Thermotropic glazings for overheating protection. I. Material preselection, formulation, and light‐shielding efficiency

Abstract

This article presents a systematic strategy for formulation and optimization of thermotropic layers for overheating protection purposes. Specifically, thermotropic systems with fixed domains (TSFD) which consist of a thermotropic additive finely dispersed in a matrix material are considered. Based on systematic material (component) preselection regarding thermoanalytical characteristics and refractive indices, numerous thermotropic layers were formulated. TSFD with thermoplastic matrix were produced by compounding and compression molding. TSFD with resin matrix were produced by UV curing. The thermotropic layers were analyzed as to solar optical properties, threshold temperature, switching process and residual transmittance in the opaque state applying UV/Vis/ NIR spectrometry equipped with a heating stage. Best performing materials exhibited solar hemispheric transmittance in the range of 72.2-84.5% and between 59.6 and 83.7% in the clear and opaque state, respectively. Threshold temperatures between 45 and 75 C were realized. Refractive index difference between matrix and additive and solar hemispheric transmittance displayed a close correlation. Hence, refractometry was shown to be an appropriate tool for material preselection. Furthermore, investigations revealed a close correlation of thermal transitions of thermotropic additives recorded by differential scanning calorimetry and threshold temperatures of thermotropic layers formulated therewith. However, thermotropic layers formulated so far have to be optimized with respect to light-shielding performance for efficient overheating protection.

Thermotropic overheating protection glazings: effect of functional additives and processing conditions on the overheating protection performance

“…For the annealed layers, the temperature-dependent behavior was inverted. Such kind of behavior was observed already in earlier studies [28][29][30]: TSFD without defects (like vacuoles or voids) showed a decrease in the solar hemispheric transmittance upon exceeding the threshold temperature. This behavior is consistent with the observed change in the relative refractive index (an increasing deviation from unity) between matrix and thermotropic additive upon exceeding the threshold temperature [28,30].…”

“…The UV-curable resin matrix consisted of 57 wt% polyester acrylate oligomer Ebecryl 800 (Allnex Belgium SA/NV, former Cytec Surface Specialities, Drogenbos, Belgium), 40 wt% reactive diluent OTA-480 (OTA) -a propoxylated glycerol esterified with acrylic acid (Allnex) -and 3 wt% photobleaching photoinitiator Lucirin TPO-L (BASF SE, Ludwigshafen, Germany). The thermotropic additive was paraffin wax Sasolwax 5005 (additive A1; Sasol Wax GmbH, Hamburg, Germany) with a melting point of 55°C [28]. The nomenclature of the thermotropic additive is consistent with previous publications [28][29][30].…”

“…The thermotropic additive was paraffin wax Sasolwax 5005 (additive A1; Sasol Wax GmbH, Hamburg, Germany) with a melting point of 55°C [28]. The nomenclature of the thermotropic additive is consistent with previous publications [28][29][30]. The functional additives were the nonionic surfactant Lutensol AT 11 (BTC Europe GmbH, Cologne, Germany) and the potential nucleating agent synthetic Indigo (Sigma-Aldrich Handels GmbH, Vienna, Austria).…”

“…The solar-weighted transmittance was determined by weighting the recorded spectral data in steps of 5 nm by the AM1.5 global spectrum [48]. The spectrophotometer was adapted by a heating stage to adjust the sample temperature within a range from ambient temperature to a maximum of 115°C [28]. The measurements were performed at room temperature and 70°C.…”

“…Recent studies regarding TSFD revealed a limited overheating protection performance of the layers due to an inappropriate scattering domain size and/or shape [17,19,26,28,30]. In a preceding study, potential remedies in order to adjust the scattering domain shape and size were outlined [30].…”