Slit Rheometer Research Articles

Molecular dynamics simulation of slit rheometer for predicting shear thinning of short polyethylene chains

Understanding the molecular basis of rheological properties is crucial from both experimental and theoretical perspectives. Slit rheometry is commonly employed to measure the viscosity of fluids. This study utilized molecular dynamics simulations to investigate isothermal contraction flow at the nanoscale. Short linear polyethylene chains were uniformly extruded by a constant-speed piston from a reservoir through an abrupt contraction slit into the surrounding vacuum. Overall, die swelling and die wetting phenomena were observed. Molecular chains were stretched within the slit, while those outside the slit shrunk. Notably, the velocity profile within the slit varied with wall slip at different extrusion velocities. The relationship between the apparent shear viscosity and shear rate exhibited two primary characteristics: the first-Newtonian plateau and the shear thinning slope. Therefore, this molecular simulation method effectively demonstrates the general non-Newtonian behavior of macroscopic polymer fluids.

Comparative study on the rheological properties of cellulose acetate and double‐base propellant

AbstractCellulose acetate (CA) and nitrocellulose (NC) are both cellulose derivatives with similar chemical structure and physical properties, and CA are non‐flammable and biodegradable, so CA has been used as a safe substitute to NC for artillery propellant research. The ester functional groups of CA and NC are different, and there are other components in the propellant, so the rheological properties of CA and propellant can be different slightly. However, the comparative study of rheological properties between CA and propellant is rarely reported. In this paper, the rheological properties of CA and double‐base propellant (DP) in extrusion process was studied by in‐line slit rheometer. The results show that the power‐law model can describe the rheological properties of CA and DP well. The shear viscosity of CA and DP decreases gradually with the increase of solvent ratio, and when the solvent ratio of CA is 0.87 and that of DP is 0.34, the rheological properties of CA and DP are most similar. The Arrhenius equation can reflect the relationship between shear viscosity and temperature of CA and DP, and the viscous activation energy of CA is about 23.68 kJ/mol, while that of DP is about 7.38 kJ/mol. In general, the effect of temperature on the rheological properties of CA is stronger than that of DP.

Comparison of shear viscosity and normal stress measurements by rotational and on-line slit rheometers with tube model predictions

In-extruder measurements of shear viscosity and normal stresses are important as these measurement techniques allow determining the rheological state of the polymer melt at processing conditions up to high shear rates. However, validation of viscosity and normal stress data obtained by in-line slit rheometers at high shear rates is difficult due to a lack of overlap of the in-line data and the off-line measurements by rotational rheometers limited to lower shear rates. Here, shear viscosity and normal stress data measured in-line at large shear rates during extrusion and off-line at low shear rates are compared to predictions of the Doi-Edwards model and the Hierarchical Multi-Mode Molecular Stress Function (HMMSF) model using linear-viscoelastic off-line small amplitude oscillating shear data of two polystyrenes and a low-density polyethylene as input parameters. For polystyrene, the results of this investigation do not only validate the experimental data obtained by rotational as well as slit-die rheometry, but also demonstrate the agreement between experiments and models up to very high shear rates, which were not experimentally accessible earlier. The low-density polyethylene shows a more complex behaviour, which follows the HMMSF model at low shear rates, but approaches the Doi-Edwards model at high shear rates.

Examination of the melt temperature stability of the mold-type slit rheometer affected by plasticizing conditions and the shear heating in the nozzle and sprue

Examination of the melt temperature stability of the mold-type slit rheometer affected by plasticizing conditions and the shear heating in the nozzle and sprue

In-line rheological properties of rubber toughened Wood Polymer Composites

Wood Polymer Composites are materials that are more and more used and studied by both academic and industrial research. Although such materials are successfully substituting natural wood in many relevant applications, still some of their drawbacks limit their widespread diffusion. One of these is their brittleness, which can be limited by blending with a toughening agent. In this study we investigated the rheological properties, in terms of the flow curve and the wall slip characteristics, of a toughened Wood Polymer Composite. Testing was performed using an in-line slit rheometer attached directly to a single screw extruder. The material used for the experiments was a blend of polypropylene-based wood polymer composite compound, containing 35 wt.% of wood flour and a suitable thermoplastic elastomer used as a toughening agent. The results showed a shear thinning behaviour and a significant slip velocity of the material that is beneficial during processing.

Simultaneous slit rheometry and in situ neutron scattering

In situ measurement of fluid structure during flow, e.g., by neutron scattering, is key to understanding the relationship between structure and rheology. For some applications, structures at high shear rates previously unreachable are of particular interest. Here, we report development of a flow cell slit rheometer for neutron scattering (μRheoSANS). The devices were used to measure the structure of a semi-dilute surfactant solution of worm-like micelles during flow. Analysis of the rheometry and scattering data allows isolation of the scattering signal from the high-shear, near-wall region of the flow cell. The reported results agree with those from the existing Couette-based RheoSANS instrument. The worm-like micelles exhibit an alignment transition at Weissenberg number (Wi) ≈ 1, coinciding with the onset of shear thinning. This transition is followed by a peak in micelle alignment at a higher shear rate, after which the degree of alignment decreases moderately. This technique can achieve higher shear rates than existing RheoSANS techniques, expanding the ability to study the structure of complex fluids at elevated shear rates.

Determination of the effect of pressure on viscosity at high shear rates by using an injection molding machine

ABSTRACTThe effect of pressure on rheological behavior of polymer melts is surely a significant phenomenon for polymer processing, and its determination has been the object of several studies which highlighted the difficulties in performing accurate measurements. Even more important is the determination of this effect at very high shear rates, which cannot easily be obtained by conventional rheometers. In this work, a slit rheometer was located at the nozzle of the injection molding machine to obtain rheological measurements by means of two pressure transducers. Pressure values were analyzed by an original method to evaluate the pressure coefficient and the viscosity at zero pressure. The zero pressure viscosity aligned with independent rheological measurements obtained in previous works. The effect of pressure on viscosity was described by means of a parameter whose values resulted to be in line with those obtained in previous works, although the highest shear rates explored in this work were at least one order of magnitude larger. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 45277.

Rheological Properties of Coating Colors Containing Biobased Latex

This paper focuses on the understanding of basic properties of water-swollen crosslinked starch nanoparticles as a function of crosslink density. The extent of their water swelling is decreased with increasing particle crosslink density and solid concentration and vice versa. This study elucidates the unique rheological properties of starch nanoparticle dispersions and paper coating formulations in comparison with water-soluble cooked starch and synthetic latex counterparts. These rheological studies extend over many decades of shear rates, using several different rheometers. Low shear viscosities were obtained using a Cannon-Fenske viscometer and a TA AR-2000 stress rheometer with double concentric cylinder geometry. Intermediate shear rate rheology was evaluated with a Hercules rheometer. High shear rates were studied with ACAV A2 ultra-high shear capillary and slit rheometers. Unlike conventional cooked and soluble starch solutions, starch nanoparticle latex dispersions are colloids that consist of internally crosslinked particles. With increasing intra-particle crosslink density these biobased colloids have been found to behave much like petroleum based synthetic latex colloids. However, at ultra-high shear their rheological properties are relatively more shear thinning compared to hard particles, including synthetic latex and pigment particles, which exhibit shear-thickening and dilatancy. The implications of the rheological data on high-speed coater runnability are discussed.

Short-Term Flow Induced Crystallization in Isotactic Polypropylene: How Short Is Short?

The so-called “short-term flow” protocol is widely applied in experimental flow-induced crystallization studies on polymers in order to separate the nucleation and subsequent growth processes [Liedauer et al. Int. Polym. Proc. 1993, 8, 236–244]. The basis of this protocol is the assumption that structure development during flow can be minimized and the rheological behavior, i.e., the viscosity, does not change noticeably. In this work we explore the validity of this assumption for short but strong flows and reveal the structure formation during the early stages of crystallization. Viscosity and structure evolution of an isotactic polypropylene (iPP, Mw ≈ 365 kg/mol and Mw/Mn = 5.4) melt at 145 °C are measured during the short-flow period (0.2–0.25 s) using the combination of a slit rheometer and fast X-ray scattering measurements. For high enough (apparent) shear rates (≥240 s–1) a viscosity rise during flow is observed; i.e., the condition for “short-term flow” is not satisfied. With a time delay with respect to the viscosity rise, the development of shish is observed at a position halfway the length of slit, along the flow direction, by means of ultrafast time-resolved SAXS measurements. Depending on the shear rate, these shish are detected during (shear rates ≥ 400 s–1) or after flow (240 s–1 ≤ shear rates < 400 s–1). For even lower shear rates of 160 and 80 s–1, the viscosity does not change significantly, and instead of shish, oriented row nuclei (X-ray undetectable) are generated. These two shear conditions qualify as short-term flow. A full understanding of the coupled flow and crystallization phenomena requires that the transient and nonhomogeneous behaviors, both in flow and in flow gradient direction, have to be taken into account. This can only be done by a full numerical model, and therefore, the results presented in this paper also provide a valuable data set for future numerical studies.

Rheology and enhancement of extrusion of linear and branched polyethylenes using low amount of organoclay

AbstractInteraction between 0.05 wt % organoclay and polyethylenes of different short chain branching (SCB) was studied. Linear rheology (van Gurp‐Palmen plot) was used to study the effect of organoclay on the rheology of polyethylenes. Organoclay had effect only on the van Gurp‐Palmen plot of linear polyethylene. Fourier transform (FT) rheology, extrusion at high‐shear rates in a slit rheometer, transient stress growth analysis, and extensional rheology were conducted to examine the potential of organoclay as a processing aid. Organoclay reduced the transient stress overshoot, normal stress difference, ηo, onset of shear thinning, and extrusion pressure of polyethylene. The reduction was more pronounced in linear polyethylene without branching. Such effects gradually decreased as the branch content increased. The trend was independent of the type of flow (shear or extensional). It was interesting to note that FT rheology was not effective in explaining the impact of organoclay on polyethylene. The work concluded with the proposition that organoclay (as low as 0.05 wt %) was a good processing aid for linear polyethylene and polyethylenes with low content of SCB. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Elongational viscosity of LDPEs and polystyrenes using entrance loss data

AbstractFor two low‐density polyethylenes and two polystyrenes, axisymmetric and planar elongational viscosities are estimated using entrance loss data from capillary and slit rheometers, respectively. The elongational viscosity is estimated by optimizing the values of various parameters in the Sarkar–Gupta elongational viscosity model such that the entrance loss predicted by a finite element simulation agrees with the corresponding experimental data. The predicted entrance loss is in good agreement with the experimental data at high flow rates. The difference in the experimental and predicted entrance loss at lower flow rates might have been caused by large error in the experimental data in this range. POLYM. ENG. SCI., 2008. © 2007 Society of Plastics Engineers

Modelling and experimental verification of pressure prediction in the in-mould coating (IMC) process for injection moulded parts

In-mould coating (IMC) is carried out by injecting a liquid low viscosity thermosetting material onto the surface of the injection moulded part after it has solidified, while it is still in the mould. The coating then cures and adheres to the substrate. Due to its successful application to exterior body panels made from compression moulded sheet moulding compound, IMC is being developed as an environmentally friendly technology that would ultimately replace painting of injection moulded thermoplastic parts. In the short term, however, we believe IMC has the potential of being a substitute for primers. In a previous paper, we presented a Hele–Shaw based mathematical model to simulate the coating flow during the IMC process assuming the coating to be a power law fluid and using the traditional no-slip boundary condition. This model adequately predicted the fill patterns but did not predict pressures correctly. This deviation has been attributed to slip (or apparent slip) at the wall, as often found in flow through microchannels, due to the microscopic length scale of the flow, and the existence of a high shear rate Newtonian plateau for the coating viscosity. In this paper we present a mathematical model that includes the slip boundary condition and the Carreau viscosity model to properly describe the rheological behaviour of the coating material at high shear rates. The rheological and slip parameters of the coating material were measured using a customized micro slit rheometer. The predicted pressures are compared with experimental results obtained using our IMC pilot facility. It was found that the predicted pressures and coating thickness agree well with the measured values.

Microfluidics and rheology of dilute carbon black suspensions for in-mould coating (IMC) applications

In-mould coating (IMC) is carried out by injecting a thermosetting dilute carbon black suspension onto the surface of an injection-moulded part after the part has solidified, but while it is still in the mould. Due to the microscopic length scale of the IMC flow (10–25 µm), a study of the slip flow and rheological properties at high shear rates of the coating liquid is critical in modelling the IMC process. A customized microslit rheometer using micrometer-sized channel gaps between 25 and 100 µm was developed and used for that purpose. A reduced viscosity of the suspension due to slip (or apparent slip) at the fluid–wall interface was found in the 25 µm gap channel. Rheological equations for slit rheometers using no-slip and slip boundary conditions were used to determine the viscosity of the suspension at the microscale level. By analysing the viscosity data using the rheological equations developed, we can determine the values of the slip parameter, known as slip length. The slip boundary condition and high-shear-rate-plateau viscosity model are then applied to model the slip flow of the suspension. By including both the wall slip and high-shear-rate-plateau viscosity in the flow model, we can better predict the pressure distribution of the suspension.

Use of Adjustable-Gap On-Line and Off-Line Slit Rheometers for the Characterization of the Wall Slip and Shear Viscosity Behavior of Energetic Formulations

It is difficult to characterize the rheological behavior of energetic suspensions due to their viscoplasticity and wall slip. The use of the rectangular slit geometry, as an on-line or off-line rheometer is advantageous provided that the surface to volume ratio of the slit die can be systematically varied to allow the wall slip corrections to be made. Here two rectangular slit rheometers designed and built to handle the rheological behavior of energetic suspensions are presented. The gap of these rheometers is variable to give the user the ability to vary the gap and hence the wall shear rate, thus enabling wall slip corrections to be made. A series of pressure transducers, flush with the wall, are used to determine the pressure drop over the fully developed flow region and hence the wall shear stress directly from the fully developed pressure gradient. The ability to independently vary the mass flow rate and the gap opening allows one to carry-out the wall slip corrections and hence obtain the wall slip velocity versus the shear stress data, which can then be used as the boundary condition during the simulation of the die and extrusion flows and at the same time allow the determination of accurate shear viscosity data. A set of data systematically collected with an on-line slit rheometer with a continuously adjustable gap to characterize the wall slip velocity as well as the shear viscosity material function of a LOVA formulation as a function of deformation rate, solvent concentration, and temperature is used to illustrate the working principles of the on-line and off-line adjustable-gap rheometers.

New rheological developments for reactive processing of poly(ε-caprolactone)

Abstract This short review aims to show how an integrated activity on reactive processing have been developed these last years in our laboratory. We can say that the originality of this approach is based on combining developments in chemistry, in line instrumentation, and rheology aspects. Our rheological works can be divided into four important contributions: rheo-physics, rheo-chemistry, rheo-mixing and rheo-processing. These different parts are illustrated from the e-caprolactone polymerisation in bulk and dispersed media. Rheo-physics studies allowed us to calculate the molecular weight distribution and chain structures of in situ polymerised poly(e-caprolactone) samples. From rheo-chemistry works, we are now able to predict the variation of the complex shear modulus versus the extent of the polymerisation. The developments of new rheological tools such as rheo-mixer enable us to investigate complex mixing situations encountered in reactive polymer blends and formulations. Lastly, a rheo-processing approach based on the in-line measurement of the viscosity in a slit rheometer at the die exit of the extruder allows us to envisage its application to the experimental control of the reactive processing in extruder. To cite this article: P. Cassagnau et al., C. R. Chimie 9 (2006) .

Geometry of a high-shear slit rheometer and its effect on the correlation with the coating process

Geometry of a high-shear slit rheometer and its effect on the correlation with the coating process

Slit‐flow ektacytometry: Laser diffraction in a slit rheometer

Deformability of red blood cells (RBCs) is a determinant of blood flow resistance as RBCs pass through small capillaries of the microcirculation. Available techniques for measuring RBC deformability often require a washing process after each measurement, which is not optimal for day-to-day clinical use. A laser diffraction technique has been combined with slit-flow rheometry, which shows significant advances in ektacytometric design, operation, and data analysis. The essential features of this design are its simplicity (ease of operation and no moving parts) and a disposable element that is in contact with the blood sample. With slit ektacytometry, the deformation of RBCs subjected to continuously decreasing shear stress in a slit flow can be quickly measured with extremely small quantities of blood. The measurements with the slit ektacytometer were compared with those of LORCA and a strong correlation was apparent. The deformability of the hardened RBCs was markedly lower than that of the normal RBCs. In addition, the young cells showed higher values of the elongation index than did the old cells. The newly developed slit ektacytometer can measure RBC deformability with ease and accuracy. In addition, the slit ektacytometer can be easily used in a clinical setting owing to the incorporation of a disposable element that holds the blood sample.

Shear and extensional properties of short glass fiber reinforced polypropylene

Shear and extensional properties of a commercial short glass fiber reinforced polypropylene were carefully investigated using commercial rheometers and a novel on-line rheometer. This on-line slit rheometer, installed on an injection molding press, has been designed to measure the steady shear viscosity, the first normal stress difference, and the apparent extensional viscosity of polymer melts and composites for high strain rates up to 105 s−1 in shear and 200 s−1 in extension. Our results show that the steady-state viscosity measurements using the on-line rheometer are in excellent agreement with those obtained using commercial rheometers. The steady-state and the complex viscosities of the composites were found to be fairly close to that of the matrix, but the Cox-Merz rule was not verified for the composites at high rates. The elasticity of the composites was found to be equal to that of the polypropylene matrix. The apparent extensional viscosity was obtained from the pressure drop in the planar converging die of the slit rheometer using the analyses proposed by Cogswell [1] and Binding [2]. The extensional viscosity of the polypropylene was found to be much larger than the shear viscosity at low strain rates with a Trouton ratio of about 40 that decreased rapidly with increasing strain rate down to the value of 4 at 200 s−1. The extensional viscosity of the composites was also found to be close to that of the matrix, with values 35 and 5% larger for the 30 and 10 wt% reinforced polypropylenes, respectively. These results are compared with the predictions of the Goddard model [3], which are shown to overpredict our experimental results. POLYM. COMPOS. 26:247–264, 2005. © 2005 Society of Plastics Engineers.

Simultaneous measurement of red blood cell aggregation and viscosity: Light transmission slit rheometer

Aggregation of red blood cells (RBCs) is a major determinant of blood viscosity. There have not been available techniques for measuring RBC aggregation and viscosity, simultaneously. A laser transmission technique has been combined with a slit rheometry, which shows significant advances in rheometer design, operation and data analysis. A laser beam traverses a blood suspension flowing through a slit and is scattered by RBCs in the volume. The transmitted light is captured by a photodiode, linked to a computer, while the differential pressure variation is measured by a pressure transducer. Both measurements of the laser-transmitted intensity and pressure with respect to time enable to determine the aggregation index and the viscosity. The advantages of this design are in its simplicity, i.e., ease of operation and no moving parts, low cost and a short operating time. In addition, the slit-flow aggregometer can be easily used in a clinical setting owing to the incorporation of a disposable element that holds the blood sample.

Formation of Barrier Ribs for Plasma Display Panel via Roll Forming of Green Tapes

In an attempt to explore the possibility of forming barrier ribs for a plasma display panel by plastically deforming a green tape with a grooved roll, the effects of green tape composition and rolling parameters on the height of the barrier ribs formed were studied. For the analysis of the results, viscosity of the green tape was evaluated using a modified slit rheometer. The results indicated that the effects of organic constituents in the green tape and processing parameters on the dimension of the barrier ribs formed are closely related with the pseudoplastic characteristics of the green tape.