The SPE Library contains thousands of papers, presentations, journal briefs and recorded webinars from the best minds in the Plastics Industry. Spanning almost two decades, this collection of published research and development work in polymer science and plastics technology is a wealth of knowledge and information for anyone involved in plastics.
Two small scale (30g and 200g full charge) recirculation flow cells have been designed, manufactured and commissioned for the study of newly synthesised novel polymers. Full field stress and velocity measurements for a number of polymer melts through two abrupt contraction dies have been made utilising stress birefringence and particle tracking velocimetry techniques. These results have been compared with those through geometrically identical contractions mounted in flow cells on 38mm and 60mm extruders in order to quantify the effects of scale up. Complimentary small angle neutron scattering (SANS) and X-ray scattering (SAXS) studies on molecular configuration and shear induced crystallisation show the usefulness of these flow cells and brief results from these experiments will be presented.
The effect of low temperature modeling of the time-temperature shift factor on the prediction of residual stress and warpage of injection-compression molded compact discs is studied for an optical grade polycarbonate. Predicted residual stress and warpage with WLF and Arrhenius shift factors truncated at different temperatures indicate that the truncation temperature has a significant effect on the predicted part qualities. A double domain approach is employed to fit the shift factor with WLF function above Tg and an asymptotic function below Tg, and the simulation results are compared with the experimental observations. The comparison shows that the double domain shift factor yields good model fit and part quality prediction of injection-compression molded compact discs.
A mechanical hole-burning (MSHB) scheme was constructed to compare the analogous observations to those from dielectric non-resonant spectral hole burning (NSHB) for glass-forming liquids near their glass transitions. We used the framework of the BKZ and Bernstein-Shokooh nonlinear viscoelastic constitutive equations to examine the modified responses in a way that does not invoke an explicit heterogeneous or homogeneous nature for the relaxation response. From the BKZ model only partial hole-burning features are observed in the modified shear modulus. The Bernstein- Shokooh model used to calculate the modified compliance shows no evidence at all of a hole-burning event. These results suggest that in addition to showing potential as a probe of dynamic heterogeneity, MSHB may also a prove to be a sensitive test for the validity of nonlinear constitutive laws.
This work presents an overview on the role of process aids on the rheological properties of rigid PVC. A new rheological approach is introduced to allow a better assessment of the role of these additives. This system comprises a combination of a Couette-type cell and a capillary rheometer. The former allows a good control of the thermo-mechanical history of the compound prior to injection into the capillary barrel where a viscosity measurement is performed. The results showed that rigid PVC undergoes a fusion and gelation processes during the early stages of processing. In this step, the particles are agglomerated under the influence of heat and mostly shear. There seems to be an optimal morphological state where the best mechanical properties are obtained. Additional work showed that the addition of high molecular weight impact modifiers which also act as “binders” in the matrix promote the fusion and gelation of PVC. The results are supported by impact testing and microscopy.
A portable rheometer has been developed for characterizing plastic melts for different measurement purposes. The rheometer is intended particularly for use with rigid PVC processing, but can be used for other materials too. Measurements showing the accuracy of the instrument and its reproducibility are discussed. Comparisons are made between measurements on a conventional laboratory capillary rheometer and ones on the rheometer developed, using polypropylene. The practical application of the rheometer is also shown. This is used in combination with a twin-screw extrusion line to evaluate the rheological data of different pressure pipe and profile PVC formulations in order to develop new die geometries.
There is a demand for the development of techniques for viscosity measurements of very small polymer samples. Traditional rheological equipment and standard tools are limited in their capabilities to measure milligram samples of polymers. This paper outlines methods and tools used to measure the melt viscosity of polymer samples as small as 5 mg. Special, small diameter parallel plates are used to quantify the shear rheology of these samples. The data is fit to several GNF models, and the melt index is calculated from these parameters. Results from this technique are compared to results from actual melt index measurements.
The addition of small quantities of plate-like nanoclay can substantially increase the polymer melt viscosity, while adding dissolved gases such as CO2 can reduce the viscosity of a polymer melt. The combined effect of nanoclay and CO2 on polymer melt rheology was investigated for an extrusion process. The shear viscosities of polystyrene/CO2/nanoclay melts were measured using an extrusion slit die rheometer with a backpressure regulator. Our results show, without the presence of CO2, that the viscosity of the nanocomposite increases with nanoclay loading. However, when the nanocomposite melt is swelled by CO2, the nanoclay acts to reduce viscosity compared to the pure polystyrene/CO2 system. A possible explanation is that a significant amount of CO2 is adsorbed on the surface of the nanoclay to lubricate the flow due to the existence of surface modifier and a unique nanoclay particle layering structure.
A novel extensional rheometer has been developed for use in characterizing the flow behavior of polymers in uniaxial extension. The device has been designed as a fixture for use on a commercial rotational rheometer and incorporates dual wind-up drums that allow for a truly uniform extensional deformation during flow measurement. The miniature unit can be accommodated within the oven chamber of almost any rotational rheometer such that the extensional flow properties of filled and unfilled polymers can be measured over a very wide range of extensional rates, deformations, and temperatures. Validation results with this robust instrument are in excellent agreement with published data in the literature. These and other results indicate the potentially invaluable impact that this novel device could have as a polymer characterization tool.
A laboratory film stretcher that closely simulates the stretching conditions encountered on the industrial biaxial tenter-frame stretching process was utilized to investigate the simultaneous biaxial stretching of isotactic polypropylene films in the partly molten sate. The effects of chain tacticity of the polypropylene resins on the biaxial deformation behavior and the resulting mechanical properties were studied. Correlations were found and explained between the deformation behavior, end-film properties and the morphological characteristics of the partly molten state.
Elongational viscosity becomes more and more important due to increasing processing velocities in industrial processing. For film blowing, blow moulding and spinning process it is inevitable to consider elongational properties. The most important requirements for industrial application areFast testing with easy useHigh selectivityElongation rates similar to processingOnline measurement.Several methods of measurement have been analyzed and some are introduced on the market but not under all aspects mentioned above.First an online elongational viscosity measurement on the basis of the so called “Rheotens” is presented in this work. This method of melt elongation is then compared with converging flow method (entrance pressure loss), were different calculations of elongational viscosity are used. Different lots of polyolefine types are selected for elongational viscosity measurement. The selectivity of both methods to material differences are compared discussing measurement uncertainty.
The relevance of transient uniaxial elongational viscosity determination of polymers to industrial processes such as film blowing and foam extrusion is now being well recognized. Elongational rheometry is also beginning to be well documented, but it remains a delicate measurement technique, with reliability of the equipment and reproducibility of the data still being of prime concern.Recent developments in experimental evaluation coupled with state-of-the art optical techniques extend a step further the capabilities of generating reliable response in elongation, especially for large Hencky strains where dimensions of the sample being stretched is subjected to uncertainty. This paper will review the current approaches used and present a new way of monitoring in real time the true elongational response of polymeric materials, which exhibits enhancement of both the accuracy and the rapidness of the data acquisition.
A semi-hyperbolic (SHPB) die attached to capillary rheometer has been proposed as a method to obtain extensional viscosity data for polymer melts at high extension rates and strains. There has been very little confirmation that data obtained from this type of device is a true measure of the extensional viscosity, ?e. Values of ?e+(transient extensional viscosity) were obtained on a Münstedt device for several polyethylenes (PE) at extension rates which overlapped with those obtained from a semi-hyperbolic die. For a highly branched PE the values obtained on a SHPB die were qualitatively in agreement with those from the extensional rheometer. However, for a linear and a sparsely branched PE the values were an order of magnitude higher than those obtained from the Münstedt device. A new analysis was carried out relating the pressure drop to the extensional viscosity which included a correction term for the variation of pressure along the die wall.
The predictive capabilities of three models (modified White-Metzner model, eXtended Pom-Pom model and newly proposed modification of the Leonov model) are tested for steady shear and uniaxial extensional flows of LDPE, mLLDPE and PVB melts. The input low-shear-rate viscosity data was measured on ARES Rheometrics parallel plate rheometer, whereas RH7-2 capillary rheometer was used for the determination of shear viscosity (capillary), first normal stress coefficient (slit die) and uniaxial elongational viscosity (Cogswell method). A newly proposed ‘effective entry length correction‘ has been applied to deal with all extensional viscosity data.
A new rheological model of polymer melt was developed to describe viscosity-shear rate curves utilizing a four parameters equation based on Guassian processes approach for regression description of melt properties. Four parameters of the model (Newtonian viscosity at “zero” shear rate, viscosity and shear rate at inflection point, and dispersion of the rate of change of the viscosity) were found to be a function of polymer chain structure, molecular weight and temperature. The model provides precision description in a wide range of conditions of shear deformation of the polymer melts, solutions, blends and alloys.
Polymer use in micro-devices, especially in the medical industry has been rapidly increasing. During assembly of micro-devices it is desirable to produce weld joints that are about 100 ?m in width. This paper reviews the use of fiber coupled laser diodes in conjunction with special lenses to produce spot sizes between 25 and 50 ?m in diameter for through transmission infrared welding technique. Studies were completed to evaluate the effect of travel speed, power density and pressure on weld quality for polycarbonate and polystyrene. It was found that process parameters are extremely critical in producing consistent welds. In addition, new testing techniques had to be developed to allow quantitative measurements to be made on weld strength due to relatively small weld area. Micrographs of the resulting welds revealed evidence of ablation at high heat inputs. Finite element analysis of the mechanical tests showed that the localized weld strength approached the yield strength of polycarbonate.
Polymer use in micro-devices, especially in the medical industry has been rapidly increasing. During assembly of micro-devices it is desirable to produce weld joints that are about 100 ?m in width. This paper reviews the modeling of heat flow during through transmission infrared micro-welding of plastic using fiber coupled laser diodes. Two models were used to predict temperature distributions within welded samples. Both models were based on a moving heat source and moving coordinate system. For the simpler model a moving point heat source was used and for the more complex model a Gaussian distributed heat source was used. It was found that the distributed model can accurately predict temperature fields in plastic laser welds for all ranges of the parameters evaluated. However, the point heat source model was only able to accurately predict temperature fields with a relatively small laser focal spot (25 ?m). In addition it was found that for micro-welding of plastics, when the dimensionless distribution parameter is less than two, a point heat source model predicts similar widths to those predicted by a distributed heat source model.
Residual stresses are detrimental to a plastic joint for a number of reasons. They lead to reduced strength and fatigue life in joints, act as stress concentrators and cause crazing, cracking when exposed to solvents. In this paper, the residual stresses in Clearweld® joints were measured using photoelasticity. This interference based technique was used in conjunction with a stress separation algorithm to quantify the maximum residual stress level and the stress distribution in the weld region. Also, the effects of process parameters like welding speed, power and ink solvents on residual stresses were evaluated. The GE solvent test was also employed for comparison with the photoelasticity results. A comparison of the residual stresses between various joining processes was also made.
In recent years, the use of high power diode lasers for through transmission laser welding (TTLW) of thermoplastics has increased rapidly due to the many advantages that they provide, mainly their compact size and low cost. The diode laser output can affect the heating rate and uniformity, making it important to characterize the diode laser in any welding application. In this study, diode laser bar characterization included measurement of the power efficiency, beam shape, and intensity distribution. Those measurements were performed with and without a proprietary fiber bundle provided by Branson Ultrasonics, which improved the uniformity of power intensity. In TTLW laser beam reflection, absorption and scattering as it passes through the transparent part affects the power that reaches the weld interface. Therefore, it was important to study the transmittance and reflectance of polycarbonate and high-density polyethylene. The effects of thickness and beam incident angle on transmittance and reflectance were also measured.
Residual stresses are detrimental to the strength of plastic joints. They lead to reduced strength and fatigue life in joints, act as stress concentrators and cause crazing and cracking when parts are exposed to solvents. In this paper, photoelasticity is used to measure the residual stresses in laser welded polycarbonate. Photoelasticity, an interference based technique, was used in conjunction with a stress separation algorithm to quantify these stresses along and perpendicular to the weld line. The GE solvent test was also employed to serve as a comparison to the photoelasticity results. A comparison of the residual stresses between laser welded samples and hotplate welded samples have also been made.
Vibration welding is a joining technique to assembly thermoplastic components. Meltdown-time profiles and assessment of weld microstructure are commonly used to characterize the behavior of polymers during vibration welding. The aim of this work is to establish relationships between the rheological properties of molten polymers and their meltdown rate during vibration welding. Two polypropylene homopolymers with different molecular weights resulting in different rheological properties were studied. Vibration welding was carried out using a butt-weld geometry and meltdown-time profiles were measured. Significant discrepancies between experimental results and theoretical predictions based on the simple model developed by Stokes suggest the presence of significant elastic effects.
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