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Capillary Rheometry: Analysis of Low-Viscosity Fluids, and Viscous Liquids and Melts at High Shear Rates

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Capillary Rheometry: Analysis of Low-Viscosity Fluids, and Viscous Liquids and Melts at High Shear Rates Reprinted from American Laboratory October 2009 Technical Article by Tien T. Dao, Allan X. Ye, Ali A. Shaito, Nick Roye, and Kaj Hedman Capillary Rheometry: Analysis of Low-Viscosity Fluids, and Viscous Liquids and Melts at High Shear Rates heology is defined as the study of the vertical capillary die when no instrument The basic components of a capillary rhe- flow and deformation of fluids. Based driving force is applied, and 2) low-pressure ometer consist of a barrel, ram (piston), Ron the need to quickly and accurately readings from the transducer during the capillary die fitted inside the barrel, pres- set and control processing conditions, opti- measurement. Many times the measured sure gauge, and force transducer (Figure mize product performance, and/or ensure pressure signal can reach the low-end limit 1). The material is located within the bar- product acceptance, accurate rheological of the pressure transducer used. rel, and then the ram (or piston) attached measurements have become essential in the with a force transducer pushes the material characterization of any flowable materials. Another concern is the required length- through the capillary die at a specified rate. By utilizing rheological measurements on to-diameter ratio (L/D) of the die for the Above the die inside the barrel is a pres- new materials, process conditions can easily measurement to be rigorous. According to sure transducer, which allows for the mea- be determined, and the final product prop- Macosko,3 an L/D of 60 or higher is required surement of the gauge pressure ΔP within erties and performance can be predicted. to eliminate the die entrance/exit effects of the barrel of the capillary rheometer. The Historically, many processes impart high- low-viscosity fluids at high-shear-rate range. force transducer attached to the ram can be shear deformation and/or high-shear-rate This constraint requires that a die with a substituted for the pressure transducer and conditions on a material. Obtaining reliable very small diameter and long length be pro- is used as the measured signal ΔP inside the and accurate data under these conditions duced, which is difficult, if not impossible, barrel. However, any friction in the system has sometimes proven to be problematic to fabricate based on normal machining not associated with the sample resistance and difficult. techniques. In addition, other concerns may to flow is also included in the measured arise in the measurement of low-viscosity output. Using the measured pressure ΔP Traditionally, capillary rheometers have fluids at high shear rates, including viscous been used to measure the shear viscosity heating of the sample during testing, and and elasticity of viscous materials at high high-kinetic-energy effects. a shear rates. More recently, extensional vis- cosity has been obtained through the work A force balance of the translation mechan- of Cogswell et al.1 The interest in high ical system shows that the measured pres- shear rates stems from the mode of defor- sure drop ΔP in the capillary die includes mation a material will undergo in processes the following components: 1) pressure drop like extrusion, film blowing, and injection due to viscous dissipation (the sample’s molding. The design and operational prin- viscosity) within the capillary Pvisc, 2) pres- ciple of a capillary rheometer are condu- sure needed to obtain the kinetic energy of cive to providing rheological data under the testing fluid Pkin, 3) pressure loss due to these unique processing conditions. The end effects including the entrance and exit moving piston/die arrangement produces pressure loss Pent and Pexit, and 4) pressure high shear rates on the order of 107 s–1. A drop for viscoelastic fluids associated with measure of elasticity can be determined elastic energy Pelastic. The overall measured b by measuring expansion of the sample at pressure drop can be written as follows3: the die exit, called die swell. Melt fracture 2 can be studied by observing the extrudate. ΔP = Pvisc + Pkin + Pent + Pexit + Pelastic Extensional viscosity can be obtained as noted above. As a result, capillary rheom- The kinetic energy pressure contribution eters are widely used in both research envi- for a capillary die is given by: ronments and quality control laboratories. 2 Pkin = ρv Capillary rheometers are versatile and widely used in the rheological measurement where ρ is the density of the fluid, and v of viscous materials where the die sizes nor- is the average velocity of the flow. The mally range from 0.5 to 2.0 mm. However, well-known Bagley correction includes capillary rheometers suffer from several lim- both viscous and elastic components and itations when attempting to measure fluids is used to account for the entrance and Figure 1 a) Schematic of complete capillary with viscosity lower than 1 Pa s: 1) sam- exit pressure effects of non-Newtonian rheometer, and b) closeup of the die structure with ple flow by gravity through the traditional viscoelastic fluids. variables. Figure 2 SmartRHEO capillary rheometer. Figure 3 Viscosity of HDPE at 190 °C using a NOVA rotational rheometer and SmartRHEO capillary rheometer. and known controlled flow rate, one can calculate the viscosity of the test sample. By controlling the velocity of the ram, one algorithm permits automatic, operator-free HDPE and PDMS samples were evaluated can control the shear rate of the sample use of the capillary rheometer. using the ESP mode. These automated ESP through the die and calculate the viscosity results were then compared with the tradi- of the sample at fixed, control shear rates. tional manual-testing mode. By varying the components of the capillary Samples rheometer, the range of shear rates and A commercial, fractional melt index testing condition can be extended to fit extrusion-grade high-density polyethylene Instruments the requirement of the experimenter. (HDPE) was evaluated at 190 °C to demon- A SmartRHEO capillary rheometer was strate the capability of the instrument for test- used to measure the rheological properties Operation of a capillary rheometer is sim- ing high-viscosity materials. To demonstrate of samples over a wide range of shear rates ple but can be more time consuming than the instrument’s ability to measure medium- and test conditions. SmartRHEO is the a rotational rheometer. This is because the viscosity samples, a polydimethylsiloxane most technically advanced laboratory cap- larger sample charge requires more time to (PDMS) gum was evaluated at 105 °C. Both illary rheometer for the determination of reach thermal equilibrium. In addition, it the HDPE and PDMS were then compared the flow behavior of a wide range of materi- can take significant time for the sample to to small amplitude oscillatory shear (SAOS) als. The rheometer can measure shear rates attain steady-state flow at each shear rate. results obtained with a NOVA rheometer from 0.01 to over 10,000,000 s–1, with force Further, the operator must assess when the (REOLOGICA Instruments) (Figure 3) at capability from 0.1 to 50 kN. A wide vari- measured stress/force or shear rate reaches comparable shear rates/frequencies using the ety of die sizes and L/Ds are available. Cor- steady state for a particular point in the Cox-Merz rule. rosion-resistant sample chambers, pistons, measurement before changing to the next and dies are also offered. Temperature con- measurement point. A low-viscosity Newtonian oil, along with a trol is better than ±0.1 °C. Other options modified oil, were evaluated to demonstrate include: 1) automatic cleaning, 2) nitrogen This paper describes the SmartRHEO cap- the system’s ability to measure low-viscosity blanket, 3) pressure-volume- temperature illary rheometer (REOLOGICA Instru- fluids at high shear rates. The role of the system, 4) thermal conductivity system, 5) ments, Bordentown, NJ) (Figure 2). The modifier is to raise the zero shear viscosity slit die geometry, 6) die swell, 7) stretch- data presented demonstrate the instru- while also imparting pseudoplastic, shear- ing unit, and 8) extension viscosity by the ment’s ability to measure not only high- thinning behavior to enhance flowabil- Cogswell method. The VisualRheo soft- viscosity materials at high shear rates, but ity and providing elasticity, leading to an ware can be run in manual, plateau, and also low-viscosity fluids. The ability to test increase in lubricity at high shear rates. automatic ESP mode. The ESP mode is low-viscosity fluids extends the usefulness a significant time saver, allowing experi- of the capillary rheometer to a wider range To demonstrate the utility and accuracy ments to be run without user intervention. of materials. In addition, the Equilibrium of the ESP algorithm in the SmartRHEO’s Other available features in the software Stress Prediction (ESP) instrument control VisualRheo operational software, both the are the Weissenberg-Rabinowitsch and Figure 4 NOVA rotational rheometer. Bagley corrections, advanced curve-fitting module, elongational viscosity module, Figure 5 Viscosity of PDMS at 105 °C using a NOVA rotational rheometer and SmartRHEO capillary wall slip module, and thermal degradation rheometer. analysis module. Data exportation to other Windows programs is included as standard. tested at 190 °C, and the viscosity was of the SmartRHEO capillary rheometer. Capillary testing and results measured over a shear rate range from 1 PDMS exhibits less flow instability at For the testing of high- and medium- to 1000 s–1. At rates greater than 1000 s–1, high shear rates, permitting accurate viscosity materials at high shear rates, the melt fracture of HDPE occurred, and the data collection at shear rates in excess SmartRHEO capillary rheometer was uti- experiment was stopped. Results of this of 5000 s–1. Results of this experiment lized with a 1.0-mm die and a die length of measurement are presented in Figure 3.
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