Giovanni Torriano

Flow properties of hydroxypropyl guar gum and its long-chain hydrophobic derivatives

Abstract This paper deals with the rheological properties under continuous shear flow conditions of aqueous systems of hydroxypropyl guar gum (HPG) and three derivatives, of the same or lower molecular weight (MW) than HPG, characterized by different contents of long-chain hydrophobic pendants. The shear-dependent behavior of HPG and of the derivative with low molecular weight and low degree of hydrophobization (LMLH) resembles that of isotropic polymer solutions; accordingly, it can be fairly well described by the Cross equation, the effect of temperature discussed in terms of activation energy for viscous flow and the dependence of the Cross parameters η0 and λ on polymer concentration expressed by simple power-laws. The other two derivatives considered, i.e. the one of low MW and high long-chain hydrophobic substitution and the other of a MW as high as that of HPG and a low hydrophobic pendant content (LMHH and HMLH, respectively) show an unusual behavior, characterized by the existence of a shear rate interval in which the shear viscosity undergoes a catastrophic breakdown, and which cannot be described by any simple rheological model. Nevertheless, as far as the temperature effect is concerned, all experimental data can be combined into one master curve by a simple shifting procedure. All systems exhibit marked time-dependent properties of the thixotropic type, which are described with a stretched exponential model and discussed in terms of the variation of the model parameters with temperature and polymer concentration. The peculiar features of the long-chain hydrophobic derivatives can be ascribed to a balance between inter- and intramolecular interactions, which is mainly governed by the local stress field.

Rheological behaviour of low-fat and full-fat stirred yoghurt

The rheological properties of a low-fat and a full-fat yoghurt were investigated under continuous and oscillatory flow conditions with a rotational coaxial-cylinder viscometer at different temperatures. In continuous shear flow tests, flow instability phenomena were detected below a critical shear rate value. Flow curves were correlated with the three parameter De Kee-Turcotte model, and the temperature dependence of the relevant parameters was demonstrated. The non-linear viscoelastic properties of both types of yoghurt were described, at first approximation, in terms of the complex modulus G∗ and phase lag δ. The effects of temperature on weakening of the yoghurt structure was illustrated.

Activity coefficients of hydrocarbons in phthalates

Abstract Activity coefficients at infinite dilution of hydrocarbons in eight phthalates at 25, 50, 75, 100 and 125 °C were determined by gas chromatography. The effects of the nature of the esterifying alcohols and of the solute and that of temperature are discussed. A non-linear behaviour of 1n γ1∞ versus 1/T was found; hence the excess partial molar heat capacities at infinite dilution of the solutes in the phthalates studied (CpE,∞) were evaluated over the temperature range 25–125 °C and a relationship between 1n γ1∞ and T (°K) was obtained.

Correlation of hydrocarbon activity coefficients with solubility parameters for phthalate esters

Abstract Eight phthalate esters (diethyl, di-n-butyl, diisobutyl, dicyclohexyl, di-2-ethylhexyl, diisodecyl, butyl benzyl and butyl ethylhexyl phthalate), commonly used as stationary phases in gas chromatography and as plasticizers for polymeric materials, have been characterized by determining the polar and non-polar contributions to the solubility parameter in accordance with the theory of Weimer and Prausnitz. The results obtained are in good agreement with the activity coefficients at infinite dilution determined previously by gas chromatography. The influence of the nature of the esterifying alcohols is discussed. Good agreement was found between the order of the “polarity indices” determined in the present work and the few data reported in the literature by Hansen.

Rheological aspects of the hardening of water-borne alkali silicate zinc-rich paints

SummaryThe kinetics of the reaction between zinc dust and alkali silicate in water-borne silicate zinc-rich paints was investigated on the basis of rheological measurements. The paints studied were formulated with the same zinc dust/silicate vehicle ratio. Different zinc dusts were employed. The vehicle was a litiumsodium silicate solution. Formulation included a rheological agent to prevent rapid settling of suspended zinc dust.Rheological tests were carried out at various time intervals after paint mixing. Each sample was subjected, stepwise, to increasing and decreasing shear rate sequences, each shear rate being applied until stress steady values were attained. Shear rates were within the range 2.35 to 1700 s−1; subsequently, hysteresis cycles were traced. Equilibrium data were fitted with the Bingham equation.Yield values and ultimate viscosities obtained at the various test time intervals were compared. Both parameters were found to increase with increasing time after mixing. Their increase in time brings out the fact that two successive processes take place; accordingly, structural hypotheses were suggested taking into account the modification set up in the system by the zincsilicate reaction.ZusammenfassungDie Kinetik der Reaktion zwischen Zinkstaub und Alkalisilikat in wäßrigen Silikat-Zink-Farben wurde mit Hilfe rheologischer Messungen erforscht. Die untersuchten Farben wurden stets mit dem gleichen Verhältnis von Zinkstaub zu Silikat-Träger formuliert. Dabei wurden verschiedene Zinkstäube eingesetzt. Der Träger bestand aus einer Lithium-Natrium-Silikat-Lösung. In der Formulierung war außerdem ein rheologischer Wirkstoff, der das schnelle Absetzen des suspendierten Zinkstaubes verhindern sollte, enthalten.Die rheologischen Messungen wurden zu verschiedenen Zeiten nach dem Farbmischen durchgeführt. Jede Probe wurde einer auf- und absteigenden Stufenfolge von Schergeschwindigkeiten unterworfen, wobei die Scherung solange konstant gehalten wurde, bis sich ein stationärer Wert eingestellt hatte. Die Schergeschwindigkeiten lagen dabei in einem Bereich zwischen 2,35 und 1700 s−1. Anschließend wurden Hysteresisschleifen aufgenommen.Die Gleichgewichtswerte ließen sich mit der Bingham-Gleichung beschreiben. Fließgrenzen und Endviskositäten, die nach verschiedenen Zeitdauern gemessen worden waren, wurden miteinander verglichen. Beide Parameter nahmen mit der Zeit zu. Daraus ist erkennbar, daß zwei aufeinanderfolgende Prozesse stattfinden. Dementsprechend werden Strukturmodelle vorgeschlagen, welche die Veränderungen in Rechnung stellen, die als Folge der Zink-Silikat-Reaktionen in dem System stattfinden.

Flow properties of oven-curing high solids white enamels

For the characterization of the rheological behaviour of a white, high solids, oven-curing enamel for household equipment with regard to its industrial application, attention was focused on the flow aspects involved in laying down process and film formation phenomena, i.e. viscosity at very high and very low shear rate, structural build-up in rest conditions, presence and magnitude of elastic components. Hence, investigations were aimed at (1) the determination of the equilibrium flow curve; the application of the Shangraw-Grim-Mattocks model led to the evaluation of parameters to describe infinite shear-rate viscosity and yield stress; (2) obtaining information about particle aggregation state in shear conditions. Quemada model gave indication that, even at very high shear, particle aggregates are stable; (3) the determination of time-dependent behaviour: elastic components were found to be almost negligible; the Trapeznikov-Fedotova procedure allowed thixotropic build-up in rest conditions to be evaluated, as concerns both amount and kinetics. Remarkable flow features found were: differences in the temperature dependence of viscosity at low shear rate and of yield value for enamels formulated with different pigment volume concentrations and the strong effect of the pigment volume concentration on the initial rate of thixotropic build-up in rest conditions.

Some Aspects of the Flow Properties of High-Build Paints

High-build paints are at present very popular on account of both technical and economical reasons. They are dispersions of one or more pigments and extenders in a low-molecular weight polymer solution; they contain additives and rheological agents; the volume fraction of the disperse phase usually ranges between 0.15 and 0.25.