A novel sliding plate rheometer for molten plastics

A novel sliding plate rheometer has been developed that is suitable for use with molten plastics, concentrated polymer solutions, raw elastomers, and other viscoelastic or thixotropic materials. It can generate steady shear rates from 0.05 to 500 s^?1 and can also be used to measure linear viscoelastic properties. In addition, it can be used to measure a broad spectrum of nonlinear viscoelastic properties such as the nonlinear relaxation modulus and the shear stress growth coefficient. In order to measure these nonlinear properties it is necessary to generate large, uniform, transient deformations Involving high strain rates. Rotational and capillary melt rheometers are not capable of generating this type of deformation, and until now it was not convenient to use sliding plate rheometers for this type of application. However, the recent development of a reliable and robust shear stress transducer makes it very convenient to use the sliding plate geometry to carry out all of these tests. The new rheometer is described, and examples of the types of data it can generate are shown.     

A biaxial extensiometer for molten plastics

While biaxial stretching plays a central role in several polymer processes, there has been no technique suitable for laboratory study of controlled biaxial stretching of molten thermoplastics at typical melt processing temperatures. The sheet inflation technique, which has been used previously to study rubbers and very high viscosity melts, has been adapted for use with molten thermoplastics by the use of oil as an inflation medium. Problems encountered in the use of the first prototype were eliminated in a second model, which is thought to produce reliable results. Stress growth data are presented for a low density polyethylene and for a polystyrene. A basic limitation of the technique is that the maximum achievable strain is in the range of 1.3 to 1.7, and this is often insufficient to bring out the nonlinear viscoelastic behavior of the melt.     

An extensiometer for molten plastics

A new rheometer has been developed to measure the extensional viscosity of molten thermoplastics at typical extrusion temperatures. It is at the same time versatile and easy to operate. One end of the test specimen is held in a fixed position by an oil-cooled chimp mounted on the end of a force transducer. The other end of the specimen is held by, a moving, oil-cooled chimp. The moving clamp is suspended from a carriage mounted on a ball nut which travels on a long lead screw. The lead screw is driven by an electric motor operating through a gear box, and its speed can be controlled to produce either a constant stress (creep) or a constant strain-rate (stress growth) experiment. Experimental results have verified that in the “constant strain rate” mode the strain rate does rise rapidly to a constant level. Measurements on several polyethylene resins indicated that the extensional viscosity rises with strain rate at low strain rates.     

Process rheometers for molten plastics: A survey of existing technology

There are many operations in the plastics industry where it is useful to be able to monitor the rheological properties of a melt as it is being processed. Such operations include polymerization, blending, compounding, and reactive extrusion. The design of such instruments poses major challenges, as there is a conflict between the need for rapid sample renewal and the desire for measurement accuracy. In-line types, which are installed directly in the process flow, have a rapid response but must operate in an environment where temperature, pressure, and flow rate fluctuate in response to process upsets. On-line types, which make use of a side stream and gear pump to feed the rheometer, allow for more freedom in the design of the rheometer but have an inherent signal delay due to the time required for melt to reach the rheometer. A variety of rheological sensors have been described in the patent and research literature, each with its unique advantages and disadvantages, and some of these are finding increased use in the plastics industry to improve product quality and process efficiency.     

A pulse rheometer for reacting foams

A foam rheometer has been developed based on the impulse theory of linear viscoelasticity. The rheometer is a large-volume parallel-plate device which operates under gap-loaded conditions, and is designed as an add-on fixture for the Materials Testing System Model 312. The rheometer provides characterization data of the pre-gel (via the zero-shear viscosity) or the post-gel (via the equilibrium modulus) properties of a foaming cellular polymer with a precision of better than ± 15%.     

A new rheometer measuring infrared dichroism in molten polymers subjected to transient and steady shear flow

A rotational parallel plate rheometer that enables simultaneous measurement of the transient or steady-state rheological properties and infrared dichroism was designed and constructed to study orientation in molten polymers. Measurements can be carried out at shear rates between 0.05 and 300 s⁻¹ and at temperatures between 20 and 300°C. Both shear stress and axial normal force together with dichroism are continuously measured during shear flow. Infrared dichroism data for polypropylene showed excellent agreement with data obtained with a FTIR-instrument. The Hermans orientation function for molten poly(dimethyl siloxane) at steady state showed a strong shear-rate dependence in the region 0.1−20 s⁻¹. Rheological data for molten poly(dimethyl siloxane) agrees with data obtained from a conventional rheometer.     

Rheological studies of ultraviolet curing with an oscillating plate rheometer

The dynamic viscoelastic behavior during UV curing was studied for thin liquid films of epoxy acrylate prepolymer by the use of an oscillating plate rheometer. The dynamic viscosity rapidly increases after a certain period of irradiation, so that the UV curing process has the minimum exposure energy required to start polymerization. The minimum exposure energy markedly increases with increasing sample thickness; this results from the inconsistency of degree of curing in the direction perpendicular to the shearing surface. Since free radicals which initiate polymerization are formed by photochemical decomposition of initiator, the ability to cure a film depends on the light intensity at a given depth of the film. the attenuation of light in the film is primarily responsible for this inconsistency. When UV light is applied through a UV filter, the curing behavior is analyzed by a single exponential decay of light with depth because the curing is induced by absorption of a monochromatic light of 365 nm. On the other hand, when UV light from a UV lamp whose output spectrum is a continuum is directly applied, the curing behavior is explained by a combination of energy absorption at different wavelengths. In both cases, the theoretical curves of dynamic viscosity predicted in relation to spectral sensitivity show a good agreement with the experimental results.     

Steady flow and dynamic viscoelastic behavior of nylon 1313 using parallel‐plate rheometer and capillary rheometer

In this study, a novel nylon with long alkane segments (also called nylon 1313), which was synthesized using 1,13‐tridecanedioic acid in our laboratory, has been characterized. Different rheological behaviors of nylon 1313 have been presented using steady shear, creep recovery, and dynamic tests. The time‐temperature effects have also been investigated. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 1643–1651, 2005     

A novel rheometer design for yield stress fluids

An inexpensive, rapid method for measuring the rheological properties of yield stress fluids is described and tested. The method uses an auger that does not rotate during measurements, and avoids material and instrument‐related difficulties, for example, wall slip and the presence of large particles, associated with yield stress fluids. The method can be used for many types of yield stress fluids, including concentrated lignocellulosic biomass. Sample preparation prior to measurement is minimal, reducing, or eliminating disruption of the sample. We show that measurements using this technique compare well with measurements obtained with a vane rheometer. A variation of the described method is proposed that would make it easier to measure time‐dependent rheological properties. © 2014 American Institute of Chemical Engineers AIChE J, 60: 1523–1528, 2014     

A study of rheological behavior of a polypyrrole modified UHMWPE gel using a parallel plate rheometer

Polypyrrole powder was added to a 6% UHMWPE decalin solution as an interfacial modifier. Rheological properties and dynamic viscoelastic properties, including shear viscosity, shear storage modulus (G′), and shear loss modulus (G″), were measured with a parallel plate rheometer using rotation and oscillation modes. The results show that these properties increase with the PPy content. In addition, the gelation temperature decreased with increasing PPy content. This suggests that the presence of PPy enhanced the entanglement network in the solution, which is similar to the role of filler in rubber.     

The limiting pressure-velocity (PV) of plastics under unlubricated sliding

The limiting pressure-velocity (PV) of plastics materials under normal contacting pressure P and sliding velocity V when the sliding goes on continuously unlubricated has been discussed theoretically in this study. The sliding friction tests between the rotating edge of a hollow cylinder against a stationary metal surface were carried out in room air and when the metals were cooled with circulating water. The results obtained were as follows:

• The relation between PV_max and the critical temperature τ_bmax of the surface above which ordinary frictional conditions are not maintained may be represented by the following formula, Where C = Hn/μk, and H is the mean total heat conductivity between materials and environment in cal/cm². °C · s, n the area ratio of heat radiation surface to frictional surface, μ the kinetic frictional coefficient, k the thermal equivalent of frictional work in cal/kg · cm · s, and τ_a the cooling or environment temperature.
• The value of C ranges from 1 to 10 in cal/kg · cm s · °C, and the value of PV_max from 45 to 750 in kg/cm² · cm/s under prevailing room temperature air cooling in this experiment, however, the value of C has been increased about 2.3 times as much as those values by circulating water of 2°C around the lower side of the test piece.

     

A tensile rheometer for solid polymers containing volatile diluents

A tensile rheometer capable of confining diluent is described. This rheometer makes it possible to characterize solid samples with imbibed diluent. Tensile stress-relaxation data were obtained as a function of temperature for dry glassy semicrystalline poly(ethylene terephthalate) (PET) and as a function of composition for semicrystalline PET imbibed with dimethlformaide (DMF). Data on PET/DMF captures the glass-rubber transition as DMF content is increased. © 1994 John Wiley & Sons, Inc.     

Heat transfer characteristics of molten plastics in a vertical falling film reactor

The heat transfer efficiency during the pyrolysis process is a key factor to be considered in the design of pyrolysis reactors. In this study, the average apparent heat transfer characteristics of molten plastic pyrolysis in a vertical falling film reactor were explored by experiments and numerical simulation and the apparent heat transfer coefficients were determined. In addition, the temperature distribution and the thickness of the liquid film in the reactor were predicted and the influences of pyrolysis temperatures on the average apparent heat transfer coefficients were discussed.     

A rheometer for concentrated polymer solutions containing volatile solvents

A new rheometer cell and integrated transducer, which is capable of performing both steady state and dynamic measurements in a controlled environment and at elevated temperatures and pressures, is described here. This cell mounts onto either the Rheometrics Mechanical Spectrometer or the Rheometrics System Four rheometer and utilizes the wide range of shear rates and frequencies available on these instruments. The unique capabilities of the rheometer and cell permit measurement of the rheological properties of concentrated polymer, solutions containing volatile solvents at temperatures above the normal boiling point of the solvent. The steady shear and dynamic properties of several polystyrene/ethylbenzene solutions having polymer concentrations greater than 50 wt percent are presented to demonstrate the performance of the rheometer.     

Inverse method for the dynamical analysis of wall shear rates using three-segment probes in parallel plate rheometer

Mass transfer measurements were conducted on a Parallel Plate Rheometer (PPR) using the limiting diffusion current method, i.e. the polarography technique. The database constructed was exploited for the validation of the inverse method under a well-controlled unsteady shear flow. This method is based on a numerical sequential estimation. It has been applied to determine of the wall shear rate in the upper disk of the PPR. It requires the numerical inversion of the convection diffusion equation in order to apply it to instantaneous mass transfer measurements. This requires the use of electrochemical probe, which allows the determination of the local mass transfer rate for known flow kinematics. A multi-segment electrochemical probe has been used. The directional characteristics of the three-segment probe were highlighted. The directional angle effect and the frequencies of oscillations effect were studied in order to test the robustness of the inverse method within the presence of such factors. The obtained results demonstrate that the inverse method allows a good determination of the shear rate, which follows the experimental one for different cases tested in a satisfactory way.     

A simple,versatile and inexpensive rheometer for polymer melts

A simple, versatile biconical rehemoter has been developed. This device provides shear creep and creep recovery data for polymer melts over a temperature range of 200–500°F. and a range of applied shear stresses from 2 × 10³ to 9 × 10⁵ dynes/cm². Extensive reheological data have been obtained for two samples each of polyisobutylene and high-density polyethylene. These illustrate the value of the device in obtaining data useful for predicting and understanding the processing properties of polymer melts.     

Hot plate welding of plastics: Factors affecting weld strength

Welded joints were made under a range of conditions in polypropylene, glass fiber reinforced polypropylene and poly (methylmethacrylate) bars. Melt flow in the weld was investigated by microscopy and by contact microradiography, and weld strengths were measured by tensile tests. The fracture toughness of the weld zone was determined by tests on double edge notched specimens. The study shows that weld strength is strongly affected by hot plate temperature, heating time and melt flow during welding. Insufficient heating or melt flow results in incomplete bonding. Excessive melt flow produces strong transverse orientation. Both reduce strength, but in different ways, which can be distinguished by fracture mechanics tests.     

A high reynolds number rotating disk rheometer

The construction and principles of operation of a rotating disk rheometer are outlined as is its use in the study of polymeric fluid drag reduction. The need for a sensitive measuring system under a wide variation of Reynolds numbers has prompted the development of a computer-controlled system that allows a systematic measurement of the turbulent fluid frictional difference between dilute polymer solutions. Also, the long-term drag-reducing stability of macromolecules in a turbulent flow field can be conveniently measured. Representative data from experimental measurements are presented demonstrating the capabilities of this instrument. © 1994 John Wiley & Sons, Inc.