James L. White

A Fundamental Study of the Rheological Properties of Glass‐Fiber‐Reinforced Polyethylene and Polystyrene Melts

Shear viscosity, shear flow normal stresses, and elongational viscosity have been investigated for glass‐fiber‐filled high‐density polyethylene and polystyrene melts. Viscosity measurements are consistent in cone‐plate and capillary instruments Normal stress coefficients increase more rapidly with glass fiber content than the shear viscosity, especially at low deformation rates. Elongational viscosities are very high at low elongation rates but decrease rapidly with increasing deformation rate. Mechanisms are proposed to explain these observations.

Flow patterns in polyethylene and polystyrene melts during extrusion through a die entry region: measurement and interpretation

Abstract Flow visualization experiments have been carried out on these melts flowing from a reservoir into a capillary die. The existence and magnitude of vortices at the die entrance have been determined over a range of extrusion conditions. The vortex size is interpreted in terms of the theory of viscoelastic fluid mechanics. It is found that the second-order fluid-perturbation solution cannot represent the observed experimental results. The data are correlated with (i) a Weissenberg number τch V L ≡ \ gt( γ w) γ w ≡ Ψ1 γ w/2η  (N1)w/ 2(σ12)w measured at the die wall and (ii) with the deformation-rate dependence of relaxation time. Interpretation of vortex formation and size in terms of elongational viscosity is offered. Several polystyrene and polyethylene melts have been rheologically characterized as part of this study with measurements of viscosity η and principal normal stress difference N1. The zero shear viscosity η0 of the polystyrenes varies with the 3.5 power of the weight-average molecular weight Mw while the principal normal stress difference coefficient Ψ1 varies with the sixth power of Mw when evaluated at a shear rate of 1 sec−1.

Investigation of failure during elongational flow of polymer melts

Abstract A basic study of the mechanisms of necking and ductile failure of polymer melts in uniaxial elongational flow has been carried out. A linear stability analysis was carried out using a White—Metzner convected Maxwell model with a deformation-rate-dependent relaxation time, which varies according to τ = τo/(1 + aτo[2trd2] 1 2 ). It was shown that filament stability and elongation to break depend upon τoE, where E is the elongation rate, and a. At fixed τoE, filament stability decreases with increasing a. At small a, stability increases with increasing τoE while for a > 1 √3 , stability decreases with increasing τoE. For a material with small a, ductile failure can occur for small τoE, but cohesive fracture should be the cause of failure at larger τoE. For a material with large a, however, ductile failure always dominates the failure mode. These results are used to interpret failure in elongational flow of low density and high density polyethylene and polypropylene melts and describe how the latter two melts exhibit ductile failure.

Instability phenomena in tubular film, and melt spinning of rheologically characterized high density, low density and linear low density polyethylenes

Abstract An experimental study of instabilities in melt spinning, ribbon drawdown and tubular film extrusion of moleculary and rheologically characterized high density (HDPE) low density (LDPE) and linear low density (LLDPE) polyethylenes is presented. The characteristics of the instabilities occuring in these processes are described. The draw resonance instability is found to occur in melt spinning and ribbon drawdown. Special attention is given to tubular film extrusion which has received little treatment in the literature. Three distinctive tubular film instabilities are described. These involve (i) an axisymmetric periodic fluctuation in bubble diameter; (ii) a fluctuation of frostline height and tension; (iii) a helical motion of the bubble. The characteristics and occurrence of these phenomena are discused. In the melt spinning and ribbon drawdown, th LDPE is the most stable and the broad molecular weight distribution HDPEs the most unstable. In tubular film extrusion, the LDPEs are again the most stable but the narrower distribution HDPEs and LLDPEs are much more unstable than the broader distribution HDPE. These results are discussed in terms of convected Maxwell model representations.

An Experimental Study of Rheological Properties of Polymer Melts in Laminar Shear Flow and of Interface Deformation and Its Mechanisms in Two‐Phase Stratified Flow

The stress state of four rheologically characterized polymer melts—low‐density polyethylene (LDPE), high‐density polyethylene (HDPE), polystyrene (PS), and poly(methyl methacrylate) (PMMA)—in laminar shearing flow was completely determined by measuring the non‐Newtonian viscosity μ and the first and second normal stress differences N1 and N2. The mean N2 function was negative and about 0.1–0.3 the value of N1. The measurements were made in a Weissenberg rheogoniometer using cone‐ and parallel‐plate geometries. Additional viscosity measurements were made in a capillary rheometer. An experimental study of interface distortion in the stratified two‐phase flow has been carried out using these melts. It was found in the stratified flow experiments that the lower viscosity melts exhibited concave interfaces and encapsulated the higher viscosity melts irrespective of the relative values of N1 and N2. The rate of interface distortion and encapsulation increases with increasing viscosity differences. The pressure ...

Polymer Melt Flow Instabilities in Extrusion: Investigation of the Mechanism and Material and Geometric Variables

A comprehensive study of the effect of material, geometric, and operating variables on the onset of melt flow instabilities in extrusion was made. Primary objectives were to compare such melt instabilities for a variety of materials and to correlate instability criteria with independent rheological parameters. Seven polymer melts were studied: two polyethylenes, polypropylene, polystyrene, two polybutadienes, and an SBR copolymer. Two distinct extrusion apparatuses were used: a die fed by a screw extruder system and a die fed by an Instron rheometer. The variables studied were temperature, entry cone angle, length/diameter ratio of the die, and flow rate. Independent rheological measurements were performed. These consisted of the evaluation of normal stresses on the Weissenberg rheogoniometer, entrance pressure drop measurements on the Instron rheometer, and viscosity measurements on both instruments and the screw extruder. In a companion study, flow visualization studies were made in the die entry region...

Rheo-optics of shear and elongational flow of liquid cystalline polymer solutions: hydroxypropyl cellulose/water and poly-p-phenylene terephthalamide/sulfuric acid

Abstract An experimental study is reported of the quiescent and flow birefringent characteristics of two liquid crystalline polymer solution systems, poly-p-phenylene terephthalamide (PPD-T) in sulfuric acid and hydroxypropyl cellulose (HPC) in water over a range of concentrations. It is shown that for the quiescent state the dilute solutions are optically isotropic while the concentrated solutions consists of negatively birefringent domains. During flow at low deformation rates, moving domains are still seen. At higher deformation rates, a homogeneous highly birefringet fluid is obtained. The birefringence increases with concentration at constant deformation rate and exhibits a major increase as the liquid crystalline state is formed. The source of the birefringence is due to (1) anisotropy of polarizability of oriented macromolecules, and (2) difference in refractive index of solvent and oriented macromolecules (form birefringence). The results are interpreted in terms of the level of polymer orientation which may be developed in flow for liquid crystalline polymer solutions as compared to solutions of flexible macromolecules.

A cell model theory of the shear viscosity of a concentrated suspension of interacting spheres in a non-Newtonian fluid

Abstract A theoretical approach to the shear viscosity of concentrated suspensions of small particles in a non-Newtonian fluid has been developed using a cell theory model involving particle-particle interaction. The cell theory of Frankel and Acrivos was first generalized to power-law fluid matrices without particle interaction. Particle-particle interaction was then taken into consideration. The theory suggests that the flow behavior of such systems at low shear rates is chiefly dependent upon non-hydrodynamics interparticle interaction such as van der Waals—London and electrostatic forces which induce flocculation and yield stresses. The flow properties at high shear rates are determined by hydrodynamics interaction essentially dependent upon particle concentration and shape.

Phase behavior and structure of liquid crystalline solutions of cellulose derivatives

SummaryStructural and thermodynamic characteristics of liquid-crystalline solutions of four cellulose derivatives in a range of solvents were studied. Basic observations were made on these systems using polarized light microscopy, small angle light scattering, dilute solution and concentrated solution viscosities. The polymers studied include hydroxypropyl cellulose (HPC), cellulose acetate butyrate (CAB), ethyl cellulose (EC), and cellulose triacetate (CT). The formation of the liquid crystalline phase was shown to strongly depend on polymer concentration, solvent type and temperature. The critical volume fraction of polymer required to form the liquid crystal phase varied significantly as the solvent changed. The critical volume fraction decreased with increasing solvent acidity and polymer intrinsic viscosity in a given solvent. The breadth of the two phase region seems to decrease with increasing acidity. The liquid crystalline phase was in most cases determined to be cholesteric. In all cases positively birefringent cellulose derivatives form negative spherulitic domains. In one case, the negativity birefringent system (cellulose triacetate) formed positively birefringent spherulitic liquid crystalline domains. This is interpreted to mean the structure organizes itself by a tangential alignment of polymer chains within the domain. SALS measurements appear to detect domains and in some cases cholesteristic pitch.

A theory of thixotropic plastic viscoelastic fluids with a time-dependent yield surface and its comparison to transient and steady state experiments on small particle filled polymer melts

Abstract Suspensions of small particles in polymer melts exhibit a time-dependent thixotropic responses. Previous theories of the behavior of such systems while including the capability of predicting non-Newtonian viscosities, normal stresses and yield values (indicating stresses below which there is no flow), have not treated thixotropy. Such a theory is presented here. A time and deformation rate dependent von Mises form yield function is proposed to represent the thixotropic and plastic characterisitcs. This is combined with a non-linear viscoelastic constitutive equation. The theory is compared with experimental data for carbon black and calcium carbonate filled polystyrene melts. Reasonably good agreement is obtained with both transient and steady-state experiments.