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SPE Library content related to rheology
A Rheological Model for Thermoplastic Resins Melt
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.
Effect of Polypropylene Rheology on the Vibration Welding Process
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.
Investigation of the Processing Characteristics and Mechanical Properties of Metallocene Catalysed Polyethylene Foams for Rotational Moulding
The object of this work is to investigate the foaming characteristics of three grades of metallocene-catalysed Linear Low Density polyethylenes for rotational moulding using both an exothermic and endothermic chemical blowing agent. This paper reports on the results of ongoing experimental investigations in which rheological and thermal parameters are related to the polymer structure and mechanical properties. Through adjustments to moulding conditions, the significant processing and physical material parameters, which optimise metallocene catalysed linear low-density polyethylene foam structure, have been identified. The results obtained from equivalent conventional grades of Ziegler-Natta-catalysed linear low-density polyethylene are used as a basis for comparison.
The Effect of Coupling Agents on Foaming with Polymer Microspheres in Rotational Molding
The use of polymer microspheres for producing microcellular foams is a new development in rotational molding. In previous studies, some reduction in mechanical properties has been found due to the immiscibility between the polymer shell and the matrix polymer. Coupling agents can act as a molecular bridge at this interface and can also affect bubble growth by altering the rheological properties of the matrix polymer. The influence of different coupling agents on the melt properties of several resins was investigated, as well as the effect of these coupling agents on the mechanical properties of foamed rotationally molded parts.
Polypropylene/Clay Nanocomposites: Characterization and Interpretation of Structure - Property Relationships
Polypropylene (PP) nanocomposites were obtained by melt compounding of PP and organoclays in the presence of maleic anhydride grafted polyolefin compatibilizer. The effects of compatibilizer type and content and the mixing sequence on the morphology and properties of the PP/clay nanocomposites were investigated. PP nanocomposites exhibit enhanced mechanical properties compared with neat PP and they also are thermally more stable. Rheological properties are sensitive to the composite structure and thus are a valuable indication of the degree of exfoliation.
Mechanisms of Clay Exfoliation in a Polymer Matrix during an Extrusion Process: A Structure-Property Relationship
Polymer-organoclay nanocomposites were prepared by melt extrusion techniques, employing different processing conditions and material formulations. The structure-morphology relationship of the nanocomposites was analysed using transmission electron microscopy (TEM) and wide-angle X-ray diffraction (WAXD). Results from the rheological study showed significant correlation between the dynamic rheology of the polymer melt during the extrusion process, and the exfoliation mechanism of the organoclay layered-silicate observed in the WAXD diffractogram and TEM analysis. Results also indicate that the exfoliation process in the extrusion environment could be achieved through different mechanisms and these mechanisms can be optimised by adoption of different extrusion processing conditions and material formulations.
Mechanism of Exfoliation of Nanoclay Particles in Epoxy-Clay Nanocomposites
In this study, a rheological interpretation of exfoliation of clay particles was proposed using experimental observations in the curing of organically treated nanoclay- Epon828 mixtures with three curing agents. The elastic force exerted by cross-linked epoxy molecules inside the clay galleries was found responsible for exfoliation of clay layers from the intercalated tactoids. Complete exfoliation of clay galleries was observed under the conditions of slow increase of complex viscosity and fast rise of storage modulus. Gel time presented an upper bound of time available for exfoliation. Faster intra-gallery polymerization, although expedited clay exfoliation, was found to be not a necessary condition.
Enhanced Failure Properties in Layered Silicate Nanocomposites Formed by Melt Processing I. Polyoxymethylene as a Model Matrix Material
Polymer / layered silicate nanocomposites have been extensively studied over the past decade as promising high performance materials. Noteworthy mechanical performance, however, would require significant improvement in strength as well as modulus over conventional composites or unfilled polymers. Our objectives extend current research based on existing models to focus on the synthesis of an idealized layered silicate nanocomposite and develop a rationale for the current limitations of intercalation and exfoliation technology in enhancing failure properties. As a first step in this process layered silicate nanocomposites have been prepared by combining polyoxymethylene (POM), an engineering thermoplastic chosen as a model polymer system for its potentially interactive oxygenated backbone structure, with clays having different types of surface treatments. Characterization of the synthesized nanocomposites is accomplished through a combination of X-ray diffraction, TGA, mechanical properties and rheology.
Effect of Reactive Additives on the Properties of Polypropylene/Unsaturated Polyester Blends
Reactive melt modification of a low molecular weight unsaturated polyester (UP) and its blends with polypropylene (PP) were studied. The rheological and morphological properties of the polyester and its blends can be greatly improved not only by adding a peroxide to initiate competing reactions within the blend components that would lead to compatibilization, but also by some organic and inorganic additives such as coagents and alkaline earth metal oxides which can generate ionic crosslinking of the polyester. Extrusion process conditions are discussed along with DSC, FTIR, SEM and other characterization methods utilized to investigate the structure of the modified products.
In-Situ Compatibilization of PP/EPDM Blends during Ultrasound Aided Extrusion
Blends of isotactic polypropylene (iPP) and uncured ethylene-propylene diene rubber (EPDM) were treated by high intensity ultrasonic waves during extrusion. Die pressure and power consumption were measured. The effects of different gap sizes, blend ratios and number of ultrasonic horns were investigated. The rheological properties, morphology and mechanical properties of the blends with and without ultrasonic treatment were studied. In-situ compatibilization of the blends was observed as evident by their stable morphology after annealing and improved mechanical properties. The obtained results indicated that ultrasonic treatment induced the thermomechanical degradations and led to the possibility of enhanced molecular transport and chemical reactions at the interfaces. Processing conditions were established for enhanced in-situ compatibilization of the PP/EPDM blends.
Effect of High Intensity Ultrasonic Waves on Polypropylene, Polyamide 6 and Their Blends
The mechanical, rheological properties and morphology of polypropylene (PP), polyamide 6 (PA6) and their blends treated by high intensity ultrasound have been investigated. A lower head pressure and better mechanical properties are simultaneously achieved in the extrusion of these thermoplastics. A competition between the ultrasonically enhanced polycondensation reaction and degradation was observed for PA6. These enhanced polycondensation and degradation have a different mechanism than the thermally induced reaction. The better strength of ultrasonically treated PA6 is attributed to this reaction, leading to higher molecular weight, higher crystallinity and more uniform crystal size distribution. For PP, the degradation at high amplitude of ultrasound was observed. The mechanical properties of treated PP are maintained at low amplitude of ultrasound. For ultrasonically treated PP/PA6 blend, a competition between degradation and in-situ compatibilization was found. At a certain level of amplitude of ultrasound and a certain blend ratio, the tensile toughness and impact strength of treated blends were almost doubled, and a more stable morphology upon shearing and heating was observed.
Thermal and Rheological Properties of Novel Thermoplastic Polyimide Blends
We report the results of our preliminary studies on the thermal and rheological behavior of a new semicrystalline polyimide (PI) type R-BAPB and its miscibility with amorphous PI type R-BAPS having similar chemical structure to the former. To ensure miscibility of the above relatively viscous PI, a prepolymer prepared by melting dianhydride and diacetyl derivatives of aromatic diamine (BAPB type) was blended with thermoplastic R-BAPS at 50/50 and 70/30 wt % ratio. At the start of the chemical reaction, the resulting mixture was completely miscible with a low viscosity of about 50 Pa?s at 300°C that subsequently increased to about 3?104 Pa?s after 1 hr at 300°C. This mixture can provide new PI blends with better processability and thermal properties than a simple thermoplastic mixture of R-BAPS and R-BAPB having the same weight ratio.
A Mathematical Model for the Increase of G’ with Time during the Irradiation of UV Curable Coatings
Another paper in this session describes a method for following the development of rheological properties of a UV-curable coating while it is being cured. An equation has been found that models the development of G’ as a function of time from that method. It is an empirical model, not one derived from theory, e.g. reaction kinetics. The model curve conforms to the data points on a plot, all the way from start to finish of curing. Standard non-linear curve fitting algorithms such as Marquardt-Levenberg and Quasi- Newton work satisfactorily. Since the G’ values cover 2 to 3 orders of magnitude, a weighted fit emphasizing lower G’ values gives better results than an unweighted fit. The model is useful for comparing and classifying cure behavior of coatings used for fiber optic waveguides.
Melt Processing of Tailored Acrylic Copolymers
Steady shear rheology of acrylonitrile (AN) terpolymers provides an indication of the melt stability of tailored AN copolymers. It has been found that AN can be copolymerized with methyl acrylate (MA) to produce a material with up to 88 mole percent AN that possesses suitable melt stability at elevated temperatures for processing into carbon fiber precursors. A third copolymer, acryloylbenzophenone (ABP), is copolymerized in 1-2 mole percent to act as a UV stabilizing agent that activates crosslinking following fiber formation. Boric acid (BA) is also added as a “free radical quencher”, which enhances the thermal stability of the terpolymer.
Flow Induced Warpage in Polypropylene/TLCP Fiber Reinforced Composite Parts
The most common belief is that warpage in injection molded fiber reinforced thermoplastics is due to residual thermal stresses associated with shrinkage and non-uniform cooling of the parts. Studies on polypropylene (PP) reinforced with pregenerated thermotropic liquid crystalline polymer (TLCP) microfibrils suggest that warpage is associated with enhanced flow induced orientation in the presence of high aspect ratio fibrils and increased frozen-in residual stresses due to increased relaxation times. Injection molded rectangular plaques of PP reinforced with pregenerated TLCP microfibrils were generated in order to study the influence that concentration and aspect ratio have on warpage and shrinkage. In an effort to relate the material parameters to warpage and shrinkage, the rheological behavior of these fiber-filled systems was investigated. The approach could be extended to glass-reinforced PP also.
Effectiveness of PPMA Compatibilizers for Nanolayer Dispersion in PP: Bound vs Free Anhydride
This study investigates the importance of maleic anhydride distribution in maleated PP compatibilizer for dispersion of nanolayers in polypropylene melts. Several grades of PPMA have been analyzed for bound fractions of maleic anhydride. The structure of the resulting nanocomposites has been investigated with X-ray diffraction and rheology. The relative viscosity of the composite relative to the silicate free mixture provides a quantitative index of the level of exfoliation of the clay. The most exfoliated nanocomposite is the one with the largest amount of covalently bound maleic anhydride, which is located predominantly at the terminus of the polymer chain in a poly (maleic anhydride) graft.
Electrically Activated Polypropylene/Clay Nanocomposites
Polypropylene(PP)/clay nanocomposites under electric field was reported to show an exfoliated structure without any compatibilizer such as maleic anhydride functionalized polypropylene(MAPP). We could regulate the degree of dispersion and exfoliation of materials by controlling the amount of clay loading, the strength of electric field, the time exposed to electric field, etc. However, a new design concept is required for a continuous production of PP/clay nanocomposites under electric field.In this talk, we will present a novel method to continuously produce PP/clay nanocomposites using electric melt pipe equipped on a twin-screw extruder. Rheological and XRD measurements guide the degree of exfoliation and the improved properties of PP/clay nanocomposites. As applying the electric field is a physical process, the approach can be easily extended to make other polymer/clay nanocomposites.
Effect of Organoclay Structure on the Rheology and Crystallization Behavior of In-Situ Polymerized PA6 Nanocomposites
PA-6 Nanocomposites containing nanometer-scale, finely dispersed silicate platelets (‘nanoclays’) have been prepared via in-situ polymerization of caprolactam with various types of organoclays. Depending on the chemical structure of the organoclay, a covalently tethered, a nontethered or a weakly tethered PA-6 Nanocomposite was obtained. While all the nanoclays showed a consistent nucleation effect on the PA-6 crystallization, the tethered nanocomposites showed a slower rate of crystallization than standard PA-6 or the non-tethered systems. The melt rheology at low shear rates reflected the clear effects of the tethering in causing a significantly higher melt viscosity and slower relaxation relative to standard PA-6.
Effect of Mixer Type on Exfoliation of Polypropylene Nanocomposites
Polypropylene/organoclay nanocomposites have been prepared by melt blending in five different mixers: an internal mixer, two lab-scale, co-rotating vertical twin-screw mixers, a 30 mm co-rotating twin-screw extruder, and a multilayer extrusion system. The effectiveness of these mixers toward the dispersion of the clay into the polymer matrix was evaluated by TEM, X-ray diffraction, and melt rheology. Mechanical properties and coefficients of linear thermal expansion (CLTE) were also evaluated for these blends. The vertical twin-screw mixer at lower shear rate appears to provide the best mixing in terms of dispersion efficiency and modulus improvement. The combination of shear rate and residence time in the mixer is discussed in order to rationalize our results.
Time-Scales of Coalescence in Polymer Processing: Study on Polypropylene/Polyamide-6 Blends
The effects of shearing time, volume fraction, shear rate, and viscosity ratio on coalescence of isotactic polypropylene (PP) and polyamide-6 (PA6) blends were studied in simple shear flow. A simple model for coalescence developed to provide characteristic times in coalescence in polymer processing operations was used to analyze experimental results. The pre-coalescence droplet morphology was created by melt blending the polymers in a twin-screw extruder at several compositions and was subjected to a simple shear flow in a cone and plate rheometer at low shear rates (0.1 and 0.5 s-1). The rheological data was analyzed after removing the effects of viscosity mismatch to leave only the interfacial effects on coalescence.
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