Rheological characterization of coatings with regard to application and film formation

Summary The rheological properties and their functional dependencies, involved in the flow behavior of coatings during application and drying, are discussed. From the available rheometrical techniques a characterization of coatings is proposed, which reflects the most important rheological parameters with respect to the application process. This characterization includes measurement of (i) the change of viscosity during the drying period, (ii) the viscosity of the coating, ready for application, in the shear range from below 1 s^–1 to more than 10000 s^–1, (iii) the thixotropic recovery of viscosity at a shear rate below 1 s^–1 following shearing at a shear rate, where all structures are broken, or being at least 3 decades higher than the measuring rate, (iv) the stress relaxation from a low shear rate where relaxation times are largest or from a realistically high shear rate, and (v) the change in the composition of the material during drying in terms of evaporation.This characterization is found to be closely related to many of the different flow problems occurring in the application process. It may be extended more fully to elucidate specific problems, since the present characterization has been limited in order to make it a useful tool in routine work. For the same reason versatile rotational viscometers, normally used in coatings laboratories, are the most advanced instruments required for the characterization. This characterization, furthermore, is consistent with theoretical approaches to rheology, thereby giving access to an extension of the validity of the measurements on theoretical grounds and to use of extensive knowledge of how well-defined materials behave and interact in the design of coatings with a certain flow behavior. The framework of the characterization is found to be crucial in relating application process and rheology, whereas the specific measurements can be made in various ways.

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Zusammenfassung Die rheologischen Eigenschaften und ihre funktionellen Abhängigkeiten, welche das Fließverhalten von Anstrichstoffen während deren Applikation und Trocknung beeinflussen, werden besprochen. Mit Hilfe von gebräuchlichen rheometrischen Verfahren wird eine Charakterisierung von Anstrichstoffen vorgeschlagen, welche die wichtigsten rheologischen Parameter in Verbindung mit dem Applikationsprozeß berücksichtigt. Diese Charakterisierung umfaßt folgende Messungen: 1. Die scheinbare Viskosität während der Trocknung, 2. die Viskosität des Anstrichstoffes unmittelbar vor dessen Applikation, im Bereich von Schergeschwindigkeiten von unter 1 s^–1 bis über 10000 s^–1, 3. die thixotrope Rückbildung der Viskosität bei einer Schergeschwindigkeit unter 1 s^–1, gemessen nach Beendigung einer Scherbeanspruchung mit einer Geschwindigkeit, welche die gesamte Struktur aufzubrechen imstande war oder mindestens 3 Zehnerpotenzen höher liegt als die Meßgeschwindigkeit, 4. die Spannungsrelaxation, ausgehend von einer niedrigen Schergeschwindigkeit, wo die Relaxationszeiten am größten sind, oder von einer realistisch hohen Schergeschwindigkeit, und 5. die Veränderung des Materials während seiner Trocknung infolge Verdampfung seiner flüchtigen Anteile.Diese Charakterisierung wurde zu vielen verschiedenen Fließproblemen, wie sie während des Applikationsprozesses vorkommen, in enger Beziehung stehend gefunden. Sie könnte erweitert werden, um noch speziellere Probleme deuten zu können, doch ist die Charakterisierung als nützliches Hilfsmittel für Routinearbeiten begrenzt. Aus dem gleichen Grunde wurden für die Charakterisierung keine Instrumente vorgeschlagen, die komplizierter als Rotationsviskosimeter sind.Die vorliegende Charakterisierung ist in Übereinstimmung mit den theoretischen Grundlagen der Rheologie, so daß die Meßergebnisse auf theoretischer Basis verallgemeinert werden können. Für die Formulierung von Anstrichstoffen mit gemessenem Fließverhalten endlich können die Kenntnisse über Verhalten und Wechselwirkung theoretisch definierter Stoffe mit Nutzen angewandt werden.Der oben gegebene Charakterisierungsrahmen erwies sich als entscheidend für die Anwendbarkeit der Rheologie auf den Applikationsprozeß, die spezifischen Messungen hingegen können auf verschiedene Weise ausgeführt werden.

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Paper, presented at the Annual Conference of the Deutsche Rheologische Gesellschaft in Berlin, May 8–10, 1978.

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With 3 figures     

Explicit constructions and properties of generalized shift-invariant systems in $L^{2}(\mathbb {R})$

Generalized shift-invariant (GSI) systems, originally introduced by Hernández et al. and Ron and Shen, provide a common frame work for analysis of Gabor systems, wavelet systems, wave packet systems, and other types of structured function systems. In this paper we analyze three important aspects of such systems. First, in contrast to the known cases of Gabor frames and wavelet frames, we show that for a GSI system forming a frame, the Calderón sum is not necessarily bounded by the lower frame bound. We identify a technical condition implying that the Calderón sum is bounded by the lower frame bound and show that under a weak assumption the condition is equivalent with the local integrability condition introduced by Hernández et al. Second, we provide explicit and general constructions of frames and dual pairs of frames having the GSI-structure. In particular, the setup applies to wave packet systems and in contrast to the constructions in the literature, these constructions are not based on characteristic functions in the Fourier domain. Third, our results provide insight into the local integrability condition (LIC).     

Electronic coupling through intramolecular π-π interactions in biscobaltocenes: a structural, spectroscopic, and magnetic study

The paramagnetic dinuclear complexes 1,8-bis(cobaltocenyl)naphthalene (2) and 1,8-bis[(pentamethyl-η(5)-cyclopentadienyl)(η(5)-cyclopentadiendiyl)cobalt(II)]naphthalene (4) were synthesized. The molecular structures were characterized by X-ray structure analysis and consisted of two cobaltocenes linked through a distorted naphthalene clamp. Electronic interactions between the two cobalt atoms were observed by cyclic voltammetric studies. Superconducting quantum interference device (SQUID) measurements of the pure compounds and diluted in their diamagnetic iron derivatives, as well as variable-temperature NMR spectroscopy experiments in solution are presented. Magnetic measurements revealed an antiferromagnetic coupling of the electrons in complexes 2 and 4. From NMR spectroscopy experiments, Curie behavior in the temperature range from -60 to +60 °C can be deduced. The electronic structure and magnetic behavior is supported by results of broken-symmetry DFT and multireference calculations along with UV/Vis spectroscopic data, which revealed an intramolecular through space π-π interaction between the cobaltocene units.     

Probing the acceptor substrate binding site of Trypanosoma cruzi trans-sialidase with systematically modified substrates and glycoside libraries

Systematically modified octyl galactosides and octyl N-acetyllactosamines were assessed as inhibitors of, and substrates for, T. cruzi trans-sialidase (TcTS) in the context of exploring its acceptor substrate binding site. These studies show that TcTS, which catalyses the α-(2→3)-sialylation of non-reducing terminal β-galactose residues, is largely intolerant of substitution of the galactose 2 and 4 positions whereas substitution of the galactose 6 position is well tolerated. Further studies show that even the addition of a bulky sugar residue (glucose, galactose) does not impact negatively on TcTS binding and turnover, which highlights the potential of 'internal' 6-substituted galactose residues to serve as TcTS acceptor substrates. Results from screening a 93-membered thiogalactoside library highlight a number of structural features (notably imidazoles and indoles) that are worthy of further investigation in the context of TcTS inhibitor development.     

Distribution of N-glycosylation sequons in proteins: how apart are they?

N-glycosylation is a common protein modification process, which affects a number of properties of proteins. Little is known about the distribution of N-glycosylation sequons, for example, the distance between glycosylated sites and their position in the protein primary sequence. Using a large set of experimentally confirmed eukaryotic N-glycoproteins we analyzed the relative position and distribution of sequons. N-Glycosylation probability was found to be lower in the termini of protein sequences compared to the mid region. N-glycosylated sequons were found much farther from C terminus compared to the N-terminus of the protein sequence and this effect was more pronounced for NXS sequons. The distribution of sequons, modeled based on balls-in-boxes classical occupancy, showed a near-maximum probability. Considerable proportion of sequons was found within a distance of ten amino acids, indicating that the steric hindrance was not a key factor in protein N-glycosylation. Interestingly, the distribution of all sequons present in N-glycoproteins showed a pattern very similar to that of glycosylated sequons. The results indicate that protein N-glycosylation chiefly follows a random design.     

Photocatalytic methane decomposition over vertically aligned transparent TiO2 nanotube arrays

We report here on a study of vertically aligned TiO(2) nanotube arrays grown by the one-step anodic oxidation technique and their photocatalytic performance for methane decomposition. Quantitative activity data as a function of film thickness is obtained.