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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Small Scale Flow Visualisation of Polymer Melts in a Recirculation Extruder
A novel small scale (< 30g of polymer) recirculation flow cell enables the study of the in-process behaviour of newly synthesised polymer melts. The system incorporates a single screw extruder, gear pump and variable geometry inserts. This paper extends our previous work by presenting results obtained using the techniques of stress birefringence and particle image velocimetry for a variety of polymeric materials including melts of LDPE, HDPE and PS, in a simple contraction flow. Process-induced change in a polymer is also observed. The new facility complements the larger extrusion studies performed in our laboratory, and comparisons between the two flow scales are also given.
Combined Infrared Spectroscopy and Ultrasound Studies of Changing Melt Composition during Single Screw Extrusion
In-Line ultrasound velocity measurements have been made simultaneously with on-line mid- and near- infrared spectroscopy during single screw extrusion of a range of blends of an HDPE and a PP, to assess the sensitivity of these techniques to blend composition. Ultrasound velocity was sampled at IHz, using in-house process monitoring technology; mid-infrared data was obtained at I 'sample' per minute (16 scans at 2cm-1) resolution). Under these conditions, it was found that the ultrasound technique could successfully resolve a change of l wt% PP in HDPE, whereas the infrared spectroscopy (using specific peak height change) could not resolve better than ~ 2wt%. The techniques showed very close agreement in monitoring the dynamics of change from one blend composition to another. The original blends were made by mixing pellets; twin-screw compounded blends of the PE/PP are being investigated, to assess any effects of blending process history.
Melt Flow Simulation and Measurement of Extensional Viscosity in Planar Hyperbolic Dies
An adaptation of the 'separable' KBKZ constitutive equation for polymer melts is presented. This permits prediction of both strain hardening and shear thinning (for a single parameter set), consistent with measurements made on branched polymers such as low density polyethylene (LDPE) melts. Simulations using a new finite-element based code are presented showing that the constitutive model can be used to predict both stress and vortex growth for planar as well as axisymmetric contraction flows. Flow visualisation has been used to study the flow behaviour of a LDPE melt as it is processed through a planar hyperbolic die on a commercial scale extruder. The die profile consists of a planar hyperbolic section that gradually merges with a parallel slit and is designed to promote constant extensional strain rates at the centre line of flow. Extensional strain rates were determined by performing particle velocimetry along the flow axis. Stress, strain and strain rate data for the low density polyethylene melt is presented, and compared to simulation; it is shown that good agreement and a constant extension rate is achieved along the centre-line.
Visualisation of Melt Interface in a Co-Extrusion Geometry
Past studies, principally confined to numerical simulations, have led to several theories being proposed to explain the source of interfacial instabilities that often occur during coextrusion of polymer melts. However the mechanisms, kinematics and rheological correlations leading to the onset of instability are not yet fully understood. Instability is thought to arise from one of two locations within a die. One location is that coincident with the point where an interface is created when the melt streams combine; the other region is a point just prior to the die exit region where the interface usually experiences a maximum shear stress. It is further suggested that interracial instability can develop when the layer thickness ratio of the melt streams exceeds a critical limit. This paper presents the results of flow visualization studies performed in order to gain a better understanding of the interracial instability phenomena of polyolefin melts within a flat film coextrusion die. Visualisation studies have been initially confined to melts of the same viscoelastic properties, using a new modular flow cell. The objective of the experiments is to determine the effects of geometry and processing on the source and nature of flow instabilities in coextrusion.
Gas Assisted Injection Moulding: 3D Finite Element Modelling Using a Pseudo Concentration Method and Experimental Studies
A 3D implementation of a pseudo-concentration method is used to simulate cavity filling in the Gas Assisted Injection Moulding process. The pseudo-concentration method represents two different fluids (i.e. gas and polymer) within the same domain. Linear quadrilateral finite elements form a fixed grid on which to generate the solution using a time stepping procedure. Simulation results are compared with specimens manufactured on an industrial Gas Assisted Injection Moulding machine. Reasonable agreement is observed between simulation and experiment. The pseudo-concentration implementation discussed here is capable of quantitative predictions of cavity filling. Simulation experiments are ongoing to determine the wall thickness relationship with temperature.
Sensitivity of Ultrasonic Velocity and Attenuation Measurements to Temperature Pressure and Filler Concentration for Magnesium Hydroxide Filled LDPE and HDPE
Samples of magnesium hydroxide filled Low density polyethylene (LDPE) melts were examined ultrasonically, over a range of temperatures and pressures, to determine the effect of melt temperature, pressure and filler concentration on ultrasonic velocity through the melt. Tests were carried out on static samples of melt under conditions of no flow. These data were used with extrusion processing data to predict filler concentration. It was found that for a unit change ultrasonic velocity was most sensitive to changes in temperature and least sensitive to changes in pressure. Signal attenuation was determined through samples of magnesium hydroxide filled HDPE over a range of temperatures and pressures.
A 'Bleed' on Line Rheometer for Polymer Melts
A process-driven on line rheometer has been designed and used to obtain at-process assessment of variation in polymer melt shear viscosity, in conjunction with statistical and dynamics studies of the extrusion process. No gear pumps are involved in this device: melt flow through a chosen die/dies is driven by the melt pressure in ,the extruder. The bleed rheometer mounts via a standard Dynisco pressure transducer fitting. Results for an LDPE and an LLDPE are presented, for differing die geometries and extrusion screw speeds. These compare well with on-line rheometry studies in our laboratory. The effects of extruder conditions on the results obtained from the bleed rheometer are discussed. It is clear that such a simple device could be used for process quality assessment.
Statistical Process Control Applied to the Extrusion Process
Statistical Process Control (SPC) is a powerful tool that can be extremely useful in assessing the performance of the extrusion process, provided it is applied properly. Histograms, normal probability plots and control charts have been used in this work to assess the operation of the extrusion process, as part of a fuller statistical and dynamic study of the extrusion process. It was observed that the extrusion process changed its behaviour with time in a way that affected statistical measures (mean and standard deviation) of key extrusion variables (melt pressure, screw speed, motor current, extrudate diameter). Such observed behaviour of the extrusion variables is categorised as statistically unstable" in classical SPC and control charts cannot be applied to the extrusion process variables directly. A treatment (a transform) is suggested for use with the extrusion variables which might show a stable behaviour allowing control charts to be used."
Processing and Morphology of Hyperbranched Polyester Polyol/Polystyrene Blends
Recent work has shown that hyperbranched polymers have promise for use as processing aids for polyolefins and as toughening agents for thermosetting resins. This promise stems from the high reactivity and unique rheological properties of hyperbranched polymers, which are attributable for the unusual molecular structure. However, there are difficulties associated with the blending of these small, branched molecules into high molecular weight polymers, and processing can be challenging. In this study, we investigate the blending of hyperbranched polyester polyols (HBP) and high molecular weight polystyrene using batch and continuous processing techniques. The overall size, or generation, of the individual hyperbranched polyols is varied, as is the reactivity of the thermoplastic matrix toward to the polyol. Large reductions in system viscosity resulted from the addition of the hyperbranched polymers to the polystyrene. High energy processing and reactive compatibilization were effective in producing finely dispersed morphology in the blends. The processing characteristics, compatibility, and morphology of the blends are reported as a function of HBP generation, reactivity, type of processing, and shear rate.
Recycling of 100% Cross-Linked Rubber Powder by High-Temperature High-Pressure Sintering
Studies estimate there are two billion scrap tires in U.S. landfills with over 270 million tires added yearly. An overview of a simple technique for recycling thermosets will be discussed. In short, it is possible to recycle rubber powders made from scrap tires with the application of only heat and pressure and achieve good mechanical properties. An investigation of the mechanical properties of typical consolidated rubber powders as a function of the molding variables is be shown. To date every type of cross-linked elastomer investigated could be sintered, including silicone rubbers, natural rubbers, ethylene-propylene-diene rubbers, styrene-butadiene rubbers and fluoroleastomers.
Bridging the Modulus Gap between LLDPE and HDPE
Developing blown film resins for the production of films that exhibit the toughness properties of LLDPE with the stiffness and tensile characteristics of HDPE has historically been difficult to achieve for both resin producers and film converters. Resin producers have attempted to push the density higher for LLDPE film resins and lower for HDPE film resins. Separately, film producers are continuously searching for the same hybrid film through blending low, linear low, medium and high-density polyethylenes. Producing successful film blends for heavy-duty applications with densities over 0.918 g/cc usually results in a high performance LLDPE film resin blended with low levels of a MDPE or HDPE. As the level of MDPE or HDPE increases, loss of critical toughness properties such as machine direction tear (MD tear) and dart impact strength is observed. There now exists a novel method of producing films that possess a full density range between 0.918 g/cc and 0.938 g/cc. This approach is based on the utilization of a new family of lower density, high molecular weight medium density (HMW-MDPE) film resins. Blending a HMW-HDPE with a resin density of 0.938 g/cc with high performance LLDPE film resins, films can be fabricated over a wide range of densities without the loss of tear and impact properties. These blends can be produced at thin or thick gauge on either high stalk or conventional blown film equipment.
Bonding of Vulcanized Rubber to Polyester Fibers with Modified RFL Adhesives
Resorcinol-Formaldehyde-Latex (RFL) adhesive and its modification were used to bond rubber to polyethylene terephthalate (PET) cord. The modification of RFL was done by adding the activating materials such as a chlorophenol condensate (DK) and chlororesorcinol condensate (CRA). The effect of heat treatment time, test temperature, aging, and cure conditions on adhesion were studied. Overcure conditions were used to simulate long service time. It was shown that adhesive strength decreases with increasing test temperature and deteriorates with overcure. Bond strength increases with increasing heat treatment time. In addition, adhesion increases with increasing ratio of DK or CR to RFL.
The Effect of Cavity Pressure Transducer Location on Process Robustness
This paper presents the results of a study on the effect of cavity pressure transducer location on process consistency, when the transducer is used for control of the velocity to pressure transfer during the injection phase of the injection molding process. Specifically, a three-stage molding process, where a velocity phase is used to pack the part out slowly, was studied. This study looks at the weight and dimensional stability, when the process is subjected to material variation and check ring wear, of parts molded with the transducer near the gate versus near the last place to fill. It is found that, in some instances, the consistency of the parts can be improved by placing the controlling pressure transducer at the last place to fill in the mold cavity.
Polyester Ionomers in Binary and Compatibilized Blends with Poly(Ethylene Terephthalate), Poly(Butylene Terephthalate) and Nylon 6,6
Binary blends of polyester ionomers with polar polymers have been prepared by both solution and melt-mixed methods to determine the effect of melt-processing on blend compatibility. The effect of metal-sulfonate groups and counterion type was evaluated by blending sulfonated and non-sulfonated forms of an amorphous polyester ionomer with both nylon 6,6 and poly(ethylene terephthalate). The thermal properties and phase behavior of the blends were determined by DSC and ESEM analysis, respectively. A comparison of the degree of compatibility for melt and solution blends suggests that polyester ionomers can interact with polyamides by strong specific interactions between the metal counterions and the amide functionalities. In contrast, polyester ionomers may become compatible with other polyesters by melt-phase transesterification (i.e., in situ copolymer formation). In blends of PBT with Nylon 6,6, sulfonated PET ionomers may be used as minor component additives for blend compatibilization. These compatibilized blends show synergistic improvements in mechanical properties with a significant decrease in minor phase domain size.
Polymer Layered Silicate Nanocomposites Prepared by a Two-Stage Method
Polymethylmethacrylate (PMMA) and polystyrene (PS) clay nanocomposites were prepared via in-situ bulk polymerization. The effects of initiators and clay surface chemical modification on the nanocomposite structures were studied. Exfoliated PMMA and PS clay nanocomposites were synthesized. A two-stage method was then used to prepare PS clay nanocomposites. The masterbatch nanocomposites with a high clay concentration and good dispersion were first prepared via in-situ polymerization. The masterbatch was then blended with a pure polymer using a compounder to lower the clay concentration to the desired level. The thermal stability of the nanocomposites was investigated.
Isobaric and Isochoric Fragility of Polymers
Data taken from the published literature were used to determine the dynamic fragilities for several polymers in both isobaric and isochoric conditions. We find that the path dependence of fragility varies widely for different polymers. For PVAc and PEA, The fragility is independent of the pressure and specific volume, the isochoric fragility and isobaric fragility are almost the same. But for PVC and PMA, the fragility is sensitive to the change of pressure and volume, and the isobaric liquid is more fragile than the isochoric one. Both the pressure dependence of the isobaric fragility, dm/dP, and the specific volume dependence of isochoric fragility, dm/dV, were determined from the data.
TTT Diagram Development of a High Performance Epoxy Resin and Prepreg
Hexcel Corporation's 8552 resin is a thermoplastic-toughened high-performance epoxy and is being used in the construction of the Army's prototype Comanche helicopter. Understanding the cure behavior of a thermosetting system is essential in the development and optimization of composite fabrication processes. A time-temperature- transformation (TTT) diagram was constructed, which characterizes the relationships between the degree of cure, temperature, time, and material processes of the 8552 resin. Torsional braid analysis (TBA) and differential scanning calorimetry (DSC) were used to develop this diagram. By using the TTT diagram, development of a solid-state cure cycle was begun. This will then be used to prepare composites for comparison to those prepared with the manufacturer's recommended cure cycle.
A Range of Processing Methodologies for Designing Adequate Tissue Engineering Scaffolds Based on Natural Origin Degradable Polymers
An ideal tissue engineering scaffold must be designed from a polymer with an appropriate degradation rate, and the processing technique must allow the preparation of 3-D scaffolds with controlled porosity and adequate pore sizes, as well as tissue matching mechanical properties. This communication revises recent work that has been developed in our laboratories with the aim of producing porous polymeric structures (from starch based blends) with adequate properties to be used as scaffolds for bone tissue engineering applications. The developed methods include a range of melt processing technologies (based on injection molding and extrusion using blowing agents and in some cases surfactants) and other innovative combined techniques, such as, solvent casting-particulate leaching and compression molding + particulate leaching. The samples produced by the different methods were characterized with respect to the morphology of the porous structures and their mechanical and degradation behavior.
A New Model for Interpreting Nanocomposite Behavior
A new model has been developed to help in understanding nanocomposite behavior. This model employs the concept of a constrained polymer region around the nano-particles. The constrained polymer region characteristics are dependent upon a number of factors that involve both the type of nano-particle but also the characteristics of the polymer. The intermolecular bonding energies of a given polymer profoundly affect the size and stability of the constrained polymer region. The model will be discussed in some detail as well as the application of the model to interpret nanocomposite data. The model has shown utility in interpreting both physical as well as permeability behavior in a variety of composites.
The Role of Fabrication Technology Inventions in the Introduction of LLDPE
Innovation and invention were key to meeting the challenges of overcoming fabrication process limitations of Linear Low Density Polyethylene (LLDPE). In the late 1970’s as Union Carbide sought to commercialize LLDPE from a gas phase reactor, we found that the “as polymerized” resin product, targeted for the blown film market, had significantly better physical properties than LDPE but poorer processibility, at least in the common equipment in use. UCC met this challenge by assigning product development and fabrication process experts to develop and implement appropriate technologies. The problems we investigated covered many of the classical polymer processing issues including melt fracture, draw resonance and film blowing instabilities. Although only solutions that were projected to be economically acceptable were pursued to completion, all of our studies helped to develop a better understanding of these classical problems. In this presentation, I will discuss some of the important scientific and technological understandings and solutions that were found or rediscovered and how they were applied to compounding and fabrication extrusion lines. A number of examples that illustrate the technical advances in supporting LLDPE commercialization will be given both from our own experience and from other researchers. We will also use these examples to show the synergistic interaction among laboratory experiments, scientific theory and production economics as a feedback loop driving innovation.
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