摘要:

A vane rheometer was used to measure the modulus of foam systems. Modulus development in a reactive polyurethane foam system was captured using a commercial stress-controlled rheometer with a four-blade vane geometry test fixture. The results were compared to those from a temperature programmed flooded parallel plate method. The vane geometry method was shown to be a convenient and accurate way to measure the physical gelation time of the reactive foaming system, without the complexity of the temperature-program method. Shaving cream was used as a calibration material to show the validity of the vane method for foam modulus measurements. For open cell foams, the error in the vane method increases due to the additional compressibility of the foam. The nature and magnitude of this error was analyzed via numerical simulation.

ISSN:0148-6055    

DOI:10.1122/1.550906

出版商:The Society of Rheology    

年代:1998

数据来源: AIP

摘要:

Planar Stokes flow of a concentrated planar suspension of rigid cylinders is simulated using the finite-element method. A constraint condition, limiting cylinder approach to a specified distance, is applied to simulate interparticle contacts. The results of these simulations reveal a stress response and ordering of the cylinders which is similar to experimental observations of suspensions of nominally spherical particles. These results include direct observations of anisotropic structure, as well as the development of a distinct particle-phase pressure resulting from the asymmetry of interparticle contacts and the consequent developed anisotropy. A simple pair-wise interaction model is proposed forab initioprediction of the particle-phase pressure for these planar suspensions of cylinders.

ISSN:0148-6055    

DOI:10.1122/1.550955

出版商:The Society of Rheology    

年代:1998

数据来源: AIP

摘要:

Isothermal melt spinning is a prototypical example of many polymer processes: The polymer melt is first subjected to simple shear flow in the extrusion die, which is followed by uniaxial extension under constant force in the spinline. In a Rheotens test, the tensile force needed for elongation of an extruded filament is measured as a function of the draw ratio. In this way, a whole set of melt spinning conditions is probed during one test run. The analysis of this complicated deformation history is simplified by the existence of Rheotens grandmastercurves. These allow a direct and straightforward description of material behavior in the fiber spinning process, and a simplified determination of an apparent elongational viscosity under the action of a constant drawdown force.

ISSN:0148-6055    

DOI:10.1122/1.550907

出版商:The Society of Rheology    

年代:1998

数据来源: AIP

摘要:

The rheology of aqueous dispersions of colloidal hematite(α-Fe2O3)rods with well-characterized shape and dimension was investigated. The effects of volume fraction (φ), aspect ratio, and Debye length(κ0−1)on the rheology were studied in particular. The particles were spindle-type bodies with aspect ratios equal to 8.4 and 4.8. They were dispersed in aqueous solutions of known ionic strength at a ζ potential, which was characterized in the Smoluchowski limit. Their rheology was compared to that of spherical hematite prepared by the method of Reference 1985. Dilute solution viscometry indicated that the effects ofκ0−1and the aspect ratio on theO(φ2)contribution to the low-shear effective viscosity were of comparable magnitude, and that the Huggins coefficient was a decreasing function of the aspect ratio. For concentrated suspensions, the dimensionless shear thinning was adequately fit by a correlation involving a critical stress, which was originally developed for spherical suspensions [Reference 1959]. The volume fraction dependence of the zero-shear viscosityη0was found to be a strong function of both the aspect ratio andκ0−1.The aspect ratio affected both the volume fraction at whichη0diverged(φm),and the strength of the singular behavior [characterized by α inη0/μ=(1−φ/φm)−α]. In contrast,κ0−1affectedφm,but not α. The effect of the aspect ratio and volume fraction on the linear viscoelasticity of suspensions forφ > φmwas quantified. The results highlight the particular ways in which the colloidal suspension rheology of moderate aspect ratio rods differs from that of spheres.

ISSN:0148-6055    

DOI:10.1122/1.550961

出版商:The Society of Rheology    

年代:1998

数据来源: AIP

摘要:

A simple superposition model was used to define the relationship between molecular weight distribution and shear viscosity for linear polymeric systems. Gel permeation chromatography data for molecular weight distributions were fitted using statistical distribution functions. A simple superposition model was then employed to calculate the shear viscosity for the systems investigated. The effect of polydispersity on the shape of the flow curves was calculated. The simplicity of the model makes feasible its use in numerical simulations of complex geometries as encountered in polymer processing equipment. The present study also sheds some light on the relationship between entanglement and disentanglement phenomena in polymeric systems.

ISSN:0148-6055    

DOI:10.1122/1.550908

出版商:The Society of Rheology    

年代:1998

数据来源: AIP

摘要:

In this work we investigate the use of an inhomogeneous structural model which explicitly takes into account flow-induced crystallization for representing the necking phenomenon in high-speed fiber spinning. For simplicity, we have considered a one-dimensional (cross-section averaged) approximation for an isothermal system with no surface tension and air drag, with or without inertia. Flory’s approach [J. Chem. Phys.15, 397–408, (1947)] is used to predict the onset of crystallization in the spinline. After the onset of crystallization, the fiber is modeled as an inhomogeneous medium with two separate (meso) phases—one semi-crystalline and the other amorphous. The amorphous phase, before and after the onset of crystallization, is modeled as a viscoelastic fluid, represented here by the extended White–Metzner model. The semi-crystalline phase is modeled as an anelastic solid. We demonstrate neck formation for a variety of processing conditions and material property values consistent with those encountered in practice. In particular, the addition of inertial effects, which can also be important in high-speed fiber spinning, shifts but does not eliminate the window in parameter space over which the inertialess model predicts neck formation. Based on these results, we propose as a mechanism for the neck formation the structural changes within the material induced by the crystallization and the ability of the semi-crystalline phase to rapidly take up high stresses.

ISSN:0148-6055    

DOI:10.1122/1.550913

出版商:The Society of Rheology    

年代:1998

数据来源: AIP

ISSN:0148-6055    

DOI:10.1122/1.550916

出版商:The Society of Rheology    

年代:1998

数据来源: AIP