Publicação
In-line rheo-optical microstructural characterization of complex polymer systems
| Resumo: | As polymers progressively face more stringent requirements in terms of processing and performance, it is often necessary to develop new complex material systems while minimizing costs and time-to-market. Thus, it seems useful to make available fast response characterization tools that, although requiring small amounts of sample, are still capable of generating adequate data on the correlations between rheological response, process-induced material structure and product properties. For this purpose, a prototype small-scale single / twin-screw extrusion system of modular construction, with outputs in the range 30-500 g/h, was coupled to a modular rheo-optical slit die designed to measure shear viscosity and normal-stress differences, as well as perform rheo-optical experiments, namely small angle light scattering (SALS) and polarized optical microscopy (POM). The extruder is equipped with ports that allow sample collection along its axis, in order to subsequently evaluate the development of melting and mixing, morphology, or chemical conversion. Also, downstream equipment is present, so that the engineering properties of the extrudates can be evaluated. The hole pressure method, which is based on the difference between the pressures measured by means of a flush-mounted pressure transducer and a directly opposed recess mounted transducer, was implemented on the rheo-optical slit die in order to measure the normal stress differences. Results are presented and the concept is validated for three different polymers. The main difficulties and sources of errors associated with the measurements of normal stresses using the hole pressure method are discussed. The optical validation of this small-scale device was performed using several volume fraction dispersions of standard size polystyrene (PS) particles in polydimethylsiloxane (PDMS). The output of this exercise is the definition of an operational window mapping the range of particle sizes and concentrations that can be correctly measured with the two complementary in-line optical techniques. The full practical potential of the mini-extrusion line was explored to study the morphological development of polystyrene/ poly(methyl methacrylate) industrial blends. The morphological evolution along the extruder, the flow-induced structures developed, the relaxation processes upon cessation of flow and the corresponding rheological characteristics are presented, together with the mechanical and structural characteristics of the extruded sheets. Finally, the in-line techniques were applied to study the flow-induced structures and structural relaxation of a cellulose-based liquid crystalline polymer, acetoxypropylcellulose (APC), at flow rates relevant to practical polymer processing. The main innovative results of the thesis are highlighted below: - the practical utility of the hole pressure method for the in-line rheological characterization of polymer melts has been assessed; for the first time, a quantitative estimation of the impact of the potential sources of error on the sensitivity of the technique has been given. - the construction and validation of a complete small scale experimental extrusion set-up (directly scalable to industrial production) offering access to the study of relationships between process parameters and the evolution of materials characteristics (structural and rheological) from the barrel up to the extrudate. - the rheo-optical characterization of a liquid crystalline acetoxypropylcellulose melt at shear rates never achieved before (10 s−1 to 1000 s−1), showing for the first time a change in the negative extrudate occurring at a critical shear rate which coincides with a change in the stress relaxation after flow cessation and which relates to a flow-induced two fluids morphology, thus giving credit to a flow model proposed earlier for this class of materials. |
|---|---|
| Autores principais: | Teixeira, Paulo Francisco |
| Assunto: | Engenharia e Tecnologia::Outras Engenharias e Tecnologias |
| Ano: | 2016 |
| País: | Portugal |
| Tipo de documento: | tese de doutoramento |
| Tipo de acesso: | acesso restrito |
| Instituição associada: | Universidade do Minho |
| Idioma: | inglês |
| Origem: | RepositóriUM - Universidade do Minho |
| Resumo: | As polymers progressively face more stringent requirements in terms of processing and performance, it is often necessary to develop new complex material systems while minimizing costs and time-to-market. Thus, it seems useful to make available fast response characterization tools that, although requiring small amounts of sample, are still capable of generating adequate data on the correlations between rheological response, process-induced material structure and product properties. For this purpose, a prototype small-scale single / twin-screw extrusion system of modular construction, with outputs in the range 30-500 g/h, was coupled to a modular rheo-optical slit die designed to measure shear viscosity and normal-stress differences, as well as perform rheo-optical experiments, namely small angle light scattering (SALS) and polarized optical microscopy (POM). The extruder is equipped with ports that allow sample collection along its axis, in order to subsequently evaluate the development of melting and mixing, morphology, or chemical conversion. Also, downstream equipment is present, so that the engineering properties of the extrudates can be evaluated. The hole pressure method, which is based on the difference between the pressures measured by means of a flush-mounted pressure transducer and a directly opposed recess mounted transducer, was implemented on the rheo-optical slit die in order to measure the normal stress differences. Results are presented and the concept is validated for three different polymers. The main difficulties and sources of errors associated with the measurements of normal stresses using the hole pressure method are discussed. The optical validation of this small-scale device was performed using several volume fraction dispersions of standard size polystyrene (PS) particles in polydimethylsiloxane (PDMS). The output of this exercise is the definition of an operational window mapping the range of particle sizes and concentrations that can be correctly measured with the two complementary in-line optical techniques. The full practical potential of the mini-extrusion line was explored to study the morphological development of polystyrene/ poly(methyl methacrylate) industrial blends. The morphological evolution along the extruder, the flow-induced structures developed, the relaxation processes upon cessation of flow and the corresponding rheological characteristics are presented, together with the mechanical and structural characteristics of the extruded sheets. Finally, the in-line techniques were applied to study the flow-induced structures and structural relaxation of a cellulose-based liquid crystalline polymer, acetoxypropylcellulose (APC), at flow rates relevant to practical polymer processing. The main innovative results of the thesis are highlighted below: - the practical utility of the hole pressure method for the in-line rheological characterization of polymer melts has been assessed; for the first time, a quantitative estimation of the impact of the potential sources of error on the sensitivity of the technique has been given. - the construction and validation of a complete small scale experimental extrusion set-up (directly scalable to industrial production) offering access to the study of relationships between process parameters and the evolution of materials characteristics (structural and rheological) from the barrel up to the extrudate. - the rheo-optical characterization of a liquid crystalline acetoxypropylcellulose melt at shear rates never achieved before (10 s−1 to 1000 s−1), showing for the first time a change in the negative extrudate occurring at a critical shear rate which coincides with a change in the stress relaxation after flow cessation and which relates to a flow-induced two fluids morphology, thus giving credit to a flow model proposed earlier for this class of materials. |
|---|