Rotary Jet Spinning of Polymer Fibres

Polymeric nanofibres can be produced from a variety of methods such as electrospinning and melt blowing, with fibres being produced having applications in many sectors such as biomedicine, composites and filtration. Existing methods are not however capable of producing nanofibres to commercial volumes in an energy efficient way. In this research we investigate a new method of producing nanofibres, namely Rotary Jet Spinning (RJS), which is a relatively new method of fibre production similar to candyfloss production, where centrifugal forces are used to expel jets of polymer from a state of melt or solution in order to produce polymeric fibres. We investigate this method in detail, initially concentrating on the comparison between electrospinning and RJS. Firstly, it was found that electrospinning produced slightly smaller fibre diameters compared to RJS over a broader range of solution concentrations. Secondly, the ability to produce high modulus fibres was investigated by means of an imidization technique, where polyamic acid solution was produced and spun into fibres before conversion to a co-polyimide fibre with an elastic modulus of around 40 GPa. In the third experimental chapter, the viscosity reliability of the RJS process was evaluated by means of computational fluid dynamics simulations, where it was shown that low viscosity (1-10 Pa.s) Newtonian fluids are required to establish fibre production. For fluids with lower viscosities, beading occurred in solution spinning and droplets were produced from melt spinning. Viscosities higher than the recommended value resulted in blockage, with no fibres being produced from either method. Lastly, the production of ceramic fibres was evaluated to establish the ability of the RJS process to produce a ceramic nanofibre. Fibres on the nanoscale were not achieved, however a variation in solvent volatility and crosslinking time were factors in fibre diameter reduction, with solvent variations highlighting the potential of this process to achieve the required fibre size from RJS and thereby demonstrating this technology as a viable option for high volume fibre production.EPSRC grant number 150219

Investigating the shear rheology of molten instant coffee at elevated pressures using the Cambridge multipass rheometer

The processing of instant coffee may involve pumping the material in a melt phase through operations at elevated pressures. The shear rheology of this material was investigated using the Cambridge multipass rheometer, which allows for shear rheometric testing under conditions of independently controlled temperature and pressure. In this work the back pressure was set to dissolve any air that was present in the melt, so that the rheology of the single phase material could be tested. Data were collected over the accessible range of conditions; shear rates from 0.01 to 1000 s −1 , temperatures from 80 to 110 °C, and pressures from 0.01 to 300 bar. The melt exhibited thixotropic behaviour at low shear rates, and the data could be fitted to a non-dimensionalised Carreau fluid model with a temperature dependence which followed an Andrade relationship. Sample to sample variation was observed, which is attributed to differing water content. The results demonstrate how the rheology of complex food materials can be accessed under process conditions.This work was supported by Mondelēz International, Jacob Douwe Egberts, and the Engineering and Physical Sciences Research Council

Pressure development due to viscous fluid flow through a converging gap

The behaviour of fluid flow in industrial processes is essential for numerous applications and there have been vast amount of work on the hydrodynamic pressure generated due to the flow of viscous fluid. One major manifestation of hydrodynamic pressure application is the wire coating/drawing process, where the wire is pulled through a unit either conical or cylindrical bore filled with a polymer melt that gives rise to the hydrodynamic pressure inside the unit. The hydrodynamic pressure distribution may change during the process due to various factors such as the pulling speed, process temperature, fluid viscosity, and geometrical shape of the unit (die). This work presents the process of designing a new plasto-hydrodynamic pressure die based on a tapered-stepped-parallel bore shape; the device consists of a fixed hollow outer cylinder and an inner rotating shaft, where the hollow cylinder represents a pressure chamber and the rotating shaft represents the moving surface of the wire. The geometrical shape of the bore is provided by different shaped inserts to set various gap ratios, ancl the complex geometry of the gap between the shaft and the pressure chamber is filled with viscous fluid materials. The device allows the possibility of determining changes in the hydrodynamic pressure as the shaft speed is altered while different fluid viscosity during the process is considered. A number of experimental procedures and methods have been carried out to determine the effects of various shaft speeds by using Glycerine at 1 to 18 °C and two different types of silicone oil fluids at 1 to 25 °C on the hydrodynamic pressure and shear rate. Viscosities of the viscous fluids were obtained at atmospheric pressure by using a Cone-plate Brookfield viscometer at low shear rate ranges. Moreover, Computational fluid dynamics (CFD) was used to develop and analyze computational simulation models that demonstrate the pressure units, which studies the drawing process involving viscous fluids in a rotating system. A finite volume technique was used to observe the change in fluid viscosity during the process based on non-Newtonian characteristics at high shear rate ranges. The maximum shaft speed used in these models was 1.5m.sec'1. Results from experimental and Computational models were presented graphically and discussed

Understanding Extrudate Swell: From Theoretical Rheology to Practical Processing

This thesis focusses on the measurement and prediction of extrudate (or die) swell of molten polymers. The overall aim of this work is to understand extrudate swell for complex, industrially relevant systems. This is performed by first understanding the causes of swelling for well defined, monodisperse polymers at a molecular level. The systems are then gradually built up in complexity from bidisperse to very polydisperse and/or branched samples. At each stage predictions for extrudate swell are obtained using the \textit{flowSolve} fluid dynamics package combined with a molecular constitutive equation and are compared to extrusion experiments using a novel Multi-Pass-Rheometer setup. The effects of both molecular weight and temperature can be ignored when shear rates are scaled by Rouse Weissenberg number as extrudate swell is a chain stretch controlled phenomenon. For monodisperse systems theoretical predictions using the Rolie-Poly constitutive equation match experimental results up to a $W_R$=7 above which simulations over-predict swelling ratios. This is justified in this work using reduction of monomeric friction at high deformation rates. Extrudate swell of polydisperse polystyrenes is successfully predicted up to high Weissenberg numbers using the Rolie-Double-Poly equation when combined with monomeric friction reduction. A slight under-prediction is seen at low Weissenberg number where the chain stretch times of long polymer chains are increased by dilution with shorter chains. Qualitatively correct but quantitatively poor predictions are obtained for highly polydisperse polyethylenes where the low shear extrudate swell is under-predicted. Branched polymers behave differently experimentally to linear samples, exhibiting extrudate swell below the Rouse time of the polymer backbone. A small amount of branching increases swelling ratios versus the linear case but moderate increases in branching above this point have little effect on the experimentally observed swelling ratios. Significantly branched polyethylenes swell more than this, especially at high shear rates. There is a similar trend in simulated results using the XPP model but only a partial agreement between simulated and experimental extrudate swell is observed

Rheological evaluation and guidelines of high-performance amorphous thermoplastics and carbon fiber reinforced composites for additive manufacturing

Although additive manufacturing (AM) has revolutionized the manufacturing industry through rapid and complex geometry fabrication capabilities at a fraction of the cost, only a small fraction of the materials used for traditional manufacturing are compatible with AM. Emerging applications in polymer AM motivate the need for production and development of new materials with a broader range of thermal and mechanical properties. Advancements in AM have also led to new system development such as Big Area Additive Manufacturing (BAAM) systems at Oak Ridge National Laboratory, capable of processing high-performance thermoplastics and composites. As the application space for three-dimensional printed components continues to grow, it is necessary to identify appropriate processing conditions and expand the current selection of high-performance thermoplastics and fiber reinforced composites for AM systems. However, there is no formal process for designing, screening, and evaluating the printability of these high-performance thermoplastics and composite systems. Traditional polymer characterization techniques utilizing thermal and rheological material properties have been effectively employed in other polymer processing methods such as injection molding to identify suitable processing conditions. Therefore, to expand the current high-performance material selection for BAAM using industrial grade pellets, these techniques are employed to establish the relationships between fundamental material properties such as thermal and rheological properties and AM processing parameters. Overall, this work is an attempt to expand the current selection of highperformance feedstock on large format AM systems such as BAAM using thermal and rheological characterization techniques. This is achieved by predicting their extrudability through the nozzle, quantifying the impact of pressure transients on extrusion, and identifying appropriate processing conditions for these materials to provide a basis for optimizing the use of current high-performance materials as AM feedstock

Development of a novel rheometer for viscoelastic fluids

In this research a new rheometer is developed and the principles of the operation are studied in order to find methodologies that enable making measurements of rheological properties of complex fluids in a "one step" manner. The flow field within the T-junction geometry of the device for fluids that exhibit elongational rheometry is non-homogeneous due to the stagnation point. In contrast with the operation of conventional rheometers, data processing from the prototype rheometer involves modelling of the flow field and the construction of the mapping algorithm to connect data obtained from the device with the underlying elastic constitutive parameters of the fluid using inverse approach

The effect of extensional flow on shear viscosity

Shear rheology is conventionally studied under pure shearing flows, rather than more realistic mixed flows. Moving parallel surfaces and capillary rheometery are examples of the former, whilst the latter occurs whenever a flow accelerates or decelerates creating an additional component of extension, e.g. on passing through an industrial extrusion die. We postulate and gather supporting evidence that shear rheology is a function of not only shear, but both shear and extension rate, a factor with important consequences for fibre spinning and extrusion operations. The direction, as well as rate, of extensional deformation is important. A novel two-phase flow, planar extension experiment is developed and the surface coatings necessary to control the interface structure identified. Shear viscosity evolution is monitored, in-situ, under extensional flow, by optically measuring shear rates either side of a test fluid – reference fluid interface; issues due to optical refraction are critically addressed. Preliminary evidence is shown for a 1.2wt% 4x10^6 MW PEO solution that parallel (+ve) extensional flow, on the order of 11.5s-1 , causes a reduction in shear viscosity, and perpendicular (-ve) causes an increase in shear viscosity, supporting the hypothesis. A framework for a comparison experiment, with the same shear history but without extension, is presented. As part of this work, design criteria for planar hyperbolic extensional channels are critically assessed. In particular, expanding a hyperbola entrance region would maximise total Hencky strain, yet this region is almost never given rationalised consideration in literature. In this region the basis for the hyperbolic profile breaks down, and a new profiling strategy and channel form are presented, which is found to only differ significantly in this inlet region. A useful design limit of 130 degrees on channel inlet angle is identified. The new profile is compared to a hyperbolic profile through the use of CFD for wall slip flow, and a slight improvement in extension rate uniformity along the centreline found. Deviations are contrasted against assumptions made in the profiling strategy: comments are made with regards the possibility for “internal” shear to occur, and non-uniform extension rates are accordingly found to exist between streamlines in these channels despite the use of full wall slip in the simulations

Rheology and Processing of Polymers

This book covers the latest developments in the field of rheology and polymer processing, highlighting cutting-edge research focusing on the processing of advanced polymers and their composites. It demonstrates that the field of rheology and polymer processing is still gaining increased attention. Presented within are cutting-edge research results and the latest developments in the field of polymer science and engineering, innovations in the processing and characterization of biopolymers and polymer-based products, polymer physics, composites, modeling and simulations, and rheology