Rheological behavior of spectrally selective coatings for polymeric solar absorbers

Since the world’s energy demands are growing rapidly, there is a constant need for new energy systems. One of the cleanest, most abundant, and renewable natural resources available is solar energytherefore, the development of surfaces with high absorption of solar radiation is increasing. To achieve the best efficiency, such surfaces are coated with spectrally selective coatings, which are strongly influenced by the pigments and resin binders. Spectrally selective paints have a very specific formulation, and since the applied dry coatings should exhibit high spectral selectivity, i.e., high solar absorptivity and low thermal emissivity, the rheological properties of liquid paints are of great importance. In the present work, we studied the effect of the rheological properties of liquid thickness-insensitive spectrally selective (TISS) paints on the spectral selectivity and adhesion of dry coatings on a polymeric substrate. The results showed that the functional and adhesion properties of dry coating on polymeric substrates is strongly dependent on the rheological properties of the binder and catalyst used for the preparation of the liquid paints. It was shown that the paints with good spectral selective properties (thermal emissivity e$_T$ 0.92) and good adhesion (5B) can be prepared for polymer substrates

Dynamická změna barevnosti multifunkčních termochromně-fluorescenčních pigmentů

The aim of presented paper was to prepare and characterise multifunctional thermochromic-fluorescent pigments with potential application in anti-counterfeiting patterns in security printing. Pigments were prepared by microencapsulation of chosen thermochromic system into melamine formaldehyde resin and the resin was modified with Uranine and Acid Red 52 fluorescent dyes, respectively. The fluorescence at low and high temperature was measured by spectrofluorometer. The concentration of 2.3 × 10–5 grams of fluorescent dye per one gram of polymer is sufficient for detection of fluorescence of modified polymeric shell. The dynamic colour change of prepared pigments was analysed in terms of cumulative colour difference obtained from reflectance measurements. Resulting multifunctional pigments exhibit much lower colour contrast and wider temperature sensitive interval in comparison with the bulk thermochromic system. However, the concept of two levels of verification based on two types of colour change embodied in one pigment has been approved.Cílem bylo připravit a charakterizovat multifunkční termochromně-fluorescenční pigmenty s možnou aplikací v prvcích proti padělání v bezpečnostním tisku. Pigmenty byly připraveny mikroenkapsulací zvoleného termochromního systému do melamin-formaldehydové pryskyřice modifikované fluorescenčními barvivy Uranine a Acid Red 52. Fluorescence při nízké a vysoké teplotě byla měřena spektrofluorimetrem. Pro detekci fluorescence modifikované polymerní obálky byla dostačující koncentrace 2,3 × 10–5 gramů fluorescenčního barviva na 1 gram polymeru. Dynamická změna barevnosti připravených pigmentů byla analyzována prostřednictvím kumulativní barvové odchylky vyhodnocené z měření reflektance. Ve srovnání se samotným termochromním systémem vykazují připravené multifunkční pigmenty výrazně menší barvový kontrast a širší interval, při němž dochází k barevné změně. Nicméně byla prokázána možnost dvouúrovňové verifikace založené na dvou typech dynamické změny barevnosti jednoho pigmentu

Kombinace kolorimetrické a termické analýzy reverzibilních termochromních systémů obsahujících lakton krystalové violeti

In present study, the intention was to investigate how the colour change of thermochromic composites is connected with their phase transitions. Crystal violet lactone was used as a colour former and bisphenol A as a developer. Two co-solvents, 1-tetradecanol and 1-tetradecanoic acid, were selected due to the same length of the aliphatic chain and different functionality. Series of seven thermochromic systems varying in co-solvent’s molar ratio were prepared for each type of the co-solvent. The colour change was measured by reflectance spectroscopy, and temperatures characterizing the dynamic colour change were computed from the cumulative colour difference function. The onset and endset temperatures of thermochromic system’s phase transitions were obtained from DSC analyses. Connections between the fastest colour changes and the decolouration limits at raised temperatures to solid–solid, solid–liquid and liquid–solid transitions are demonstrated for both heating and cooling, respectively. Different behaviour of compared systems with alcoholic and acidic co-solvent is discussed as well.Cílem studie bylo zjistit, jak je barevná změna souvisí s fázovými změnami termochromních systémů na bázi leuko barviv. Jako barvivo byl použitý lakton krystalové violetia jako vývojka bisfenol A. Použité byli dvě rozpouštědla shodující se ve velikosti alifatického řetězce ale s jinou funkční skupinou, a to konkrétně 1-tetradekanol a kyselina 1-tetradekanová. S každým rozpouštědlem bylo připraveno sedm termochromních systémů s různým poměrem rozpouštědla. Dynamická změna barevnosti byla měřena pomocí reflexní spektroskopie a vyhodnocena pomocí kumulované barvové odchylky. Teploty charakterizující fázové změny byly vyhodnoceny z DSC analýzy. Je popsána spojitost mezi teplotami charakterizujícími změnu barevnosti při limitu odbarvení a teplotami charakterizujícími fázové přechody v této oblasti. Rovněž je diskutován rozdíl v chování systémů s využitím kyseliny a alkoholu jako rozpouštědla

In-depth rheological characterization of tungsten sol-gel inks for inkjet printing

The inkjet printing of the functional materials prepared by the sol-gel route is gaining the attention for the production of the variety of the applications not limited to the printed boards, displays, smart labels, smart packaging, sensors and solar cells. However, due to the gelation process associated with the changes from Newtonian to non-Newtonian fluid the inkjet printing of the sol-gel inks is extremely complex. In this study we reveal in-depth rheological characterization of the WO$_3$ sols in which we simulate the conditions of the inkjet printing process at different temperature of the cartridge (20–60 °C) by analyzing the structural and rheological changes taking place during the gelation of the tungsten oxide (WO$_3$) ink. The results provide the information on the stability of the sol and a better insight on the effects of the temperature on the gelation time. Moreover, the information on the temperature and the time window at which the inkjet printing of the sol-gel inks could be performed without clogging were obtained. The WO$_3$ ink was stable in a beaker and exhibited Newtonian flow behavior at room temperature over 3 weeks, while the gelation time decreased exponentially with increasing temperature down to 0.55 h at 60 °C

Structural, optical and thermal properties of benzofluoran-based thermochromic composite

Thermochromic composites, consisting of leuco dye, colour developer and organic solvent indicate reversible thermochromism, with colouration in the solid state and discoloration in the molten state. These functional materials, the so-called smart materials, are of considerable interest in graphic art technology for applications such as temperature sensors, security markers, thermal printing, cutlery and clothing. Thermochromic composites of 6‘- (diethylamino)-1‘-2‘-benzofluoran (BF) dye, 2,2-bis(4-hydroxypheniyl) propane (BPA) developer and 1-octadecanol (OD) solvent were investigated in this work. Composites with four different molar ratios of BPA were analysed. The chemical structure, thermal and optical properties of samples were analysed by UV-VIS and FTIR spectroscopy, colorimetric measurements and differential scanning calorimetry. The applied methods enable to study the properties of the material below and above the phase-changes. Much interest was devoted to analyse how different molar ratio of developer influence on the structural, optical and thermal properties of the composites

Dependence of binder and photocatalyst in photocatalytically active printing ink

Photocatalytically active printing ink changes its colour when exposed to UV light. The ink is based on photocatalyst which decomposes organic dye in presence of UV light causing the changes in colour. Such process can be used as a simple UV dosimeter as its colour change depends on duration to UV light exposure. We have prepared such an ink based on redox dye 2,6-dichloroindophenol (DCIP), photocatalyst titanium dioxide (nanodimensional anatase), reducing agent (glycerol) and two different water based binders – hydroxyethylcellulose (HEC) and polyvinyl alcohol (PVA). Prepared ink was applied onto a substrate, exposed to UV light and analysed by UV/Visible spectra. When the ink was exposed to UV light, the colour of the redox dye DCIP was changing from blue to colourless that belongs to the dihydro DCIP form. The colour change depends on several factors: on the amount of TiO2 and UV exposure, as well as on the type of binder. In this work the influence of two selected binders and the amount of photocatalyst on the properties of ink was observed. The results show that the ink based on polyvinyl alcohol has a greater initial absorption and absorbs at higher wavelength. Colour changes were faster when higher amount of photocatalyst and HEC as a binder were used. Nevertheless, it has been confirmed that both binders are appropriate for preparation of UV active functional printing inks