plateau regions followed by shear-thinning “power-law” regions. Considerable reduction of viscosity is observed as the carbon dioxide content is increased. Classical viscoelastic scaling methods, employing a composition-dependent shift factor to scale both viscosity and shear rate, were used to reduce the viscosity data to a master curve at each temperature. The dependence of the shift factors on polymer chain density and free volume were investigated by comparing the shift factors for PDMS-CO₂ systems to those obtained by iso-free volume dilutions of high molecular weight PDMS. This comparison suggests that the free volume added to PDMS upon swelling with dissolved carbon dioxide is the predominant mechanism for viscosity reduction in those systems. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys, 35: 523–534, 1997     

Melt fracture of polyisobutylenes

The processability of different grades of polyisobutylene (PIB) was investigated using a capillary rheometer. Direct focus was given to the occurrence of melt fracture phenomena, such as sharkskin and gross melt fracture (GMF). The influence of molecular weight (MW) of PIB, temperature and die entrance angle on melt fracture was examined in detail. Due to their highly elastic nature, high MW PIBs were found to exhibit gross melt fracture instability even at low shear rates, rendering their processing an impossible task. An increase in temperature resulted in postponing both instabilities (sharkskin and gross) to higher shear rates, thus making their processing easier. Finally, decreasing the entrance angle below a critical value resulted in postponing the onset of GMF to higher shear rates.     

EFFECT OF PAN-MILLING ON RHEOLOGICAL PROPERTIES OF HIGH DENSITY POLYETHYLENE*

The effect of pan-milling on the rheological properties of high density polyethylene (HDPE) was studied. Aninnovative milling apparatus, viz. an inlaid pan-mill, was used. Melt indexer, capillary rheometer, Haake Rheocord 90 single-screw extruder and Brabender rheometer were used to evaluate the rheologieal properties of HDPE. HDPE with higher initialmolecular weight and larger particle size was easier to degrade under pan-milling stress, as indicated by the melt index.Pressure oscillation in capillary flow occurred at significantly higher shear stress and shear rate for milled HDPE than forunmilled HDPE. The apparent shear viscosity of HDPE decreased with increasing times of milling. After milling, the flowactivation energy decreased and thus the sensitivity of viscosity to temperature was reduced. Die pressure and torque duringsingle screw extrusion were reduced significantly after milling. Plasticizing time as measured in a Brabander mixer decreasedmarkedly with increasing milling times.     

顺丁橡胶挤出过程中熔体不稳定流动的研究

<正> 在高聚物挤出过程中,当挤出速率在一定条件下增大到某一数值时,挤出物表面将出现竹节状或鲨鱼皮状、扭曲等现象,此即为熔体破裂或不稳定流动。它严重影响着高聚物制品的外观质量,并限制了生产速率的进一步提高。所以,深入探讨不稳定流动产生的机理,进而建立判定不稳定流动的临界条件,对指导高聚物压出成型加工过程参数的选择及过程控制,具有实际意义。     

Kinetic study of γ-radiation-induced polymerization of ethylene in the presence of acetylene

The effects of acetylene on the γ-radiation-induced polymerization of ethylene were studied from the viewpoint of kinetics. The experiments were carried out under a pressure of 150–400 kg/cm²; the temperature was 30°C; the dose rates were 2.7 × 10⁴ and 1.1 × 10⁵ rad/hr; the acetylene content was 0–2.21%. Both the polymer yield and the molecular weight increased acceleratively with the reaction pressure in the polymerization containing 0.18% acetylene. The yield increased almost proportionally with the dose rate, and the molecular weight was found to be almost independent of the dose rate in the polymerization containing 2.21% acetylene. The polymerization rate and the molecular weight increased with reaction time, but the increment decreased with increasing acetylene content. The degree of increase in the molecular weight also decreased with increasing time. These results were analyzed by using a graphical evaluation method for kinetics, and the effects of acetylene on each elementary step in the polymerization discussed.     

Extrudate swell behavior of PS and LLDPE melts in a dual die with mixed circular/slit flow channels in an extrusion rheometer

Extrudate swell behaviors of polystyrene (PS) and linear low‐density polyethylene (LLDPE) melts in a dual channel die, having mixed circular/slit flow channels, in a constant shear rate rheometer were examined. The extrudate swell ratio for PS melt was observed to be higher than that for LLDPE melt for all cases, this being associated with the differences in molecular structures that could be described in terms of power law indexes and secondary flows near the die entrance. In single channel die, the extrudate swell of both PS and LLDPE melts in circular flow channel die was greater than that in slit flow channel, whereas, in dual channel die the slit channel exhibited a higher extrudate swell ratio, the results being explained by revealing the flow patterns of the melt in the barrel and die of the rheometer. It was found that the dimensionless size of the vortex flows near the entrance, and the extent of disentanglement of molecular chains on entering the die were the important factors for the differences in the extrudate swell ratios of the melts at the die exit influenced by the die designs used. Copyright © 2003 John Wiley & Sons, Ltd.     

Cavitation of polyethylene during extrusion processing instabilities

We present observations of cavitation that occur inside a capillary die during extrusion of polyethylene. This phenomenon was observed over the last 1.5 mm of the capillary tube immediately upstream of the exit. We observed spontaneous formation of voids near the wall that grew to a typical length and width of 150 μm, and then shrank and disappeared over a time frame of approximately 20 ms. From velocity measurements of these structures, we concluded that their width in the radial direction was smaller than in the axial and lateral directions, and they were near the wall. The shape of the cavities was highly irregular. We assessed the roles of extensional stress and shear stress at the exit region and concluded that they were not the direct cause of cavitation. Rather, cavitation occurs in conjunction with an upstream rupture of the polymer that occurs in the contraction region leading into the capillary tube (gross melt fracture). We argue that the exit region does, however, serve as the initiation point of the cavitation because of a combination of the reduced pressure and extensional stress. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2791–2799, 2002     

The temperature window of minimum flow resistance in melt flow of polyethylene. Further studies on the effect of strain rate and branching

The existence of a narrow temperature window (150–153°C) of smooth extrudability coupled with a minimum in flow resistance (extrusion pressure) in high-molecular weight polyethylene (>4 × 10⁵ g mol^?1) was the subject of a previous article where it was associated with strain-induced formation of the mobile hexagonal mesophase. The new findings of this note show that this minimum in flow resistance only sets in above a critical strain rate; this is interpreted in terms of the requirement of a critical strain rate in order to stretch molecules to their fully extended configuration. Furthermore, this critical strain rate is shown to be higher for lower molecular weight materials, in agreement with a priori considerations. Additionally, the temperature at which the pressure minimum occurs in a polyethylene containing methyl branches shifts to a significantly lower value than that for the linear material. This is interpreted in terms of the ? CH₃ groups raising the crystal free energy, thereby lowering the temperature at which the transition to the hexagonal phase occurs.