Capillary Flow Behavior Research Articles

A relationship between extrudate swell ratio and entry stored elastic strain energy during die flow of tyre compounds

The elastic strain energy stored in the channel entrance flow and the extrudate swell ratios are important characteristics of elasticity during channel extrusion of polymer materials. There should be some form of relationship between them, especially in the case of the use of a short or mid-length die. The viscoelastic behavior and mechanisms in capillary extrusion flow of rubber compounds were investigated in the present paper. Under the operation conditions with temperature of 90 °C and apparent shear rate range of 10–10 3 s −1, the rheological properties of two tyre compounds, cavity stock and buffer stock, were measured. It was found that the shear flow of both the tyre compounds obeyed the power law, and the non-Newtonian index values were close to each other. The entry pressure drop (Δ P 0), entry elastic stored energy ( e) and die-swell ratio ( B) increased roughly linearly with increasing shear stress at the channel wall. The B value for cavity stock was greater than that for buffer stock, whereas it was the reverse for Δ P en. Also, the correlation between the die-swell ratio and entry elastic stored energy is discussed. The results showed that B was a linear function of e.

STUDY OF THE ANOMALOUS CAPILLARY BAGLEY FACTOR BEHAVIOR OF THREE TYPES OF WHEAT FLOUR DOUGHS AT TWO MOISTURE CONTENTS

ABSTRACTThe flow properties of wheat flour dough were evaluated using a capillary rheometer as a function of flour type (soft, hard and durum wheat flours) and dough moisture content (43 and 48% wb). The description of experimental flow properties was conducted using the classical Bagley approach and assuming linear relationships between pressure drop and L/D ratio (R2 ranging from 0.795 to 0.993). The entrance pressure drop (Po) increased (from 40 to 1870 kPa) with an increase in shear rate (from 40 to 1333 s−1) and a decrease in dough moisture content. Shear viscosities (7 to 160 Pa.s) and elongational viscosities (41 to 2840 kPa.s) demonstrated shear‐thinning behaviors that were characterized using power law models. Elongational and shear flows were compared by the Trouton ratio. The characterization of the capillary flow behavior for wheat flour dough was then discussed in regard with a possible nonlinear behavior of Bagley plots and related to possible changes in dough structure (i. e. dough hardening, strain hardening and dough deorientation).

Influence of Mixing Conditions and Rest Time on Capillary Flow Behavior of Wheat Flour Dough

ABSTRACTThe capillary flow properties of wheat flour dough were evaluated as a function of mixing time (3–60 min) and rest time (0 or 120 min) after mixing. The description of experimental flow properties was conducted using the classical Bagley approach. Long mixing times were more efficient in decreasing the resistance to elongational flow than in decreasing the resistance to shear flow. The effects of rest time on the rheological properties were estimated by calculating the percent changes in values during resting and appeared to depend on the mechanical history of dough (mixing conditions). The observed changes in rheological properties were related to possible changes in dough structure. Exponential models were used to describe the changes in farinograph consistency (R2 = 0.981) and extensional viscosity (R2 = 0.744 to 0.949) as a function of the mechanical energy input during mixing.

Temperature-dependent instabilities in the capillary flow of a metallocene linear low-density polyethylene melt

The capillary flow behavior of a metallocene linear low-density polyethylene was studied in a wide temperature range. The critical shear stress for the onset of the unstable spurt flow was found to be dependent on temperature in a nonlinear fashion and it showed a minimum value at a critical temperature, at which unusually long period pressure oscillations were observed. For temperatures above the critical one, the observed decrease of the critical shear stress with decreasing temperature is explained on the basis of an increase in the distance between entanglements. At temperatures below the critical one, the increase in the critical shear stress and the eventual suppression of pressure oscillations as the temperature is further decreased are suggested to be the result of a flow-induced phase change that ends on complete crystallization and suppression of flow. The flow-induced crystallization phenomenon and the extrudate quality were dependent on the contraction ratio. Elimination of surface extrudate distortions took place at low temperatures when using a contraction ratio of 30, this fact can be attributed to the flow-induced phase change. Finally, a decrease in the activation energy for flow or “easy flow” was observed at temperatures below the critical one. Such “easy flow” is perhaps the precursor of the “temperature window” of low-flow resistance reported by Keller and co-workers [Kolnaar, J. W. H. and A. Keller, Polymer 35, 3863–3874 (1994); Waddon, A. J. and A. Keller, J. Polym. Sci. Polym. Part B: Polym. Phys. 28, 1063–1073 (1990)].

Narrow-pore Flow Behavior of Blood Cells in Cardioplegic Suspension

Cardioplegia alters the ionic composition of the myocardium and also the blood in a way that may influence the cellular capillary flow behavior. We measured changes in RBC volume and narrow-pore flow resistance of blood cardioplegia versus crystalloid medium. Potassium, magnesium and sodium as osmotic control caused an expected cell shrinkage and reduced the flow resistance through 3 microm pores; however, stressing the osmosis further resulted in increased resistance. No major effects were seen with the 5 microm filters. Twenty percent blood cells in the cardioplegic medium caused a 360% increase in 5 microm pore resistance. There were no obvious additional filterability effects of the cardioplegic additives other than their osmotic patterns. There may be a theoretical advantage in having a cell-free medium in terms of flow resistance. Using blood cardioplegia, a limited hypertonicity may be beneficial in reducing the capillary flow resistance of RBC.

Wall slip and absence of interfacial flow instabilities in capillary flow of various polymer melts

We conduct a comprehensive experimental study of melt/wall interfacial slip behavior of various polymer melts in capillary flow. Polymers under study include two polystyrenes of different molecular weights (MW=280 000 and MW=2×106, respectively), two low-density polyethylenes (LDPE), poly(ethylene vinyl acetate) (EVA), and polypropylene (PP). The experimental results reveal an important finding that has greatly extended our previous knowledge of the roles of melt/wall interfacial interactions and molecular entanglements in dictating capillary melt flow behavior. None of the six polymers exhibits an interfacial stick–slip transition or shows any sign of wall slip in bare aluminum dies. Yet, all are found to display a sizable wall slip when the strength of polymer/surface adsorption is reduced by lowering the die wall surface energy with a fluoropolymer coating. The degree of wall slip as characterized by the Navier–de Gennes extrapolation length is demonstrated to be explicitly proportional to the melt viscosity, such that the magnitude of wall slip in the coated die decreases with increasing shear thinning. Diminishing of a measurable wall slip at high stresses explains the well-known absence of spurt, flow oscillation, and sharkskin phenomena in capillary flow of commonly encountered polymers such as PS, LDPE, PP.

Capillary flow behaviour of microcrystalline wax and silicon carbide suspension

Suspensions of ceramic particles in low or high molecular weight polymers are shaped into various three-dimensional parts using various moulding and extrusion technologies. Such bodies are subsequently fired-up and sintered to remove the binder. The utilities of such three-dimensional ceramic bodies depend on the restrictions related to the shapeability of the ceramic suspension, hence to the flow and deformation behaviour of the suspension. In this study, factors affecting the flow and deformation behaviour of a 50% by volume of silicon carbide in a wax binder was investigated. Consistent with the previously observed behaviour of other highly filled materials, the ceramic suspension exhibited viscoplasticity, plug flow and wall slip. Furthermore, flow instabilities associated with the axial migration of the low viscosity binder under the imposed pressure gradient were observed. These results pinpoint to the various difficulties associated with the collection of rheological data and emphasize the relevance of various flow mechanisms, including wall slip and mat formation and filtration based flow instabilities, which would also occur in processing/shaping flows of such ceramic suspensions including extrusion and moulding.

Interfacial phenomena in the capillary extrusion of metallocene polyethylenes

Metallocene catalysts are known to produce homogeneous random polyolefin copolymers with narrow molecular weight distribution and controlled long/short-chain branching. Two such linear low-density polyethylenes were studied by using both constant-stress and capillary rheometry, in order to assess their rheological and processing behavior, as well as to identify critical conditions for the onset of flow instabilities. It was found that these polymers are thermorheologically complex liquids, apparently due to the presence of long-chain branching. Compared with conventional linear low-density polyethylenes, these metallocene polyethylenes exhibit quite unusual behavior in capillary flow, not previously reported to our knowledge. Specifically, we have encountered long transients in start-up of capillary experiments, and in some cases, the capillary reservoir had to be loaded several times before a steady-state pressure was obtained. In addition, we found that these polymers slip at shear stresses higher than about 0.05 MPa. This critical value is much smaller than the critical value of 0.10 MPa reported for other conventional polyethylenes. A simple qualitative model is proposed to rationalize these findings. It suggests that the molecular mechanism of slip is dominated by a competition between flow-induced chain detachment from the wall and disentanglement of the chains in the bulk from those attached to the wall.

A viscosity function for thermoplastics blended with a thermotropic liquid crystalline polymer

The capillary flow behavior of thermoplastic blends containing thermotropic liquid crystalline polymers (TLCP) was studied both theoretically and experimentally. Significant viscosity reductions were observed for a polyethersulfone (PES) blended with a TLCP at 10 and 30 wt %, respectively. A viscosity function was developed and tested to evaluate the relationship between the blend viscosity reduction and weight fraction including some property-related parameters: η = η0[1 − φ + φ/δ + K(1 − φ)(1 − λ)φϵ]−1, where η and η0 stand for blend and matrix viscosity, respectively. ϕ is the weight fraction; δ, the viscosity ratio of the two parent components; λ, the critical stress ratio; and ϵ, an exponent. Interfacial slippage in the blend has been suggested as the flow mechanism by which a substantial reduction in the melt viscosity takes place upon the addition of a small amount of TLCP. The fact that experimental data could be fitted to the viscosity function with satisfactory accuracy should make the function an acceptable way of modeling, analyzing, and parameterizing the experimental rheological data of these blends. © 1996 John Wiley & Sons, Inc.

Stick−Slip Transition in Capillary Flow of Polyethylene. 2. Molecular Weight Dependence and Low-Temperature Anomaly

The present study focuses on molecular characteristics and temperature dependence of a superfluid-like stick−slip transition in a series of highly entangled high-density polyethylene (PE) resins and their rheological behavior in pressure-driven capillary flow. It is found that at sufficiently high stresses the capillary flow characteristics are largely dictated by the stick−slip transition occurring at the PE/die wall interfaces. The transition magnitude is found to have an explicit and strong molecular weight Mw dependence reminiscent of bulk chain entanglement, i.e., the extrapolation length bc ∝ Mw3.4. In addition to its normal temperature dependence for T ≥ 200 °C as described in paper 1, the critical stress for the stick−slip transition ceases to scale linearly with T from 200 down to 160 °C for all four molecular weights studied. The extrapolation length bc at the transition also becomes much larger at a lower T. The anomaly is clearly interfacial in nature and may involve a flow-induced ordered mes...

Capillary flow behavior of bicomponent polymer blends

The flow behavior of bicomponent polymer blends of four types of polymers (polypropylene, polystyrene, high-density polyethylene and polymethyl-methacrylate) was examined using a capillary extrusion rheometer. The viscosity of the blend was generally less than the value calculated by the theoretical or empirical additivity rules proposed in previous reports, whereas the entrance pressure loss, which is considered to be an effect of elasticity, was larger than the estimated value. Thus the variation of the viscosity with blending ratio was inversely proportional to the variation in the elastic property. The cross-section of the material extruded in a roughly dispersed state showed an annularly stratified flow pattern in which the lower viscosity component polymer appeared to form the outer skin layer. However, the observation that the viscosity of the properly blended material at certain blending ratios was sometimes lower than that of either homopolymer could not be explained.

Anomalous Bulk Viscosity of Cholesteric Systems Exhibiting Blue Phases

Abstract The bulk viscosity of two cholesteric mixed systems cholesteryl nonanoate/cholesteryl chloride and CB 15/ZLI 1612 has been measured using Ubbelohde capillary viscometers. A viscosity maximum close below the clearing point only has been observed in such mixtures exhibiting blue phases (BP). Cholesteric systems with larger pitches in which no BPs exist show a nematic-like capillary flow behaviour.

Capillary and slit flow behavior of concentrated solutions of a rod-like polymer

This paper is concerned with the flow behavior of isotropic solutions of the rod-like polymer, poly(p-phenyleneterephthalamide) (PPT), in 100 percent sulfuric acid. Studies include entry flow visualization in a slit die and solution fracture, and die swell in capillaries and a slit die. It was observed that solutions of PPT exhibit nearly negligible die swell, a slip-stick type of fracture that disappears at high shear rates, and radial entry flow patterns similar to Newtonian fluids. Fracture was associated with the plateau in the shear stress vs shear rate curve. Because values of the wall shear stress (τw.) obtained from capillary measurements were in good agreement with those obtained from a cone-and-plate rheometer and values of the loss modulus (G″) obtained from small-strain dynamic oscillatory measurements, it is believed that the rheological properties lead to the flow instability. These results are in agreement with the predictions of a recent theory by Doi and Edwards for concentrated solutions of rod-like molecules. Data are also presented for a flexible chain polyamide, nylon 6,6, in 100 percent H2SO4 for the purpose of comparing the flow characteristics of rigid and flexible chain polymers.

A Study of the Flow Behavior of Fillers Dispersed in a Polymer Melt

The present study is an experimental investigation of the effects of the kinds and dimensions of dispersed fillers on the capillary flow behavior of a polypropylene. Two kinds of fillers were used: glass-fibers with two different fiberlengths, and glass-beads with two different diameters.The results may be summarized as follows:(1) The coefficient of viscosity of melts with dispersed fillers under a constant shear rate of 1 sec-1 increased with the increase of content of fillers, and the effects of filler content on the viscosity in the case of glass-fiber dispersed melts were generally larger than for melts containing glass beads. And the relation between the increasing of viscosity of fillers and volumetric content of fillers in the case of longer glass-fiber was larger than the calculated value using theoretical equations, such as those of Einstein and of Guth and Smallwood.(2) The viscosity increase with the dispersion of fillers decreased noticeably with the increase of shear rate.The relation between the relative viscosity and shear rate was represented by a master curve, which was obtained by moderate shifting on the horizontal axis of shear rate.(3) The pressure loss at the capillary inlet increased with the increase of filler content at low temperature such as 180°C, but the effects of fillers upon the pressure losses were generally complex at high temperatures such as 210°C.(4) The die swell ratio for filled melts was smaller than that of non-filled melts. The decrease of the die swell ratio of filled melts seems to be affected not only by the decrease of the volume ratio of resin in the system but also by the accelerated orientation of filler induced by flowing in the case of glass-fiber fillers.

The Effect of Hydrostatic Pressure on the Capillary Flow Behavior of a Polymer Melt

The present study is an experimental investigation of the effects of hydrostatic pressure on the capillary flow behavior of a polystyrene melt by using an Instron type rheometer with a two-stage capillary.The results may be summarized as follows:(1)The relations between the pressure losses in the first and second capillaries are given by the following equationsΔp1=Δptot-Δp2 (a)Δp2=pn (b)where, Δp1 and Δp2 are the pressure losses in the first and second stages, respectively, Δptot is the total pressure loss, which nearly equals the applied pressure p, and pn is the hydrostatic pressure induced by the second capillary. Since the sample is compressed in the first stage by the pressure pn, which is estimated by equation (b), it is possible to make an experimental study of the effects of hydrostatic pressure.(2) The ratio of the viscosity coefficients for hydrostatic pressure flow to that for atmospheric pressure flow η′n/η′o is approximately given by the following equation._??_ (c)where, K1 and N are material constants.(3) The pressure loss due to the elasticity effect of the specimen at the capillary inlet, under the influence of the hydrostatic pressure, is given by the following eqution_??_ (d)where, ΔpE.0 is pressure loss at the capillary inlet under hydrostatic pressure, ΔpE.0 is that under stmospheric pressure, Δυ/υ0 is the volumetric strain, A and K2 are material constants.

The Critical Shear Stress in Capillary Flow of Polymer Melts

The objective of the present study is to compare experimentally the capillary flow behavior determined by using a KOKA-type flow tester with the concentric cylindrical flow behavior by using a rotational viscometer at the critical condition of the irregular flow of high-density polyethylene and polypropylene melts.In general, the yield shear stress τy obtained by the rotational viscometer test is lower than the critical shear stress τw* obtained by the capillary flow test. This is because the yield shear stress decreases considerably with increasing temperature while the critical stress increases slightly with temperature in the higher temperature region.The characteristic curves, or the relationship between the shear rate and shear stress, for the concentric cylindrical and capillary flow tests under a constant temperature showed discontinuity in the temperature range of the present study, and it is reasonably considered that one of the reasons for such difference between the flow curves is the effect of the hydrostatic pressure in the reservoir of the capillary flow test.

The Viscometric Behavior of Solubilized Calf Skin Collagen at Low Rates of Shear

Abstract Special Ostwald viscometers with unusually long capillaries were used to study the viscometric behavior of calf skin collagen solubilized in citrate buffer at pH 3.44. Graphs of shearing stress with respect to rate of shear show that at very low rates of shear this system displays Newtonian behavior in capillary flow. Under this condition extrapolation of reduced viscosity to zero concentration gives a value of intrinsic viscosity of 27 dl per g, which is higher than previously accepted values determined under non-Newtonian conditions. The energy of activation of flow of this system is positive and increases with concentration to a maximum value of 5.8 kcal. Entropy of flow is negative and decreases with concentration.