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.
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Organic Solar Cells - Stabilization by Plastic Encapsulation
Due to the simple chemical tuning of the electronic properties of organic molecular semiconductors these materials are investigated in optoelectronic devices, covering the field from laser diodes to solar cells. In all these applications the stability of the active organic layer is crucial. Photo-oxidation is one of the biggest problems when using organic molecules for solar cells. We show how encapsulation affects the lifetime and performance of such devices. We present examples of organic solar cells based on poly (para) phenylene vinylene (PPV) and sexiphenyl (PHP). Its spectral response and electrical characteristics were tested under laboratory conditions and under AM1.5 conditions in the desert.
Thermal Expansion and its Effect on the Normal Force for a Modified Force Rebalance Transducer
Prior work in this laboratory [Niemiec, et al, J. Rheology, 40, 323-334 (1996)] showed that anomalous normal forces could arise in rotary shear measurements when thermal expansion of the force rebalance transducer (FRT) superimposed a squeezing flow on the shear flow. Transducer heating results from the current to the magnetic coils in the FRT necessary to counteract the applied torque. Partly due to this work, the manufacturer redesigned the transducer by replacing stainless steel components with Invar, an alloy with a very low coefficient of thermal expansion. Tests on the new transducer show that the thermal expansion is significantly reduced. The behavior of the new transducer is described.
A Nonlinear Fluid Standard Reference Material: Progress Report
A new NIST Standard Reference Material (SRM 2490 - Nonlinear Fluid for Rheological Measurements) demonstrates shear thinning and normal stresses typical of polymeric fluids. SRM 2490 consists of polyisobutylene dissolved in 2,6,10,14-tetramethylpentadecane (pristane), giving a stable fluid with a wide temperature range. NIST will certify the linear viscoelastic behavior and the shear-rate dependence of the viscosity and first normal stress difference between 0 °C and 50 °C. NIST will also use the fluid in a round robin to help the polymer community identify sources of variability in rheological measurements. Here we report progress on the project.
Pressure Flow in the Barrel: The Relationship of Fluid Elements in the Cavity to Original Barrel Locations
Pressure-driven flows dominate the injection molding process. A pressure flow is established in the barrel of the injection molding machine initially when the screw moves forward to inject material into the cavity. The laminar shear flow established by this action exhibits a parabolic velocity profile with the highest velocity in the center of the barrel and zero velocity at the barrel walls (Figure 1). With plastic melts the velocity profile is somewhat “flattened” due the non-Newtonian, i.e. nonlinear, rheological behavior of the melt. Such a velocity profile is not only characteristic of the basic flow in the barrel, but it also applies to the pressure-driven shear flows in the nozzle, runners, gates and cavities. The shear flow in the barrel is unique since the origin of the pressure energy is the screw displacement during injection. In order to maintain the highest velocity in the center of the various flow channels, material needs to be constantly added at the pressure source. This occurs in the barrel in the region closest to the screw. As the screw moves down the barrel, slow moving melt adjacent to the screw tip and near the barrel wall, is forced to the fast-moving center region (Figure 2). This sequence continues until the screw bottoms out or until the injection is stopped. Experiments with colored tracers have verified the laminar flow conditions and the volume-element relationship.
A Study of the Long-Term Crystallization Process in Polypropylene and Acetal
Over the years there has been a great deal of discussion regarding the length of time required for certain semi-crystalline materials to achieve full potential crystallinity. Many suppliers of flexible chain crystalline materials have studied the phenomenon of post mold shrinkage. In addition, many suppliers of these materials recommend an extended conditioning period for molded test specimens before conducting physical property tests. It is supposed that it can take two to three weeks for properties to fully develop. Yet even within supplier organizations there seems to be no consensus on the necessity for this practice and little documentation exists to support either a long-term or a short-term approach to sample conditioning. This study uses tensile stress-strain properties and dynamic mechanical analysis to track the property development of test specimens molded in polypropylene and acetal. Unfilled homopolymers and copolymers were used in both material families.
The Application of New Spectral Birefringence Technique to Characterize the Structural Changes That Take Place in Heat Setting of Preoriented PET and PETG
With the intent of simulating the heat-setting process of a tenter film stretching process, the birefringence development of PET and PETG films are monitored on-line using spectral birefringence technique. In this technique, a white light source is used on an optical train with a heating chamber equipped with rapid sample insertion capability. The full visible spectrum is utilized and essentially, this technique has no limitations on measurements of retardation to very high values. Spectral birefringence technique also enables one to detect the changes in the trend in birefringence (such as decrease as a result of relaxation, or increase as a result of crystallization) and to investigate changes that occur very rapidly in the order of seconds. The effects of stretch ratio on the kinetics of the structural changes in PET and PETG films are investigated. It is found that heat setting at temperatures where crystallization rates are slow results in partial relaxation followed by a rapid rise in birefringence, particularly for samples with low stretch ratios. In this paper, only the results for PETG films are presented due to the limited space. However the presentation will cover the results for both PET as well as PETG films.
Mesoscopic Simulation of Glass Transition Temperature and Comparison with Entropic Difference Model
Mesoscopic simulations that lie between atomistic and macroscopic simulation is used to calculate the glass transition(s) for partially miscible copolymers in blends. The predictions are compared with the entropic difference model presented earlier for predicting the multiple glass transition temperatures observed in these systems. The molecular weight effects on the free volume were not considered in this study. The temperature dependence on heat capacity was included in the entropic difference model by assuming a power series expansion and then a power function representation. The power function indeed is found to be a better representation of the experimental data compared with the power series expansion. The physical interpretation of the ??Sm calculated using experimental data is discussed.
Ballistic Impact Energy Measurement of Novel Coextruded PC/PMMA Multilayer Composites
Ballistic impact energy measurements of coextruded polycarbonate/polymethylmethacrylate (PC/PMMA) multilayer composites were performed. Until PC composition reaches approximately 70%, ballistic kinetic energy absorption capability of these composites is relatively low and differences within the region are negligible. They all failed in brittle fashion. However, ballistic performance improved drastically above 80% of PC composition, with ductile mode of failure. The individual layer of PMMA appeared to be the dominant factor in determining the ballistic performance as well as mode of failure of these coextruded composites. Ballistic impact energy values were significantly increased when the PMMA individual layer thickness was reduced to approximately 0.36 µm or thinner. Ductile mode of failure, which is predominant in the monolithic PC, occurred in these corresponding PC/PMMA multilayer composites.
Self-Reinforced Composites of Various Polyesters with PET/HBA Based LCP
Four different polyesters, PET, PBT, PEN, and copolyester, were each blended with LCP based on poly(ethylene terephthalate)/p-hydroxybenzoic acid, PET/HBA, by using a single screw extruder with a static mixer followed by a die. Unidirectional sheets and fibers of the blends of various LCP contents were prepared at various operating conditions including die geometry, die temperature, and extension ratio to find optimum processing conditions required for improvement of mechanical properties. Depending on matrix and processing conditions, blends revealed different LCP domain deformation, size, shape, and, consequently, mechanical properties. Among four different polyesters, PET matrix was the most efficient in obtaining uniform LCP phase distribution and deformation in blends. However, blends based on the copolyester matrix showed the best mechanical properties. The mechanical properties of injection moldings made by reprocessing thermoplastic/LCP fibers at the melt temperature below the LCP melting point were measured. Among various blends, the LCP fibrils preexisting in PEN/LCP fibers were best preserved in moldings.
Control of Melt Temperature on Single Screw Extruders
Melt temperature in extrusion affects most aspects of the process, including polymer integrity, ability to handle and cool the extrudate and surface finish of the final product. Attempting to control the melt temperature via adjusting extrusion settings on a single screw extruder has a varying success rate, depending on such factors as: screw speeds being attempted, screw design, polymer viscosity and screw tip pressure. This paper will give the results of a study including the above mentioned parameters and the melt temperature control possible on a single screw extruder. Most extrusion engineers have a feel for their ability to control melt temperature with their machines and polymers, but an encompassing study will clarify the degree of control possible on this machine under a broad array of conditions.
Structure/Property Relationships in Polyetheretherketone Vibration Welds
Vibration welds were made in polyetheretherketone (PEEK) using different pressures, with a variety of welding times, in order to produce welds with a range of mechanical properties. Polarised Fourier transform infrared (FTIR)-microspectrometry was used to measure crystallinity and molecular orientation, and transmitted light microscopy was used to study morphology. Tensile and tensile-impact tests were carried out, and the broken test specimens were examined by scanning electron microscopy (SEM) and transmitted light microscopy to establish where failure occurred, and to examine the fracture surfaces of broken test specimens.
Mathematical Modeling of Three-Dimensional Die Flows of Viscoplastic Fluids with Wall Slip
The mathematical modeling of the continuous processing of filled polymers, and concentrated suspensions in screw extruders and dies of complex shapes is undertaken. The simulation of the flow of such filled systems in complex geometries is rendered complicated by the occurrence of wall slip at fluid/solid boundaries. The incorporation of wall slip in the analysis of three-dimensional flows including flows through dies, single/twin-screw extruders and other processing geometries is currently lacking. Here we present an analysis of three-dimensional flows with wall slip, and demonstrate the procedure using the flow of a Herschel-Bulkley fluid in a tapered die.
Squeeze Flow Rheometer
The squeeze flow rheometer is widely used especially for the rheological characterization of composites and polymer melts. However, since it incorporates both shear and extensional deformations its use is not straightforward. Here the two-dimensional constant-speed squeezing flow of viscoplastic fluids between two approaching surfaces in relative motion is solved using the Finite Element Method. Slip at the wall, a condition generally encountered with viscplastic fluids at solid surfaces, is incorporated in the model. The analysis is applicable to the rheological characterization and testing of parameters of constitutive equations for filled polymers and elatomers that exhibit a yield stress and will expand our understanding of the squeeze flow rheometer. The numerical analysis was focused on the determination of conditions under which 1-D analysis is valid.
Particulate Based Conductive Composites Exploiting Percolation-Range Microstructure
The electrical and tensile properties of graphite based conductive composites were investigated as a function of mixing conditions and the specific energy input incorporated during the mixing process. The microstructural features were characterized employing wide-angle x-ray diffraction. A tri-block copolymer with polystyrene end blocks and poly(ethylene-butylene) mid block was used as the primary matrix material. Graphite powders with controlled particle size distributions were used as conductive fillers. Systematic studies were carried-out varying the volume percent and mixing distribution characteristics of the filler particles. The volume resistivity of the composites exhibited a significant increase with the increased specific energy input. The increase was associated with the enhanced coating of the conductive particles and the better distribution of the matrix polymer as the specific energy input increased.
Viscoelasticity of Polyethylenes Produced with Single Site Metallocene Catalysis
Linear low density polyethylene (LLDPE) produced using metallocene catalysts is gaining prominence as a class of new polyethylenes with superior performance. Two recently commercialized metallocene-catalyzed linear low density polyethylene resins were characterized in terms of their storage and loss moduli, shear viscosity, shear stress growth, stress relaxation upon cessation of steady shear, and first normal stress difference material functions. Overall the rheological behavior reflects the relatively narrow molecular weight distributions of the resins. The oscillatory shear and relaxation moduli data were employed to determine the parameters of Wagner model. Various material functions, determined on the basis of this model in conjunction with the fitted parameters, agreed reasonably well with the experimental results. The reported data and parameters should facilitate an improved understanding of the processability characteristics of these two new LLDPEs.
3-D Analysis of Fully Flighted Screws of Co-Rotating Twin Screw Extruder
A new mathematical model of the flow and heat transfer occurring in the fully-flighted regular flighted" screw elements of the co-rotating twin screw extruder is developed. The method avoids the unwrapping of the screws and solves the conservation equations using a realistic replication of the actual channel geometry. The numerical solution of the conservation equations is accomplished using the three dimensional Finite Element Method. The technique allows a wide range of processing features including the effects of the gap thickness between the barrel and the screws to be investigated. Here typical isothermal results are presented for two generalized Newtonian fluids i.e. Bingham and Ostwald-de Waele "Power-Law" fluids."
Assessment of Particle Migration Effects in Capillary Rheometry of Filled Polymers
It is important to assess the conditions under which the migration of particles becomes important during rheological characterization of filled polymers. Such migrations may become important during nonhomogeneous flow where gradients in shear rate induce particles to move away from high shear rate regions resulting in nonhomogeneous concentration distributions and the blunting of velocity distributions. Using the mathematical model of Phillips et al., a finite difference numerical solution was developed to assess the importance of particle migration effects in pressure-driven viscometric flows.
Statistics of Mixing Distributions in Filled Elastomers Processed by Twin Screw Extrusion
Continuous processing of filled elastomers by twin screw extrusion and achievement of viable mixing distribution characteristics present formidable challenges. In this work a thermoplastic elastomer, HyTemp, was plasticized with DOA and filled with ammonium perchlorate powder and additives. It was found that the extruder geometry, the order of ingredient addition and die pressurization have profound effects on the mixing distribution characteristics of the elastomer based extruded profiles. The mixing distribution characteristics were quantitatively determined by x-ray diffraction techniques.
Elastomeric Coating of Filler Powders by Slurry Precipitation
The work presented here involves the coating of inert particles by an elastomer. The plasticized elastomer is first dissolved in an organic solvent and is mixed with water in which the inert particles are slurried. Due to the miscibility of the organic solvent and water, the elastomer gels and coats the inert particles. The coated particles further coalesce to form the molding powder of desired size range. This paper addresses the dynamics of the coating operation and the role of process parameters on the extent of coating and particle size distribution.
Hot Water Resistance of Glass Fiber-Reinforced Thermoplastics
The hot water resistances of three kinds of short glass fiber or glass bead-reinforced plastics [polyphenyleneether (PPE), polyphenylenesulfide (PPS), and polyoxymethylene (POM)] were studied by hot water immersion tests and tensile tests. It was found that the tensile strengths of these plastics decreased and that the change of the strength was most remarkable in glass fiber-reinforced PPS (GF-PPS). Scanning electron microscope (SEM) observations of the tensile fracture surface revealed that the change in tensile strength was attributable to the deterioration of the interface between the glass fiber and the matrix resin. Although the change in the tensile strength of glass fiber-reinforced PPE (GF-PPE) was small compared with that of GF-PPS, debonding between the glass fiber and the matrix resin and surface cracks were observed on the surface of the GF-PPE specimens.
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