Elongational viscosity of polymer melts: A lubricated skin-core flow approach

Previous work by this research group has shown that the use of a lubricated skin/core flow of polymer melts and a hyperbolic converging die results in an essentially pure elongational flow at a constant elongational strain rate in the core. The previous work was carried out on a laboratory-scale coextrusion system in a planar slit die; tracer particles and an image analysis system were used to confirm the predicted behavior. In this work, the technique was implemented first on the coextruder assembly, as a planar elongational rheometer, and then on a commercial capillary rheometer, as a uniaxial elongational rheometer for polymer melts. The later is achieved by replacing the standard capillary die with a hyperbolic axisymmetric die. A two-laycred billet is prepared for placement in the rheometer barrel by completely encapsulating the core polymer (the polymer to be analyzed) with a low-viscosity polyethylene skin. Commercial grades of polypropylenes, syndiotactic polystyrene, and nylon-66 were analyzed using this technique. Elongational viscosity at high extensional rates can be determined with this method; values in excess of 500 s⁻¹ have already been achieved. © 1996 John Wiley & Sons, Inc.     

Flow-induced crystallization in polymer melts: how Winter’s gelation concept fits into the picture

Nobody could expect that gelation times of many hours, which were observed in quiescent polymer melts below the melting temperature of the spherulites, would play a role in flow-induced crystallization. However, this role became obvious for an iPP, when special polarization optics were employed in the well-known arrangement of the Linkam CSS450 apparatus (parallel glass plate rheometer). With the aid of a λ-plate, the interference color red of first order was created in white polarized light. This sensitive interference color changes quickly into blue, when a small phase difference is added (with subtraction a yellow color is obtained). With the aid of the great number of blue halos, which came up at 145 °C after the cessation of flow, an oriented gelation was identified. It spreads faster over the sample than the spherulites. The unwieldy gelation times, which have been found in quiescent melts, seem to be the consequence of a shortage of nuclei.     

Kinetics of polymer crystallization under processing conditions: transformation of dormant nuclei by the action of flow

In previous papers [Janeschitz-Kriegl H, Ratajski E, Stadlbauer M. Flow as an effective promotor of nucleation in polymer melts: a quantitative evaluation. Rheol Acta 42 (2003) 355-364; Stadlbauer M, Janeschitz-Kriegl H, Eder G, Ratajski E. New extensional rheometer for creep flow at high tensile stress. Part II. Flow induced nucleation for the crystallization of iPP, J Rheol 48 (2004) 631-639. [1] and [2]] two types of experiments were carried out: (a) rapid quenches of quiescent melts of i-PP from their state of equilibrium to a series of rather low temperatures and (b) short term shearing or extension of melts at only mild degrees of undercooling. Strong undercooling as well as high mechanical loads cause similar tremendous increases in the number densities of nuclei by many decades. Unexpectedly, an extremely non-linear dependence of the said number densities on the loading times has been found.In the present paper an explanation for this non-linear relation is tried. The assumption is made that in a quiescent melt there is a huge reservoir of badly organized aggregates (local alignments) of chain molecules, which as such can become effective nuclei only at rather low temperatures. These aggregates are assumed to grow by the action of flow after being oriented in the flow direction. In this way a large amount of low quality dormant nuclei can be transformed into nuclei of a better quality, which are active at higher temperatures. Starting at individual aggregates this growth can lead to thread-like precursors initiating shish-kebab structures.Instead of the flow time the specific mechanical work has been found to be a useful universal parameter. Some measurements of the optical retardation are quoted, which strongly support the basic assumption.     

Rheological Behavior and Mechanical Properties of Blends of Poly(vinyl chloride) with CP-POSS

In this work, a polyhedral oligomeric silsesquioxane which contains 3-chloropropyl groups (CP-POSS) was synthesized. The rheological behavior of CP-POSS/PVC blends was investigated by torque rheometer and capillary rheometer. Mechanical properties were investigated by electronic material tester. Influences of blending composition, shear rate and shear stress on melt apparent viscosity and non-Newtonian index (n) were discussed. The results show that the plastic time decreases and melt viscosity increases with increasing content of CP-POSS. CP-POSS has a good compatibility with PVC. The blend has the best impact strength when the content of CP-POSS is 7 wt%. The CP-POSS can be used as a processing aid and impact-resistant aid for PVC.     

Effect of combining boron nitride with fluoroelastomer on the melt fracture of HDPE in extrusion blow molding

Boron nitride (BN) is a new polymer processing aid which not only eliminates surface melt fracture in the extrusion of molten polymers, but also postpones the critical shear rate for the onset of gross melt fracture to significantly higher values that depend on resin type and additive concentration. In this work, the influence of BN as a polymer processing additive is first examined in the extrusion blow molding of high‐density polyethylene (HDPE) resins in order to evaluate its usefulness and performance in operations other than continuous extrusion. The equipment used includes both a Battenfeld/Fisher 50‐mm extrusion blow molding machine and a parallel‐plate rheometer. Two types of HDPE, which are blended with boron nitride at various concentration levels, are tested accordingly. It is found that the degree of BN dispersion, characteristics of the HDPE resins, extrusion temperature, and induction time play an important role in eliminating melt fracture. Finally, the influence of combining BN with fluoroelastomer, as an enhanced and potentially better processing aid on the melt fracture of a third HDPE is examined. It is found that such a combination is a superior processing aid that allows extrusion blow molding at very high shear rates.     

流变仪旋转叶片法在自由基聚合反应中的应用研究Ⅰ:丙烯酰胺自由基聚合反应的空闲时间测试方法

使用装备自制的四叶片旋转叶片的应力控制流变仪研究过硫酸铵(APS)和四甲基乙二胺(TEMED)氧化还原体系作为引发体系引发丙烯酰胺(AM)自由基聚合过程的模量变曲线,交联剂为N,N’-亚甲基双丙烯酰胺(MBAM)。通过与传统的平行板夹具比较说明了旋转叶片法测试丙烯酰胺聚合是准确、方便的。从丙烯酰胺聚合过程的模量曲线和温度曲线可以得到体系聚合的空闲时间(idle time)。     

A new technique to determine rate constants for growth and agglomeration with size- and time-dependent nuclei formation

Fluidized bed spray agglomeration is a particle size enlargement process where particles stick together with the help of spraying binder. High impact forces between particles lead to attrition. Attrition may be modeled as poly-disperse nucleation. Furthermore, particulate event like over-spray leads to the formation of particles in a wide range of volume. A new technique for the determination of agglomeration, growth and nucleation parameters is presented in this work. The model is based on a previous approach which takes mono disperse nuclei formation in the smallest class into account. Frequently in crystallization processes, nucleation is assumed to be mono-disperse. The technique presented here incorporates nuclei formation in a certain range of volume. It is quite general and applicable to consider size- and time-dependent nuclei formation. For two particular cases of growth and agglomeration including size-dependent nuclei formation, simulation data was generated by continuous feeding of nuclei in a certain range to demonstrate the capability of parameter extraction of the model. Further, the new technique is applied to extract rate constants from experimental data measured during fluidized bed spray agglomeration. This technique is also useful for the prediction of bimodal behavior of particle size distributions (PSD).     

Combining boron nitride with a fluoroelastomer: An enhanced polymer processing additive

The effect of a new processing additive (boron nitride powder in combination with a fluoroelastomer) on the rheology and processability of molten polymers is studied. The equipment used includes an Instron capillary rheometer equipped with a special annular die (Nokia Maillefer wire coating cross‐head) and a parallel‐plate rheometer. Metallocene polyethylenes with and without boron nitride (BN) and fluoroelastomer are tested in extrusion. First, it is demonstrated that BN is a superior processing aid compared to conventional fluoropolymer ones. Second, it is found that the combination of BN powders with a small amount of a fluorelastomer improves even further the processability of molten polymers (melt fracture performance).     

Influence of long-chain branching on the rheological properties of low density poly(ethylene)

The objective of this work was to correlate rheological parameters with basic molecular parameters such as different molecular weights and level of long-chain branching (LCB) in low density polyethylene. LCB is known to exert a considerable influence on resin's melt flow characteristics which have been useful in predicting resin end-use properties (especially processability). In the scope of the work steady flow measurements were done with an Instron Rheometer and dynamic oscillatory measurements using a balance rheometer. The molecular weight data were obtained using gel permeation chromatography, light scattering technique, and solution viscosimetry. A correlation matrix was calculated for 34 measured properties. The results showed that good correlations may be obtained especially between LCB-parameters, die swell and optical properties of the polymer. The LCB properties may also be quite correctly predicted from dynamic viscosity data at low frequencies.     

A new processing aid for the extrusion of polyolefins

The influence of a new processing additive (a composition of fine particles of boron nitride) on the rheology and processability of molten polymers is studied. The equipment used includes both an Instron capillary rheometer with special annular dies (Nokia Maillefer wire coating crosshead) attached to the rheometer and a parallelplate rheometer. Two metallocene polyethylenes with several types of boron nitride powders varying in particle size distribution are tested at various concentration levels. The powder having the finest particle size was found to have the greatest influence in the processing of polyolefins using crosshead dies and tips. As a result, its use eliminates surface melt fracture and postpones the critical shear rate for the onset of gross melt fracture to significantly higher values depending on the additive concentration. The performance of the boron nitride in eliminating melt fracture is compared with that of a typical fluoroelastomer. It is shown that boron nitride is a superior processing aid.     

Melt flow behavior of blends from poly(vinyl chloride) and epoxidized natural rubber

Blends from poly(vinyl chloride) (PVC) and epoxidized natural rubber (ENR) were prepared in a Brabender plasticorder by the melt blending technique. The melt flow behavior of these blends with respect to blend ratio and temperature has been examined using a melt flow indexer and capillary rheometer. ENR decreases the Brabender torque, increases the melt flow index (MFI), and decreases the melt viscosity of PVC in the blends. Arrhenius plots were used to study the effect of temperature on melt flow index (MFI) and viscosity. Moreover, the flow behavior index (n′) obtained from capillary rheometer data was found to be dependent on temperature and blend ratio.     

Determination of the pressure dependence of polymer melt viscosity using a combination of oscillatory and capillary rheometer

For an accurate simulation of a high-pressure injection molding process by using the CAE software, it is important to understand the pressure sensitivity of a polymer's melt viscosity. The current work describes a method for the determination of the pressure dependence parameter D₃ of the Cross-WLF model. It uses a combined rheological technique using both dynamic and capillary rheometers. Three grades of polycarbonate homopolymers were studied in this work and their complex viscosities were measured using a dynamic shear rheometer. The dynamic data were used to obtain six out of the seven parameters of the Cross-WLF, except D₃. A capillary rheometer fitted with a counter pressure chamber was further used to characterize the pressure dependence of the zero shear viscosity and to determine the D₃ parameter. Finally, the derived parameters were validated by carrying out injection molding with a box tool and comparing the actual pressure profiles with simulation results using the Autodesk® MoldFlow® software. The validation results indicated that actual pressure profiles from the simulation were found to be less than 10% than that of injection molding. POLYM. ENG. SCI., 60:517–523, 2020. © 2019 Society of Plastics Engineers     

New developments in melt rheology aid formulating rigid vinyl compounds with improved extrusion processing

This paper describes a new approach to the rheological characterization of PVC compounds using a capillary rheometer. Since the control of the thermal degradation rate of rigid PVC is very critical in extrusion, a new quantitative method of measuring the true degradation rate of PVC is proposed for formulating rigid vinyl compounds. The usefulness of this new technique is demonstrated in formulating PVC compounds with the lowest rate of degradation measured on the Kayeness capillary rheometer. This study shows a very good correlation between the capillary rheometer data and the extrusion melt temperature, melt viscosity, and the rate of degradation of PVC. The ideal melt temperature for extrusion is determined from the rheology data of each PVC compound to attain the longest run time. The study also shows how to measure the true melt temperature in extrusion, an important variable, to control the rate of degradation in processing. The results of this study show how the new rheology technique can be used as a practical tool for product development, quality control of the PVC compounds, and the development of optimum extrusion parameters.     

Characterization of impregnated particles via powder rheology

This work presents a specific application of a new powder rheometer prototype, for the characterization of liquid-impregnated particles which are used in chemical engineering for gas treatment on fixed beds. This new type of rheological characterization can provide useful information about the surface state of the particles and could contribute to the optimization of the impregnation process. The rheometer, consisting of a stress-imposed rheometer coupled with a vibrating cell, generates a Brownian-like motion at a macroscopic scale which makes the sample behave as a non-Newtonian condensed fluid, and allows rheological property measurements of powders which are very sensitive to changes on their surface. Their rheological behavior and its relationship with the impregnation ratio have been analyzed for several samples having different impregnation ratios, thanks to a free volume structural model taking into account the shear rate, the frictional stress, the granular pressure, the vibration frequency, the vibration energy, the free volume distribution and the mass of the samples. The resulting interpretation suggests a stepped kinetic of impregnation: first the liquid adsorbs on the surface of the silica matrix, then it fills progressively the pores, and finally it coats the outer surface of the particles.     

Über die vernetzung eines ungesättigten äthylen-propylen-terpolymer-kautschuks

The various aspects of the sulfur vulcanization chemistry and mechanism of ?Nordel”? hydrocarbon rubber, an ethylene-propylene terpolymer, are discussed. The chemical nature of the reactions undergone by the accelerators has been found to be analogous to those observed for natural rubber and SBR. During vulcanization approximately 50% of the double bonds are saturated, and approximately one crosslink is formed for each two double bonds initially present. The type of crosslink formed has been examined and the vulcanization kinetics studied with the aid of a Monsanto rheometer.     

Biopolyester melt behavior by torque rheometry

A torque rheometer method has been developed to evaluate the melt behavior of biopolyesters. The main features of this technique include: (i) a modified equipment configuration, so as to maintain the melt temperature within ± 1°C pf the set temperature after the first few minutes of polymer melting and thermal equilibration, and (ii) a kinetic equation that can be used to determine the melt behavior from the torque-time data. Applying this methology to a poly(3-hydroxybutyrate-co-3-hydroxyvalerte) copolymer shows that the degradation kinetics follow Arrhenius behavior with temperature, that the degradation rate increases with increasing hear, and that the activation energy for random chain scission is independent of shear rate. This effect of shear on the degradation process may be a result of viscous heating. The degradation rate is increased because of the increased internal energy of the polymer chains as a result of the mechanical deformation. There is no evidence of direct mechanical degradation. Last, the viscosity values obtained with the torque rheometer are consistent with those obtained by capillary rheometry.     

A rheological characterisation technique for fast UV-curable systems

Using a rheometer coupled with a UV-light generator, a photo-rheometry set-up has been developed to study the viscoelastic properties of UV-coating systems during fast curing. Due to a high reaction rate, the viscoelastic properties have to be evaluated using a special procedure. This technique was found suitable to obtain reliable rheological data during the fast photo-reaction, allowing the determination of gel points occurring within less than 1 s.     

聚丙烯/碳化硼共混体系的流变性能

在成纤聚合物中混入不同的无机物粉末,可研制一系列具有特殊性能的新型纤维。本文采用转矩流变仪和毛细管流变仪相结合的方法对碳化硼含量为5～35%（重量比）的聚丙烯/碳化硼共混体系的流变性能进行了研究,并对实验结果进行了处理,得出了有价值的结论,为研制防中子辐射纤维提供了实验依据。     

A new approach to capillary viscometry of thermoset transfer molding compounds

A new capillary rheometer incorporated in an instrumented transfer molding press has been developed. This rheometer is effective for determining the viscosity characteristics of thermoset molding compounds under both isothermal and typical molding conditions. In examples of the rheometer's utility, the power law indices of two commercial epoxy molding compounds have been determined to be approximately 0.7. Additionally it has been shown that preconditioning typical epoxy compounds at 47 percent relative humidity causes a viscosity decrease of about 40 percent owing to plasticization of the epoxy resin.