Polymeric Thermotropic Glazings for Overheating Protection: Systematic Polymer Science Based Optimisation of Performance Characteristics

Andreas Weber

Research output: ThesisDoctoral Thesis

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Abstract

This dissertation deals with the optimisation of the performance characteristics of polymeric thermotropic glazings for overheating protection purposes employing a systematic polymer scientific approach. The thermotropic glazings investigated - preferably exhibiting a transmittance reduction upon exceeding a temperature threshold - are intended for use as overheating protection glazings for buildings and especially for solar thermal collectors. More specifically, focus was on formulation of thermotropic systems with fixed domains (TSFD), consisting of a thermotropic additive as the active component finely dispersed (forming scattering domains) in a polymeric matrix material. From scattering theory the refractive index difference between matrix and additive as well as the scattering domain size were recognised to be the most important parameters affecting the light-shielding efficiency of TSFD. Thus, structure-property-relationships between thermo-refractive properties of TSFD constituents, the light-shielding efficiency and the internal material structure of the established TSFD was of major interest and were studied employing an systematic material formulations strategy based on sound polymer physical characterisation of numerous TSFD constituents and of the established TSFD. The thermo-refractive properties of TSFD constituents were (majorly) sufficient in order to achieve TSFD with efficient overheating protection performance. However, the actually obtained overheating protection performance was limited. This was ascribed to inappropriate shape and/or size of scattering domains on the one hand side and to defects formed in parts of the TSFD on the other hand side. TSFD with inappropriately shaped and sized scattering domains exhibited a moderate reduction in solar hemispheric transmittance. TSFD with appropriately shaped (spherical) but inappropriately sized scattering domains showed only minor changes in solar hemispheric transmittance in general. TSFD with inappropriately sized spherical scattering domains and with defects displayed an increase in solar hemispheric transmittance. The formation of different scattering domain shapes was ascribed to differences in the interactions between the individual matrix materials and the thermotropic additives. Additives which were soluble in the matrix resin when they were liquid, were subject to crystallisation from a homogeneous mixture upon TSFD formulation yielding formation of an energetically favourable shape (i.e. non-spherical) and size. For insoluble thermotropic additives - forming spherical scattering domains -, the different viscosities of matrix material and thermotropic additive in the liquid state were suspected to prevent formation of appropriately sized scattering domains. The defect formation in several TSFD was attributed to limited adhesion at the interface of matrix material and thermotropic additive and to thermally induced effects. These thermally induced effects were either stronger expansion/contraction of the thermotropic additive compared to the matrix due to apparent temperature conditions during processing or thermally induced diffusion of the thermotropic additive in the molten state. The prevention of defect formation by systematic optimisation of processing conditions and material formulation improved the solar hemispheric transmittance change of a specific TSFD: Whereas the initial layer with defects showed a significant transmittance increase, the layer lacking defects showed a moderate transmittance reduction upon exceeding the threshold temperature. Nevertheless, the diameters of the spherical scattering domains were inappropriate. Adjustment of the scattering domain size via a specifically developed photo-initiated miniemulsion polymerisation mediated encapsulation process for the thermotropic additive resulted in a significantly enhanced transmittance reduction of the layer for
Translated title of the contributionPolymere thermotrope Überhitzungsschutzverglasungen: Systematische polymerwissenschaftsbasierte Optimierung der Leistungscharakteristika
Original languageEnglish
QualificationDr.mont.
Supervisors/Advisors
  • Pinter, Gerald, Assessor A (internal)
  • Kern, Wolfgang, Assessor B (internal)
  • Resch-Fauster, Katharina, Supervisor (internal)
Publication statusPublished - 2014

Bibliographical note

embargoed until 14-11-2018

Keywords

  • thermotropic glazings
  • overheating protection
  • thermotropic systems with fixed domains
  • photopolymerization
  • miniemulsion
  • scattering domains
  • morphology
  • structure-property-relationships
  • spectroscopy

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