On-line Rheometry Research Articles

Evolution of dispersion along the extruder during the manufacture of polymer–organoclay nanocomposites

Although correlations between the final dispersion level of organoclay nanocomposites and melt mixing conditions in a twin screw extruder have been proposed, the actual lengthwise evolution of dispersion in these machines is still not well understood. Despite their practical importance such studies are difficult, due to the possibility of inducing degradation and/or morphology changes during material collection and/or sample preparation. This work uses on-line rheometry and in-line near-infrared spectroscopy (NIR) to monitor the evolution of dispersion of a PP/PP-g-MA/organoclay system along the axis of twin screw extruder. Regardless of the screw profile utilised, dispersion develops quickly in the first part of the machine, and then it levels-off or decreases, even if the final dispersion levels are significant. In order to validate this data, Scanning Transmitted Electron Microscopy (STEM), X-ray diffraction (XRD) and mid-FT-IR analyses are carried out on samples collected from the same locations where the on-line/in-line measurements were performed. The results confirmed the previous measurements. Thermogravimetric analysis (TGA) and Time of Flight-Secondary Ion Mass Spectrometry (TOF-SIMS) on clay, matrix and nanocomposite showed that thermal degradation of the clay surfactant and of the polymer matrix could contribute to the observed dispersion reversal.

Monitoring the Production of Polymer Nanocomposites by Melt Compounding with On-line Rheometry

Abstract Polymer nanocomposites are often prepared by melt compounding due to the suitability of the latter to industrial scale production. Even though monitoring the production process for quality control and/or optimization purposes is generally done off-line, the possibility of using on-line oscillatory rheometry has many inherent advantages. This work illustrates the use of a prototype rheometer to monitor the production of polymer nanocomposites by making measurements at specific locations along the extruder axis. The device is presented and its operation is explained. Examples of its use to characterize polypropylene and polyamide matrix nanocomposites with organoclays and carbon nanotubes are discussed, thus demonstrating the usefulness of the device.

Physical and Biological Study of Cell Cultures in A Bioreactor: On-Line and Off-Line Rheological Analyses

Rheological behaviour of culture broth stands as a fundamental parameter in bioprocess performances because it affects simultaneously the heat and mass transfer as well as the flow pattern. On-line measurements of rheological behaviour are hardly compatible with the operating condition with respect to accurate and stringent conditions imposed by cell culture strategy. Our scientific and technical objectives are (i) to develop and identify an experimental device enabling on-line rheometry and (ii) to discuss and compare on-line and off-line measurements. In this aim, a bioreactor was equipped with a derivation loop including a specific on-line rheometric device as well as additional physical and biological measurements (specific density, mass flow rate, electrical conductivity, pH, pO2 and temperature) during microbial cell cultures. In a first time, friction curves of calibrated ducts were established with Newtonian and non-Newtonian shear-thinning fluids. In a second time, axenic cultures with two microorganisms (bacteria and yeast exhibiting different sizes) were investigated in pure oxidative culture in order to produce biomass under high cell concentrations: ~ 40 to 110 gCDW/l for E. coli (bacteria) and ~ 75 to 105 gCDW/l for Y. lipolytica(yeast). Cell broths exhibited Newtonian behaviour for E. coliand shear-thinning behaviour for Y. lipolytica, which were both dependant on biomass concentration. On-line and off-line rheological measurements are consistent for E. coli and Y. lipolytica, but significantly differed. On-line estimated viscosity appears higher than off-line apparent viscosity. Several assumptions in relation with microorganism physiology and metabolism (size, morphology, surface properties, concentration, biological activity) could be formulated in agreement with scientific literature. On-line rheology brings new insight to investigate complex interaction between physical and biological phenomena.

Measuring the rheological properties of polymer melts with on-line rotational rheometry

Since the rheological response of a polymer system to an imposed stress is sensitive to composition, morphology, degree of mixing or temperature, on-line rheometry carried out along the length of the processing or compounding equipment - and able to follow the evolution of physical–chemical processes - is of great technical and scientific significance. This work presents and validates a computer controlled on-line rotational rheometer capable of quickly collecting material samples from within an extruder at different axial locations and performing the usual measurements of typical bench top commercial instruments. Validation of the prototype is made by direct comparison of its measurements with those carried out conventionally, including off-line measurements at room and high temperatures and on-line measurements. In order to illustrate its usefulness, the instrument is then used to monitor the evolution of the rheological behavior of three different materials along the axis of a twin screw extruder.

Extrusion Foaming of Poly(Lactic acid) Blown with CO2: Toward 100% Green Material

This paper presents a thorough investigation of the continuous extrusion foaming of amorphous poly(lactic acid) (PLA) using carbon dioxide (CO2) as the blowing agent. Detailed results describing the plasticization induced by CO2 dissolution as measured from two different methods, on-line rheometry and in-line ultrasonic technique, are given. Characteristics of the foams obtained from extrusion performed under various processing conditions are also reported. Extrusion of PLA foams in the 20–25 kg/m3 density range was achieved. However, the associated processing window was very narrow: CO2 content lower than ca. 7 wt% did not lead to significant foam expansion while foams blown at CO2 contents larger than 8.3 wt% showed severe shrinkage upon ageing.

Simultaneous determination of shear and extensional viscosities and wall slip by on-line rheometry with parameter fitting: an application to EPDM rubber

Shear and extensional viscosities and wall slip are determined simultaneously under extrusion processing conditions using an on-line rheometer. Because it is not possible to independently control flow rate and temperature, classical methods for interpretation of capillary data cannot be used with on-line rheometry. This limitation is overcome using computational optimization to fit parameters in a flow model. This consists of three parts, representing shear viscosity, extensional viscosity, and wall slip. Three-parameter, power law forms, based on local instantaneous deformation rates and including temperature dependence, are used for each, and analytic solutions applied for entry flow and flow in the capillary. For entry flow, the Cogswell–Binding approach is used, and for developed flow in the capillary a solution incorporating wall slip is derived. The rheometer, with interchangeable capillaries, is mounted in place of the die on a rubber profile extrusion line. Pressure drops and temperatures for extrusion of an EPDM rubber through 2 mm diameter capillaries of length 0, 2, 3, 4, and 5 mm are logged and flow rates determined for a range of extruder speeds (5 to 20 rpm). Pressures ranged from 60 to 75 bar and temperatures from 86 to 116 °C. Mean flow velocity in the capillaries was between 5 × 10−3 and 5 × 10−1 m s−1. The nine material parameters are optimized for best fit of the analytic pressure drops to experimental data, using about 100 data points, with the Levenberg–Marquardt method. It is concluded that flow is dominated by extension and wall slip. Shear flow appears to play little part. The slip model indicates that slip velocity increases much more rapidly than the wall shear stress (in the range 0.5–1 MPa) and decreases with temperature for a given stress level. Results for the (uniaxial) extensional viscosity represent an engineering approximation to this complex phenomenon at the high strains (approximately 200) and high extension rates (up to 800 s−1) applying in the extrusion. Results indicate a slight extension hardening and a decrease with temperature. Results are put into the context of the available studies in the literature, which, particularly with regard to wall-slip and extensional flow, consider conditions far removed from those applying in industrial extrusion. The present methods provide a powerful means for flow characterization under processing conditions, providing data suitable for use in computer simulations of extrusion and optimization of die design.

Degradation of polyethylene during extrusion. II. Degradation of low‐density polyethylene, linear low‐density polyethylene, and high‐density polyethylene in film extrusion

AbstractThe degradation of different polyethylenes—low‐density polyethylene (LDPE), linear low‐density polyethylene (LLDPE), and high‐density polyethylene (HDPE)—with and without antioxidants and at different oxygen concentrations in the polymer granulates, have been studied in extrusion coating processing. The degradation was followed by online rheometry, size exclusion chromatography, surface oxidation index measurements, and gas chromatography–mass spectrometry. The degradations start in the extruder where primary radicals are formed, which are subject to the auto‐oxidation when oxygen is present. In the extruder, crosslinking or chain scissions reactions are dominating at low and high melt temperatures, respectively, for LDPE, and chain scission is overall dominating for the more linear LLDPE and HDPE resins. Additives such as antioxidants react with primary radicals formed in the melt. Degradation taking place in the film between the die orifice, and the quenching point is mainly related to the exposure time to air oxygen. Melt temperatures above 280°C give a dominating surface oxidation, which increases with the exposure time to air between die orifice and quenching too. A number of degradation products were identified—for example, aldehydes and organic acids—which were present in homologous series. The total amount of aldehydes and acids for each number of chain carbon atoms were appeared in the order of C5>C4>C6>C7≫C2 for LDPE, C5>C6>C4>C7≫C2 for LLDPE, and C5>C6>C7>C4≫C2 for HDPE. The total amounts of oxidized compounds presented in the films were related to the processing conditions. Polymer melts exposed to oxygen at the highest temperatures and longest times showed the presence dialdehydes, in addition to the aldehydes and acids. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 1525–1537, 2004

On-Line Rheometry for Twin-Screw Extrusion (Along the Extruder) and its Applications

Abstract Due to a number of practical difficulties, both in- and on-line measurements of the rheological properties of complex systems during extrusion are usually performed at the end of the extruder, under very specific experimental conditions. This makes this type of instruments more useful for quality control than for process optimisation, since information about the influence of the geometry and/or processing conditions on the evolution of the material characteristics inside the extruder is not easily gathered. Recently, however, the authors have developed an on-line capillary rheometry system that overcomes most of the existing problems and allows small amounts of sample to be tested in very near real time, along the extruder. The present work aims at illustrating the usefulness of this concept for the study of physical compounding processes and some reactive systems. Two very different systems will be used for that purpose: a reactive extrusion process (the peroxide-induced thermal degradation of polypropylene) and the dispersive mixing involved in the preparation of thermoplastic/carbon fibre composites.

In-Process R eometry Studies of LDPE Compounds

Abstract The second part of a comprehensive study into the investigation and validation of in-process measurements on a low density polyethylene compound during extrusion is reported. A commercial grade of branched low density polyethylene was used in the study, compounded with various levels of magnesium hydroxide ame retardant filler, up to 0 by weight. In-line and on-line rheometry were used to monitor melt ow behaviour during single screw and twin screw extrusion; off-line capillary rheometry was performed on the compound before and after extrusion, to examine any change in rheology. Morphological observations were made using scanning electron microscopy, and molecular characterisations carried out using gel permeation chromatography. In-process rheometry was found to be able to detect the presence of filler at low and high loadings, but was unable to discriminate between intermediate levels. Off-line rheometry indicated that twin screw extrusion altered the rheology of the compound, and capillary wall slip was found to increase after extrusion. Morphological observation indicated that filler in the compound was better dispersed after extrusion, especially at high volume fractions. Molecular weight and polydispersity of the LDPE were shown to increase after extrusion, more so in the case of low filler loadings. Overall, in-process measurements provided useful rheological data, and the compound was found to exhibit complex, process-dependent rheology.

Modelling of Capillary Rheometer Data and Extrapolation of the Viscosity Function into the Linear Viscoelastic Region

Abstract 3- and 4 parameter models (Carreau-Winter, Sabia, Yasuda) are used for fitting capillary rheometer tests. These fitted data are then compared with rotational visco-simeter test data in the region of small deformation speeds. The results clearly demonstrate that the extrapolated capillary rheometer data acc. to the Sabia and Yasuda models yield good practice-relevant results. A good correlation was achieved between the calculated “zero viscosity” and the mean molecular mass and between the model curvature parameters and the width of the molecular mass distribution. For quality assurance in laboratory rheometery and on-line rheometry these obtained good results are of high importance.

The Use of On-Line Rheometry to Characterize Polymer Melts Containing Physical Blowing Agents*

The application of on-line rheometry is considered with particular reference to polymer melts plasticized by physical blowing agents. Experimental investigations were performed on polystyrene containing 0-15 wt% of HCFC-142b or HFC-134a or 0-5 wt% of carbon dioxide at various temperatures and shear rates. It was shown that results could be used to indicate the solubility limit and related to the glass transition temperature. Rheological modelling and data fits were compared indicating that a time/ temperature-composition (i.e. relative concentrations of polymers/blowing agent mixtures) superposition principle was valid. The model was extended to consider effects using a semi-crystalline polymer (polypropylene). It was concluded that an on-line retum-to-stream process control rheometer could satisfactorily measure and monitor mixtures of polymers and physical blowing agents used in thermoplastic foam extrusion. Appropriate viscosity measurements could be directly linked to a number of critical variables.

Applications of on‐line rheometry in reactive compounding

AbstractOn‐line rheological measurements are usually employed to monitor the reactive modification of single polymers during extrusion. This article provides data on the use of an on‐line rheometer to monitor the reactive compatibilization of polyamide/polyolefin blends in a corotating twin‐screw extruder. The type of functionalized compatibilizer and the choice of compounding conditions are among the variables examined. On‐line and off‐line rheological data are correlated with the morphologies of the extrudates and the mechanical properties of injection‐molded samples in attempts to establish general relationships between on‐line rheology, microstructure, and properties of polymer blends. © 1995 John Wiley & Sons, Inc.