Effect of formulation and processing conditions on light shielding efficiency of thermotropic systems with fixed domains based on UV curing acrylate resins

Abstract

Within this study relationships between material formulation and processing parameters and the morphology (vacuole formation) of thermotropic systems with fixed domains (TSFD) for overheating protection purposes were investigated. Main aim was on improving light shielding efficiency of TSFD based on UV curable acrylate resins by optimization of selected key parameters including photo-initiator type and content, type of reactive diluent, radiation intensity/dose, and thermal treatment of layers during manufacturing. Variations of type of reactive diluent and thermal treatment had a minor effect on overheating protection performance. Utilization of photo-bleaching photo-initiator of acylphospine oxide type instead of a blend of conventional Type I (a-hydroxy ketone type) and Type II (benzophenone) photo-initiators enabled reduction of radiation dose to achieve properly cured layers. The results revealed that a significant reduction of radiation intensity/dose prevented formation of vacuoles. Consequently, light shielding efficiency of TSFD was enhanced significantly. Nevertheless, obtained scattering domain size was inappropriate for optimum light shielding efficiency and requires further optimization strategies.

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].…”

“…This finding is supported by the observation that the addition of AT 11 was not effectual in order to enhance the difference between solar hemispheric transmittance at room temperature and at 70°C. For a layer lacking any AT 11 (specifically if that was layer M7A1-OTA-p3-RT-0.008 [29], which represents a treatment combination of AT 11: 0 mol kg -1 ; Indigo: 0 mol kg -1 ; treatment: RT; annealing: no; see Table 2), a solar hemispheric transmittance of 81.2 and 78.5% was achieved below and above the threshold temperature, respectively. Upon the application of an AT 11 concentration of 0.01 mol kg -1 , the solar hemispheric transmittance was recorded to be 81.9 and 81.1% below and above the threshold temperature, respectively.…”

“…To address the aspects discussed above, TSFD M7A1-OTAp3-RT-0.008 -which was already developed in a preceding publication [29] -was selected as the prototype Table 1 Factors and factor levels employed for formulation of thermotropic systems with fixed domains (TSFD). system.…”

“…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 addition of nucleating agents to mixtures of a matrix material and an additive may probably yield a reduction in the size of scattering domains by increasing the number of crystallization loci and by affecting the free energy for crystallization. Furthermore, the temperature conditions applied during the manufacturing process may significantly affect the overheating protection performance of TSFD [14,29], due to temperature-affected variations in nucleation of crystallization, which probably induce changes in the scattering domain size distribution. Furthermore, an annealing step employed after curing probably enhances the homogeneity of the TSFD (as was observed for other TSFD that were produced in the authors' laboratory).…”