Boris Orel

Structural Properties and Antibacterial Effects of Hydrophobic and Oleophobic Sol−Gel Coatings for Cotton Fabrics

In a continuation of previous studies, the wetting properties of the hydrophobic diureapropyltriethoxysilane [bis(aminopropyl)-terminated polydimethylsiloxane (1000)] (PDMSU) sol-gel hybrid, which forms washing-resistant water-repellent finishes on cotton fabrics, were further investigated. The addition of 1H,1H,2H,2H-perfluorooctyltriethoxysilane (PFOTES) to PDMSU resulted in a highly apolar low-energy surface on aluminum with gammaStotal equal to 14.5 mJ/m2 and a DetlaGiwi value of -82 mJ/m2. Mixed PFOTES-PDMSU finishes applied on cotton fabrics increased the water contact angles (thetaw) from approximately 130 degrees (PDMSU) to 147 degrees, also imparting oleophobicity (thetadiiodomethane=130 degrees, thetan-hexadecane=120 degrees) to the finished cotton fabrics. Washing caused breakage of the coatings integrity as established from SEM, which was attributed to the partial removal of PFOTES from the composite films, also shown by subtractive IR attenuated total reflectance (ATR) and XPS spectral measurements made on washed and unwashed fabrics. The antibacterial properties of the PFOTES-PDMSU-finished fabrics were assessed with the transfer method (EN ISO 20743:2007), revealing that the reduction of Escherichia coli bacteria on unwashed cotton fabrics was nearly 100%. Moreover, for washed (10 times) cotton fabrics a much higher bacterial reduction was noted for the PFOTES-PDMSU finishes (60.6+/-10.8%), surpassing PDMSU (30.4+/-6.1%) and commercial fluoroalkoxysilane (FAS) (21.9+/-5.7%) finishes. The structure of PFOTES-PDMSU gels, xerogels, and the corresponding coatings was investigated by analyzing the 29Si NMR and IR ATR spectra and comparing them with the spectra of PFOTES and octameric (T8) PFOTES based polyhedra. The results revealed the tendency of PFOTES to condense in octameric silsesquioxane polyhedra (T8), coexisting in the PDMSU sol-gel network with cyclic tetramers (T4(OH)4) and open cube-like species (T7(OH)3). The presence of -OH-functionalized PFOTES silsesquioxanes, established even in coatings heat-treated at 140 degrees C (15 min), also explained the excellent washing fastness of PFOTES finishes on cotton fabrics. The regenerative nature of the water- and oil-repellent properties of washed PFOTES-PDMSU-finished cotton fabrics was attributed to the surface mobility of the T8 PFOTES based polyhedra, ousted from the coating interior during consecutive washings.

Application of the Kubelka-Munk theory for the determination of the optical properties of solar absorbing paints

Abstract The possibility to predict the solar absorptance gradient by colorimetry is described. The study is performed on black spectrally selective paints used as absorber in photothermal conversion of solar energy. The Kubelka-Munk absorption and scattering coefficients of all considered paints are determined. The relation between the degree of dispersion in the paint and Kubelka-Munk coefficients in the visible spectral region is discussed. The coefficients are used further to calculate the transparency values according to principles of colorimetry. The measured solar absorptance gradient for a very thin paint layer reversibly depends upon calculated transparency for different types of paints. This result helps to express the solar effectiveness of spectrally selective paint by the transparency in the visible.

Optical and electrochemical properties of CeO2 and CeO2TiO2 coatings

Thin solid films of CeO2 and mixed CeO2TiO2 were prepared by the sol-gel route via the dip-coating technique. Particulate sols of ceria were made from inorganic ((NH4)2Ce(NO3)6) precursor which were used for preparation of CeO2 thin solid films while CeO2TiO2 coatings have been made by using mixed organic-inorganic (Ti(OiPr)4 and CeCl3·7H2O) precursors. The solar transmission values (Ts) of both coatings are in the range 0.6–0.8 and depend on coating thickness. Cyclic voltammetric (CV) measurements show that the CeO2/LiOH system exhibits higher overall electrochemical reversibility when compared to the CeO2TiO2/LiOH system. The CeO2/LiOH system is also less sensitive with regard to the coating thickness. Coulometric measurements show that CeO2 exhibits a larger storage capability which was determined as a function of the coating thickness. “In situ” UV-VIS spectroelectrochemical measurements which have been performed on CeO2 and CeO2TiO2 coatings revealed that both types of samples exhibit electrochromic effect in the spectral range 500 < λ < 330 nm but remain unchanged in the visible spectral range.

Ion storage properties of CeO2 and mixed CeO2/SnO2 coatings

Ion storage CeO2 and CeO2/SnO2 coatings were prepared by the sol-gel dip-coating method using an aqueous-based process. The influence of added SnO2 in the CeO2 oxide coatings on the inserted/extracted charge was determined by chronocoulometric measurements. It was found that for 60 nm thick film, the inserted/extracted charge was twice (Q=10 mC cm−2) as large for films containing 17 mol% SnO2, if compared to pure CeO2. The effect of the addition of SnO2 to the mixed oxide coatings on their optical properties and structural characteristics was studied.

The surface modification of cellulose fibres to create super-hydrophobic, oleophobic and self-cleaning properties

The surface modification of cellulose fibres was performed with the use of low-pressure water vapour plasma, followed by the application of a pad-dry-cure sol-gel coating with the water- and oil-repellent organic-inorganic hybrid precursor fluoroalkyl-functional siloxane (FAS), with the aim of creating the “lotus effect” on the cotton fabric surface. The tailored “lotus effect” was confirmed by measurements of the contact angle of water (154°) and n-hexadecane (140°), as well as by measurements of the water sliding angle (7°), which were used to identify the super-hydrophobic, oleophobic and self-cleaning properties of the modified fibres. The chemical and morphological changes caused by modifications of the fibres were investigated by XPS, FTIR, AFM and SEM. The results show that the plasma pre-treatment simultaneously increased the surface polarity, average roughness, and surface area of the fabric. The application of the FAS coating after plasma pre-treatment caused only a slight increase in the surface roughness, accompanied by a decrease in the surface area, indicating that the architecture of the surface was significantly changed. This result suggests that the surface pattern affected the “lotus effect” more than the average surface roughness. The plasma pre-treatment increased the effective concentration of the FAS network on the fabric, which resulted in enhanced repellency before and after repetitive washing, compared with that of the FAS-coated fabric sample without the plasma pre-treatment. Despite the fact that the plasma pre-treatment increased the concentration of the oxygen-containing functional groups on the fabric surface, this phenomenon insignificantly contributed to the adhesion ability and, consequently, the washing fastness of the FAS coating.

Radiative cooling efficiency of white pigmented paints

Abstract Nocturnal temperature measurements of various infrared selective radiators performing radiative cooling have been carried out. Measured temperature differences between the radiators and ambient temperature were about 10°C. Radiating surfaces have been prepared by applying different kinds of paints on aluminium substrates. It was shown that the addition of a BaSO4 extender in the paint dispersion increased the cooling performance of the paint radiators. Comparisons between predicted cooling performances by calculations and measured ones are discussed in detail.

Conductive SnO2/Sb powder: preparation and optical properties

Preparation of an SnO2 semiconducting powder doped with antimony (x=2.38 mol%) was achieved by co-precipitation. The unit cell parameters of the doped SnO2 powders were measured and their changes with dopant concentration were determined. Four-point sheet resistance measurements, together with optical and infrared spectra of the powder were taken in order to obtain a highly-conducting, low-emitting powder which could be used for antistatic paint preparation. Evolution of the phonon bands corresponding to Sn-O stretching modes as a function of dopant concentration were followed, and a model calculation based on an extended four-parametric Kurosawa relation was applied to the reflection spectra of differently doped powders. It was found that the frequency of the plasma oscillations shifts with dopant concentration, and the intensity of the reflectivity peaks was correlated with plasmon-phonon interactions. An additional negative reflection peak in the range 1100 to 1200 cm−1 was found in the reflection spectra of highly doped powders and was attributed to the coupled modes between the plasma oscillations and one of the phonon combinational or overtone modes of SnO2.

Structural and electrochemical properties of CeO2 and mixed CeO2/SnO2 coatings

Abstract CeO2 and mixed CeO2/SnO2 coatings were prepared via the sol-gel method using the aqueous-based process. The addition of SnO2 to the mixed oxides coatings on their structural characteristics and optical properties were studied. The influence of added SnO2 in the CeO2 oxide coatings on the inserted/extracted charge was determined by chronocoulometric measurements. It was found that for 60 nm thick films the inserted/extracted charge was twice as large (Q = 10 mC/cm2) for films containing 17 mol% SnO2 if compared to CeO2.

Optimum thickness determination to maximise the spectral selectivity of black pigmented coatings for solar collectors

Abstract A simple method for theoretical prediction of optimum thickness to maximise the spectral selectivity of the absorber in photothermal conversion of solar energy is presented. The optical properties of the absorber are described in terms of Kubelka—Munk theory. The calculation is applied on black thickness-sensitive spectrally selective paint coatings. The obtained optimum thicknesses are compared with values which have been established by experimental optimisation.

Thickness sensitive spectrally selective (TSSS) paint for solar collectors: optical properties, surface characterization and application properties

Abstract A black paint was prepared which is suitable for the automatic application technique known as coil-coating. Different formulated black paints were applied on aluminium at different thicknesses (1–20 μm), and the properties of the corresponding coatings were evaluated. The influence of the paint formulation on the optical and surface properties of the TSSS paint coating aluminium is discussed in detail. Moderate spectral selectivity ( a s = 0.92, e T = 0.42 ) was achieved for a coating with thickness 3–4 μm).